LIST OF CONTENTS - University of Glasgow :: School of...

Final Year Project Jorge Lorente Mallada [0701914]

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LIST OF CONTENTS

1. SUMMARY……………………………………………………...….4 2. LIST OF OBJECTIVES…………………………………………..…5

3. INTRODUCTION……………………………………………….….6

4. LITERATURE REVIEW……………………………………….…..7

THEORETICAL CONCEPTS……………………………………….....7 PREVIOUS WORK………………………………………………….....8

5. CURRENT TEST METHODS……………………………………..9 INTRODUCTION…………………………………………………......9 TESTING…………………………………………………………..…12 ANALYSIS OF THE RESULTS…………………………………..…31 CONCLUSIONS…………………………………………………..….38

6. BIAXIAL BIAS EXTENSION TESTS……………………..…....39

INTRODUCTION…………………………………………….……...39 TESTING…………………………………………………………......42 ANALYSIS OF THE RESULTS………………………………..…....48 CONCLUSION…………………………………………………..…...52

7. DESIGN OF A BIAXIAL BIAS EXTENSION RIG………….....53

INTRODUCTION…………………………………………………....53 PRELIMINARY DESIGN CONCEPT……………………………....53 DESIGN SPECIFICATIONS………………………………………...54 DESIGN CONCEPT……………………………………………….....56

8. REFERENCES……………………………………………............59 9. APPENDICES…………………………………………………….61


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LIST OF FIGURES FIGURE 1: Warp and Weft of reinforced composite material. FIGURE 2: Non-consolidated material. FIGURE 3: Consolidated material. FIGURE 4: Picture Frame Test Rig. FIGURE 5: Uniaxial Bias Extension Rig. FIGURE 6: Instrom tensile testing machine. FIGURE 7: Zwick-Roell tensile testing machine. FIGURE 8: Tape of the composite material used during the project. FIGURE 9 a: Picture Frame Rig disassembled. FIGURE 9 b: Picture Frame Rig assembled. FIGURE 10: Picture Frame sample. FIGURE 11: Set of Picture Frame sample and rig ready for being tested. FIGURE 12: Layout of Picture Frame Test. FIGURE 13: Picture Frame sample after carrying out the test. FIGURE 14: Load-Extension graph for Picture Frame tests. FIGURE 15: Load-Theoretical shear angle graph for Picture Frame tests. FIGURE 16: Experimental-Theoretical shear angle graph for Picture Frame tests. FIGURE 17: Load-Experimental shear angle graph for Picture Frame tests. FIGURE 18: Normalised load-Experimental shear angle graph for Picture Frame tests. FIGURE 19: Sample of Uniaxial Bias Extension test. FIGURE 20: Sample of Uniaxial Bias Extension clamped within the UBE rig. FIGURE 21: Sample of Uniaxial Bias Extension test after carrying out the test. FIGURE 22: Load-Extension graph for Uniaxial Bias Extension tests. FIGURE 23: Load-Theoretical shear angle graph for Uniaxial Bias Extension tests. FIGURE 24: Load-Extension graph for Uniaxial Bias Extension tests. FIGURE 25: Load-Theoretical shear angle graph for adjusted data of Uniaxial Bias Extension tests. FIGURE 26: Experimental-Theoretical shear angle graph for adjusted data of Uniaxial Bias Extension tests. FIGURE 27: Load-Experimental shear angle graph for Uniaxial Bias Extension tests. FIGURE 28: Normalised load-Experimental shear angle graph for Uniaxial Bias Extension tests. FIGURE 29: Horizontal bar for Picture Frame tests. FIGURE 30: Detail of parallel lines to Picture Frame rig .


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FIGURE 31: Difference between adjusted data and non-adjusted data graphs. FIGURE 32: Expected result of Normalised load graph while carrying out Picture Frame tests. FIGURE 33: Example of Picture Frame problem (case 1). FIGURE 34 a: Picture Frame squared sample. FIGURE 34 b: Picture Frame sample partially sheared before the test. FIGURE 35: Example of Picture Frame problem (case 2). FIGURE 36 a: Picture Frame squared sample. FIGURE 36 b: Picture Frame sample partially sheared. FIGURE 37: Increase of “d” distance on Uniaxial Bias Extension tests. FIGURE 38: Wrinkling problems. FIGURE 39: Generic sample of Biaxial Bias Extension tests. FIGURE 40: Sample geometries while carrying out Biaxial Bias Extension tests. FIGURE 41: Development of a Biaxial Bias Extension test. FIGURE 42: Sample dimensions according to model geometry c). FIGURE 43: Sample dimensions according to model geometry a). FIGURE 44: Sample with c) dimensions placed within the rig. FIGURE 45: Sample with a) dimensions placed within the rig. FIGURE 46: Sample of Biaxial Bias Extension tests deformed after carrying out the test. FIGURE 47: Kinematics graph of Biaxial Bias Extension test. FIGURE 48: Drawing of starting position of sample for Biaxial Bias Extension test. FIGURE 49: Drawing of ideal behaviour expected on the sample after carrying out the test. FIGURE 50: Drawing of real behaviour expected on the sample after carrying out the test. FIGURE 51: Analysis of “d” distance in Biaxial Bias Extension tests. FIGURE 52: New geometry proposed for model geometry c). FIGURE 53: Buduka N Amadi pre-design of a biaxial testing device. FIGURE 54: Table where the final design will be implemented. FIGURE 55: Gordon Pettigrew design concept. FIGURE 56: Bill of materials. FIGURE 57: Final assembly of the design proposed.


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1. SUMMARY Within this project an in-depth analysis of reinforced composites shear behaviour has been carried out. The main objective was to develop characterisation tests in order to identify the parameters that were offering a better understanding of the work, as those parameters could be used later for the implementation of constitutive models through finite element. Those constitutive models are the main goal of research within the composites area, as could help to improve manufacture and design processes while working with these materials. The shear behaviour of the material helps to understand its rheological analysis, which is something that many companies are looking for nowadays. For carrying out this analysis, two characterisation tests using non-consolidated composite material were carried out, Picture Frame Tests and Uniaxial Bias Extension Tests. The objective followed was to obtain similar results while carrying out both tests, and also to analyze in-depth each of them in order to identify which one was offering a closer behaviour to the ideal one. While carrying out those tests, some difficulties were encountered, what depicted that although they were offering good results, its analysis increased its grade of complexity. This was the reason why a third type of test was analysed, the Biaxial Bias Extension Tests, which in theory were supposed to offer a closer response to the ideal behaviour, but which proper development within the Materials Laboratory was not possible because there was no machinery available. As a result of the interest that the Biaxial Bias Extension Tests could offer, a design for carrying out this test was developed. The design, which was based on a Solid Edge model, was done according to the facilities that were available in the Materials Laboratory, where the test could be carried out once the manufacture of the design is done. This will offer more information while developing characterisation tests, and therefore will help to the implementation of future constitutive models of reinforced composites.


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2. LIST OF OBJECTIVES As within the whole project two big different stages have been identified, the objectives planned for each of them have been classified in two different sections: Objectives of the first stage of the project

- Analysis of characterisation tests carried out on composites materials, particularly Picture Frame and Uniaxial Bias Extension Tests, which will help in future to the development of constitutive models.

- Carry out characterisation tests for analysing shear behaviour of composite materials.

- Comparison between the tests considered, analysing the information that each of them offers, and highlighting their main advantages and disadvantages.

- Analysis of the third characterisation test that allow understanding of reinforced composites, Biaxial Bias Extension tests.

- Highlight the theoretical differences between this test and the ones analysed previously.

Objectives of the second stage of the project

- Design a Biaxial Rig that will allow the development of Biaxial Tests in future in the Materials Laboratory of the University of Glasgow.

- Develop a CAD model of the whole rig. - Manufacture of the rig

Objectives reached From further readings on the testing of reinforced composites area, the main concepts of characterisation tests have been understood and applied while carrying out the testing. Moving on, previous work from other students helped while both analysing characterisation tests, and carrying out the tests. Furthermore, a whole 3-D design has been developed in a CAD model, which will allow to the future development of Biaxial Bias Extension tests. However, the manufacture of the model has not been finalised for the moment although the plans have already been sent to the Workshop Unit, where they will be manufactured before the end of the term.


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3. INTRODUCTION Nowadays, working with composites is becoming more and more important as those materials can offer high mechanical properties combined with low weight, which is depicted in really high strength-density and stiffness-density relations [1, page 27]. In addition, these materials offer cost advantages while comparing them to other strength materials such as titanium. All these characteristics make these materials to be used in a very wide range of industrial applications [2, page 295], although this is not the only area where they are being used, as many commercial applications such as sports items have already started to work with reinforced composites [1, page 52], making their use a symbol of innovation, while the work with aluminium and steel is becoming obsolete. On the other hand, materials such as aluminium and steel have been used in the industry since many years, so a deep knowledge on them is understood thank to characterization tests such as tensile or compression tests that allow understanding of the materials’ behaviour under different loads and boundary conditions. Those characterization tests are not so developed in the reinforced composites area, and their analysis represents one of the main important goals within this project, as they are going to facilitate a better understanding on the materials’ properties and they are also going to allow prediction of materials’ behaviour or response while being exposed to different conditions. Characterization tests The main object followed while developing characterisation tests, is that can help to the development of new constitutive models, or otherwise to determine the relevant fitting parameters of existing ones that help to the simulation process of reinforced composites while predicting accurately its rheological behaviour [3]. This is key factor for composite industries, as they lack on knowledge within this area, and a better understanding of the materials’ rheological behaviour can lead to better design and manufacture methods. The first step while carrying out characterization tests is to identify the parameters that are offering deeper and more interesting information about the complex behaviour of the material. Those parameters can be analyzed either directly or indirectly from the tests, as for example the normalized load applied to a material can be directly known because it is an output data from the testing machines used while developing the tests. However there are other parameters such as the shear angle that have to be calculated either by Image Analysis or by using specific software. Comparison between materials Once the characterization tests have been set up, different composite materials can be tested in order to classify them following different criteria such as strength, deformation, or temperature for instance. This offers the advantage of comparing those materials for identifying which one offers the preferred behaviour for any special condition or situation.


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4. LITERATURE REVIEW 4.1 THEORETICAL CONCEPTS Reinforced composite materials are formed from woven fabrics which at the same time are a set of thousands of long and thin fibres possessing high strength and stiffness which are aligned in two different directions forming the warp and the weft of the fabric [4, page 9] (See Figure 1)

Figure 1: Warp and weft of reinforced composite material

In addition there is an important factor such as the material consolidation characteristics that has to be considered and that must be taken into account in order to develop a good analysis of the material tested, as both stiffness and strength vary depending on the one that is analysed [4, page 11].

• Unconsolidated material: Considers just the fibres that form the bulk of the material. (See Figure 2)

• Consolidated material: In this case, the fibres have been mixed with a resin for consolidating the material. The consolidation takes place in an autoclave where compaction of the resin and the fibres happen because of the effect of heat and pressure. (See Figure 3)

Figure 2: Non-consolidated material Figure 3: Consolidated material Moving on, woven fabrics have been used in the production of composite products due to ease of handling, the variety of woven patterns available and the relative stability of their pattern and shape during lay up [5]. However, the efficient use of these materials is limited by the problems associated with their design and manufacture, like for example draping of the fabric[6], which means that it can undergo large in-plane deformations, changing the fibre orientations and the lamina thickness, alterations that have a


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significant impact on the strength and stiffness of the final structure [6]. As a result of this fact, accurate models of the draping characteristics of the fabric are needed to design the preform correctly, helping to predict the properties of the finished component [7]. Considering the concept of the deformations that can cause alterations in the final structure, two principal mechanisms of fabric deformation during draping have been identified: pure shear and shear at the fibre cross-over [6]. Between both of them, the mechanical behaviour that is going to be analyzed further in this project is pure shear, as it comprises yarn slippage (which is a phenomenon that reduces the local strength of the fibre in a process of shear deformation [8]) and relative movements between warp and weft [6]. Basically, pure shear consists of a trellising action that can cause the block of the fibres which can lead to wrinkling behaviour [5] although this response will depend on the spaces between the yarns and the load applied. Finally, according to [9] The main aspects that must be focused while analyzing those materials are:

• Non-linearities of behaviour due to the undulations of the yarns • The increase in shear rigidity with the angle between warp and weft

yarns especially when the shear angle becomes large. 4.2 PREVIOUS WORK Within this project previous analysis carried out by some researchers within the reinforced composites area such as Harrison, Potluri, Sharma and many others has been considered. Apart from that, this project has been based on three more final year projects developed by students of the University of Glasgow as follows:

- Nisarga Chaudari (2007): This Msc student developed a final year project “Formation and Mechanical Behaviour of Composite Material” based on the analysis of shear behaviour of non-consolidated material by carrying out characterisation tests such as Picture Frame Tests.

- Leo Martin (2007): This BEng student developed a final year project “Investigation Into The Shear Testing Of Reinforced Composites” where he analysed shear behaviour of consolidated material and its friction reponse. In addition he developed the design of a Uniaxial Bias Extension rig that was used in this project for the development of characterisation tests.

- Buduka N Amadi (2007): The title of the final year project that this Msc student carried out was “Design of a Biaxial Testing Device” and has been really useful in the second stage of this project while designing the biaxial rig, as some concepts had already been covered within his project.

- Gordon Pettigrew: This BEng student developed an analysis of a thermoforming station for carrying out tests with reinforced composites. The design carried out helped especially for the designing stage of this project.


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5. CURRENT TEST METHODS 5.1 INTRODUCTION There are three main tests methods for understanding shear behaviour on fibre reinforced composites, Uniaxial Bias Extension, Picture Frame Tests [3] which are the one’s that are going to be analysed on the first stage of the project and Biaxial Bias Extension Test [10]. The deformation of the bias extension experiments is measured from a central gauge area while on Picture Frame it is measured from the whole sample. 5.1.1 PICTURE FRAME TEST In this test, the material tested is clamped into a squared rig that comprises four arms of identical length as shown in Figure 4. The material tested must be square-shaped so, once clamped the four arms form two couples of parallel arms. While preparing the sample of material it must be checked that it is cut in 45 º, so the warp and the weft will be parallel to the arms of the rig once the sample is clamped [9, 11]. For carrying out the test, the rig with the sample is placed in a tensile testing machine that applies a tensile force by pulling the rig from the top joint causing the picture frame to move from an initially square configuration into a rhomboid. Consequently, the whole specimen within the picture frame is theoretically subjected to pure shear [5]. According to [3], while clamping the specimen to the rig, two different boundary conditions can be distinguished:

• Tightly clamped conditions: In this case, the material, usually thermosetting is tightly clamped to the rig so no slippage could occur between the rig and the sample. This condition has a slight inconvenience as tightening the material can cause damage on it what can lead to bad test results.

• Loosely clamped conditions: In this situation, the material, usually

thermoplastic is not clamped so tight, so it is only needed to check that the sample is correctly placed and held within the rig, but without tightening it.

One of the main advantages of this test is that the analysis of the results is going to be easier, because this device it is shown as the most satisfactory to impose a homogeneous shear deformation in a woven reinforcement [12].


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Figure 4: Picture Frame Test Rig

5.1.2 UNIAXIAL BIAS EXTENSION TEST This test is simpler than the Picture Frame as the sample and the rig used have simpler shapes as shown in Figure 5. In this case the rig where the sample is clamped is formed by two rectangular plates that also have clamping pins to tight the material correctly. This test involves clamping a rectangular piece of woven material so that warp and weft directions of the tows are orientated initially at 45º to the direction of the applied tensile force [9]. The specimen dimensions are characterized by the sample’s length/width ratio, which is represented by � [9] and considering this ratio, the length of the material sample (Lo) must be at least twice the width (wo) according to the normalisation method that is going to be followed [3].

a b

Figure 5: Uniaxial Bias Extension Test; before (a) and after (b) carrying out the test The theoretical reason why a length/width ratio equals to 2 has to be reached is because for those conditions the sample can be divided into three different sections (A, B and C) where section A undergoes pure shear behaviour, section B is characterized because the


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shear angle there is half that in region A and section C remains un-deformed throughout the test and consequently does not contribute to shear force [3]. However, the use of this test usually brings out a complex combination of shear and tension, which makes it difficult to isolate the shear deformation in the test and hence complicates the characterization of pure shear behaviour of textile composites [13]. This is the main reason why the picture frame test has originally been designed as an alternative for the bias extension test, in order to obtain a closer deformation to the pure trellis mode [14]. In this test, and as well as happened in the Picture Frame Test, the tensile force is exerted by a tensile testing machine that pulls up the rig through the top plate. 5.1.3 ADVANTAGES AND DISADVANTAGES OF BOTH TESTS

Prior to the analysis of the main advantages and disadvantages of both tests, a couple of common features have been identified for both tests:

- Both of them are simple to perform and the results are reasonable repeatable [3].

- The forces applied to the material are resolved into forces along and parallel to the tow directions [13].

Moving on from the theoretical concepts analyzed and the knowledge of both Picture Frame and Uniaxial Bias Extension tests, a first and simple comparison based on the advantages and disadvantages of each tests can be seen as follows: a) Picture Frame Test The main advantage of this test is that as said above, the deformation of the material is essentially homogeneous throughout the deforming sample(edge effects being ignored), so the kinematics of the material deformation are readily calculated [11], which leads to an easier analysis of the results. Another advantage is does this test does not pull the material apart, so no damage is caused to the sample. On the other hand, the main disadvantage of this test is the boundary conditions that are imposed to the sample, so in case of loosely clamped conditions the results may fail to induce the required kinematics, whereas in tightly clamped conditions the sample can be induced to a slight misalignment [3] which can lead to mistaken results. This misalignment can be of either warp or weft, or both of them, so the process of placing and clamping the sample within the rig is crucial and it must be carried out carefully in order to avoid damaging the sample and checking that both warp and weft are parallel to the arms.


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b) Uniaxial Bias Extension Test The main advantage of the test is that boundary conditions are much less relevant to the test result [3], besides the process of clamping the sample is quite easy, so wrong test results won’t be expected because of misalignment. On the other hand one of the main disadvantages of the test is that, as explained before, three different areas must be identified within the sample in order to analyze shear behaviour, what makes the analysis quite complicated and confusing as the deformation field within the material is not homogeneous [3]. In addition, these tests are likely to suffer both shear response and intra-ply slip, what in terms of rheological analysis presents an extra difficulty, as the energy associated to the deformation will be dependent of both concepts, not only of shear as it happens in Picture Frame Tests. The intra-ply slip phenomenon provides an alternative mechanism by which the sample can accommodate the imposed material extension depending on the resistance offered by the shear deformation mechanism. Thus, at some point during the development of a bias extension test, intra ply slip becomes a significant deformation mechanism and eventually becomes in accommodating sample extension, so the sample can increase in length without further increase in shear angle [3]. Another important disadvantage is that in bias extension tests and due to the high forces experimented; the material does pull apart from either the top or bottom plate. As soon as this happens the analysis of the sample has no importance, as the sample is being damaged 5.2 TESTING The main objective that is followed within this section is to describe the main steps needed for the correct development of the characterisation tests that are going to be carried out. In addition a brief summary of the devices available on the laboratory is elaborated. 5.2.1 DESCRIPTION OF THE PROCESS While carrying out these characterisation tests, some main steps were identified and described as follows: 1) IDENTIFYING PARAMETERS [14] The tests are developed using a tensile testing machine which works with a specific software that shows technical parameters such as the load applied or the displacement experienced. However, those parameters are not the one’s that are going to be used to develop constitutive models, and to understand the shear behaviour because they don’t offer as much information related to the shear behaviour as for example the shear angle, measured either theoretically or experimentally. As those parameters can not be directly identified from the tensile testing machine, some theoretical concepts that involve both Picture Frame and Uniaxial Bias Extension tests must be taken into account.


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Picture Frame test calculations The data obtained from the computer during the test is used to determine the shear angle of the sample for a particular displacement, by using the following formula [3, 6] that relates the shear angle to the displacement experienced by the sample: � = (� / 2) – 2*arcos [(1/�2) + (d1/2*L1)] Where, � = Theoretical shear angle d1 = Extension of the sample L1 = Length of the picture frame rig’s arm Uniaxial Bias Extension test calculations In this case, the method that is followed is the same as for Picture Frame Test, and only differs in the definition of one of the parameters used as it is shown [3, 6]: � = (� / 2) – 2*arcos [(1/�2) + (d1/2*L1)] Where, � = Theoretical shear angle d1 = Extension of the sample L1 = Length of the central region before deformation For both tests, this formula is used for analysing the kinematics of the tests, which means the analysis of the shear angle, depending only on the displacement data. So, if the displacement used comes from theoretical analysis, the shear angle obtained would be representing the shear angle for the theoretical analysis and in the case of the experimental analysis. The kinematics analysis named before is founded on geometrical mapping algorithms based on the formula explained before. 2) IMAGE ANALYSIS In order to obtain experimental results of the tests, image analysis allows to data acquisition of the experimental parameters that are going to be calculated such as experimental shear angle and displacement. The basis is just to record the tests by using a video camera like many others investigators such as Sharma et al and Chang et al have done. This will allow establishing comparisons between the theoretical and the experimental behaviour of the material, what will help to develop constitutive models. The procedure that is followed to obtain the experimental values is to record the tests, so the experimental shear angle could be directly obtained by measuring directly from a computer screen, and the experimental displacement could be calculated once known the length of the test and the speed of the tensile testing machine. The layout of the


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digital camera reveals as a key factor, as it must be placed perpendicularly to the tested sample. 3) NORMALISATION Once all the parameters have been identified, it is necessary to plot them for understanding the shear behaviour. However, if the results obtained want to be compared with the ones corresponding to other materials and tests, a process of normalisation of the results must be carried out, because according to [3], the determination of material properties in a characterisation test should be independent of the test method and the sample size. Talking in these terms, the shear force produced during the testing of a material should be independent of both sample dimensions in the case of picture frame and bias extension tests and length/width ratio in the case of bias extension tests. The normalisation procedure followed by Harrison et al is based on normalising the force exerted by the tensile testing machine, and it is different for Picture Frame tests and for Uniaxial Bias Extension Tests:

- Normalisation procedure of Picture Frame Tests: In this case, the parameter that can vary is the size of the sample, and as it is square-shaped, the normalised value of the applied force will be calculated by dividing the applied load by the side length of the picture frame rig [3].

- Normalisation procedure of Uniaxial Bias Extension Test: For this particular test, the parameter that varies depends on the dimensions of the sample as well, but in this case it is the length of the central gauge area, which is the one that undergoes pure shear response. As a result, the normalised load will be calculated by dividing the applied one by the length of the central gauge area [3].

5.2.2 SUMMARY OF THE DEVICES AVAILABLE IN THE LABORATORY The materials laboratory within the Mechanical Engineering Department offers the use of the following devices:

- Two tensile testing machines: One of them is an Instrom machine where the first tests were carried out. At the beginning this machine was the one used because the other one was not available at that moment. The other machine is a Zwick-Roell which was much more new and which offered a greater analysis of the results. Anyway no difference in accuracy has been considered in the results, as the main advantage between the new one and the old one is that the development of the tests was much easier and quicker. Both machines can be seen in Figure 6 and Figure 7.


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Figure 6: Instrom machine Figure 7: Zwick-Roell machine - Material: In this project, an unconsolidated woven perform with self-

reinforced polypropylene (PP) tape from Don&Low Ltd was used. The material is a self reinforcing composite (SRC) which is a new class of thermoplastic composite, in which the matrix and fibres are composed of the same polymer [17]. (See Figure 8).

Figure 8: Tape of the composite material used during the project


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- Picture Frame Rig: This device was available on the lab as a result of the final project of the BEng student Murray Whyte. It is shown later within the Picture Frame Test description in Figures 9 a and 9 b.

- Uniaxial Bias Extension Rig: As well as the picture frame one, this rig was the result of the final project of another BEng student Leo Martin. It is shown later within the Uniaxial Bias Extension Test description in Figure 17.

5.2.3 PICTURE FRAME TESTS For avoiding misunderstanding of the results obtained, three different tests were carried out, so a better analysis of the results could be done. 5.2.3.1 Preparing the samples The material that was used for both tests was non-consolidated polypropylene provided by Don&Low, a leader company in the composites area. The size of the sample had to be prepared according to the dimensions of the picture frame rig available on the laboratory (See Figures 9 a and 9 b), which meant a tested area of 170x170 mm2, so the area of the whole sample needed to be greater for allowing a proper clamp to the rig (See Figure 10).

Figure 9 a: Picture Frame rig disassembled Figure 9 b: Picture Frame rig assembled


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Figure 10: Picture Frame sample ready for clamping into the rig (corners were removed in order to allow a correct clamping)

Once the sample was prepared it had to be placed within the rig, so both warp and weft had to be parallel to the arms of the rig as explained before [9, 11]. As can be seen in Figure 11, where the set of rig and sample are prepared for testing, a cross had to be drawn in the middle of the sample for further analysis of experimental results. In addition, the corners of the sample were removed in order to place the sample correctly within the rig, and some holes were made in the sample for clamping it properly.

Figure 11: Set of sample and rig ready for being placed in the tensile testing machine


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5.2.3.2 Testing After having prepared the set, it was set up into the tensile testing machine which operating conditions were: (See Figure 12)

- Room Temperature - Speed: 200 mm/min - Load applied: 1 kN load cell. - Displacement: 100 mm - Thickness of the sample: 0.5 mm

This machine uses software which inputs are the operating conditions and which outputs are going to be the load applied and the corresponding displacement, as they are the one’s that offer the information required for further analysis of the theoretical results.

Figure 12: The sample is ready for testing. In the image can be seen the computer which software interfaces with the tensile testing machine for giving the corresponding results of the test.

After setting up the machine, the test was carried out. Firstly, the sample was square-shaped, but once the top joint was pulled up by the tensile testing machine, the sample changed into a rhomboid shape as the material was being deformed by the load applied [3]. In the end, the sample was totally deformed as shear deformation had occurred. (See Figure 13)


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Figure 13: Image of the sample after carrying out the test. It doesn’t seem really damaged if compared with the original one shown in Figure 10, because the effect of shear causes damage within the fibre, affecting to its properties. Only the extremes of the sample seem damaged because of the clamping action within the rig.

Once the test was done, the information that the machine offered was, a graph were load applied versus its corresponding displacement was plotted, and the corresponding outputs measured while carrying out the test, presented in an excel file. However, all that information was theoretical, as it was what the machine measured because of the effect of the load applied by the load cell, so it was necessary to contrast it with the real deformation that the sample was experiencing. For obtaining this information it was necessary to carry out Image Analysis, as explained previously in the testing section, so the test was recorded by using a digital camera. 5.2.3.3 Analysis of the results

a) Theoretical Results: with the data obtained from the machine and the theoretical calculations explained before in the testing section, the theoretical shear angle could be obtained.

The graphs that have been plotted display the relationships between the load applied, the extension of the sample and the theoretical shear angle. Those graphs are shown in Figure 14 and Figure 15.


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LOAD-EXTENSION PICTURE FRAME

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Figure 14: This graph shows the relationship between the load applied from the top joint of the picture frame rig and the displacement (measured vertically) that the whole rig is pulled up. The different colours represent three different tests that have been carried out.

LOAD-THEORETICAL SHEAR ANGLE PICTURE FRAME

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Figure 15: In this graph it is shown the relationship between the load applied from the top joint and the theoretical shear angle that the sample experiences measured in degrees. The different colours show again the results of three different tests.

b) Experimental Results: the way that the experimental shear angle was determined was by measuring how it varied within time intervals (See section 5.2.1). In addition, in this case, once known the time and speed of the test, the displacement could be determined, and therefore the theoretical shear angle for those experimental conditions.


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Once the experimental and theoretical shear angles were determined, the experimental load applied could be determined by interpolating the data obtained in the theoretical results.

With all these data, two graphs were displayed as shown in Figure 16 and Figure 17.

EXPERIMENTAL-THERORETICAL SHEAR ANGLE PICTURE FRAME

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Figure 16: This graph shows the relationship between the experimental shear angle that the sample is experiencing and the theoretical one that is supposed to suffer. The ideal situation is, therefore, when both shear angles are similar, which means that they follow the 45º line drawn in the graph, what actually happens as can be seen.

LOAD-EXPERIMENTAL SHEAR ANGLE PICTURE FRAME

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Figure 17: This graph shows the relationship between the load applied through the top joint and the experimental shear angle.


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5.2.3.4 Normalization of the results This is an important procedure that is based on normalizing the results of the experimental load applied to the sample, what will allow further comparisons between different materials and different operating conditions of the tests as was explained before in the normalisation section. In Figure 18 it is shown the graph with the normalised results.

NORMALISED LOAD-EXPERIMENTAL SHEAR ANGLE PICTURE FRAME

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Figure 18: Normalization of the load applied for picture frame tests. 5.2.4 UNIAXIAL BIAS EXTENSION TESTS For avoiding misunderstanding of the results obtained, three different tests were carried out, so a better analysis of the results could be made. 5.2.4.1 Preparing the samples The material that was used for both tests was non-consolidated polypropylene provided by Don&Low, a leader company in the composites area. Firstly, the main important factor that had to be considered while preparing the sample was that it had to be cut in 45º, so once it was clamped, both warp and weft would be forming 45º respect the plates of the bias extension rig [9]. (See Figure 19).


- 23 -

Figure 19: In this image, the whole sample is shown. The angle between the warp and weft can be seen as well as the lines that had to be drawn in order to identify the zone of pure shear conditions (the central one).

Secondly, the size of the sample had to be prepared according to the theoretical concepts related to Uniaxial Bias Extension Tests (See section 5.1.2), which meant considering a length/width ratio equals to 2, so the length of the tested area had to be twice the width. The dimensions of the tested area were 220x110 mm2, but the dimensions of the whole sample had to include the clamp area shown in figure 16. Once the sample was prepared, and before clamping it to the rig, some lines had to be drawn in order to identify the area under pure shear conditions (See section 5.1.2). Later, and before clamping the sample within the rig, some holes had to be made in the sample in order to clamp it properly. (See Figure 20)


- 24 -

Figure 17: In this image, the sample is ready for placing into the tensile testing machine.

5.2.4.2 Testing After having prepared the sample within the rig, the set was set up into the same tensile testing machine used for Picture Frame Test, considering similar operating conditions as well. (See section 5.2.3.2):


After setting up the machine, the test was carried out. At the beginning in the central area analyzed, the lines representing both warp and weft were perpendicular, but once the top joint was pulled up by the tensile testing machine, the sample became thinner and longer, as well as those lines stopped being perpendicular. After a few seconds, the material started to pull apart from either the top or the bottom plate, hence, the material was totally deformed as shear deformation and wrinkling had occurred. (See Figure 21)


- 25 -

Figure 21: Here can be seen the result after the test, where a part of the sample has been completely pulled apart (left side of the image), but the central area where pure shear conditions where acting had remained without pulling.

Once the test was done, the information that the machine offered was the same one as for PF tests, a graph were load applied versus its corresponding displacement was plotted, and the outputs measured while carrying out the test, presented in an excel file. However, as well as happened with the Picture Frame Test, all that information was theoretical, as it was what the machine measured because of the effect of the load applied by the load cell and the predetermined displacement, so it was necessary to contrast it with the real deformation that the sample was experiencing. For obtaining this information it was necessary to carry out Image Analysis, as explained before in section 5.2.1, so the tests were recorded again with a digital camera. 5.2.4.3 Analysis of the results

a) Theoretical Results: with the data obtained from the machine and the theoretical calculations explained before in the testing section, the theoretical shear angle could be obtained.

The graphs that have been plotted display the relationships between the loads applied, the extension of the sample and the theoretical shear angle. Those graphs are shown in Figure 22 and Figure 23.


- 26 -

LOAD-EXTENSION BIAS EXTENSION

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Figure 22: This graph shows the relationship between the load applied from the top joint of the uniaxial bias extension rig and the displacement (measured vertically) that the whole rig is pulled up. The different colours represent four different tests that have been carried out.

LOAD-THEORETICAL SHEAR ANGLE BIAS EXTENSION TEST

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Figure 23: In this graph it is shown the relationship between the load applied from the top plate and the theoretical shear angle that the sample experiences measured in degrees. The different colours show again the results of four different tests.


- 27 -

b) Adjustment of the theoretical results: While analyzing the theoretical results it

could be shown that the three tests didn’t offer the same plot, which was expected. That happened because some samples had experienced a wee displacement prior to the test caused probably while placing the sample, or while allocating it in the tensile testing machine. This displacement made the sample to be partially sheared, so in order to obtain a consolidate data to compare with the picture frame results, an adjustment of the data had to be done. So, considering the induced shear angle and basing on the formula used to obtain the shear angle explained in the theoretical concepts section, the induced displacement that was causing a induced shear angle could be calculated, and could be used as an input for the theoretical displacement data obtained from the tensile testing machine. The final adjusted results obtained are shown in Figure 24 and Figure 25:

Figure 24: Relation Load-Extension with adjusted data


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- 28 -

LOAD-THEORETICAL SHEAR ANGLE BIAS EXTENSION TEST

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Figure 25: Load-Theoretical angle with adjusted data

c) Experimental Results: the way that the experimental shear angle was determined was by measuring how it varied within time intervals. In addition, and as well as happened with the PF test, once known the time and speed of the test, the displacement could be determined, and therefore the theoretical shear angle for those conditions. Once the experimental and theoretical shear angles were determined, the experimental load applied could be determined by interpolating the data obtained in the theoretical results

With all these data, two graphs were displayed as shown in Figure 26 and Figure 27.


- 29 -

EXPERIMENTAL-THEORETICAL SHEAR ANGLE BIAS EXTENSION

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Figure 26: This graph shows the relationship between the experimental shear angle that the simple is experiencing and the theoretical one that is supposed to suffer. The ideal situation is, therefore, when both shear angles are similar, which means that they follow a 45º line drawn in the graph, what actually happens as can be seen.

LOAD-EXPERIMENTAL SHEAR ANGLE BIAS EXTENSION TEST

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Figure 27: This graph shows the relationship between the load applied through the top joint and the experimental shear angle.


- 30 -

5.2.4.4 Normalization of the results The method followed is the same as for PF tests and the results can be seen in Figure 28 where it is shown the graph corresponding to the normalization of the uniaxial bias extension tests that have been carried out.

NORMALIZED LOAD-EXPERIMENTAL SHEAR ANGLE BIAS EXTENSION

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Figure 28: Normalization of the load applied for picture frame tests.


- 31 -

5.3 ANALISYS OF THE RESULTS 5.3.1 PROBLEMS ENCOUNTERED WHILE CARRYING OUT THE TESTS While describing both Picture Frame and Uniaxial Bias Extension tests, a comparison between both tests was developed, basing on theoretical concepts, and now that some tests had been carried out using those characterisation tests, some other results that give more evidence of the difference existent between both tests were obtained despite both of them have the same focus.

5.3.1.1 Picture Frame Test

While carrying out the first tests, two main problems had to be faced, the possible misalignment of the fibre while clamping the specimen, and the correct squared position before starting the tests. According to [14], only 0.5 mm of misalignment is enough to cause a significant divergence in response. The reason why misalignment could be occurring to the fibre was because while clamping the specimen, the clamp was moving causing also the material to move, and as it was unconsolidated material, it was necessary to be really careful with it, otherwise it could be damaged. The other main problem was to place the rig in a squared position prior to start the test, because otherwise the material could be suffering either tension or compression depending on the angle deviation, and this could adversely affect to the results, as it is explained later in section B.

Solution: The way that those problems were dealt was by using a manufactured bar available on the laboratory (See Figure 29), that suited with the extents of the picture frame rig, allowing a perfect squared position while clamping the specimen and while placing the rig within the tensile testing machine.

Figure 29: Horizontal bar used for the analysis of PF tests in order to avoid both

misalignment and wrong start position of the set rig-sample

Another aspect that was considered while carrying out the picture frame tests was that one of the objectives was to check if the specimen was pulling out from the clamps or


- 32 -

not, because as it was said in the theory, picture frame tests shouldn’t be affected by this fact, but this had to be proved. So, basically some lines were drawn parallel to the clamps before carrying out the test, so once the test was done it could be analyzed if pulling out had occurred or not by checking if those lines had remained parallel to he clamp, what was proved as shown in figure (See Figure 30).

Figure 30: The arrows show the lines that were drawn parallel to the PF rig.

5.3.1.2 Uniaxial Bias Extension Tests

Firstly, these tests didn’t show any particular problem to deal with as the material had just to be clamped within the rig. However, the results obtained showed that although there was no change in the sample dimensions, there was an important difference between their graphs. In the end a possible cause was found; as the sample had to be drilled, the material could be being damaged by distortion of the fibres. Apparently, there was not a real solution apart from being careful while drilling the sample. Apart from that, in those tests as well as in the Picture Frame one’s the problem of damaging the sample while clamping it was also present, so the specimen could be partially sheared at the beginning, aspect that has been covered before in section 5.2.4.3 b), (Adjustment of the theoretical data). The difference between the normal data and the adjusted one can be seen in Figure 31.


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Figure 31: Difference between the adjusted data (left) and the original one (right)


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- 33 -

5.3.2 PROBLEMS CAUSED BY WRONG START POSITION IN PF TESTS While analyzing the normalisation force of the Picture Frame Tests, the result that it is expected is similar to the one seen in Figure 32 as at the beginning there is low force to apply because the material is starting to shear, and in the end the force increases as a result of the friction response of the fibres.

NORMALISED LOAD-EXPERIMENTAL SHEAR ANGLE PICTURE FRAME

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Figure 32: Expected result of the normalised load while carrying out picture frame tests.

However, and as shown in the following figures, two other situations can be analyzed.

1. As shown in Figure 33, in this case the force at the beginning is really high

because the material was already suffering tensile strength, so the shape of the specimen at the beginning was similar to Figure 34 b rather than to Figure 34 a. The main reason why this happened is because in Figure 34 b the fibres are already sheared, so the frictional resistance is higher.

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Figure 33: Example of case 1 Picture Frame problems


- 34 -

b) a) Figure 34: On the left it can be seen figure b) where the fibres are already sheared, and on the right it can be seen figure a) with a perfect squared sample. 2. As can be seen in Figure 35, in this case the load at the beginning is really

low, what suggests the idea that the material was under compression, and therefore the frictional resistance was lower. Talking in terms of shape, this means that the sample was squashed, so was looking like Figure 36 b rather than like Figure 36 a.

NORMALIZED LOAD-EXPERIMENTAL SHEAR ANGLE PICTURE FRAME TC

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Figure 35: Example of case 2 Picture Frame problem


- 35 -

b) a) Figure 36: On the left it can be seen the b) figure where the sample is under compresion, and on the right it can be seen figure a) with a perfect squared sample. Therefore, in order to develop this test correctly, it was necessary to check that no misalignment had occurred to the fibre, so the sample is not damaged. Another way of doing it is by using the horizontal bar mentioned before that guarantees a perfect squared shape of the sample.

Considering that the results obtained shear behaviour is not always the expected one, so a question is wondered for cases 1 and 2 studied. What is happening in both cases?

- Analyzing the first one, if the sample is under tensile stress, the central area

does not experiment either pure shear response or wrinkling, as the fibres start to enlarge causing an extension of the fibre.

- Analyzing the second case, when the sample is squashed, the central area starts to wrinkle at a low force, increasing the wrinkling rate as the force increases, so no pure shear behaviour is happening.

5.3.3 CONSIDERATION OF D DISTANCE AND THICKNESSES As part of the shear behaviour analysis, some other parameters apart from the shear angle (either theoretical or experimental) were considered. Firstly, and as part of Bias Extension Tests, there is a region between both top and bottom plate that shouldn’t experience any shear behaviour, and therefore should remain constant during the development of the test. However, results shown in Figure 37 give evidence that this distance increases while carrying out the test, something that shouldn’t happen theoretically, but that happens experimentally indeed. A first solution was thought; as the material was wrinkling, the specimen wasn’t experiencing the real shear behaviour, which showed another reason why biaxial bias extension tests were going to be developed, in order to look for a closer result to the ideal behaviour.


- 36 -

D DISTANCE- EXPERIMENTAL SHEAR ANGLE

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Figure 37: Increase of D distance analysed on the Uniaxial Bias Extension Test.

However, this was not the only parameter analyzed, because for some tests, the thickness of the sample was also measured in order to check which parts of the specimen were experiencing a greater deformation, and therefore could be under more tension. Obviously, it was expected a lower thickness of the sample as long as the test was carried out because a tensile force was being applied so the fibres were re-ordering. Therefore, as a response to the enlargement of the fibre, and the energy associated to it, the fibres had to reduce its thickness. Although this is not going to be part of the shear behaviour analysis, it was something that was going to help to understand what was happening to the fibre, and why, as some regions within the same sample could be experiencing greater thickness deformation. The thickness was analysed for Picture Frame Tests (See Appendices Section), but not for Uniaxial bias Extension as in these tests the sample after carrying out the test was completely deformed so it was not possible to measure its thickness. 5.3.4 CONSIDERATION OF WRINKLING Wrinkling of the specimen is a phenomenon that often occurs during textile composite forming and is a major problem for manufacturers [15]. The prediction of this defect is therefore, of major importance for the design and optimization of textile composite structures. The way that can be predicted is, either when the in-plane shear exceeds a measured locking angle (for kinematic models) or when in-plane compressive forces occur along the tow direction (for mechanical models) [15]. An example of how wrinkling happens can be seen in Figure 38.


- 37 -

a) b)

Figure 38: In these images can be seen the effect of wrinkling in both Uniaxial Bias Extension (a image on the left) and Picture Frame Tests (b image on the right) As it has been analysed before, wrinkling of the specimen is a major example of non-pure shear behaviour of the sample while carrying out characterisation tests such as PF and UBE and can also cause misinterpretation of the results. The way that this effect was affecting the tests was associated with the level of the compressive stresses which were being developed during the shearing of fabrics, with the thickness of fabric, and with its stiffness and its microstructure [5].


- 38 -

5.4 CONCLUSIONS Despite many analyses were carried out to understand the behaviour of both characterisation tests, only the experimental results offered further information about the shear behaviour of the sample. The objectives that were followed were a close response to the ideal behaviour while analysing the shear angle (which was mostly obtained), and a similar normalised load applied for both tests. Considering the last aspect, the results showed that the normalised load was a wee bit greater for the picture frame tests than for the uniaxial bias extension tests. This difference is believed to be due to the contribution of a high tensile loading of fibres in the picture frame test configuration as fibre yarns inside the picture frame apparatus, in contrast to the bias-extension test, are directly constrained on either fibre end and so even a small amount of misalignment may yield a significant contribution to the fibre stretching and, therefore, to a large global load value [14]. However, once the reason of this wee greater value of the load had been explained, for the final analysis both load values were considered as similar, which helped while being coherent with the analysis, and with the fact that a similar force should be obtained while carrying out both tests. Picture Frame Tests, had revealed as the closest tests to pure shear behaviour as no wrinkling or extension of the fibre was happening, but this was managed after carrying out many tests and identifying different problems as explained before, so they are not the easiest one’s to analyze. On the other hand, Uniaxial Bias Extension tests are much easier to analyze, but all the samples were experiencing wrinkling prior to pulling apart from the clamps, so although shear behaviour was happening, it was necessary to avoid the appearance of those wrinkles. Considering those conclusions and focusing on the uniaxial bias extension test, that seems to be the easiest one, an improvement has to be made in order to avoid those wrinkles; this is why Biaxial Bias Extension Tests were analyzed in the next stage of the project.


- 39 -

6. BIAXIAL BIAS EXTENSION TESTS 6.1 INTRODUCTION Considering previous work, those tests seemed to offer a solution to the problems encountered with the development of Picture Frame Tests and with Uniaxial Bias Extension Tests, because, according to [16], this test can potentially avoid many of the aforementioned problems associated with the PF and UBE tests as the results are less influenced by the test’s boundary conditions and test samples are less prone to intra-ply slip. On the other hand, this test also shows some difficulties, especially while carrying out its analysis as the deformation field induced throughout the test sample is not homogeneous, what in fact leads to added complexity when normalising the test data [16].

Moving on, the basis of this tests is to exert force in two different perpendicular axis, pulling the specimen that it is tested with four independent arms were the sample is clamped, expecting a shear response from the material [7]. Considering the force exerted by each axis, it is obvious that this force should be different, as otherwise the sample wouldn’t move. Basically that means that the force exerted by one of the axis will be static while the other one will increase gradually. This also means that the velocity with which the sample is pulled in one axis and the other is going to be different [7]. An example of this including a first approximation to the shape of the sample is shown in Figure 39.

Figure 39: Example of a generic sample of Biaxial bias Extension Test. the area remarked

with an arrow is the one that is going to experience pure shear behaviour.

After carrying out a deep research, it has been shown that in this kind of tests, the geometry of the sample analyzed becomes a key factor, as can affect both the kinematics (considering an energy approach) [10] and also the normalisation procedure [16]. The way that kinematics could be affected by the geometry of the sample is related


- 40 -

to how the yarn crimps or extents depending on the direction where the load gradually increases [10]. On the other hand, normalisation procedure can be affected because as explained before, the normalisation of the sample depends on the sample dimensions. However, there is no specific restriction on the geometry used, and for the tests that have been carried out, the geometry selected has been based on a normalisation procedure developed by Harrison et al that is explained later. Roughly, this normalisation procedure suggests three different geometries for a biaxial testing sample as can be seen on Figures 40 a, 40 b, and 40 c.

a) b)

c)

Figure 40: According to [16], here are shown the three main sample geometries (a, b and c)

while carrying out Biaxial Bias Extension Tests. Analysis

As part of the research carried out for analysing this test, a trip to the University of Manchester was done together with the supervisor of the project in order to have a better understanding on the material as the Materials Department within that university was already carrying out this type of tests. Further information about this trip is given in the Appendices

Moving on, and focusing on the analysis that wanted to be developed within the Materials Laboratory were the other tests had been carried out, there was no machinery available to develop this test. However, it has been decided to go ahead and carry out some tests manually just to obtain some quick results to check the differences that can be noticed between this test and the others. The way that those tests are going to be


- 41 -

carried out is by exerting a load manually in the direction where the material is not clamped using hands as “clamps”, and the arms strength as the force exerted by an actuator as can be seen in Figure 41. It is obvious that this procedure is quite imprecise, but it could give a first idea of the results that are being looked for.

Figure 41: Example of how the previous tests were carried out in the materials laboratory. Previously, considering the work developed by some researchers, some assumptions

have been made:

• The fabric is assumed to deform only through the trellis shear mechanism i.e. the tows of the fabric are considered inextensible and therefore all the deformation energy is considered to be due to shearing rather than extension of the fibres [16]

• Intra-ply slip is assumed negligible [16]. • The deformation is homogeneous characterizing the deformation of the

whole fabric [10]. • The warp and weft yarns remain in contact during loading and there is no

yarn slippage at yarn crossovers [10].


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6.2 TESTING This section is going to cover the same aspects considered before in section 5.2 6.2.1 DESCRIPTION OF THE PROCESS While carrying out these characterisation tests, some main steps were identified and described as follows: 1) IDENTIFYING PARAMETERS [14] The tests are carried out using a tensile testing machine which works with a specific software that shows parameters as the load applied or the displacement experienced. But those parameters are not the one’s that are going to be used to develop constitutive models because they don’t offer as much information related to the shear behaviour as the shear angle measured either theoretically or experimentally. For identifying those parameter, the same formula used for Uniaxial Bias Extension is used here.

Biaxial Bias Extension test calculations In this case, the method that is followed is the same as for Uniaxial Bias Extension Test, using the same formula as for the mentioned test that relates the shear angle to the displacement experienced by the sample [3, 6]: � = (� / 2) – 2*arcos [(1/�2) + (d1/2*L1)] Where, � = Theoretical shear angle d1 = Extension of the sample L1 = Length of the central region before deformation

2) IMAGE ANALYSIS

The objective of this step is the same as the corresponding one for Picture Frame Test and Uniaxial Bias Extension Test (See section 5.2.1).

3) NORMALISATION The basis of this procedure follows the same theoretical concepts explained before for PF and UBE, but it also depends on some new aspects. The normalisation work followed for this particular test, is based on the different geometries that can be used for carrying out the tests as explained before (see introduction to biaxial tests), and also according to [16] on the total axial force measured in a BBE test, Fa, which is comprised of two components; one is due to the material response to deformation, Fn, and the other contribution, Fr, is a reaction force due to transverse force Fc, applied across the specimen. However, such a level of complexity within the normalisation of this test is not going to be considered, mainly because as the force is going to be exerted manually, it is not


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going to be possible to quantify it, so the normalisation of this force could never be done.

6.2.2 BIAXIAL BIAS EXTENSION TESTS For avoiding misunderstanding of the results obtained, three different tests were carried out with two different sample geometries corresponding to Figures 40 a and 40 c, so a better analysis of the results could be made. 6.2.2.1 Preparing the samples The material that was used for both tests was non-consolidated polypropylene provided by Don&Low, a leader company in the composites area. The size of the sample had to be prepared according to the sample specifications shown in figure 36. In this case, as they are Bias Extension Tests, the length/width ratio had also to be considered, but in this case and according to sample specifications it was 1, instead 2 as happened for the Uniaxial Bias Extension Tests. (See Figures 42 and 43).

Figure 42: Example of sample dimensions according to Figure 40 c sample geometry.


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Figure 43: Example of sample dimensions according to Figure 40 a sample geometry.

Once the sample was prepared, and before clamping it to the uniaxial rig, some lines had to be drawn in order to identify the area under pure shear conditions, as can be seen in the pictures above. Later, and prior to clamping the sample within the rig, some holes had to be made in the sample in order to clamp it properly. (See Figures 44 and 45).


- 45 -

Figure 44: Sample c) ready to carry out the biaxial test.

Figure 45: Sample a) ready to carry out the biaxial test.


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6.2.2.2 Testing After having prepared the sample within the rig, the set was set up into the same tensile testing machine used for Picture Frame Test and Uniaxial Bias Extension Tests, considering similar operating conditions as well. (See section 5.2.3.2):


After setting up the machine, the test was carried out. At the beginning in the central area analyzed, the lines representing both warp and weft were perpendicular, but once the top joint was pulled up by the tensile testing machine, the sample became thinner and longer, as well as those lines stop being perpendicular. At the same time, the lines drawn became wee bit curvilinear giving evidence that pure shear behaviour was not happening. After a few seconds, the material started to pull apart from either the top or the bottom plate, hence, once the test was carried out, the material was totally deformed (See Figure 46).

Figure 46: Sample of BBE Test totally deformed after carrying out the test.

6.2.2.3 Analysis of the results Once the test was done, it was necessary to analyze the results and compare them with the ideal ones, and with the ones obtained for PF and UBE tests. However, in this case, as the load exerted in one axis was manual, there was no sense in analyzing the theoretical results. Therefore, for the analysis of the results of this test, only the kinematics of the model has been considered. The way that the experimental data was obtained was the same as explained before for PF and UBE tests.


- 47 -

Experimental Results: the experimental shear angle was measured through time intervals from the video recorded with a digital camera. In addition, in this case, once known the time and speed of the test, the displacement could be determined, and therefore the theoretical shear angle for those conditions. However, as in this case load was applied manually in one axis, the plot relating both load and normalised load with experimental shear angle couldn’t be calculated as the values of the manual load are unknown. With all these data, the graphs showing the relation between experimental and theoretical shear angle were displayed as shown in Figure 47.

EXPERIMENTAL-THEORETICAL SHEAR ANGLE BIAXIAL SHAPE 3

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Figure 47: This graph shows the relationship between the experimental shear angle that the

simple is experiencing and the theoretical one that is supposed to suffer. The ideal situation is, therefore, when both shear angles are similar, which means that they follow a 45º line drawn in the graph, what actually happens as can be seen.


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6.3 ANALYSIS OF THE RESULTS

6.3.1 PROBLEMS ENCOUNTERED WHILE CARRYING OUT THE TESTS

The first samples that were analyzed carrying out Biaxial Bias Extension Tests were prepared according to the third geometry proposed by Harrison et al, because it was the easiest to prepare and also to analyze. Those tests gave some results that differed from the theoretical ones that were expected, and some problems were encountered:

- Firstly, the area that was used for exerting the force manually was not big

enough to pull the material correctly. To sort out this problem it was decided to increase this area, by preparing new samples.

- The material was wrinkling from the beginning, something that it was supposed to be avoided while carrying out these tests. However, a possible solution to this problem was found; as not enough force was been exerted through one axis, it was obvious that the wrinkles were going to appear.

- The shear behaviour was not the ideal one. The way that those results were differing can be seen in Figures 48, 49 and 50, where three major areas (A, B and C) are identified. The figures show the differences of the shape for the start position and for the real and ideal behaviour of the sample after the carrying out the test.

- The A area represents the central zone where pure shear behaviour is expected. - The B area B shows where the material is expected to shear half than region A. - The C region is the one that remained constant without being deformed.

Figure 48: Drawing of the sample before carrying out the test identifying the three main

areas (A, B and C).


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Figure 49: Shows the expected shape of the sample after carrying out the test

Figure 50: Shows the real shape of the sample after carrying out the test.


- 50 -

As can be seen while comparing the drawings, the ideal behaviour does not look like the real one which was expected. Unexpectedly, region A was experiencing pure shear, but region B was not experiencing shear in the whole area, just in half of it, and region C was not behaving as ideally, as it was partially shearing. 6.3.2 CONSIDERATION OF D DISTANCE As mentioned above, the ideal behaviour of these tests does not look like the real one, so pure shear behaviour is no happening. As a result of this, it was decided to measure the “d” distance previously considered in UBE tests. The results can be seen in Figure 51, where a significant increase of its value is observed. This corroborates that the ideal behaviour is not happening as this value should remain constant during the development of this characterisation test.

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SHAPE A (3)

SHAPE C

Figure 51: Increase of “d” distance analysed on the Biaxial Bias Extension Test.

6.3.4 ENERGY ANALYSIS Furthermore, another phenomenon occurring to the sample while carrying out these tests was encountered, because it was proved that the sample was changing the direction of the fibre on a curvilinear way while developing the tests, so the fact that the energy associated with the test is only related to the shear behaviour should be reconsidered, as part of this energy could also be going to change the direction of these fibres. Besides, this was something that could also affect to PF tests, because in those tests, it was being assumed that all the energy was causing the shear behaviour in the specimen, but it had not been checked if there was a change on the fibre direction of the sample in the clamping area, something that could also represent an inconvenient of this test if it were proved. However, this could only be proved by developing further analysis on energy terms.


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6.3.5 IMPROVEMENTS After carrying out the first tests and the solutions to the main problems were dealt, a second set of tests were carried out considering the proposed improvements in order to obtain a better approach to ideal results. By considering those improvements, the first problems related to the size of the sample and to wrinkling were avoided, but referring to shear response no improvement was made as the B area was still divided in two different sections. This fact made us to think about the real contribution of this section to the shear behaviour; as this section was not clamped to the rig it shouldn’t be shearing as it wasn’t being affected by other fibres. In the end, we came up with the idea that this half section wasn’t contributing at all to shear behaviour, so the third geometry proposed for the analysis of Biaxial Bias Extension Tests considered in [16] could be better represented as the one shown in Figure 52.

Figure 52: New geometry of the sample. The new areas identified are the ones with a circle. This area was supposed to be a “B” area but research has shown that behaves as a “C”one.


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6.4 CONCLUSION With the development of “homemade” biaxial bias extension tests, a non-wrinkling sample was expected, and also an idea of what it was been looked for with the development of real Biaxial Bias Extension tests, closer results to ideal behaviour. However, the development of previous biaxial tests revealed two important aspects as seen before:

- The analysis of the different geometries that suit this test, which has improved its analysis.

- The impact of dissipated energy in the shape of the final sample, something that had not been considered in previous tests, and that can be a focus of further work in this area.

Those results have encouraged us much more with the final purpose of developing biaxial tests, as the feeling of getting closer to the ideal results is becoming higher. At the same time, it has served to open a new research area of those tests related to energy dissipation.


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7. DESIGN OF A BIAXIAL BIAS EXTENSION RIG 7.1 INTRODUCTION Previously it has been stated that Biaxial Bias Extension tests can avoid some of the results encountered in both Picture Frame and Uniaxial Bias Extension tests [16]. However, the results obtained with the “homemade” biaxial tests did not show this, mainly because the way that the tests were carried out was not the correct one, as the force exerted through one of the axis was been exerted manually, but its development was really useful to understand both ideal behaviour followed with those tests, and possible design concept of the final rig. 7.2 PRELIMINARY DESIGN CONCEPT In this section, the concept design analysed by a Msc student is explained as it is relevant to the development of the biaxial rig.

- Buduka N. Amadi: This student developed a pre-design of a biaxial testing device with the same purpose as the biaxial rig that wants to be developed within this project. His design was based on a pulley system attached to the table were the tests were going to be carry out, that connected the clamps with the actuator in one axis and with a load system in the other axis. The actuator exerted a load that was transmitted through a wire via the pulley system to the clamps, while at the same time a load exerted by some weights was exerted on a perpendicular axis via the pulley system as well. Apart from that, in order to avoid friction during the tests, the table was going to be based on an air bed on its central part, so air exerted by a pump could lift the specimen, and some bearings attached to the table in the two axis would lift the clamps and shafts that were connecting them to the pulley system. The basis of this design can be seen in Figure 53.


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Figure 53: Pre-design carried out by Msc student Buduka N. Amadi

7.3 DESIGN SPECIFICATIONS Prior to the design, some key concepts were established in order to develop the correct design of the bias extension rig.

- The rig was going to be placed in one of the tables available on the materials laboratory, like the one shown in Figure 54.

Figure 54: Table where the Biaxial Rig is going to be implemented.


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- Image Analysis is needed, but placing the video camera over the table difficult the duty of recording the test, so following the idea of Potluri et al at the University of Manchester, a scanner will be placed between the specimen and the table in order to record the test.

Considering these conditions, it is obvious that the idea showed by the student Buduka Amadi can not be considered as the air bed is going to be strong enough for lifting the specimen, but not the clamps. Therefore, a solution had to be adopted in order to lift the whole pulley and rig system. For doing it, a concept developed by another Msc student was considered:

- Gordon Pettigrew: This student developed a rig for attaching a shaft system to a tensile testing machine with the idea of changing the height of the whole device when necessary while the whole system was attached to the machine through a clamp. For doing it, a bicycle tube seat was attached to a flat surface, so by using the bicycle seat connection, the height could be changed at any time, guarantying a reliable joint at any height. The idea of this concept it is shown in Figure 55.

Figure 55: Design concept developed by Msc student Gordon Pettigrew


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7.4 DESIGN CONCEPT In this section, the final concept is presented considering ideas from both Msc students explained before, as its combination gave a great result which fulfilled the conditions of the design. 7.4.1 COMPONENTS The components needed for the final designed exposed are shown in Figure 56 which represents the Bill of Materials of the project. A detailed description of each component is done in the appendices section, presented with its correspondent technical specifications if required. BILL OF MATERIAL UM = Unit of MeasureName: Biaxial Testing Rig MB = (M-manufactured; B-bought)LEVEL ITEM NO. DESCRIPTION UM MB QUANTITY NUMBER

zero 1000 Biaxial Bias Extension Rig OF PART IN PARENT1 1100 Table EA M 1 11 1200 Rig System EA M 1 1

2 1210 Shaft EA B 1 43 1211 Shaft X-axis 12 mm EA B 0.5 13 1212 Shaft Y-axis 12 mm EA B 0.5 1

2 1220 Clamp EA M 1 43 1221 Male Part EA M 1 13 1222 Female Part EA M 1 13 1223 Fasteners EA B 1 5

4 1224 Bolt M8 15 mm EA B 1 12 1230 Bearing EA B 1 2

3 1231 Bearing EA B 1 13 1232 Housing EA M 1 1

1 1300 Pulley System EA M 1 12 1310 Top System EA B 1 1

3 1311 Pulley EA B 1 43 1312 Shaft 5 mm EA B 0.12 43 1313 Attachment Shaft EA B 1 83 1314 Attachment Pulley EA B 1 8

2 1320 Bottom System EA M 1 13 1321 Pulley EA B 1 53 1322 Shaft 5mm EA B 0.12 53 1323 Attachment Shaft EA B 1 103 1324 Attachment Pulley EA B 1 103 1325 Extension Plate EA M 1 2

4 1325.1 Fasterners EA B 1 45 1325.2 Bolt M5 (15 mm) EA B 1 15 1325.3 Nut M5 EA B 1 1

3 1326 Plate EA M 1 14 1326.1 Fasterners EA B 1 4

5 1326.2 Bolt M6 (15 mm) EA B 1 15 1326.3 Nut M6 EA B 1 1

Figure 56: Bill of Materials (goes on in next page)


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1 1400 Sujection System EA M 1 12 1410 Table Extensions X-axis EA M 1 2

3 1411 Extension Plate X-axis EA M 1 13 1412 Top Plate X-axis EA M 1 13 1413 Bottom Plate X-axis EA M 1 13 1414 Fasteners EA B 1 8

4 1415 Bolt M8 (75mm) EA B 1 12 1420 Table Extensions Y-axis EA M 1 2

3 1421 Extension Plate Y-axis EA M 1 13 1422 Top Plate Y-axis EA M 1 13 1423 Bottom Plate Y-axis EA M 1 13 1424 Fasteners EA B 1 8

4 1425 Bolt M8 (75mm) EA B 1 12 1430 Bicycle joint EA B 1 4

3 1431 Bicycle seat EA B 1 13 1432 Fasteners EA B 1 4

4 1433 Bolt M5 (15 mm) EA B 1 14 1434 Nut M5 EA B 1 1

2 1440 Top of Extensions EA M 1 43 1441 Top Extension plate EA M 1 13 1442 Bearing EA B 1 13 1443 Housing for bearing EA M

1 1500 Actuator sujection EA M 1 12 1510 Horizontal Bars EA M 1 12 1520 Bottom table EA M 1 12 1530 Hold actuator EA M 1 12 1540 Fasteners EA B 1 8

3 1521 Bolt M8 (25mm) EA B 1 1

Figure 56: Bill of Materials 7.4.2 ASSEMBLY The final design can be presented through some systems as shown before in the Bill of Materials. Talking on those terms, the Rig system is the one that has to be attached to the specimen and to the table as well. For doing it, the four clamps (that hold the specimen) are connected to a shaft which passes through a bearing that makes the whole system to be lifted a few centimetres. The other extent of the shaft is connected to carbon fibre, which acts as the wire needed to transmit the load from the actuator to the shaft through a complex pulley system. The fastening system is based on bicycle seat tubes that allow the whole device to vary the height of the rig system. At the same time, the bicycle tube is fastened to a couple of steel plates that are located close to the edges of the table acting as extensions. In the end, the whole pulley system allows transmission of the load exerted by the actuator, which is fixed to avoid its movement through the axis from which the load is exerted. The design concept can be seen in Figure 57. The different stages of the assembly are shown in Appendices section


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Figure 57: Assembly of the final concept Developer for carrying out Biaxial Bias Extensión Tests.


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8. REFERENCES [1] Jones, Robert M. Mechanics Of Composite Materials [Philadelphia, Pa.: Taylor &

Francis, c1999. Second edition] [2] Hull, D. and Clyne, T.W. An Introduction to Composite Materials [Cambridge:

Cambridge University Press, 1996. Second edition] [3] P. Harrison, M.J. Clifford *, A.C. Long. “Shear characterisation of viscous woven

textile composites:a comparison between picture frame and bias extension experiments” Composites Science and Technology 64 (2004) 1453–1465.

[4] Vasiliev, Valery V. and Morozov, Evgeny V. Mechanics and Analysis of

Composite Materials. [Amsterdam ; London : Elsevier, c2001.] [5] U. Mohammed, C. Lekakou*, L. Dong, M.G. Bader. “Shear deformation and

micromechanics of woven fabrics” Composites: Part A 31 (2000) 299–308. [6] S.B. Sharma, M.P.F. Sutcliffe, S.H. Chang. “Characterisation of material

properties for draping of dry woven composite material” Composites: Part A 34 (2003) 1167–1175.

[7] S.H. Chang, S.B. Sharma, M.P.F. Sutcliffe. “Microscopic investigation of tow

geometry of a dry satin weave fabric during deformation” Composites Science and Technology 63 (2003) 99–111.

[8] J. Page, J. Wang. “Prediction of shear force and an analysis of yarn slippage for a

-weave carbon fabric in a bias extension state” Composites Science and Technology 60 (2000) 977-986.

[9] Jean Launay, Gilles Hivet, Ahn V. Duong, Philippe Boisse. “Experimental

analysis of the influence of tensions on in plane shear behaviour of woven composite reinforcements” Composites Science and Technology 68 (2008) 506–515.

[10] T.V. Sagar, P. Potluri and J.W.S Hearle. “Energy Approach to predict

Uniaxial/Biaxial Load-Deformation of Woven Preforms” University of Manchester Institute of Science and Technology (UMIST) From ECCM 10, 2002.

[11] McGuiness GB, O_Bradaigh CM. “Development of rheological models for

forming flows and picture-frame shear testing of fabric reinforced thermoplastic sheets” J Non-Newton Fluid Mech1997;73:1–28.

[12] Bassem Zouari, Jean-Luc Daniel, Philippe Boisse. “A woven reinforcement

forming simulation method. Influence of the shear stiffness”. Computers and Structures 84 (2006) 351–363

[13] X.Q. Penga, J. Cao, J. Chen, P. Xue, D.S. Lussier, L. Liu. “Experimental and

numerical analysis on normalization of picture frame tests for composite materials” Composites Science and Technology 64 (2004) 11–21


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[14] A.S. Milani, J.A. Nemes, R.C. Abeyaratne, G.A. Holzapfel. “A method for the approximation of non-uniform fiber misalignment in textile composites using picture frame test” Composites: Part A 38 (2007) 1493–1501

[15] H. Lin, J. Wang, A.C. Long, M.J. Clifford, P. Harrison. “Predictive modelling for

optimization of textile composite forming” Composites Science and Technology 67 (2007) 3242–3252

[16] P. Harrison, P. Potluri, K. Bandara and A.C. Long. “A Normalisation Procedure

For Biaxial Bias Extension Tests”


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APPENDICES

8.1 RESULTS OF PREVIOUS TESTS…………………………………62 8.2 ASSEMBLY STAGES……………………………………………...XX 8.3 STRENGTH CALCULATIONS……………………………………XX 8.3 INFORMATION OF PURCHASED MATERIALS………………..XX 8.4 COSTING…………………………………………………………....XX 8.5 CAD DRAWINGS…………………………………………………..XX 8.6 GANNT CHART………………………………………………….....XX

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