Effect of Loading Rates on Short-Beam Shear Testing of Fiberglass Reinforced Polyester Sucker-Rods

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ENGT 3303

Effect of Loading Rates on Short-Beam

Shear Testing of Fiberglass

Reinforced Polyester Sucker-Rods C.G. Cruz

24 SEP 2014

Abstract

One of the quality testing methods of fiberglass reinforced sucker-rods is measuring the

interlaminar shear rate by applying a constant load on a specimen. In testing of these parts different

loading rates are used yielding different results in the lab. Our test is to determine whether loading rate

has a direct effect on final strength and if so, if it is statistically relevant. The method in which we tested

involved and ASTM standard short-beam shear test to measure interlaminar shear rate with loading

rates of 0.4 in/min, 0.2 in/min, and 0.05 in/min. Five Specimens are tested for each loading rate and

data is recorded and analyzed to determine variance. Testing determined that loading rate did have a

direct effect on final strength and that it was statistically relevant. Examination of data determined that

0.2 in/min yielded the closest set of data with only .95% variance and that the ASTM testing standard of

0.05 in/min yielded the greatest variance with 1.57%. Further analysis of the ASTM standard test reviled

possible discrepancies in the exceptions we made to accommodate our specimen of sucker-rod. Further

testing must be conducted in order to prove the exceptions made to the ASTM standard account for

final strength variance under different loading rates. A ratio concept is talked about as a possible testing

model.

Introduction

Fibergrate Composite Structures Company in Stephenville, Texas is currently testing a new

fiberglass composite sucker-rod to be used predominantly in the oil field industry. New fiberglass

reinforced material in this application offers certain strength, weight, and chemical resistance

improvements over the current steel sucker-rod in use by industry today. As development and larger

production of sucker-rod moves forward, certain quality management questions must be analyzed in

order to ensure a consistently strong and safe product for consumers. One of the main concerns of

quality control is a consistent interlaminar shear rate that meets industry standard. The transferring of

loads within the fibers that reinforce the structure is directly related to the adherence of the polymer

matrix to the fibers. In order to test such adhesion, a short-beam shear test is adapted from ASTM

D2344-13 (“D2344”) [1]. In testing the material, three different loading rates have been used yielding

different but similar results. In some loading scenarios, the measured interlaminar shear of fiber-

reinforced composites can be affected by the loading rate that is applied during a test such as the short

beam shear test. Our Hypothesis is that different loading rates during a short-beam shear test have a

direct effect on part failure measurements and if so, if it is statistically relevant. Further testing will

determine the effects of strain rate on the interlaminar shear properties of Fibergrate sucker rod due to

loading rates of 0.4, 0.2 and 0.05 inches per minute.

Effect of Loading Rates on Short-Beam Shear Testing of Fiberglass Reinforced Polyester Sucker-Rods C.G. Cruz 9/24/2014

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Literature Review

Prior to testing, further background on the product and testing methods were gathered in order

to adequately analyze the results and understand the material that was being investigated. Research

gathered included: 1) Background on glass fiber production, data and properties in fiberglass reinforced

plastics. 2) Background on the pultrusion manufacturing process, resin impregnation, pulling systems,

finishing methods and reinforcement performance. 3) Characterization of physical and mechanical

properties of fiber glass composites in other tests. 4) Effects of loading rates on ultimate strengths in

other composite testing.

In order to effectively analyze what is happening physically and chemically within our specimen

during testing, a deeper understanding of fiberglass reinforced plastics must obtained. This was done by

reviewing a glass fiber reinforcements section of Fundamentals of Composites Manufacturing [2].

Once a better understanding of the material was achieved, an insight into how the specimen

was manufactured was needed. Prior specs from Fibergrate indicated that the fiberglass-reinforced

composites were produced using a basic pultrusion process. Although a basic understanding of the

pultrusion processes is known, deeper insight into the method was gained in order to ensure that all

variables in the specimen are considered. Information into the process was gathered in the pultrusion

section of Fundamentals of Composites Manufacturing [3].

After reviewing both the basic properties of fiberglass-reinforced composites and the

manufacturing of the specimen, the question arose of whether similar testing has been done into the

loading rates on shear strength in similar material. After reviewing the Tarleton State University

collection of scientific journals, several promising articles were reviewed in order to determine

relevancy. The first of these reports was the Characterization of Physical, Mechanical and Fire properties

of Fiber Glass Composite due to Weight fraction [4]. Although this article did not discuss the short-beam

shear test it was helpful in discussing the effect of material selection and ratio of fiberglass

reinforcements/ polyester composites, as well as, an in-depth look at controlled testing methods of

materials and their reactions.

The second article, Effect of the Loading Rate on Ultimate Strength of Composites [5], analyzed

the breaking loads of a pressurized fiber composite vessel in a slow burst test. This test showed a

positive correlation in loading rates versus specimen failures. Although little data from this test can be

directly applied to our testing, positive background into stress transfers in a matrix between fiberglass

reinforcements is gained. Understanding and insight into the viscoelastic nature of certain fiberglass

reinforced plastics can also be gathered from the article and can be applied as knowledgeable

background to the test as well.


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Although no articles were obtained that encompassed a more direct relevancy to our testing,

strong background into the nature of our material, its manufacturing processes, and similar testing

behaviors, provided us with a healthy understanding of our specimen allowing us to make more credible

observations.

Methodology

Testing Machine- The testing machine used was a Page-Wilson testing machine

calibrated using D2344 standards [6].

Loading Nose and Supports- An exception to the ASTM standards was made in this

aspect. D2344 calls for certain flat or curved supports to match a flat or curved

specimen. However, our specimen is a cylindrical object with a diameter of .995.

Because of this, a loading nose and two supports were fabricated to match the

specimen. Complete dimensional specifications for loading nose and supports where not

gathered for this test but can be obtained from Fibergrate Composite Structures

Company. Set up dimensions and inside loading diameter are shown Figure 1 below:

Figure 1

Measurements- All dimensional measurements were taken using a dial caliper that has

been calibrated by an accredited organization when purchased.

Environment- The environment for testing was maintained at ambient testing laboratory

conditions and constant operating temperature as described in D2344 [6].

Sampling- Five specimens were tested for each of the three loading rates. Each

specimen was loaded the same orientation into the machine, measured for dimensional

consistency, labeled appropriately to ensure data matched specimen and photos of

failure were taken for visual analysis.


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Specimen Geometry- The geometry of the test specimen differed from the ASTM

specification. Instead of testing a small section of flat or curved material, a section of the

sucker-rod being produced was tested for quality management. The geometry of test

specimens is described in Figure 2 below:

Figure 2

Specimen Material- All specimens were produced from the same material, the same

day, same time, and from the same machine. Each was virtually identical and did not

show any visual abnormalities upon prior examination. The material is a fiberglass

reinforced polyester made with 77% glass that was pultruded from the same machine.

Further material properties were documented at Fibergrate but not gathered for this

test.

Configurations- All test specimens contained a marked line on the “top” of the length

side. This line represented the uppermost section of the sucker-rod as it was being

produced at Fibergrate. This line was orientated upward upon being loaded into the

machine to ensure clocking consistency.

Specimen Preparation- All specimens were prepared using the ASTM standard provided

within D2344 [7].

Speed of Testing- The speed of testing was set at a rate of crosshead movement at three

different intervals for five specimens each. The first test was done at a rate of 0.4

in/min, the second at 0.2 in/min and the third at a rate of 0.05 in/min.

Loading- Load was applied to the specimen at each specified rate wile data is recorded

ten times every second and documented using the testing machines program. Load will

be continued on the part until either of the following occurs:

o A load drop-off of 10% (This is a different amount than the specified

30% described in D2344 [8] due to material behavior)

o two-piece specimen failure, or

o the head travel exceeds the specimen nominal thickness.


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Data Recording- Load versus crosshead displacement data was recorded ten times a

second using testing machine software. Maximum load, final load and the load at any

obvious discontinuities in the load-displacement data were recoded per D2344 standard

[8]

Failure Recording- Visual part failures can be seen going laterally along the grain pattern

after testing. In order to record these, parts are loaded into a vise and pictures are taken

with the face of the specimen that has its identification labeled. A backlight source and

high resolution camera were used for this process.

Calculations

All formulas were obtained from D2344 [9] and calculated using an Excel Spreadsheet program

due to all data being loaded to the spreadsheet after test.

Short-Beam Strength- Short-Beam Strength was calculated using the equation that

follows:

𝑭𝒔𝒃𝒔 = 𝟎. 𝟕𝟓 ×𝑷𝒎

𝝅𝒓𝟐

Where:

𝐹𝑠𝑏𝑠 = Short-beam strength, MPa (psi) 𝑃𝑚 = Maximum load observed during the test, N (lbf) 𝑟 = Radius of specimen face *The formula for area (𝜋𝑟2) replaced the standard area equation (𝑏 × ℎ) described in the standard due

to specimen geometry.

Statistics- For each set of test specimens average value, standard deviation, and

coefficient of variation is calculated as follows:

�̅� = (∑ 𝑥𝑖

𝑛

𝑖−1

) /𝑛

𝑠𝑛−1 = √(∑ 𝑥𝑖

2 − 𝑛(�̅�)2𝑛𝑖−1 )

𝑛 − 1

𝐶𝑉 = 100 × 𝑠𝑛−1/�̅�

Where:

�̅� = Sample mean (average) 𝑠𝑛−1 = Sample standard deviation 𝐶𝑉 = Sample coefficient of variation, % 𝑛 = Number of specimens 𝑥𝑖 = Measured or derived property


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Results

The tests were performed on Tuesday, September 16, 2014 in the Tarleton State University

testing lab in the time span of 2:00pm-4:00pm. All coupon configurations were standard and tests were

performed by members of the Industrial Materials class of the university.

Variations to the ASTM standard test where made to accommodate the geometric nature of the

specimens and are documented in the methodology.

The material being tested is a fiberglass reinforced polyester rod that is 77% glass and is

manufactured by a pultrusion processes from Fibergrate Composite Structures in Stephenville, Texas. Its

geometry is listed in the methodology and all other material data can be obtained from the company.

Calibration dates and methods for all measurements and test equipment can be obtained

through Tarleton State University, Engineering Technology Department.

Sample 1 (0.4 in/min)

The first set of test performed was at a load rate 0.4 in/min. A measurement of load (lbs) and

vertical displacement was taken ten times every second. Data gathered by the testing machine is

recoded below in Graph 1:

Graph 1

0.00

2000.00

4000.00

6000.00

8000.00

10000.00

12000.00

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20

Load

(lb

s)

Extension (in)

0.4 IPM Load Rate

Specimen 1 Specimen 2 Specimen 3 Specimen 4 Specimen 5


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The maximum loads are recorded in Table 1.1:

Table 1.1

Sample Pounds

1.) 10257

2.) 10338

3.) 10573

4.) 10527

5.) 10299

Other calculations are recorded in Table 1.2:

Table 1.2

Calculation Value

Mean 10398.8 lbs

Std. Deviation 126.9

Coef. Of Variance 1.22%


The second set of test performed was at a load rate 0.2 in/min. A measurement of load (lbs) and



Graph 2

0.00

2000.00

4000.00

6000.00

8000.00

10000.00

12000.00

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

Load

(lb

s)

Extension (in)

0.2 IPM Load Rate


Breakage is consistent in most specimens; sample

1 from the 0.4 IPM group is shown as the most

prominent:


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The maximum loads are recoded in Table 2.1:

Table 2.1

Sample Pounds

1.) 10166

2.) 10144

3.) 9974

4.) 10083

5.) 9919


Table 2.2

Calculation Value

Mean 10057.2 lbs


Coef. Of Variance .95%


The final set of tests performed was at a load rate 0.05 in/min. A measurement of load (lbs) and



Graph 3

0.00

2000.00

4000.00

6000.00

8000.00

10000.00

12000.00

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

Load

(lb

s)

Extension (in)

0.05 IPM Load Rate


Breakage is consistent in most specimens, sample

2 from the 0.2 IPM group is shown as the only

outlier with 3 break lines:


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The maximum loads are recorded in Table 3.1:

Table 3.1

Sample Pounds

1.) 9950

2.) 9949

3.) 9625

4.) 10051

5.) 9746


Table 3.2

Calculation Value

Mean 9864.2 lbs


Coef. Of Variance 1.57%

Other Calculations

Short-beam Strength was calculated for each sample and documented in the Table 4 below:

Table 4

Sample 0.4 0.2 0.05

1 9898.418 9810.6 9602.151

2 9976.587 9789.369 9601.186

3 10203.37 9625.312 9288.513

4 10158.98 9730.501 9699.62

5 9938.95 9572.235 9405.283

Mean 10,035.26 9705.62 9519.35

Breakage is consistent in every specimen, sample 1

from the 0.05 IPM group is shown as a

representation of the group:


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An Analysis of Variation was also calculated using the formula provided by the ANOVA: Single

Factor function in excel. Results are as follows in Table 5:

Table 5

Anova: Single Factor

SUMMARY Groups Count Sum Average Variance

Column 1 5 51994 10398.8 20136.2 Column 2 5 50286 10057.2 11514.7 Column 3 5 49321 9864.2 30158.7

ANOVA

Source of Variation SS df MS F P-value F crit

Between Groups 732894.5333 2 366447.2667 17.7859394 0.000257624 3.885293835 Within Groups 247238.4 12 20603.2

Total 980132.9333 14

Discussion

The first testing set involved five specimens to be loaded at a rate of 0.4 in/min. The specimens

where first measured, then labeled with their designated testing set. Diameter and length were

recorded to ensure testing consistency. Each specimen proved to be equal in diameter, length and other

material properties. Visual inspection of each sample was then conducted to ensure that no chipping,

scaring or other physical deformations that would interfere with the end results existed. Upon

verification that there were no deformations of the part visible, specimen 1 was then selected to be

loaded into the machine. A blue stripe was orientated upward into the machine to ensure each part was

loaded in the same region. This stripe derived from the uppermost section of the rod as it was taken

from the pulltrusion processes. Once loaded, a constant increasing load was placed upon the specimen

until first breakage at 10257 lbs. Because the first breakage did not exceed the set 10% load drop off

weight a constant load was again applied for another four breaks. After the load drop of was met, the

machine stopped and finalized the data. The testing machine was then jogged back, to allow the part to

be removed. Once removed, the maximum load was recorded on the face end of the specimen and

loaded into a vise for pictures. This process was then repeated for specimens 2-5 that yielded maximum

loads described in Table 1.1. Specimen failure among the samples indicates a consistent lateral breakage

demonstrating positive interlaminar shear. This visually expresses the correct failure we are looking for

in this test. However, the expanse of breakage varies extensively from specimen to specimen. The

greatest outlier of which involved specimen 1 with five prominent lateral breaks. Compared to the group

average of two break lines, this indicates a great inconsistency in breaking.


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The second testing set again involved five specimens to be loaded at a rate of 0.2 in/min. The

same precautions, set-up, measurements and inspections took place for each specimen. Under a slower

load rate, sample 2 demonstrated similar physical qualities, demonstrating a mean maximum load value

of 10057.2 lbs. When comparing this to sample 1, we see a 341.6 lb. difference in the means. A similar

drop in value due to lower loading rate is seen in short-beam strength when comparing the means. In

this test we see a 329.64 lb. drop in short-beam strength after lowering the loading rate. The coefficient

of variance in the maximum load values also falls .27% demonstrating a more consistent grouping of

data in our test. However, visual failures of each part continue to vary in number of breaks, thickness of

gapping, and area in which failures occur among specimens.

The final testing set involved five specimens to be loaded at a rate of 0.05 in/min. The same

precautions, set-up, measurement, and inspections took place for each specimen. At the slowest loading

rate, sample 3 exhibited a considerable fall in in maximum load when compared to sample 1 (534.6 lbs.),

and sample 2 (193 lbs.). Short-beam strength was similarly lowered when compared to sample 1 (515.91

lbs.), and sample 2 (186.27 lbs.). This correlation in the reduction of final strength with lowering of

loading rates could indicate that other internal forces in the specimen are being affected with a constant

load and not just the bonding of the matrix to which we are trying to test. However, upon reviewing the

breakages in each specimen, we see a more consistent family of breaks at each face. Breaks in sample 3

are all similar in size, gapping, and area of occurrence from a purely observational standpoint.

Conversely, the coefficient of variance of these set of maximum loads are the highest out of all the

sample sets. At 1.57% the variance of maximum loads illustrates an inconsistency in data when

compared to other loading rates.

Short beam strength of each specimen in each sample collected in Table 4 was then entered into

the Anova: Single Factor function in Excel in order to give us an analysis of variance in data gathered. If

variation of each sample set proves to be statistically relevant in this function, we must reject a

hypothesis that they are the same. The threshold for this particular equation is computed as F Crit

(3.88). If F is computed to be above threshold then data is statistically relevant. The F value computed is

to be 17.785, therefore data is statistically different in variance. This supposition is shown in Table 5.

In reviewing our exceptions to the D2344 standard, certain factors changed may account for

variations of the data plot at 0.05 in/min. In D2344 the shear test is conducted on either predominately

flat or mildly curved specimens that are comparatively thin (.25in). The loading nose and supports match

the specimens in surface contact area. In measuring interlaminar shear on the thin specimens the matrix

and fibers are loaded in an even area. In our modified test we used a specimen with a diameter of .995in

and a length of 4.09in. According to D2344, the test method is not limited to dimensions of the

specimens given, but must maintain a length-to-specimen thickness ratio of 4.0 on flat specimens [10].

Further testing is required to determine if the volume of the sucker rod and loading area has a direct

effect of the shear stresses within the specimen.


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Conclusion

Fibergrate Composite Structures Company tests interlaminar shear rates of sections of their

new fiberglass reinforced polyester sucker-rods for quality and data analysis purposes. In testing these

samples, different loading rates have been used, thus yielding different results in the lab. The question

presented to our testing lab enquires as to if loading rates of 0.4 in/min, 0.2 in/min, and 0.05 in/min

have any direct correlation to final strengths. Our hypothesis is that different loading rates during a

short-beam shear test have a direct effect on part failure measurements.

In testing our hypothesis three sample sets were set up having five test specimens each. After

conducting testing we analyzed the data and specimens to determine a correlation and possible

reasoning. In doing so we noticed a steady fall in final strength as loading rates decreased. This fall in

strength was proven to be statistically relevant by analyzing the variance using the Anova: Single factor

function in excel shown in Table 5. Upon reviewing the coefficient of variance in each sample, the

tightest grouping of data comes from out 0.2 in/min sample set at .95%.

After reviewing our exceptions we made to D2344, we see a possible discrepancy to our

method. In the ASTM standard we test flat or curved specimens with a thickness of 0.25in to 0.08in. We

modified our test to fit a .995in diameter rod. When using the 0.05in/min loading rate that is called for

in D2344 we see a large variance in data of 1.57%, the largest of our sample sets. We propose that

further testing must be done to examine the possibility of a ratio to surface area versus loading rate. We

would do this by setting up a ratio of surface area of specimens and loading rates. Since both surface

areas of specimens are known and our ASTM specification gives us a loading rate on the thinner

specimen, we can accurate propose a ratio like the basic function below:

𝑆𝑎1

𝐿𝑟1=

𝑆𝑎2

𝐿𝑟2

Where:

𝑆𝑎1 = Surface Area of specimen 1 (𝑊𝑖𝑑𝑡ℎ × 𝑇ℎ𝑖𝑐𝑘𝑛𝑒𝑠𝑠) 𝑆𝑎2 = Surface Area of specimen 2 (𝜋𝑟2) 𝐿𝑟 = Loading Rate Further research should be done in order to find an equation that better links these two values in order to better account for other variables that are present in the test. Once the ratio is complete, a similar test to the one we performed should be conducted to test if the loading rate that comes from the ratio yields a similar variance to the standard D2344 test on a flat specimen.


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References

[1] ASTM International (2013) Standard Test Method for Short-Beam Strength of Polymer

Matrix Composite Materials and Their Laminates. Designation: D2344/D2344M-13

[2] Strong A. (1989) Fundamentals of Composite Manufacturing: Materials, Methods, and

Applications. Society of Manufacturing Engineers. 2nd Edition, Pp. 203-207. ISBN

0872633586

[3] Strong A. (1989) Fundamentals of Composite Manufacturing: Materials, Methods, and

Applications. Society of Manufacturing Engineers. 2nd Edition, Pp. 453-460. ISBN

0872633586

[4] Hasan E., Helal M., Nour M., Shokry K. (2014) Characterization of Physical, Mechanical and

Fire Properties of Fiber Glass Composite Due To Weight Fraction. Key Engineering

Materials. Vol. 600. Pp. 547-557. Trans Tech Publications. DOI 10.4028

[5] Bunsell A., Chou H., Mair G., Thionnet A. (2013) Effect of The Loading Rate on Ultimate

Strength of Composites. Aplication: Pressure Vessel Slow Burst Test. Composite Structures.

Elsevier BV. Vol. 104, p144. ISSN 0263-8223


Matrix Composite Materials and Their Laminates. Pp.2-3 §7. Designation: D2344/D2344M-

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Matrix Composite Materials and Their Laminates. Pp.3 §8.3. Designation: D2344/D2344M-

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Matrix Composite Materials and Their Laminates. Pp.7 §11.6.1. Designation:

D2344/D2344M-13


Matrix Composite Materials and Their Laminates. Pp.7 §12 Designation: D2344/D2344M-

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Matrix Composite Materials and Their Laminates. Pp.2 §5.3 Designation: D2344/D2344M-

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Effect of Loading Rates on Short-Beam Shear Testing of Fiberglass Reinforced Polyester Sucker-Rods

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