MSM2111 Tensile Test Prac Guide 2012

Compiled by: BH RobertsRevision: 2012

Practical Guide

Tensile Testing of Steel

Faculty of Engineering, the Built Environment & ITTechnology for tomorrow

Department of MechatronicsNorth Campus

Strength of Materials 2MSM2111

Faculty of Engineering, the Built Environ. & IT Doc ID:

MSM2111Prac1

Department of Mechatronics Rev.: 2012

Tensile Testing of Steel By: BHR

1. PURPOSE / OBJECTIVES OF THE PRACTICAL EXERCISE

The purpose of the practical in the context of the Mechatronics learning program and the Strength of Materials 2 module is to provide an opportunity for learners to:

Investigate certain aspects of the syllabus of the module from a practical perspective

Obtain an understanding of certain mechanical properties and how they are measured

Write technical reports and demonstrate their understanding Apply and compare theory and calculations with physical observation and

measurement Use calculations of measured data to deduce material properties etc. Draw and interpret graphs Analyze and evaluate/discuss observations/results, and draw apt conclusions Learn how to use certain testing and measuring equipment and apply it to the task Work as part of a team Effectively apply PC based word editing and generate graphs using Exel

In addition the exercise provides an opportunity for the learners’ abilities in the above areas to be assessed.

2. OVERVIEW OF PRACTICAL EXERCISE

These guidelines are to give you an overview of the practical and what is expected from you. Guidelines are given, but it is up to you to document the procedure and organize the data into useful results etc., according to the objectives of the practical. Thus, use your own words and interpretation, making sure you cover all relevant aspects. For example, it is up to you to describe the apparatus used and the test procedure, and to select the relevant theories and compare them with the practical results, etc. Certain specific requirements will however be outlined to you, as indicated below. A mark of zero will be awarded where there is evidence of plagiarism. Rather use fewer words, but your own, than many words from someone else, which may or may not be relevant anyway.

The report must be typed in Microsoft Word. The entire report should be in one document – all graphs are to be imported from Exel into the Word document. Remember to use the third person passive voice typical of technical report writing.

You are advised to read this whole Practical Guide document before starting your report.

One report must be provided per group.

3. REPORT SECTIONS

The report must contain the following sections, typical to laboratory report writing (include section numbering (and sub-section numbering where applicable)):

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Aim/ObjectiveApparatusMethodTheoryObservations / Results / CalculationsDiscussion / Conclusions

Further instructions on each of these are given below.

Aim / Objective

The aim of the practical is to investigate certain properties of a steel sample, namely:

Young's Modulus of ElasticityYield PointUltimate Tensile StrengthPercentage ElongationPercentage Reduction in Area

Apparatus

List and briefly describe the apparatus used. Describe the test specimens, and indicate why they are shaped like they are. Provide a simplified diagram (e.g. using the Word ‘draw’ function, not hand drawn) of the test apparatus arrangement. Photographs can also be included. Could the equipment and the test specimens be improved upon? Suggest possible improvements, but do this in the discussion/conclusion section.

Method

Explain in your own words the method used for each investigation / procedure. Your description should be clear enough that someone who did not witness the practical would have a clear understanding of what was done. Essentially we will be testing two samples, one mostly in the elastic range, and the other over the entire strain range, to failure. Both tests will be performed using the Hounsfield tensile testing machine.

Theory

Indicate and explain clearly in this section all relevant theory and how it is used in your analyses. Show pertinent formulae and explain how they are used / what their purpose is, in this practical. Sample calculations are normally provided in the observations/results/calculations section.

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Observations / Results / Calculations

It is not necessary in this instance to include the whole Exel spreadsheet of result data. Do however show relevant portions of the spreadsheet data, e.g. data used to calculate stress, yield point, etc. For the purpose of this practical exercise, tables should show raw and manipulated/calculated data, so that the assessor can follow your working better. Preferably place relevant extracts from the spreadsheets in the body of your report where the data from each extract is being referred to, not at the end, in this case, for ease of marking.

Show relevant graphs in this section.

Refer the reader to the relevant formulae indicated in the theory section, and show sample calculations. Indicate all observations made during the practical, in this section (some of these may perhaps need to be mentioned in the theory section – use your discretion).

Errors are also generally calculated in this section, and discussed further in the discussion/conclusion section.

Note: The discussion / answers to at least some of the questions given below would generally be in the discussion/conclusions section of a practical report. The questions are listed in this section (results) so as to indicate to what data/results they are applicable. For this exercise, address the questions, where appropriate, in the results section as you proceed, as opposed to the discussion/conclusion section. Note that there are some additional questions listed under Discussion / Conclusions in this document. Also note that there is not necessarily one specific correct answer to each question, and some of them are purposefully of an ‘open ended’ nature.

The following must be included in this section. Letters have been used with each bullet, for reference purposes. Please show the letter that a question falls under, and include the actual question text from this guide in your report – in italics (or in a different colour if your hard copy is in colour), so I can see what is Question and what is Answer (sometimes there is more than one instruction/question under a letter, so you may have to separate these out from each other – just make sure the relevant letter appears whenever you are dealing with an item under that letter; do this for both the questions requiring answers and for the graphs and values that you are determining).

A) Show the graphs obtained from the test data (spreadsheet obtained from the instrument). Include the following, assuming the relevant data are all available (or calculate from the data if necessary; indicate what is directly from the data, and what is calculated – i.e. clarify in the column headings in your tables whether it is a raw data column or a calculated column): Force vs. Extensometer displacement; Stress vs. Extensometer strain; Force vs. Cross-head displacement; Stress vs. Cross-head strain (to obtain an approximate strain from the cross-head displacement, consider that the full parallel length of the specimen is between the jaws of the testing machine). Superimpose the

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Stress/Strain graph obtained from the extensometer, with that obtained from the cross-head (in the region of overlap i.e. where strain is relatively small).

B) The technician will have done a first pull at a low load, to embed the grippers firmly into the sample, then relaxed the force on the sample by jogging the mechanism back, and then ‘zero-ing’ the force and displacement readings, after which he should have done a pull to somewhere in the elastic range, and then verified this by taking force back down to zero and observing whether the strain also returned to zero; then he should have pulled the specimen to just past yield. Use the second pull data (i.e. to somewhere in elastic range) for comparison with the next pull (i.e. to yield) – see whether the graphs overlap nicely and compare the E’s calculated for each pull (only do the regression analysis on the one to yield – just do a simple calculation for the elastic range pull, for comparison purposes).]

C) Determine the yield point using visual observation of the turning point on the Force vs. Extensometer displacement graph (or Stress vs. Extensometer strain), and by the 0.2% strain offset method. Superimpose the offset line onto the graph and hence determine the yield stress for the material. Note that the distance between the gripping points of the extensometer is manufactured to be 50mm when it is in its most closed position (as opposed to its most extended position). Note: If a different extensometer was used, it may have a different closed length. Did the technician make sure that the extensometer was fully closed before starting the test? Did he also ‘zero’ the extensometer reading (and force reading) on the PC before starting? If so, why? Is this legitimate? Did he make sure there was a slight tension in the specimen before commencing testing? Explain why this might be necessary (look up the definition of and consider the phenomenon of ‘backlash’ occurring in machinery and how this might affect readings). Explain how the initial distance between the gripping points of the extensometer would affect the determination of E – would it appear too large, too small, or would it be unaffected, if the gauge was actually say 51mm open at the start as opposed to 50mm as it is supposed to be (assuming the user thought the extensometer was at its most closed position at the start)? Explain. Compare the yield point determined experimentally to data obtained from at least two reference books and one internet site.

D) Calculate E using the test table data (and compare it to the value given from the test PC, if this value is shown in the PC results page). Which extension readings should be used to find E, i.e. extensometer or cross-head readings? Use an Excel curve fitting function to obtain the best fit linear relation between force and extension, and between stress and strain, in the elastic region (to find E). Compare this to using only two points, i.e. a single data point very near the upper end of the elastic region, and a single point very near the lower end of the elastic region. Is it legitimate to use zero force, zero displacement as a data point in calculating E? Explain. Does this point (0;0) appear in the test PC tables? Suggest a reason for the answer. Also compare the above to a linear regression determination of the slope to determine E, by manual calculation (show all formulae used and explain procedure). For the latter, use about 4 or 5 points well spread over the range of the linear elastic part of the force/extension (or stress/strain) graph. Obtain these from the table data. Tabulate these data points and show them on a graph, together with the

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regression line determined. Determine and discuss, using regression analysis, the correlation co-efficient (between stress and strain or between force and displacement/extension).

E) Compare the value of E obtained above to a value calculated from the cross-head readings. Is it valid to use the cross-head data to determine a value for E? Explain. (Consider what the displacement on this graph really means – what displacement is being measured / where the displacement is being measured; also, what length/dimension should you use for the ‘gauge’ length (to calculate strain) if attempting to use this table/data? - explain.)

F) Compare the value of E obtained from the tensile test (extensometer data), to typical values for E for mild steel (obtain from at least two reference books and one internet reference).

G) Was there a clear proportional point and yield point (i.e. were they distinct?) H) Determine the ultimate tensile strength and the fracture strength from the

table data, and the corresponding strains. Determine the true stress at the fracture point, and the corresponding true strain (= ln (L/L0)). Could you determine the true stress (and strain / true strain) at the maximum force point, from the measurements taken? Explain (consider whether a suitable dimension(s) of the specimen was/were taken to achieve this; if not, would you be able to estimate the dimensions you need, from the fractured specimen?) Would you expect true stress to be larger or smaller, in general, than ‘engineering’ stress? Explain. Would you expect true strain to be larger or smaller than ‘engineering’ strain? Explain. Compare the above values obtained (consider conventional and ‘true’ values), to data obtained from at least two reference books and one internet site.

I) Consider the specimen at fracture. How well does the extension determined from the PC readings at fracture (on the relevant graph/table) compare with the extension measured using the actual specimen (i.e. using the original gauge length and the final length after fracture)? Would you expect a significant difference and, if so, to what could this be attributed? If you in general wanted to use the cross-head displacement data to determine relatively accurate strains throughout the test range (perhaps because you do not have an extensometer available), what changes to the test sample dimensions would you recommend, and why? (Consider length and cross-sectional area). What dimension of the test specimen would you use as the ‘gauge’ length in this scenario? Why?

J) Determine the percent elongation and the area reduction ratio, at fracture, using the Vernier measurements taken before and after fracture. Which in your opinion gives a more meaningful indication of ductility? Explain. Is it meaningful to specify the percent elongation of a material without reference to a gauge length? Explain. Would you say that the test sample, from the data obtained, is a ductile or brittle material? Why? (What typical value(s) of elongation or area reduction is normally used to indicate the ‘end’ of the range of brittle materials and the ‘beginning’ of the range of ductile materials? – obtain from at least one reference book and one internet site).

K) Compare and discuss your answers for the calculated elongation and area reduction, to data obtained for mild steel from one reference book and one internet site. Does the material conform to typical mild steel values (or do you

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think the sample was not mild steel)? What percentage carbon would you estimate the steel to have, given the reference material data?

Discussion / Conclusions

In general, discuss assumptions, observations, results, deductions, and conclusions, in this section. Also discuss errors and possible sources of error. Suggest ways to reduce these. Discuss anomalies observed or obtained from the results. Suggest possible ways to improve the practical (both from the point of view of accuracy, and from the point of view of relevance for the learner and making it more interesting/useful).

Further questions to be answered:

L) Did the testing machine gradually increase the strain/extension in a steady controlled fashion at a pre-determined rate (and hence measure what corresponding force developed), or did it gradually steadily increase the applied force at a steady rate (and hence measure what corresponding strain was developed)? Explain.

M) Given the above question, how would these two possible methods affect which variable is seen as the independent variable, and which is seen as the dependant variable? Explain.

N) In your opinion, which (strain or force) should be regarded, in terms of best understanding the material properties, as the independent and which as the dependant variable? Explain.

O) If the testing machine operated in the alternative way to your answer above, (i.e. in terms of which variable is controlled), what difference(s) do you think might be observed in the experiment? Consider, for example, whether fracture would be more immediate once the ultimate tensile (or shear) strength stress value has been passed.

Add any other relevant discussions/conclusions.

References

List all references used, in the format specified by the department.

4. ASSESSMENT

As a general guide, assessment shall be based on the following. Note however that due to the nature of the report, certain aspects may be reported under different sections, or spread over sections, therefore the weightings indicated below are a guide only and discretion will be applied during assessment.

Apparatus: 5%Method: 6%Theory: 8%Observations/Results/Calculations: 71%

Tables: (17%)

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Graphs: (17%)Calculations and sample calculations: (17%)Answering of questions (explanation type) (20%)

Discussion/Conclusions: 10%

Total: 100%

For tables and graphs, clear labeling and headings are important. Presentation is considered within the various categories, but in total will contribute about 5% to the overall mark. This includes report structure and heading / sub-heading labels and numbering, use of suitable and consistent fonts, etc.

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MSM2111 Tensile Test Prac Guide 2012

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