Fatigue Testing: Rotating BendType of assignment

ME5600 – Fracture Mechanics and Fatigue Analysis AY17/18

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College of Engineering, Design & Physical Sciences

Dept. of Mechanical, Aerospace and Civil Engineering

SED01: ASSIGNMENT/COURSEWORK FORM

Module Code: ME5600

Assessment Title: Fatigue Testing: Rotating Bend

Module Leader: Dr Tom De Vuyst

Main objectives of the assessment: You are required to generate S-N curves, or Wöhler curves for three engineering materials manufactured according to DIN 50113, by considering the regime bounded between 103 - 105 cycles to failure. Brief Description of the assessment: Experiments on fatigue are an integral part of the ME5600 module. The Rotating Bend Test provides an opportunity to consolidate understanding of lectured material on fatigue by planning and performing experimental tests to determine Wöhler curves for three common engineering materials.

Learning outcomes for the assessment (refer to the appropriate learning outcomes) Knowledge and understanding: • Key principles underpinning fatigue response of

different engineering materials.

Cognitive skills: • Generate and evaluate experimental data

Other skills and attributes: • Plan and execute safely a series of experiments • Analyse / present results in a technical report.

Assessment and marking criteria Submit a report which meets objectives, format and deadline as specified in this pro-forma.

Assessment method by which a student can demonstrate learning outcomes: Presentation (20%)

• Clarity of presentation, report structure (including figures, tables, referencing) Test (30%)

• Description of the problem, including the objective, experimental set-up, samples and test procedure (eg. choice of control parameters etc).

Critical analysis and interpretation of results (50%)

• Analysis of results, including physical interpretation, engineering context / comparison with literature, together with physical parameters to plot, critical appraisal of method used and results obtained.

Weighting: This assignment accounts for 20% towards your overall mark for ME5600.

Format for the assessment/coursework: (Guidelines on the expected format and length of submission): • A formal report is required, whereby its length should not exceed ten to fifteen (10-15) pages. • For detailed guidance of the report content, please refer to Section 3: “Individual Report”. Distribution date to students: 7th November, 2017 Submission Deadline: 6th December, 2017 Feedback to students (3 week turnaround): 24th January, 2018 Indicative Reading List: Please refer to module lecture notes and recommended reading

Other Information: • Please ensure you wear appropriate PPE (Safety Shoes) before undertaken any testing. • Testing can only be performed under supervision of authorised user.

Checked and approved by: Internally checked by Module Lead

Date: 6th November, 2017

SED01- V2

ME5600 – Fracture Mechanics and Fatigue Analysis AY17/18

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1. Introduction

Fatigue results in progressive and localised damage within a material when subject to cyclic loading, resulting in micro-cracks growing to a sufficient critical size, which will propagate suddenly, causing a component to fracture / fail. Failure can occur at magnitudes where the cyclic stress amplitude is less than yield, or ultimate tensile strength limits found in material handbooks. A commonly used test to measure the fatigue limit (or fatigue endurance) is the rotating bend, or reversed bending test, which applies a constant sinusoidal load through the application of a bending moment. For further information on the Zwick machine, please refer to the User Manual [1]. With this piece of equipment, the bending moment is applied by adding mass to a 1m horizontal lever arm. Using Engineer’s Theory of Bending, the magnitude of cyclic stress amplitude (S) generated at the surface of the specimen can be calculated. By determining experimentally the number of cycles that may / may not cause failure (N), the fatigue results are presented as S-N, or Wöhler curves, shown schematically in Figure 1. From figure 1, an endurance limit can be defined, which is based upon the highest cyclic stress amplitude a material can withstand in order to reach a minimum number of cycles (>107 to 108 cycles) and not fail. The endurance limit needs to be determined experimentally and depends upon the material being tested.

Figure 1 – Typical S-N, or Wöhler Curves for two materials where the endurance limit may not be so precisely

defined

2. Tasks

You are required to generate S-N curves for three engineering materials, which have the following properties, using specimens manufactured according to DIN 50113 [1].

• 16mm shoulders, Ø10mm Round Specimens

Material Type Approximate Yield Stress (MPa) Approximate UTS (MPa) 6082 Aluminium 90-280 250-340 EN8 Mild Carbon Steel 280-465 550-850 CZ121 Brass 230-350 360-500

Table 1 – Material Types to be tested You are required to work in small groups. Due to the time taken to perform a single test, each person must individually test two materials (minimum) and be prepared to share your results with the rest of the group(s), in order to generate a sufficient (and even distribution) of data points for your individual report.

ME5600 – Fracture Mechanics and Fatigue Analysis AY17/18

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As a suggestion for the breakdown of tests per material type, please refer to Table 2, as this will provide five to six data points per material.

MCS EN8 Steel Al6082 CZ121 Brass Group 1 (three people) 2 2 2 Group 2 (three people) 2 2 2 Group 3 (four people) 2* 2* 2*

Table 2 – Suggested number of test per material type (* additional tests required – chosen by group)

PPE:

a) You must wear appropriate safety shoes (as testing requires handing various weights up to 5kg). b) Heat resistant gloves when removing failed specimens from the machine.

MACHINE SETUP:

a) Please read Section 4 (pages 17-22 and 30) in the Zwick User Manual in order to gain familiarity with the test machine [1].

b) Confirm with an authorised machine user that the test machine is ready for operation (power, air and computer connected).

c) The machine must be turned on (and ready for operation) before launching TestExpert software.

d) Ensure specimen to be tested is gripped securely in the collets and close the safety door fully.

e) Wearing appropriate gloves, add the required mass to the hanger to load the specimen to required stress.

f) Press the “ON” button (on control panel under machine) to activate the drive train.

g) Define the test rpm and maximum number of cycles for the test through the Wizard in TestExpert. Leave other options such as data sampling rates at their default values.

h) Once you are ready to perform the test, select the “start” icon, which will start the machine.

i) If for any reason the test needs to be aborted, press the “stop” icon, or push the red emergency stop button (located on the control panel under the table supporting the machine).

SPREADSHEET:

a) You are recommended to create an Excel spreadsheet for your tests, as you will need to determine the magnitude of the cyclic stress, as a function of applied mass to the 1m lever arm.

b) At various rpm (250 -> 6000rpm), calculate the number of cycles per second and estimate the time taken to reach 1K, 10K, 50K and 150K cycle, etc.

c) Due to the limited time available for testing, a single test should not exceed 25-30 minutes (and can be shorter*), which means your planned tests should include 103 - 105 cycles to failure.

d) Time permitting, one test per material type should be conducted between 100K -> 250K cycles to failure

NOTE:

* - You need to think carefully about the loading rpm, as tests need to be isothermal. Applying the cyclic loading too quickly will result in adiabatic conditions, whereby the heat generated will severely affect your results and not be a true reflection of the number of cycles to failure.

ME5600 – Fracture Mechanics and Fatigue Analysis AY17/18

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CYCLIC LOADING: What tests need to be performed?

Due to the time taken to perform the tests, it is not possible to characterise a complete S-N curve for all three material types, from 103 cycles through to the endurance limit (between 107 - 108) cycles.

As an exercise, estimate how long it would take to reach the endurance limit at the maximum 6,000rpm of the machine. Comment on the value calculated.

Suggested Steps:

a) Conduct an initial test at an alternating stress level equivalent to ± 90%UTS and record the number of cycles to failure, the time taken to perform this test and the rpm applied. (HINT: You will need to consider carefully the rpm assigned, as you need to ensure the test is isothermal).

a) For each specimen, observe the failure surfaces. (Be careful when recovering the specimen as it may be hot / contain sharp edges).

b) Reduce the applied cyclic stress to generate a sufficient and even distribution of data points for each material type. It is recommend that testing to take place between 0.6-0.9UTS in order to complete the tests in the time available.

c) Please share your results between both groups. d) Test data should be presented as an S-N curve with an even distribution of points.

3. INDIVIDUAL REPORT: You are required to submit an individual report (max ten-fifteen pages). Please ensure that your report is your own work, including your assessment of result / failure surfaces for the shared data. As a minimum, your report should contain the following: 1. Brief, but comprehensive overview of the background underlying the rotating bend test, which includes a

discussion of the bending moment / stress distribution applied across the cross-section of the test specimen. This overview should not exceed two pages.

2. Explain the test procedure followed (max one page) and clearly state which two material tests you are responsible for.

3. For each material type, a table presenting the applied load, cyclic stress and rpm, together with the corresponding number of cycles to failure for all load levels used. Explain clearly your justification for the load levels selected (as this may influence your assessment of the results).

4. Complete S-N curves for all three material types. 5. In your analysis section, provide images of the failure surfaces observed for all three material types and

comment on their appearance and the fatigue performance of all three material types.

Please ensure your report is appropriately structured and referenced.

4. REFERENCES:

[1] Zwick / Roell, “Section 4 – Instruction / User Manual for material testing machines BRA37002000.1”, V5.4, 2010. (Available through Blackboard and hard copy provided in Material Testing lab)