1. PHF110: Basic Physics and Materials Dr Mark A. E. Jepson Room: S227 m.a.e.jepson@lboro.ac.uk

2. PHF110: Basic Physics and Materials Lecture 2 Mechanical Testing

3. Materials selection  Screwdriver is made of 2 materials  Steel and Perspex  What properties do they have which make them suitable?

4. Definitions – What are they describing?  A level of stress at which there is a significant change in the state of the material  The ratio of the stress divided by the strain  A measure of a material’s ability to undergo plastic deformation before fracture  A measure of a materials ability to resist deformation by indentation or abrasion (scratching)  A measure of the amount of energy a material absorbs as it fractures Strength Stiffness Ductility Hardness Toughness

5. Intended Learning Outcomes  Understand why we need to test materials in a reproducible way  Explain the processes of hardness, toughness and tensile testing  To appreciate that different materials need different testing considerations

6. Why do we test Materials?  What can we learn? Properties of materials, integrity of part, resistance to environment, objective achieved?  Why do we need to know? Compare materials for selection, choosing the best material, understanding of uses…  Who may use the Information? Designers, scientists, students, inventors…

7. Types of testing  Destructive  Determines ultimate properties of materials  Inspect broken pieces and / or process of destruction  Non-destructive  Used to detect flaws or defects  To determine properties of materials standards must be followed

8. Hardness A measure of a material’s ability to resist deformation by indentation or abrasion (scratching)  How could we test hardness? (clue – last week’s notes)  Scratching and Indentation tests are used  When would you use a scratch test?  Now we will look at some indentation tests

9. Hardness indentation tests  Brinell  Vickers  Rockwell  These all measure the same property but are conducted in slightly different ways

10. Brinell  Hardened steel (or tungsten carbide) ball  Ball D = 10 mm  Lower loads for softer materials Load 3000 kg 10 – 15s

11. Brinell d

12. Brinell Limitations  Can not test very hard materials as the ball may deform  Different loads can give different results – due to large deformations involved  Range of use: 0.25D ≤ d ≤ 0.5D  Large indent causes substantial damage

13. Vickers  Similar to Brinell  Uses a load of 1 – 1000 g  Diamond indenter  Pyramid shaped  Every indent is similar regardless of depth  Same formula as Brinell 136° d1d1 d2d2

14. Vickers limitations  Indents are much smaller than Brinell: good and bad… why?  Measurements of indent can suffer from large error  Sample must be polished in order to see the indent properly  Good for small specific areas of samples rather than large bulk measurements  Micro (and nano) hardness testing is available too and based on Vickers testing

15. Rockwell  Applies a major and a minor load  Conical diamond indenter or steel ball  Depth is measured rather than diameter Minor load Minor load + Major load Minor load

16. Considerations  Indents near an edge are not reliable. Can you think why?  Other indent proximity  Accuracy is dependent on human ability to measure indent size (except for Rockwell)  Relationship to other properties  Hardness is approximately proportional to strength for iron – not true for all

17. Impact testing - Toughness A measure of the amount of energy a material absorbs as it fractures  How could we measure toughness?

18. Impact testing  Represent the most severe deformation conditions relative to fracture potential  Low temperature deformation  High strain rate  Triaxial stress (i.e. in the presence of a notch)  The fracture toughness of a material can be calculated  Impact testing is good for comparisons

19. Impact testing h h'

20. Impact testing

21. Impact Testing  Charpy and Izod testing  Differences are in the sample geometry and way the piece is struck CharpyIzod

22. Videos  Charpy & Izod testing Charpy & Izod testing

23. Tensile testing  Tensile “shopping list”  A way to hold the sample  A way to apply force  Accurate measurement  (dimension change)

24. Tensile testing  Tensile “shopping list”  A way to hold the sample  A way to apply force  Accurate measurement  (dimension change)

25. Sample geometries  There are standards for geometries  Important measurements are  Gauge length  Width  Radius Thickness Parallel length

26. Tensile Testing Ceramics Refer to the general properties table in your notes What adjustments would you need to make to a tensile test machine for different materials?

27. 3-point bending for ceramics  Ceramics cannot be tensile tested easily. Why?  They are very brittle, so gripping is difficult  They must be perfectly aligned  During tensile testing, there is very little elongation  Sample preparation near impossible So, how can we test them?

28. 3 – Point Bending  Ceramics are very strong in compression  By bending the sample, we can force the outside edge into tension  Rupture strength is approximately double tensile strength Tension – not possible

29. Stress-strain curves (repeat)

30. Consider important properties for these items…  Safety goggles?  Pen (think of the different parts)?  Fizzy drinks bottle?  Touch screen of a smart phone?  What properties do they require and why?

31. My ideas GogglesPenBottlePhone Toughness(ball) Hardness StrengthElectrical conductivity Transparency(body) Stiffness Cost? Recyclabilty? Strength

32. Intended Learning Outcomes  Understand why we need to test materials in a reproducible way  Explain the processes of hardness, toughness and tensile testing  To appreciate that different materials need different testing considerations