1. MECHANICAL TESTING

2. Why are metals tested ? Ensure quality Test properties
Prevent failure in use
Make informed choices in using materials
Factor of Safety is the ratio comparing the actual stress on a material and the safe useable stress.

3. Two forms of testing
Mechanical tests – the material may be physically tested to destruction. Will normally specify a value for properties such as strength, hardness, toughness, etc
Non-destructive tests (NDT) – samples or finished articles are tested before being used.

4. HARDNESS TESTING
Hardness is the ability to withstand dents or scratches

5. Hardness testing machine
The indenter is pressed into the metal
Softer materials leave a deeper indentation

6. Brinell hardness test Uses ball indentor.
Cannot be used for thin materials.
Ball may deform on very hard materials
Surface area of indentation is measured.

7. Vickers hardness test Uses square pyramid indentor. Accurate results.
Measures length of diagonal on indentation.

8. Rockwell hardness tests
Gives direct reading.
Rockwell B (ball) used for soft materials.
Rockwell C (cone) uses diamond cone for hard materials.
Flexible, quick and easy to use.

9. Impact Tests
Toughness of metals is the ability to withstand shock load and impact.
It will not fracture when twisted.

11. Izod test Strikes at 167 Joules. Test specimen is held vertically.
Notch faces striker.

12. Charpy impact test Strikes form higher position with 300 Joules.
Test specimen is held horizontally.
Notch faces away form striker.

13. Tensile Testing
Uses an extensometer to apply measured force to an test specimen. The amount of extension can be measured and graphed.
Variables such as strain, stress, elasticity, tensile strength, ductility and shear strength can be gauged.
Test specimens can be round or flat.

14. Extensometer

17. Producing graphs Two basic graphs: Load – extension graph.
Stress – strain graph.

18. Load - extension graph for low carbon steel

21. Draw graph for this tensile test?

22. Identify the straight line part of the graph.

23. Youngs Modulus (E) E = Stress
Strain
Stress = Load
Cross section area
Strain = Extension
Original length

24. Youngs Modulus for stress – strain graph
Select point on elastic part of graph
Calculate Youngs Modulus with this point
E = Stress
Strain

25. Youngs Modulus for Load – extension graph

26. Proof Stress The stress that causes a % increase in gauge length.
It can be found by drawing a line parallel to the straight part of the graph.
A value can be taken from the vertical axis.

27. Proof stress for Load – Extension graph

28. Proof stress for Stress – Strain graph

29. Tensile Strength Tensile strength = Maximum Load Cross section area
Maximum load is the highest point on the graph.
Often called Ultimate Tensile Strength (UTS)

30. Creep
When a weight is hung from a piece of lead and left for a number of days the lead will stretch. This is said to be creep.
Problems with creep increase when the materials are subject to high temperature or the materials themselves have low melting points such as lead.
Creep can cause materials to fail at a stress well below there tensile strength.

31. Fatigue Fatigue is due to the repeated loading and unloading.
When a material is subjected to a force acting in different directions at different times it can cause cracking. In time this causes the material to fail at a load that is much less than its tensile strength, this is fatigue failure. Vibration for example is a serious cause of fatigue failure.

32. Fatigue Fatigue can be prevented with good design practice.
A smooth surface finish reduces the chance of surface cracking.
Sharp corners should be avoided.
Corrosion should be avoided as this can cause fatigue cracks.