MECHANICAL TESTING

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.

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.

HARDNESS TESTING Hardness is the ability to withstand dents or scratches

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

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.

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

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.

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

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

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

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.

Extensometer

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

Load - extension graph for low carbon steel

Draw graph for this tensile test?

Identify the straight line part of the graph.

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

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

Youngs Modulus for Load – extension graph

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.

Proof stress for Load – Extension graph

Proof stress for Stress – Strain graph

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

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.

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.

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.