Mechanical Properties

2. Yield Strength, Ultimate Tensile Strength
The stress-strain diagram showed to you reveals two other important parameters: Yield Strength and Ultimate Tensile Strength. Up to a certain loading, a material can recover it’s strain during the un-loading phase defining the presence of an elastic limit, which a designer should not cross in most of the applications. However, from a practical point of view it is difficult to identify the elastic limit accurately. A more useful approach comes from the observation that for most of the metals the safe level of strain is about 0.002 (0.2%). Hence, if you draw a line staring from this value parallel to the initial part of the stress-strain diagram, the point of intersection of this line with the stress-strain diagram is defined as the Offset Yield Strength or simply the Yield Strength of the material – σ_y expressed usually in MPa (10⁶ Pa).

The other important parameter specifically for metals and metallic alloys is Ultimate Strength. It is defined as the point of maximum stress that a material can withstand. It may be noted that such a point could readily be obtained from the engineering-stress diagram while the true stress-strain which is based on actual cross-sectional area at every stage of loading would not show this point.

3. Fracture Toughness
From the experience of many catastrophic failures, the designers now know that it is simply not good enough to keep the average stress level of a component or a structure below a safe stress level. In reality, there are flaws present everywhere (like pores, micro-cracks etc.) and there is a good possibility of overstressing at these locations. Hence, the important question here to ask is that whether the flaw will increase in size and cause failure or not while loading.

Performing tests on specimens having known flaw-size one can determine the critical Stress intensity factor K that would cause the growth of flaw and failure.
This is denoted as K_c, more specifically as K_IC for critical stress-intensity factor corresponding to a plane-strain scenario. The unit of K_IC is in MPa-m^1/2.