Yield stress and ultimate stress Yield stress:
Yield stress can be defined as lowest stress at which extension of test piece increases without increase in the load. It is stress corresponding to yield point. For the ductile material yield point is defined well whereas for brittle material it can be obtained by offset method. It is called as yield strength.
Yield Stress = Lowest stress = Yield Point Load/ Cross sectional Area
Ultimate stress: The ultimate stress or Ultimate strength corresponds to highest point of stress strain curve. It is ratio of maximum load to original area of cross section. At this ultimate point, lateral strain becoms localized resulting into formation of neck.