figure no:1 showing the stress-strain diagram for aluminum alloy. we can notice from the stress-stress diagram that aluminum has a considerable ductility. despite the absence of explicit yield point as in structural steel. the initial portion of the stress-strain curve is linear with a recognizable proportional limit. the proportional limit for aluminum alloy ranges from 70 to 410 Mpa (10 to 60 Ksi). aluminum alloy undergoes large strain before failure. ultimate stress ranges from 140 to 550 Mpa (20 to 80 Ksi). Yield stress for aluminum alloy can be determined by drawing offset line parrel to the linear portion of the stress-strain curve. the straight line offset by a standard strain such as 0.002. the intersection point with the stress-strain curve is the point of yield stress. figure no:2 illustrates the concept of offset method. yield stress acquired by offset method should be distinguished from true yield stress. therefore it is called offset yield stress. Figure-3 showing stress-strain diagram for rubber. the stress-strain diagram for rubber remained linear for a relatively high strain value of 0.1 to 0.2 at the proportional limit. beyond proportional the behavior depends on rubber types. for example, soft rubber will undergo enormous elongation without failure but hard rubber will resist the load with minimal stretching. therefore the curve will go up sharply as shown in figure no:3.