The slope representing the ratio between strain and stress expresses the elastic modulus (E) and is almost identical for all steels (approximately 30,000 by 100,000 pounds per square inch [PSI]). Therefore, a mild steel wire and a spring steel wire with the same cross sections will deflect the same amount under the same load, regardless of their yield or tensile strengths. However, higher-strength steels can carry higher loads and will remain elastic under loads that would permanently deform lower-strength steels.
Assume that Material 1 in Figure 2 is aluminum and has a different stress/ strain slope from steel. The different angle of the slope means that the aluminum's elastic modulus is different from steel's—actually, about one-third of steel's—so under the same load, all aluminum specimens deflect about three times as much as steel.
Not all materials display such a definite yield stress point as mild steel. It is generally accepted that the stress which creates 0.2 percent elongation will be the yield stress.
Elongation (e) normally is expressed as the ratio of the permanent elongation divided by the original specimen length. The measured permanent elongation includes the increased elongation at the neck, so shorter specimens provide relatively larger elongation. North American standards specify that elongation should be measured in 2- or 8-inch lengths.
