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There are numerous alloys with markedly different ranges of strengths. Thus, strength is an important factor in the selection process that determines the copper alloy to be used for a connector spring. As described in the topic "Contact Force," the importance of strength is its relevance to contact force. Young's modulus of elasticity is a measure of the stiffness of a spring member. Stiffness is an important factor in spring design because it, too, affects contact force. This is readily seen from the equations that relate deflection to contact force. For example, with a cantilever beam design (fixed at one end, load at opposite end) contact force is directly proportional to the modulus. Clearly, this is an important consideration in the design of a connector spring.
The elastic modulus of copper alloys ranges from 16 to 20 million pounds per square inch (about 110 to 138 kMpa). This range is representative of practically all of the copper alloys used for connector springs. While this variation is quite a lot less than that for strength and conductivity, it should not be ignored. The values for each alloy are tabulated in various handbooks and data sheets.
Young's modulus of elasticity is defined as the relationship between stress and strain for stresses below the proportional limit. At high design stresses, the use of this modulus in the equations that relate contact force and deflection will not precisely predict actual behavior. Temper, orientation, and stress mode affect the modulus of elasticity, but only slightly. Such detailed measurements of this characteristic have not been thoroughly reported, and usually a single value of Young's modulus is associated with each copper alloy. This further validates the importance of prototypes.
Another, related, property is the Poisson's Ratio. This number is a material property which relates the strain in the transverse direction in a tension test to the strain in the longitudinal direction. It is nearly a constant for most copper alloys, and a value of 0.3 can be assigned to all connector alloys without harm to any important design parameter.