[PSUBS-MAILIST] definition

[Sean T. Stevenson via Personal_Submersibles]

Strength is a stress value at which the part fails, typically expressed in psi or MPa. There are two failure strengths typically quoted for a material: 

The first is yield strength, which is the maximum stress (force per unit area) at which the deformation is entirely elastic (i.e. recoverable). Stress beyond yield can be carried by the material, but it will deform plastically and not return to its original dimension. Within the yield limit, stress vs strain exhibits a linear relationship. Stretch a material twice as far, and you will develop twice as much stress in it etc. 

The second is the ultimate strength, or the maximum stress that can be developed in a material. If you strain a material beyond its yield point, what you typically see is the end of the linear stress  / strain relationship, as the curve flattens to exhibit a slow rise up to the ultimate strength, and then a slow fall beyond that limit as you continue to stretch the material to failure. 

Modulus is the slope of the linear portion of the curve within the yield point (stress/strain).  The modulus is essentially a measure of the stiffness of the material. Modulus has the same units (psi or MPa) as strength, because strain is dimensionless (actually it is length change over original length, as in if I have a 10 cm rod that I stretch 0.5 cm beyond its unstretched length, I have developed 0.05 strain, or 5% strain in the material). Ceramics are high modulus materials, elastomers are low modulus.

By convention, most material properties quote the tensile strength values. For materials which vary significantly between tension and compression (or for which the primary strength is in compression, such as concrete), the compressive strengths and modulus will be used.

Sean


On June 20, 2017 3:39:59 PM PDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hi All,What is the difference between compressive strength and
>compressive modulus?Hank
>
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