Stress-to-strain proportionality

Stress-to-strain proportionality is limited to the elastic region, for very low strains where Hooke s law applies. 

To calculate the modulus of elasticity E in the elastic region, use the following [7-9]  

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In the proportional (elastic) region, Hooke s law applies, and Yoimg s modulus E = o/e. Beyond the elastic limit there is no stress-to-strain proportionality, and Hooke s law does not apply. Beyond the elastic limit, stress can be constant while strain continues to increase. A viscoelastic solid plastic has viscous fluid characteristics beyond the elastic limit. 

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