Learning Forum

[Zhenhao Qian]

When I fitting the shear relaxation curve, I found I must define the elastic modulus, but I don't know which modulus should I define, Instantaneous modulus or equilibrium modulus?

[John Doyle]

Assuming this is a small strain application, the base material can be defined with the instantaneous linear elastic modulus and Poisson's ratio.

If this is a large strain application for a polymer type material, consider defining  base material as a hyperelastic strain energy density function.

[Zhenhao Qian]

 

Thank you for your reply! And I’ m sorry about my late reply.

I also have some questions

 

1. Would I interpret small strain as the part before plastic stage? 
2. Before I do stress relaxation test about my material, I do uniaxial compression test to confirm its linear elastic stage. Because as far as I know, the stress relaxation test must ensure that it is within the linear elastic stage of material. Due to limitations, I can only apply load at a low speed (about 10mm/min to 60mm/min), I ‘m not sure how to confirm the linear elastic stage, because before 0.2 strain, the uniaxial curves don’ t seem to be completely ‘ linear‘ . I’ m not sure whether my understanding is correct.
   The figure shows the uniaxial compression curves. Red line = 10mm/min, Blue line = 60mm/min.