Fracture Toughness Test ASTM D5045

Hello dear colleagues,

I have to perform a test for a brittle polymer and I am using the standard ASTM D5045*.

In the section 7. there is a geometrical requirement which is at the same time a validation of the reliability of the obtained results. It involves the calculation of the following quantity:

2.5*(Kq/σy) -------- it gives an idea of the size of the plastic zone

Kq is the fracture toughness calculated. σy is the yield stress. My doubt is that in my case my material does not show yielding. Should I just take the tensile strength instead?

*Standard test methods for plane-strain fracture toughness and strain energy release rate of plastic materials.