Hi all,

I'm trying to understand the Tsai-Wu failure criterion for orthotropic materials. On the wikipedia page I found a way to calculate F12 however I don't understand an assumption they make. They say:

if sigma1=sigma2=sigma12 and sigma1=sigma3=sigma13,... but if this is the case why is sigma12 not equal to sigma13 since both are equal to sigma1?

All help is appreciated.