viscoelastic material characterization and modeling

Xi Liu's picture


I used TA DMA tested the viscoelastic material properties (stress relaxation test). When I did Time-Temperature Superposion to create the mater curve, i found some of the shift-X values were negative (those points after Tg), which cannot be fit to WLF function. ANybody knows the reason behind this?  I excluded those point when fit the WLF coefficients, is this acceptable?

And I found the C1 and C2 of one material are negative, can i still use it at high temperature? thank you