Compliance, sonic

During the creep of PET and PpPTA fibres it has been observed that the sonic compliance decreases linearly with the creep strain, implying that the orientation distribution contracts [ 56,57]. Thus, the rotation of the chain axes during creep is caused by viscoelastic shear deformation. Hence, for a creep stress larger than the yield stress, Oy,the orientation angle is a decreasing function of the time. Consequently, we can write for the viscoelastic extension of the fibre... 

Ef is the sonic modulus of the fibre. From Eq. (13.167) it follows that measurement of and 0(relation between the compliance and the orientation parameter (sin20)E/[l+erF/(2gd)]- This relation has been verified experimentally by a combined sonic modulus-X-ray diffraction experiment on a PpPTA fibre (Baltussen and Northolt, 2002), as shown in Fig. 13.100. Linear regression yields eA = 240 GPa and gd = 1.60 GPa. 

FIG. 13.100 The sonic compliance versus the orientation measured by X-ray diffraction during extension of a low-modulus PpPTA fibre. From Northolt and Baltussen (2002). Courtesy John Wiley Sons, Inc. 

---