The comparison of the modulus Ep
evaluated from the ultrasonic propagation speed measurements with the static modulus Es
from the tensile tests and the examination of change in Ep
with tensile deformation for polyethylene terephthalate (PET) fibers and glass beads filled polystyrene composites revealed the following peculiarities in Ep
was not linearly dependent upon the crystallinity but dependent upon the cube root of crystallinity, while Es
was directly dependent upon the crystallinity. In the case of PET fibers, the moduli, Ep
, were satisfactorily described by a parallel two-phase structure model with crystalline and amorphous regions.
(2) The two-phase structure model also enabled to be applied to composite materials. Ep
of the glass beads filled polystyrene composites was not linearly dependent upon the volume fraction of fillers but was dependent upon the cube root of volume fraction of fillers. In the case of glass beads filled polystyrene composites in which the adhesion between fillers and matrix was improved by silan-treating, the modulus Es
was described by the series model. In the case of untreated composites, Es
(3) In the case of low oriented PET fiber, Ep
decreased by drawing in spite of an increase in birefringence. It is suggested that the disorder of secondary bonds are closely related to the ultrasonic propagation mechanism.
for PET fibers increased in stress relaxation after a small deformation but decreased in stress relaxation after a large deformation.