NUMERICAL STUDY ON CREEP DAMAGE OF COMPOSITES AT HIGH TEMPERATURE

A unit cell model is applied to study the creep damage behavior after fiber fractures in the fiber-reinforced composites at high temperature. The user subroutine CREEP has been programmed for ABAQUS. The fiber breakage results in a new crack. The results show that the stress concentration factor resulted from the fiber breakage increases with the creep time. The creep damage takes place near the crack, and then grows in the matrix along a certain angle, up to the total failure. The influences of the ratio of modulus of the fiber to the matrix (Ef/Em) on the creep deformation, damage and stress distributions have been studied. With the increasing Ef/Em, the damage in the matrix increases. Analysis on the different ductility of matrix shows that the creep damage of low ductile matrix composites is higher than high ductile matrix composites.

STUDY ON THE EFFECTS OF BREAKAGE OF SINGLE FIBER ON CREEP BEHAVIOR OF FIBER REINFORCED COMPOSITES

A 3-D micro cell model with multi-fibers has been presented to study the effects of breakage of single fiber on the whole creep behavior of fiber reinforced composites by finite element method (FEM). Before the fiber breakage, the stresses of all fibers are identical. With the creep time increasing, stress in fiber increases but stress in matrix decreases. It is assumed that the fiber breakage occurs when the stress in fiber reaches a critical value. The stress redistribution resulted from the breakage of fiber has been obtained. The influence on the axial stress of the broken fiber is local. The stress in the all fiber sections is not uniform. There is a local stress concentration in the matrix. And this stress concentration in the matrix is more and more serious with the creep deformation. The stress transference of the loading due to the fiber breakage has been studies numerically. It is found that the fibers near to the broken fiber will take over more loading.

Pb-DOPING EFFECTS ON THE DIELECTRIC AND PYROELECTRIC PROPERTIES OF(Sr,Pb)TiO_(3)SYSTEM

Lead strontium titanate(Sr_(1-x)Pbx)TiO_(3)(0.20≤x≤0.45,step=0.05)ceramics were prepared by conventional mixed oxide method.The X-ray diffraction patterns indicate that the prepared samples have perovskite-type structure.With the increase of Pb content,there is a tendency from the cubic to tetragonal structure.The scanning electron microscopy micrographs reveal that the addition of Pb can affect microstructure.The dependent temperature dielectric permittivity and dielectric loss were investigated in the frequency range from 100 Hz to 1MHz.The maximum peak of the dielectric permittivity versus temperature curve was broadened and a frequency dispersion of the dielectric permittivity was observed for the(Sr_(0.8)Pb_(0.2))TiO_(3)ceramics.The results obtained at the frequency of 10 kHz reveal the Curie temperature linearly increased with the lead content.Thefitted curves of temperature versus inverse dielectric permittivity at 10 kHz for(Sr_(1-x)Pbx)TiO_(3)ceramics are consistent with Curie-Weiss law.The Pyroelectric properties were also investigated.The high pyroelectric coe±cients andfigure of merits indicate that the SPT ceramics are potential materials for pyroelectric sensors.