Chapter 12:Multiscale Modeling for Damage Analysis 535 When the interface between the matrix and the fiber debonds,this relieves the tensile residual stresses in the matrix.Due to different stresses in the matrix and the fiber at the interface,the fibers slide on the interfacial surface.Subsequently,the sliding surfaces cause degradation of material due to frictional wear at the interface.Pullout and pushback tests are useful in determining the stress required to cause interfacial sliding.This mostly depends upon the strength of the adhesive bond between the matrix and the fiber at the interface. Fiber microbuckling When a unidirectional composite is loaded in compression,the failure is governed by the matrix and occurs through a mechanism known as micro- buckling of fibers.There are two basic modes of microbuckling deformation: extensional”and“shear'”modes[5l],as shown in Fig.l2.3,depending upon whether the fibers deform“out of phase”or“in phase.”The com- pressive strength corresponds to the onset of instability and is given as Extensional Mode Shear Mode Fig.12.3.Extensional and shear modes of microbuckling [51]When the interface between the matrix and the fiber debonds, this relieves the tensile residual stresses in the matrix. Due to different stresses in the matrix and the fiber at the interface, the fibers slide on the interfacial surface. Subsequently, the sliding surfaces cause degradation of material due to frictional wear at the interface. Pullout and pushback tests are useful in determining the stress required to cause interfacial sliding. This mostly depends upon the strength of the adhesive bond between the matrix and the fiber at the interface. Fiber microbuckling When a unidirectional composite is loaded in compression, the failure is governed by the matrix and occurs through a mechanism known as micro￾buckling of fibers. There are two basic modes of microbuckling deformation: “extensional” and “shear” modes [51], as shown in Fig. 12.3, depending upon whether the fibers deform “out of phase” or “in phase.” The com￾pressive strength corresponds to the onset of instability and is given as Fig. 12.3. Extensional and shear modes of microbuckling [51] Chapter 12: Multiscale Modeling for Damage Analysis 535