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The failure of laminates usually is initiated by the failure of the plies with the maximum transverse stresses. The failure of those plies generally reduces the load carrying capability of the laminate, even though they carry only a small part of the external load. The analysis of failure under off-axis stresses accordingly is essential for the prediction of the strength of laminates. This is taken into account in the "mechanisms based failure criteria" e.g. by Puck. Most of these inter fiber failure criteria are not based on the micromechanics of the failure process but on prescribed stress interaction functions. The failure of plies under transverse loading obviously is governed by the bond strength between fiber and matrix fiber. The interfacial debonding was studied e.g. by Paris et al. and Corea et al.. The bond strength between fiber and matrix usually is measured by using micromechanical tests e.g. pull-out, push-out or fragmentation tests. Stress analyses as well as fracture mechanical analyses show that the interfacial failure over a large range is dominated by shear stresses (Kim and Mai, Pisanova et al, Marotzke and Qiao). The stress distribution arising in those experiments however differs from the stresses acting in a lamina under transverse loading. In transverse failure of a lamina, radial stresses as well as shear stresses are dominating the failure process of the interface. In addition longitudinal shear stresses are present. Experimental work concerning off-axis loading of single fibers was done e.g. by Tandon and Kim and by Ogihara and Koyanagi. They studied the influence of the fiber alignment by using a specimen in form of a cruciform with skew wings.
The debonding of a fiber in a glass fiber / epoxy composite under transverse loading is studied. The stress field in the interface as well as the energy release rate are analysed for two fiber volume fractions.
The circumferential propagation of an Interface crack starting at the center of a fiber which is located within a hexagonal fiber array is studied. The analysis is performed by a finite element Simulation under plane stress conditions. Two fiber volume fractions are considered, this is 30% and 70%. The stress distribution before and during crack propagation is calculated for different stages of the
crack. The crack opens by dominating tensile stresses in the first phase while it closes when propagating along the interface. The total energy release rate as well as the mode I and mode II parts are calculated. In the first phase the crack is driven by an increasing mode I energy release rate, indicating an unstable crack propagation. Then the mode II energy release rate increases rapidly and dominates the debonding process while the mode I part decreases and finally vanishes. Subsequently also the mode II part decreases, indicating stable crack propagation. In the last phase the crack becomes unstable again due to a strongly increasing mode I energy release rate. During the debonding process a remarkable change the mode ratio takes place.
The main features of the debonding process are similar for low and high fiber volume fraction. In case of the high fiber content, however, the mode I part is more pronounced and the maximum of the total energy release rate is shifted to lower crack angles.
The strength of laminates distinctly depends on the transverse strength of the laminas. The transverse failure is dominated by the adhesion between fiber and matrix. Usual strength criteria however do not take into account the adhesive strength explicitly. The determination of the interface strength is performed on the micromechanical scale using single fiber specimens. The fibers are loaded under off-axis loading while the debonding is monitored under a microscope. The stresses acting in the Interface are calculated by finite element analyses. It is found that for off-axial angles up to 35° interfacial debonding is the dominant failure mode while fiber breakage takes place at lower angles. The occurrence of fiber breakage and debonding under off-axis loading shows that the restriction to two potential failure planes - perpendicular or parallel to the fibers - as applied in common failure criteria has to be put in question.