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Eingeladener Vortrag
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Failure of fiber reinforced composites – role of the matrix behaviour and the interface strength
(2016)
Failure of fiber reinforced composites initiates at microdefects in the matrix or at the interface. The interface strength is calculated from the stress field directly before crack initiation by nonlinear finite element simulations. Large deformations and plastic material behaviour are taken into account. Microscopical inspections of the fibers after failure show only small regions with adhering matrix material. This indicates that the interface strength is below the matrix strength.
The aim of the paper is to analyse the failure process taking place on microscale in fiber reinforced composites. The failure is initiated at microscopical defects presumingly in the interface. The circumferential propagation of interface cracks starting at microdefects in unidirectional plies under transverse loading is studied using a representative volume element. First a linear elastic fracture mechanical analysis of interface cracks in a carbon fiber reinforced epoxy resin is performed by calculating the mixed mode energy release rate using the virtual crack closure method. The influence of the fiber spacing is studied by varying the fiber volume fraction. The total energy release rate rapidly grows after crack initiation indicating an unstable crack propagation. The first phase is dominated by mode I. The mode II energy release rate starts slightly delayed but surpasses the mode I part after a short crack length. By increasing the fiber volume fraction the maximum of the energy release rate shifts to smaller crack angles.
Off-axis tests are widely used for the determination of the anisotropic mechanical properties of composite materials, this is, the longitudinal E-modulus and the strength. The mechanical properties are evaluated from the test data under idealising assumptions such as homogeneity of the material as well as a homogeneous stress distribution. In real off-axis tests, however, significant deviations from the idealising assumptions are present. The reduction of the accuracy of the results strongly depends on the off-axis angle.
In this paper a series of off-axis tests is analysed. The stresses on potential failure plane are calculated by finite element analyses. By varying the geometrical parameters it is shown how small deviations from the ideal case influence the measured elastic constants and the strength. Especially at low off-axis angles the mechanical properties strongly vary with the angle. In addition, fracture surfaces for different shear/normal stress combinations are studied and typical phenomena are identified.