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Eingeladener Vortrag
- nein (92)
The breakage of a fiber and its effect on the strain energy is studied in a model composite, this is, a single fiber embedded in a dogbone specimen. The fracture process is recorded by an acoustic emission device, the crack patterns are monitored under a microscope. A finite element analysis is performed in order to estimate the energy released by different failure scenarios. The abilities and limitations of the acoustic emission analysis in characterizing different failure types in fiber reinforced composites are evaluated.
The debonding of fibers under mixed-mode loading is studied on model composites containing fibers with different orientations with respect to the loading direction. The spacing of the fibers is large, accordingly no significant fiber interaction takes place. For any fiber orientation, interfacial debonding occurs, even though at different load levels. Only for the low off-axis angle, additional fiber fracture is found. Using a rectangular specimen, the interface crack starts at the edges propagating into the center at low speed. In the first phase, only the central part of the interface debonds. After the whole fiber is debonded, the crack propagates in circumferential direction. The superimposition of fiber breakage and debonding shows that the restriction of two potential failure planes - perpendicular or parallel to the fibers - has to be put in question.
The debonding of a fiber in a glass fiber / epoxy composite under transverse loading
is analysed. The stress field in the interface as well as the energy release rate are
analysed for various fiber volume fractions between 5% and 85%. The circumferential
propagation of an interface crack in a hexagonal fiber array is studied. The analysis
is performed by a finite element simulation under plane stress conditions. The
stress distribution during crack propagation is calculated for different crack phases.
The crack starts under dominating mode I stresses. After propagating some 60° the
mode II part reaches a maximum while the mode I part vanishes and the crack
closes. The further crack propagation is driven by mode II stresses only. In addition,
the stress transfer into the neighbouring fibers during debonding is studied.
The failure of fiber reinforced composites under transverse loading is investigated on a
representative volume element. The volume element is composed of a twelve fiber hexagonal
array. The debonding of the central fiber is studied under transverse shear as well as under
transverse compressive loading by calculating the mode I and mode II part of the energy
release rate. The initiation of failure is assumed to take place at the location of maximum
tensile or maximum shear stresses, respectively. In order to determine the failure initiation
point, the stress field in the interface before crack initiation is calculated.