Fracture behavior and acoustic emission in bending tests on single-fiber composites

The bending fracture mechanisms and interfacial behavior of single-fiber composites (s.f.c.) with different fiber surface treatments and embedded fiber positions were investigated in three-point bending with simultaneous acoustic emission monitoring. Microfractures occurring at fiber breakages were examined by AE parameters and observations by a polarized microscope. As a result, it was found that AE signals in a bulk resin specimen were almost not detected, while many AE events were monitored in the s.f.c. bending specimens. The number of AE events was in good agreement with the number of fiber breakages, except for specimens with an embedded fiber near the compressive surface. Using AE parameters, especially the peak frequency and its power energy obtained by a power spectrum analysis, failure modes can be identified. A transition of failure mode from fiber break accompanied by a matrix crack and debonding to buckling is observed when the stress in the embedded fiber changes from tension to compression. The debond length is very long near the loading point for the specimens with a fiber near the tensile surface, but it decreases with increasing distance from the loading point. The debond length is small for the specimen with an embedded fiber near the neutral plane since the strain in the fiber decreases. Furthermore, a model for debonding failure is proposed and the maximum interfacial shear stress is derived. It is confirmed that fiber fragment lengths for the specimens with a fiber near the tensile side can be also expressed by the Weibull distribution as done in s.f.c. tensile tests.