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A Finite Element Study of Dynamic Stress Concentrations Due to a Single Fiber Break in a Unidirectional Composite



In high-performance unidirectional composites, even under static tensile loading, breaking of a fiber is a locally dynamic process which leads to propagation of stress waves in the interface, matrix and neighboring fibers. Current Finite Element modeling approaches consider the fiber break to be a quasi-static process and do not account for these wave propagation mechanisms. To gain better understanding of this event, a fiber-level finite element model of a 2-dimensional array of S2-glass fibers embedded in an elastic epoxy matrix with interfacial cohesive traction law is developed. The brittle fiber fracture results in release of stored strain energy as a compressive stress wave that propagates along the length of the broken fiber at speeds approaching the axial wave-speed in the fiber (6 km/s). This wave induces an axial tensile wave with a dynamic tensile stress concentration in adjacent fibers that diminishes with distance. Inclusion of these dynamic effects shows that there is not only a significant increase in the peak stress concentrations, but also a 5x increase in the the zone-of-influence of a single fiber break. The FE modeling approach adopted here enables us to visualize the dynamic wave propagation mechanisms associated with brittle fracture within the composite. The results are qualitatively compared with experimental observations of damage evolution within unidirectional composites and they are consistent with the experiments.

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