SYMPOSIA PAPER Published: 01 January 1997

Modeling Crack Extension in Chopped-Fiber Composites


A finite element model for predicting the effect of fiber bridging, fiber properties, and fiber-matrix interface strength on the crack-tip stresses and crack propagation potential of a chopped-fiber composite is proposed. The method of virtual crack extension was used to model crack growth in a micromechanics composite material model. The purpose of this study was to investigate the effects of fiber bridging of a crack and fiber-matrix interface strength on the stress intensity and strain energy release rate in virtual crack extension.

A model of an aligned, 1% by volume fiber loading, chopped-fiber composite with a preexisting crack was developed to represent a portion of a fracture toughness or fatigue crack propagation specimen. Nonlinear contact elements were used to model fiber-matrix interface strengths. The von Mises stress at the crack tip was calculated for each configuration before crack extension, and the strain energy release rate was calculated for each crack step.

The presence of fibers without bridging of the crack did not greatly affect the stress at the crack tip. However, fiber bridging of the crack reduced the crack tip stress by a factor of seven. The magnitude of strain energy release rate was greatly reduced and the sign of the slope of the strain energy release rate versus crack length curve was changed from positive to negative by fiber bridging. In accordance with the theory of tough fiber reinforcement of brittle matrices, the results of applying nonlinear contact elements with varying coefficients of friction predict that an intermediate fiber-matrix interface strength will be most effective in toughening a brittle composite.

Author Information

Friis, EA
Orthopaedic Research Institute, Inc., Wichita, KS
Hahn, DL
Voranado, Wichita, KS
Cooke, FW
Orthopaedic Research Institute, Inc., Wichita, KS
Hooper, SJ
Wichita State University, Wichita, KS
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Developed by Committee: D30
Pages: 364–378
DOI: 10.1520/STP19937S
ISBN-EB: 978-0-8031-5353-0
ISBN-13: 978-0-8031-2411-0