DISCONTINUOUS MODE OF CRACK EXTENSION IN UNIDIRECTIONAL COMPOSITES.
Technical rept. (Final),
ILLINOIS UNIV URBANA DEPT OF THEORETICAL AND APPLIED MECHANICS
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In the microscopic examination of cracks in fiberglass, it was observed that the crack made microscopic skips across the fiber. The direction of skipping is strongly influenced by the tensile stress perpendicular to the fiber. As a consequence, the crack skipping has a preferred orientation under skew-symmetric loading. It was observed that when the direction of shear was reversed the preferred crack skipping direction was reversed also. Due to the crack skipping mode of crack propagation, the usual analysis for a straight crack is no longer valid. A mathematical model was developed to describe this mode of crack extension. The fiberglass stringers were replaced by their equivalent forces and the crack is reduced to a straight line. Experiments in propagating the crack with full reversal of the shear force substantiate the validity of the mathematical model. With this stringer force model, it can be seen that, at least quantitatively, the constant strain energy release rate can be a fracture criterion for cracks under combined symmetric and skew symmetric loads. The major usefulness of this stringer force model probably lies in the analysis of crack propagation under cyclic loading. With this model, the influence of mean stress can be analyzed and the mechanism of crack growth can be more fully investigated. Author
- Ceramics, Refractories and Glass
- Laminates and Composite Materials