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Yielding stress of matrix and interface is chosen to represent the influence of the chemical, mechanical, and thermodynamical nature of the bonding process between matrix and fiber materials. The shear stress-strain behavior of interface and matrix material is modeled as elastic-perfectly plastic. A local stress analysis model is used to represent the influence of interface and matrix yielding on stress distribution resulting from fiber fractures. The model is combined with the concept of multiple fiber fractures in the development of a mechanistic representation of tensile strength. In this strength formulation, fibers are assumed to be a statistical quantity, and as the material is loaded, fibers fracture randomly throughout the body causing localized stress concentration. The accumulation of these breaks results in overall failure. The resulting analysis is applied to numerical studies of the influence of interface/matrix yielding on stress and damage characteristics of unidirectional composites under quasistatic uniaxial loading. A discussion of optimal design parameters for composites is also presented.
The predicted tensile strength clearly shows that there exists an optimum of composite strength at a certain level of interface and matrix yielding stress. A value of yielding stress larger than optimum causes a loss of composite strength as a result of the dominant increase in local stress concentration, and a value smaller than optimum causes a loss of composite strength as a result of the dominant increase of the region of influence of the local disturbance (increased bundle length). This phenomenon has also been observed by other researchers in their experimental studies of the effect of surface treatment on the tensile strength of composites. Furthermore, by comparison with experimental data, it is shown that the influence of interfacial bond on tensile strength of composites may be secondary to other effects such as change of fiber strength as a result of surface treatments or friction effects engendered by resin shrinkage or residual thermal stress.
Alexander Giacco professor, Virginia Polytechnic Institute and State University, Blacksburg, VA
Assistant professor, Clarkson University, Potsdam, NY
Stock #: CTR10099J