Strengths and Thermal Residual Stresses of Unidirectional Composites

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Abstract
Failure strengths of unidirectional composites are theoretically obtained by the employment of certain basic assumptions and the consideration of the strengths of the constituents and the fiber-matrix interface. The most impor tant assumption is that macro-failure occurs when one of the micro-stress levels in composites reaches a micro-failure criterion. Thermal residual stresses based on temperature-dependent material properties are considered as the initial state. An experimental verification of the residual stresses is then carried out by means of photoelasticity. A second phase of analysis involves the resolution of an external loading in an arbitrary direction into six basic loadings in the xyz-coordinates. Failure strengths of UD-composites are determined after all the stress components are superposed. The target material utilized is Carbon-Epoxy. Four kinds of micro-criteria are adopted. It is found that the Tsai-Wu theory with the minimum F12 can be best ex plained by the maximum principal stress criterion in the matrix. The thermal residual stresses do not have predominant effects on the strengths if an inter face normal strength is large.