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A micromechanics-based constituent progressive damage model was proposed in this study to predict macroscopic failure behavior of composite laminates under multi-axial mechanical loadings as well as thermal influences. For this purpose, a micromechanics-based failure theory, named the micromechanics of failure, has been further developed not only to account for the constituent failure but also to progress damage. We first modeled the unit cell of the microstructure of a UD lamina both to derive ply properties from constituent properties and to calculate micro stresses within fiber and matrix from macro-level ply stresses using stress amplification factors. Independent failure criteria dedicated to each constituent are employed to judge failure modes of fiber, matrix, and fiber--matrix interface. Good agreement between theoretical predictions and test data was achieved. [ABSTRACT FROM AUTHOR]

Micromechanical approaches are employed to investigate the influence of different fiber arrangement on the mechanical behavior of unidirectional composites (UD) under various loading conditions. A micromechanical model with a random fiber array is generated and used in a finite element analysis together with two frequently used representative volume elements (RVE), or unit cell models of square and hexagonal arrays. The algorithm for generating the random fiber array is verified by comparing the comprehensive performance of a unit cell based on our random array and that of a unit cell based on a real fiber distribution in the UD cross-section. Performance of the random and regular fiber arrays is also evaluated through frequency distributions of stress invariants in matrix and tractions at the fiber-matrix interface due to various loading types. The effects of different loading angles on the overall response of regular arrays to various loading conditions are investigated thoroughly. Finally, the Weibull distribution of the maximum normal interfacial traction in random array is compared with the cumulative probability distribution of transverse strength data acquired from experiment, and good agreement is achieved. [ABSTRACT FROM AUTHOR]

A micromechanical approach is developed to determine the micro stress within a unidirectional composite under various mechanical and thermal loading conditions. Based on linear stress-strain relations, the concept of a stress amplification factor is introduced, and the correlations between macro stress and micro stress are explicitly expressed in mathematical equations. Three unit cell models, square, hexagonal, and diamond fiber arrays, are analyzed and compared using three-dimensional finite element methods. Subsequently, effective material properties, the distribution of micro stress in the fiber/matrix, as well as traction distribution at the fiber-matrix interface, and the effect of different interfacial stiffness, are obtained. [ABSTRACT FROM AUTHOR]

A three-dimensional finite element analysis is performed to investigate the effects of fiber arrangements on the residual thermal stresses in unidirectional composites of various fiber volume fractions (FVFs). The fiber arrangements include the regular fiber arrays (square and hexagonal arrays) and a random fiber array. Normal, tangential, and shear stresses at the fiber--matrix interface are first obtained using unit cells of the regular square and hexagonal fiber arrays. To simulate better real fiber arrangements, random fiber distribution is modeled and analyzed using a finite element analysis. Statistical distributions of residual thermal stresses are obtained for various FVFs and compared with the results from regular fiber arrays. The effects of constituent material properties of composites on thermal stresses are also taken into consideration. The results indicate that the random arrangement of fibers has a significant influence on residual thermal stresses especially at high FVFs. The mode stresses of the random fibers are well estimated using the square array whereas the mean stresses are better predicted from the hexagonal array. It is shown that predicted coefficients of thermal expansion are not influenced by the microstructure of composites. [ABSTRACT FROM AUTHOR]

The micromechanical approach was used to investigate the interfacial strain distributions of a unidirectional composite under transverse loading in which fibers were usually found to be randomly packed. Representative volume elements (RVEs) for the analysis were composed of both periodic fiber arrays, such as a square array and a hexagonal array, and a random fiber array. The finite element analysis was performed to analyze the normal, tangential, and shear strains at the interface. In order to verify the RVE for describing a random fiber array, the generated RVE was statistically compared with the one constructed from typical images of a transverse cross-section of a unidirectional composite. Since the strata distributions at the interface experienced periodic characteristics along with its circumference, the Fourier series approximation with proper coefficients was utilized to evaluate the strain distributions at the interface for the periodic and random fiber arrays with respect to fiber volume fractions. From the analysis, it was found that the random arrangement of fibers had a significant influence on the strain distribution tit the interface, and the strain distribution in the periodic arrays was one of special cases of that in the random array. [ABSTRACT FROM AUTHOR]