An integrated theory is developed for predicting the hygrothermomechanical (HGTM) response of fiber composite components. This integrated theory is based on a combined theoretical and experimental investigation. In addition to predicting the HGTM response of components, the theory is structured to assess the combined hygrothermal effects on the mechanical properties of unidirectional composites loaded along the material axis and off-axis, and those of angle-plied laminates. The theory predicts values in good agreement with measured data at the micromechanics, macromechanics, laminate analysis, and structural analysis levels.
Statistical analysis and multiple regression were used to determine and quantify the significant hygrothermomechanical (HGTM) variables which influence the tensile durability/life (cyclic loading, fatigue) of boron-fiber/epoxy-matrix (B/E) and high-modulus-fiber/epoxy-matrix (HMS/E) composites. The use of the multiple regression analysis reduced the variables from 15, assumed initially, to 6 or less with a probability of greater than 0.999. The reduced variables were used to derive predictive models for compression and intralaminar shear durability/life of B/E and HMS/E composites assuming isoparametric fatigue behavior. The predictive models were subsequently “generalized” to predict the durability/life of graphite-fiber/resin-matrix composites. The “generalized” model is of simple form, predicts conservative values compared with measured data, and should be adequate for use in preliminary designs.
Criteria have been developed for identifying, characterizing, and quantifying fracture modes in high‐modulus graphite‐fiber/resin unidirectional composites subjected to off‐axis tensile loading. Procedures are described which use sensitivity analyses and off‐axis data to determine the uniaxial strength of fiber composites. It was found that off‐axis composites failed by three fracture modes, which produce unique fracture surface characteristics. The stress that dominates each fracture mode and the load angle range of its dominance can be identified. Linear composite mechanics is adequate to describe quantitatively the mechanical behavior of off‐axis data are comparable to those measured in uneasily tests.
Energy-absorbing mechanisms are identified and evaluated using several approaches. The energy-absorbing mechanisms considered are those in unidirectional composite beams subjected to impact. The approaches used include mechanistic models, statistical models, transient finite-element analysis, and simple beam theory. Predicted results are correlated with experimental data from Charpy impact tests. The environmental effects on impact resistance are also evaluated. Working definitions for energy-absorbing and energy-releasing mechanisms are proposed and a dynamic fracture progression is outlined. Possible generalizations to angleplied laminates are described.
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