It now is recognized widely that stiffness changes during the service loading of composite laminates can be significantly large, especially as those changes affect deflections, dimensional changes, vibration characteristics, and load or stress distributions. Several generic sources of stiffness change can be identified, in various degrees, in fibrous composite materials. The source which occurs quite early in the life of a specimen or component is matrix cracking, the subject of this paper. While most laboratories now report stiffness changes, very little systematic philosophy has been developed to account for and explain such stiffness changes. The complexity of this situation requires systematic study, and motivates the search for a model, or models, which can describe the behavior and predict unfamiliar response. The present paper reports the results of an experimental program and an analytical modelling exercise which indicate that much of the observed matrix cracking can be predicted and the effects on stiffness calculated with various degrees of accuracy depending upon the sophistication of the model used.

The mechanisms by which subcritical and critical damage develops in several lamination geometries of T300/5208 and T300/914C graphite/epoxy material during tension-tension fatigue were closely examined. A damage analogue in the form of stiffness reduction was used to provide a framework by which the sequence of damage development could be correlated with mechanical response. Stiffness reduction, measured continuously during the course of cyclic loading, was shown to provide a reproducible characteristic correlation with percent of life expended. The relationship was observed to differ markedly among lamination geometries, but for a given geometry was found to clearly indicate the partition of the mechanical response into distinct regions in these characteristic curves. These regions, moreover, were shown to be predominated by particular damage mechanisms—some already discussed in the literature, others less well-recognized. Results of the observed damage development sequence for cross-ply and quasi-isotropic laminates are presented along with a preliminary association between this damage and the characteristic stiffness reduction curves for these geometries. The geometries used were characterized by distinct, predominant, early subcritical damage conditions. This secondary and subsequent damage development was examined in relation to known, predictable beginning state. Of particular emphasis in each case was the role of this developing damage state in the fracture of fibers in the 0-deg plies. Damage detection and characterization was accomplished using both nondestructive and microscopic techniques. Two techniques proved to be of considerable utility: penetrant-enhanced stereo X-ray radiography and scanning electron microscopy of coupons taken from penetrant-enhanced deplied, damaged specimens. A number of significant damage conditions, not heretofore reported, were observed: the production of interior delaminations at the 0/90-deg interfaces of [0,902]s laminates by the gradual growth of longitudinal cracks in the 0-deg plies; the existence of a dense distributed microcrack condition at all distinct interfaces of [0,90,±45]s laminates; the segregation of 0-deg fiber breaks in all laminates into zones coincidental with cracks in the adjacent plies; and, the appearance of shear fracture in 0-deg fibers associated with the passage of longitudinal splits. Mechanisms for each of these damage conditions are proposed in terms of the micromechanics of the predominant damage condition with which they are associated and the global stress state.

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