A1 Delamination (Interlaminar Failure)

Topic

This laminate, composed of several stacked layers, undergoes delamination—the separation of two of those layers—when an out-of-plane stress arises between them: a stress acting perpendicular to the plane of the layers, rather than within the plane the laminate was designed to withstand. Because this stress acts out-of-plane rather than in-plane, it forces the layers apart instead of subjecting them to tension or compression within their own plane; this interlayer separation constitutes the delamination.

However, the presence of delamination between two layers of this laminate does not necessarily mean it will grow; its propagation depends on the interlaminar energy release rate in Mode I—the opening mode, where the two layers pull directly apart from one another. If the Mode I interlaminar energy release rate exceeds a specific threshold, the delamination propagates, extending along the interface between the two layers; if it remains below that threshold, the delamination stays the same size without propagating. This threshold is precisely what defines the criterion for delamination propagation.

However, such delamination does not always propagate in pure Mode I; it is frequently combined with Mode II—the sliding mode—in which the two layers slide over each other rather than separating directly. The fact that delamination propagates through a combination of Mode I and Mode II means that the pure Mode I propagation criterion alone is insufficient to predict such propagation; instead, the mixed-mode Benzeggagh-Kenane criterion must be used. This criterion combines the contributions of both modes—based on their respective shares of the total energy release rate—to determine whether the combination exceeds the effective propagation threshold.

Consequently, this model—addressing the out-of-plane stress-induced delamination with which the analysis began—describes a failure mode that has no analogue in metals or monolithic ceramics. Since neither of those materials is composed of layers, there is no interface where interlaminar separation could occur, making this failure mode unique to laminates. The interlaminar shear strength provided by another model in this set—the one describing the corresponding test—supplies the data for this model's propagation criterion; meanwhile, the delamination itself, once propagation begins, is experimentally detected by yet another model in the set: the one describing C-scan ultrasonic inspection.

ID:4417