First ply failure load

Suppose we replace the 90° layers with a laminae in an attempt to increase the axial stiffness and to increase the first-ply failure load as in Figure 7-61. The load-deflection curve slope after first-ply failure also increases when a laminae replace the 90° layers. However, the energy absorption decreases with such a stacking sequence change. The associated fatigue lives are not known unless both laminates are made and subjected to fatigue loading. 

These values are determined by experiment. It is, however, by no means a trivial task to measure the lamina compressive and shear strengths (52,53). Also the failure of the first ply of a laminate does not necessarily coincide with the maximum load that the laminate can sustain. In many practical composite laminates first-ply failure may be accompanied by a very small reduction in the laminate stiffness. Local ply-level failures can reduce the stress-raising effects of notches and enhance fatigue performance (54). 

If no laminae have failed, the load must be determined at which the first lamina fails (so-called first-ply failure), that is, violates the lamina failure criterion. In the process of this determination, the laminae stresses must be found as a function of the unknown magnitude of loads first in the laminate coordinates and then in the principal material directions. The proportions of load (i.e., the ratios of to Ny, to My,/ etc.) are, of course, specified at the beginning of the analysik The loaa parameter is increased until some individual lamina fails. The properties, of the failed lamina are then degraded in one of two ways (1) totally to zero if the fibers in the lamina fail or (2) to fiber-direction properties if the failure is by cracking parallel to the fibers (matrix failure). Actually, because of the matrix manipulations involved in the analysis, the failed lamina properties must not be zero, but rather effectively zero values in order to avoid a singular matrix that could not be inverted in the structural analysis problem. The laminate strains are calculated from the known load and the stiffnesses prior to failure of a lamina. The laminate deformations just after failure of a lamina are discussed later. 

Unlike ductile metals, composite laminates containing fiber-reinforced thermosetting polymers do not exhibit gross ductile yielding. However, they do not behave as classic brittle materials, either. Under a static tensile load, many of these laminates show nonlinear characteristics attributed to sequential ply failures. One of the difficulties, then, in designing with laminar composites is to determine whether the failure of the first ply constitutes material failure, termed first-ply failure (FPF), or if ultimate failure of the composite constitutes failure. In many laminar composites, ultimate failure occurs soon after first ply failure, so that an FPF design approach is justified, as illustrated for two common laminar composites in Table 8.9 (see Section 5.4.3 for information on the notations used for laminar composites). In fact, the FPF approach is used for many aerospace and aircraft applications. 

Most of the criteria discussed so far focus on predicting some form of failure onset, typically first-ply failure. As referred to above, this does not necessarily coincide with final failure or collapse of a laminate. There may be significant additional load carrying capability, which can be very important for damage tolerance considerations. 

In the general approach, the loads are applied incrementally until first-ply failure occurs. The type of failure, matrix or fiber, determines which properties of the failed plies must change to reflect the damage created. This is subjective and can cover a range of possibilities. The most conservative approach would completely discard affected properties for the failed plies. So for fiber failure, E would be set to zero. For matrix failure, E22 and G12 would be set to zero. Then, the loads would be incremented until another ply fails, and the procedure would be repeated to complete failure of the laminate. Less conservative approaches attempt to only partially discount stiffness values of the failed ply and even differentiate between tension and compression moduli. These methods can be reasonably accurate if they are accompanied by selected tests that help better define adjustment factors for the stiffness properties of failed plies. However, they are limited in applicability and accuracy because they are affected by the first-ply failure criterion used to trigger the failure sequence and because they do not correctly capture damage modes such as delamination and the interaction between them such as matrix cracks causing delaminations in adjacent ply interfaces. 

Francis PH, Wahath DE, Weed DN. First ply failure of G/E laminates under biaxial loadings. Fiber Sci Technol 1979 12 97-110. 

Angle-ply [ 0]2s laminates of T300/5208 graphite/epoxy composite were analyzed using the proposed failure criterion. In ensence, Tsai-Hill, interfacial failure, and surface-ply failure loads were obtained first and the laminate strength was taken to be the lowest failure load among the three. 

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