low applied strain or even cure shrinkage stress. These cracks can propagate as delaminations in fatigue (Rogers et al., 1990).
Movement of the laminate prior to cure, such as occurs during compaction of thick sections, can change the fiber length required by the final geometry. Since the fiber will not stretch or compress elastically, because the actual fiber stress is low, the fiber will either bridge, causing a void, or buckle, yielding local fiber curvature. Either way, significant resin stresses are induced as the fiber tries to straighten out under tension loading or buckle under compression loading. The matrix will crack, and the fracture will propagate under fatigue loading as a delamination in tension or as fiber crimping in compression. The latter failure mode is catastrophic (Morse and Piggon, 1990).
Heat is produced by the chemical reaction of cross-linking for all plastic matrix systems. This phenomenon is called exotherm. The matrix gells and becomes rigid at a given location as a function of time at temperature. The exotherm can raise the internal temperature of a laminate well above the surface temperature, which causes gelling to occur at different locations and different times. The fiber and matrix generally have different thermal coefficients of expansion, which results in residual stresses on cool down after cure. Filament winding processes introduce residual fiber stresses in the fiber matrix assembly that are retained through cure. Each of these phenomena can result in local residual stresses of significance in the matrix. Procedures for computing these stresses were developed under U.S. government contract and reported by Rai and Brockman (1988); thus, the code is available on request.
The design phase is not complete until the effects of these conditions have been determined and their stress effects included in fatigue analysis.
Fatigue failure in GRP composite materials, especially carbon, is a matrix-dominated phenomenon (see, for example, Lagace, 1985). Matrix strength in the composite is low compared with the fiber strength. Matrix failure through the thickness of a ply is termed transverse cracking or splitting, depending on whether the primary laminate load is transverse to or aligned with the fiber direction of the ply. Matrix failure between plies is termed delamination. Delamination can induce unstable sublaminate buckling in compression (Chai et al., 1981). However, all of these matrix failure modes cause local points of strain concentration on the fiber (Rogers et al., 1990). The fiber can be described as brittle since its stress-strain response is linear to failure (Prandy and Hahn, 1990). Therefore, fiber failure will occur when these local strain concentrations, together with the global strain, exceed either the fiber tensile strength or compression instability strength (O'Brien, 1980). Fiber failure, once initiated, generally propagates catastrophically across the laminate (Reifsnider, 1980).
Transverse cracking in the matrix occurs in plies that are off the axis of the load-carrying plies due to their strain. But transverse cracking and delamination also occur as a result of loads that are out of the plane of the laminate (Chan et al., 1986; O'Brien, 1980). These failure modes both occur in areas of curved fibers. The fiber possesses linear strain response and excellent fatigue properties. The matrix is viscoelastic, thereby absorbing energy in each strain cycle and yielding very poor fatigue properties (Mandell et al., 1985).
Any attempt to design long-life blades must consider minimization of matrix strain. Any design that contains curved fibers or any process that cannot maintain fiber alignment will result in fatigue-limited characteristics that are not represented by the coupon-generated fatigue data unless the coupon contains the same defect in a controlled manner.
Examination of the failed blades presented to the committee revealed quality variations that exceed good practice. A portion of the disappointment in blade life must be attributed to lack of quality control.