The application of classical fracture mechanics to laminated com posites is discussed. A convenient method is presented for predicting the static strength of a flawed specimen. Theoretical predictions are compared with experimental data for specimens containing two types of flaws.

As part of an overall effort to develop durability and damage tolerance methodology for graphite-epoxy composites, coupon specimens have been developed to measure the fundamental static fracture and subcritical growth behavior of delaminations. Two basic designs, one for the tensile opening mode (Mode I) and one for the forward shear mode (Mode II), are described. These specimens were used to characterize the behavior of two types of interfaces (0/0 and 0/90) for static fracture, constant amplitude fatigue, and spectrum fatigue. Fracture mechanics technology was applied through the principles of strain-energy release rate. Three-dimensional finite-element analyses were employed to interpret the experimental results. A simple growth law was shown to correlate the constant-amplitude and spectrum-growth data. It was found that the applied cyclic load must be nearly equal to the critical static load to obtain observable growth in the tensile opening mode. On the other hand, the graphite-epoxy delamination growth rate in the forward shear mode is comparable to the aluminum growth rate in tension, which suggests that shear is the chief subcritical growth mode for graphite-epoxy.

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A modified X-ray nondestructive testing technique was used to study matrix cracks parallel to fibers and delaminations between plies in graphite-epoxy composite material. A tetrabromoethane (TBE) opaque additive applied at the source of the damage zones enhanced the flaw image. Center-slit specimens were fabricated from Modmor II/Narmco 5208 graphite-epoxy laminates with three different ply orientations. Tensile ramp and sawtooth cyclic loadings at different levels were applied to these specimens. Periodic X-ray monitoring was conducted to observe the initiation and growth of cracks and delaminations at the slit tips. The initiation of damage zones appeared as fiber separation in the ±45-deg directions tangent to the semicircular periphery of the cutout tips. As the load level was increased, this fiber separation continued while fiber separation at other locations also appeared. Delamination first occurred at approximately the same time as fiber separation. The delaminated area was found to surround the most severe fiber separation region and grew in size as the load level was increased. Limited test results indicated a slow and an accelerated damage growth rate during ramp loading. For cyclic loading the majority of growth occurred on the first load cycle. Subsequent cyclic loading to the same level contributed little additional growth in the majority cases.

The objective of this paper is to describe the bolted joint analysis method developed and used by LM Aeronautics (named IBOLT) including the theoretical basis, required input, and coupon-level strength-prediction validation. IBOLT performs a fracture-mechanics-based static strength prediction for a rectangular composite joint element subjected to any combination of biaxial membrane loads, shear loads, and an off- axis bolt load. Out-of-plane bending moments are also treated. Joint configuration effects (single- or double-shear) are modeled by a beam-on-elastic-foundation analysis to account for the thickness and stiffness of the joint members as well as the bolt bending and shear stiffness. Empirical equations are included to account for fastener head geometry, effects of filled versus open holes, fastener/hole clearance, and fastener torque. Required input, in addition to the usual geometry, loads, fastener, and substrate elastic properties, includes a variety of special laminate-level joint properties. Uni-axial notched, un-notched, and bolted-joint coupon tests are conducted to develop these properties. In addition to these uniaxial coupon tests, bearing/bypass tension and bearing/bypass compression tests have been performed to validate the accuracy of IBOLT strength predictions. An overview of these validation results is provided, along with a brief summary of IBOLT's strengths and weaknesses as a practical bolted joint stress analysis tool.