FACTORS INFLUENCING THE FATIGUE BEHAVIOR OF FERROUS-BASED LAMINATED COMPOSITES

Metallic-based laminates offer promise as damage tolerant materials under conditions of fatigue loading. It has long been known that laminated metals can exhibit improved impact properties due to delaminations and the associated crack arrest phenomena that have been observed to occur at weak interlaminar boundaries. To extend this crack arrest behavior to conditions of fatigue loading, a broad based experimental program has been conducted using ultrahigh carbon steel as a base material.;In the first phase of the experimental program, a shear test was used to evaluate factors influencing the interlaminar bonding strength between adjacent steel plies. A knowledge of bond strength is critical if crack arrest mechanisms are to be optimized. It was found that the bond strength among steel plies remains quite high over a wide range of thermomechanical processing routes. A desire to obtain lower interlaminar bond strengths led to the introduction of secondary metals to be used as weak interleafs between the high strength ferrous plies. Two materials--copper and iron containing three percent silicon--were evaluated and found to be suitable interleaf candidates.;In the second phase of the experimental program, a variety of laminates consisting of layers of steel interleaved with copper or iron-silicon alloy were prepared. The fatigue behavior of laminates tested in the crack arrest orientation showed that a range of crack growth behavior is possible--ranging from crack arrest at each interleaf to uninhibited crack propagation. For laminates containing a copper interleaf, it was shown that crack arrest and life improvement are optimized when ply thickness is increased, when interleaf thickness is decreased, or when ferrous ply yield strength is increased. No life improvement was obtained through use of the ferrous interleaf.;Finally, a micromechanical model has been introduced which explains the experimentally observed crack growth behavior. By seeking an understanding of near crack-tip stresses as a fatigue crack propagates toward a weak interface, factors influencing local delamination and crack arrest can be fundamentally understood. A previously developed applied mechanics model was used for this purpose. The model used supports the experimental results and has emerged as a potentially useful tool for predicting crack growth behavior in other materials systems.