Fatigue Crack Propagation in 2090 Aluminum-Lithium Alloy: Effect of Compression Overload Cycles

Fatigue crack propagation behavior has been examined in a commercial 12.7 mm thick plate of Al-Cu-Li-Zr alloy, 2090, with specific emphasis on the effect of single compression overload cycles. Based on low load ratio experiments on cracks arrested at the fatigue threshold (ΔKTH), it is found that crack growth at ΔKTH can be promoted through the application of periodic compression cycles, of magnitude two times the peak tensile load. Similar to 2124 and 7150 aluminum alloys, such compression-induced crack growth at the threshold decelerates progressively until the crack re-arrests, consistent with the reduction and subsequent re-generation of crack closure. The compressive loads required to cause such behavior, however, are far smaller in the 2090 alloy. Such diminished resistance of aluminum-lithium alloys to compression cycles is discussed in terms their enhanced “extrinsic” crack growth resistance from crack path deflection and resultant crack closure, and the reduction in the closure from the compaction of fracture surface asperities by moderate compressive stresses.