Ranking 7XXX Aluminum Alloy Fatigue Crack Growth Resistance Under Constant Amplitude and Spectrum Loading

Abstract

Fatigue crack growth (FCG) experiments were conducted on controlled variations of Type 7075 and 7050 aluminum alloys. Alloy FCG resistance was ranked under constant amplitude and simple variable amplitude load spectra. Fracture mechanics and fractographic approaches were used to interpret causes for variation in ranking of 7XXX aluminum alloy FCG resistance with loading conditions. The interpretation is built around clarification of a controlling FCG mechanism that is dependent upon interaction of microstructure and load history. This clarification represents a necessary first step toward knowing which microstructure or which design (test) procedure is optimum for a particular class of application, for example, fighter as opposed to bomber or transport aircraft.

Micromechanisms of fatigue crack growth retardation following overloads

“…For example, the retardation or crack arrest resulting from a high-low block loading sequence is found to be much greater than that observed for a single overload of equivalent r OL (22,26,29). The fractographic features associated with high-low block loading in a 2024-T3 aluminum alloy are illustrated in Fig.…”

“…ref. 29), that crack branching is more significant at higher values of r OL and lower ~KB and occurs only above a certain threshold r OL ). Moreover, the applied load is effectively lowered at the tip of a branched crack during the entire fatigue cycle, and the Mode II sliding accompanying crack branching also enhances the closure stress level.…”

“…-9-As the kinked or forked crack branches out along one of the shear bands (29,(33)(34)(35), it tends to change direction on intersecting a grain boundary.and on trying to orient itself along the pre-overload main crack (33,35) (Fig. 2c).…”

“…overload ratio r OL ' baseline stress intensity range ~KB and the alloy system (29,33). (It appears, from the information available in the literature (e.g.…”

“…Fig. 8 shows the fracture surface appearance due to pre-overload crack growth by a striation mechanism As it is known that in moist environments, enlarged corrosion products formed on fracture surfaces due to fretting and rubbing at low growth rates can result in additionaZ crack closure (29,(55)(56)(57)(58), the role of such additional closure processes is found to be a strong function of alloy composition and heat treatment in aluminum alloys (59,60).…”