The Growth of Small Corrosion Fatigue Cracks in Alloy 7075

Abstract The corrosion fatigue crack growth characteristics of small (>35 μ m) surface and corner cracks in aluminum alloy 7075 is established. The early stage of crack growth is studied by performing in situ long focal length microscope (500×) crack length measurements in laboratory air and 1% sodium chloride (NaCl) environments. To quantify the “small crack effect” in the corrosive environment, the corrosion fatigue crack propagation behavior of small cracks is compared to long through-the-thickness cracks grown under identical experimental conditions. In salt water, long crack constant K max growth rates are similar to small crack da/dN. 1.0 Introduction The propagation of small fatigue cracks from surface defects (5 μm to 10 μm) constitutes a large percentage (50% to 90%) of the total fatigue life of structural components [1]. Thus, accurate prediction of small crack da/dN is required for damage tolerant based life predictions. A problem arises because small cracks propagate at unpredictable accelerated rates compared to long fatigue cracks [2]. The difference in long and small crack growth behavior is due to numerous effects (i.e., diminished crack-tip shielding (closure) effects at small crack sizes). Compounding the small crack problem are complex environmental effects. Research on high strength steels in salt water has shown that chemically short (< 500-μm) cracks propagate 1.5 to 500 times faster than long cracks subjected to the same mechanical driving force [3]. Limited data on 2000 series alloys exposed to deaerated salt water suggest little chemical-crack-length effect [4]. The purpose of this work is to study the chemical crack length effect in environmentally sensitive alloy 7075-T6.