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Surface Condition and Fatigue Crack Growth in an Aluminum Alloy

The fatigue crack growth in Aluminum 2024-T3 alloy specimens whose surface were scratched has been investigated. The results in such specimens indicated an increased fatigue crack growth rate. Whereas, in scratched-shot peened specimens, crack growth rates were nearly equal those in as-received material. This useful influence occurred at low ΔΚ. However, as Δ Κ increased growth rates gradually increased and finally, merged with those of scratched specimens. When a through width scratch was combined with a through thickness scratch, crack growth rates exhibited characteristics of small crack behavior. INTRODUCTION In recent years, a large amount of work investigating the propagation of fatigue cracks was accomplished [1-5]. Researchers have generally divided cracks according to their lengths, small ( < 0 . 5 mm) and long cracks ( > 2 mm). Studies [1-5] have confirmed that small crack behavior is distinct from that of long cracks. That is small cracks exhibit lower threshold stress intensity factor and grow faster than long cracks at the same ΔΚ. The cited reasons for this anomalous behavior are: reduced crack closure, three-dimensional nature of small cracks, and microstructural interaction. Among small cracks Vol. 5, No. 4, 1994 Surface Condition and Fatigue Crack Growth in an Aluminum Alloy are two types, microstructural-short and physically-small. Short crack growth is significantly influenced by its interaction with microstructural barriers [5-7] and the abnormal behavior of small cracks is partly attributed to crack closure [8,9]. Yang [10] showed that a large change of the crack shape occurs during early growth of very small surface cracks. This change is largely due to the transition from unstable to stable geometry. During transportation, machining of materials, and assembly of components, scratches could be induced on the surface by sharp instruments. Recent studies [11,12] have demonstrated that scratched specimens of aluminum-lithium alloys under cyclic loading have lower fatigue lives than non-scratched specimens. Little study however, has been done on the influence of these surface scratches on crack propagation. The elimination or reduction of the scratch depth by such methods as shot peening, grit, or sand blasting may result in substantial improvement of fatigue life. These methods will not only reduce the effective scratch depth but will introduce compressive residual stresses that enhance the fatigue strength and corrosion characteristics of metals [13]. Furthermore, the residual stresses will significantly affect crack growth rates [14]. This study will present and compare results of fatigue crack growth rates in scratched and scratchedshot peened to non-scratched specimens of 2024-T3 aluminum alloy. EXPERIMENTAL PROCEDURE Specimens 5"x1"x0.04" of 2024-T3 bare aluminum were cut in the rolling direction of the metal sheet. Scratches of 90 degrees angle (Fig 1) were generated onto different positions of specimens Fig 1 Cross section of a scratch.