Fatigue and Crack Growth in 7050 T7451 Aluminum Alloy Under Constant and Variable Amplitude Loading

Abstract

The 7050 aluminum alloy is used in many aerospace structural applications. Previous studies have identified that fatigue cracks develop very rough cracksurface profiles, which cause very high crackclosure levels due to a combination of plasticity, roughness and debris. Previously, tests were conducted on compact, C(T), specimens to generate crackgrowthrate data from threshold to near fracture over a wide range in stress ratios (R). New threshold testing methods, based on compression precracking, were used to generate the data in the nearthreshold regime. The plasticityinduced crackclosure model, FASTRAN, was used to correlate the data over a wide range in stress ratios and crackgrowth rates from threshold to near fracture. To account for the very high crackclosure levels, a very low constraint factor, like planestress conditions, had to be used in the model. In addition, the crackopening loads were measured during these tests using a local straingauge method to generate another خ”Keffrate curve. These two curves differed only in the nearthreshold regime. Herein, fatiguecrackgrowth tests were conducted on C(T) specimens under spike overloads and simulated aircraft spectrum loading. Fatigue tests were also conducted on singleedgenotch bend (SEN(B)), specimens over a wide range in loading conditions (constant amplitude and three aircraft spectra). All specimens were machined from a single forged block of 7050T7451. However, no residual stresses were measured in both the SEN(B) and C(T) specimens. Two European standard spectra were used, but modified to have only tensiontension loading. The purpose of this paper was to evaluate the two different effective stressintensity factor curves for making crackgrowth and fatiguelife predictions. Smallcrack theory was used to make fatiguelife predictions using inclusionparticle sizes from the literature. Fatigue predictions on the SEN(B) specimens agreed fairly well (آ±30%) using a 12micrometer semicircular initial flaw located at the semicircularedge notch under all loading conditions, except the model was unconservative (factor of three) on one of the severe aircraft spectra (MiniTWIST+, Level 1). For the C(T) specimens subjected to singlespike overloads, the lifeprediction code also produced much more retardation than observed in the tests. However, the predicted cracklengthagainstcycles under the MiniFalstaff+ spectrum were only about 15% longer than the tests. The discrepancy under the singlespike overloads and the severe aircraft spectra was suspected to be caused by the low constraint factor and/or crack paths meandering around overload plastic zones. Ideally, a roughnessinduced crackclosure model; in addition to the plasticity model, would be needed to obtain more reasonable results.