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 crack-suiface profiles, which cause very high crack-closure levels due to a combination of plasticity, roughness and debris. Previotisly, tests were conducted on compact, C(T), specimens to generate crack-growth-rate 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 near-threshold regime. The plasticity-induced crack-closure model, FASTRAN, was used to correlate the data over a wide range in stress ratios and crack-growth rates from threshold to near fracture. To account for the very high crackclosure levels, a very low constraint factor, like plane-stress conditions, had to be used in the model. In addition, the crack-opening loads were measured during these tests using a local strain-gauge method to generate another AK^,ff-rate curve. These two curves differed only in the near-threshold regime. Herein, fatigue-crack-growth tests were conducted on C(T) specimens under spike overloads and simulated aircraft spectrum loading. Fatigue tests were also conducted on single-edge-notch bend (SEN(Bj), specimens over a wide range in loading conditions (constant amplitude and three aircraft spectra). All specimens were machined from a single forged block of7050-T7451. 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 tension-tension loading. The purpose of this paper was to evahtate the two different effective stress-intensity factor curves for making crack-growth and fatigue-life predictions. Small-crack theory was used to make fatigue-life predictions using inclusion-particle sizes from the literature. Fatigue predictions on the SEN(B) specimens agreed fairly well (±30%) using a 12micrometer semicircular initial flaw located at the semicircular-edge notch under all loading conditions, except the model was unconservative (factor of three) on one of the severe aircraft spectra (Mini-TWIST-^, Level 1). For the C(T) specimens subjected to single-spike overloads, the life-prediction code also produced much more retardation than observed in the tests. However, the predicted crack-length-against-cycles under the Mini-Falstaff-\-spectrum were only about 15% longer than the tests. The discrepancy under the single-spike 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 roughness-induced crack-closure model: in addition to the plasticity model, would be needed to obtain more reasonable results."plasticity-induced" crack closure, other closure mechanisms have been identified, such as roughness-, fretting-product-, and oxide-debris-induced closure. These mechanisms have greatly improved our understanding of the complex interactions that occur dur...

Closure measurement and analysis for small cracks from natural discontinuities in an aluminium alloy

“…The low α value was also reported to work well when applied to FASTRAN analysis for spectrum loading on 7050-T7451 [17] and also β-annealed Ti-6Al-4V [20,21] . Previous research efforts [20] have focused on modelling the RICC contribution explicitly, combined with plasticity induced closure modelling with α values around the expected and separately justified range of about 1.8-2.0.…”

“…The compliance based crack opening data were very important in this case because previous studies using FASTRAN to analyse crack growth in 7050-T7451 found that low values (around 1.3) of the three dimensional constraint factor, α, were required to correlate well with both CA and spectrum loading [16][17][18]. The value of α used in FASTRAN is typically about 1.8-2.0 for most materials [5,6,[31][32][33], which is consistent with three-dimensional elastic-plastic FEA studies [14,15].…”

“…FASTRAN analysis had compared very well using the low values of constraint in other cases for 7050-T7451 material [17,18], but not in [26]. This was because the work in [26] used the most recent and improved version of FASTRAN, Version 5.42 [11].…”

“…Limitations of the PICC only approach as used in FASTRAN have become evident when applied to materials including 7050-T7451 and β-annealed Ti-6Al-4V which exhibit very rough crack surfaces [16][17][18][19][20][21]. Newman et al [16] applied local strain gages ahead of the crack tip in 7050-T7451 aluminium alloy compact tension (C(T)) specimens to measure crack-opening loads using…”

Airline and Aircraft Reliability

“…A lot of papers and books examine these problems and offer possible solutions [1] - [9] where the Markov chains (MC) and semi-Markov process with reward (SMPW) theories [10] - [12] are offered to solve these problems, using exponential approximation of fatigue crack size growth function, (1) where  , Q are parameters of fatigue crack trajectory (PFCT).…”