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-surface profiles, which cause very high crack-closure levels due to a combination of plasticity, roughness and debris. Previously, tests were conducted on compact, C(T), specimens to generate crack-growth 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 crack-closure 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-gage method to generate another ΔKeff-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. Also, fatigue tests were conducted on single-edge-notch 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 7050-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 evaluate 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 12-micrometer semi-circular initial flaw located at the semicircular-edge notch under all loading conditions, except the model was unconservative (factor of 3) 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 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.

Improved test method and analytical modelling for fatigue crack growth in coarse‐grain titanium alloy with rough fatigue surfaces

“…Another very important issue is to determine whether the precracking method (tension or compression) has any influence on subsequent fatigue crack growth under spectrum loading. Materials that have been found to exhibit sensitivity under CA loading, such as 7050‐T7451, have been found to have no such sensitivity when the subsequent fatigue cycling is under spectrum loading …”

Weight functions and stress intensity factors for pin‐loaded single‐edge notch bend specimen

“…The present non‐dimensional f ‐factor of Eqs and is compared with the results determined by Ziegler and Newman using the boundary element method . As can be seen from Fig.…”

“…a). Recently, a new specimen that has a higher stress concentration factor and can be pin‐loaded was proposed by Newman and Ziegler . The new specimen centred on modifying the traditional SENT specimen is called pin‐loaded single‐edge notch bend (PSENB) specimen (Fig.…”

“…To conduct experiments and analysis for small cracks in the PSENB specimen, stress intensity factor (SIF) solutions for the applied pin‐loading as well as the so‐called Dugdale loading, that is, a uniform crack face pressure segment acting in the immediate wake of the crack tip are indispensible. An SIF equation for a through‐thickness edge crack emanating from the semi‐circular notch of the PSENB specimen for the pin‐load case was developed by Ziegler and Newman by curve fitting to their results obtained using boundary element method . These 2D SIFs were further used for 3D part‐through surface cracks based on a similitude approach, that is, using the available 3D SIFs for the SENT specimen and multiplying the ratio between the 2D SIFs (or stress concentration factors) of the SENT and PSENB specimens.…”

“…Furthermore, SIFs for the Dugdale load case, which is required by the strip‐yield model based crack‐closure analysis, have not been determined. The purpose of the present paper is therefore twofold, the first being to determine the SIF for the applied pin‐load case and compare with the available solution of Ziegler and Newman, and the second being to provide accurate analytical weight function for the PSENB specimen for calculating SIFs for more complicated (or even arbitrary) load cases including the Dugdale stress and other important fracture mechanics parameters such as the crack tip plastic zone sizes and crack opening displacements, which are required by the plasticity‐induced crack‐closure model. In the present study, two weight function methods, that is, the classic analytical weight function method and the recently developed numerical weight function complex variable Taylor series expansion (WCTSE) method are employed.…”