Fatigue crack growth in 7249‐T76511 aluminium alloy under constant‐amplitude and spectrum loading

Abstract

Fatigue crack growth tests were conducted on compact, C(T), specimens made of 7249‐T76511 aluminium alloy. These tests were conducted to generate crack growth rate data from threshold to near fracture over a wide range of load ratios (R). Four methods were used to generate near threshold data: (1) ASTM E‐647 load reduction (LR), (2) compression pre‐cracking constant‐amplitude (CPCA), (3) compression pre‐cracking LR, and (4) constant crack mouth opening displacement LR method. A crack closure analysis was used to develop an effective stress‐intensity factor range against rate relation using a constraint factor (α = 1.85). Simulated aircraft wing spectrum tests were conducted on middle crack tension, M(T), specimens using a modified full‐scale fatigue test spectrum. The tests were used to develop the constraint‐loss regime (plane strain to plane stress; α = 1.85 to 1.15) behaviour. Comparisons were made between the spectrum tests and calculations made with the FASTRAN life prediction code; and the calculated crack growth lives were generally with ±10% of the test results.

Estimation of the opening load under variable amplitude loading

“…The 2% offset criterion is typically used, and it is considered to provide good results. 36,[42][43][44] 3.2 | Applicability of the ASTM compliance offset method for random loading data Despite the apparent easiness to apply of ASTM method, some difficulties have been found in the determination of the opening load level from the real measurement data under the type of random loading considered here.…”

“…They are based on identifying the point where the slope of the compliance curve deviates by some small amount, eg, 2% offset, from the linear upper part of the curve. The ASTM recommended procedure is based on the linear fitting and compliance offset approach and is widely used . A detailed description of the compliance offset procedure can be found in the standard; only a brief summary is outlined here.…”

“…A preset deviation of the compliance offset from the fully open value (the so‐called compliance offset criterion) defines the crack opening load. The 2% offset criterion is typically used, and it is considered to provide good results …”

An overview of fatigue in aircraft structures

“…63 Fracture toughness decreases with increasing strength. 68 The substitution of older by newer, improved materials, is of interest in refurbishing exercises, as a substitution of wings described in 69 where the selection of 7249-T6511 is discussed in the context of the wings of the US Navy aircraft, Lockheed P-3C. [64] or more generally by ref.…”

“…7075 is a common alloy for aero structures, where multi-site fatigue crack nucleation is of interest. 68 The substitution of older by newer, improved materials, is of interest in refurbishing exercises, as a substitution of wings described in 69 where the selection of 7249-T6511 is discussed in the context of the wings of the US Navy aircraft, Lockheed P-3C. This aircraft is involved in an extensive life-extension program being conducted by the US Navy, including the replacement of the 7075 T651 wings with the 7249-T76511 alloy due to its improved corrosion resistance over the 7075 alloy.…”