Parametric (Non)-Variance of the Mid-Regime Fatigue Crack Propagation in an Aluminium Alloy AA6056-T6

Abstract

Extent of experiment-related variability in fatigue crack propagation is essential to be known while comparing the performance with other materials, for assessment of welds or damage tolerance analysis. Since such a variability can be dependent on material or parameter investigated, and may originate from the size effect, thin sheets of the relatively new airframe alloy AA6056-T6 have been investigated for baseline data using C(T) and M(T) specimens. As a screening step, various parameters such as surface cladding, crack plane orientation, specimen width and thickness, and initial notch length have been varied on C(T) specimens. The material also stems from different heats. The maximum crack opening displacement (CODmax), obtained from a displacement gauge, is calibrated against the optical crack length for the indirect crack length measurement and its precision is increased by acquiring an individual calibration curve for each specimen. The variability in the mid-regime fatigue crack propagation range is found to be relatively low (± 35%). With this range as the tolerance, the alloy is found to exhibit mostly a parametric non-variance, since, except for the specimen type, other parameters investigated are found to be non-influential. M(T) specimens are found to yield conservative fatigue crack propagation data.

Fatigue crack propagation into the residual stress field along and perpendicular to laser beam butt‐weld in aluminium alloy AA6056

“…However, the averaged stress level was very low (σ x ≤−25 MPa) and σ y being tensile in nature (≤ 20 MPa) should not affect crack propagation of the base material significantly. This was confirmed within the experimental tolerance limit 49 . It is as well likely that these residual stresses are resulting from micro‐stresses, because comparable values were found in heat treated (peak‐aged), but non‐welded base material.…”

Utilising CODmax as an Indirect Fatigue Crack Length Measurement Parameter for M(T) Specimens of an Airframe Aluminium Alloy AA6056

“…Therefore, following Sullivan and Crooker [24], we tried the maximum crack opening displacement value COD max from a displacement gauge as a correlating parameter for the indirect crack length measurement. This method is found to be very suitable for C(T) specimens [13]. Here its suitability is verified for M(T) specimens of the base material.…”

“…In all specimens a nearly 1:1 correlation was observed, not only for small width (Figure 6a), but also for large width specimens (Figure 6b), and the resolution was comparable to that for the optical method. Thus, as for C(T) specimens [13], COD max turns out to be a very suitable parameter also for the indirect fatigue crack length measurement of M(T) specimens.…”

“…Since the material available was limited at that time, C(T) specimens were the first choice. However, conservative data than possible on C(T) specimens [8][9][10][11][12][13] were required and were to be acquired on M(T) specimens and at a low load ratio (R = 0.1). The topics turned up were possibility of the size effect (specifically, specimen width) and the method to be used for the indirect crack length measurement.…”

“…This would limit the accuracy of data acquired. Moreover, specimen geometry can affect a fatigue crack propagation curve such that compared to C(T) specimens, M(T) specimens provide more conservative data [8][9][10][11][12][13].…”