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

Abstract

Accuracy of indirect fatigue crack length measurement by potential drop method or by compliance technique may be affected at low load ratio due to fracture surface contact, crack closure or mixed mode fracture. As an alternative, the maximum value of crack opening displacement, CODmax, from a clip gauge was utilized. Middle crack tension M(T) specimens were used to obtain conservative data at a low load ratio (R = 0.1). Thin sheet specimens (B = 3.2 mm) with different widths (100 mm ≤ W ≤ 400 mm) of AA6056-T4 were investigated in the mid-regime (Paris regime), which is of interest for damage tolerance analysis. The use of CODmax is found to provide crack lengths equivalent to those measured optically. Hence, the method is very suitable for indirect crack length measurement. Furthermore, small width specimens provided data equivalent to large width specimens. Insofar, the size effect is found to be absent, and fatigue crack propagation data can be acquired on small width specimens when material availability is limited.

Improving interfacial properties of a laser beam welded dissimilar joint of aluminium AA6056 and titanium Ti6Al4V for aeronautical applications

“…Usually, scatter in fatigue crack propagation may vary up to a factor 3 [58]. However, the improvement is more than the scatter level found in AA6056-T6 [57] or the specimen-to-specimen difference (shading in Fig. 9a).…”

“…The mid-regime (Paris regime) of AA6056-T6 is found to exhibit low scatter (±35 %) and is also insensitive to a number of parameters [57]. Residual stresses on the Al-side should be tensile [3] and thus also not favourable for resistance against fatigue crack propagation.…”

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

“…Crack length was monitored continuously and indirectly using an on‐top (clip‐on) displacement gauge for C(T) specimens (attached to the cut at the specimen top; Fig. 2) and a load‐line displacement gauge for M(T) specimens, and calculated from the maximum crack opening displacement (COD max ) as described in detail elsewhere 46 . In brief, the COD max value was recorded continuously at a signal interval of 20 mV.…”

“…In that case, the applicability and the accuracy of potential drop or compliance technique would be adversely affected. In this context the COD max technique 46 was found to be less susceptible.…”

“…2) and a load-line displacement gauge for M(T) specimens, and calculated from the maximum crack opening displacement (COD max ) as described in detail elsewhere. 46 In brief, the COD max value was recorded continuously at a signal interval of 20 mV. Occasionally, crack length was also measured directly on opposite specimen surfaces using optical microscope, and the averaged optical value was used for calibration of the corresponding COD max value.…”