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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.

Hybrid welding is, as regards selection of welding parameters, a tolerant process. On the other hand, the weld seam and the heat affected zone may tend to be wider as in an arc process, which may be detrimental to mechanical stability. With CO 2 laser beam buttwelds as a basis for comparison, properties of butt-welds of a peak-aged airframe alloy AA6013-T6/3.2 mm, produced by Nd:YAG-MIG hybrid, are investigated. A filler wire (AA4047; AlSi12) was used for both types of welding. It is found that the hybrid process coarsened the microstructure in the fusion zone partly. Furthermore, although the weld seam and the heat affected zone were wider, mechanical properties were not at all adversely affected. Changes in hardness and tensile strength were marginal and the fatigue performance was in fact improved for the hybrid welds. These results are presented and discussed. It is concluded that the hybrid welding deserves consideration for fatigue critical applications.

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.