Fatigue Behaviour of Welded Joints Made of 6061-T651 Aluminium Alloy

Abstract

The purpose of this chapter is to present the main results of an investigation concerning the assessment of the fatigue behaviour of welded joints made of the 6061-T651 aluminium alloy. The 6061 aluminium alloy is one of the most common aluminium alloys for heavyduty structures requiring good corrosion resistance, truck and marine components, railroad cars, furniture, tank fittings, general structures, high pressure applications, wire products and pipelines. Many of these applications involves variable loading, which makes very relevant the study of the fatigue behaviour of this aluminium allow. In particular, the study of the fatigue behaviour of welded joints is of primordial importance since welds are intensively used for structural applications. The proposed investigation focuses in four types of welded joints, made from 12 mm thick aluminium plates, namely one butt welded joint and three types of fillet joints: T-fillet joint without load transfer, a load-carrying fillet cruciform joint and a longitudinal stiffener fillet joint. Traditionally, the fatigue assessment of welded joints, including those made of aluminium alloys, is based on the so-called S-N approach (Maddox, 1991). This approach, which is included in main structural design codes of practice, adopts a classification system for details, and proposes for each fatigue class an experimental-based S-N curve, which relates the applied stress range (e.g. nominal, structural, geometric) with the total fatigue life. Alternatively to this S-N approach, the Fracture Mechanics has been proposed to assess the fatigue life of the welded joints. It is very often claimed that welded joints have inherent crack-like defects introduced by the welding process itself. Therefore, the fatigue life of the welded joints may be regarded as a propagation process of those defects. A relation between the Fracture Mechanics and the S-N approaches is usually assumed. The slope of the S-N curves is generally understood to be equal to the exponent of the power relation governing the fatigue crack propagation rates of fatigue cracks. More recently, the local approaches to fatigue have gaining added interest in the analysis of welded joints (Radaj et al., 2009). In general, such approaches are based on a local damage definition (e.g. notch stresses or strains) which makes these approaches more adequate to model local damage such as the fatigue crack initiation. In this sense, the Fracture Mechanics can be used to complement the local approaches, since the first allows the computation of the number of cycles to propagate an initial crack until final failure of the component. The present research seeks to understand the significance of the fatigue crack initiation, evaluated using a local strain-life approach, on the total fatigue life estimation for four types