PSI - Issue 75

Carolina Payares-Asprino et al. / Procedia Structural Integrity 75 (2025) 489–500 C. Payares-Asprino et al./ Structural Integrity Procedia (2025)

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1. Introduction For years, the go-to material for aerospace applications was aluminium. However, aerospace manufacturers have started investigating alternatives to aluminum, such as stainless steel. Stainless steel possesses three main advantages over aluminum: 1) corrosion resistance, 2) stronger and more resistant to wear and tear, and 3) handle scratches/impact damage much better. For example, airplane manufacturers are increasingly relying on stainless steel for parts that have higher performance requirements. Fuel tanks on airplanes and spacecrafts are made from stainless steel since they are exposed to extremely corrosive materials but also must be able to withstand high temperatures and resist structural damage. Modern aerospace vehicles are much more likely to be built with stainless steel airframes or fuselages that can offer excellent strength-to-weight ratios. Duplex stainless steel (DSS) 2205 is a two-phase, ferritic, austenitic metal. Because of its high content of chromium (22%), molybdenum (5%), and nitrogen (5 to 6%), it offers excellent localized and uniform corrosion resistance. Its unique microstructure allows for superior abrasion, erosion, and fatigue resistance, and mechanical strengths higher than standard 304 and 316 stainless steels, plus it provides for very good weldability. Joining duplex alloys is challenging due to several embrittling precipitates and metallurgical changes that occur during welding. Generally, the quality of a weld joint is strongly influenced by the welding parameters where welding conditions and the imbalanced phase ratio of austenite/ferrite leads to solidification cracking, corrosion susceptibility, and lower ductility. The phase balance is important because it determines the overall properties of the alloy: the ferrite phase imparts strength, chloride stress corrosion cracking resistance, ferromagnetic properties and solidification cracking resistance and the austenite phase imparts toughness, corrosion resistance and hydrogen cracking resistance and is non-ferromagnetic. The optimum alloy properties are achieved when the ferrite content is around 50%. This can be achieved routinely during parent material manufacture. However, welding can change the phase balance by differing amounts, depending on the welding parameters that change the phase balance during the welding process due to the rapid cooling involved in most welds’ thermal cycles [ Muthupandi et al. (2003), Sathiya et al . (2009)] Mechanical components can fail under cyclic loading over a period of time, known as the fatigue phenomenon. To prevent fatigue induced failures, material behavior are investigated to determine the endurance limit of the material for safe design and to ascertain that no damage will occur once a constant amplitude load drops below the endurance limit (Infinite life), thus leading to reducing the cost and loss in human lives. Fatigue is observed to be the major cause of failure in metal structures and components and is expected be responsible for approximately 90% of all metallic failure [Callister (1991)]. Butt welded specimens also showed that the fatigue strength decreased with increasing yield strength [Kuma et al. (2018)]. It has been assumed that as long as the behavior of the material does not exceed the yield point, the material will not fail, but this argument is not always correct [Shigley et al. (2011)]. The problem of fatigue failures is more complex and dangerous in welded structures. Numerous complications that require consideration in evaluating and defining potential fatigue failures in welded structures are as follows [Atzori et al. (2009)]: • Defining material properties, which vary throughout the weld joint, specifically in the two zones of welds as they are the fusion zone (FZ) and the heat affected zone (HAZ) • Presence of high residual stresses, both local (due to the weld itself) and structural (due to the assembly process of the structure) which vary throughout the weld joint; • Defining precisely, the weld bead geometry (bead size and shape) and radius at the toe of the weld, that vary even in well-controlled manufacturing and are the most important factors affecting the design and engineering of welded structures. The fatigue strength of welded steels is affected by the applied load mean stress and the residual stress in the vicinity of the weld. Since residual stresses induced during manufacturing can be detrimental to the integrity and service behavior of a component, stress relief operations are an integral step in many production sequences. Beneficial residual stress distributions can relax during service by these same mechanisms, leading to loss of their worth. Special emphasis is placed on fatigue-induced relaxation. In general, compressive residual stress at the surface of a component is beneficial: it tends to increase fatigue strength and fatigue life [McMahon (2021)]. Residual stress relaxation should be studied in more detail, especially for different joint types with varying stress concentrations. Furthermore, future studies should address residual stress from post-weld treatment methods