An overview of fatigue in aircraft structures

Abstract

From safe‐life to fail‐safe and damage‐tolerance approaches, the last one emerged as the main design philosophy for aerostructures, allowing weight savings and at the same time increasing reliability and structural integrity in the presence of damages which may have occurred during the manufacturing process or during service. The application of damage‐tolerance philosophy requires extensive know‐how of the fatigue and fracture properties, corrosion strength, potential failure modes and non‐destructive inspection techniques, particularly minimum detectable defect and inspection intervals. To face scatter in material properties, conservative approaches considering the worst scenario or statistical methods dealing with the variability of material have been employed in the fatigue assessment of structures. The fatigue life estimation can display substantial variability, illustrating the need for a probabilistic assessment in practical applications. As an example, the variation in the fatigue life for the common scenario of a through crack in an Al alloy plate was evaluated taking into account scatter of properties. The 2010 FAA rule establishing a limit of validity (LOV) puts a bound in the indefinite operational life allowed for by earlier regulations. This requirement, together with the diminishing role of aluminium in airframes, will certainly shape the directions of fatigue, fracture and damage mechanics research in years to come, expanding the knowledge base upon which substantiation of LOV values is made, and ensuring safety under sustainable conditions.

Improving the fatigue performance of airframe structures by combining geometrical modifications and laser heating

“…The fatigue crack is assumed to initiate from the welding joint of the middle string exactly at the centre of the panel. Although the link‐up of coplanar cracks in the realistic cases of multiple‐site damages could dramatically reduce the number of cycles to reach the critical crack size, only single‐crack initiation is considered here. Because the alignment of the stringers with the major loading axis will make it hardly possible to link up the fatigue cracks initiated at the welding joints, which would all propagate in the direction perpendicular to the stringers.…”

“…In analogy to the welding of a thin plate, T max at a distance r from the heating line is considered inversely proportional to r : where Q is the heat input, c is the specific heat, ρ is the density, and h is the thickness of the plate. By substituting Equation 3 into Equation 2 for the case T y ≤ T max ≤ 2T y , one can obtain the attenuation of the absolute value of the inherent strain, ε * (r) , with increasing r , which has the following form, as shown in Equation : where A and B are constants. Thus, the inherent strain field produced by LH is assumed to have a transverse profile, as shown in Figure , which remains nearly constant along the heating line.…”

“…The fatigue of airframe structures is an extremely important concern in the safety of aircraft. According to the statistics, the reason for approximately 60% of the failures in aircraft components is fatigue damage . The evolution of such damage includes 2 stages.…”

“…This approach permits the existence of a certain extent of fatigue damage and requires periodic inspection of the airframe structure to detect and repair small cracks before they grow into a thread. The detectable size of fuselage cracks during inspection processes is typically in the range of millimetres, and their critical size, at which the residual strength of the cracked structure falls below a prescribed limit, can be hundreds of millimetres depending on the structural component and stress state being considered . For such long cracks considered in the damage tolerance approach, the crack growth rate can be well formulated by empirical equations such as Paris law and the more complicate NASGRO equation as embedded in the computer codes NASGRO and AFGROW …”

Fatigue assessment of refill friction stir spot weld in AA 2024‐T3 similar joints

“…Furthermore, the airframe design is closely correlated with interactions between cracks and fastened components . The extensive use of structural welded joints represents an important challenge considering the damage tolerance design, where a full comprehension of the crack initiation and crack growth is mandatory for applying these solid‐state processes in the aerospace industry …”