Failure analysis of a cracked cylinder block assembly of an aircraft brake system

Abstract

This study contains the analysis of a cylinder block assembly of an aircraft brake system that failed due to the formation of cracks. The failure analysis was performed using a two-prong strategy (i) experimental, which included the analysis of the fracture surfaces along with microstructure and composition, and (ii) numerical (finite element analysis, FEA), to find the stress state. Visual inspection revealed relatively long cracks emanating from a sharp corner (junction) which traversed 87 mm in one direction and 52 mm in another. At a mirror-symmetric location, another smaller crack (∼16 mm) revealed the initial growth stage. During detailed fractography of the region containing long cracks, clear beach marks indicating progressive failure were observed pointing towards the same sharp corner. Microstructure and compositional analysis revealed the cylinder block assembly material to be heat treated Al 2024 having no inherent nonconformities. The hardness was also found to be conformant to the typical of the Al 2024. The stresses are produced as a consequence of (a range of) hydraulic pressures acting on its pistons which were estimated using FEA. The stress was confirmed to be concentrated (having Kt ∼ 2.25) at the same junction(s) as identified during fractography. Due to the cyclic nature of the applied loads, the resulting stresses initiated the fatigue crack(s) at the stress concentration locations (junctions). Fatigue life calculation using unnotched SN diagram (Kt = 1, R ∼ 0)) showed that failures could potentially occur between 2.83 E5 to 1.12 E5 load cycles, depending on the value of stress amplitude, and it matched with the estimated life in our case. Thus, a combination of cyclic brake applications and the presence of stress concentration locations resulted in fatigue crack initiation and propagation leading to the ultimate rejection of the cylinder block.