PSI - Issue 57

C2 - Confidential

4

Hayder Y Ahmad et al. / Procedia Structural Integrity 57 (2024) 478–486 Ahmad et al./ Structural Integrity Procedia 00 (2023) 000 – 000

481

The unexpected, yet interesting, part of this case study is the propagation of the crack happened under static load. This can be explained either by the nature of the crystal structure of the Mg alloy, which is HCP, the effect of the hydrogen embrittlement or the effect of stress corrosion crackling (SCC). Following this failure, a full mechanical stress calculation was carried out to evaluate the tensile hoop stress level on the surface of the Mg outer cylinder. A comprehensive examination of the Mg alloy’s chemical composition and mechanical properties were carried out in this study. Moreover, microstructure examination and the EDX findings were also carried out in this investigation. In addition to this, a full fractography of the fracture surface is examined in the research. 3. Mechanical Model The hoop stresses from the interference fit ’s thermal load were calculated using the following set of equations Dornfeld (2004): The interface pressure between the two cylinders can be calculated following equation 1: = ( 2 2 + − 2 2 + )+ ( 2− 2− 2 2 + ) (1) Once the pressure is calculated from equation 1, the hoop stresses at the interface can be calculated following two equations: A) The ID of the outer cylinder is σ oh tensile: ℎ = 2 + 2 2 − 2 (2) B) The OD of the inner cylinder is σ ih compressive: ℎ =− 2 + 2 2 − 2 (3) If heating or cooling a part to achieve a shrink fit, the required radial interference is:  R = δ r = R α  T (4) Where R is the interference radius, α is the coefficient of thermal expansion and  T is the temperature change. The inner cylinder is made out of Iron and the outer cylinder is cast Mg alloy. In addition to the interference fit, the two cylinders are fixed together by bolts. Therefore, the fixing hole acts as an area of stress concentration. The dimensions of the two cylinders are in table 1 below:

Table 1. Cylinders dimensions Cylinder

Materials

Length in mm

Thickness in mm

inner outer

Iron

61 75

20

Mg alloy

4

The properties of the materials of the two cylinders are listed in table 2: