PSI - Issue 76

Christina Mamagkinidou et al. / Procedia Structural Integrity 76 (2026) 82–88

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fractographic investigations were conducted to correlate the locations of crack initiation with the corresponding residual stress at these points. 2. Materials and experimental methods 2.1. Material The investigated material is a martensitic stainless steel sheet (14-7PH) with a thickness of 1.8 mm that has been precipitation hardened at 550 °C. The chemical composition and mechanical properties of the material are provided in Tables 1 and 2, respectively.

Table 1. Chemical composition in wt-%. C Cr Ni Si

Mo

Cu

Ti

Mn

P

S

0.04

13.81

6.89

1.41

0.79

0.71

0.33

0.27

0.026

0.001

Table 2. Mechanical properties. Sheet thickness (mm)

Tensile strength (MPa)

0.2 % proof stress (MPa)

Elongation (%)

Vickers hardness, HV 10 (kgf/mm²)

1.8

1430

1430

6.5

454.4 ± 2.0

2.2. Residual stress measurement X-ray diffraction (XRD) measurements were conducted with a SEIFERT XRD Charon S-XL diffractometer employing CrK α radiation. The measurements were performed incrementally by electrolytically removing successive material layers from a localised area, starting from each of the two opposite surfaces of the sheet at offset positions and gradually progressing toward the mid-thickness of the sheet. At each depth, the residual stress components parallel as well as perpendicular to the rolling direction were determined. The sin² ψ method was applied to the {211} crystallographic plane of the martensitic matrix, using a diffraction angle of 2 θ = 156°. 2.3. Fractographic investigation Field-emission scanning electron microscopy (FE-SEM) was used to analyse the fracture surface of the fatigue specimens tested to determine the S-N curve shown in Fig. 1. Energy-dispersive X-ray spectroscopy (EDS) was applied to determine the chemical composition of the crack-initiating inclusions. The positions of these inclusions were measured with respect to their distance to the surface, as illustrated in Fig. 2.

Fig. 2. Measurement of crack initiation locations on the fracture surface.