PSI - Issue 76

Afshin Khatammanesh et al. / Procedia Structural Integrity 76 (2026) 115–122

116

1. Introduction For a safe fatigue design, it is essential to conduct experiments with specimens extracted from components with the relevant microstructure and surface finish, under close-to-operation loading conditions. Failing to consider the effect of process-related residual stresses, or performing tests at irrelevant load ratios, for example, may produce inaccurate or difficult-to-interpret results. The aim of the present work is to study the high and very high cycle fatigue (HCF/VHCF) properties of precipitation-hardened martensitic stainless steel sheets with ground surface condition under fully-reversed tension-compression loading. Data on the cyclic strength of steel sheets in the VHCF regime are limited due to experimental challenges. Especially under reversed tension-compression loading, the specimens tend to buckle, which results in limitation for sheet thickness and specimen geometry. An efficient method to study the VHCF properties is ultrasonic fatigue testing, which operates at around 20 kHz and enables testing up to one billion cycles within less than one day. Few investigations have been reported employing the ultrasonic fatigue testing technique for thin sheets under fully reversed tension-compression loading (Stanzl-Tschegg et al ., 1993, Müller-Bollenhagen et al ., 2010), Tofique et al ., 2017), tension-tension loading (Mayer et al ., 2014), and reversed bending (Ishii et al ., 2003). In the present paper, ultrasonic fatigue test results obtained under fully-reversed tension-compression loading are presented. Fractographic investigations were conducted, and the data are analysed using fracture-mechanics principles. 2. Materials and experimental procedure The materials investigated are precipitation-hardening martensitic stainless steels 14-7PH with the chemical compositions listed in Table 1. Three different sheets with thicknesses of t = 1.8 mm, 2.3 mm, and 3.1 mm were employed. Materials A (1.8 mm), B (2.3 mm), and C (3.1 mm) were precipitation hardened at temperatures of 550 °C, 570 °C, and 552 °C, respectively. The measured Vickers hardness and the mechanical properties are given in Table 2. The surfaces of the sheets were ground in rolling direction resulting in a roughness of around Rz = 2 µm.

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

Si

Mo

Cu

Ti

Mn

P

S

A B C

0.04 0.04 0.04

13.81 13.80 13.72

6.89 6.80 6.98

1.41 1.53 1.50

0.79

0.71 0.63 0.71

0.33 0.39 0.29

0.27 0.47 0.27

0.026

0.001 0.001 0.001

-

0.02

0.76

0.023

Table 2. Mechanical properties. Material Sheet thickness (mm)

Tensile strength (MPa)

0.2 % proof stress (MPa)

Elongation (%)

Vickers hardness, HV 10 (kgf/mm²)

A B C

1.8 2.3 3.1

1430 1529 1462

1430 1503 1462

6.5 6.0 5.7

454.4 ± 2.0 469.9 ± 2.6 463.2 ± 2.2

Test specimens, as shown in Fig. 1, were extracted from the sheets by water-jet cutting or milling with the longitudinal axis parallel to the rolling/grinding direction. They feature a rectangular gauge section with a length of 8 mm and a width of 5 mm. The total length, l , of the sheet specimens was slightly different for materials A, B, and C to ensure resonance condition at around 19 kHz. The surfaces in the gauge section produced by specimen machining were ground parallel to the longitudinal direction, but the original surface was not further polished. The specimens were brazed to a connecting part including a thread as depicted in Fig. 1, which allows them to be attached to the load train. Experiments were performed with the high-accuracy ultrasonic fatigue testing equipment developed at BOKU University. Tests were conducted under fully-reversed tension-compression loading, i.e., at a load ratio of R = − 1. Intermittent loading with pulse lengths between 100 ms and 1 s and cooling pauses between 300 ms and 1 s was utilised to avoid self-heating of the specimen during high-frequency testing. In addition, the specimens were cooled