PSI - Issue 47

Jan Patrick Sippel et al. / Procedia Structural Integrity 47 (2023) 608–616 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Test Setup and Materials The fatigue tests are performed using an ultrasonic (US) piezoelectric fatigue testing setup described in more detail in Spriestersbach et al. (2018) and Heinz et al. (2013). The test setup consists of a generator, a piezoelectric converter, three boosters, also called Mason horns and the specimen. The boosters as well as the fatigue specimen shown in Figure 1 are designed to reach a resonance frequency of approximately 20 kHz to achieve a longitudinal standing wave with the maximum applied stress forming in the center of the specimen. The tests are performed with a stress ratio of R = -1, in pulse pause mode, with additional air cooling to prevent a self-heating of the specimen with ∆T max < 40 K, up to an ultimate number of 10 9 cycles.

Fig. 1. Specimen geometry.

The investigated materials are the two high-strength steels AISI 52100 (German designation: 100Cr6) and AISI 4140 (German designation: 42CrMo4) in quenched and tempered state. The fatigue results of the AISI 4140 specimens are published in Khayatzadeh et al. (2022). The respective chemical compositions of the steels are given in Table 1. The specimens were manufactured with additional material before heat treatment and the final specimen geometry is manufactured after heat treatment. Both steels were austenitized in a vacuum furnace at 840 °C for 20 min, quenched in oil to room temperature and finally tempered at 180 °C for 2 h resulting in a martensitic microstructure.

Table 1. Chemical composition of the tested materials in wt. %.

Tested Material

C

Si

Mn

P

S

Cr

Mo

Al

Ti

Fe

AISI 52100 AISI 4140

0.95 0.43

0.28

0.27

0.003 0.012

0.001 0.039

1.45

0.02

0.03

9ppm

balance balance

0.259

0.743

1.060

0.207

-

-

The specimen from AISI 52100 and AISI 4140 have a hardness of 775 HV 10 and 645 HV 0.5, respectively. The fracture toughness is given as K C,AISI 52100 = 18 – 19 MPa·m 1/2 in Beswick (1989) and as K C,AISI 4140 = 32.7 MPa·m 1/2 in Wahab et al. (2014) for AISI 52100 and AISI 4140, respectively. The Phillips XL 40 scanning electron microscope was used for the fracture surface analysis. To measure the roughness of the fracture surface areas the confocal microscope µ surf explorer by NanoFocus was used. The roughness was measured according to DIN EN ISO 4287 both along the crack propagation direction and circularly around the non-metallic inclusion. 3. Results Although all the tests for both materials are performed on the same test setup, with identical specimen geometries and with the same conditions, regarding cooling and pulse pause mode, differences between the fracture surfaces of the two materials are evident. The differences described below are found for all specimens of the respective materials. In this chapter those differences are first described and additionally analyzed via roughness measurements using confocal microscopy on two representative specimens to make them more tangible. In addition, the fracture surface morphologies are analyzed using SEM. To explain the underlying mechanism for the differences regarding the crack