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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propagation, the natural frequency as well as the applied amplitude during the late crack propagation shortly before final failure are evaluated and must be considered. 3.1. Fracture surface comparison The fracture surfaces of an exemplary specimen for each of the two tested materials are shown in Figure 2. The differences in fracture surface are common for all the failed specimens and the selection is based on comparable stresses, lifetime as well as inclusion size and position. The AISI 52100 specimen on the left was tested with a maximum stress amplitude of 1100 MPa and failed after 567,486 cycles, while the AISI 4140 specimen on the right was tested with a maximum stress amplitude of 900 MPa and failed after 900,226 cycles. Both specimens failed with crack initiation at an internal non-metallic inclusion and the formation of a smooth fracture surface area, the so-called fisheye, which is typical for high-strength steels in the high cycle fatigue regime. Additionally, in case of the AISI 52100 specimen a more or less homogeneous final force fracture, indicating unstable crack propagation, is observed outside of the fisheye with the height of the crack ridges increasing with increasing crack length.

Fig. 2. Fracture surface of an AISI 52100 (left) and a AISI 4140 specimen (right) after ultrasonic fatigue test.

Unlike the characteristics of the left fracture surface, in case of AISI 4140, unexpectedly several distinct beach marks are observed outside the fisheye. Additionally, the fracture surface area is heterogeneous with an alternating pattern like structure. In the middle of the specimen a bright fracture surface area is observed and is assumed to be the result of relatively fast crack propagation. Assuming a constant stress amplitude, we would expect an increase in the stress intensity factor (SIF) with growing crack length as described by Murakami et al. (1989) and therefore an increase in topography height analogously to the behavior of the AISI 52100. However, in case of AISI 4140 we observe two darker areas, separated by a slightly brighter one before final force fracture surface is found at the bottom of the specimen. Since the cause of the optically divergent fracture surface areas with alternating brightness outside the FiE is unknown, they must first be described in more detail. To make those optically differences more tangible and quantifiable a roughness measurement via confocal microcopy was performed. 3.2. Roughness measurement via confocal microscopy The AISI 52100 specimen is examined as a reference for the roughness development along the crack path. Figure 3 shows the respective position of the measured profiles (left) as well as the results (right). Seven roughness profiles along the crack propagation direction were measured, with their respective positions being displayed by the dotted lines in the fracture surface on the left. The results of those longitudinal profiles correspond to the symbols connected with dotted lines in the right diagram. We observe an increase in roughness with increasing distance to crack initiation