PSI - Issue 7

B.M. Schönbauer et al. / Procedia Structural Integrity 7 (2017) 492–496 B.M. Schönbauer et Al./ Structural Integrity Procedia 00 (2017) 000–000

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investigated precipitation-hardening chromium-nickel-copper stainless steel 17-4PH shows high static as well as fatigue strength in combination with high toughness and good corrosion resistance. It is therefore widely used in applications where these properties are required, for example, it is a standard material for steam turbine blades in the low pressure part of turbines. Several investigations have been performed in recent years with the aim to study the fatigue properties of 17-4PH stainless steel. Fatigue lifetimes as well as fatigue crack growth rates were determined in different environments with focus on fatigue crack initiation at corrosion pits (Schönbauer et al. (2015)). The influence of non-metallic inclusions (Schönbauer et al. (2016)) and various types of small surface defects (Schönbauer et al. (2017a)) on the cyclic properties were studied. The most recent investigations deal with the variable amplitude loading (Schönbauer et al. (2017c)) and the torsional fatigue behaviour (Schönbauer et al. (2017b); Schönbauer et al. (2017d)). In this paper, the influence of small defects under tension-compression and torsional loading is studied. Results of smooth specimens as well as specimens containing artificial defects are compared. The applicability of fracture mechanics models for fatigue limit prediction is discussed considering the role of notch root radius of defects. 2. Material and experiments The investigated material 17-4PH is a chromium-nickel-copper stainless steel in the precipitation hardened condition H1150. The ultimate tensile and yield strength are 1030 MPa and 983 MPa, respectively. More details on the material can be found in Schönbauer et al. (2016). The results reported in this study were obtained from fatigue experiments using rotating bending, servo-hydraulic (tension-compression and torsion) and ultrasonic (tension-compression and torsion) testing machines. The respective test frequencies were 50-67 Hz, 25-60 Hz and 19 kHz (in the same order as above). The load ratio for all tests was R = −1. The experimental procedure s as well as the specimen shapes are described in detail in Schönbauer et al. (2017a) and Schönbauer et al. (2017d).

Fig. 1. Geometries of artificial defects.

Artificial defects were introduced into the gauge sections of specimens. The geometries of the defects are shown in Fig. 1. Circumferential notches, corrosion pits, 1-hole defects and pre-cracked 1-hole defects were used for tension-compression tests. For torsional tests, 1-hole, 2-hole. 3-hole and pre-cracked 2-hole defects were employed. The orientation of 2-hole and 3-hole defects as well as pre-cracks used for torsional testing was perpendicular to the direction of major principal stress, i.e. at an angle of 45° with respect to the specimen axis. Before fatigue testing, all specimens were stress-relief annealed at 600 °C for one hour to reduce any residual stresses that could possibly have been generated during specimen preparation and introduction of defects.