PSI - Issue 57

Felix-Christian Reissner et al. / Procedia Structural Integrity 57 (2024) 411–419

413

F.-C. Reissner et al. / Structural Integrity Procedia 00 (2023) 000–000

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Fig. 1. Geometry of the specimens

In total, three shot peening test series are conducted. The specimens of test series 1 (SP1) are treated with the first shot peening treatment, where a deflector is used. The deflector is a rod with a sharp tip, where the surface of the tip has a 45° angle. The shot is blasted on the tip of the rod and is deflected to the surface of the cross bore, while the rod is moving through the cross bore. For the second test series (SP2) the second treatment is additionally conducted after the first treatment. This is a common procedure in shot peening. It is used to smooth the surface with the second shot peening. While the first and second treatments are conducted with a deflector, the third treatment is carried out manually, i.e. a worker manually blasts the cross bore from the outside. The third shot peening is conducted to induce residual stresses, particularly in the chamfer. In addition, there are two test series of untreated (UT) and thermally stress relieved (SR) specimens which are used as references to evaluate the effect of the shot peening.

Table 1. Shot peening parameters. Treatment

Almen intensity

Procedure

Test series

SP1

SP2

SP3

1. Shot peening 2. Shot peening 3. Shot peening

0,23 - 0,33 mmN 0,17 - 0,23 mmN 0,02 - 0,025 mmA

deflector deflector manual

x

x x

x x x

(approx. 0,06 – 0,075 mmN)

The cyclic fatigue tests are performed at two test rigs. The EN-GJS-700-2 (cast iron) specimens are tested on the PWY NS mechanical test rig from SCHENCK. The 34CrNiMo6+QT (steel) specimens are tested on a hydraulic test rig at the Fraunhofer Institute for Structural Durability and System Reliability (LBF). Due to of the limitations of the mechanical test rig, the cast iron specimens are tested to fracture. Whereas the steel specimens are tested to 20 % loss of rigidity. Both criteria can be considered as comparable as the loss of 20 % rigidity is almost equivalent to fracture. 2.2. Experimental Results The cyclic fatigue tests are evaluated by the maximum likelihood method according to Störzel and Baum gartner (2021), Table 2. However, in accordance with the FKM-Guideline by Rennert et al. (2020), the slope after the knee point is used as a fixed parameter. The test results of the shot-peened (SP) and untreated (UT) steel specimens indicate that shot peening has no significant effect on the fatigue strength of the steel specimens, Figure 2 (b). On the other hand, the test results of the shot-peened cast iron specimens show a strength-increasing effect of shot peening. While the strength-increasing effect is up to 40 % in the very high cycle fatigue domain ( N > 10 6 ), the effect decreases to 0%at N = 10 5 cycles, Figure 2 (a). The absence of a strength-increasing effect in the case of the steel specimens is probably for the same rea son as the slowly decreasing effect in the case of the cast iron specimens. The strength-increasing effect is mainly based on compressive residual stresses which are induced by shot peening. However, due to the cyclic loading to which the specimen is subjected, the residual stresses tend to disappear. In the case of the steel