PSI - Issue 68

Pascal Franck et al. / Procedia Structural Integrity 68 (2025) 119–125 P. Franck et al. / Structural Integrity Procedia 00 (2025) 000–000

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3.2. Constant amplitude tests (CAT) Following the previously described MAT, further constant amplitude tests (CAT) were conducted to allow a first estimation of a Woehler curve for the LBL material. The results of the MAT have been directly incorporated into the experimental campaign of the CAT since the first two load levels were chosen based on the first load level that showed a remarkable material reaction and a lower load level that did not lead to a visible plastic material response. Thus, the load levels of 35 and 55 MPa were chosen and multiple tests were conducted for these two loads in CAT. Once again, the load ratio was chosen to be R = 0.1, and a test frequency of f = 3 Hz was applied.

(a)

(b)

Fig. 4. (a) Hysteresis loops at different testing times of the constant amplitude test with a maximum stress of 35 MPa and (b) undamaged specimen after reaching the desired 2∙10 6 cycles.

Due to the applied lower load of 35 MPa, the specimen was capable of reaching the desired 2∙10 6 cycles without failure. During the CAT, the hysteresis loops were recorded every 30 min and for sufficient documentation the recording frequency of the displacement sensors was set to 25 Hz for 20 s, thus recording a total of 60 load cycles. Fig. 4 (a) shows recorded hysteresis loops at 4 different times of the test. It can be seen, that the hysteresis loops are not opened up at the beginning nor the end of the procedure. The loops are just slightly moving to higher strains, but the occurring strains do not seem to lead to remarkable damage of the specimen. Since the loops are not opened up, a nearly elastic material behavior can be assumed. Only the described movement of the loops is an indication of a negligible plastic material response. The assumption is backed up by the representation of the test specimen after the material test shown in Fig. 4 (b). It becomes apparent, that the specimen is not visually damaged in the testing area. Even the characteristic cracks in the curves of the bone shape, that occurred in nearly every test conducted and which did not necessarily lead to the immediate failure of the specimen, did not appear. For the load level of 35 MPa fatigue strength can therefore be confirmed for the material. The second load level had a maximum stress of 55 MPa and was the first level in which the LBL showed a remarkable material reaction during the MAT. Because of the higher load, the expected shortage of the number of cycles to failure appeared during the tests. The presented specimen was able to withstand about 1.8∙10 5 cycles until fracture, which is close to 10% of the 35 MPa load level. Thus, the material reaction was more pronounced right from the beginning of the testing procedure. In the DIC recordings of the specimen surface, it can be seen, that the nodal area (top right in Fig. 5 a) once more could be identified as a high-loaded area. This fits the assumption, that the nodal areas are weakening points of the material and tend to be initiation points for failure. When looking at the hysteresis measurement for the higher loaded test, it becomes evident, that the increased load led to a plastic deformation of the material right from the start of the tests. The hysteresis loops are opened up from the first cycle and the opening gets