PSI - Issue 35

M. Faruk Yaren et al. / Procedia Structural Integrity 35 (2022) 98–105 Yaren M. F. et al / Structural Integrity Procedia 00 (2021) 000 – 000

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Fig. 1. Experimental setup and an example of recorded picture.

To observe the effect of the block size on crack propagation life, different variable amplitude loading tests were performed after pre-cracks are generated. Two different load blocks were used, and these blocks are applied repeatedly until the specimens fracture. 13 kN overload block was applied during “n” cycles between two 7 kN constant amplitude fatigue loading with 1000 cycles, and this process is repeated until failure. The number of cycles of the overload block, "n", is defined as a variable. The number of cycles "n" of the overload block is changed as 10, 20, 50, 200 500 cycles in each test, respectively, while the 7 kN fatigue load is always applied through 1000 cycles between the overload blocks. Details of the tests are given in Table 1.

Table 1. Details of the block overload tests.

Load for Precrack (kN)

Load for FCG (kN)

a (avg) (mm)

Test

F OL /F da/dN Cycle

Block loading behavior

1 0.64 - 6.4 24.612 0.7 - 7.0 2 0.64 - 6.4 24.829 0.7 - 7.0 3 0.64 - 6.4 24.976 0.7 - 7.0 4 0.64 - 6.4 25.320 0.7 - 7.0 5 0.64 - 6.4 24.925 0.7 - 7.0

1.86 1.86 1.86 1.86 1.86

16488 1000x7kN + 500x13kN+ 1000x7kN … 17902 1000x7kN + 200x13kN+ 1000x7kN … 11529 1000x7kN + 50x13kN + 1000x7kN … 18401 1000x7kN + 20x13kN+ 1000x7kN … 17902 1000x7kN + 10x13kN+ 1000x7kN …

4. Results The changes in crack propagation life due to the overload block size are seen in Fig. 2. Crack propagation life was observed as 14500 cycles under a constant amplitude 7 kN load without applying overload block loading. If overload blocks are applied as n = 50, 20 and 10 cycles, the crack propagation life becomes 20000, 32000 and 48000 cycles, respectively. As seen in Fig. 2., the number of cycles in overload block affects crack growth life significantly. If the overload block includes significantly higher number of cycles (i.e., n = 500 and 200), the bigger-size plastic zone generated in the first cycle of the block overload is traveled through faster due to higher number of available overload cycles in the same block overload, which accelerates the crack growth. In the case of a short cycle of block overload (i.e., n = 50, 20, 10), retardation is clearly observed in crack growth rate. This is because, in contrast to the above, there are less number of load cycles in the overload block to pass through the larger plastic zone size faster. As a consequence, longer crack growth lives are seen in comparison to the 7 kN constant amplitude loading test. The generated larger plastic zone size by block overload dominates the crack growth behavior. As a result, the crack propagates very slowly under 7 kN fatigue loading except at the 13 kN overload block. Thus, when the number of load cycles for block overload is not too high, longer crack propagation lives are obtained in the experiments compared to constant-amplitude load conditions.