PSI - Issue 57

Ahmad Qaralleh et al. / Procedia Structural Integrity 57 (2024) 649–657 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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5

4. Experimental Results The test results of the steering knuckle under constant and variable loads are shown in Fig. 4 a and b. In the case of the constant amplitude loading, the S-N curves run parallel with a slope of k= 6. In this area, the mean stress sensitivity is M= 0.26. It is worth noting that the pulsating tensile load (R= 0) exhibits a knee point at N k = 4×10 5 cycles. In contrast, the test points for the alternating load (R= -1) align linearly, making it challenging to identify a knee point to the high fatigue region. However, both load cases show very low scatter in the test results. Fig. 5 shows a comparison between the cyclic curves under a strain-controlled test for the load ratios R ε = -1 and R ε = 0. The effect of superimposed mean strains has minimal influence on the strain-life curve of the steering knuckle, as shown in Fig. 6. On the other hand, stress-strain behavior is affected by the maximum stress level, resulting in apparent distinctions between alternating and pulsating loads. Under pulsating loads the cyclic stress-strain curve flattens after transitioning from macroscopically elastic to elastic-plastic material behavior. This suggests that smaller load amplitudes are enough to achieve the same magnitude of strain amplitudes. a.) b.) 70

R F = 0 k = 6 N k = 3·10 6 F a,k = 28 kN T S = 1:1.08

R F = 0 k = 6 N k = 4·10 5 F a,k = 16kN T S = 1:1.01

40

60

50

30

20 Force Amplitude F a [kN] (log) R F = -1 k = 6 N k = 1·10 6 F a,k = 17.3 kN T S = 1:1.1

30 Force Amplitude F a [kN] (log) R F = -1 k = 7 N k = 1·10 7 40

F a,k = 31 kN T S = 1:1.13

20

10 4

10 5

10 6

10 7

10 4

10 5

10 6

10 7

Cycles to Failure N (log)

Cycles to Failure N (log)

Fig. 4 S-N curves of the steering knuckle. a.) constant amplitude loading; b.) variable amplitude loading

10 2

E' = 192.5 GPa K' = 1463.7 n' = 0.1074 R p0,2 ' = 751 MPa R e = -1

E' = 192.5 GPa s f ' = 1528.8 MPa

1000

b = -0.0849 e f ' = 1.4999 c = -0.7909 R e = -1

10 1

800

10 -1 Strain Amplitude e a [%] 10 0

200 Stress Amplitude s a [MPa] 400 600

E' = 192.5 GPa s f ' = 1151.9 MPa

E' = 192.5 GPa K' = 1093.6 MPa n' = 0.07118 R p0,2 ' = 703 MPa R e = 0

b = -0.0581 e f ' = 2.0731 c = -0.8161 R e = 0

0

10 -2

0.0

0.2

0.4

0.6

0.8

1.0

10 0

10 1

10 2

10 3

10 4

10 5

10 6

10 7

Strain Amplitude e a,t [%]

Cycles to failure N (log)

Fig. 5 Comparison of the cyclic stress-strain curve under alternating, R ε = -1, and pulsating loads, R ε = 0.

Fig 6. Comparison of strain life curves under alternating R ε = -1, and pulsating loading, R ε = 0

4.1. Cyclic material behavior To investigate the influence of variable amplitudes on the cyclic material behavior, incremental step tests were carried out (Landgraf, et al., 1969). Table 2 shows the key parameters obtained from the Incremental Step Test for describing the cyclic stress-strain behavior of the material.