PSI - Issue 19

Thorsten Voigt et al. / Procedia Structural Integrity 19 (2019) 4–11

7

Dr.-Ing. Thorsten Voigt/ Structural Integrity Procedia 00 (2019) 000 – 000

4

F = a (w⁄o NaCl) a (w NaCl) = 1,45 in load direction

20

constant Amplitude var. Amplitude w/o salt var. Amplitude with salt Required Load run out

required variable amplitude load: a = 3,2kN ; 16200 cycles

amplitude (log) [kN]

2

1,E+02

1,E+03

1,E+04

1,E+05

1,E+06

cycles (log)

Figure 7: Control arm, results of constant amplitude and variable amplitude tests

A damage calculation against the Woehler curve resulting from the constant amplitude tests shows that the control arm can easily withstand the nominal load program since the resulting damage sum is ≈ 0 . In an analogous way, it is determined that with a load level scaled by a factor of 5, failure should occur before 10 6 load cycles are reached. The first variable amplitude test was therefore carried out with a scaled maximum load of max = 23.3 kN (towards vehicle interior). The load ratio of = −0.36 between the smallest and the largest load peaks was kept constant. In the following tests, the load level was reduced to 87% and 70%, respectively. The results of the variable amplitude tests with and without salt spray are shown in Figure 7 (dashed and dash dotted line). The slope of the Gassner curve is similar to the Woehler curve. The damage sums of the three experiments: 1 = 2.18; 2 = 1.01; 3 = 0.36 may show greater dispersion. On the whole, they are above the minimum value of = 0.3 expected for fatigue strength tests, and on average even quite close to the theoretical value of = 1 . 3.3. Variable amplitude tests with salt spray For salt spray tests, the load level of the load signal was reduced once more. The aim was to achieve a failure only at about 10 6 load change to give the salt spray sufficient exposure time, and thus to influence the fatigue strength behaviour of the material. Figure 5 shows a built-in sample under a salt crust. The evaluation of the experiments shows that under the influence of salt spray, the same cracking and fracture behavior occurs as in the experiments under a normal atmosphere. Under corrosion, the control arm fails at a lower load level, as expected. Relating the two Gassner curves of variable amplitude tests to each other, a load factor between test without salt spray and tests with salt spray of 1.45 can be derived (see Figure 7). An overview of all variable and constant amplitude tests on control arms is shown in Table 3. Table 3: tests on control arm

No. of test

amplitude

salt spray

max. inbord force

load amplitude

cycles to failure

[kN]

[kN] 10,3 14,5 12,3 14,6 13,4 11,2

#1 #2 #3 #4 #5 #6 #7 #8 #9

constant constant constant variable variable variable variable variable variable variable

no no no no no no

15,2 21,3 18,2 23,3 20,3 16,3 13,8 11,5 12,9 13,2

318056

575

27941

154955 540056 5320000 317800 2297400 2057720 750540

yes yes yes yes

9,4 7,9 8,8 9,0

#10

3.4. Application of the local concept for the validation of the test results

In order to validate the tests on the control arms and to obtain further information on the cyclic strength of the material used at the breaking point, an FEM simulation of the test load case was carried out. The node with the highest stress under a lateral load is in the region of the typical cracking point under in endurance tests (see Figure 8). For a unit load of = 1 kN , a main stress of