PSI - Issue 13

Marco Schmidt et al. / Procedia Structural Integrity 13 (2018) 91–96 M. Schmidt et al. / Structural Integrity Procedia 00 (2018) 000–000

94

4

(a1)

(b1)

-0.45

0.00

0.00

0.28

(a2)

(b2)

0.00

0.26

-0.49

0.00

Fig. 2. (a) 1. Principal strain; (b) 2. Principal strain; (1) F 1 : F 2 = − 3 : 1; (2) F 1 : F 2 = − 7 : 1.

The influence of negative stress triaxialities on the damage behavior has been investigated experimentally by di ff erent, throughout the experiment constant loading ratios F 1 : F 2 . Shear loading caused by F 2 is superimposed by compression loading caused by F 1 i.e. with more elevated negative load factor F 1 (from 0 to − 8) more compression is superimposed which causes more negative stress triaxialities. The experimental evaluation was made with a digital image correlation system. The corresponding displacement measure u 2 of the force-displacement-curves (see Fig. 1(b)) was extracted by two points on the surface of the specimen with a distance of 6 mm whereas one evaluation point was on the right and one on the left side of the notch. The experimental results are compared with the corresponding numerical simulations performed with Ansys Classic. In this present paper, a detailed evaluation of the load cases F 1 : F 2 = − 3 : 1 and F 1 : F 2 = − 7 : 1 is considered. Fig. 1(b) shows the load-displacement curves of both load cases in shear direction. Both load cases show a very good agreement between experiment and numerical simulation. With the load combination F 1 : F 2 = − 3 : 1 a maximum force of 910 N and a maximum displacement of 1 . 18 mm is achieved. With increasing compression, load case F 1 : F 2 = − 7 : 1 shows that the maximum force drops to 510 N and a displacement of 1 . 05 mm is achieved. The experimental curves of both load cases indicate similar behavior at the end of the test. The high compression makes it di ffi cult to perform the experiments due to shear and friction mechanisms. Furthermore, there are very good agreements in the comparison of the principal strains between the experiments and numerical simulations, as one can see in Fig. 2. The left column shows the first principal strain and the right column shows the second one. For each comparison, the DIC evaluation of the experiment is on the left and the evaluation of the numerical simulation on the right. The strains are evaluated at 2 3 of the maximum displacement u 2 and it can be seen in both load cases that a strain band develops for the first and second principal strain from bottom left to top right. Besides the good agreement between experiment and numerical simulation, the localization of the strains can be easily recognized. This localization gives an idea of the orientation of the crack leading to failure of the specimen. In the load case F 1 : F 2 = − 3 : 1 the first principal strain reaches a maximum of 0 . 28 and the second one a minimum of − 0 . 45. For the load case F 1 : F 2 = − 7 : 1 the maximum of the first principal strain is 0 . 26 whereas the minimum of the second one is − 0 . 49. In order to assess the influence of the stress state on damage behavior, the stress triaxiality and the Lode parameter of the numerical calculations were evaluated in a cut plane passing though the center of the notch at 2 3 of the final displacement u 2 , see Fig. 3. At load case F 1 : F 2 = − 3 : 1 the distribution of stress triaxiality and the Lode parameter in the middle of the fracture surface is relatively homogeneous. It becomes more inhomogeneous towards the outside. The minimum stress triaxiality of η = − 0 . 39 is not in the center of the cut plane and has the associated Lode parameter