PSI - Issue 13

Christopher Schmandt et al. / Procedia Structural Integrity 13 (2018) 799–805 C. Schmandt, S. Marzi / Structural Integrity Procedia 230 (2018) ECF22

802

4

before applying the warmed-up adhesive. This improves the connecting process between adhesive and primer molecules. Longer curing times should be avoided because they yielded a chemical passive primer layer that was no longer able to connect to the adhesive. Shorter curing times yielded adhesively failed regions as well, probably due to an incomplete curing process of the primer layer or too much difference in temperature between substrates and adhesive before application. It should be taken care that the recommended adhesive temperature of 50 °C is constant over the whole application process. Figure 3(a) illustrates evaluated J over u for v -controlled tests as well as the corresponding derivatives of J with respect to u , describing the restraining cohesive stress σ (traction-separation-laws). For reasons of clarity, just one test for each investigated constant controlled crack opening velocity is exemplarily displayed. Comparing different tests in the initial region of elastic material behavior, curves seems to be not affected by loading rate and viscous effects appear to be negligible here. On the other hand, fracture energy J c is significantly increasing with higher crack opening velocities as well as the corresponding critical crack opening displacement at the point of complete failure in the traction-separation-laws. Both values seem to plateau at the highest tested crack opening velocity. Cohesive strength σ 0 as plateau stress within the nearly trapezoidal traction-separation-laws seems to slightly increase with loading rate. There might be a marginally decrease of σ within the plateau region. Oscillations are resulting from low-pass-filtering prior to derivation and were no result from experiments. Fig. 3 (b) illustrates evaluated cohesive law parameters for fracture energy J c , cohesive strength σ 0 and cohesive stiffness k. There was no convincing dependency of cohesive stiffness on crack opening displacement observed. Therefore, cohesive stiffness k = (25 ± 3.3) N/m 3 of the adhesive joint is assumed to be constant over the investigated range of crack opening velocities. This confirms the previously stated assumption, that the elastic region of traction-separation-laws is not affected by loading rate. The analytical approaches � � �� �� � ������ � � � �� (4) � � � �� � ��� ��� � � � � �� (5)

Fig. 3. (a) J over u for various constant controlled v ; (b) Evaluation of characteristic parameters for cohesive law with analytical functions.