PSI - Issue 42

652 Niklas Ladwig et al. / Procedia Structural Integrity 42 (2022) 647–654 Author name / Structural Integrity Procedia 00 (2019) 000 – 000 measuring window of the DIC system. The chosen test control depended on the selected mode ratio and the respective adhesive. In tests with dominant mode I components ( ≤ 45⁡° ), a constant displacement rate of ̇ = 0.2⁡mm/min in y -direction was introduced. With the mixed-mode ratio, which is defined in equation (2), the resulting portion is used to calculate the setpoint for . This set point was used to control the torsional actuator of the testing machine. In tests with dominant mode III components ( > 45° ) an angular displacement rate ̇ = ⁡0.3⁡°/min was introduced and the tensile actuator was controlled to the desired setpoint . 4. Results and discussion 4.1. Fracture envelope Figure 4 shows the evaluated fracture envelops in mixed-mode I+III for the two tested adhesives. To identify the crack initiation, the maximum in the axial force over the displacement of the test machine was used. This kind of proceeding differs from the one recommended in relevant standards, e.g. ISO25217 [1], but since an approach independent of crack length has been applied to calculate fracture energies, no effort has been undertaken neither to monitor nor to measure the current crack length. In addition, the sensitivity of the global specimen compliance matrix with respect to measured loads and displacements depends on the particular mode mix, which lead to the decision to assume the start crack of crack propagation at maximum force for the main reason of simplicity. However, no indication was observed that the particular choice of definition of the start of crack propagation would have a significant impact on the values given in figure 4. 6

Figure 4 - Fracture Envelope for both adhesives

Both adhesives show a significant increase in the critical energy release rate (ERR) close to pure mode III loading with an approximate factor of four in comparison to mode I. It seems that, whereas the brittle EP4510 exhibits an approximately constant increase in the ERR from 0 ° to approximately 75 °, the ERR remains relatively constant for the elastic-plastic SP498 between 0 ° and approximately 60 °. The effect of having a rising critical ERR close to mode III has already been reported by Loh and Marzi [8] and are well-known in case of mixed-mode I+II.