PSI - Issue 54

Rahul Iyer Kumar et al. / Procedia Structural Integrity 54 (2024) 164–171 Iyer Kumar, De Waele / Structural Integrity Procedia 00 (2023) 000–000

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Fig. 1: DCB specimen with discontinuity at the steel-adhesive interface

Fig. 2: A schematic representation of the DCB specimen and the piano hinges through which fatigue load P is applied to the specimen

2.2. Experimental procedure

As previously stated, there is no standard available that describes the experimental procedure for testing DCB specimens with metal adherends and thick adhesive bondline. The standard ASTM D6115 − 97 ’Standard Test Method for Mode I Fatigue Delamination Growth Onset of Unidirectional Fiber-Reinforced Polymer Matrix Composites’ is used as a guideline for the current research. It recommends applying fatigue load under displacement control and the maximum cyclic displacement ( δ max ) applied during the test is given by Eqn. 2, where ( δ crit ) avg is the average critical displacement for quasi-static delamination growth. G I c is the corresponding critical strain energy release rate (SERR) under mode I loading. This results in δ max being approximately equal to 70 . 71%( δ crit ) avg . δ 2 max ( δ crit ) 2 avg = G I max G I c = 0 . 5 (2) Quasi-static tests on identical DCB specimens were performed at TUDelft who are one of the QUALIFY project partners (Saleh et al., 2020, Report D1.3.2). The force-displacement curve of the tests shows an initial positive linear relationship followed by a marked decrease in force. This discontinuity in the force-displacement curve is considered to be the critical point at which the crack starts to propagate. Averaging the corresponding displacement values of all specimens yields the value of ( δ crit ) avg . The standard also advises that the displacement-controlled fatigue test be stopped if either the compliance of the specimen reaches a certain value or if the desired maximum number of cycles has been reached. The fatigue test can then be followed by a residual quasi-static test. Due to the limited number of test specimens, it was resolved to employ force-controlled fatigue loading to construct the fatigue crack growth rate (FCGR) curve. Additionally, it was also resolved to implement a block-loading fatigue scheme where the load on the specimen is increased after a certain number of cycles, similar to the test procedure employed in our previous works (Iyer Kumar et al., 2021, 2022). A block-loading fatigue scheme gives the flexibility of starting at a low load close to the threshold SERR G I th and also having enough data points in the stable crack growth region. Depending on the loads and specimen there is a possibility that also a few points in the unstable crack growth region are obtained. Similar to the procedure described in the standard, here, the average value of the critical force ( F crit ) avg at which the crack starts to propagate during