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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3. Results and Discussion

Fatigue experiments are performed on DCB specimens where a block loading scheme is applied to the specimen as elaborated in Section 2.2. The crack length is determined using images captured during the fatigue test at regular intervals. The load-displacement data from the servo-hydraulic machine is also recorded along with the images. Using these data in the Kanninen-Penado model formulation, the SERR in the specimen is calculated. The FCGR at the adhesive-steel interface is shown in Fig. 4, where the shaded red region is the range of critical fracture toughness G I c as obtained by quasi-static tests performed on identical DCB specimens (Saleh et al., 2020, Report D1.3.2). The equation of the fitted line (power law) is the da dN = 3 . 86 × 10 − 21 ( ∆ G ) 5 . 91 .

Fig. 4: The fatigue crack growth rate curve for a DCB specimen with an initial crack at the adherend-adhesive interface.

3.1. Validity of the Kanninen-Penado model

The Kanninen-Penado model has been developed for a DCB specimen where the initial crack is located at the plane of symmetry and the crack tip is loaded under pure mode I condition. However, the DCB specimen in this research has a crack at the steel-adhesive interface and this introduces asymmetry at the crack tip. In order to verify the validity of the Kanninen-Penado model for the current DCB specimen configuration, the length of the process zone was calculated from Eqn. 4 and compared with the length obtained from the strain patterns obtained using the DIC technique. Additionally, these strain patterns at the crack tip are analysed to evaluate the loading condition (i.e., is it mode I dominated?) at the crack tip. 3.1.1. Process zone length The process zone length is defined as the length beyond the crack tip over which positive peel stresses are dis tributed and is dependent on the thickness and Young’s modulus of the adhesive and the adherend materials. DIC analysis does not give information regarding stresses but rather displacement and strain fields on the surface of the test specimen. With the knowledge that positive stresses result in positive strains, the length from the crack tip over