PSI - Issue 83

R.J.B. Rocha et al. / Procedia Structural Integrity 83 (2026) 187–195

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3. Results 3.1. Failure modes

The experimental tests showed different failures modes. Fig. 3 (a and b) refer to SLJ whereas Fig. 3 (c and d) refer to SJ. Fig. 3 (a and c) present a cohesive failure and (b and d) show an adherend failure. Cohesive failure is the desired outcome, as it demonstrates that the adhesive’s full load-bearing capacity has been mobilized.

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b)

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Fig. 3. Failure modes: a) Cohesive in the adhesive (SLJ PLA with 7752), b) adherend (SLJ ABS with 2015), c) cohesive (SJ PLA with 7752) and d) adherend (SJ PETG with 7752).

The CZM methodology enables assessing adhesive degradation by tracking the SDEG (stiffness degradation) parameter, which ranges from SDEG=0 (indicating undamaged material) to SDEG=1 (representing failure). Plastic deformation in the adherends is monitored using the PEEQ (equivalent plastic strain) variable. Numerically obtained joint failure was observed either as cohesive failure within the adhesive or as plasticization of the adherends, leading to tensile failure. Fig. 4 (a) illustrates the cohesive failure in the adhesive layer of the SLJ with PLA adherends ( L O =20 mm) bonded with 2015. With continued loading, SDEG rises and damage propagates toward the center of the adhesive layer, eventually resulting in adherend separation. During this process, the adherends experienced plasticization up to 6.3%, which, in this scenario, did not alter the primary failure mechanism in the adhesive. However, alternative outcomes may arise under different geometric or material conditions. Fig. 4 (b) presents adherend failure.

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b)

Fig. 4. Failure of the adhesive layer (a) and adherend failure (b).

Cohesive failures were often accompanied by plastic deformation of the adherends. For PLA joints, regardless of the adhesive type or L O , failure consistently initiated as cohesive within the adhesive at the overlap edges and then propagated throughout the adhesive layer. In contrast, ABS joints bonded with 2015 always failed in the adherends. Specifically, for L O =5 mm, plasticization extended across the entire adhesive layer, while for L O =10 and 20 mm, plastic deformation was confined to the overlapped ends. When using 7752 and L O =5 mm, cohesive failure occurred in the adhesive, but for L O =10 and 20 mm, failure shifted to the adherends at the overlap edges. In PETG joints,