PSI - Issue 68

Ihssane Kididane et al. / Procedia Structural Integrity 68 (2025) 358–364 Ihssane Kididane et al. / Structural Integrity Procedia 00 (2024) 000–000

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additional 5 mm in terms of a eq . While the plot on the left side of Fig. 5 looks unremarkable, the plot on the right gives more insight into the observed fracture behavior. Once a certain small amount of mode II load is applied, the portion of fracture energy in mode I drops to about half of its pure mode I value, while the total fracture energy remains constant. As the mode-mixity further changes towards mode II, the relationship between J Ic and J IIc appears to be linear. Fig. 6 correlates the fracture envelopes to the observed fracture patterns using the example of a typical sample.

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Fig. 5. Fracture envelopes for di ff erent adhesive layer thicknesses.

The initial mode I loading (see subsection 3.2) leads to perfect cohesive fracture within the adhesive layer. As soon as the mode-mixity changes towards mode II, the crack moves into the interface between substrate and adhesive, which is not an uncommon observation for structural adhesive joints (Marzi et al., 2009, 2011). Consequently, the fracture envelopes have to be treated again (ref. to sub-chapter 3.1) as properties of the interface. This observation further correlates with the drop in J Ic in Fig. 5 (right) caused by a change from cohesive to adhesive failure and the fact that no significant di ff erences were obtained between the individual layer thicknesses. It should be noted that care should be taken when comparing the data from Fig. 3 (left) with those from Fig. 5 (left), as the former are the maximum values that occurred during crack propagation.

mode I fracture δ 2 -> δ 2,0

mixed-mode I+II

Cohesive fracture inside adhesive layer

Adhesive fracture in the interface

Direction of crack growth

Fig. 6. Typical fracture pattern of a CMMB sample.

4. Summary

The aim of the work was to use a novel dual-actuator fracture test to determine the entire fracture envelope of a structural adhesive joint in mixed-mode I + II from a single experiment. This approach is intended to significantly reduce experimental the e ff ort by reducing the number of samples required. The method was used to determine the dependence of the mixed-mode I + II fracture envelope on the adhesive layer thickness. Furthermore, values obtained at a specific mode-mixity of φ = 41 ◦ should validate the method by comparing with SLB reference measurements from the literature. Fully cohesive failure of the joints only occurred under pure mode I loading. In mixed-mode, the cracks moved to the interface between the adhesive layer and the substrate and propagated there. This resulted in lower fracture