PSI - Issue 57

Cristian Bagni et al. / Procedia Structural Integrity 57 (2024) 859–871 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The stiffness drop failure criterion is more flexible compared to the other two options, since it allows the definition of failure to be tailored to the needs/requirements of each company and/or application, that are not necessarily always the same. For example, it is possible to correlate a given stiffness drop to the appearance of the first visible crack on the surface of the adhesive or on the surface of the adherends close to the mechanical joints, or to select a value of stiffness drop that guarantees a minimum allowable residual fatigue life. Furthermore, it is important to decide whether it is more appropriate to adopt the first or the second methodology proposed in Section 2, i.e. whether to consider the fatigue life of the hybrid joint as the life of the adhesive only, or as the sum of the life of both the adhesive and the mechanical joints. Once the methodology to adopt and the most appropriate failure criterion are chosen, it is possible to determine a series of load-life (LN) datapoints from the testing of hybrid joint specimens as representative as possible of the production parts and processes of interest. Focussing on the first methodology (or first step of the second methodology), the LN datapoints can be reverse engineered into stress-life (SN) datapoints by calculating the peel stress range, ∆σ peel , through linear superposition as follows: ∆ peel = peel_max_unit load ∙ ∆ = peel_max_unit load ∙ max ∙ (1− ) (5) where peel_max_unit load is the maximum peel stress, obtained from the FE model (with a unit load applied), at the centroid of the membrane shell elements wrapped around the solid elements modelling the adhesive (the element experiencing the highest peel stress is likely to corresponds to the location where failure should initiate). max is the maximum of the applied sinusoidal load for each test, and = min max ⁄ is the load ratio. Finally, through statistical analysis of the SN datapoints it is possible to derive mean (50% certainty of survival, 50% confidence interval) and design (e.g. 97.7% certainty of survival, 95% confidence interval) SN curves and corresponding parameters. At the time of writing, an extensive testing programme in collaboration with an electric vehicle manufacturer, NIO Performance Engineering Ltd, involving both lap shear and coach peel hybrid joint specimens, is underway at Hottinger Brüel & Kjær ’s ‘Advanced Materials Characterisation & Testing’ laboratory in the United Kingdom . To illustrate the process described above, some preliminary test data from the ongoing testing programme is presented. However, it is noted that this data represents a very limited part of the overall testing programme and is not sufficient to derive SN parameters. Therefore, it must be considered for illustration purposes only. The data presented below was obtained by testing lap shear specimens with self-piercing rivets under sinusoidal axial loading. The test set-up is shown in Fig. 6. Dimensions of the specimens and materials cannot be disclosed for confidentiality reasons.