PSI - Issue 37

Shirsha Bose et al. / Procedia Structural Integrity 37 (2022) 131–138 Bose et al., 2021/ Structural Integrity Procedia 00 (2019) 000 – 000

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and load-bearing capability. Such devices are multi-layered structures, with a stretchable or flexible base polymer (e.g., polyethylene terephthalate, polyether sulfone), a middle adhesive layer and a topmost conductive metallic layer. Due to the mismatch in the stiffness and layer thickness of the softer base polymer and the metallic layers, a tendency to exhibit interfacial delamination was observed in stretching of such system (Xu et al., 2010). Other mechanical factors responsible for the interfacial delamination of the polymer-metallic interface include wrinkling and buckling (Pan et al., 2014). Generally, flexible electronics should sustain large deformations (>>1%) without exhibiting failure in their mechanical stability (Lucchini et al., 2016). Still, several studies applied strain levels limited to ~1% in computations (Liao et al., 2017; Lin et al. , 2020; Kang et al., 2021), basically limiting their physical deformation at low failure strain. Other studies employed strain levels in the range of 2.5-5% to predict the interfacial delamination between the stiffer metallic layers and softer substrates (Peng et al., 2011; Li et al., 2019). However, it is desirable to apply high strain levels (10%) while simulating super-stretchable polymeric substrates such as silicone exhibiting a failure strain >100% (Graz et al., 2011). Collagen film in dry state has a failure strain of about 23.5% (Bose et al., 2020a), which is comparatively low as compared to the super-stretchable polymers. Previous studies showed the tremendous change in the mechanical properties (such as strength, modulus and failure strain) of collagen film tested in wet environment (Bose et al., 2020a; 2020b). Thus, it might be reasonable to consider the effect of wet condition during the preparation and characterisation of collagen-based flexible sensors. Though the researchers were successful in fabricating collagen-based flexible electronics, still, there is a lack in the understanding of the adhesion at collagen-metallic interfaces. Thus, this study focusses on the interfacial behaviour between collagen and metallic layers to predict performance of systems with two different geometries. This paper first provides details on the 3D model developed in the finite-element software Abaqus Simulia 2018, followed by results and discussions. A brief overview of the effect of hydration is also included. 2. Model description To analyse the interfacial delamination between collagen and metallic layers, a 3D computational model was formulated in the commercially available finite-element software Abaqus CAE 2018. 2.1. Model dimensions Two different geometries were considered for a better understanding of the interfacial behaviour of collagen metallic system. The schematic of the square and circular micro-islands were used from the work of Moreno et al. (2015) and are shown in Fig. 1. These micro-islands are multi-layered structures with the same layer arrangements: the bottom layer consists of collagen film as substrate (indicated in grey, 50 µm and 100 µm thickness in case of dry and wet environment, respectively), whereas, the metallic layers of chromium (middle-layer in blue, 30 nm) and gold (top-layer in yellow, 170 nm) are used as the adhesive and conductive layers, respectively. 2.2. Material formulations Elastic-perfectly plastic behaviour was assigned to the metallic layers of gold and chromium as these are very thin with insignificant strain-hardening behaviour. The focus was on the yield strength in these layers. The collagen substrate was modelled as hyperelastic material with Ogden model (N = 3), material parameters obtained from Bose et al. (2020) for both environments. The material properties are presented in Table 1. The gold (slave) and chromium (master) interfaces were connected using a surface-to-surface tie constraint while a surface-based cohesive-zone model was used to model the interface of collagen- chromium as it acts as a “sticky” contact (chromium is an adhesive layer). The onset of damage is analysed in terms of the strength-based criterion (CSMAXSCRT). Another important damage paramter CSDMG signifies the failure of the elements from the bonded surface.