MECHANICAL CHARACTERIZATION OF PRESSURE-SENSITIVE ADHESIVE (PSA) BONDED ASSEMBLY
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This study focuses on comprehensive empirical and mechanistic understanding of the mechanical behavior of adhesive joints bonded with pressure sensitive adhesives (PSAs). PSAs are capable of very large deformation. The stress-strain and creep behavior of such joints are complex due to constant competition between cavitation dynamics in the bulk and at interfaces; and fibrillation and nonlinear visco-plastic behavior of the PSA material. The behavior is further altered by the presence of flexible or semi-rigid carrier layers because they alter the stress field within the joint and also provide additional interfaces for sequential cavity nucleation and growth. These mechanisms are known to result in multiple phases and transitions in their stress-strain and creep curves. The number of transitions depends on the presence (or absence) of carrier layers and the severity of the secondary transitions depends on the flexural compliance of the carrier layers). The effective mechanical response of the PSA joint is therefore affected by this complex set of events during slow deformation process, including the final stage when the PSA starts to debond from the substrate and/or carrier layer. This morphological evolution of the PSA depends on the adhesive material properties, joint configuration (joint aspect ratio and presence/absence of carrier layers), bonding substrate surface properties (surface energy, roughness and presence of contaminants), carrier layer properties (surface energy, surface roughness and flexural rigidity) and loading conditions (loading rate, stress level and temperature). This study consists of experimentation and mechanistic modeling. In the experimental study, bonded PSA test specimens were fabricated for selected PSA/substrate combinations, after detailed parametric study to gain insights into the influences of the lamination conditions (bonding pressure, bonding time, bonding pressure, post-cure and aging protocols). The joint performance parameters of interest for this parametric study include (i) tensile strength, ductility and creep resistance; (ii) peak stress and peak strain; and (iii) number of transitions and severity of transitions. These specimens were subjected to mechanical tests on a dynamic mechanical analyzer (DMA) to measure stress-strain response and creep response for different loading conditions. In the modeling phase, mechanistic models are developed to provide fundamental insights about the dominant deformation mechanisms in PSA bonded assemblies This has the added advantage of reducing the enormous amount of physical testing that engineers would need to conduct to empirically characterize every PSA-substrate combination of interest over all the loading conditions of interest The predictive mechanistic model is based on enhancement of a simple ‘block’ model that has been proposed in the literature for simulating the stress-strain and creep behavior of the PSA/substrate at different loading conditions. This model acts as a virtual test, predicting the mechanical response of a PSA bonded assembly by explicitly accounting for the PSAs’ nonlinear visco-plastic material properties, cavity dynamics in the bulk and at the interfaces, fibrillation dynamics, and other system configurations such as bonding substrate surface properties and carrier layer properties. This model is shown to be able to predict the stress-strain and creep behavior of PSA bonded assemblies under a broad range of operating conditions, after proper calibration by a few corner cases of physical tests. The predictive model can become a virtual testing method that for real-time prognostic health management (PHM) for PSA bonded assemblies. Test equipment includes a commercially available Dynamic Mechanical Analysis (DMA), to conduct the constant speed stress-strain test and constant force uniaxial creep test on the sample of selected PSA bonded assemblies at selected loading conditions. An observation fixture is also designed for studying the morphological evolution of PSA layer by video recording the cavitation and debonding at the PSA-substrate interface during tensile deformation of a PSA bonded assembly. Complexity in the study includes: (i) structural change of PSA system due to cavitation and fibrillation; (ii) sequential cavitation and fibrillation due to additional interface introduced by carrier layer; (iii) joint parameter (material, configuration, surface roughness and surface energy); (iv) nonlinear rate-dependent plastic material properties of bulk PSA; and (v) implementation of new material model into commercial FEA tools.