Publications

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Time-dependent, viscoelastic responses of materials like polymers and glasses have long been studied. As such, a variety of models have been put forth to describe the behavior including simple rheological models (e.g. Maxwell, Kelvin), linear "fading memory" theories, and hereditary integral based linear thermal viscoelastic approaches as well as more recent nonlinear theories that are either integral, fictive temperature, or differential internal state variable based. The current work details a new LINEAR_THERMOVISCOELASTIC model that has been added to LAME. This formulation represents a viscoelastic theory that neglects some of the phenomenological details of the PEC/SPEC models in favor of efficiency and simplicity. Furthermore, this new model is a first step towards developing "modular" viscoelastic capabilities akin to those available with hardening descriptions for plasticity models in LAME. Specifically, multiple different (including user-defined) shift-factor forms are implemented with each being easily selected via parameter specification rather than requiring distinct material models.

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In this work, we investigated microstructural features of elastomeric foam with the goal of identifying descriptors other than porosity that have a significant effect on the macroscale mechanical response. X-ray computed tomography (XCT) provided three-dimensional images of several flexible polyurethane foam samples prior to mechanical testing. The samples were then compressed to approximately 80% engineering strain. Stereo digital image correlation was used to measure the three-dimensional surface displacement data, from which strain was determined. The strain data, which were calculated with respect to the undeformed coordinates, were then overlaid on the corresponding surface generated from XCT. Heterogeneities in the strain-field were cross-correlated with topological quantities such as pore size distribution. A statistically significant correlation was identified between the distance transform of the pore phase and strain fluctuations.

In this work, we applied the in-situ X-ray Computed Tomography (XCT) mechanical testing method that coupled the in-situ mechanical loading with the XCT imaging to study the damage mechanism of GMBs inside the Sylgard as the material was subject to mechanical loading. We studied Sylgard specimens with different volume fraction of GMBs to understand how they behave differently under compression loading and how the volume fraction of GMBs affect the Sylgard failure. Both high resolution (1.5 μm/voxel) and low resolution (10 μm/voxel) XCT imaging were performed at different loading levels to visualize the GMB collapse during the compression of Sylgard with different volume fraction of GMBs. Feret shape of GMBs were calculated from the high resolution XCT images to determine whether the GMBs were intact or fractured, as well as the relationship between the size distribution of GMBs and their Feret shapes. Through these quantitative analysis of the high resolution XCT data, we were able to understand how the size and volume fraction of GMBs affect their failure behavior. The Digital volume correlation (DVC) technique was applied to the low resolution XCT images to calculate the local deformation of Sylgard specimen, which enabled us to understand the different failure propagation and failure mechanisms of Sylgard with different volume fraction of GMBs.

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To elucidate the damage mechanisms in syntactic foams with hollow glass microballoon (GMB) reinforcement and elastomer matrices, in situ X-ray computed tomography mechanical testing was performed on syntactic foams with increasing GMB volume fraction. Image processing and digital volume correlation techniques identified very different damage mechanisms compared to syntactic foams with brittle matrices. In particular, the prevailing mechanism transitioned from dispersed GMB collapse at low volume fraction to clustered GMB collapse at high volume fraction. Moreover, damage initiated and propagated earlier in closely-packed GMBs for all specimens. Both of these trends were attributed to increased interaction between closely-packed GMBs. This was confirmed by statistical analysis of GMB damage, which identified a consistent, inverse relationship between the probability of survival and the local coordination number (Nneighbor) across all specimens.

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Silicone elastomer filled with glass micro balloons (GMB) is an elastomeric syntactic foam used in electronics and component packaging for encapsulation, potting, stress-relief layer, and electrical insulation purposes. Under mechanical loading, the reinforcing phase, namely the GMBs embedded in the elastomer matrix, may break or delaminate, leading to internal damage and macroscale stiffness degradation, which can alter the material's protective capacity against mechanical shock and vibration. The degree of damage is controlled by the loading history, delamination, and failure behavior of the GMBs. We investigate the GMB failure behavior in this work wherein we present an indentation experiment to measure the force required to fail individual GMBs that are either embedded in the elastomer matrix or adhered to the surface of an elastomer layer. The indentation apparatus is augmented with an inverted optical microscope to enable in situ imaging of the GMB. Failure modes for the embedded or non-embedded GMBs are discussed based on the morphology of the broken GMBs and the measured failure forces. We also measure the adhesion energy between the glass balloon and the elastomer, based on which the possibility of delamination between the GMB and the surrounding elastomer matrix during the failure process is evaluated. Our results can facilitate the development of a failure criterion of GMBs which is necessary for establishing a physics-based constitutive model to describe the macroscopic damage mechanics of elastomeric syntactic foams.

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In this report, we investigate how manufacturing conditions result in the warpage of moderate density PMDI polyurethane foam (12-50 lb/ft 3 ) when they are released from a mold. We have developed a multiphysics modeling framework to simulate the manufacturing process including resin injection, foaming and mold filling, gelation of the matrix, elevated cure, vitrification, cool down, and demolding. We have implemented this framework within the Sierra Mechanics Finite Element Code Suite. We couple Aria for flow, energy conservation, and foaming/curing kinetics with Adagio for the nonlinear viscoelastic solid response in a multi-staged simulation process flow. We calibrate a model for the PMDI-10S (10 lb/ft 3 free rise foam) through a suite of characterization data presented here to calibrate the solid cure behavior of the foam. The model is then used and compared to a benchmark experiment, the manufacturing and warpage over 1 year of a 10 cm by 10 cm by 2.5 cm foam "staple'. This component features both slender and thick regions that warp considerably differently over time. Qualitative agreement between the model and the experiment is achieved but quantitative accuracy is not. 2