Diagnostics and Models for Birthing Stress

Abstract

Understanding the stress development in fluids as they transition to solids is not well-understood. Computational models are needed to represent "birthing stress" for multiphysics applications such as polymer encapsulation around sensitive electronics and additive manufacturing where these stresses can lead to defects such as cracking and voids. The local stress state is also critical to understand and predict the net shape of parts formed in the liquid phase. In this one-year exploratory LDRD, we have worked towards a novel experimental diagnostic to measure the fluid rheology, degree of solidification, and the solid stress development simultaneously. We debugged and made viable a "first-generation" Rheo-Raman system and used it to characterize two types of solidifying systems: paraffin wax, which crystalizes as it solidifies, and thermoset polymers, which form a network of covalent bonds. We used the paraffin wax as a model system to perform flow visualization studies and did some preliminary modeling of the experiment, to demonstrate the inadequacy of the current modeling approaches. This work will inform an advanced fluid constitutive equation that includes a yield stress, temperature dependence, and an evolving viscosity when we pursue the full proposal, which was funded for FY20.