A Triaxiality-Dependent Constitutive Model For Predicting Damage-Induced Softening in 3D-Printed Soft Suture Layer

Adhesive interfacial layers are ubiquitous in both engineering and natural material systems. They are essential in governing the mechanical behaviors of these materials. Especially, in the field of biological and bio-inspired materials, soft interfacial layer plays very important role in mechanics of them. The rapid development of 3D printing enables high quality bonding between dissimilar materials, providing an opportunity to fabricate biomimetic composites with arbitrary morphology. Along with this new development, new challenges appear in constitutive modeling and the prediction of damage initiation and evolution of 3D printed materials. To meet this need in the field, the model and methodology developed in this thesis is the first effort made in constitutively modeling the damage initiation and evolution of 3D printed soft interfacial layer. In this thesis, by expanding the concept of virtual internal bond (VIB) theory to macroscale, a methodology is developed to use a strain energy based hyperelastic softening model to predict the constitutive behavior and damage evolution of hyperelastic adhesive layers with damage-induced softening under mixed-mode loading. A user subroutine (ABAQUS/VUMAT) is developed for numerical implementation of the model. 3D-printed wavy soft rubbery interfacial layer is used as a material system to verify and validate the methodology. The wavy morphology provides a mixed Mode I/II loading to the material inside the layer. The Arruda-Boyce hyperelastic model is incorporated with the strain energy based softening model to capture the nonlinear pre-and post- damage behavior of this material under mixed Mode I/II loads. The model is able to accurately predict the overall damage-initiation and evolution of the 3D-printed rubbery interfacial layer under mixed-mode loading without any pre-defined failure criteria. To characterize the material model parameters of the 3D-printed rubbery adhesive layer, a series of modified scarf-joint specimens were designed, which enables systematic variation of the mixed mode loading condition via a single geometric parameter, the slant angle. Beaks and butterfly geometries were introduced to effectively reduce the stress concentration in the scarf-joint specimens. To verify model prediction, mechanical experiments of both compact tension and uni-axial tension are performed on 3D-printed biomimetic suture specimens with different morphology and material combination. By applying the material model developed, FE simulations were performed to compare with the experiments. To explore the strain rate effects of 3D-printed soft interfacial layers, a visco-hyperelastic softening model is developed. To characterize the model parameters, a series of uniaxial tension tests were performed under various loading rate (0.001/s-0.1/s). The proposed visco-hyperelastic softening model is further verified by the uniaxial tension experiments on 3D printed sutures with sinusoidal wavy interfacial layer