Compliance-Tailored Multilayers: Theory, Computation, 3D Printing and Testing

Abstract

Mitigation of stress concentrations in multilayered materials and structures, particularly at freeedges, represents a significant opportunity for performance enhancement of such systems. Undesirable edge effects can be mitigated by appropriately spatially tuning the material properties. Emergence of additive manufacturing (AM) technologies, especially multi-material 3D printing, offers the possibility of cost-effective automation of the fabrication process and provides greater flexibility for locally tailoring the material architecture in three-dimensions across scales. This study is therefore focused on theoretical and computational modeling, additive manufacturing and experimental evaluation of compliance-tailored multilayers. Multilayers having discrete and continuous changes in elastic properties are explored computationally and experimentally via 3D printing, considering different design configurations to illustrate the potential of stiffness-tailoring approach towards generating superior multi-material systems. The dependence of structural performance, focusing on strength while maintaining stiffness, experimentally reveals a 100 % increase in strength and a 70 % increase in toughness for stiffness-tailored designs in comparison with their homogeneous compliant counterparts. This performance increase is shown to be due to a reduction in peak peel and shear stresses/strains near the free-edges of the interlayer. The mechanics of interfacial stress-transfer through the stiffness-tailored interlayer is elucidated through a series of theoretical models for different planar and cylindrical multilayers adequately accounting for bending deformations based on a variational method. Material-tailoring strategies considering both continuous and discontinuous functions are identified for the optimal design of such systems. The results from the analytical models are validated with those from analogous finite element models. A probabilistic mechanics framework is also presented for predicting the behavior of multilayers under material uncertainty considering spatially varying stiffness as a stochastic field. The change in failure mode/mechanism, and delay in growth of multi-site cracks, due to tailoring as observed from experiments provides design opportunities to increase both strength and toughness, properties that are often mutually exclusive. Manipulating materials at ever smaller scales, in three-dimensions, allows for strain engineering through compliance-tailoring towards unparalleled mechanical performance, but also opens up new opportunities in fabrication and additional functional engineering.

Date of Award	Dec 2017
Original language	American English