Multimaterial Topology Optimization of Adhesive Backing Layers via J-Integral and Strain Energy Minimizations

Strong adhesives often rely on reduced stress concentrations obtained via specific functional grading of material properties. This can be seen in many examples in nature and engineering. Basic design principles have been formulated based on parametric optimization, but a general design tool is still missing. We propose here the use of topology optimization to achieve optimal stiffness distribution in a multimaterial adhesive backing layer, reducing stress concentration at selected (crack tip) locations. The method involves the minimization of a linear combination of (i) the J-integral around the crack tip and (ii) the strain energy of the structure. This combination is due to the compromise between numerical stability and accuracy of the method, where (i) alone is numerically unstable and (ii) alone cannot eliminate the crack tip stress singularity. We analyze three cases in plane strain conditions, namely, (1) double-edged crack and (2) center crack, in tension, as well as (3) edge crack under shear. Each case evidences a different optimal topology with (1) and (2) providing similar results. The optimal topology allocates stiffness in regions that are far away from the crack tip, and the allocation of softer materials over stiffer ones produces a sophisticated structural hierarchy. To test our solutions, we plot the contact stress distribution across the interface. In all observed cases, we eliminate the stress singularity at the crack tip, albeit generating (mild) stress concentrations in other locations. The optimal topologies are tested to be independent of the crack size. Our method ultimately provides the robust design of flaw tolerant adhesives where the crack location is known.