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Tensile and shear strength tests of metal/metal and polymer/polymer joints featuring a new functionalized nanofiber/epoxy composite adhesive were conducted. Strength increase is not as high as we expected (only up to 30%) although we used GCNF-ODA reactive linkers to improve the interface. The moderate strength increase is due to high interfacial stress developed in nanocomposites because of the high stiffness property mismatch, and inefficient interfacial shear stress transfer through shear-lag mechanism. In order to design strong nanocomposite materials, continuous or at least aligned nanofibers/nanotubes should be employed.

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A comprehensive investigation of the fiber pushout test reveals that the existence of free-edge stress singularities hinder the accurate measurement of interfacial shear strengths. An integrated analytical, numerical, and experimental investigation is conducted to remove the free-edge stress singularities by modifying the edge design in a model fiber pushout test. The proposed interfacial joint angles are applicable to most composite material systems. The microdroplet test is recommended over the fiber pushout test since the convex shape of the microdroplet matrix makes it a natural choice.

An integrated experimental and numerical investigation was conducted for removing the free-edge stress concentrations in dissimilar material joints. A convex interface/joint design, inspired by the shape and mechanics of trees, allows for least stress concentrations at bi-material corners for most engineering material combinations. In-situ photoelasticity experiments on convex polycarbonate-aluminum joints showed that the free-edge stress concentration was successfully removed. As a result, the new design not only improves the static load transfer capacity of dissimilar material joints, but also yields more reasonable interfacial tensile strength evaluation. For convex polycarbonate-aluminum and PMMA-aluminum joint specimens, the ultimate tensile load increased up to 81% while the total material volume reduced by at least 15% over that of traditional butt-joint specimens with severe free-edge stress concentrations.