Spatial programming of defect distributions to enhance material failure characteristics

Abstract

Defects play a major role in determining the mechanical properties of materials. Examples of this span from dislocations, grain boundaries, and precipitates in metallurgy to voids and imperfect interfaces in natural composites. These can enable complex failure modes and balance competing mechanical properties. With the increasing adoption of additive manufacturing in both research and industry, there are unprecedented opportunities for controlling the internal composition and structure of a material system. However, the very nature of forming a material in increments produces a set of characteristic defects (e.g., void formation due to incomplete merger of new material), which results in local heterogeneity and anisotropy. Rather than seeking to prevent these characteristic defects, we utilize them to improve the damage resistance of printed structures by systematically controlling their distribution within a single material. Inspired by the well-known Bouligand structure found in tough natural materials, we use a helical build sequence in which each layer is added at a defined pitch angle relative to the previous layer. The resulting helical defect distribution can guide the crack tip during fracture and enhance damage resistance. Microstructural evidence from crack surface images and X-ray microtomography reveals intricate mixing of twisting and branching cracks during failure, and is explained via an analytical model. Since these improvements are obtained solely from the defect distribution within a single material, the findings of this work could improve the failure characteristics of a broad range of printed materials (metals, ceramics, polymers, and composites).