Characterizing the effect of print orientation on interface integrity of multi-material jetting additive manufacturing

Relatively few engineering devices and structures are monolithic, as combinations of materials are often needed to meet the necessary functionality, performance, weight, and cost requirements. Progress in additive manufacturing (AM) now allows multiple materials to be produced in a single manufacturing process, opening new opportunities for expeditiously achieving functional and performance targets. Just as interactions at interfaces have long been of interest in the area of adhesive bonding, similar issues need to be addressed for printed composite materials, including how print orientation may affect failure. In this study, acrylic photopolymers were printed in a multi-material jetting process to produce fracture specimens consisting of an elastomeric layer sandwiched between two stiffer strips. Findings are offered as contributions to the three process modeling challenge topics conveyed in a recent NIST/NSF Measurement Science Roadmap (Pellegrino, 2016), as follows: Specimen configuration guidelines for standards for validation, certification, and qualification of models/parts: Several test specimen configurations based on the double cantilever beam method were fabricated and evaluated to measure the fracture resistance of the assembled layers, leading to a recommended configuration that minimizes material usage and spurious dissipation. Understanding interfacial science of AM polymers: Substantial differences are reported in the measured fracture energy and locus of failure (though nominally occurring at or near material interfaces) as a function of the print direction and interface architecture. Non-equilibrium material/process measurements and models: All tests showed increased fracture energy at higher crack propagation rates; power-law relationships proved useful for comparing specimen configurations and describing this rate-dependent nature of the interface and materials involved. These studies, which additionally include encouraging preliminary results with graded multi-layers, also suggest opportunities for designing printed interfaces with improved performance and durability for multi-material AM products.