Abstract
The aim of the study is to investigate the mechanics of additively manufactured polymers. Initially, an experimental study is performed to investigate the dynamic fracture behavior of additive manufactured Acrylonitrile Butadiene Styrene (ABS). A single edge notched bending specimen with three orientations, namely horizontal builds with 45°/-45° (H45),0°/90° (H90) raster orientations, and vertical builds with layers perpendicular to the pre-crack(V0) are considered for this study. In addition, a novel toughening mechanism is explored by changing the surface topology to deflect the crack paths. Fracture initiation toughness is increased by 138% for a V0 specimen configuration compared to H90. Introducing a surface pattern to the specimen increases the fracture toughness by 58% as compared to control specimens. Additionally, higher fracture initiation toughness is achieved by increasing the size of the pattern and the changing the shape of the pattern. Later, an experimental investigation is performed to observe the electro-mechanical response of carbon black/ Acrylonitrile butadiene styrene additively manufactured composites under quasi-static (tensile, shear, and mode-I fracture) and dynamic (mode-I fracture) loading conditions for the potential in situ damage sensing. A modified four-point probe technique coupled with a high-resolution data acquisition system is employed to obtain the real-time electrical response. In the case of tensile loading, +45o/-45oprinted specimens show a nonlinear change of electrical response due to a nonlinear failure mode. Under the shear loading, the electrical response remains unchanged during the elastic deformation. Filaments debonding is the major failure mode for 0oprinted specimens under both tensile and shear loading. For mode-I fracture under both static and dynamic fracture loading, a minimal change of electrical response is observed before crack initiation due to the cancellation effect of the tension and compression on both sides of the neutral surface. To understand the mechanics of multi-materials, a comprehensive experimental investigation is performed to observe the interfacial fracture toughness of bi-material additively manufactured composites. Six different process parameters are considered to investigate the influence of their effect on fracture toughness. Due to the increase of crystallinity of the polymer, the fracture toughness decreases by about 40% when the bed temperature increases from °60 C to 100 °C. However, with improved molecular diffusion, the fracture toughness is enhanced by95% with the increase of the printing temperature. The printing speed has no significant impact on fracture toughness. For the effect of layer height, thinner layers provide a better bond strength and polymer wetting, resulting in a higher fracture initiation toughness compared to thicker layers. Finally, an experimental investigation is performed to observe the effect of postprocessing heat treatment on the interfacial fracture toughness of bi-material additively manufactured semi-crystalline polymer composite. An asymmetric double cantilever beam and single-leg bending specimens made of polylactic acid and Nylon are considered for the mode-I and mixed-mode fracture characterization, respectively. Specimens are isothermally heated in a forced convection oven for a wide range of temperatures and durations. It is observed that fracture toughness decreases significantly for both mode-I and mixed-mode conditions when specimens are annealed below the 160 °C. An increase of the crystallinity at the high temperature annealing prevents the polymer chain mobility, hinders the neck growth, and provides poor intermolecular diffusion resulting in fracture toughness decreased by 88% as compared to the unannealed specimen. Annealing at the 160 °C improves the bi-material interfacial fracture toughness by a maximum of 1225% via sufficient interfacial wetting, higher molecular diffusion, and a longer polymer chain entanglement process.