Amanda Wu

First-principles calculations within the density-functional-theory (DFT) approach are conducted in order to explore and explain the effect of small amounts of titanium on phase stability in the U-6Nb alloy. During rapid quenching from high to room temperature, metastable phases α′ (orthorhombic), α″ (monoclinic), and γ0 (tetragonal) can form, depending on Nb concentration. Important mechanical properties depend on the crystal structure and, therefore, an understanding of the effect of impurities on phase stability is essential. Insights on this issue are obtained from quantum-mechanical DFT calculations. The DFT framework does not rely on any material-specific assumptions and is therefore ideal for an unbiased investigation of the U-Nb system.

Silicone elastomers have broad versatility within a variety of potential advanced materials applications, such as soft robotics, biomedical devices, and metamaterials. A series of custom 3D printable silicone inks with tunable stiffness is developed, formulated, and characterized. The silicone inks exhibit excellent rheological behavior for 3D printing, as observed from the printing of porous structures with controlled architectures. Herein, the capability to tune the stiffness of printable silicone materials via careful control over the chemistry, network formation, and crosslink density of the ink formulations in order to overcome the challenging interplay between ink development, post-processing, material properties, and performance is demonstrated.

Direct ink writing enables the layer-by-layer manufacture of ordered, porous structures whose mechanical behavior is driven by architecture and material properties. Here, we incorporate two different gas filled microsphere pore formers to evaluate the effect of shell stiffness and Tg on compressive behavior and compression set in siloxane matrix printed structures. The lower Tg microsphere structures exhibit substantial compression set when heated near and above Tg, with full structural recovery upon reheating without constraint. By contrast, the higher Tg microsphere structures exhibit reduced compression set with no recovery upon reheating. Aside from their role in tuning the mechanical behavior of direct ink write structures, polymer microspheres are good candidates for shape memory elastomers requiring structural complexity, with potential applications toward tandem shape memory polymers.

Direct ink writing enables the layer-by-layer manufacture of ordered, porous structures whose mechanical behavior is driven by architecture and material properties. Here, we incorporate two different gas filled microsphere pore formers to evaluate the effect of shell stiffness and T g on compressive behavior and compression set in siloxane matrix printed structures. The lower T g microsphere structures exhibit substantial compression set when heated near and above T g , with full structural recovery upon reheating without constraint. By contrast, the higher T g microsphere structures exhibit reduced compression set with no recovery upon reheating. Aside from their role in tuning the mechanical behavior of direct ink write structures, polymer microspheres are good candidates for shape memory elastomers requiring structural complexity, with potential applications toward tandem shape memory polymers. The 3D printing process employed in this work, also known as direct ink writing (DIW), enables the layer-by-layer manufacture of ordered, porous structures whose mechanical behavior is driven by architecture and material properties. We pursue hierarchical porosity as a means of lightweighting, tailoring mechanical response and introducing functionality into 3D printed silicones. Hierarchical porosity is achieved by a combining printed structural porosity with intrastrand porosity, obtained by adding hollow, gas-filled microspheres to the ink. Aside from their role in tuning the mechanical behavior of 3D printed architectures, polymer microspheres are good candidates for shape memory applications requiring structural complexity with the ability to achieve both open or closed cell porosity. Here, for the first time, we demonstrate that shape memory can be achieved in 3D printed porous elastomers simply by the addition of polymer microspheres with controlled shell glass transition temperatures. Process development and fabrication of stochastic elastomeric foams is driven by diverse applications requiring advanced structural performance facilitated by both closed cells (e.g., shock absorption, acoustic damping and thermal insulation) and open cells (e.g., biocompatible membranes, tissue engineering scaffolds, semiper-meable membranes for materials separation and food processing) 1–6. This application space has benefitted from structural control, enabled by a family of emerging technologies, broadly known as 3D printing. Recently, 3D printing of silicones has been used to create mechanical energy absorbing materials with negative stiffness 7 , vas-cularized tissue constructs 8 , stretchable sensors 9 , soft robotics 10 , and shape morphing materials 11. These advances are made possible by the flexible and stretchable nature of silicone elastomers, combined with the unique structural and compositional control enabled via 3D printing. Applications benefitting from structurally engineered porosity accommodated by 3D printing include engineered tissue scaffolds 12 , photolithographic patterned nanowire growth for tailored electronics 13, 14 , and nanoli-thography metamaterials with a negative refractive index for cloaking and superlensing applications 15 , engineered with unit cells smaller than the wavelength of light 16. In addition to their predictability, repeatability and potential for architectural complexity, ordered porous structures are desirable over stochastic foams from a long-term mechanical performance standpoint, due to their minimization of local stress concentrations which can result in localized material failure 17. Further spatial and temporal control can be achieved by 3D printing with shape memory polymers 18. Since their development, in the 1960's 19 , polymers with shape memory behavior 20 have found applications in self-repairing components 21 , high performance textiles 22 , and surgical medicine 23. More recent advancements in

ABSTRACT Additive manufacturing (AM) technology provides unique opportunities for producing net-shape geometries at the macroscale through microscale processing. This level of control presents inherent trade-offs necessitating the establishment of quality controls aimed at minimizing undesirable properties, such as porosity and residual stresses. Here, we perform a parametric study into the effects of laser scanning pattern, power, speed, and build direction in powder bed fusion AM on residual stress. In an effort to better understand the factors influencing macroscale residual stresses, a destructive surface residual stress measurement technique (digital image correlation in conjunction with build plate removal and sectioning) has been coupled with a nondestructive volumetric evaluation method (i.e., neutron diffraction). Good agreement between the two measurement techniques is observed. Furthermore, a reduction in residual stress is obtained by decreasing scan island size, increasing island to wall rotation to 45 deg, and increasing applied energy per unit length (laser power/speed). Neutron diffraction measurements reveal that, while in-plane residual stresses are affected by scan island rotation, axial residual stresses are unchanged. We attribute this in-plane behavior to misalignment between the greatest thermal stresses (scan direction) and largest part dimension.

... Amanda S. Wua, Tsu-Wei Chou*b, John W. Gillespie Jr.a, David Lashmorec and Jeff Riouxc. a Center for Composite Materials, University ... 36, ZF Zhang, QW Li, TG Holesinger, PN Arendt, JY Huang, PD Kirven, TG Clapp, RF DePaula, XZ Liao, YH Zhao, LX Zheng, DE Peterson ...

The practice of upgrading metal parts with composites in large structures has led to an increased use of composite joints, particularly mechanical fastenings, due to ease of assembly and replacement. A drawback of mechanical joints is that damage is difficult to detect visually. In this research, an embedded carbon nanotube network has been used to modify the conductivity of bolted

ABSTRACT This investigation into the rate-dependent tensile behavior of carbon nanotube (CNT) fibers provides insight into the role of strain rate and specimen gage length on tensile strength. Chemical vapor produced CNT continuous fibers made of single and dual wall CNTs are evaluated and the potential for fiber improvement by post-process stretching to improve alignment is explored. Post-processed CNT fibers exhibit significantly higher strengths (3–5 GPa) and moduli (80–200 GPa) than untreated fibers. During dynamic tension evaluation, real-time electrical measurements provide correlations between high rate deformation/damage mechanical behavior and electrical resistance of the fiber specimens. Furthermore, this first look into the dynamic tensile behavior of CNT fibers demonstrates their potential to serve as sensors in high rate applications.

ABSTRACT A major challenge in the damage assessment of materials under dynamic, high strain rate loading lies in the inability to apply most health monitoring methodologies to the analysis and evaluation of damage incurred on short timescales. Here, we present a resistance-based sensing method utilizing an electrically conductive carbon nanotube film in a fiberglass/vinyl ester composite. This method reveals that applied strain and damage in the form of matrix cracking and delamination give rise to electrical resistance increases across the composite specimen; these can be measured in real-time during high strain rate loading. Damage within the composite specimens is confirmed through pre- and post-mortem x-ray micro computed tomography imaging.