Time-resolved x-ray diffraction (XRD) was used to examine the behavior of Ce under shock loading to stress states up to 22 GPa that span the shock-melt transition. Experiments reported here observed Ce held at a steady state for ∼500 ns prior to being uniaxially released to ambient pressure. Time-resolved XRD shows a constant diffraction pattern over the duration of the steady state with rapid solidification occurring on release. Cerium was found to remain crystalline as Poisson's ratio (ν) increases in the α-phase with incipient melt observed in XRD once ν reaches 0.5. Diffraction results along with sound speed measurements limit melt completion to be between 12 and 14 GPa, significantly lower than previously expected. The XRD results add confidence to previous methods used to define incipient melt and help to define a method to constrain the melt region along the Hugoniot independent of a light source.
Solid-state cold spraying (CS) of metals and respective blends is becoming increasingly attractive compared to conventional high temperature processes due to the unique properties such as increased yield strength, low ductility, and differences in tensile and compressive strengths that result from microstructural features due to the CS process. Here we report the results of plate impact experiments applied to CS deposits of tantalum (Ta), niobium (Nb), and a tantalum- niobium blend (TaNb). These methods allowed for definition of the Hugoniot for each material type and allowed for assessment of the Hugoniot Elastic Limit (HEL). Scanning electron microscopy was used on recovered samples to characterize the fracture mechanism during spallation.
Researchers at Sandia National Laboratories have recently conducted a series of experiments on novel cold spray deposited materials to understand dynamic material properties at high strain rates.
Talley, Samantha J.; Branch, Brittany A.; Welch, Cynthia F.; Park, Chi H.; Watt, John; Kuettner, Lindsey; Patterson, Brian; Dattelbaum, Dana M.; Lee, Kwan S.
Cellular silicone reinforced with silica filler prepared using additive manufacturing (AM) have been used widely for vibrational damping and shockwave mitigation. The two most commonly printed cellular silicone structures, simple cubic (SC) and face-centered tetragonal (FCT) display distinctly different static and dynamic mechanical responses dependent upon structure. In this work, the relationship between filler size and composition with mechanical response is investigated using polydimethylsiloxane-based silicones filled with aluminum oxide, graphite, or titanium dioxide. SC and FCT structures of porous, periodic silicone pads were printed using new direct ink write (DIW) resin formulations containing up to 25 wt% of functional filler (TiO2, Al2O3, or graphite). All AM pads were characterized using mechanical techniques (DMA, compression). Dynamic compression experiments coupled with time-resolved X-ray phase contrast imaging were performed to obtain insights into role of filler interactions in the in situ evolution of shockwave coupling in these functional, periodic porous polymers.
With the recent advances in additive manufacturing (AM), long-range periodic lattice assemblies are being developed for vibration and shock mitigation components in aerospace and military applications with unique geometric and topological structures. There has been extensive work in understanding the static properties associated with varying topology of these lattice architectures, but there is almost no understanding of microstructural affects in such structures under high-strain rate dynamic loading conditions. Here we report the shock behavior of lattices with varying intrinsic grain structures achieved by post process annealing. High resolution 316L stainless steel lattices were 3D printed by a laser-powder bed fusion machine and characterized by computed tomography. Subsequent annealing resulted in stress-relieved and recrystallized lattices. Overall the lattices had strong cubic texture aligning with the x-, y- and z-directions of the build with a preference outside the build direction (z). The recrystallized sample had more equiaxed polygonal grains and a layer of BCC ferrite at the surface of the structure approximately 1 grain thick. Upon dynamic compression the as-deposited lattice showed steady compaction behavior while the heat-treated lattices exhibit negative velocity behavior indicative of failure. We attribute this to the stiffer BCC ferrite in the annealed lattices becoming damaged and fragmenting during compression.
Dattelbaum, Dana M.; Ionita, Axinte; Patterson, Brian M.; Branch, Brittany A.; Kuettner, Lindsey
The advent of additive manufacturing (AM) has enabled topological control of structures at the micrometer scale, transforming the properties of polymers for a variety of applications. Examples include tailored mechanical responses, acoustic properties, and thermal properties. Porous polymer materials are a class of materials used for shock and blast mitigation, yet they frequently possess a lack of structural order and are largely developed and evaluated via trial-and-error. Here, we demonstrate control of shockwave dissipation through interface-dominated structures prepared by AM using 2-photon polymerization. A fractal structure with voids, or free surfaces, arranged less than 100 μm apart, allows for rarefaction interactions on the timescale of the shockwave loading. Simulations and dynamic x-ray phase contrast imaging experiments show that fractal structures with interfaces assembled within a “critical” volume reduce shockwave stress and wave velocity by over an order of magnitude within the first unit cell.