One of the many advantages resulting from the advent of three-dimensional (3-D) printing, otherwise known as additive manufacturing (AM), is that it makes possible the fabrication of highly complex structures that were not possible with traditional techniques. For example, lattices for use as waveguides or similar functions can be created with customized properties. However, a good understanding of the dynamic wave propagation behavior in these novel and complex structures is necessary for their optimal use in practical applications. A quartet of researchers conducted a series of in situ experimental observations of wave propagation in AM-fabricated polymer lattice structures using high-speed phase contrast x-ray imaging at the U.S. Department of Energy’s Advanced Photon Source (APS), complemented by direct numerical simulations. Further similar work should open the path to practical applications of these unconventional materials.
Recent work revealed that when subjected to dynamic compression, which creates mechanical shock waves that propagate throughout the material, polymer lattices display an elastic precursor wave in addition to plastic waves. Because of the complex structure of such lattices, including the presence of free surfaces within them, this mechanical shock wave behavior cannot easily be compared to waves in bulk solids. These complicating factors led to additional questions regarding the behavior of the elastic precursor wave under various impact conditions, including its speed, decay, and propagation distance.
To investigate these issues more closely, the research team from the Lawrence Livermore and Los Alamos national laboratories tracked the evolution of the elastic precursor wave in a 4 x 8 x 12 octet AM-fabricated polymer lattice using x-ray phase contrast imaging at the Dynamic Compression Sector 35-ID x-ray beamline at the APS (the APS is an Office of Science user facility at Argonne National Laboratory). To induce dynamic compression of the samples, two types of gas-gun driven flyers were used: PMMA and Al-6061. The researchers compared their experimental results with direct numerical simulations.
Elastic precursor waves are transmitted atom by atom through a lattice, and although they can travel faster overall than plastic waves, the pressure they transmit is much lower than plastic waves evidenced by their low particle speeds. If dynamic compression is very strong, elastic waves may be overtaken by plastic waves (which are pressure dependent), a condition known as “overdriving.” The elastic precursor wave is usually seen to decay significantly in bulk materials over a propagation distance of several millimeters, even with far higher pressure and amplitude in metals compared to polymers. This can make the characterization of elastic precursor waves quite challenging, especially when comparing to polymer lattices.
Nevertheless, the experimenters managed to measure the movement of lattice nodes with sufficient resolution to visualize the elastic precursor wave as it moved through the sample. They utilized five different impact conditions, with one condition close to the point of overdriving. The researchers observed an elastic precursor wave under four of the conditions (Fig. 1). This wave propagated through more than 10 unit cells without fully dissipating, contrary to behaviors seen in bulk solids or in granular and porous materials. No elastic precursor was seen under the near-overdriving condition, but the investigators note that this may be due to the resolution limits of the experimental setup.
Numerical simulations compared quite well with the experimental data and made it possible to substantiate the speed of the elastic precursor. The elastic wave was shown to be essentially independent of the impact conditions or the material properties of the lattice, and as in a bulk solid, is considered to travel at the sound speed in the material.
The research team notes that the presence of elastic and plastic waves at the same time in a particular material is a complex phenomenon to study, particularly in a unique structure such as the octet lattice examined in these experiments. However, the fact that this work demonstrated these kinds of classical mechanical phenomena in a novel AM structure was interesting and unexpected, and promises that a better understanding of the dynamic behavior of these unique materials is within reach. — Mark Wolverton
See: Jonathan Lind1*, Brian J. Jensen2, Matthew Barham1, and Mukul Kumar1, “In situ dynamic compression wave behavior in additively manufactured lattice materials,” J. Mater. Res. 34(1), 2 (January 14, 2019). DOI: 10.1557/jmr.2018.351
Author affiliations: 1Lawrence Livermore National Laboratory, 2Los Alamos National Laboratory
Correspondence: * lind9@llnl.gov
This work was performed under the auspices of the U.S. Department of Energy (DOE) by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and by Los Alamos National Laboratory under contract DE-AC52-06NA25396. This publication is based in part upon work performed at the Dynamic Compression Sector, which is operated by Washington State University under the DOE/National Nuclear Security Administration Award No. DE-NA0002442. This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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