Materials at extremes

To fully characterize a material’s mechanical properties, one must study their response to a range of testing conditions. The high strain and strain-rate behavior of a material is of particular importance in the understanding of nonlinear viscoelastic properties, material failure, blast mitigation, and other such extreme phenomena. In order to investigate these properties, we utilize two techniques; the Laser-Induced Particle Impact Test (LIPIT) and laser-induced shock waves.  

Laser-Induced Particle Impact Testing (LIPIT)

High velocity ballistic impact phenomena range in scale from catastrophic planetary impact events, impacts related to explosives and firearms,  to micron-scale additive manufacturing. Microparticle impact often poses a problem by causing erosion, but can also find use in powder blasting and cold spray manufacturing.  In medicine and biology, ballistic microparticles are used for needle–free gene and drug delivery. Despite the wide range of applications, the dynamics of microparticle impact remain unexplored. While macroscale projectile impact has been studied in real time using high–speed imaging techniques, investigations of microscale impact have been limited to post–mortem analysis of impacted samples. For this purpose we have deloped an optical benchtop platform, the Laser Induced Particle Impact Test (LIPIT). 

LIPIT involves focusing a high-energy laser pulse onto the back face of a “launch pad”. Launch pads consist of spherical microparticles dispersed on an elastomer-coated metallic film. Ablation of the metallic film leads to ejection of a microparticle. Microparticles are launched with speeds reaching several kilometers per second. A multi-frame high-speed camera is used to capture the impact history with nanosecond temporal and micrometer spatial resolution. The experimental tool will be capable of guiding the modeling effort and supporting the design of novel high-performance materials through direct experimental validation.

Review paperVeysset, David, et al. "High-velocity micro-projectile impact testing." Applied Physics Reviews 8.1 (2021): 011319.

Fig. (i) Schematic representation of LIPIT. A picosecond laser pulse is focused on a launching pad on top of which micro-particles are deposited. Particles are accelerated upon laser ablation to supersonic speed (3 km/s). (a) Multi–frame sequences with 5 ns exposure time showing single–projectile impacts on (a) a protein-base hydrogel, (b) a poly(urethane urea) (PUU) elastomer, and (c) an aluminum substrate. (a) Impact of a silica sphere (7.4 um diameter) at 500 m/s followed by projectile penetration. (b) Impact of a silica sphere (7.4 um diameter) on PUU at 670 m/s followed by projectile rebound. (c) Impact of a aluminum particle (40 um diameter) at 800 m/s followed by particle adhesion.

Laser-induced shock wave studies

Shocks are stress waves associated with high-pressures and high-strain rates, characterized by a discontinuous jump in state variables across the “shock front”. Shock loading is associated with strain rates on the order of 106 to 109 s-1 and stresses exceeding several gigapascals. The study of shock waves is of importance in understanding the effects of extreme phenomena, including blast-waves from explosive materials and primary traumatic brain injury as well as in the generation of exotic states of matter through strain. 

Our laser-induced shock wave platform serves as a benchtop, all-optical technique for the study of shock wave induced phenomena. The technique involves focusing of high-energy laser pulses onto thin material layers (<100 μm) sandwiched between two glass slides. Absorption of the laser energy leads to rapid thermal expansion that generates a shock wave traveling laterally in the plane of the sample. Pressures are varied by tuning the input laser energy, with pressures of up to tens of gigapascals readily achieved. To date, this technique has been used to study high pressure generation in liquids and solids, pressure induced fracture and phase transitions, acoustic wave induced cell permeability for drug delivery, as well as shock induced cavitation in soft materials. 


Fig. (left) Cartoon representation of the laser induced shock wave technique. High energy pump pulses focused onto samples launch shock waves in the plane of the sample via rapid thermal expansion. Samples consist of thin layers of material sandwiched between two substrates.  (right) laser-induced shock wave in water, probed by femtosecond imaging. The dark region on the left hand side is the laser-excitation region. In this image, the excitation pulse was focused to a line, launching a planar shock wave traveling towars the right. 


The lab is located in NE47-580.

see below for more experimental details. Representative publications listed at bottom of page. 


Laser Induced Particle Impact Testing
Dynamic stiffening of polyurethanes and polyureas studied via LIPIT.  We show that dynamic stiffening and dynamic strengthening depend strongly on the segmental mobility of mobile-phase PTMO and that dynamic strengthening additionally depends on the fraction of rigidphase PTMO. The comparison of polyurethane and polyurea demonstrates that both the soft segment length and hydrogen bonding type strongly influence the impact response. The determinants of dynamic stiffening and strengthening identified in this study provide insight into the optimization of elastomeric materials
LIPIT has been used extensively to study metallic bonding. Insights from these studies have revealed the effect of jetting on metallic particle bonding. LIPIT can also be used to extract an effective dynamic yield strengh of materials. (Sun et. al. App. Phys. Lett. 2019)