U.S. Patent 5, 438, 402; Aug, 1995

A method for characterizing interfacial adhesion is through laser driven shocks of thin films. This is a fundamental technique that quantifies interfacial adhesion. In the laser spallation experiment (shown in figure to the right) developed to measure the tensile strength of a thin film interface, a 2.5 nanosecond (ns) long Nd:YAG laser pulse is impinged over a 3 mm-dia area on a 0.5 µm thick Al film sandwiched between the back surface of a substrate disc (25 mm diameter and 1 mm thick) and a 50 to 100 µm thick layer of SiO2. The melting- induced expansion of Al under confinement generates a compressive stress wave (with 1 ns rise time) directed towards the test coating, which is deposited on the substrate's front surface. The compression stress pulse reflects into a tensile wave from the coating's free surface and leads to its spallation (complete removal) at a sufficiently high amplitude. During this film separation process, the transient free surface velocity of the coating is continuously recorded using a state-of-the art optical interferometer, a schematic of which is shown in the figure to the right. Optical fringes corresponding to stress pulses of 1-2 ns rise-time and 16-20 ns total duration are recorded in a single shot mode, with a resolution of 5 ps! In addition, the patented design of the interferometer can handle reflection from optically rough surfaces.

Next, the recorded fringe information is reduced to transient free surface velocity profile v(t), and is used to calculate the normal interfacial tensile stress in the following manner. For a coating of density ρ and thickness h, the interface stress σ is calculable from the measured transient velocity v(t).Because of high strain rate loading conditions, all inelastic effects are largely suppressed during interface decohesion, and thus, the measured strengths are intrinsic and related directly to the atomic microstructure and chemistry of the interfacial region. This was exemplified by measuring the tensile strengths of Nb/sapphire interfaces whose structure and chemistry were characterized using high resolution transmission electron microscopy and modified by a combination of heat treatment and deposition of 5 to 40 Å thick interlayers of Cr and Sb. Thus, atomic-scale tailoring of the interfacial region for either maximizing or reducing adhesion can be accomplished. Using this technology, interfacial strengths in the range of 0.1 GPa (14.5 ksi) to 2.5 GPa (362 ksi) have been measured in a variety of engineering systems (paints, multilayer electronic devices, engines, tribology) involving metal, ceramic and polymeric coatings deposited on metal, semiconductor and ceramic substrates.

A laser spallation experiment has been developed to measure the strength of planar interfaces between a substrate and a thin coating (in the thickness range of 0.3-3 µm). In this technique a laser pulse of a high enough energy and a pre-determined duration is converted into a pressure pulse of a critical amplitude and width that is sent through the substrate toward the free surface with the coating. The reflected tensile wave from the free surface of the coating pries-off the coating. The critical stress amplitude that accomplishes the removal of the coating is determined from a computer simulation of the process. The simulation itself is verified by means of an interferometry process that is capable of mapping out the profile of the stress pulse generated by the laser pulse. Interface strength values ranging from 0.1 to 2.5 GPa have been determined for various thin film systems system.

The dynamic tensile strengths of various joints have been analyzed. One study included E-glass composite/polymer and polymer/steel interfaces within the E-glass composite/polymer/AL-6XN stainless steel joint were measured using a laser spallation technique. Values were obtained for the polymer/composite interface while a much higher value was obtained for the steel/polymer interface. Because of the transient nature of the stress pulse, the strain rate changes continuously as the interface stress builds up. A peak strain rate of 5 x 10⁵ s^-1 was estimated. The effect of moisture on the tensile strength of the E-glass/polymer interface was also examined. The effect was found to be minimal, with the tensile strength stabilizing after 30 days of exposure to a 90%RH, 50°C environment. When comparing the strengths of corresponding interfaces in an epoxy-bonded joint from a previous study, it was concluded that our polymer results in a much stronger and durable joint.

The deformation of metals subjected to ultra-high strain rates, in excess of 10⁷ s^-1, is associated with a variety of unique physical phenomena, not normally observed in metals deformed at lower strain rates. Additionally, nanostructured materials are now recognized to exhibit extreme strength, reversible plasticity and dislocation-free twin boundary motion. In this study, a combination of laser-generated shockwave experiments, TEM observations, and Molecular Dynamics (MD) simulation are used to study the dynamic response of several nanostructured fcc metals when loaded at extremely high strain rates. MD simulation conditions are remarkably close to those achieved experimentally, allowing for direct interpretation of collective atomic motion and associated defect structures. The combination of nano-geometry and nanosecond rise-time stress waves is able to deform the material at stress (50 GPa) and strain rate (in excess of 10⁷ s^-1) levels never accomplished before. Two different loading setups are shown below.

ch An apparatus and test procedure for fabrication and loading of single crystal metal nanopillars under extremely high pressures (>1 GPa) and strain rates (>10⁷ s^-1), using laser-generated stress waves, are presented. Single-crystalline Cu pillars (~1.20 µm in tall and ~0.45 µm in diameter) prepared via focused ion beam milling of Cu(001) substrates are shock-loaded using this approach with the dilatational stress waves propagating along the [001] axis of the pillars. Transmission electron microscopy observations of shock-loaded pillars show that dislocation density decreases and that their orientation changes with increasing stress wave amplitude, indicative of dislocation motion. The shock-loaded pillars exhibit enhanced chemical reactivity when submerged in oil and isopropyl alcohol solutions, due likely to the exposure of clean surfaces via surface spallation and formation of surface steps and nanoscale facets through dislocation motion to the surface of the pillars, resulting in growth of thin oxide films on the surfaces of the pillars.

An experimental study to bend FIB-prepared cantilevered single crystal Cu nanopillars of several hundred nanometers in diameter and length at ultrahigh strain rate is presented. The deformation is induced by laser-generated stress waves, resulting in local strain rates exceeding 10⁷ s^-1. Loading of nano-scale Cu structures at these extremely short loading times shows unique deformation characteristics. At a nominal stress value of 297 MPa, TEM examination along with selected area electron diffraction characterization revealed that twins within the unshocked Cu pillars interacted with dislocations that nucleated from free surfaces of the pillars to form new subgrain boundaries. MD simulation results were found to be consistent with the very low values of the stress required for dislocation activation and nucleation because of the extremely high surface area to volume ratio of the nanopillars. Specifically, simulations show that the stress required to nucleate dislocations at these ultrahigh strain rates is about one order of magnitude smaller than typical values required for homogeneous nucleation of dislocation loops in bulk copper single crystals under quasi-static conditions.

Our research began by introducing a polymer as an adhesive for joining E-glass and stainless steel substrates. A recipe was discovered that resulted in a joint stronger than one of its adhered. Experiments to characterize the intrinsic and total fracture energies of the joints under both static and dynamic loads have been studied. The effect of hygrothermal loads is also discussed and shows that joints with this polymer remain virtually undamaged by such loads. Next, ultrahigh strain rate constitutive behavior of our polymer is presented at peak strain rates of 10⁷ s^-1 under pure tension and combined pressure-shear and tension-shear loads by subjecting thin polymer layers to laser generated stress waves of several nanoseconds in duration. We also studied the application of the polymer in multilayer armor design by showing two unusual effects. First, a sample flipping effect, in which polymer/foam bilayer specimens show almost a 40% reduction in their impact resistance depending upon which face bears the impact, and a second, resonance effect, when thin polymer layers are sandwiched between acrylic and polycarbonate plates.

Our research on this polymer started nine years ago. In the beginning, the research was motivated by the desire to find a durable adhesive to join AL-6XN stainless steel plates and E-glass composite sections for lightweight ship construction that could withstand static and dynamic loads during ship operation. To that challenge, a polymer chemistry was designed that resulted in failures within the E-glass composite section, away from the joint, when the bonded specimens were separated using a double cantilever beam (DCB) experiment. Additionally it was shown that hygrothermal loads did not degrade the mechanical integrity of the steel/polymer/E-glass joints as their fracture energies were reduced from 1232 ± 15 J/m² to 1070 ± 35 J/m² when subjected to an accelerated 90% RH, 38°C environment for 30 days. In contrast, the fracture energy of the same joints constructed using the Hysol 9394 epoxy which is widely used in the aerospace industry underwent a 90% reduction from 950 J/m² to 88 J/m² when subjected to the same conditioning loads. To separate the size-related inelastic component of the fracture energy, DCB experiments were also carried out at the cryogenic temperature of 77 K to determine the intrinsic fracture energy value which can be used as a local failure criterion in large scale structural simulation of polymer joints.

Since these joints are expected to perform under blast loads, their ability of resist dynamic loads was also studied by measuring their tensile strength and fracture energy at extremely high rates of loading. This was accomplished by separating the joints using well-characterized laser-generated stress waves of several nanoseconds in duration. Because of the sharp rise-time of the stress pulse, failures in both experiments were accomplished at a peak strain rate approaching 10⁶ s^-1. Like the intrinsic fracture energy, these values can be used to set the local failure criterion in design simulations of polymer joints subjected to dynamic loads. To further aid the designer in determining the durability of these joints, these parameters were also measured in conditioned samples. Together with the chemical recipe, the measurements of both the intrinsic and total fracture energies under static as well as dynamic loads provide a complete set of parameters for designing and constructing durable polymer joints for large scale ship building and other structural applications. Following studies were also extended to measure the dynamic tensile strength and fracture energy of the polymer by essentially using the same procedures except that the polymer layer on the steel substrate was chosen thick enough such that the returning tensile wave caused cohesive failure within the polymer volume. An average tensile strength of 93.1±5 MPa at a peak strain rate of 1.67 x 10⁷ s^-1 and fracture energy of 6.75 J/m² at a peak strain rate of 1.2 x 10⁷ s^-1 were obtained.

After developing and characterizing polymer joints that were stronger than one of the substrates (E-glass composites), the project focus shifted to understanding the constitutive and fracture behavior of polymer at ultrahigh strain rates, exceeding 10⁵ s^-1. This was motivated by the need for developing polymer-based armors against weapon in modern warfare, such as shaped charges and explosively formed projectiles, which can attain speeds in the range of 2,500 to 9,000 m/s and therefore cause dynamic loading within 50 ns upon impact. Since behavior at such high rates of loading cannot be studied even with the most sophisticated plate-impact and split-Hopkinson bar setups, our goal was to first develop experimental procedures similar to those utilized in these tests and then study the polymer behavior at such rates under pure tension, combined tension-shear, and combined pressure-shear loadings. Two major effects were uncovered. First, it was demonstrated that the Time Temperature Superposition principle holds for the polymer even at strain rates as high as 10⁵ s^-1. This strain rate is two orders of magnitude higher than those reported by Zhao et al. (2008) for the similar phenomenon. This important finding suggests that blast simulations of large-scale structures and armors involving the polymer can be based on constitutive data gathered under quasi-static conditions and translated to higher rates using the TTS principle.

To study this, thin polymer layers were sandwiched between acrylic, glass, steel and Al plates and loaded with stress waves of 50 ns duration. The stress waves exiting the sections were compared with those entering the sections by the use of interferometry. An unusual resonance phenomenon was uncovered when the polymer layers were sandwiched between the acrylic and polycarbonate plates suggesting a substantial increase in the transmitted stress wave amplitudes contrary to a reduction that was expected. The expected results were however obtained when the polymer was sandwiched between Al and steel plates, each showing a dramatic reduction in the transmitted stress wave amplitude. Beside this ultrahigh rate testing, the effectiveness of the polymer in managing more traditional impacts in the low strain rate regime (10³ s^-1, or impulse loading over 1-50 ms range) were also studied by carrying out tests in a standard drop tower facility. To start, dense polymer sheets of 1-2 mm thickness were combined with commercially available foams to prepare bilayer specimens. A sample flipping effect was discovered which showed that the impact force was lower by up to 40% when the dense polymer layer was away from the striking impacter head. This observation, along with the discovered resonance effect, is very important to consider when designing polymer-based multilayer armors. Details pertaining to this research can also be found in Youssef & Gupta (2012) and Youssef & Gupta (2011).

Professor Gupta's interview regarding use of a UCLA developed polymer in football helmets:

Video of helmet (left) and polymer (right) impact testing at the Dynamic Research Institute (DRI):

Bacterial biofilm-related infections are a burden on the healthcare industry. The effect of laser generated shockwaves through polycarbonate, a flexible polymer, is explored for its ability to generate high peak stresses, and also for its ability to conform to complex wound surfaces. Shockwave pulses in Al coated polycarbonate substrates and a resulting peak stress of greater than 60 MPa was measured which should provide sufficient pressure to kill bacteria.

Reported mechanical properties of orbital connective tissue and fat have been too sparse to model strainstress relationships underlying biomechanical interactions in strabismus. We have performed rheological tests to develop a multi-mode upper convectedMaxwell (UCM) model of these tissues under shear loading. From 20 fresh bovine orbits, 30 samples of connective tissue were taken from rectus pulley regions and 30 samples of fatty tissues from the posterior orbit. Additional samples were defatted to determine connective tissue weight proportion, which was verified histologically. Mechanical testing in shear employed a triborheometer to perform: strain sweeps at 0.52.0 Hz; shear stress relaxation with 1% strain; viscometry at 0.010.5 s¹ strain rate; and shear oscillation at 1% strain. Average connective tissue weight proportion was 98% for predominantly connective tissue and 76% for fatty tissue. Connective tissue specimens reached a long-term relaxation modulus of 668 Pa after 1,500 s, while corresponding values for fatty tissue specimens were 290 Pa and 1,100 s. Shear stress magnitude for connective tissue exceeded that of fatty tissue by five-fold. Based on this data, a multimode UCM model was developed with variable viscosities and time constants, and a damped hyperelastic response that accurately described measured properties of both connective and fatty tissues. Model parameters differed significantly between the two tissues. Viscoelastic properties of predominantly connective orbital tissues under shear loading differ markedly from properties of orbital fat, but both are accurately reflected using UCM models. These viscoelastic models now facilitate realistic global modeling of EOM behavior in binocular alignment and strabismus.

Professor Gupta and a few of his graduate students took to using the lab's high speed video camera in order study the biomechanical effect of the axe-handle. Professor Gupta's report can be found here.