Laser-generated Stress Waves Research Articles

A new BEM for modeling and simulation of 3T MDD laser-generated ultrasound stress waves in FGA smart materials

A new BEM for modeling and simulation of 3T MDD laser-generated ultrasound stress waves in FGA smart materials

Microstructure Evolution In Metal Nanostructures Under Extreme Conditions Of Temperature And Strain Rate

This paper explores a fundamental connection between ductility and domain size in metallic solids under extreme conditions of cryogenic temperatures and strain rates (10 8 s –1 ). A series of novel experiments, backed by multiscale modelling and transmission electron microscopy (TEM) analysis, are presented that involve loading of TEM-ready single crystal nanopillar samples of Cu of varying lengths (50 nm to 1 mm) and aspect ratios (50 nm to 100 nm in diameter) by laser-generated stress waves of sub-nanosecond rise times, under extreme conditions of strain rate (>10 8 s –1 ) and temperature (100K). The nucleation stress for Shockley partials, which can be taken as a proxy for the onset of ductile deformation, was measured to be only 1 GPa. This is an order of magnitude lower than the previously measured values of 35 GPain bulk geometries. TEM observations show remarkable ability of the material to re-arrange itself through motion of dislocations to form subgrain boundaries within a very short duration of only few nanoseconds.

Ultrahigh strain-rate bending of copper nanopillars with laser-generated shock waves

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.

In-situ measurement of solder joint strength in board-mounted chip-scale packages using a quantitative laser spallation technique

A previously developed laser spallation (LS) technique for measuring the interface strengths of planar films is modified to determine the interfacial tensile strengths of solder joints, in situ, in board-mounted chip-scale packages (CSPs). The new technique is demonstrated on packages with bare Cu pads and Pb-free solders. The solder/pad interface strength is determined by first using a laser-generated stress wave to separate the geometrically heterogeneous interface, in situ, and then quantifying the failure stress by a combination of interferometry and wave mechanics simulation. The solder to pad interfacial strengths were found to be 705 ± 102, 510 ± 71, and 369 ± 55 MPa for packages stressed by baking at 150 °C for 0, 48, and 100 h, respectively. The interface strengths were found to be roughly proportional to the critical laser energies for separation of solder joints. Thus, it may suffice to use the critical laser energy as a parameter for the purposes of material selection and solder joint quality inspection during manufacturing. The quantitative capability of the LS test for testing board-level packages now provides an opportunity to enhance the board-level drop test that is widely conducted in the industry today.

The influence of laser-induced nanosecond rise-time stress waves on the microstructure and surface chemical activity of single crystal Cu nanopillars

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.

Resonance in Polyurea-Based Multilayer Structures Subjected to Laser-Generated Stress Waves

We report resonance associated with polyurea layers sandwiched between identical acrylic and polycarbonate plates when subjected to laser-generated stress waves of several nanoseconds in duration. Transmitted stress wave amplitudes almost 16 times the incident stress wave amplitudes are observed. An elastodynamics simulation identified the thickness of the polyurea layer as the key parameter controlling the amplitude of the transmitted stress wave. This resonance effect was found absent when the polyurea was sandwiched between the steel and aluminum plates of similar thickness. However, for these samples, a dramatic reduction in the amplitudes of the transmitted stress waves was recorded. A finite element analysis of the wave propagation through the sandwiched polyurea layer in these samples tied the large amplitude reduction to the large acoustic impedance mismatch and the viscoelastic dissipation within the polyurea layer.

Dynamic tensile strength of polyurea

Abstract

Dynamic response of polyurea subjected to nanosecond rise-time stress waves

Shaped charges and explosively formed projectiles used in modern warfare can attain speeds as high as 30,000 ft/s. Impacts from these threats are expected to load the armor materials in the 10 to 100 ns timeframe. During this time, the material strains are quite limited but the strain rates are extremely high. To develop armors against such threats it is imperative to understand the dynamic constitutive behavior of materials in the tens of nanoseconds timeframe. Material behavior in this parameter space cannot be obtained by even the most sophisticated plate-impact and split-Hopkinson bar setups that exist within the high energy materials field today. This paper introduces an apparatus and a test method that are based on laser-generated stress waves to obtain such material behaviors. Although applicable to any material system, the test procedures are demonstrated on polyurea which shows unusual dynamic properties. Thin polyurea layers were deformed using laser-generated stress waves with 1–2 ns rise times and 16 ns total duration. The total strain in the samples was less than 3%. Because of the transient nature of the stress wave, the strain rate varied throughout the deformation history of the sample. A peak value of 1.1×105 s−1 was calculated. It was found that the stress-strain characteristics, determined from experimentally recorded incident and transmitted wave profiles, matched satisfactorily with those computed from a 2D wave mechanics simulation in which the polyurea was modeled as a linearly viscoelastic solid with constants derived from the quasi-static experiments. Thus, the test data conformed to the Time-Temperature Superposition (TTS) principle even at extremely high strain rates of our test. This then extends the previous observations of Zhao et al. (Mech. Time-Depend. Mater. 11:289–308, 2007) who showed the applicability of the TTS principle for polyurea in the linearly viscoelastic regime up to peak strain rates of 1200 s−1.

Dynamic fracture energy of polyurea-bonded steel/E-glass composite joints

We report dynamic fracture energy measurements for E-glass/polyurea/steel joints at a peak strain rate of 1.2 × 10 7 s −1. Experiments were done by initiating a crack at the steel/polyurea interface by a laser-generated stress wave, whose profile was recorded using optical interferometry. The critical energy release rate or the energy delivered by the propagating stress wave to the crack-tip region at crack initiation was computed by using a wave dynamics simulation. An average value of 359 (±19) J/m 2 was obtained. The effect of moisture on the fracture energy values was also examined. Results showed that the values reduced by only 3% to 349 ± 19 J/m 2 in samples that were exposed to 75% RH at 65 °C for 30 days. In addition to using the data as a local failure criterion in large-scale simulation of structural joints, it can be used to estimate the residual fracture energy of the E-glass/polyurea/steel joints in service.

Thermodynamic and stress analysis of laser-induced forward transfer of metals

On the basis of a two-temperature model taking into account the laser-generated stress waves, we have performed an analysis of heating and melting dynamics in different metallic films (Au, Zn, Cr) irradiated by femtosecond laser pulses for the experimental conditions on laser-induced forward transfer. The modeling results have allowed puzzling out why, even within the same family of materials, substances can be transferred in the different phase states under rather similar laser-irradiation conditions.

Glycosaminoglycan degradation reduces mineralized tissue–titanium interfacial strength

Although the localization of the proteoglycan/glycosaminoglycan (GAG) complex at the bone-titanium implant interface has been implied, the role of proteoglycans on the establishment of bone-titanium integration is unknown. The hypothesis to be tested was that proteoglycans play an important role in establishing bone-titanium interfacial adhesion. The objective of this study is to investigate the effect of proteoglycan knockdown by GAG enzymatic degradation on the interfacial strength between mineralized tissue and titanium having different surface topographies. Rat bone marrow-derived osteoblastic cells were cultured on either a machined titanium disk or an acid-etched titanium disk. At day 21 of culture, one of the three following GAG degradation enzymes was added into the culture; chondroitinase AC, chondroitinase B, or keratanase. After 3 days of incubation (at day 24 of culture), the laser spallation technique was applied to the samples in order to assess the tissue-titanium interfacial strength. In this technique, a laser-generated stress wave is used to separate the tissue-titanium interface, and the interfacial strength is determined interferometrically by recording the transient free surface velocity of the tissue. Mineralized tissue cultured on the acid-etched titanium showed 20-30% higher tissue interfacial strength than that cultured on the machined titanium (p < 0.0001). For both the machined and acid-etched surface cultures, administration of the enzyme reduced the interfacial strength by 25-30% compared with the untreated control cultures (p < 0.0001). There were no differences in the effect among the three different enzymes tested. A nanoindentation study revealed that the enzyme treatment did not affect the elastic modulus of the mineralized tissue. Scanning electron microscopic and energy dispersive spectroscopic analyses revealed less post-spallation tissue remnant on the titanium substrates when treated with the enzymes. The tissue remnant was greater in amount on the acid-etched surface than on the machined surface. The results suggest that there exists not only mechanical interlocking but also biological interfacial adhesion between the mineralized tissue and titanium, in which the proteoglycan/GAG complex is involved.

Study on the interface strength of zirconia coatings by a laser spallation technique

Tensile strength of interfaces between zirconia coatings and stainless steel substrates were measured using a laser spallation technique. In this technique, a laser-generated compression stress wave reflects into a tensile wave from the free surface of the coating and pries off its interface at a critical amplitude. Zirconia coatings were deposited using the gas tunnel-type plasma spraying process which was previously shown to result in denser and harder deposits compared with those produced by conventional spray methods. The interface strength was found to increase with a decrease in the spray distance and traverse speed, and with an increase in the traverse number, and coating and substrate thickness. The variation in the measured strength with these variables could be explained through their effect on controlling the temperature of the particles and that in the interfacial region. Higher temperature in the interfacial region during coating deposition is expected to increase adhesion by enhancing chemical and physical bonding mechanisms. Depending upon the processing conditions used, the tensile strength for the zirconia/steel interface system was found to vary anywhere between 206 and 501 MPa.

Mixed-mode failure of thin films using laser-generated shear waves

A new test method is developed for studying mixed-mode interfacial failure of thin films using laser generated stress waves. Guided by recent parametric studies of laser-induced tensile spallation, we successfully extend this technique to achieve mixed-mode loading conditions. By allowing an initial longitudinal wave to mode convert at an oblique surface, a high amplitude shear wave is generated in a fused silica substrate and propagated toward the thin-film surface. A shear wave is obtained with amplitude large enough to fail an Al film/fused silica interface and the corresponding shear stress calculated from high-speed interferometric displacement measurements. Examination of the interfaces failed under mixed-mode conditions reveals significant wrinkling and tearing of the film, in great contrast to blister patterns observed in similar Al films failed under tensile loading.

Glass-modified stress waves for adhesion measurement of ultra thin films for device applications

Laser-generated stress wave profiles with rarefaction shocks (almost zero post-peak decay times) have been uncovered in different types of glasses and presented in this communication. The rise time of the pulses was found to increase with their amplitude, with values reaching as high as 50 ns . This is in contrast to measurements in other brittle crystalline solids where pulses with rise times of 1 –2 ns and post-peak decay times of 16 –20 ns were recorded. The formation of rarefaction shock is attributed to the increased compressibility of glasses with increasing pressures. This was demonstrated using a one-dimensional nonlinear elastic wave propagation model in which the wave speed was taken as a function of particle velocity. The technological importance of these pulses in measuring the tensile strength of very thin film interfaces is demonstrated by using a previously developed laser spallation experiment in which a laser-generated compressive stress pulse in the substrate reflects into a tensile wave from the free surface of the film and pries off its interface at a threshold amplitude. Because of the rarefaction shock, glass-modified waves allow generation of substantially higher interfacial tensile stress amplitudes compared with those with finite post-peak decay profiles. Thus, for the first time, tensile strengths of very strong and ultra thin film interfaces can be measured. Results presented here indicate that interfaces of 185-nm-thick films, and with strengths as high as 2.7 GPa , can be measured. Thus, an important advance has been made that should allow material optimization of ultra thin layer systems that may form the basis of future MEMS-based microelectronic, mechanical and clinical devices.

A hydrophobic self-assembled monolayer with improved adhesion to aluminum for deicing application

A previously developed laser-generated stress wave procedure for measuring the tensile strength of ice/solid interfaces was used to study the effectiveness of a self-assembled monolayer of dimethyl- n-octadecilcholorsilane (DMOCS) in providing a hydrophobic surface to a 6061 Al alloy. In this experiment, a laser-induced compressive stress wave travels through a substrate disc that has a layer of ice grown on its front surface. The compressive stress pulse reflects into a tensile wave from the free surface of ice and pulls the ice/Al interface apart at a sufficiently high amplitude. The interface strength was calculated using a wave mechanics simulation, with the interferometrically recorded free surface velocity of a bare Al substrate (measured under identical ice spallation conditions) as an input. The tensile strength of the ice/DMOCS interface at −10°C was found to be 131 MPa, which is the lowest among all surfaces tested to date, which include, as-received Al surfaces (274 MPa), surfaces prepared with chemical and mechanical polishing (181 MPa), and PMMA-coated (190 MPa) and polyimide-coated (177 MPa) Al surfaces. The two methyl groups attached to the outer end of the carbon chain in the DMOCS coating provide a completely hydrophobic surface, while its silane group at the bottom most end reacts with the adsorbed hydroxyl groups on the aluminum surface to form a strong Si to Al covalent bond. Thus, DMOCS coatings present an appealing alternative to various presently available ice mitigating surface strategies, by providing a longer-lasting surface with the highest hydrophobicity possible.

Effect of humidity and temperature on the tensile strength of polyimide/silicon nitride interface and its implications for electronic device reliability

A new strategy for improving, predicting, and optimizing the mechanical reliability of interfaces appearing in electronic devices, substrates, and their packages, is introduced. An essential feature of this strategy is to measure the fundamental tensile strength of interfaces devoid of any material plasticity and geometry effects and quantify changes in the measured strengths by exposing interfaces to varying levels and duration of in-situ temperature rise and relative humidity. These fundamental strength charts can now to be used in conjunction with simulations capable of predicting time-dependent stress concentrations, moisture accumulation, and temperature rise at critical interfaces during processing and service, and thus, predict device reliability from a fundamental standpoint. Since these latter simulations are already well developed and largely available, implementation of the proposed strategy requires development of largely unavailable database on the degradation of the interfaces’ intrinsic tensile strengths. As a start, this paper presents such a data for the polyimide/Si 3N/Si interface system, which besides serving as an exemplar, has importance in device fabrication. The strength data is gathered using the laser spallation technique, in which a laser-generated stress wave on the backside of the substrate pries off the coating deposited on its front surface. This work extends this technique for multilayer testing. Interestingly, the degrading effect of each variable was found to fall in two separate regimes of moderate and strong effect. Uncovering of such transition zones is of obvious importance in reliability studies.

Observations of transonic crack velocity at a metal/ceramic interface

A system for measuring the interface crack velocity is presented, and applied to photolithographically-generated 90–100 μm long cracks between a thin (1 μm) Nb strip of 60 μm width and a sapphire substrate. The cracks were loaded using a laser-generated stress wave. The duration of crack advance used for determining the average crack velocity was taken as the time during which the interface tensile stress exceeded that needed for crack initiation. This was numerically calculated using the interferometrically-measured free surface velocity of the sapphire as an input. Equi-spaced Al lines of 200 Å thickness were deposited in front of the crack tip using photolithography and used as markers to determine the total crack advance by viewing the shock-loaded samples in a scanning electron microscope. The maximum crack propagation speed of 11,346 m/s was determined which is close to the dilatational wave speed of 11,090 m/s in the stiffer sapphire along its basal plane axis. These observations are the first report of crack speeds approaching the dilatational wave speed of the stiffer bimaterial component.

Laser-generated stress waves and their effects on the cell membrane

High-power lasers can generate well characterized stress (pressure) waves. The characteristics of the stress waves can be controlled by the appropriate choice of the laser parameters and the properties of the target material. Laser-generated stress waves can alter the structure and function of cells in vitro. Furthermore, they render the cell membrane permeable. Molecules present in the extracellular medium diffuse into the cytoplasm under the concentration gradient. Subsequently, the plasma membrane reseals, keeping the exogenous molecules inside the cell. Laser-generated stress waves can provide a potentially powerful tool for drug delivery.

Cell loading with laser-generated stress waves: the role of the stress gradient.

To determine the dependence of the permeabilzation of the plasma membrane on the characteristics of laser-generated stress waves. Laser pulses can generate stress waves by ablation. Depending on the laser wavelength, fluence, and target material, stress waves of different characteristics (rise time, peak stress) can be generated. Human red blood cells were subjected to stress waves and the permeability changes were measured by uptake of extracellular dye molecules. A fast rise time (high stress gradient) of the stress wave was required for the permeabilization of the plasma membrane. While the membrane was permeable, the cells could rapidly uptake molecules from the surrounding medium by diffusion. Stress waves provide a potentially powerful tool for drug delivery.

Role of laser-induced stress and shock waves in modification of the photoconductivity of films

An influence of laser-induced stress and shock waves on photoelectric properties of epitaxial n- (CMT) films with a cellular structure was studied. Irradiation of CMT films with nanosecond ruby laser pulses resulted in a modification of photoconductivity spectra and an increase in the photosensitivity because of segregation of electrically active point defects at the cell boundaries (acting as sinks) and also because of changes in their electron states as a result of action of the laser-generated stress and shock waves. Using the calculations of the temperature of the samples, the depth of shock wave formation and the amplitude of shock wave at laser irradiation, it was established that the strongest photosensitization of CMT took place in deep-seated layers where the shock wave was formed. This deduction was confirmed by an increase in the photosensitivity of purpose-designed samples coated with copper foil in such a way as to exclude the photo- and thermal effects under laser irradiation of CMT solid solutions.