Tong Jiao

Effects of Matrix Composition, Microstructure, and Viscoelasticity on the Behaviors of Vocal Fold Fibroblasts Cultured in Three-Dimensional Hydrogel Networks

Vocal fold diseases and disorders are difficult to treat surgically or therapeutically. Tissue engineering offers an alternative strategy for the restoration of functional vocal folds. As a first step toward vocal fold tissue engineering, we investigated the responses of primary vocal fold fibroblasts (PVFFs) to two types of collagen and hyaluronic acid (HA)-based hydrogels that are compositionally similar, but structurally variable and mechanically different. Type A hydrogels were composed of mature collagen fibers reinforced by oxidized HA, whereas type B hydrogels contained immature collagen fibrils interpenetrated in an amorphous, covalently cross-linked HA matrix. PVFFs encapsulated in either matrix adopted a fibroblastic morphology and expressed genes related to important extracellular matrix proteins. DNA analysis indicated a linear growth profile for cells encapsulated in type B gels from day 0 to 21, in contrast to an initial dormant, nonproliferative period from day 0 to 3 experienced by cells in type A gels. At the end of the culture, similar DNA content was detected in both types of constructs. A reduction in collagen content was observed for both types of constructs after 28 days of culture, with type A constructs generally retaining higher amounts of collagen than type B constructs. The HA content in the constructs decreased steadily throughout the culture, with type A constructs consistently exhibiting less HA than type B constructs. Using the torsional wave analysis, we found that the elastic moduli for type A constructs decreased sharply during the first week of culture, followed by 2 weeks of matrix stabilization without significant changes in matrix stiffness. Conversely, the elastic modulus for type B constructs increased moderately over time. It is postulated that PVFFs residing in gels alter the matrix organization, chemical compositions, and viscoelasticity through cell-mediated remodeling processes.

High Strain Rate Response of an Elastomer

Pressure‐shear plate impact experiments are used to study the nonlinear dynamic response of an elastomer at shearing rates of 105 – 106 s−1. Samples with thicknesses in the range 100 μm – 400 μm are cast between two hard steel plates. Because of the comparatively low impedance of the elastomer, longitudinal waves reverberating through the thickness of the sample — and recorded with a laser interferometer — are used to determine the isentrope of the material under uniaxial strain compression. Once the sample is fully compressed a shear wave arrives and imposes a simple shearing deformation. From the transverse velocity, measured interferometrically at the rear surface of the sandwich target, the shear stress and the transverse velocity at the rear surface of the sample are determined. These measurements provide an indication of the shearing resistance of the material under pressure. When the longitudinal unloading wave arrives from the rear surface of the target, these same measurements provide an indicati...

PRESSURE‐SENSITIVITY AND CONSTITUTIVE MODELING OF AN ELASTOMER AT HIGH STRAIN RATES

Pressure‐shear plate impact experiments have been conducted to study the pressure dependence of the shearing resistance of an elastomer (polyurea) at very high strain rates: 105–106 s−1. Two impact configurations were used. In the first, an unloading longitudinal wave reflected from the rear surface of the target assembly arrives at the sample midway through its loading by the incident shear wave. In the second, an unloading wave reflected from the free rear surface of the flyer arrives at the sample prior to the arrival of the incident shear wave. As a result, the sample is sheared at high strain rates—at both high and low pressure—during a single experiment (first case) or at high strain rates and low pressures (second case). Based on the experimental results, a constitutive model has been developed that involves a hyperelastic spring acting in parallel with an elastic spring and viscoplastic dashpot acting in series. The viscoplastic dashpot is modeled by means of a thermal activation model in which th...

PRESSURE‐SENSITIVITY AND TENSILE STRENGTH OF AN ELASTOMER AT HIGH STRAIN RATES

Pressure‐shear plate impact experiments have been conducted to study the mechanical response of an elastomer (polyurea) at very high strain rates: 105–106 s−1. Thin samples are cast between two hard steel plates. Longitudinal waves reverberating through the sample are used to determine the slope of the isentrope at compressive stresses greater than, say, 500 MPa—the initial pressure at impact. Shear waves measure the shearing resistance at the pressure attained after the “ring‐up” of the pressure in the sample is complete. In the current work, release wave experiments and plane wave simulations are used to extend the isentrope into the tensile regime—and ultimately to failure. The previous work is also extended to determine the pressure‐sensitivity of the materials shearing resistance at high shearing rates and low pressures. To achieve the latter, the impact configuration is designed so that an unloading longitudinal wave reflected from the rear surface of the target assembly arrives at the sample midwa...

Shearing resistance of aluminum at high strain rates and at temperatures approaching melt

High-temperature, pressure-shear plate impact experiments have been conducted to investigate rate-controlling mechanisms for plastic deformation of high-purity aluminum at high strain rates (106 s-1) and at temperatures approaching melt. The objective of these experiments was to look for a possible change in the rate-controlling mechanism of dislocation motion from thermally activated motion of dislocations past obstacles to phonon drag as the temperatures become high enough that thermal activation becomes relatively unimportant. The experimental results show an upturn in shearing resistance with increasing temperature at high temperatures, suggestive of a change in ratecontrolling mechanism. However, the upturn is too steep to be described by a usual phonon drag model with a drag coefficient that is proportional to temperature. Simulated results show that the modeling of strain rate hardening based on a phonon drag model leads to too strong an increase in flow stress with increasing strain rate in the dr...

HIGH RATE PLASTICITY UNDER PRESSURE USING A WINDOWED PRESSURE‐SHEAR IMPACT EXPERIMENT

An experimental technique has been developed to study the strength of materials under conditions of moderate pressures and high shear strain rates. The technique is similar to the traditional pressure‐shear plate‐impact experiments except that window interferometry is used to measure both the normal and transverse particle velocities at a sample‐window interface. Experimental and simulation results on vanadium samples backed with a sapphire window show the utility of the technique to measure the flow strength under dynamic loading conditions. The results show that the strength of the vanadium is approximately 600 MPa at a pressure of 4.5 GPa and a plastic strain of 1.7%.

Experimental and computational investigation of the response of an elastomer at pressures up to 18 GPa and strain rates of 105 −106 s−1

Pressure-shear plate impact (PSPI) experiments have been conducted to study the mechanical response of an elastomer (polyurea) at high pressures and high strain rate. The previously determined isentrope has been extended to 18 GPa. At this pressure, the high-strain-rate shearing resistance of polyurea is approximately 1 GPa. From the PSPI tests, it is evident that the shearing resistance of polyurea is quite sensitive to pressure. Based on the experimental results, a quasilinear viscoelasticity model is introduced and implemented in ABAQUS to simulate the response of polyurea P1000 under the impact conditions of the various PSPI experiments. Results of these simulations are compared with the experimental results to gain insight into the viability of the proposed model.

Pressure-shear plate impact experiment on soda-lime glass at a pressure of 30 GPa and strain rate of 4·10^7 s^(-1)

Recent modifications of a powder gun facility at Caltech have enabled pressure-shear plate impact (PSPI) experiments in a regime of pressures and strain rates that were previously unaccessible. A novel heterodyne diffracted beam photonic Doppler velocimeter (DPDV) has also been developed for simultaneous measurement of the normal and transverse particle velocity histories using the ±1st order diffracted beams produced by a 400 lines/mm diffraction grating deposited onto the polished rear surface of the impacted target plate. We present and interpret the results of a PSPI experiment conducted on a 5 µm thick soda-lime glass sample subjected to a normal stress of 30 GPa and a shear strain rate of 4 · 10^7 s^(–1). Transverse particle velocity measurements reveal a peak shear stress level of 1.25 GPa up to a shear strain value of 2.2, followed by a precipitous drop in stress and complete loss of shear strength.

IR temperature measurement in pressure-shear plate impact experiments

Pressure-Shear Plate Impact (PSPI) experiments on samples sandwiched between two hard plates have been developed previously for measuring the shearing resistance of materials at high strain rates, large inelastic shear strains, and high pressures. To enhance the value of such experiments in developing constitutive models for the dynamic response of materials, concurrent temperature measurements are being pursued by monitoring the infrared radiation emitted from the sample/window interface. The emitted radiation is collected by fast HgCdTe detectors through a pair of 90° off-axis parabolic reflectors. ZnSe is used as the rear plate (window) because its transmission band (0.6 μm -14 μm) covers an exceptionally wide range of wavelengths — extending beyond the cutoff wavelength of the IR detector. Because temperatures generated in PSPI experiments are modest, the emissivity of the interface is increased by lapping the sample surface to a ‘matte’ finish. Pilot experiments are assessed for their potential and limitations.

3 – Testing, Experiments and Properties of HSREP

Normal-incidence and pressure-shear plate impact experiments have been conducted to study the mechanical response of an elastomer, polyurea P-1000, at high strain rates – 105–106 s−1 – and high pressures – up to 18 GPa. Configurations with samples sandwiched between two hard plates have been used to conduct constant-pressure, pressure-change, and low-pressure experiments to investigate the pressure dependence of the shearing resistance. A symmetric pressure-shear plate impact configuration has been used to measure directly the thickness-averaged nominal strain rates of the sample – as well as the tractions on both of its interfaces with linear elastic plates. Release wave experiments have been used to capture the behavior at compressive stresses below 0.5 GPa, and still lower until tensile failure occurs. From these experiments, the quasi-isentrope of polyurea is obtained as well as its high-strain-rate shearing resistance at pressures up to 18 GPa. The experimental results show that the shearing resistance of polyurea increases strongly – essentially proportionally – with increasing pressure. Based on these experimental results, a quasi-linear viscoelasticity model is introduced to capture the observed nonlinear pressure-volume behavior, the strong dependence of shearing resistance on pressure, and the strong relaxation of deviatoric stresses. This model has been implemented in Abaqus to simulate the response of polyurea P-1000 under the impact conditions of the various experiments. Results of these simulations are compared with the experimental results to gain understanding of the viability of the proposed model. Finally, an assessment is made of current understanding of the mechanical response of polyurea P-1000 and what further high-strain-rate and high-pressure research is required. Certain elastomers, when present on the front (strike-face) side of steel, significantly enhance the resistance to ballistic penetration. The mechanism is primarily twofold: (1) impact induces a viscoelastic phase transition in the polymer, with consequent large energy absorption; and (2) the transient hardening of the rubber (by approximately. three orders of magnitude) spreads the impact force laterally, reducing the local pressure. The result is armor having improvements in ballistic performance of approximately 50% without additional weight (or substantial reductions in areal density without loss of penetration resistance). Variations on this approach, using multiple rubber or steel layers, or replacing the elastomer coating with a laminate composed of thin elastomer and metal layers, are described. The question of the range of applicability of the time–temperature superposition principle or of time–temperature equivalence has often been questioned. Specifically, the assurance that the “extrapolation” of data acquired at strain rates and time scales appropriate for quasistatic deformations may be appropriate under conditions involving very high rate or extremely short time scales has not been guaranteed in the past. Evidence is presented here showing that the use of data derived from quasistatic deformation histories is applicable to time scales that are shorter by factors of 106–108 which cover most of the deformation rates germane to explosive environments.