In Situ Deformation Characterization of Density-Graded Foams in Quasi-Static and Impact Loading Conditions

Digital image correlation is utilized to characterize deformation and strain fields developed within the layers of density-graded multilayered foam structures subjected to uniaxial quasi-static and dynamic compression. Three-layered graded structures fabricated from rigid polyurethane foams with nominal densities of 160, 240, and 320 kg/m³ are subjected to both quasi-static and dynamic loading. The quasi-static measurements show that, irrespective of the loading direction, the densification initiates in the lowest density layer and propagates into other layers later once the first layer is fully densified. The deformation mechanisms are seen to be different in the case of dynamic loading conditions compared to the quasi-static loading. The deformation mechanism, in the case of dynamic loading, depends on the sample orientation relative to the direction of the applied load. In cases where the higher density layers are impacted, the propagation of the elastic and compaction waves leads to partial deformation of the lowest density layer. Sample deformation continues in all layers upon the reflection of the stress waves from the distal end of the sample. In cases where the lowest density layer is oriented towards the impact face, a completely different deformation response is observed. A detailed full-field analysis of strain and stress is performed. The mechanisms associated with the formation and propagation of stress waves from the impact ends to the distal ends of the samples are discussed

Multiscale Deformation And Failure Behavior Of Polymer Bonded Explosives Subjected To High Rate Loading

Polymer bonded explosives (PBX) are heterogeneous granular composites with a high-volume fraction of solid. Typically, they contain 80-95% explosive crystals and 5- 20% soft polymer binder. These materials are subjected to different loading conditions during their service life. The main function of the soft binder is to reduce shock sensitivity to prevent an accidental explosion. However, there have been inadvertent detonations in these materials during transportation and handling. The reason for such unintentional explosion is not well understood. It is commonly accepted that the formation of local hightemperature regions, called ‘hot spots’, is the primary cause. Hotspot formation is associated with the local energy dissipation mechanisms in the material system during dynamic loading. There is a large knowledge gap in understanding the dynamics of local failure mechanisms in polymer bonded explosives subjected to loading at different time scales. The primary focus of the present work is to understand the local deformation mechanisms in polymer bonded explosives subjected to high rate and impact loading. An experimental method is developed based on high-speed photography and digital image correlation (DIC). The experimental setup helps to observe and quantify the deformation mechanisms in-situ at a spatial and temporal resolution of 10.66 µm/pixel and 200 ns, respectively. The capability of the experimental setup is validated in two heterogeneous materials system at strain rates varying from 150-1000 s-1. v In this study, polymer bonded sugar (PBS), a mechanical simulant of PBX is used. PBS contains sugar solid crystals and plasticized hydroxyl-terminated polybutadiene (HTPB) as the soft binder. Two different dynamic loading configurations are studied, simulating high strain rate and high impact loading conditions. The first loading configuration involves the dynamic loading of PBS specimens at a strain rate from 150 to 1000 s-1. High temporal resolution dynamic deformation measurements are conducted at macro and meso (local) length scales. From these experiments, global and local deformation mechanisms and failure behavior are studied in detail. The effects of strain rate and particle volume fraction on the deformation mechanisms are studied for a comprehensive understanding of the material behavior. These experiments reveal the link between the macroscale shear band formation and its microscopic origin. The second case involves, a direct impact loading utilizing a gas-gun with impact velocity varying from 50 to 100 m/s. From the images captured during loading, a quantitative analysis of the compaction wave dynamics is performed at two length scales. The particle velocity, compaction wave velocity, and wave thickness are calculated from the macroscale experiments. In addition. spatial stress distribution is determined from the equilibrium equations using the full displacement data obtained from DIC. From stress and strain rates, the total energy dissipated during compaction wave propagation is estimated. Finally, mesoscale experimental observations are used to identify the main local failure and deformation mechanisms associated with the energy dissipation

High Strain Rate Response of Adhesively Bonded Fiber-Reinforced Composite Joints A Computational Study to Guide Experimental Design

Adhesively bonded carbon fiber-reinforced epoxy composite laminates are widely used in aerospace applications. During a high energy impact event, these laminates are often subjected to high strain rate loading. However, the influence of high strain rate loading on the response of these composite joints is not well understood. Computational finite element (FE) modeling and simulations are conducted to guide the design of high strain rate experiments. Two different experimental designs based on split Hopkinson bar were numerically modeled to simulate Mode I and Mode II types loading in the composite. In addition, the computational approach adopted in this study helps in understanding the high strain rate response of adhesively bonded composite joints subjected to nominally Mode I and Mode II loading. The modeling approach consists of a ply-level 3D FE model, a progressive damage constitutive model for the composite material behavior and a cohesive tie-break contact element for interlaminar delamination

Mesoscale shock structure in particulate composites

Multiscale experiments in heterogeneous materials and the knowledge of their physics under shock compression are limited. This study examines the multiscale shock response of particulate composites comprised of soda-lime glass particles in a PMMA matrix using full-field high-speed digital image correlation (DIC) for the first time. Normal plate impact experiments, and complementary numerical simulations, are conducted at stresses ranging from $1.1-3.1$ GPa to elucidate the mesoscale mechanisms responsible for the distinct shock structure observed in particulate composites. The particle velocity from the macroscopic measurement at continuum scale shows a relatively smooth velocity profile, with shock thickness decreasing with an increase in shock stress, and the composite exhibits strain rate scaling as the second power of the shock stress. In contrast, the mesoscopic response was highly heterogeneous, which led to a rough shock front and the formation of a train of weak shocks traveling at different velocities. Additionally, the normal shock was seen to diffuse the momentum in the transverse direction, affecting the shock rise and the rounding-off observed at the continuum scale measurements. The numerical simulations indicate that the reflections at the interfaces, wave scattering, and interference of these reflected waves are the primary mechanisms for the observed rough shock fronts

Strength of magnesium at high pressures and strain rates

The strength and deformation mechanisms in magnesium can be significantly affected by anisotropy, high strain rates, and pressure. In this study, pressure shear plate impact (PSPI) experiments are conducted to measure the strength of extruded polycrystalline magnesium at pressures varying from 5 to 10 GPa at a nominal strain rate of 10⁵ s⁻¹. The experimental technique enables to first shock load the material sample to the desired normal stress in one direction, and then shear the material in a perpendicular direction. A recently developed hybrid analysis method for PSPI experiments is used to extract the stress–strain curves of magnesium from the particle velocity records measured at the rear surface of the target. The PSPI experimental results reveal a slower twinning saturation at high pressures. To better understand the material behavior under the combined stress states in the PSPI experiments, the results were compared with that of a specimen deformed by a two-step process of quasi-static compression followed by dynamic shear loading at relatively low pressure. The two-stage loading at low pressure, and the calculation of temperature rise in the PSPI experiment revealed that the combined effect of the reorientation and the temperature rise lower the flow strength of magnesium at high pressures under multiaxial loading

Generalized linear prediction method in phase-shifting interferometry in the presence of noise

The effectiveness of phase-shifting interferometry (PSI) techniques employing piezoelectric device PZT in the estimation of phase depends largely on the accuracy with which the phase shifts are imparted to the device and the noise influencing the measurement. Several effective algorithms have been proposed to compute the phase shifts imparted to the device and subsequently obtain the phase using least-squares estimation technique. In this paper, we propose a generalized approach, which accurately estimates the phase shifts in the presence of noise. The method is based on the idea of linear prediction and explores the fact that sampling more data frames yields a reliable phase step estimate in a least-squares sense. We also compare our method with a commonly used generalized phase-shifting method based on histogram analysis and show that our proposed approach is highly effective. We also present simulation and experimental validations of our proposed method. (c) 2007 Elsevier Ltd. All rights reserved

An investigation of shock-induced phase transition in soda-lime glass

There exists a large body of evidence from experiments and molecular dynamics simulations to suggest the occurrence of phase transitions in soda-lime glass (SLG) and other silica glasses subject to shock compression to pressures above 3 GPa. In light of these findings, the current work investigated the existence of phase transition in SLG using shock and release experiments. The experiments employed symmetric SLG-SLG impact to achieve complete unloading to zero stress after shock compression to stresses in the range of 3-7 GPa. The stress-strain response and the Lagrangian release wave speed behavior of SLG obtained from these experiments are seen to reveal a mismatch between the loading and unloading paths of the pressure-strain curve for the material, which serves as compelling evidence for the occurrence of a shock-induced phase transition in the material at relatively low pressures. Furthermore, the release wave speed vs strain data obtained from experiments were used to construct a methodology for modeling the shock and release behavior of SLG. This scheme implemented in numerical simulations was able to capture the release behavior of shock compressed SLG, for which a robust and satisfactory model was previously unavailable.</p