Hydroxyl-terminated polybutadiene (HTPB) is a widely used polymeric binder in polymer-bonded explosives (PBXs) and solid rocket propellants. Even though used in small fractions, the elastomeric binder absorbs much of the impact energy and therefore requires careful modeling of its mechanical behavior to accurately simulate the response of PBXs when they are subjected to large strains and strain rates. While the normal response of HTPB has been characterized under uniaxial stress and uniaxial strain loading, shear strength measurements under large pressures and large shear strain rates have not been made so far. Such measurements are critical for modeling localization and failure in PBXs and accurate prediction hotspot formation. In this study, pressure-shear plate impact (PSPI) experiments have been used to measure the shearing resistance of HTPB at different pressures. The shearing resistance of HTPB is found to be strongly pressure dependent. A quasi-linear viscoelastic model with pressure-dependent shear wave speeds and shearing resistance is used to describe the experimentally observed dynamic response of HTPB. The model consists of an instantaneous elastic response and viscoelastic relaxation of the elastic response.
An experimental technique is reported, which can image the deformation fields associated with dynamic failure events at high spatial and temporal resolutions simultaneously. The technique is demonstrated at a spatial resolution of ~1 μm and a temporal resolution of 250 ns, while maintaining a relatively large field of view (≈ 1.11 mm × 0.63 mm). As a demonstration, the technique is used to image the deformation field near a notch tip during initiation of a shear instability in polycarbonate. An ordered array of 10 μm diameter speckles with 20 μm pitch, and deposited on the specimen surface near the notch tip helps track evolution of the deformation field. Experimental results show that the width of the shear band in polycarbonate is approximately 75 μm near the notch-tip within resolution limits of the experiments. The measurements also reveal formation of two incipient localization bands near the crack tip, one of which subsequently becomes the dominant band while the other is suppressed. Computational simulation of the experiment was conducted using a thermo-mechanically coupled rate-dependent constitutive model of polycarbonate to gain further insight into the experimental observations enabled by the combination of high spatial and temporal resolutions. The simulation results show reasonable agreement with the experimentally observed kinematic field and features near the notch-tip, while also pointing to the need for further refinement of constitutive models that are calibrated at high strain rates (~105/s) and also account for damage evolution.
Commercially available boron carbide ceramics typically have heterogeneous microstructures that contain distributions of processing‐induced inclusions. The inclusions that are rich in carbon (i.e., carbonaceous) govern the underlying mechanisms of brittle fracture through wing crack formation, and thus dictate the mechanical response of the ceramic. In this study, we investigate the dynamic failure of five boron carbide ceramic materials with different inclusion populations. All of the materials were prepared by hot‐pressing; four of these boron carbides contained different sizes and concentrations of carbonaceous inclusions, while one contained no carbonaceous inclusions. The heterogeneity distributions were characterized in some detail for statistical analysis using scanning electron microscopy and quantitative image analysis. A modified compression Kolsky bar setup with in situ ultra‐high‐speed microscopic imaging (10 million frames per second) was then used to study the influence of the inclusion distributions on the dynamic failure processes in these materials, at nominal high strain rates of 102–103 s−1. The in situ ultra‐high‐speed microscopy highlighted the link between micro and macroscale failure processes and demonstrated that the carbonaceous inclusions are indeed the preferential sites for nucleation of wing cracks, as previously hypothesized based on post‐mortem observations. The relative orientation of an inclusion with respect to the compression axis was shown to affect the likelihood that it would participate in crack nucleation. All of the ceramics were also found to have orientation‐dependent peak compressive stress, regardless of the presence of carbonaceous inclusions, suggesting that grain orientation distributions are also important.
The mechanical behavior of polymer-bonded explosives (PBXs) and polymer-bonded simulants has been widely studied under normal impact. However, their shearing response under large pressures and shear strain-rates has not been explored, to the best of our knowledge. Such measurements are crucial in informing constitutive models that aim to predict hot-spot formation and ignition behavior of PBXs subject to multi-axial loading. Pressure-shear plate impact experiments have been conducted on a composite of hydroxyl-terminated polybutadiene (HTPB) binder and sucrose (an energetic simulant) under normal stresses of 3–10 GPa nominally and shear strain-rates of the order of 105 s−1. The shear strength of the composite shows a dramatic drop after reaching a critical shear strain. Such a drop could be due to fracture or localization in either of the two phases, further compounded by other mechanisms such as friction between the resulting interfaces or collapse of voids. It is also observed that the shear strength of the composite is highly pressure-sensitive, increasing from 176 to 453MPa as the normal stress increases nominally from 3 to 9.5 GPa. Comparison of the shearing behavior of the composite with that of a granular aggregate of sucrose grains at low normal stresses (∼3 GPa) indicates that the binder plays the role of a lubricant between the sucrose grains at those normal stresses. However, at normal stresses of ∼9–10 GPa, the shear strength of HTPB becomes similar to sucrose, and beyond these normal stresses, a transition in the failure mode and its location is expected.
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