Volume 13, Issue 2 -

Magnesium alloy is very attractive in many industrial applications due to its low density. The structure-property relationships of the magnesium alloy under quasi-static loading have been extensively investigated. However, the dynamic behavior, particularly the mechanism of high-rate plastic deformation, of the magnesium alloy requires more in-depth investigations. In this paper, the effect of aging treatment on the quasi-static and dynamic properties of a typical rare earth Mg-Gd-Y magnesium alloy is investigated. In particular, the plastic deformation mechanism under dynamic compression loading is discussed. Split Hopkinson Pressure Bar (SHPB) was used to carry out dynamic compression tests with controllable plastic deformation by using stopper rings. The experimental results demonstrate that both static and dynamic properties of the Mg-Gd-Y alloy vary under various aging treatment conditions (under-aged, peak-aged and over-aged conditions), due to two different kinds of second phases: remnant micro size phase from solid solution treatment and nano precipitation from aging treatment. The results of microstructure characterization and statistic analysis of the metallographic phase are presented. The area fraction of the twinned grains increases due to aging treatment and dynamic loading. The main plastic deformation mechanism of the rare earth Mg-Gd-Y magnesium alloy is possibly dislocation slip, rather than twinning for the conventional AZ31 magnesium alloy under high strain rate loading.

The validity of the constant propagation fracture energy postulation for dynamic fracture is discussed. As shown from existing direct and indirect experimental results, this assumption may not represent the physical reality. For spontaneous fractures, the fracture energy was shown to increase linearly with the crack length, and for dynamic fractures driven by known amplitude impulsive loading (generated by planar impact), the fracture energy was not a constant either. Despite of its phenomenogical origin, the Broberg’s theory developed for self-similar crack growth works well for both spontaneous fractures and dynamic fractures produced by well defined dynamic loading. In this theory, the fracture energy is not a constant. Furthermore, with given far-field loading or equivalent far-field loading, the crack speed is uniquely determined by a strength-like material parameter. This parameter is related to the cohesive strength as proposed by H. J. Gao for hyperelastic materials in the crack-tip process zone. It is proposed in this work that the strength-like parameter (or equivalently the constant fracture speed) is a better material parameter to describe the dynamic fracture propagation process for most dynamic fractures.

In order to make effective measure for the stress, strain and strain rate in the specimen, a tensile split Hopkinson bar (TSHB) was optimized in the first loading duration and the thermoviscoplastic models for a pure copper were determined. The deformation of the pure copper bar during the multi-tensile loading including the necking process in the optimized TSHB testing was recorded by a high speed camera and was used in providing an extensional check of the determined thermoviscoplastic models of the pure copper. The thermoviscoplastic models determined in a certain range of strain were employed in numerically simulating the large deformation of the pure copper bar to adjust the thermoviscoplastic model to describing large strain range. The instability strain of thermoviscoplastic materials in simple tension given by Batra et al was compared with the experimental strain at diffuse necking of the pure copper bar and the computed instability strain based on the load-average strain of the pure copper bar. The local necking of the pure copper bar in the optimized TSHB testing was numerically simulated using the micro-mechanical data given by Ragab and was compared with the recorded necking.

The penetration depth of rigid projectiles is investigated using the modified spherical cavity expansion theory and the Holmquist–Johnson–Cook (HJC) model for the concrete targets, in which the model parameters used in the Mohr–Coulomb Tresca-limit yield criterion are obtained by the triaxial compression tests. By comparing the cavity expansion pressures of the concrete samples with two different types of aggregate, the influence of the aggregate hardness on the penetration processes is discussed. With the analysis of the tractions acted on the projectile nose, the influences of the projectile and target material properties as well as the projectile structure on the transition impact velocity are also discussed. By comparing the theoretical results with the experimental data, two penetration mechanisms are demonstrated for the ogive-nose projectile penetration into concrete with the striking velocities up to 2.0 km/s.

A spherical cavity expansion model for concrete is first proposed by using an elastic-brittle-plastic material law with Hoek–Brown strength criterion. The constitutive model can capture the basic features of the mechanical response of concrete materials including the effects of strain-softening and pressure hardening (pressure-dependent shear strength) and all the parameters used in the model can be determined from material tests. The forcing function obtained from the spherical cavity expansion analysis is then employed to construct a penetration model for concrete targets struck by ogival-nosed projectiles. It transpires that the present model predictions are in good agreement with experimental observations in terms of penetration depth and ballistic limits/residual velocities in the case of perforation.

Microscale SiC particles with multiwalled carbon nanotubes grow on were filled in epoxy resin to construct multiscale hybrid composite. Static and dynamic compressive responses of composites were investigated. A dynamic experimental technique, split Hopkinson press bar (SHPB), was introduced to study the constitutive laws of materials at high strain rates. SEM observations show good dispersion of carbon nanotubes and SiC particles, but bad link between SiC particles and carbon nanotubes exists. Experimental results of static and dynamic tests verify that increase of strength happens with SiC/CNT as reinforcement Increment of strength under dynamic loading is bigger than under static loading.

The interaction between an eccentric matrix crack and a circular inclusion in composites was investigated numerically to explore the strain rate effects on the crack deflection/penetration behavior at the matrix-inclusion interface. It is found, with the increase of strain rate, that there will occur in turn the crack deflection, the double-crack fracture and the crack penetration processes for a certain material. Moreover, both higher relative interfacial strength and smaller eccentric angles can redound to the crack penetration. Additionally, it is demonstrated that the strain rate effect on the composite strength is a local structural response characteristic rather than an intrinsic material property, which sounds in agreement with that proposed by previous authors.

A multiscale model is used to analyze the compaction processes in granular HMX beds composed of different particle sizes (coarse particles, d=40 μm and microfine particles, d=4 μm). The localization strategy of Gonthier is extended to include changes in thermal energy induced by compression. The variation in yield strength caused by solid-liquid phase change is also considered. Analysis of the steady-state wave structure indicates that the compaction behavior of a porous material is dependent on particle size. For solid volume fraction φ s < 0.88 , the fine particle beds provide greater resistance to compaction than the coarse particle beds, and they propagate compaction waves that travel at faster speeds. When φ s > 0.88 , the physical state of the compacted bed has become very similar for the two materials. For subsonic compaction waves, the evolution of the grain temperature shows that large particles lead to large hot spots and high temperature and coarse particles are more shock sensitive at low shock pressures. For supersonic compaction waves, compression induced changes in thermal energy play an important role in localization strategy. It increases the localization sphere center radius. The dissipated energy is deposited over a larger localization volume so that the grain temperature near the intergranular contact surface is reduced significantly. The localization center radius further increases because of the decrease in the yield strength caused by solid–liquid phase change. Consequently, the peak grain temperature is reduced further.

In this paper, an Eulerian–Lagrangian two-phase flow model for liquid-fueled detonations is constructed. The gaseous mixture is described by an Eulerian method, and liquid particles in gaseous mixture are traced by a Lagrangian method. An improved space-time conservation element and solution element (CE/SE) scheme is applied to the simulations of detonations in liquid C 10 H 22 -O 2 /air systems. Different fuel droplet sizes and equivalence ratios are considered in the present study. Interestingly, the numerical results show that liquid-fueled detonations have some difference with gaseous detonations. Especially, a deficit in the propagation velocity compared to the gaseous detonation velocity is observed in mixtures with lean fuel and larger droplet sizes, while an increase in the propagation velocity compared to the gaseous detonation velocity is observed in the mixtures with very rich fuel. The surprising phenomenon is analyzed and discussed with the aid of detailed numerical results. In addition, the formation and propagation of two-phase detonation waves are characterized by series of results and the influence of particle radii is also discussed. All numerical results show that the present model can describe the gas-particle two-phase system accurately, and can be applied to numerical simulations of liquid-fueled detonations.

A series of shock initiation experiments are performed on the PBXC03 explosives in different formulations to understand the influence of the explosive particle size on the shock initiation, and the in-situ pressure gauge data are obtained which show that shock sensitivity decreases with the explosive particle size under the test condition used in this paper. Moreover, a mesoscopic reaction rate model which is calibrated by the experimental data on a medium formulation PBXC03 explosive is adopted and then applied to predict numerically the shock initiation of other PBXC03 explosives in different formulations. The numerical results are in good agreement with the experimental data.

The spall strength of cast iron has been investigated by means of planar plate impact experiments conducted in a vacuum. A single stage gas gun was utilized to drive projectiles to velocities between 100 and 300 m/sec, resulting in low to moderate shock loading of the cast iron specimens. Measurement of the stress histories were made with the use of commercial manganin stress gauges that were imbedded between the back face of the cast iron specimen and a low impedance backing of polycarbonate. Spall strength values were calculated utilizing the measured peak stress and minimum stress pullback signals captured in the stress history. Spall Strength values were found to vary between 0.98 and 1.45 GPa for the cast iron tested. Post-Mortem analysis of recovered specimen has provided insight into the evolution of spall failure in cast iron and shed light on the varying nature of the spall strength values calculated. It was determined that the lower bound of strength values was associated with small scale micro-failure, while the upper bound values corresponded to complete spall fracture.

As an indispensable part of transportation system, urban underpass can alleviate traffic congestion and reduce traffic accidents effectively. In order to acquaint the hydrodynamics of shockwave in underpass and assess the potential damage and risk of bombing accidents, a numerical simulation was established to calculate the blast effects originating from the 13Kg TNT. Based on the location of detonation point, the shape of corridor section and the exit’s cover, peak pressure distributions were predicted. The results indicate that the propagation of shockwave in horizontal direction switches to the inclined top of ground in a near-field due to the ground reflection and the set of exit’s cover increases the risk of explosion to the structure and the users.

In order to investigate rock dynamic mechanical properties of amphibolites, sericite-quartz schist and sandstone under the different strain rates varying from 30 s -1 to 150 s -1 , the specimens were subjected to axial impact at different projectile speeds by using the split Hopkinson pressure bar (SHPB) of 100 mm in diameter. The optimal experimental size of rock samples is verified by analyzing the stress equilibrium of cylindrical rock samples in different thicknesses. It has studied the mechanic properties of these three rocks which under impact loadings; and analysed the dynamic compressive strength, failure modes, energy dissipation variation with the strain-rate and the strain-rate hardening effect from the perspective of material microstructure. Experimental results show that the dynamic Young’s modulus of rock samples increase with strain-rate slightly. The dynamic failure modes of different rock samples are always different. When at a lower strain-rate, the damage of sandstone takes a peeling off the external radial tensile failure mode, but that of amphibolites takes axial splitting mode; when at a higher strain-rate, sandstone takes granular crushing failure mode, and that of amphibolites and of sericite-quartz schist take massive crushing mode. Significant strain-rate effect can be represented by a linear relation between the specific energy absorption and the strain-rate , or between the dynamic strength increase factor η and .