Plate Impact Tests Research Articles

Numerical simulation of multiscale damage-failure transition and shock wave propagation in metals and ceramics

Statistical theory of evolution of typical mesoscopic defects revealed specific type of criticality - structural-scaling transitions and allowed the development of phenomenology of damage and plastic flow in materials under intensive loading, which established characteristic multiscale collective modes of defects responsible for formation of plastic waves and damage-failure transition. Original approach based on wide range constitutive equations was developed for simulation of multiscale damage-failure transition mechanisms and shock wave propagation in metals and ceramics in range of strain rate 103 − 108 s−1. It was shown that mechanisms of a plastic relaxation and damage-failure transitions are linked to multiscale kinetics of mesodefects collective modes with the nature of solitary waves and blow-up dissipative structures consequently. Numerical simulation of original plate impact tests showed that the model describes shock wave loading for metals and ceramics, and allowed us to explain the effect of power law phenomena of plastic wave fronts formation, its self-similar features under reloading and unloading. Analysis of shock wave profiles in ceramics for different thicknesses of specimens in terms of self-similar variables supports the universality of shock wave fronts.

Effect of Crystalline Structure on Intergranular Failure During Shock Loading

The effect of crystalline structure on intergranular failure during shock loading has been examined. A suite of dynamic tensile experiments, using plate-impact testing, were conducted on copper (face-centered cubic) and tantalum (body-centered cubic) specimens with different grain sizes (30–200 μm). These experiments were designed to probe void nucleation, growth, and coalescence processes that for these materials are known to lead to failure. For the grain sizes examined in the study, post-impact metallographic analyses show that in copper specimens, during the early stages of deformation, voids were present primarily at general or low-coincidence, high-angle grain boundaries (GBs), irrespective of grain size. In tantalum, while some voids developed along the GBs, an increasing amount of transgranular damage was observed as the grain size increased. A scenario based on the availability of potential nucleation sites and number of slip systems inherent to each crystalline structure is discussed. The role that this availability plays in either promoting or hindering plastic processes leading to damage nucleation and growth is then examined.

Modeling the Confinement and Prestress Effects on Dynamic Failure of Ceramic Deep Beam

The present study performs numerical experiments to investigate the confinement and prestress effects on the dynamic failure of a ceramic deep beam. Johnson-Holmquist ceramic model (JH-2) is implemented into LS-DYNA through its user subroutines. The implementation and the effectiveness of the model are validated by a single element compression test and available plate impact test results. Failure of ceramics is isolated into two modes, i.e. tension induced cracking and compression generated crushing. Three different setups, i.e. bare ceramic beam, the beam with lateral constraints, and the beam with lateral constraints and a 100 MPa pre-applied pressure are considered in the numerical experiments. The results show that, in lower velocity impact, few tension induced cracks connecting the impactor head and the supports happened on the specimen, minor crush zone is found around the supports. Under higher velocity impact, however, a relative larger crush zone is also observed near the impact head, spreading tension cracks occurred in a triangular zone on the specimen. The lateral confinement affects the tension induced failure under both lower velocity and higher velocity impacts. The pre-applied pressure has only minor effect on the failure of the deep beam in low velocity impact. In higher velocity impact, however, the pre-applied stress may change the distribution profile of the tensile cracks. Both pre-stress and the lateral constraints have negligible effect on the compression crush happened near the impactor head in the higher velocity impacts. The dynamic failure patterns of the ceramic deep beam and the confinement and prestress effects are demonstrated clearly in the present study. Energy transformation mechanism during the impacts is briefly discussed.

Shielding performances of the designed hybrid laminates impacted by hypervelocity flyer

With increasing threat of man-made debris in outer space, a shielding panel designed for spacecraft or satellite should withstand hypervelocity impact of debris, with speed up to 9.0km/s. In this study, shielding performances of 2024 aluminum alloy panel, carbon fiber reinforced polymer composite (CFRP) panel and their hybrid laminates with different stacking configurations were investigated by ballistic tests of the materials impacted by Mylar flyer at 9.0km/s. The results are supposed to guide the design of shielding laminate panel. It was found that the stacking combination of CFRP and aluminum alloy significantly reduced the peak shock pressure induced by hypervelocity impact, and the increase of the layer number enhanced the shielding performance of the hybrid laminate. The five-layered aluminum alloy/CFRP laminates resisted the impact of the flyer without perforation. Furthermore, the extent of the damage of an impacted laminate was related to the velocity profile on its free surface. The planar plate impact testing employing a Doppler laser velocity interferometer is a feasible approach to quantitatively evaluate the shielding performance of structural material.

Modelling of Shockwave Propagation in Orthotropic Materials

Modelling of shockwave propagation in orthotropic materials requires an appropriate description of material behaviour within elastic and plastic regimes. To deal with this issues, a finite strain constitutive model for orthotropic materials was developed within a consistent thermodynamic framework of irreversible process in this paper. The important features of this material model are the multiplicative decomposition of the deformation gradient and a Mandel stress tensor combined with the new stress tensor decomposition generalised for orthotropic materials. The elastic free energy function and the yield function are defined within an invariant theory by means of the introduction of the structural tensors. The plastic behaviour is characterised within the associative plasticity framework using the Hills yield criterion. The complexity was further extended by coupling the formulation with the equation of state (EOS) to control the response of the material to shock loading. This material model which was developed and integrated in the isoclinic configuration provides a unique treatment for elastic and plastic anisotropy. The effects of elastic anisotropy are taken into account through the stress tensor decomposition and plastic anisotropy through yield surface defined in the generalized deviatoric plane perpendicular to the generalised pressure. To test its ability to describe shockwave propagation, the new material model was implemented into the LLNL-DYNA3D code. The results generated by the proposed material model were compared against the experimental Plate Impact test data of Aluminium Alloy 7010. A good agreement between experimental and simulation was obtained for two principal directions of material orthotropy.

Explosive Laboratory Devices for Dynamic Shock Loading of Materials in Mbar Pressure Region

Dynamic methods utilizing strong shock waves are used for investigating the properties of condensed matter at very high pressures and temperatures. Explosive driven plate impact tests have been conducted to find out the shock Hugoniot of materials up to pressure of 2 Mbar. Explosive cascading utilizing overdriven detonation waves in high explosives produced by flyer impact from the first stage to accelerate comparatively thin flyer plates to very high velocities have been demonstrated. Numerical simulations using Autodyn 2D/3D have been performed to optimize the various parameters in two stage explosive assemblies to accelerate flyer to velocities exceeding 10 km/s. Shock pressure up to 20 Mbar has been successfully measured using explosive assemblies in a convergent flow. Expertise and infrastructure available in TBRL for launching metal flyers to high velocity and monitoring its in-flight velocity and profile have been discussed in this paper.

Modelling of dynamic behaviour of orthotropic metals including damage and failure

A physically based material model for metals, with elastic–plastic and damage/failure orthotropy is proposed in this paper. The model is defined within the frameworks of irreversible thermodynamics and configurational continuum mechanics and integrated in the isoclinic configuration. The use of the multiplicative decomposition of deformation gradient makes the model applicable to arbitrary plastic and damage deformations. To account for the physical mechanisms of failure, the concept of thermally activated damage initially proposed by Klepaczko (1990) was adopted as the basis for the new damage evolution model. This makes the proposed damage/failure model compatible with the Mechanical Threshold Strength (MTS) model (Follansbee and Kocks, 1988; Chen and Gray, 1996; Goto et al., 2000; Gray et al., 1999; Chen et al., 1998) which was used to control evolution of flow stress during plastic deformation. In addition the constitutive model is coupled with a shock equation of state which allows for modelling of shock wave propagation in the material. The new model was implemented in DYNA3D and our in-house non-linear transient SPH code, MCM (Meshless Continuum Mechanics).Parameters for the new constitutive model for AA7010 (a polycrystalline aluminium alloy, whose orthotropy is a consequence of grain morphology), were derived on the basis of the tensile tests and Taylor anvil tests. The tensile tests were performed for the range of temperatures between 223.15K and 413.15K, and strain rates between 6.4×10-4s-1 and 6.4×101s-1.The new model was validated in two stages. The first stage comprised a series of single element tests design to separately validate elasticity, plasticity and damage related parts of the model. The second stage comprised a series of numerical simulations of Taylor anvil and plate impact tests for AA7010 and comparison of the numerical results with the experimental data. The numerical results illustrate the ability of the new model to predict experimentally observed behaviour.

Ultrasonic detection of spall damage nucleation under low-velocity repeated impact

Repeated plate impact testing with impact stress well below the threshold spall-stress (2.6 GPa) on medium carbon steel was carried out to the identical target plate by impacting the flyer plate. Occurrence of spall damage under low-velocity repeated impact was evaluated nondestructively with a low frequency scanning acoustic microscope. We observed the spall damage distribution by the B- and C-scan images. In order to initiate the spall damage (voids in a ductile material or cracks in a brittle one) the particular value of threshold spall-stress should be exceeded what already belongs to a commonly accepted knowledge. Generally, the spall damage development is dependent on the amplitude and the duration of the stress pulse. If the stress is high and duration is long enough to create tensile failure of material, the voids or cracks nucleate along the spall plane, and consequently, they form macrocracks. Therefore, the spall damage does not create when the first impact stress is less than the threshold spall-stress. However, after the fifth low-velocity repeated impact test, the generation of the spall damage was detected, even if the impact stress (1.1–1.7 GPa) was lower than the threshold spall-stress (2.6 GPa).

Effects of Two Different Dynamic Loading Conditions on Spall and Damage of 7075 Aluminum Alloy

The fracture behaviors of the 7075 aluminum alloy under two different dynamic loading conditions are investigated by means of a light-gas gun. The fracture surfaces obtained in the spall test are compared to the fracture surfaces obtained with a blunt projectile struck to the aluminum alloy plate. Optical and scanning electron microscopes are used in the investigation. For the plate-impact test, spall of the target was attributed to intergranular fracture caused by the tensile stress. The fracture behavior during projectile penetration is complex and consists of several fracture modes in addition to that the fracture is also of dynamic character. The penetration process of aluminum alloy target included: plugging stage, the microcracks nucleation stage, and the final tensile fracture stage. Mixed intergranular brittle/ductile fracture was observed, and brittle fracture played a dominate role.

Newly Developed Unsteady Wave Sensing System for Polycarbonate

Stress-strain relationships of polycarbonate (PC) were determined over a very wide range of strain rates including shock wave regime. High-velocity plate impact tests, drop-weight tests, and quasi-static tests using universal and Instron testing machines were used for the high strain rate (107 s-1), medium strain rate (102 s-1) and low strain rate (10-4 s-1) tests, respectively. The revised unsteady wave sensing system (UWSS) for plate impact tests was newly developed to determine the stress-strain relationships and Hugoniot linear relation of PC. The system consists of a powder gun for plate impact tests and three polyvenylidene fluoride (PVDF) gauges embedded in the PC utilizing a newly developed nanosecond UWSS. As originally proposed, UWSS is aimed in obtaining experimental inputs for the Lagrangian analysis used to determine the dynamic behavior of materials. The new method to determine also the shock Hugoniot stress-strain curves is proposed for PC at medium particle velocities up to about 1 km/s. The revised, unsteady wave sensing system (M-UWSS, which we proposed before) using plate impact experiment with three PVDF gauges embedded is applied to construct stress-strain curves under shock loading up to Hugoniot stress σH and Hugoniot strain εH. Linear relationship between shock velocity Us and particle velocity Up: Us = C0 + S x Up, where C0 and S are material constants, is used to determine the constant S, since the constant C0 is determined as bulk sound velocity at ambient pressure. By using the momentum conservation and the mass conservation relations, S = (1 - C0 / CH) /εH, is derived from the linear relationship described above, where , ρ is density and CH ≈ Us.

Bird Impact Analysis of Wing Leading Edge Structure Based on SPH Method

In this paper, a new bird model which combined the visco-elastic material properties and Smooth Particle Hydrodynamics (SPH) method has been proposed to simulate the bird features in high-speed conditions. A rigid plate impact test was adopted to compare the reaction of SPH and Lagrangian bird model. Compared with the results of Lagrangian bird model, new SPH model was more in line with the experimental data, and has better computational efficiency. The SPH bird model was further used to bird impacting with the wing leading edge by using finite element program LS-DYNA. Two kinds of leading edge structural enhancement programs have been proposed and carried out simulation of bird impact. Basis of calculation, the design parameters of experimental structure was determined and was produced to wait for final testing.

Ultrasonic evaluation of spall damage accumulation in aluminum and steel subjected to repeated impact

This paper shows that the spall damage accumulated in target plates during repeated impact tests can be effectively evaluated by invoking ultrasonic techniques. The experimental data obtained for commercially available, pure aluminum and medium carbon steel subjected to three times repeated plate impact test, serve as a proof of efficiency and accuracy of our method. The spall damage was evaluated using a low frequency scanning acoustic microscope (C- and B-scan images) as well as measurement of ultrasonic velocity, attenuation (amplitude change of B2–B1 echo) and backscattering intensity. We provide here a detailed account of the damage history in essentially ductile aluminum. This proves that the voids nucleated during the first impact form the defect array undergoing coalescence when the stress of second impact is lower than the level applied in the first one. The case of aluminum contrasts the behavior of carbon steel, governed by the development of microcracks generated already during the initial impact. Our conclusions are based on the output of B- and C-scan images consistent with the results of ultrasonic measurements. While providing insights into the evolution of the spall damage, the non-destructive ultrasonic technique promoted by the present authors offers advantages of efficiency, direct applicability to the components of commercial structures, and for early damage prediction.

Impact on HDPE and PVC plates – Experimental tests and numerical simulations

This paper presents first material tests on HDPE and PVC, and subsequently impact tests on plates made of the same materials. Finally, numerical simulations of the plate impact tests are compared with the experimental results. A rather comprehensive series of mechanical material tests were performed to disclose the behaviour of PVC and HDPE in tension and compression. Quasi-static tests were carried out at three rates in compression and two in tension. Digital image correlation, DIC, was used to measure the in-plane strains, revealing true stress–strain curves and allowing to analyze strain-rate sensitivity and isotropy of Poisson's ratio. In addition, dynamic compression tests were carried out in a split-Hopkinson pressure bar. Quasi-static and dynamic tests were also performed on clamped plates made of the same PVC and HDPE materials, using an optical technique to measure the full-field out-of-plane deformations. These tests, together with the material data, were used for comparative purposes of a finite element analysis. A reasonable agreement between experimental and numerical results was achieved.

Transient stress and failure analysis of impact experiments with ceramics

At Ernst-Mach-Institut (EMI), ductile and brittle materials are studied both experimentally and numerically with the focus on shock and impact loading and associated damage effects. Experimental investigations using plate impact, edge-on impact, penetration and perforation on light gas accelerators provide input for numerical studies and allow verification of constitutive models. Hydrocodes are the standard numerical tool for the simulation of transient processes and basis of the modelling in the present paper. Besides use of commercial codes with mesh-based Lagrangean and Eulerian spatial discretization, an own meshfree SPH code has been developed at EMI. Edge-on impact experiments, plate impact tests and deep penetration in confined targets are simulated to study different loading histories and failure conditions. Significantly different material models based on continuum damage approaches as well as microstatistical modelling of inhomogeneities have been implemented. Their performances in reproducing damage fracture patterns and continued fragment loading in different ceramic types are analysed and compared to edge-on impact experiments.

Quasi-brittle fracture during structural impact of AA7075-T651 aluminium plates

The stress–strain behaviour of the aluminium alloy 7075 in T651 temper is characterized by tension and compression tests. The material was delivered as rolled plates of thickness 20 mm. Quasi-static tension tests are carried out in three in-plane directions to characterize the plastic anisotropy of the material, while the quasi-static compression tests are done in the through-thickness direction. Dynamic tensile tests are performed in a split Hopkinson tension bar to evaluate the strain-rate sensitivity of the material. Notched tensile tests are conducted to study the influence of stress triaxiality on the ductility of the material. Based on the material tests, a thermoelastic–thermoviscoplastic constitutive model and a ductile fracture criterion are determined for AA7075-T651. Plate impact tests using 20 mm diameter, 197 g mass hardened steel projectiles with blunt and ogival nose shapes are carried out in a compressed gas-gun to reveal the alloy's resistance to ballistic impact, and both the ballistic limit velocities and the initial versus residual velocity curves are obtained. It is found that the alloy is rather brittle during impact, and severe fragmentation and delamination of the target in the impact zone are detected. All impact tests are analysed using the explicit solver of the non-linear finite element code LS-DYNA. Simulations are run with both axisymmetric and solid elements. The failure modes are seen to be reasonably well captured in the simulations, while some deviations occur between the numerical and experimental ballistic limit velocities. The latter is ascribed to the observed fragmentation and delamination of the target which are difficult to model accurately in the finite element simulations.

Dislocation mechanics of copper and iron in high rate deformation tests

Different dislocation processes are shown to be operative under high rate loading by impact-induced shock tests as compared with shockless isentropic compression experiments (ICEs). Under shock loading, the plastic deformation rate dependence of the flow stress of copper is attributed to dislocation generation at the propagating shock front, while in shockless ICEs, the rate dependence is attributed to drag-controlled mobile dislocation movement from within the originally resident dislocation density. In contrast with shock loading, shockless isentropic compression can lead to flow stress levels approaching the theoretical yield stress and dislocation velocities approaching the speed of sound. In iron, extensive shock measurements reported for plate impact tests are explained in terms of plasticity-control via the nucleation of deformation twins at the propagating shock front.

同一標的板に対し衝撃毎に異なる厚さの飛翔体を用いた平板衝撃試験におけるスポール損傷変化の非破壊評価

同一標的板に対し衝撃毎に異なる厚さの飛翔体を用いた平板衝撃試験におけるスポール損傷変化の非破壊評価

Editorial

Stress–strain relationships of polycarbonate (PC) are determined over a very wide range of strain rates, including a shock wave regime. Plate impact tests, drop-weight tests, and quasi-static tests using universal and Instron testing machines are used for the high strain rate (107 s−1), medium strain rate (102 s−1) and low strain rate (10−4 s−1) tests, respectively. A newly modified unsteady wave sensing system (NM-UWSS) for plate impact tests is developed to determine the stress–strain relationships of PC. The system consists of a powder gun for plate impact tests, three embedded polyvenyliden fluoride (PVDF) gauges, and NM-UWSS. As originally proposed, UWSS is aimed at obtaining experimental inputs for the Lagrangian analysis used in determining the dynamic behavior of materials. We revise this standard system (UWSS) twice to gain a higher time resolution. In the past, the conventional charge mode (Q2 method) was used. The first modified system (M-UWSS) has been used to study two classes of materials: (1) metallic materials and (2) polymeric materials, where the Q1 method coupled with a transient differential equation for the equivalent circuit of the measurement circuit for the PVDF stress gauge was used. The latest method (Qt method) for gaining the highest time resolution of shock wavefront structure by considering the effects of a piezofilm's thickness is proposed for PC at particle velocities of up to 1 km/s. Here we show from basic equations of piezoelectricity that the charge density q, i.e., the charge release per unit area, of the active electrode is proportional to the ratio of the thickness of the shocked region to the total thickness of the piezofilm. It is demonstrated that the rise time of shock charge density q in the piezofilm induced by such shock in the Q2, Q1 and Qt methods, in this order, is becoming much shorter. The latest Qt method has the highest accuracy among these three methods. Power law relations between stress and strain rate are observed again with PC under conditions of uniaxial strain over a very wide range of strain rates, i.e., 10−4–107 s−1 including a shock wave regime. For the PC, the effects of strain rate on the stress–strain relationships are estimated using empirical formula.

Stress–strain relationships of polycarbonate over a wide range of strain rate, including a shock wave regime

Stress–strain relationships of polycarbonate (PC) are determined over a very wide range of strain rates , including a shock wave regime. Plate impact tests, drop-weight tests, and quasi-static tests using universal and Instron testing machines are used for the high strain rate (10 7 s −1 ), medium strain rate (10 2 s −1 ) and low strain rate (10 −4 s −1 ) tests, respectively. A newly modified unsteady wave sensing system (NM-UWSS) for plate impact tests is developed to determine the stress–strain relationships of PC. The system consists of a powder gun for plate impact tests, three embedded polyvenyliden fluoride (PVDF) gauges, and NM-UWSS. As originally proposed, UWSS is aimed at obtaining experimental inputs for the Lagrangian analysis used in determining the dynamic behavior of materials. We revise this standard system (UWSS) twice to gain a higher time resolution. In the past, the conventional charge mode ( Q 2 method) was used. The first modified system (M-UWSS) has been used to study two classes of materials: (1) metallic materials and (2) polymeric materials, where the Q 1 method coupled with a transient differential equation for the equivalent circuit of the measurement circuit for the PVDF stress gauge was used. The latest method ( Q t method) for gaining the highest time resolution of shock wavefront structure by considering the effects of a piezofilm's thickness is proposed for PC at particle velocities of up to 1 km/s. Here we show from basic equations of piezoelectricity that the charge density q , i.e., the charge release per unit area, of the active electrode is proportional to the ratio of the thickness of the shocked region to the total thickness of the piezofilm. It is demonstrated that the rise time of shock charge density q in the piezofilm induced by such shock in the Q 2 , Q 1 and Q t methods, in this order, is becoming much shorter. The latest Q t method has the highest accuracy among these three methods. Power law relations between stress and strain rate are observed again with PC under conditions of uniaxial strain over a very wide range of strain rates, i.e., 10 −4 –10 7 s −1 including a shock wave regime. For the PC, the effects of strain rate on the stress–strain relationships are estimated using empirical formula.

Discontinuous Growth Mechanisms of Mode II Crack under High-Speed Impact Conditions

A recoverable plate impact testing technology has been used for studying the growth mechanisms of mode II crack. The results show that interactions of microcracks ahead of a crack tip cause the crack growth unsteadily. Failure mode transitions of materials were observed. Based on the observations, a discontinuous crack growth model was established. Analysis shows that the shear crack grows unsteady as the growth speed is between the Rayleigh wave speed cR and the shear wave speed cs; however, when the growth speed approaches 2cs, the crack grows steadily. The transient microcrack growth makes the main crack speed to jump from subsonic to intersonic and the steady growth of all the sub-cracks leads the main crack to grow stably at an intersonic speed.