Dynamic Impact Experiments Research Articles

Propagation of shock waves in dry and wet sandstone: Experimental observations, theoretical analysis and meso-scale modeling

Abstract Methods of experimental observations, theoretical analysis and meso-scale modeling were used to study the propagation processes of shock waves in dry and wet sandstone under dynamic impact in this paper. According to the results from the dynamic impact experiments with velocity of 0.2–0.5 km/s, it was found that the velocity of shock wave increases linearly with water content. Additionally, the velocity of the shock wave in the sandstone showed a linearly increased regularity with the increasement of the impact velocity, which was proved by theory in this paper. Furthermore, meso-scale simulation models were performed and the simulation results showed that sandstone's porosity reduced the shock waves velocity compared to nonporous materials. Pore space filled with water counteracts the effects of porosity, resulted in larger shock wave velocity.

Anisotropy and Strain Localization in Dynamic Impact Experiments of Tantalum Single Crystals

Deformation mechanisms in bcc metals, especially in dynamic regimes, show unusual complexity, which complicates their use in high-reliability applications. Here, we employ novel, high-velocity cylinder impact experiments to explore plastic anisotropy in single crystal specimens under high-rate loading. The bcc tantalum single crystals exhibit unusually high deformation localization and strong plastic anisotropy when compared to polycrystalline samples. Several impact orientations - [100], [110], [111] and [bar{1}49] - are characterized over a range of impact velocities to examine orientation-dependent mechanical behavior versus strain rate. Moreover, the anisotropy and localized plastic strain seen in the recovered cylinders exhibit strong axial symmetries which differed according to lattice orientation. Two-, three-, and four-fold symmetries are observed. We propose a simple crystallographic argument, based on the Schmid law, to understand the observed symmetries. These tests are the first to explore the role of single-crystal orientation in Taylor impact tests and they clearly demonstrate the importance of crystallography in high strain rate and temperature deformation regimes. These results provide critical data to allow dramatically improved high-rate crystal plasticity models and will spur renewed interest in the role of crystallography to deformation in dynamics regimes.

Dynamic mechanical behaviors and failure thresholds of ultra-high strength low-alloy steel under strain rate 0.001/s to 106/s

In this study, the dynamic mechanical behaviors of the typical ultra-high strength low-alloy martensite steel 35CrMnSiA under strain rate 0.001/s～106/s were studied through quasi-static compressive tests (0.001/s), SHPB tests (2000/s～5000/s) and planar plate impact tests together with DISAR tests (105/s～106/s). The XRD analysis and metallographic observation were conducted to investigate the microstructure evolutions and failure mechanism of 35CrMnSiA under different stress states. Under uniaxial stress state, the adiabatic shear failure occurs as long as the rise rate of plastic strain energy density reaches 10.58×106 J·m−3 µ s−1 and the plastic strain energy density is above 4.51×108 J·m−3. For 35CrMnSiA samples under uniaxial strain state, the critical pressure for reversible phase transformation (α→ε, BCC→HCP) falls in the range of 17.57–19.19 GPa. Characterized by prominent temperature rise and volume shrinkage, the α→ε phase transformation induced by continuous dynamic recrystallization contributed to increasing the strength but weakening the ductility of 35CrMnSiA. In addition, the Hugoniot coefficients for 35CrMnSiA under a wide range of pressures have been determined. Moreover, the failure thresholds for 35CrMnSiA were obtained by dynamic fracture experiments and high velocity impact experiments: 35CrMnSiA projectiles fractured over impact pressure of 2.59 GPa, and when the impact pressure exceeded 21.25 GPa above which 35CrMnSiA suffered phase transformation, the projectiles had severe mass abrasion.

Effect of interface chemistry and strain rate on particle-matrix delamination in an energetic material

In this work, interface mechanical strength of a set of Hydroxyl-terminated polybutadiene (HTPB)-Ammonium Perchlorate (AP) interfaces is characterized using dynamic microscale impact experiments at strain rates up to 100 s−1. The experiments were conducted on the interfaces with varying amount of binding agent Tepanol with an impacter of radius 1 µm. Measurements of strain rates and plastic-residual depths were correlated to obtain the interface level strains and stresses. A power law viscoplastic constitutive model was fitted to the stress–strain-strain rate data in order to predict rate dependent constitutive behaviors of interfaces, particle, and matrix. An in-situ mechanical Raman spectroscopy (MRS) setup was used to analyze the effect of binding agent on interface level stress variation at different temperatures. The MRS setup is also used to obtain cohesive fracture separation properties in the analyzed samples including interface delamination strength and interface fracture energy. The measured cohesive fracture parameters capture the effect of interface chemistry variation. The cohesive parameters and the viscoplastic model obtained from the experiment were implemented in a cohesive finite element scheme to simulate dynamic crack propagation as well as delamination in model energetic material samples. Results quantify influence of the rate of loading and interface binding agent variation on rate dependent fracture. Results show that the time at which the interface delamination starts increases linearly with the increase in cohesive strength and decreases exponentially with an increase in loading rate.

Dynamic Compressive Properties of Polymer Bonded Explosives under Confining Pressure

AbstractPassively confined dynamic impact experiments on PBX1314 specimens are performed by employing aluminum jackets with split Hopkinson pressure bar. The axial and radial stress history curves are measured in the experiments, and the characterizations of the behavior of PBX1314 under dynamic multi‐axial loads are studied. A constitutive relation is developed for modeling the dynamic mechanical responses of PBX1314 by using the Boltzmann superposition principle with a Prony series representation. The material parameters of PBX1314 can be obtained by fitting the modulus master curve. Detailed finite element simulations of the passively confined tests are carried out to evaluate the measure accuracy of the device to the material mechanical behavior. The correctness of the constitutive relation is verified by comparison the finite element simulations with the experiments. Good agreements are obtained.

On design of multi-cell thin-wall structures for crashworthiness

Multi-cell thin-wall structures have drawn increasing attention and been widely applied in automotive and aerospace industries for their significant advantages in high energy absorption and lightweight. The number of cells and the topological configurations of the multi-cell thin-wall structures have a significant effect on the crashworthiness. In order to investigate the effect of the number of cells and the topological configurations of multi-cell structures on their crashworthiness characteristics, this paper first sets up the simulation models of multi-cell tubes and verifies their accuracy with the quasi-static and dynamic impact experiments. Second, it compares the energy absorption characteristics of the multi-cell structures with different numbers of cells and topological configurations under dynamic impact condition using finite element analysis (FEA). The results show that the mean crushing force (MFC) and specific energy absorption (SEA) increase with the increase in the number of cells of the multi-cell tubes, among which the five-cell tube has the best energy absorption characteristics. Third, a parametric study is carried out to investigate the effects of wall thickness and topology configurations on the crashworthiness of five-cell tubes. Finally, the multiobjective Non-dominated Sorting Genetic Algorithm (NSGA-II) is used to further optimize the five-cell tube for maximizing specific energy absorption and minimizing peak crushing force (PCF). The optimal five-cell tube has excellent crashworthiness and is a potential energy absorber.

Experimental studies of the resultant contact forces in drillbit–rock interaction

The understanding of drillbit–rock interaction is essential to the optimisation of the percussive drilling technology. It is obvious that rock fragments mainly as a result of the contact forces developed during impacts. In addition, the modelling of the dynamic response of a drilling module is possible if the contact law can be described in terms of the force and penetration. In this paper, the resultant contact force versus penetration is examined for a drillbit with conical and spherical inserts in contact with a rock. Quasi-static indentation and dynamic impact experiments are conducted on sandstone, granite and basalt. A power-law relationship is obtained between the measured resultant contact force and the penetration. The relationship is in good agreement with existing theoretical results for elastic-ideally plastic solids.

Dynamic impact force and association with structural damage to the knee joint: An ex-vivo study

No systematic, histologically confirmed data are available concerning the association between magnitude of direct dynamic impact caused by vertical impact trauma and the resulting injury to cartilage and subchondral bone. The aim of this study was to investigate the association between dynamic impact and the resulting patterns of osteochondral injury in an ex-vivo model. A mechanical apparatus was employed to perform ex-vivo controlled dynamic vertical impact experiments in 110 pig knees with the femur positioned in a holding fixture. A falling body with a thrust plate and photo sensor was applied. The direct impact to the trochlear articular surface was registered and the resulting osteochondral injuries macroscopically and histologically correlated and categorized. The relationship between magnitude of direct impact and injury severity could be classified as stage I injuries (impact <7.3MPa): elastic deformation, no histological injury; stage II injuries (impact 7.3-9.6MPa): viscoelastic imprint of the cartilaginous surface, subchondral microfractures; stage III injuries (impact 9.6-12.7MPa): disrupted cartilage surface, chondral fissures and subchondral microfractures; stage IV injuries (impact >12.7MPa): osteochondral impression, histologically imprint and osteochondral macrofractures. The impact ranges and histologic injury stages determined from this vertical dynamic impact experiment allowed for a biomechanical classification of direct, acute osteochondral injury. In contrast to static load commonly applied in ex-vivo experiments, dynamic impact more realistically represents actual trauma to the knee joint.

Fabrication of magnetic nano liquid metal fluid through loading of Ni nanoparticles into gallium or its alloy

In this study, Ni nanoparticles were loaded into the partially oxidized gallium and its alloys to fabricate desired magnetic nanofluid. It was disclosed that the Ni nanoparticles sharply increased the freezing temperature and latent heat of the obtained magnetic nano liquid metal fluid, while the melting process was less affected. For the gallium sample added with 10vol% coated Ni particles, a hysteresis loop was observed and the magnetization intensity decreased with the increase of the temperature. The slope for the magnetization-temperature curve within 10–30K was about 20 times of that from 40K to 400K. Further, the dynamic impact experiments of striking magnetic liquid metal droplets on the magnet revealed that the regurgitating of the leading edge of the liquid disk and the subsequent wave that often occurred in the gallium-indium droplets would disappear for the magnetic fluids case due to attraction force of the magnet.

Dynamic Initiation and Propagation of Multiple Cracks in Brittle Materials.

Brittle materials such as rock and ceramic usually exhibit apparent increases of strength and toughness when subjected to dynamic loading. The reasons for this phenomenon are not yet well understood, although a number of hypotheses have been proposed. Based on dynamic fracture mechanics, the present work offers an alternate insight into the dynamic behaviors of brittle materials. Firstly, a single crack subjected to stress wave excitations is investigated to obtain the dynamic crack-tip stress field and the dynamic stress intensity factor. Second, based on the analysis of dynamic stress intensity factor, the fracture initiation sizes and crack size distribution under different loading rates are obtained, and the power law with the exponent of −2/3 is derived to describe the fracture initiation size. Third, with the help of the energy balance concept, the dynamic increase of material strength is directly derived based on the proposed multiple crack evolving criterion. Finally, the model prediction is compared with the dynamic impact experiments, and the model results agree well with the experimentally measured dynamic increasing factor (DIF).

Dynamic mechanical behavior of 6061 al alloy at elevated temperatures and different strain rates

The compressive stress-strain relationships of 6061Al alloy over wide temperatures and strain rates are investigated. The dynamic impact experiments are performed using an improved high temperature split Hopkinson pressure bar apparatus. The experimental results are compared with those obtained by the modified Johnson-Cook constitutive model. It is found that the dynamic mechanical behavior depends sensitively on temperature under relatively low strain rates or on strain rate at relatively high temperatures. The good agreement indicates that it is valid to adopt the parameter identification method and the constitutive model to describe and predict the mechanical response of materials.

Experimental study of the spatial discontinuity of dynamic damage evolution

In this paper, the damage evolution of high purity aluminum under shock loading is investigated experimentally. The surface profile measurement technique based on white light axial chromatic aberration is used to measure the cross-section of sample which is soft-recovered from dynamic impact experiments. Then, the cross-section image and 3-D surface topography are obtained by reconstruction of the data, the quantified damage is also calculated based on the data. The results show that in the early stage of damage evolution the spatial distribution of relative void volume is not continuous, which results from nucleation affect, size affect and stress relaxation. The damage curves show not only the maximum damage but also a second peak. In the late stage of damage evolution, the spatial distribution of damage increment is discontinuous, which results from the coalescence of voids. The damage of the coalescence region rapidly increases and the secondary peak of the damage curve disappears.

Split Hopkinson resonant bar test for sonic-frequency acoustic velocity and attenuation measurements of small, isotropic geological samples

Mechanical properties (seismic velocities and attenuation) of geological materials are often frequency dependent, which necessitates measurements of the properties at frequencies relevant to a problem at hand. Conventional acoustic resonant bar tests allow measuring seismic properties of rocks and sediments at sonic frequencies (several kilohertz) that are close to the frequencies employed for geophysical exploration of oil and gas resources. However, the tests require a long, slender sample, which is often difficult to obtain from the deep subsurface or from weak and fractured geological formations. In this paper, an alternative measurement technique to conventional resonant bar tests is presented. This technique uses only a small, jacketed rock or sediment core sample mediating a pair of long, metal extension bars with attached seismic source and receiver-the same geometry as the split Hopkinson pressure bar test for large-strain, dynamic impact experiments. Because of the length and mass added to the sample, the resonance frequency of the entire system can be lowered significantly, compared to the sample alone. The experiment can be conducted under elevated confining pressures up to tens of MPa and temperatures above 100 [ordinal indicator, masculine]C, and concurrently with x-ray CT imaging. The described split Hopkinson resonant bar test is applied in two steps. First, extension and torsion-mode resonance frequencies and attenuation of the entire system are measured. Next, numerical inversions for the complex Young's and shear moduli of the sample are performed. One particularly important step is the correction of the inverted Young's moduli for the effect of sample-rod interfaces. Examples of the application are given for homogeneous, isotropic polymer samples, and a natural rock sample.

Dynamic Mechanical Properties of Porous Rock under Impact Loading

Dynamic impact experiments of man-made rock were carried out with the Split Hopkinson Pressure Bar (SHPB) apparatus in this paper. The impact process was analyzed and the influence of rock porosity on dynamic mechanical behavior was investigated. The stress-strain curves in rock were obtained by the one-dimensional stress wave theory. The curve lays foundation for numeric simulation of rock fracture under impact loading. The damage profiles of rock specimen under the impact loading show that the man-made rock exhibits obvious shear damage under the impact loading because it is a typical porous medium containing large quantities of defects such as pores, cracks and grain boundaries at the microscale. The experimental results also indicated that rock porosity plays an important role in dynamic mechanical behavior.

Experimental investigation on constitutive behavior of PVB under impact loading

During automotive related accidents, PVB plays an important role in both pedestrian and passenger protection as an interlayer of automotive windshield. In this paper, dynamic constitutive behavior of PVB material is thoroughly studied. Firstly, a set of dynamic compression impact experiments on PVB specimens using SHPB (Split Hopkinson Pressure Bar) method are conducted at strain rates from 700/s to 4500/s. Details of the constitutive response is analyzed based on the validation of experiment data. Stress-strain curve of PVB is then divided into two parts, i.e., “Compaction Stage” and “Hardening Stage”. Dislocations and entanglements among molecules are major reasons for the two-stage phenomena. Constitutive behaviors are different in low and high speed impacts, leading to three times more energy absorption ability of PVB in high speed impact scenario. Further, data fitting models based on both Mooney–Rivlin and Ogden Model are studied and then compared. Mooney–Rivlin Model is found to be more appropriate to describe PVB material. Moreover, PVB is proved to be a rate-dependent material with the failure strength intensify factor β ≈ 4. PVB material shows little viscoelasticity after comparison of the both models with and without the viscoelasticity part. Results offer critical experimental data, constitutive models and analysis of PVB material to further study of automotive crashworthiness and pedestrian/passenger protection.

Compressive behaviour of axially loaded spruce wood under large deformations at different strain rates

Impact limiting components of packages for the transport of radioactive materials are often designed as wood filled steel constructions. Wood absorbs major part of the impact energy in order to minimise the impact load acting upon the containment. Dynamic impact experiments with wood filled impact limiters showed different crushing mechanisms for axially loaded wood depending on their lateral constraint. Tests on spruce wood samples (Picea abies) were performed in order to clarify the influence of strain rate from static to 30 s−1 on a) compression strength, b) stress at a global strain level of 50%, and c) energy absorption capacity at 50% deformation, including statistical evaluation of the results. Results were as follows: strain rate increase led to significantly higher compression strength, stress and strain energy at a strain level of 50%. Lateral strain restriction had no effect on compression strength; it had a significant effect on stress and strain energy at strain level of 50%. Therefore, the definition of a general yield curve for wood under large deformations is not possible, the yield curve has to be chosen taking into account lateral constraints.

The Dynamic Impact Experiments Under Active Confining Pressure and the Constitutive Equation of PP/PA Blends at Multi‐Axial Compressive Stress State

AbstractSummary: A special active hydraulic confining pressure installation matched with Φ14.5 mm SHPB apparatus was developed. A series of active confining pressure impact experiments for PP/PA blends are performed in this special SHPB system under two kinds of axial strain rate: 8.0*102, 1.4*103 s−1 and the active confining pressure of 0 MPa, 4 MPa, 8 MPa, 12 MPa, 15 MPa, 20 MPa. The axial strain‐time profile, the axial stress‐time profile and the hoop strain‐time profile of the specimen are recorded online respectively. According to the equilibrium equation, the complete state of principal stress and principal strain of PP/PA blends under multi‐axial stress state is analyzed.The experimental results reveal that the axial stress‐strain curves all are related to the confining pressure and the strain rate. It can also be seen that under a constant effective strain rate the effective stress‐ effective strain curves at different confining pressures are coincident basically. This manifests that under a certain effective strain rate there is only one unique effective stress‐ effective strain curve. The multi‐axial constitutive equation for PP/PA blends is suggested finally as: where σconf is the confining pressure value. For 113 PP/PA blends, E = 2.98 GPa, α = −31.15 GPa, β = 93.31 GPa, θ2 = 8.54 µS, E2 = 0.82 GPa.

Constitutive model on the description of plastic behavior of DP600 steel at strain rate from 10−4 to 103s−1

The quasi-static and dynamic impact experiments for dual phase 600 (DP600) steel at strain rate range from 10 −4 to 1600 s −1 have been performed to study the rate-dependent mechanical behavior. The results show the obvious rate-dependent plastic behavior exists for DP600 steel. The fracture strains at high strain rates are apparently smaller than those at low strain rates although, the strength increases with the increase of the strain rates. A new constitutive model based on the Khan–Huang rate-dependent constitutive model has been proposed to describe the dynamic mechanical behavior of DP600 steel. The incremental formation of this model has been given and implemented into ABAQUS by using the user material subroutine VUMAT. The tensile deformations of the specimens at high strain rates were simulated. They are in good agreement with the experimental data, which show that the new constitutive model can describe the mechanical behavior of DP600 steel at various strain rates perfectly.

Analytical and Experimental Elastic-Plastic Impact Analysis of a Magnetic Storage Head-Disk Interface

As the use of hard disk drives in mobile applications increases, the susceptibility of disk damage due to high velocity slider-disk impact presents a serious challenge. The impact could result in extremely high contact stresses, leading to the failure of the head-disk interface. An elastic-plastic contact-mechanics-based impact model was developed and implemented to study the impact between a slider corner and a disk. The impact model is based on the contact of a rigid sphere on a deformable half-space. The effect of slider corner radii and impact velocities on the contact parameters was initially investigated for a homogeneous disk substrate. To examine the effects of thin-film layers on the disk, the model was extended to a realistic layered disk, where the actual layered mechanical properties were directly measured. At high impact velocities and/or small slider corner radii, the impact was found to be dominated by the substrate and the effect of layers was negligible. At low impact velocities and/or large slider corner radii, the effect of nanometer thick layers could be clearly seen, as these layers are stiffer than the substrate protecting the disk from potential damage at lighter loads. Realistic dynamic impact experiments involving a slider and a spinning thin-film disk were performed using an operational shock tester. The impact damage was characterized in terms of residual penetration depth caused by the impact force of the shock and the impact velocity of the slider. However, the results were inconclusive in correlating with the impact model. To better control the experimental parameters, quasistatic nanoindentation experiments were performed on actual thin-film media and were successfully compared with the model predictions.

A strain-based porosity model for use in hydrocode simulations of impacts and implications for transient crater growth in porous targets

Numerical modelling of impact cratering has reached a high degree of sophistication; however, the treatment of porous materials still poses a large problem in hydrocode calculations. We present a novel approach for dealing with porous compaction in numerical modelling of impact crater formation. In contrast to previous attempts (e.g., P-alpha model, snowplow model), our model accounts for the collapse of pore space by assuming that the compaction function depends upon volumetric strain rather than pressure. Our new ɛ-alpha model requires only four input parameters and each has a physical meaning. The model is simple and intuitive and shows good agreement with a wide variety of experimental data, ranging from static compaction tests to highly dynamic impact experiments. Our major objective in developing the model is to investigate the effect of porosity and internal friction on transient crater formation. We present preliminary numerical model results that suggest that both porosity and internal friction play an important role in limiting crater growth over a large range in gravity-scaled source size.