Uniaxial Impact Loading Research Articles

Dynamic constitutive model of saturated saline frozen soil under uniaxial impact loading

The soil matrix, salt crystals, ice crystals, and pore solutions constitute the composite geological material of saturated saline frozen soil. The destruction mode and dynamic constitutive model of saturated saline frozen soil need to be studied because infrastructure construction is increasingly being extended to regions with saturated saline frozen soil. Based on the split Hopkinson pressure bar device, uniaxial impact compression tests were conducted on frozen soil samples with different salt contents under different strain rates. The strain rate of saturated saline frozen soil must be emphasized based on the results. The gradient of the elastic segment and maximum stress of the soil are negatively correlated with the salt content increase. To further explore the failure mechanism, the study examined the damage and failure behavior of saturated saline frozen soil, along with the absorption energy in the failure process. According to the test results, the saturated saline frozen soil was similar to a particle-reinforced composite. Subsequently, the debonding damage of the ice–salt eutectic and the mechanical–chemical damage of the soil matrix were considered. The test results could be predicted accurately from the results of the model, verifying that the influences of the salt content and strain rate are reasonably considered by the constructed model.

Dynamic behavior of frozen soil with prefabricated holes under uniaxial impact loading

Dynamic behavior of frozen soil with prefabricated holes under uniaxial impact loading

Dynamic properties and failure mechanism of granite with non-persistent joints subjected to impact load

A Split-Hopkinson pressure bar was used to carry out a series of impact tests on prefabricated granite specimens containing joints with different inclination angles and at different joint persistency values. The results were used to determine the degradation effect of the non-persistent joints on the specimens and investigate the failure mechanism of the granite subjected to impact load. The effect of varying the length and inclination angle of the joint on the dynamic characteristics of the granite was systematically analyzed based on a consideration of the specimens’ dynamic mechanical properties. A continuum-discrete coupled model was subsequently established based on the results of the laboratory tests and used to investigate the failure mechanism and crack propagation behavior occurring on a microscopic scale. The results show that the geometric parameters of the joints and the loading rate have a significant effect on the dynamic response of the granite. Moreover, there is a good power function relationship between the dynamic increase factor and loading rate of the prefabricated jointed rock specimen when the loading rate is low to medium. As the inclination angle of the joint increases, the failure mode of the specimen transitions from tensile splitting failure → mixed tensile–shear failure → main shear failure → micro-damage failure. As the penetration of the joint increases, the failure mode of the specimen evolves from micro-damage failure → tensile splitting failure → mixed tensile–shear failure. When subjected to uniaxial impact loading, the main cracks in the jointed rock specimen are usually wing cracks and coplanar cracks. Anti-wing cracks are not generated by themselves but appear alongside other cracks as a supplementary means of achieving stress release. The present results offer a sound basis for evaluating the stability of jointed rock mass under the dynamical loading.

Dynamic Constitutive Model of Saturated Saline Frozen Soil Under Uniaxial Impact Loading

Dynamic Constitutive Model of Saturated Saline Frozen Soil Under Uniaxial Impact Loading

ASB induced phase transformation in high oxygen doped commercial purity Ti

Given that commercial purity titanium (CP-Ti) doped with a high oxygen concentration has recently been reported with high mechanical performance, the present work investigated the failure behavior and its mechanisms of the material under uniaxial impact loading. Besides the strong hardening effect of oxygen, it was found that increasing oxygen content led to increased propensity for adiabatic shear failure in CP-Ti. The texture or grain orientation was also found to have profound influence on the formation of adiabatic shear band (ASB). Microstructural examinations on the postmortem high oxygen CP-Ti suggested that uniform and equi-axed nano-grains were produced within the ASB. The orientations of these nano-grains were analyzed using the precession electron diffraction (PED) technique. It provides direct evidence of phase transformation occurring in ASB. Then we demonstrate that the nano-grains in ASB formed from parent grains by phase transformation, indicating that the α→β→α phase transformation process takes part in ASB evolution and is an underlying mechanism of grain refinement. As such, this result is a supplement for traditional well-accepted dynamic recrystallization mechanism of ASB evolution.

A damage and elastic-viscoplastic constitutive model of frozen soil under uniaxial impact loading and its numerical implementation

We analyzed the uniaxial impact compression due to different strain rates on frozen soil specimens at different temperatures using split-Hopkinson pressure bar (SHPB) and obtained the dynamic stress–strain curves under the corresponding conditions. The experimental results showed that the dynamic mechanical behavior of frozen soil under impact loading shows an obvious strain-rate effect and temperature effect, as well as obvious elastic-viscoplastic deformation characteristics. Furthermore, the dynamic stress–strain curves of frozen soil showed instability with the evolution of internal micro-cracks, micro-voids, and shear bands in frozen soil. The dynamic mechanical behavior of frozen soil was described by introducing an advanced rate-dependent continuous damage evolution model and an elastic-viscoplastic constitutive model based on the Chaboche unified viscoplastic constitutive theory and using the Drucker–Prager yield criterion. To obtain a numerical solution of the elastic-viscoplastic constitutive equation, the constitutive equation was discretized into an incremental form by Euler's method, and the closest point projection algorithm was used as the numerical integral to update the stress state during the deformation process of frozen soil. The constitutive model was numerically solved, and the theoretically calculated curves agreed well with the experimentally measured curves. Thus, we verified the rationality and applicability of the uniaxial dynamic constitutive model constructed for this study and developed its numerical integral algorithm.

Atomic insights into shock-induced spallation of single-crystal aluminum through molecular dynamics modeling

This study investigated on the spalling behavior of single-crystal aluminum under uniaxial impact loading at the micro-scale. Under the framework of non-equilibrium molecular dynamics, the influences of impact velocity on dynamic damage evolution were comprehensively studied. The radial distribution function revealed that the material melted or partially melted when Vp was greater than 2.0 km/s; thus, the impact velocity was used as an indicator to make a distinction between releasing melting (2.0 km/s < Vp < 4.0 km/s) and compression melting (Vp > 4.0 km/s). It was found that the spallation behavior could be characterized by void nucleation, growth, and coalescence. However, the mechanism of void nucleation and growth during classical spallation was completely different from that in micro-spallation. The former was mainly accompanied by the accumulation of countable void nucleation and growth, and consequently, the duration of the spalling process was longer. On the contrary, the micro-spallation of single-crystal aluminum was dependent on a substantial amount of void nucleations; thereby, the fracture time was less. In addition, the dependence of spallation on high strain-rate history was explored. Furthermore, the free surface velocity was determined to predict the spall strength, and it was observed that strain rate and temperature had significant effects on the spall strength. Therefore, the obtained results highlight the efficiency of the proposed analytical model for characterizing the spalling response of single-crystal aluminum induced under uniaxial impact loading.

Damage mechanism and dynamic constitutive model of frozen soil under uniaxial impact loading

The dynamic mechanical properties of frozen soil at different temperatures and high strain rates were tested using a split Hopkinson pressure bar (SHPB), and the variation of the wave impedance of the frozen soil was analyzed. Viscoelastic theory confirmed that an increase in the wave impedance in frozen soil over short times is the result of unfrozen water relaxation. Unfrozen water is an important factor that increases the peak stress of frozen soil under impact loading. Based on the compression failure mechanism of frozen soil, the internal microcracks were classified as random or vertical microcracks. The mesoscopic parameter (microcrack density) and the macroscopic physical quantity (wave velocity) were connected by the effective medium theory, and the longitudinal wave velocity was selected as the damage variable. The effects of the low-frequency parameters in the ZWT (Zhu–Wang–Tang) model (Wang, 2003) when applied to frozen soil were evaluated. Under impact-loading conditions, with the initiation and expansion of the internal microcracks in the frozen soil, the Maxwell element represented by the low-frequency parameters lost its function rather than degenerating into a simple spring, and thus, it continued to contribute to the macroscopic mechanical properties of the frozen soil. Finally, damage was introduced into the improved ZWT model to establish a dynamic constitutive model of the frozen soil. The predicted and experimental results agreed well, which verified the applicability of the model.

Double shock mode in graded cellular rod under impact

This paper presents an analytical study of the effect of the gradient in quasi-static plateau stress on the dynamic behavior of graded cellular rods under impact loading. Finite element (FE) simulations of graded hexagonal and circular cells’ chains under uniaxial impact loading are first carried out by using ABAQUS/EXPLICIT to observe their dynamic deformation modes. To build an analytical model, a graded cellular rod, whose quasi-static plateau stress varies along the axial direction, is supposed to be impinged by a rigid mass G with initial velocity V0. Only one shock front appears when the gradient is positive, while two shock fronts appear in the rod with negative gradient. Analytical expressions of the dynamic response parameters for the graded cellular rod are theoretically derived by using the one-dimensional shock theory originally proposed by Reid and Peng (1997). Closed form solution is found for the single shock (SS) mode, while finite difference method is employed to obtain solutions for the double shock (DS) mode. Densification velocity, at which the graded rod is just fully crushed at the end of its dynamic response, is determined; and accordingly, the maximum energy-absorbing capacity of the graded cellular rod is determined. The weakest part of the graded rod is suggested to be placed at the impact end to achieve higher energy absorption. The theoretical models are then extended to the second scenario in which the graded cellular rod together with attached mass G impinges onto a rigid target. Similar to Scenario I, the gradient significantly influences the capacity of the graded cellular rod when the ratio of mass G to the mass of the rod is relatively small. The comparison between the FE simulation results and analytical predictions shows good agreement, which validates the theoretical model.

Impact Behavior of Composite Hollow Sphere Structures

Porous composite materials constitute an innovative group of lightweight materials which combine high specific stiffness, good damping properties, and thermal insulation with the ability to absorb large amounts of energy at a low constant stress level. In the scope of this study, adhesively bonded metallic hollow sphere structures (MHSS) fully embedded within the adhesive matrix are considered with aim to determine their macroscopic behavior under uniaxial impact loading conditions by means of parametric computational simulations. The base material properties have been determined by quasi-static and dynamic experiments. Three topologies of syntactic hollow sphere structures of various dimensions are considered, namely the cubic primitive, the body centered cubic and the face centered cubic topology. Results of computational simulations show significant influence of topology and strain rate sensitivity on the composite structure behavior, while the influence of metallic hollow sphere wall thickness is less pronounced. Computational simulations show that it is possible to combine the MHSS topology, metallic hollow sphere wall thickness and strain rate sensitivity to achieve any desired dynamic response of MHSS adapted to a given engineering problem.

Dynamic behaviour of regular closed-cell porous metals &amp;ndash; computational study

The paper describes computational modelling of regular closed-cell cellular materials behaviour when subjected to impact loading conditions. Parametric computational simulations have been carried out to evaluate influences of the relative density, strain rate, pore gas and gas type on the macroscopic dynamic behaviour of cellular materials. The behaviour of the model under uniaxial impact loading conditions and large deformations has been analysed with the LS-DYNA code, which is based on the finite element method. This study helps to clarify which effects are indeed important and would have to be considered in developing new homogenised constitutive relationships for analysing impact problems with use of general computational codes. Additionally, the detailed computational models provide an insight into behaviour of cellular material accounting for pore filler and basic constitutive relations for further development of homogenised models under impact conditions and large deformations. Furthermore, they allow for determination of most appropriate geometrical and material parameters of cellular materials in regard to individual engineering application demands.

Behaviour of Cellular Structures under Impact Loading a Computational Study

New multiphysical computational models for simulation of regular open and closed-cell cellular structures behaviour under compressive impact loading are presented. The behaviour of cellular structures with fluid fillers under uniaxial impact loading and large deformations has been analyzed with the explicit nonlinear finite element code LS-DYNA. The behaviour of closed-cell cellular structure has been evaluated with the use of the representative volume element, where the influence of residual gas inside the closed pores has been studied. Open-cell cellular structure was modelled as a whole to properly account for considered fluid flow through the cells, which significantly influences macroscopic behaviour of cellular structure. The fluid has been modelled by applying a Smoothed Particle Hydrodynamics (SPH) method. Computational simulations showed that the base material has the highest influence on the behaviour of cellular structures under impact conditions. The increase of the relative density and strain rate results in increase of the cellular structure stiffness. Parametrical numerical simulations have also confirmed that filler influences the macroscopic behaviour of the cellular structures which depends on the loading type and the size of the cellular structure. In open-cell cellular structures with higher filler viscosity and higher relative density, increased impact energy absorption has been observed.

Computational studies on path independent integrals for non-linear dynamic crack problems

Recently the authors have derived various new types of path independent integrals in which the theoretical limitations of the so-calledJ integral are overcome. First, for elastodynamic crack problems, a path independent integralJ′ which has the physical meaning of energy release rate was derived. Later, more general forms of path independent integralsT * andT were derived, which are valid for any constitutive relation under quasi-static as well as dynamic conditions. This paper presents the theoretical and computational aspects of these integrals, of relevance in non-linear dynamic fracture mechanics. An efficient solution technique is also presented for non-linear dynamic finite element method in which a factorization of the assembled stiffness matrix is done only once throughout the computation for a given mesh pattern. Finite element analyses were carried out for an example problem of a center-cracked plate subject to a uniaxial impact loading. The material behavior was modeled by three different constitutive relations such as linear-elastic, elastic-plastic, elastic-viscoplastic cases. The applicability of theT * integral to non-linear dynamic fracture mechanics was shown with the numerical results.

Path independent integrals in nonlinear dynamic fracture mechanics.

Recently the authors have derived various new types of path independent integrals in which the theoretical limitations of the so-called J integral are overcome. First, for elastodynamic crack problems, a path independent integral J' which has the physical meaning of energy release rate was derived. Later more general forms of path independent integrals T and T were derived, which are valid for any constitutive relations under quasi-static as well as dynamic conditions. This paper presents the theoretical and computational aspects of these integrals, of relevance in nonlinear dynamic fracture mechanics. Finite element analyses were carried out for an example problem of a center-cracked plate subject to a uniaxial impact loading. The material behavior was modeled by three different constitutive relations such as linear-elastic, elastic-plastic, and elastic-viscoplastic cases. The applicalility of the T integral to nonlinear dynamic fracture mechanics was shown with the numerical results.