Intense Dynamic Loading Research Articles

Influence of aggregate size and free water on the dynamic behaviour of concrete subjected to impact loading

Concrete is a material widely used in civil engineering. Thus the knowledge of its mech anical behaviour is a major safety issue to evaluate the ability of a stru cture to resist to an intense dynamic loading. In this study, two experimental techniques have been applied to a m icro-concrete and a co mmon concrete to assess the influence of the aggregate size on the dynamic response. First, spalling tests o n dry and wet specimens have been performed to characterize the tensile strength of concrete at strain rates in the range 30 - 150/ s. T hen, e dge-on i mpact t ests i n s arcophagus c onfiguration ha ve be en conducted. The cracking pattern of the micro-concrete and the concrete plates in wet and dry conditions have been compared to appraise the influence of aggregate size and free water on the damaging process.

Computer simulation of the relation between mechanical behavior and structural evolution of oxide ceramics under dynamic loading

Computer simulation is used to investigate the deformation and damage processes taking place in brittle porous oxide ceramics under intense dynamic loading. The pore structure is shown to substantially affect the size of the fragments and the strength of the materials. In porous ceramics subjected to shock loading, deformation is localized in mesoscopic bands having characteristic orientations along, across, and at ∼45° to the direction of propagation of the shock wave front. The localized-deformation bands may be transformed into macroscopic cracks. A method is proposed for a theoretical estimation of the effective elastic moduli of ceramics with pore structure without resorting to well-known hypotheses for the relation between elastic moduli and porosity of the materials.

On flexible protection and stiff protection for structure safety under explosive/impact loading

Some examples of both flexible protection and stiff protection were discussed in connection with the authors research results to explore influences of stress wave effects and material strain-rate effects on structure safety protection under intense dynamic loads. Results display that in studies on structure safety protection under intense dynamic loading, either the loading is due to shock waves or projectile impact, wave propagation effects and material strain-rate effects for the structure itself and the around media, as well as the interaction between them, should be taken into account.

Elastic-Plastic Dynamic Response of the Tube with Free Boundary Condition Subjected to Impact

Some experimental results of the free-free tubes laterally impacted by the missile were given and the finite element program LS-DYNA was used to simulate this dynamic response process. The instantaneous deformation of the circular shell given by experiments and computer simulation were compared and discussed. It can be seen that when the impact occur the local dents firstly appear at the beginning of impact. With time increase, the depth of the dents increase, the scope of the deformation of the tube wall is enlarged; the total stiffness of the cross-section of the tube is weaken and decreases at the impact point, the beam-like bending deformation take place and the rigid-body translations occur. Through the computer simulation the exchanged energy between the missile and the tube were acquired. The impact energy of the missile is transferred to internal energy and kinetic energy of the tube. The ratio of the internal energy with the kinetic energy of the tube is great for the weakness rigidity of the tube wall, which is opposite to that of a free-free beam. This research made us deeply understand the character of the response when studying the elastic-plastic behavior of the free circular shell under intense dynamic loading.

Dynamic overloads and negative damping in mechanical linkage: Case study of catastrophic failure of extrusion press

Productivity of extrusion presses can be enhanced by increasing their cycling rate. The productivity can be further increased by modifications of the driving linkages wherein the rates of speed of necessary auxiliary motions, not associated with production loads, are increased even more. However, such advancements increase importance of dynamic processes with which the designers are not very familiar. The paper describes a practical study of a large (2 × 600 ton), long stroke (560 mm), high speed (50 spm) extrusion press with a drag-link linkage drive which was designed without proper attention to dynamics of its linkage. The press experienced numerous catastrophic failures, such as cracking and breaking of large bronze bearing bushings, breaking of a massive link, fast wear of slide guideways, etc. The attempts to reduce intensity of the dynamic loads were concentrated mostly on reducing the cycling rate of the press. These attempts were not successful until the cyclical rate had been reduced down to 25 spm. The paper presents the measured data on the dynamic loads and presents a basic dynamic analysis showing that the intense dynamic loads were caused by repetitive impacts in the bearing clearances, mostly at the reversal points of links’ kinematic parameters (position, velocity, acceleration). Effects of these impacts were reinforced by the intervals of dynamic instability of the linkage, resulting in generation of negative damping terms. Some design recommendations aimed to reduction of the dynamic loads are formulated.

On the development of a knowledge-based design support system for energy absorbers

Energy absorbers (EA) dissipate kinetic energy during impact or intense dynamic loading. They have been widely used in a variety of industrial and engineering applications. To design EAs, the conventional approach uses “trial and error” iterations, which hence may be very time consuming. To improve the design methodology, a knowledge-based design support system is proposed in this paper. With this system, designers can obtain the optimal design directly with the EA knowledge base by using intelligent rule-based reasoning techniques. On the other hand, when no data are available for reasoning, the knowledge base can provide a calculation tool to generate the design result. In addition, the relevant supporting knowledge is integrated in the system in the form of real-time images, text and video demonstration to support the decision-making task.

Influence of high intensive dynamic strain on the change of titanium nickelide acousto-elastic properties during phase transitions

The change of the rate and damping of ultrasonic vibrations in low-temperature titanium nickelide alloys during phase transitions in the material initial state and after the impact of high intensive dynamic loading has been studied. It has been established that the differences in dynamics of acoustic and elastic property changes correspond to phase transitions of different types. The influence of dynamic loading in elasto-plastic strain intervals causes the increase of the scale of the given below acousto-elastic anomalies that is progressing with the growth of load intensity: increase of elastic modulus defect and buildup of attenuation factor of acoustic waves in temperature intervals of phase transitions. The physical nature of acousto-elastic effects of titanium nickelide structural transformations has been discussed.

Simply approximate estimation of local failure position of a free-free slender shell

Dynamic plastic failure characteristics of a space free-free slender shell subjected to intense dynamic loading of suddenly applied pressure unsymmetrical triangle distributed along its span was studied. Both rigid perfectly plastic (r-p-p) analytical method and finite element method based elastic perfectly plastic (e-p-p) material idealization and shell element model were adopted to predict the local failure position in the structure. It was shown that both r-p-p and e-p-p model could estimate a plastic “kink” taking place in the slender shell, which reflects the strain localization of deformation. The comparison for the position of “kink” predicted by using r-p-p and e-p-p methods is found to be reasonable good.

Computer-aided design of high-strength gradient materials operating under dynamic loads

We consider a procedure of computer-aided design of materials with high strength characteristics based on using multicomponent systems with a gradient distribution of the concentration of components. It is shown that, in accordance with the synergetic definition of an impact-deformable material as an open system, with the help of gradient materials one can exercise control of the behavior of a loaded material in nonequilibrium state. The use of gradient materials under intense dynamic loads (impact, explosion) allows improving the stability of the system due to a decrease in the total increment of entropy that is confirmed by the Prigozhin–Klimontovich theorem.

Dynamic plastic damage of simply and doubly connected elliptic plates

This paper studies the dynamic behavior of simply and doubly connected elliptic ideal rigid‐plastic plates with simply supported or clamped contours under short‐time intensive dynamic loads. It is shown that there are several mechanisms of dynamic deformation of plates. For each mechanism, equations of the dynamic behavior are obtained. Operating conditions of these mechanisms are analyzed. Analytical expressions for the ultimate, “high” and “superhigh” loads and the maximum final deflection are obtained. Numerical examples are given.

Analytical model for a rigid, linear-hardening plastic tube subjected to impulsive torsional moment

An analytical model of a rigid, linear-hardening plastic tube subjected to impulsive torsion at its free end is suggested. It is shown that an impulsive torsional moment at the free end of the tube gives rise to a torsional disturbance of strong discontinuity that propagate at a constant speed and the plastic deformation is concentrated at the front of the disturbances. Two examples are given to illustrate the characterization of the structure under the intense dynamic pure torsional loading condition.

A further study of plastic shear failure of impulsively loaded clamped beams

As early as in 1973, Menkes and Opat (Exp Mech 1973; 13:480–6) conducted an experimental investigation on the dynamic plastic response and failure of fully clamped metal beams subjected to a uniformly distributed impulsive loading and identified three basic failure modes: large inelastic deformation (Mode I), tensile tearing (Mode II) and transverse shear failure at the supports (Mode III). A rigid-plastic analysis was later carried out by Jones (Trans ASME J Eng Ind 1976; 98 (B1): 131–6), in which an elementary failure criterion was adopted to estimate the threshold impulsive velocities at the onset of Mode II or Mode III failure. A deep understanding of these three basic failure modes is of fundamental importance to failure analyses of various structures under intense dynamic loading. The present paper re-examines the plastic shear failure (Mode III) of impulsively loaded clamped beams, with focus on two effects: (i) the interaction between the shear force and bending moment; and (ii) the weakening of the sliding sections during the failing process. A dimensional analysis is first performed to obtain a general form of the threshold impulsive velocity, which overlooks succeeding concrete analyses. The elementary failure criterion is then modified to incorporate the sliding sections’ weakening effect. Interaction between the shear force and the bending moment at the supporting ends is considered by using circular yield curve (Robinson, Int J Solids Struct 1973; 9:819), Hodge's curve (J Appl Mech 1957; 24:453–6), or a yield condition based on slip-line solutions. By taking into account the variation with time of the shear force and the bending moment over the failing cross-sections, the plastic deformation and failure process of the beams are traced and the ratio of plastic shear dissipation to the total plastic dissipation is thus calculated. This is followed by a discussion on a shear strain failure criterion. Finally, the predictions from various approaches are compared with each other as well as with relevant experimental results.

Failure of plastic structures under intense dynamic loading: modes, criteria and thresholds

This paper surveys the failure modes, criteria and thresholds of plastic structural members (beams, rings, plates and thin shells, etc.) subjected to intense dynamic loads, such as impulsive velocities or impact. Besides the three basic failure modes identified by Menkes and Opat [7] for impulsively loaded beams, namely, large inelastic deformation (Mode I), tensile tearing (Mode II) and transverse shear failure at the supports (Mode III), more complicated failure behaviours observed in the cases of complex structural members and/or dynamic loading conditions are summarized and discussed. All the failure criteria used in the relevant theoretical analyses including elementary failure criterion, energy density criterion and quasi-static equivalent energy criterion, etc., are commented upon. The threshold impulsive velocities for the onset of Mode III failure of fully clamped beams predicted from various approaches are compared with each other, as well as with the relevant experimental results. Particular attention is paid to structural members with macro imperfections (for example, containing cracks or notches), where failure which usually originate from the imperfections are fracture-dominant so that local fracture criteria, such as the J-integral criterion, should be incorporated into the global structural failure description. In addition, some most recent results of the authors’ studies are briefly presented.

Failure modes and criteria of plastic structures under intense dynamic loading: A review

This paper reviews the failure modes and criteria of plastic structural members (beams, rings, plates and thin shells, etc.) subjected to intense dynamic loads, such as impulsive velocities or impact. Besides the three basic failure modes identified by Menkes and Opat [7] for impulsively loaded beams, namely,large inelastic deformation (Mode I),tensile tearing (Mode II) andtransverse shear failure at the supports (ModeIII), more complicated failure behaviours observed in the cases of complex structural members and/or dynamic loading conditions are summarized and discussed. All the failure criteria used in the relevant theoretical analyses includingelementary failure criterion, energy density criterion andquasi- static equivalent energy criterion, etc, are commented. Emphasis is particularly put on structural members with macro imperfections (for example, containing cracks or notches), where failures which usually originate from the imperfections are fracture-dominant so that local fracture criteria should be incorporated into the global structural failure description.

Void-containing nonlinear materials subject to high-rate loading

A new concept of dynamic potential (macrostress potential or macrostrain-rate potential), which has two components, i.e., deformation and kinetic potentials for heterogeneous materials under intense dynamic loading, is proposed using a micromechanical approach. In particular, for voided nonlinear materials, the approximate expression of the macrostress potential is derived through an upper bound approach. Simplified physical models for ductile porous materials (aggregates of voids and ductile matrix) are employed with the matrix material taken as power-law thermal viscoplastic and incompressible. Approximate velocity fields for the matrix are adopted to derive the dynamic loading criterion and an approximate functional form of the dynamic loading criterion is developed. The inertial, rate sensitivity, and thermal effects are directly included in the expressions of the macrostress potential and the dynamic loading function. Various features of the loading function are numerically investigated in detail. Numerical analysis reveals that both the dynamic growth of voids and the loading surface have strong rate dependence and temperature dependence. Experimental data, theoretical analysis, and numerical modeling indicate that the inertial and thermal effects play a very important role in the dynamic behavior of the voided nonlinear materials.

Transient deformation modes of square plates subjected to explosive loadings

The phenomenon of transient deformation of square plates under intense dynamic loading has not been studied fully, either in experiment or theory. In this paper the experimental method for obtaining transient deformation profiles of square plate under explosive loading is demonstrated and evaluated. An optical technique proved to be reliable and accurate in obtaining the transient deformation mode of the square plate. Elastoplastic numerical analysis which includes finite deflection, material elasticity and strain hardening was carried out for one of the tests on an aluminium alloy plate to reproduce the transient response of the plate under measured pressure. The transient deformation profiles have also been obtained successfully by the numerical method and show good agreement with the experimental results. Numerical results are also presented for the impulsively loaded rectangular plates tested by Jones et al [Jones, N., Vran, T. and Tekin, S. A. (1970). Int. J. Solids Structures6, 1499–1512]. The combined experimental and numerical investigations provide valuable information for the understanding of transient behaviour of thin plates under explosive loading and for the development of an approximate procedure using the Rigid Plastic Method.

Dynamic damage and failure in visco-plastic materials

In this paper, a dynamic damage model in ductile solids under the application of a dynamic mean tensile stress is developed. The proposed model considers void nucleation and growth as parts of the damage process under intense dynamic loading (strain rates ϵ ⩾ 10 3 s −1). The evolution equation of the ductile void has the closed form, in which work-hardening behavior, rate-dependent contribution and inertial effects are taken into account. Meanwhile, a plate impact test is performed for simulating the dynamic fracture process in LY12 aluminum alloy. The damage model is incorporated in a hydrodynamic computer code, to simulate the first few stress reverberations in the target as it spalls and postimpact porosity in the specimen. Fair agreement between computed and experimental results is obtained. Numerical analysis shows that the influence of inertial resistance on the initial void growth in the case of high loading rate can not be neglected. It is also indicated that the dynamic growth of voids is highly sensitive to the strain rates.

Evolution of microcracks in brittle solids under intense dynamic loading

An evolution equation of microcracks for brittle cracked materials subjected to an applied dynamic tensile pressure is presented. The influence of inertial effects (or kinetics) on the dynamic growth of microcracks, which is a major characteristic of intense dynamic loading, is taken into account. Numerical analysis of the evolution equation clearly indicates that microcrack growth at high strain rate (ε̇≥103 s−1) is unstable. The critical condition for microcrack dynamic growth is obtained. It is predicted that the threshold stress for dynamic growth of microcracks is dependent on the average strain rate at super-high strain rate (ε̇≥106 s−1); however, numerical result is also shown that when the average strain rate ≤105 s−1, the threshold stress can be, approximately, taken to be a constant.

Spall damage in aluminum alloy

A void growth relation for ductile porous materials under intense dynamic general loading conditions is presented. The mathematical model includes the influence of inertial effects, material rate sensitivity, as well as the contribution of void surface energy and material workhardening. Numerical analysis shows that inertia appears to resist the growth of voids. The inertial effects increase quickly with the loading rates. The theoretical analysis suggests that the inertial effects cannot be neglected at high loading rates. Plate-impact tests of aluminum alloy are performed with light gas gun. The processes of dynamic damage in aluminum alloy are successfully simulated with a finite-difference dynamic code in which the theoretical model presented in this paper is incorporated.

Dynamic failure in ductile porous materials

In this paper, a mathematical model of dynamic fracture in porous ductile materials under intense dynamic general loading is developed. The mathematical model includes the influence of inertial effects and material rate sensitivity, as well as the contribution of surface energy of a void and material work-hardening. In addition, the condition of the void compaction is considered as well. The threshold stresses for the void growth and compaction are obtained. A simple criterion for ductile fracture which is associated with material distention and plastic deformation is adopted. As an application of the theoretical model, the processes of two-dimensional spallation in LY12 aluminum alloy are successfully simulated by means of two-dimensional finite-difference Lagrangian code.