International Journal of Impact Engineering最新文献

Existing flexible rockfall barrier systems are frequently exposed to repeated rockfall impacts during use, yet research addressing the cumulative damage sustained by these systems remains limited. A novel numerical simulation method is proposed to study the effects of repeated impacts on flexible barrier systems, which considers the damage and deformation accumulation of components through a complete restart method. Two full-scale sequential impact tests were conducted to validate this numerical simulation method's effectiveness. The impact conditions for both tests were service energy level (SEL). The deformation behavior and energy dissipation mechanism of the flexible barrier system subjected to repeated impacts were examined. The findings indicate that the net serves as the primary component undergoing deformation during rockfall impacts, with the residual deflection of the wire-ring net accounting for approximately 61 % and 58 % of the system's overall residual deflection in the respective tests. Furthermore, the energy dissipators emerge as the principal components responsible for energy dissipation, constituting approximately 71 % and 64 % of the system's energy dissipation in the two tests, respectively. Considering that both the net and the energy dissipator are key components influencing the barrier system's ability to withstand rockfall impacts, they are also prone to experiencing the most severe damage. Methods for calculating the damage of the components have been devised. The residual deflection of the wire-ring net and the energy dissipated by the energy dissipators are employed as parameters for assessing damage. A method for estimating structural damage is developed using a two-parameter model for deflection and energy dissipation. A parametric analysis was conducted to evaluate the performance of the flexible barrier system under repeated impacts spanning impact energies from 100 kJ to 2000 kJ. The cumulative damage in both the barrier and its components is thoroughly investigated. A simplified criteria for assessing cumulative structural damage incurred as the barrier undergoes multiple repeated impacts is proposed. The study findings indicate a linear relationship between the number of impacts and both component and structural damage, with the slope of this relationship positively correlating with impact energy. Structural damage can be characterized by damage of the net and the energy dissipators, with the latter as the primary influencing factor. The findings presented in this paper offer valuable insights for informing engineering maintenance decisions.

Bird strike is one of the major threats in the aerospace industry as it can result in serious structural damages and fatal incidents. The current study addressed the bird strike to the airplane windshield both experimentally and numerically. The windshield was considered to comprise glass and polycarbonate (PC) layers as well as TPU interlayer. The bird mass and speed upon collision to the windshield were considered 80–180 m/s and 1.8 kg, respectively. The experimental findings and numerical simulation results showed a proper agreement. The results also revealed the higher strength of the windshield with polycarbonate layer compare to the one with glass main layer. Bird strike with W-1 at the velocity of 160 m/s showed no fracture while the windshield with pure glass main layer broke upon bird strike at velocities below 120 m/s. The influence of various layouts was also assessed. Finally, it can be concluded that the incorporation of polycarbonate enhanced the strength of the windshield.

Thin-shell structures are widely used in various engineering applications. It is essential to investigate the impact resistance of thin-shell structures, to provide theoretical support for engineering applications. Numerous impact tests have been conducted on thin spherical shells using ballistic guns. The effects of the impact velocity and shell thickness on the deformation and fracture of thin spherical shells are summarized. Moreover, a novel damage model based on statistic damage mechanics is proposed to better predict dynamic responses of thin shells impacted by projectiles. Considering that fracture surfaces are formed by void evolution and are affected by the stress states, the damage level is defined as the ratio of the statistical cross-sectional area of the voids to the cross-sectional area of the representative elements. Utilizing statistical methods, the incorporation of continuous void nucleation, ellipsoidal void growth, and the acceleration of dynamic void evolution are introduced into the novel damage model. Subsequently, numerical investigations of the dynamic response of spherical shells under impact are conducted based on the proposed damage model. The numerical results are consistent with the experimental results in terms of the depression deformations and strain signals. The effects of shell thickness and double-layer structures on the dynamic response of spherical shells are investigated via numerical simulations considering the novel damage model in detail. The results demonstrate that the proposed model can accurately predict the dynamic response of spherical shells impacted by flat-nose projectiles, thus serving as a valuable reference for engineering design.

The fabrics made of high strength and toughness fibers are increasingly used in the structural and individual protection. The research on energy dissipation mechanism and the improvement methods for improving energy dissipation of the fabric under impact, as well as the change of ballistic limit velocity under pre-stress, is relatively mature. However, the transmission mode and velocity of transverse wave in the fabric, as well as the method of accurate finite element simulation is still lacking. In this paper, the transverse impact response of ultra high molecular weight polyethylene (UHMWPE) fabric subjected to fragment impact is studied experimentally and numerically. A novel biaxial pre-tension fixture was developed, and fragment impact test of the fabric in pre-tension state was realized using a gas gun. The displacement-time history of each point on the fabric in three-dimensional space was characterized through 3-D DIC technology, and the process of transverse wave transmission on the fabric after fragment impact was obtained. The calculated results of the finite element model established by truss element agrees reasonably well with the experimental results. The experimental and numerical results indicate that the impact velocity of fragment affects the velocity of transverse wave in the fabric, but does not change its transmission mode. With the increase of pre-tension amount, the propagation mode changes from a cross shape to a rhombic shape and finally to a circular shape, and the wave velocity increases greatly. The wave transmission process in yarns at a mesoscopic level was analyzed through finite element simulation, and the transmission path was determined. It was found that pre-tension accelerated the wave velocity on the stepwise path, causing the transverse wave to reach a farther position in the diagonal direction of the fabric, resulting in different modes of wave transmission. The research content may provide more ways for the application of fabrics in protection.

In some practical projects, two neighboring tunnels are often constructed in natural rock bodies that have a significant number of fractures with unpredictable inclination angles and locations. To study dynamic fracture characteristics of the surrounding rock mass of the twin tunnels containing a crack with different crack inclinations and crack locations, an experimental model of a cracked twin tunnel was designed and proposed, dynamic experimental analyses were carried out by using a traditional split Hopkinson pressure bar (SHPB) device and digital image correlation (DIC) method, followed by simulation conducted by using LS-DYNA code. The findings indicate that the spandrels and corners of the twin tunnels nearest the pre-cracks are more vulnerable to failure under dynamic loads, resulting in tunnels joining the ends of the pre-cracks. With various crack inclinations and crack locations, the initial cracks are typically produced at the roof and floor of the tunnel, as well as at the points of the pre-cracks. The specimen's kind of crack and its emergence site are connected. Crack inclination and location have little influence on the maximum displacement value in the twin tunnels. The study's findings could offer a fresh theoretical direction for evaluating the stability of twin tunnels when there are numerous joints and flaws.

In the experiment of scaled models to dealing with the impact of reinforced concrete large structures, the strain rate hardening effect can lead to significant differences between the dynamic responses of prototype and model. In the present study, this thorny problem is solved by adjusting the impact velocity. Based on the mechanisms of different dynamic responses, the similarity laws for the global and local responses of reinforced concrete beams subjected to impact loading are proposed by considering the strain-rate effect of materials through Buckingham Π-theorem, where constitutive models of different materials are employed for inferring dynamic stress. In dimensional analysis, multiple control factors need to be considered simultaneously, such as the strain-rate effects of steel and concrete, since reinforced concrete components are typical composite structures. The similarity law cannot be unified when considering multiple control factors. In view of this, it is recommended to estimate the dynamic response of reinforced concrete components subjected to impact loading through the upper and lower bounds of similarity law. The upper bound of similarity law is based on the strain-rate effect of concrete, while the lower bound of similarity law is based on the strain-rate effect of steel bar. The numerical models of reinforced concrete beams are established based on the experimental background. The numerical simulations with different scaling factors indicate that the model modified by the lower bound of similarity law can better predict the global response of prototype, while the model modified by the upper bound of similarity law can more accurately predict the local response of prototype. In addition, the influence of dynamic elastic modulus of reinforced concrete on the similarity law is discussed through comparative analysis.

Due to technical restrictions, the construction of a scaled structure (model) with all required dimensions can pose challenge in certain experimental impact tests. Consequently, one of the dimensions of the structure may be distorted, infringing the prerequisite conditions for the $\Pi$ theorem, and rendering similarity unattainable. In the present work, a method that changes the striker initial velocity in order to compensate for geometric distortion is analysed. The factors calculated in this technique allow estimating the real-size structure (prototype) by using scaled models. Initially, a structure comprising two plates clamped together and subjected to impact by a mass is employed to illustrate the method. Subsequently, two models with distorted thickness are tested through numerical simulations: a clamped tube struck by a rigid mass; a buffer bow colliding with a rigid wall. A comprehensive analysis of the results and the limitations of the method is done to ascertain the sources of errors and determine which structures types can benefit from this technique.

Experimental and numerical investigations on the small-size butt-welded plates subjected to air blast loads were conducted to highlight the effect of welded joints on the blast proof structures. The inherent characteristics of welded joints, including geometry, mechanical properties and welding residual stress were evaluated and discussed in the computation of blast loading. The geometry shape was assessed through macrographic of the welded joints. The distribution of mechanical and thermal physics property was determined through basic experiments using the specimens extracted from different zones of a welded joint. Welding residual stress was calculated in a thermo-mechanical coupled model. Conventional Weapons Effects Program (CONWEP) is more economic but limited compared with Arbitrary-Lagrangian–Eulerian (ALE) method. ALE and CONWEP methods were applied to simulate the air blast load applied on the plates. Effectiveness and efficiency of the methods were discussed. The results of the two programs could both coincide well with the experiment measurements. The models under six conditions were calculated to uncouple and discuss the effect of material property distribution and welding residual stress on the dynamic response of the welded structure. Permanent deflections were considered to assess the capacity of welded structures. Welding residual stress fields and the local weak materials are advantageous to the bending deformation. The phenomenological expressions of permanent deflection across thickness under different welding conditions were established based on simulation results. The effect of aspect ratio of the welded structures was also be discussed.

We present an approach for quantifying the flow stress of metals under dynamic loads, based on experiments that involve distinct but related physical phenomena. In modified Taylor tests, a stress-wave generated velocity–time signal is measured, which indirectly provides information on the plastic deformation behavior of the tested material at high strain rate. The Johnson–Cook plasticity model is calibrated for a high-strength steel on the basis of such measurements in combination with quasi-static and dynamic tensile test data. The plasticity model parameters are found with differential evolution through the inverse optimization of material test simulations. A consistent set of model parameters is identified that reproduces measurements from all types of tests. The obtained plasticity model features a small initial yield stress, which is compensated by large strain hardening so as to produce a realistic engineering yield stress. An independent calibration method is employed, by regression of the model on quasi-static and dynamic tensile test results, that confirms the validity of the plasticity model parameter values.

Inclusions are prevalent in both natural and synthetic materials. Gaining a comprehensive understanding of their dynamic response and interaction with the surrounding matrix is essential in fundamental mechanics and engineering applications. This study aims to achieve such understanding through theoretical analysis, experimental investigations, and numerical simulations, focusing on the dynamic destruction of inclusions and the underlying mechanisms. Firstly, the circumferential, radial, and shear stresses around elastic heterogeneous inclusions were derived by the wave function expansion and Duhamel integration methods. Subsequently, a set of experiments on sandstone specimens containing three types of inclusions (plaster, epoxy resin, cement) were conducted utilizing the Split Hopkinson Pressure Bar (SHPB) system in conjunction with high-speed photography and Digital Image Correlation (DIC) techniques to obtain the surface deformation of inclusion specimens. Eventually, a series of Finite Element Method (FEM) numerical simulations adopted suitable materials were also carried out to investigate the fracture process of inclusion and matrix under dynamic impact. The results demonstrate that the stress distributions and fracture mode of matrix and inclusion are highly dependent on the physical and mechanical properties of the inclusions and the surrounding matrix, specifically, density ρ, elastic modulus E, Poisson's ratio v, and strength. With an increase in the E and v, there is a reduction in the concentration of circumferential stress, while the radial and shear stresses experience an increase. The experimental and numerical results corroborated the theoretical findings and indicated that the localized dynamic stress concentrations induced by wave scattering around the inclusions directly dominated both local and overall specimen failure. Due to the dissimilarities in elastic parameters and strength between the inclusion and the matrix, the stress state of the inclusion and the interface is not homogeneous. Under dynamic loading, the weaker inclusion experiences tensile cracking at both ends of the loading, and as the mechanical properties (ρ, E, v, and strength) of the inclusion rise, a transition from tensile- hybrid-shear failure occurs within the inclusion. The findings of this study help us understand the dynamic failure mechanisms of dissimilar inclusions in brittle material.