Initial Impact Velocity Research Articles

Investigation of the Fracture Characteristics of a Cement Mortar Slab under Impact Loading Based on the CDEM

For brittle and quasi-brittle materials such as rock and concrete, the impact-resistance characteristics of the corresponding engineering structures are key to successful application under complex service environments. Modeling of concrete-like slab fractures under impact loading is helpful to analyze the failure mechanism of an engineering structure. In this paper, simulation models of impact tests of a cement mortar slab were developed, and a continuum-discontinuum element method (CDEM) was used for dynamic analysis. Concretely, the cracking simulations of a mortar slab when considering the hammer shape and impact velocity were conducted, and the impact process and failure results of the slab structure were analyzed. The results showed that the top fracture area of the mortar slab was significantly smaller than that of slab bottom under impact loadings of the drop hammer. The impact velocity was an important factor that affected the mortar slab's cracking. With the increase in the initial impact velocity, the effective fracture area of the slab structure increased significantly; the impact force and rupture degree of the mortar slab also showed a linear growth trend. The shapes of the impact hammerhead also had a significant effect on the crack model of the mortar slab. The effective fracture zones of slab structures were close under circular and square hammers, while the effective fracture zone was significantly larger under a rectangular hammer impact. The peak value (45.5 KN) of the impact force under a circular hammer was significantly smaller than the peak value (48.7 KN) of the impact force under the rectangular hammer. When considering the influence of the stress concentration of the impact hammerhead, the maximum impact stress of the rectangular hammer was 147.3 MPa, which was significantly greater than that of the circular hammer impact (maximum stress of 87.5 MPa). This may have meant that the slab structures were prone to a directional rupture that mainly propagated along the long axis of the rectangular hammerhead. This impact mode is therefore more suitable for rehabilitation and reconstruction projects of slab structures.

In-Plane Multi-Directional Dynamic Crushing of Hexagonal Honeycomb

Hexagonal honeycomb is widely used in structural passive safety protection because of its low density, high specific strength and stable deformation process. The effects of cell wall thickness, initial impact velocity and impact direction on the deformation modes and crush characteristic of the hexagonal honeycomb are investigated with an impact finite element model (FEM), in which the cell wall thickness and out-of-plane thickness of the hexagonal honeycomb are variable. The results showed that, when the hexagonal honeycomb was impacted in the transverse plane and longitudinal plane, the impact end of the structure always shrank inward until the middle of the hexagonal honeycomb was compacted, and finally the whole structure was compressed. When it was impacted in the 60∘ oblique plane, there was no inward shrinkage, and the whole structure was compressed and deformed from the impact end toward the fixed end. Under the same initial impact velocity in different impact directions, the initial peak force (IPF) and specific energy absorption (SEA) of the hexagonal honeycomb increased with the cell wall thickness. When the cell wall thickness was constant, the IPF and SEA of the hexagonal honeycomb increased with the initial impact velocity. Then empirical formulas for IPF and SEA of the hexagonal honeycomb crushing were obtained and verified by simulation. It was found that the errors of proposed empirical formulas for IPF and SEA of the hexagonal honeycomb both were within 10%, which means the empirical formulas can be used to predict the crashworthiness of the hexagonal honeycomb.

Ballistic resistance of high-strength armor steel against ogive-nosed projectile impact

The advanced high-strength armor steel with the superior strength-to-weight ratio can extremely improve the ballistic resistance and mobility of military equipment and structures. Aiming to assess the ballistic resistance including the ballistic limit and failure modes of two types of advanced high-strength armor steels, the projectile perforation test and numerical simulations were conducted. Firstly, ten shots of 15 mm-caliber ogive-nosed projectile perforation test were performed under the initial impact velocities of 164–428 m/s. The failure mode and out-of-plane displacement (OPD) response of target plate were recorded by utilizing the high-speed 3D digital image correlation system. Then, the classical finite element (FE) and mesh-free Smoothed Particle Galerkin (SPG) methods were adopted to conduct the numerical simulations, respectively. The predicted residual velocities of projectile were compared with the test data to verify the accuracy of the calibrated constitutive model parameters and numerical analysis approaches. By further comparing the failure modes and OPD–time histories of target, it was drawn that the FE method is preferable to reproduce the failure mode of target, while the SPG method is recommended to quickly predict the residual velocity of projectile. Finally, by adopting the validated numerical analysis approach, the residual velocities for two types of high-strength armor steels under the initial impact velocity range of 150–600 m/s were obtained. The reliabilities of the existing seven analytical models in predicting the ballistic limit and residual velocity for high-strength armor steels were further evaluated.

Numerical study on effects of large curved superhydrophobic surfaces on droplet post-impact dynamics

Droplet impact on curved solid surfaces is a widely observed phenomenon. Curved surfaces with large object-to-droplet ratios, termed size factor, are expected to impose influence on the droplet post-impact dynamics. In this work, droplet impact on curved surfaces with a range of size factors is simulated. The simulation results show that both the size factor and initial impact velocity can significantly affect the post-impact dynamics of the droplet. Three post-impact dynamics states, stick state, bounce state, and shatter state are identified and found to highly depend on the size factor and impact velocity. Besides, the spreading dynamics, contact time for bouncing droplets, and the detachment velocity also depend on the size factor and initial impact velocity to some degree. Splashing of droplet at higher impact velocity is also discussed. The current study could inspire the design of functional surfaces with water-resistant and anti-icing properties in relevant applications.

Bidirectional Transport of Split Droplets

Simple and smart directional transport of impacting droplets is of great significance for future life and industry, but there are still many challenges due to complex hydrodynamics. Inspired by Setaria viridis leaves, we propose an efficient strategy to manipulate the split and transport of impacting droplets based on the simple design of liquid-repellent surfaces decorated with a ridge, and we report the large-scale, high-speed, and fast-response bidirectional transport of split droplets. Upon impacting on a wirelike ridge, the droplet splits into two parts, which have considerable horizontal transport velocity up to 34% of the initial impact velocity. The transport distance can exceed 1 order of magnitude longer than the droplet size and the response time is reduced by 50% compared to the contact time on flat surfaces. The splitting of the impacting droplet is the precondition for this bidirectional movement, and the time required for splitting strongly correlates to the horizontal transport velocity. A model is developed to accurately predict the splitting time, and a scaling law is proposed to illustrate the quantitative relationship between the horizontal transport velocity and impact parameters. These findings provide a route for using macrostructures to achieve precise droplet manipulation for diverse applications, such as anti-icing and chemical detection.

Low velocity perforation of thick magnesium alloy AM60 plates impacted by rigid conical-nose impactor

The impact resistance behaviour of the plate made of the AM60 magnesium alloy at the low velocity impact perforation mode is here investigated at room temperature using a numerical approach based on experimental results. Dynamic tests were performed using an impact digital tower on the 10.0 mm thick AM60 magnesium alloy plates using cylindrical impactors with conical-nose shapes of a nominal diameter of 12.0 mm, and a nominal mass of 5.77 kg. The plates were impacted with velocities ranging from 7 to 16 m/s. During the experiments, the failure of the target plates was evaluated. Finite element (FE) model was validated using experimental results. FE simulations of the conducted experiments were performed with ABAQUS software. In simulations the strain rate dependent Johnson–Cook yield criterion with a strain hardening law was accompanied with either the ductile fracture criterion or stress triaxiality-dependent JC fracture criterion to describe the target material properties. The stress and strain distributions for different impactor velocity considered were calculated using initial impact velocity data obtained from the experiments. A range of parameters, like element size, the fracture initiation strain, friction coefficient, etc. which play an important role in the simulation, were studied. The results of numerical simulation were compared with those from the experiment obtained. A good agreement between them was achieved. The failure process of AM60 target-plate revealed that in the case of the conical-nose impactor, the ductile hole enlargement occurs during the initial stage of the impact, and subsequently, a through-thickness fracture develops causing its shear plugging failure.

Durability Assessment Method of Hollow Thin-Walled Bridge Piers under Rockfall Impact Based on Damage Response Surface

Continuous rigid-frame bridges across valleys are often at risk of rockfalls caused by heavy rainfalls, earthquakes, and debris flow in a mountainous environment. Hollow thin-walled bridge piers (HTWBP) in valleys are exposed to the threat of impact from accidental rockfalls. In the current research, ANSYS/LS-DYNA is used to establish a high-precision rockfall-HTWBP model. The rockfall-HTWBP model is verified against a scaled impact test performed in previously published research. A mesh independence test is also performed to obtain an appropriate mesh size. Based on the rockfall-HTWBP model, the impact force, damage, and dynamic response characteristics of HTWBP under a rockfall impact are studied. In addition, a damage assessment criterion is proposed, based on the response surface model, combined with the central composite design method and Box–Behnken design method. The main conclusions are as follows: (1) the impact force of the rockfall has a substantial impulse characteristic, and the duration of the impulse load is approximately 0.01 s. (2) The impacted surface of the pier is dominated by the final elliptic damage, with conical and strip damage areas as the symmetry axis. The cross-sectional damage mode is from compression failure in the impact area and shear failure at the corner. (3) The maximum displacement occurs in the middle height of the pier. The maximum displacement increases with impact height, impact velocity, and rockfall diameter and decreases with the uniaxial compressive strength of the concrete. (4) The initial impact velocity and diameter of the rockfall are the most significant parameters affecting the damage indices. In addition, a damage assessment method, with a damage zoning diagram based on the response surface method, is established for the fast assessment of the damage level of impacted HTWBP.

Effect of Impact Velocity and Angle on Impact Wear Behavior of Zr-4 Alloy Cladding Tube.

In the pressurized water reactor nuclear power plant, 316L SS chips were captured by the support grid and continued to affect the Zr-4 cladding tube, causing the fuel rods to wear and perforate. In this work, a 60° acute angle cone of 316L SS was used to simulate the cyclic impact of debris on a Zr-4 alloy tube with different initial impact velocities and impact angles. Results showed that increasing the initial impact velocity will generate a wear debris accumulation layer with a wear-reducing effect, but also promote the extension and expansion of fatigue cracks, resulting in the delamination of Zr-4 alloy tubes. The inclination of the impact angle increases the energy loss. The energy loss rate of the 45° impact is as high as 69.68%, of which 78% is generated by the impact-sliding stage. The normal force is mainly responsible for the wear removal and plastic deformation of Zr-4 alloy tubes. Tangential forces cause severe cutting in Zr-4 alloys and pushes the resulting wear debris away from the contact surfaces.

Crashworthiness design of graded cellular materials: Experimental verification of the backward design strategy

Graded cellular materials with a suitable design may improve the crashworthiness of protective structures. A backward design strategy considering the loading-rate sensitivity of cellular material has been proposed in the companion paper to guide the crashworthiness design of graded cellular material for protecting a moving object. Detailed experimental results are presented in this study to verify the feasibility and estimate the accuracy of this design strategy. A rate-dependent, rigid–plastic hardening (D-R–PH) idealization was employed to model cellular materials. A two-specimen test method was developed to determine the dynamic material parameters and their relationship with the relative density for the crashworthiness design. A power-law relationship between the dynamic material parameters and the relative density for the closed-cell foam with acrylonitrile butadiene styrene (ABS) plastic as the matrix material was obtained and employed in the crashworthiness design. Additive manufacturing was employed to fabricate the designed graded cellular specimens with ABS plastic, and the Taylor–Hopkinson pressure bar experimental technique together with high-speed photography was applied to perform impact tests. Graded ABS closed-cell foam specimens for different design conditions, e.g., the initial impact velocity, impact mass, and design target of impact force, were designed by the D-R–PH backward design strategy and prepared, and the corresponding impact tests were conducted. The impact force applied on the mass remains relatively stable and meets the crashworthiness requirement in all test cases. Therefore, the feasibility and reliability of the crashworthiness backward design strategy are verified, and this design strategy can be used to guide the crashworthiness design in engineering practice.

Lateral motion of a solid with a heterogeneous wettability surface driven by impacting droplets

When a droplet impacts a solid with a heterogeneous wettability surface, the generated asymmetric forces can manipulate the droplet, and its counterforces can also actuate the solid in theory. In this study, a water droplet impacting a movable hydrophobic substrate, which is decorated with a hydrophilic stripe and restrained by two linear dampers, is studied numerically. After preliminarily checking the effects of the solid mass and damping coefficient of linear dampers, the dynamic mechanisms of solid motion are explored by analysing the variations in the lateral force and instantaneous displacement distance of the solid. After that, the effects of the impact parameters on the solid lateral motion are mainly investigated, including the initial droplet diameter, impact velocity and offset distance between the impact point and hydrophilic stripe. On this basis, the reciprocating solid motion under successive droplet impacts is studied, and periodic motion with different amplitudes can be realized under appropriate impact conditions. The obtained results can shed some fresh insight into the potential applications of droplet–solid interactions, which are valuable for the collection and utilization of energy from natural environments.

Investigation on the transient impact characteristics of fast switching valve during excitation stage

Fast switching valve (FSV) inevitably experiences high-speed impact while obtaining high dynamics, which affects its control accuracy and service life. In this article, the impact characteristics of the FSV during the excitation stage are investigated. Firstly, a nonlinear mathematical model considering the magnetic saturation is established and verified. Then, the influences of three types of driving voltages on the initial impact velocity and the opening time of the FSV are analyzed, which proves that only the driving voltage featured by an interval of negative voltage (−24V) can obtain a good balance between the initial impact velocity and opening time. In addition, finite element simulations of transient impact characteristics show that smaller initial impact velocity is beneficial to reduce the impact displacement, equivalent stress, and wear volume. The influences of impact times on the flow area of the valve seat are quantitatively analyzed under different impact velocities. Theoretical results indicate that, compared to the high driving voltage, the variation of flow area per million impacts is reduced by 27.3% under the presented driving voltage. Finally, experimental results demonstrate that with the presented driving voltage, the average acceleration and the average noise are reduced by 65.7% and 12%, respectively. And the acceleration frequency spectrum and the modal simulation both reveal the composition of impact noise, in which the noise heard by the human ear is mainly impacted noise, not the ringing noise.

Contact time of a droplet impacting hydrophobic surfaces

A droplet impacting hydrophobic surfaces is explored via lattice Boltzmann method simulations. Dynamics of the spreading and retraction processes of the impacting droplet with different surface wettability (θ = 106°–145°) at various Weber numbers (We = 15–35) and Reynolds numbers (Re = 235–359) are investigated. It indicates that the spreading time, the time interval from a droplet just touching the surface to reaching the maximum spreading diameter, is a fixed value that is not related to the above factors. The retraction time, the time interval from a droplet reaching the maximum spreading diameter to rebounding the surface, is just closely dependent on the surface wettability. Therefore, the contact time, the sum of the spreading time and the retraction time, is mainly related to the retraction process and dependent on the surface wettability. The time is commonly normalized as the non-dimensional time, τ = t/(ρR03/σ)1/2, where R0 is the initial droplet radius and σ is the surface tension. The emphasis of this work is placed on establishing a relationship of the contact time with the surface wettability. Based on simulation research and theoretical modeling, the surface wettability is incorporated into the scaling law of contact time, and a modified scaling law, tc ∼ (R0/V0)We1/2(1 − cosθ)−1/2, where V0 denotes the initial impact velocity, is established for a droplet impacting hydrophobic surfaces of θ = 106°–145°and We = 15–35. It is demonstrated that the scale relationship can be well applied to calculate the contact time of a droplet impacting hydrophobic surfaces for moderate Weber numbers.

Experimental study on dynamic response of 6082-T6 aluminum alloy circular tubes under lateral low-velocity impact loading

Aluminum alloy components have been increasingly used in spatial structures in recent years for their advantages of high strength to mass ratio, great corrosion resistance, and low environmental pollution. In this study, impact tests on the 6082-T6 aluminum tube with circular hollow section were carried out to reveal its dynamic response under lateral impact loading, considering the accidental impact or terrorist attack that the aluminum alloy spatial structures may suffer. Four failure modes were observed from the test results, including “three plastic hinges” mode, crack at the impact position, crack at the support, and fracture at the support. The impact process was divided into four stages for the aluminum tubes without fracture: initial collision, coupling deformation, global deformation, and elastic recovery stages. Meanwhile, three stages including initial collision, coupling deformation, and failure stages were identified for the fractured tubes. The impact force, displacement, and surface strain were recorded and discussed in detail. Moreover, the impact energy transmitted to the aluminum tube was calculated based on the test results to investigate the energy absorption process of the tube under lateral impact loading. Finally, the influence of geometric parameter (section size and span of the tube), impact energy (initial impact velocity and hammer weight), impact position, and hammer shape on the dynamic behavior of the aluminum tube under lateral impact loading was analyzed based on the impact test results.

Ballistic performance of a lightweight nacre-inspired armour panel – a numerical study

This paper provides a numerical investigation on the ballistic performance of ceramic composite armour panels with porous architectures inspired by nacre from mollusc shells. The development of such armour panels can lead to reduced weight and improve ballistic performance through projectile rotation induced by non-uniform contact stresses. The porous tablets of nacre have the potential to reduce the mass of armour panels without compromising their ballistic performance. Preliminary simulations were conducted to assess the performance of several porous bio-inspired structures that have the potential to survive projectile impact with less mass. Several bio-inspired panels composed of various porous designs were generated based on the different void architectures of nacre's tablets. Their performances were benchmarked against a monolithic panel under the same projectile impact condition and having the same 4.5 mm overall panel thickness but higher mass. It was found that for the cases considered, the biomimetic panels with porous architectures possess better ballistic performance compared to the corresponding monolithic panel by arresting the projectile having an initial impact velocity of 500 m/s, whilst reducing its mass by up to 18%. A further study was conducted on bio-inspired panels with a higher thickness but the same mass as the solid monolithic panel under the same projectile impact condition. It was found that the bio-inspired panel exhibits significantly improved ballistic performance when subjected to a fragment simulating projectile having an initial velocity chosen to be the ballistic limit velocity of the corresponding monolithic panel.

Influence of Inter-Particle Friction and Damping on the Dynamics of Spherical Projectile Impacting Onto a Soil Bed

This study investigates the dynamics of a spherical projectile impact onto a granular bed via numerical simulations by discrete element method (DEM). The granular bed is modeled as an assembly of polydisperse spherical particles and the projectile is represented by a rigid sphere. The DEM model is used to investigate the cratering process, including the dynamics of the projectile and energy transformation and dissipation. The cratering process is illustrated by tracking the motion of the projectile and granular particles in the bed. The numerical results show that the dynamics of the projectile follows the generalized Poncelet law that the final penetration depth is a power-law function of the falling height. The numerical results can match well the experimental data reported in the literature, demonstrating the reliability of the DEM model in analyzing the impact of a spherical projectile on a granular bed. Further analyses illustrate that the impact process consists of three main stages, namely the impact, penetration and collapse, as characterized by the evolution of projective velocity, strong force chains and crater shape. The initial kinetic and potential energy of the projectile is dissipated mainly by inter-particle friction which governs the projectile dynamics. The stopping time of projectile decreases as the initial impact velocity increases. The final penetration depth scales as one-third the power of total falling height and is inversely proportional to the macroscopic granular friction coefficient.

A phase field model for high-speed impact based on the updated Lagrangian formulation

High-speed impact problems usually undergo a form of highly localized plastic deformation under high strain rate with intense shock waves. In this paper, we present a coupled phase field model to simulate fracture in these situations. Our model considers the dynamic finite deformation with both strain hardening and strain rate hardening by the Johnson–Cook plasticity model. In particular, the formulation is posed in an updated Lagrangian framework and the constitutive update is realized with a return-mapping algorithm. Considering the tension–compression asymmetry, a new history variable is developed to enforce the irreversibility condition of crack propagation. The effectiveness of the proposed method is validated with a numerical example of 45 steel impact experiment along with two numerical simulations of split Hopkinson bar experiments on concrete and aluminum alloy specimens, where strong shock waves are generated. Besides, the effect of a few parameters is studied, namely, the regularization length parameter, the energy release rate, the initial impact velocity, and parameters of the Johnson–Cook model. These show the correct trend in terms of the time and extent of crack propagation.

Dynamic similitude technique for predicting the behaviour of structures subjected to impact loads

To predict the dynamic behavior of large-scale structures subjected to impact loads by similitude theory is an important design tool for engineers. However, structures under impact loads usually do not follow the usual similarity laws due to material strain-rate effects and geometrical distortion. This paper proposed a dynamic similitude method to predicting the behavior of structures subjected to impact loads where material strain-rate sensitivity and geometrical distortion are both considered. In this work, we firstly deduced a loss function of the dynamic yield stress to obtain the scaling correction factor to remove the strain rate effects by correcting initial impact velocity. Then a distortion factor related to the geometrical distortion was introduced to obtain a new velocity correction factor, which made it possible to reduce the effects of material strain-rate sensitivity and geometrical distortion together. The efficiency of the scaling approach presented in this paper was illustrated by the numerical simulations and experiments of a T-beam model under impact loading and the results showed a small deviation between the predictions of the model and the prototype.

Hygro-thermo-elastic behavior for stiffened metal doubly-curved shallow shells with porous microcapsule coating under low-velocity impact considering in-plane initial load

Low-velocity impact response of stiffened metal doubly curved shallow shells with porous microcapsule coating in hygrothermal environment is analyzed considering the in-plane initial load. A modified nonlinear Hertzian contact law which ignores the influence of friction is considered for the low velocity impact problem. Based on the classical shell theory considering the von Kármán geometric nonlinearity, the governing equations of motion for this structure are derived from the Hamilton’s variation principle while the finite difference method (FDM) and Newmark-β algorithm are used to solve the equations. The effects of temperature and moisture, initial velocity of impactor, properties of metal substrate, initial load, geometry parameters of microcapsules, geometry parameters of the stiffened shell and porosity on the transient impact response of the stiffened shell are examined and analyzed. Initial impact velocity, properties of metal substrate and porosity have a significant effect on contact force and central deflection. Normal stress is greatly affected by temperature, properties of metal substrate, geometry parameters of the stiffened shell and porosity. Moreover, the impact resistance of stiffened shells is enhanced by the in-plane initial tensile load while the effect of in-plane initial compressive load is opposite.

Non-destructive evaluation of metal plasticity using a single impact microindentation

We present a non-destructive method for determination the metal plasticity using dynamic microindentation with a spherical indenter. Based on the impact loading curve, expressions for its quantitative characterization are derived. Theoretical results are supported by multiple experiments with 10 metals and alloys. The values of the plasticity parameter calculated from the depths of the indenter penetration are close to the values obtained from the energy parameters of the indentation. We have analyzed the influence of the strain and the strain rate on the value of the plasticity parameter as well as the dependence of the dynamic plasticity on the change of the initial impact velocity. Obtained results are important for the express analysis of the plastic properties of metals in out-of-lab conditions.

Determination of the plasticity of metals by impact indentation of a spherical indenter

The problems of measuring the plastic characteristics of metals are considered. It is shown that the characteristics of materials used to compare their plasticity are not comparable and depend in the different degrees on the values of strain, strain rate, and modulus of elasticity. At the same time, the value of plasticity is more physically substantiated, which is determined by the ratio of plastic strain to total strain. It is shown that one of the optimal methods for measuring plasticity (plasticity index) is indentation. The possibility of using impact microindentation for this purpose is studied and expressions are proposed that allow calculating the plasticity based on the results of a single indentation of a spherical indenter. The specialties of the calculation of strain for this type of testing are shown. It was found that the values of plasticity obtained from the ratios of the depths of the plastic and elastic penetration of the indenter are equivalent to the values calculated from the energy ratios upon impact. Experimental studies have been carried out on metals with different hardness and type of crystal lattice. For the first time, the effect of strain rate, deformation, and impact energy (initial impact velocity) on the calculated value of plasticity when a sphere is impressed with strain rates of ~ 103 s–1 is shown. It is shown that when the strain corresponding to the onset of full plasticity during indentation is reached, the maximum sensitivity of the measured plasticity parameter for various metals is achieved.