Initial Stage Of Impact Research Articles

Experimental study of dam-break-like tsunami loads on the vertical structure with overhanging horizontal slab: Flat slab

Tsunamis present a significant risk to coastal infrastructure. This study conducts a comprehensive experimental investigation into the effects of tsunami impacts on a vertical structure equipped with an overhanging horizontal slab. Dam-break waves were generated in the laboratory to simulate tsunami bore. The uplift pressure of the tsunami bore on the horizontal suspended slab and the horizontal pressure on the vertical wall were analyzed by combining experimental data with water flow patterns. The results revealed that the impact process could be categorized into four stages: initial impact, run-up, quasi-steady, and recession. Two characteristic pressures were identified: a maximum pressure during the initial impact stage and a quasi-steady pressure with a longer duration. The maximum uplift pressure was found to increase with the relative position and tsunami bore height. However, this trend was influenced by the slab height and gravity, particularly on the side of the slab closest to the wall. As the slab height increased, the water flow reflection area expanded, diminishing the focusing effect caused by the slab and decreasing the quasi-steady uplift pressure. The uplift coefficient was observed to decrease with an increase in the relative slab height, and a new uplift coefficient envelope was proposed based on experimental data and published articles. Equations for the uplift pressure distribution and a novel conversion method between uplift pressure and horizontal pressure were introduced. Furthermore, based on the measured data and existing theories, equations for estimating the maximum and quasi-steady uplift pressures are presented and validated using published data. These findings provide valuable insights into understanding and estimating the impact of tsunami on structures.

Impact resistance performance and simplified calculation method of web-opened steel beams under impact loads

To investigate the impact resistance performance of web-opened steel beams (WOSBs) under impact loads, 26 specimens of WOSBs were designed and performed the falling-hammer impact tests, focusing on the impact energy, inter-opening space, opening height, and steel strength. The test results showed that the local deformations of WOSBs exhibited Vierendeel shear deformation and out-of-plane lateral buckling of the web under impact loads. At the initial impact stage, the time delay effect between the peak impact force and the peak reaction force accounts for approximately 9.77% of the total impact duration. The increase in impact energy and opening height had an adverse effect on the impact resistance performance of WOSBs. That is, as the impact energy (or opening height) increases, the average residual displacements for H1, H2, and H3 (H1, H2, and H3 represent WOSBs corresponding to three different steel materials: Q235, Q550, and Q690, respectively) increase by approximately 23.85% (18.07%), 34.05% (24.00%), and 33.20% (37.20%), respectively. The average impact efficiencies for H1, H2, and H3 are 77.38%, 72.90%, and 72.74%, respectively. This indicates that the unique geometric characteristics of WOSBs can be effectively utilized in the design of energy-absorbing structures. In conjunction with current steel structure design standards, a calculation method for the impact resistance performance of WOSBs was proposed by introducing the dynamic amplification factor and energy absorption ratio.

Experimental Study on Failure Mode of Precast Assembled Segmental Beams under Impact Loading

Precast segmental girder bridges are threatened by impact loads during service, but there is currently little research on the impact behavior of such bridges. This study investigated the dynamic responses and failure modes of precast assembled segmental beams experimentally and numerically. 10 beam specimens were designed for static and impact tests, respectively. Numerical simulations were conducted in LS-DYNA and verified by comparing them with the test results. Failure mechanisms were analyzed by verified models. The results show that the local shear failure mechanism is prominent under impact loading. Two typical failure modes were obtained: oblique section failure and joint section direct shear failure, which are related to the relative positions of joint and impact point. Confining stress can effectively improve the static bearing capacity and impact resistance, but has little effect on the early response and final failure mode. The main failure cracks occur near the joint bottom in the initial stage of impact, when the impact force is resisted by the inertia force, resulting in a characteristic “N” shape in shear force distribution. Shear failure of the joints dominates the beam failure mode. A dynamic shear span ratio can evaluate the early force characteristics of the joint section.

Ballistic impact induced wave propagation and dislocation of three-dimensional graphene origami/copper nanocomposites

Graphene (Gr)-reinforced nanocomposites have great potential as impact protection materials due to their exceptional mechanical properties. However, the intrinsic brittleness of Gr originating from its two-dimensional (2D) geometry considerably hinders its application. In this work, a three-dimensional (3D) graphene origami (GOri) reinforced copper (Cu) nanocomposite is designed and its wave propagation characteristics under a high-velocity ballistic impact are studied using the molecular dynamics (MD) method. It is found that the intrinsic brittleness of Gr can be alleviated by transforming its original 2D shape into 3D origami shape and embedding GOri in Cu matrix can lead to effectively increased out-of-plane wave propagation speed thereby boost the dissipation of impact energy by promoting rapid dislocation propagation within the Cu matrix during the initial stage of impact.

Damage behavior and assessment of aeronautical PMMA subjected to high-velocity water-jet impact

Rain erosion of forward-facing aircraft components such as windshields can lead to a significant reduction in performance, hence a potential hazard to air safety, especially for supersonic fighters exposed to precipitation. To understand such threats, the damage behavior of two types of most commonly used aeronautical PMMA, i.e., oriented and non-oriented, under water droplet impact is investigated. A single-impact jet apparatus based on the one-stage gas gun was established to simulate the rain droplet impact. Two critical variables were evaluated, including water-jet velocities and incident angles. Typical damage characteristic features including annular depression and circumferential cracks were observed on non-oriented PMMA targets. In contrast, on oriented PMMA targets, massive subsurface craze delamination was observed without any sign of surface cracks. The distinct damage features are determined by the different damage behavior of the oriented and non-oriented PMMA targets during the initial stage of the impingement and the shearing effect of the lateral jet. Increasing impact velocity leads to surface peeling and bulk removal, escalating the damage severity. The damage mechanism of subsurface failure was revealed by observing stress wave interaction and the FEM simulation. An impulse method was developed to quantify each impact event and assess the damaged area. A linear relationship between the damaged area and the initial impact stage impulse was established, which can be applied for preliminary evaluation and prediction of the damage caused by high-velocity water droplet impact on aeronautical PMMA.

Jetting and shock wave during oblique hypervelocity impact of spherical projectile

Abstract Space debris impacts on whipped shields are dominantly non-vertical. During the initial impact stage of a projectile on a target, wave propagation and evolution occur in their interior with co-dominant material fragmentation. In this study, the effects of the impact conditions (impact velocity and attack angle) on the critical conditions for jet generation were examined based on the asymmetric jetting theory. In the geometric propagation model (GPM), the effect of the attack angle was considered, and a wave front deflection angle parameter was introduced. The modified GPM could describe the geometric features and position of a wave front during an oblique impact. Combined with smoothed particle hydrodynamics numerical simulations, the interior of projectiles, fragmentation features and pressure attenuation were studied. It was found that in large attack angle cases, the projectile material is more likely to reach the critical conditions for jet generation. The modified GPM is an oblique elliptic equation that is a function of the equivalent speed, impact velocity, attack angle, time and deflection angle. It may be applicable to hypervelocity events involving any monolithic material as long as the equivalent speed and deflection angle can be provided from numerical simulations. The impact conditions exhibit a quantitative relationship with the pressure attenuation in a projectile, among which the impact velocity has a more significant effect. This study established a quantitative analysis method for the initial impact stage of the oblique hypervelocity impact of a spherical projectile on a flat plate.

FEM-SPH adaptive method for analysis protection mechanism of B4C ceramics

To simulate the collision of a tungsten heavy alloy projectile with different velocities on the B4C ceramic target, an adaptive conversion model from Lagrangian elements to SPH particles is developed due to the limitations of the traditional finite element method (FEM) with the element erosion technique. By comparing the simulation results with experimental data and FEM simulation results, the accuracy and effectiveness of the model were validated. According to the damage morphological characteristics, the target damage was classified into the central crush area, cone cracks area, radial/circular area, and fragment falling area by analyzing the damage nephogram, and the formation causes of each area were discussed in detail. According to the simulation results, the impact process between the projectile and target is split into three stages: initial impact stage, erosion stage, and crack propagation stage, and the energy dissipation in each phase was computed and examined. Finally, the protection mechanism is summarized in detail. This study contributes to improve the protection mechanism system of B4C ceramic target.

Experimental study on flow kinematics of dam-break induced surge impacting onto a vertical wall

Tsunami surges are frequently simulated by dam-break flows over dry beds. The purpose of this study is to quantitatively investigate the flow kinematics and turbulent characteristics of a surge impacting onto a vertical wall. To quantify the flow kinematics, the particle image velocimetry technique was used in the non-aerated region, while the bubble image velocimetry technique was employed to measure the impact-induced turbulent flow with air entrainment. The measured velocity fields of the impact-induced splash confirmed the feasibility of Ko and Yeh's [Coastal Eng. 131, 1–11 (2018)] model employing a solid-body motion assumption of splash that estimates the impact force by bores and surges at the initial impact stage. Velocity fields and streamlines revealed that the main water body overturned backward and formed a large two-phase vortex, while a small counter rotating vortex was also formed at the corner of the wall-bed junction. The mean velocity magnitude of the small corner vortex is about two-thirds that of the main water body. The mean turbulent intensity of these aerated regions is about 3.4 times that of the non-aerated regions. Based on a wavelet transform-based method, the result reveals that the mean turbulence length scale of the aerated region is about two-thirds that of the non-aerated region. This study reveals for the first time the quantitative flow field results of the surge impact process, which deepen insight of tsunami risk in coastal engineering, thus improving the accuracy of post-damage prediction in coastal areas.

Numerical simulation on the dynamic response of bonded unidirectional prestressed concrete slabs subjected to low-velocity impact

In this study, the dynamic response of bonded unidirectional prestressed concrete slabs subjected to low-velocity drop hammer impact is numerically investigated by using finite element software LS-DYNA. The feasibility of the finite element models is verified using experimental results available in the literature. With the verified FE models, the dynamic response of bonded unidirectional prestressed concrete slabs is analyzed in terms of the impact force, displacement, energy dissipation, and failure modes by changing the parameters of prestressing degree, drop hammer weight, and impact velocity. The results show that the response of the prestressed concrete slab under the impact of the drop hammer can be divided into five stages, which are the initial impact stage, the inertial loading stage, the inertial unloading stage, the secondary loading stage, and the impact unloading stage. The deformations of the slabs consist of global deformation and local deformation. Impact kinetic energy is the key parameter that determines the dynamic response of the specimen, which could be used to derive the calculation formula of the specimen. With the increase of the impact kinetic energy, the main response of the specimen is changed from global response to local response. The concrete dissipates most of the impact energy under the impact of the drop hammer, which is above 67%, and the dissipation effect of the prestress bars is neglectable, which is less than 5%. The increase of unidirectional prestress can improve the impact stiffness and the penetration resistance of the slabs, and increase the proportion of local response in the specimen.

Numerical study of flow and heat transfer during a high-speed micro-drop impact on thin liquid films

Numerical simulation of high-speed micro-droplet impingement on thin liquid film covering a heated solid surface has been carried out. Effect of droplet Weber number and liquid film thickness on the characteristics of flow and heat transfer has been investigated using the coupled level set and volume of fluid method. The code is validated against both the experimental and numerical results from the literature. Results show that the crown dynamics is mostly affected by variations in the initial film thickness but is weakly influenced by changes in the Weber number. The liquid within the film can be categorized as three regions based on the heat transfer distribution: the static film region, the transition region, and the impact region. The transient local wall temperature shows three stages: first stage when the temperature decreases rapidly, followed by a second stage in which the temperature starts to rise and then becomes almost constant in the third stage. After drop impact, the local Nusselt number continuously increases until reaching a maximum value, and then decreases approaching the initial impact stage. Our analysis of the change in Weber number shows that larger Weber number contributes to intense temperature variation at the crater core relative to other radial locations. Lastly, the results reveal that the thinner liquid film leads to lower wall temperature and hence, higher average Nusselt number.

Gravity effect on water entry during an early stage

Abstract

Tsunami loads on slab bridges

Coastal bridges serve as lifelines connecting affected areas to the outside world after extreme events such as tsunamis. It is thus important to predict tsunami loads applied to bridges vulnerable to tsunami attack. Most previous studies have focused on the horizontal force on bridges due to tsunamis. However, the overturning moments applied to bridges also contribute to the flexural failure of the bridge and have not been thoroughly investigated. Continuing efforts should be made to quantify tsunami-induced loads (forces and overturning moments) on coastal bridges. This study presents a series of experiments undertaken to study the tsunami loads on slab bridges using tsunami bores. Apart from the average horizontal pressure (Ph) and the horizontal (Fx) and vertical (Fz) forces, the overturning moments about the base of the pier (Myb) and the pier-deck connection (Myp) were analysed, which are closely related to the flexural failure of bridges in tsunamis. Four different pier heights were tested, as well as seven tsunami bore cases for each bridge set-up. Results show the maximum “area-adjusted momentum flux” (the product of the flow velocity squared and the contact area of the bridge in the direction of the flow) provides a better predictor for the maximum horizontal force than the maximum momentum flux, which has been used to calculate horizontal forces on emergent structures with a uniform width in the vertical direction. When a bridge is fully submerged under a tsunami, the overturning moments Myb and Myp applied to the bridge are largest at a specific pier height. Empirical equations are proposed to calculate the maximum loads (Ph, Fx, Fz, Myb and Myp) at the initial impact stage. The threshold of jump inception when a tsunami impacts a bridge has been discussed.

Dynamic behavior and damage mechanisms of reinforced concrete piers subjected to truck impact

In this paper, the dynamic behavior and damage mechanisms of reinforced concrete (RC) piers subjected to truck impact are investigated using finite-element (FE) simulations in LS-DYNA. Firstly, high-fidelity FE models of trucks and RC piers are established and verified against previous experimental data together with a real traffic accident. Then, an intensive parametric study is carried out to evaluate the effect of truck type on the impact force, failure mode, internal force, energy absorption, and reinforcement stress of RC piers. The simulation results show that the impact force is greatly affected by the structural stiffness and mass distribution of the trucks. Big differences exist in dynamic responses and failure modes of the piers under the impact of the different trucks. Besides, the concrete plays a dominant role in the energy absorption of the piers in the initial impact stage, and the energy dissipated by the reinforcements mainly depends on the lateral deformation of the pier in the following stage. Finally, two typical failure processes of RC piers subjected to truck impact are identified based on the interaction of shear and flexural responses.

The load exerted on a rigid wall by a highly porous cylinder during impact

The paper shows the directions to improve the assessment of the external load on a rigid structure during the crash of an aircraft. Our estimates describe the modeling of initial stage of impact (deformation of the nose compartment). The main goal is to evaluate the correctness and applicability of the Riera approach to mechanical equivalent of onboard equipment. Volume-perforated samples of the aluminum alloy D16T were used as mechanical equivalents in numerical and experimental studies. Both our model which uses constitutive equations of high-porous elastoplastic medium (Carroll-Holt model) and direct 3D modeling of the structure of volume-perforated samples by finite element method in the LS-DYNA allows us to obtain the history of load on a rigid wall. We demonstrate the features and differences between this approaches and the Riera approach.

Spatiotemporal characteristics of the dam-break induced surge pressure on a vertical wall

ABSTRACT In this study, the impact process on a vertical wall due to the dam-break-induced surge was investigated experimentally. The entire surge impact process is classified into three stages, i.e. the initial impact stage, the reflected stage, and the second impact stage. Meanwhile, the wall is vertically divided into the bottom impact zone and the upper outer zone. In the initial impact stage, the upper turbulent and the lower steady splash-ups occur. Only the steady splash-ups induce horizontal pressures on the wall. In the reflected stage, a pressure oscillation occurs, which can be estimated using the pressure oscillation model. Pressure distribution in the impact zone has a large vertical gradient only in the initial impact stage. Surge force exerting on the bottom impact zone remains large in the entire process, whereas surge force in the outer zone is important only in the reflected stage. The maximum total force is equally contributed by the impact and outer zone components. A modified water wedge model was proposed, which predicts the pressure-rising process reasonably. The solid body assumption is suitable only for the turbulent splash-ups in the outer zone, which however is oversimplified for the steady splash-ups movement in the impact zone.

Power-Law Scaling of Early-Stage Forces during Granular Impact.

We experimentally and computationally study the early-stage forces during intruder impacts with granular beds in the regime where the impact velocity approaches the granular force propagation speed. Experiments use 2D assemblies of photoelastic disks of varying stiffness, and complimentary discrete-element simulations are performed in 2D and 3D. The peak force during the initial stages of impact and the time at which it occurs depend only on the impact speed, the intruder diameter, the stiffness of the grains, and the mass density of the grains according to power-law scaling forms that are not consistent with Poncelet models, granular shock theory, or added-mass models. The insensitivity of our results to many system details suggests that they may also apply to impacts into similar materials like foams and emulsions.

Numerical and experimental study on effect of braiding angle on low-velocity transverse punch response of braided composite tube

In this study, the performance of braided composite tubes under low-velocity transverse impact loading at mid-span was investigated using both numerical and experimental methods. Three types of braided composite tubes with different braiding angles (30°, 45°, and 60°) were manufactured. The transverse punch behavior of the tubes was examined on a low-velocity imspact test bench. A meso-level finite element model of the composite tube was also established for identifying the damage initiation and development. The numerical results showed a good correlation with the experimental data. The mechanical response including force–time histories, force–displacement histories, and fracture morphologies was compared between three types of composite tubes for analyzing the influence of braiding angle on the impact response and failure mode. Although suffering from the low bending stiffness depends on fiber volume fraction at initial impact stage, the braided tube with 30° angle engaged more portion to resist impact loading in subsequent process and thus presented higher peak loading than the one with large angle. In addition, there are distinct different failure modes between composite tubes with various braiding angles. Shear yarn breakage underneath the punch was prone to occur in 30° sample because the braiding yarn was closer to the axial direction of tube. In contrast, the resin was deboned severely from the braiding yarn and then the braiding yarn exhibits plastic deformation in 60° sample due to the stress concentration caused by the large braiding angle.

Effect of Liquid Compressibility at a Jet Impact on a Wall

The liquid compressibility effect during impact of a liquid jet at a speed of 250 m/s on the surface of a solid wall is investigated, depending on the jet shape. The study is conducted by the comparison of the numerical solutions obtained with and without taking into account the liquid compressibility (by the CIP-CUP and BEM methods, respectively). It is shown that the manifestation of the liquid compressibility during impact substantially depends on the jet shape. For example, the neglect of the compressibility in the case of impact of a cylindrical jet with a plane end strongly distorts the impact pattern over a long time. In the case of a cylindrical jet with a hemispherical end, a distorted picture appears only at the short-term initial stage of impact. For a pointed cone-shaped jet with an angle between the cone surface and the wall equal to π/4, the flow patterns obtained with and without allowing for the liquid compressibility are almost identical.

Numerical study on phase change behavior of liquid nitrogen droplets impinging on solid surface

The distinct physical properties of liquid nitrogen make liquid nitrogen spray cooling a promising technique in aerospace engineering, the electronic industry, superconductor cooling, cryobiology, etc. In-depth study of the dynamics and thermodynamic behavior of liquid nitrogen droplets impinging on the wall surface is helpful to understand the heat transfer mechanism of spray cooling technology with liquid nitrogen. Therefore, the mathematical model of single-liquid nitrogen droplet impacted solid surface is developed by Level Set-VOF method. The effects of wall wettability (30°－150°), initial velocity (0.1, 1.6 m/s) and wall temperature (300－500 K) on the phase change behavior during the evolution of droplets are investigated, and the mathematical model of film thickness is established. The results show that enhancing the wall wettability and increasing the impact speed facilitate the spreading of the droplets in the radial direction, thereby increasing the heat exchange area and reducing the thermal resistance. Ultimately, the heat exchange performance is significantly improved. Increasing the wall temperature results in an increase in the difference between temperatures of the solid surface and the liquid, thereby significantly increasing the wall heat flux density. The lower thermal resistance at the three-phase contact line results in a higher heat flux density at the edge than in the center; the difference among the heat flux distributions on different wetted walls decreases due to the increase of initial velocity, showing a significant velocity effect. In the film boiling region, the heat transfer process is mainly concentrated in the initial stage of impact, and the gas film is the main heat transfer resistance. Based on conservation of mass and energy, a numerical model of film thickness is developed in this paper. The model predictions are in good agreement with the simulation results of this paper and others.

Breaking wave impact on a floating body with air bubble effect

The hydrodynamic problem of a breaking wave impacting on a floating body with air bubble effect is modelled based on the incompressible velocity potential theory, which is solved using the boundary element method in the time domain. To avoid the numerical inaccuracies due to the sharp temporal and spatial variations of velocity and pressure at the initial stage of impact, a dual system is adopted. The simulation close to the impact zone is conducted in a stretched coordinate system, while away from the impact zone the deformation and propagation of incoming overturning wave is simulated in the physical coordinate system. The continuities of both pressure and velocity are enforced at the interface of two zones. When the impact zone is no longer small, the dual systems will be merged and the simulation will be undertaken in one single domain. The air bubble trapped between the breaking wave and the solid surface is taken into account based on the assumption that the trapped air undergoes an adiabatic process. An auxiliary function method is used to decouple the nonlinear mutual dependence of fluid loading, body motion and bubble deformation. Simulations are undertaken for cases related to breaking wave impact in various engineering problems, including a solid coastal wall, a freely floating ship cross section, a floating breakwater and a tension leg platform. Detailed results for pressure, free surface profile, bubble deformation and body motion are provided, and their physical implications are discussed.