High Velocity Impact Loading Research Articles

Numerical and Theoretical Study of Concrete Spall Damage under Blast Loads

Spall damage is a typical failure mode of concrete structures under blast or high velocity impact loads. At the opposite side from which the structural element was impulsively loaded, spall will occur if the net primary stresses over an area exceed the dynamic tensile strength of concrete. Fragments of structural element could eject with large velocities, and this kind of damage can cause severe threats to equipment and personnel. In the present study, reinforced concrete columns subjected to the blast loading is investigated and the numerical study of concrete spall is conducted. The spall depth is recorded and compared with the theoretical results derived from wave propagation theory. The parameters that affect the concrete spall damage are investigated.

Numerical study of concrete spall damage to blast loads

Spall damage is a typical damage mode of concrete structures under blast or high velocity impact loads. Blast and impact loads generate a stress wave propagating in the structure. At the opposite side from which the structural element is impulsively loaded, spall will occur if the net primary stresses over an area exceed the concrete dynamic tensile strength and the resistance force such as the material dynamic bond and interlock. Fragments of structural element generated from spall damage could eject with large velocities, and impose significant threats to equipment and personnel even it does not necessarily greatly reduce the load carrying capacity of the structural components. In the present study, spall damage of generic reinforced concrete columns subjected to blast loads is investigated numerically. Three-dimensional numerical models are developed to predict the concrete spalling under blast loads. The accuracy of the numerical simulations is verified with blast testing data reported by other researchers. Intensive numerical simulations are then carried out to investigate the influences of the column dimensions and reinforcement mesh on concrete spall damage. Based on numerical simulation data, empirical relations are suggested to predict concrete spall damage based on explosion scenarios, column dimensions and reinforcement conditions.

Fragmentation and debris evolution modeled by a point-wise coupled reproducing kernel/finite element formulation

A point-wise evolutionarily coupled reproducing kernel (RK)/finite element (FE) formulation was developed to model fragmentation processes induced by blast and impact loadings. In this coupled approach, each integration point defines a coupling zone so that very localized deformations and damage can be treated. A triggering criterion, such as a damage index for concrete or a critical effective plastic strain for steel, is employed to initiate the fragmentation process, which is realized by changing the morphology of the zone where fragmentation is indicated from a FE model to a meshfree model. The evolutionary coupling, whereby the FE zones can morph into meshfree zones, provides a convenient and more accurate domain from which to track the debris mass and velocity distributions, which is of great interest in many applications. To facilitate the fragmentation modeling, a flexible-to-flexible meshfree contact algorithm was implemented so that high velocity (up to 3000 m/s) impact penetration problems can be effectively analyzed by the coupled Lagrangian RK/FE formulation. Numerical results are shown for structures subjected to quasi-static, blast, and high velocity impact loadings using this coupled formulation and employing widely accepted constitutive models for concrete and steel. The examples show that the coupled formulation can be used to model material fragmentation and the evolution of debris using damage indicators provided by any validated material constitutive model.

The Effect of Hybridization on the GFRP Behavior under Quasi-Static Penetration

Quasi-static punch tests using three different penetrator shapes, namely blunt-ended, hemispherical, and conical, were conducted to investigate the mechanical behavior of glass/polyester laminates hybridized with a layer of Kevlar/polyester at four different positions. It was found that the addition of the Kevlar layer at any position improves the toughness (strength and ductility) of the composite laminates, which led to enhancement of the penetration resistance. Generally, it was also verified that the back face is the most appropriate location for the Kevlar layer for laminates penetrated quasi-statically. The results also indicate that the laminates response and damage mechanism are found to be highly sensitive to the penetrator geometry. The procedure and main findings can serve as a design guideline for dynamic applications, namely, high velocity impact loading.

Damage in preloaded glass/vinylester composite panels subjected to high-velocity impacts

This paper examines the influence of in-plane preloading on the damage of thin composite panels under high-velocity impact loading. The composite was a tape laminate made with a glass-fibre and vinylester matrix. Impact on a preloaded laminate was analysed experimentally, comparing their behaviour with the condition in which the laminate was load-free. Two preload cases representative of actual structures were selected, uniaxial and biaxial load cases. An experimental device was developed to apply the load in two perpendicular directions. This device was combined with a gas gun to carry out impact tests in a broad range of impact velocities. The static preload altered the perforation-threshold velocity and the damage area in the laminate. Decrements of the both variables were detected in the preloaded specimens, both with uniaxial and biaxial loads. The reduction of the damage area was greater for impact velocities close to the perforation-threshold velocity in all the cases analysed.

A Rate Dependent Stress-Strain Relationship Model for Normal, High and Ultra-High Strength Concrete

High and ultra-high strength concrete are becoming popular for many applications, including critical infrastructure subjected to high strain rate loading such as blast and impact. A strain rate dependent material model that is applicable to a range of strengths, varying from normal strength to ultra-high strength concrete, is presented in this paper. The results from a comprehensive experimental study conducted to investigate the strength and deformation capacity of concrete cylinders under high-velocity impact loading using a Split Hopkinson Pressure Bar (SHPB) test setup is reported. Unconfined 50 mm diameter concrete cylinders with compressive strengths varying from 32 MPa (4640 psi) to 160 MPa (23 200 psi) were tested to derive the dynamic properties of concrete at strain rates up to 300 s−1. The SHPB test data were analysed to obtain the stress-strain relationships and strength dynamic increase factors (DIFs) for these concrete specimens under dynamic axial compression.

Study of foam density variations in composite sandwich panels under high velocity impact loading

This study investigates the effect of foam density variations in sandwich structure under high velocity impact loadings. The structure consists of composite facing made from glass fiber woven roving reinforced unsaturated polyester resin and rigid polyurethane foam core with density of 37, 49, 70, 95, 105 and 240 kg/m3. Smooth bore gas gun in velocity range of 100–150 m/s and 10.7 g semi-spherical tip steel projectile was used for high velocity impact tests. Results showed 49 kg/m3 foam density attained highest performance in term of ballistic limit velocity and energy absorption for fully perforated specimens. Optimized foam core density resulted in projectile yawing and side impact to back face and higher energy absorption. SEM analysis and morphological study revealed, low ballistic performance in low density foam core (below 40 kg/m3) in the sandwich structure may be associated with foam's low cell wall thickness and strut. Similar analysis for the 40–70 kg/m3 foam core densities showed increase in foam's cell wall thickness with no significant change for the strut and also highest strut thickness for the above 70 kg/m3 foam density with no change in cell wall thickness.

Shear band formation in AISI 4340 steel under dynamic impact loads: Modeling and experiment

In this study, the occurrence of the adiabatic shear bands in AISI 4340 steel under high velocity impact loading was investigated using finite element analysis and experimental tests. The cylindrical specimen subjected to the impact load was divided into different regions separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower in the region where the probability of strain localization is high based on prior experimental results in order to initialize the formation of the adiabatic shear bands. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the time to reach the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for the formation of the transformed band in the alloy and the cracks were initiated and propagated along the transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by the strain-rates and the initial material microstructure. The simulation results obtained were compared with the experimental results obtained from the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

Experimental and modeling study of dynamic mechanical properties of cement paste, mortar and concrete

Understanding the strain rate effects on cement-based materials are important for accurately modeling concrete structure damage to high-velocity impact and blast loads. This paper reports experimental results of the strain rate effect on paste, mortar and concrete. A pulse-shaped split Hopkinson pressure bar (SHPB) was employed to determine the dynamic compressive mechanical responses and failure behavior of paste, mortar and concrete under valid dynamic testing conditions. Quasi-static experiments were conducted to study material strain rate sensitivity. Strain rate sensitivity of the materials is measured in terms of the stress–strain curve, elastic modulus, compressive strength and critical strain at peak stress. Empirical relations of dynamic increase factor (DIF) for the material properties are derived and presented. A compressive constitutive model with strain rate and damage effects was modified to accurately describe the dynamic compressive stress–strain curves of the materials.

강섬유 및 FRP Sheet로 보강한 2방향 RC 슬래브의 고속 충격저항성능에 대한 해석적 평가

This paper presents high-velocity impact analysis of two-way RC slabs, including steel fibers and strengthening with fiber reinforced polymer (FRP) sheets for evaluating impact resistance. The analysis uses the LS-DYNA program, which is advanced in impact analysis. The present analysis was performed similarly to the high-velocity impact tests conducted by VTT, the technical research center of Finland, to verify the analysis results. High-velocity impact loads were applied to mm size two-way RC slab specimens, using a non-deformable steel projectile of 47.5kg mass and 134.9m/s velocity. In this research, extra impact analysis of material specimens was carried out to verify the material models used to the analysis. The elastic-plastic hydrodynamic model, concrete damage model and orthotropic elastic model were used to simulate the non-linear softening behavior of steel fiber reinforced concrete (SFRC), and material properties of normal concrete and FRP sheets, respectively. It is concluded that the suggested analysis technique has good reliability, and can be effectively applied in evaluating the effectiveness of reinforcing/retrofitting materials and techniques. Also, the Steel fiber and FRP sheet strengthening systems provided outstanding performance under high-velocity impact loads.

Occurrence of Dynamic Shear Bands in AISI 4340 Steel under Impact Loads

In this study, occurrence of adiabatic shear bands in AISI 4340 steel under high velocity impact loads is investigated using finite element analysis and experimental tests. The cylindrical steel specimen subjected to impact load was divided into different sections separated by nodes using finite element method in ABAQUS environment with boundary conditions specified. The material properties were assumed to be lower at the section where the adiabatic shear bands are expected to initialize. The finite element model was used to determine the maximum flow stress, the strain hardening, the thermal softening, and the critical strain for the formation of adiabatic shear bands. Experimental results show that deformed bands were formed at low strain rates and there was a minimum strain rate required for formation of transformed band in the alloy. The experimental results also show that cracks were initiated and propagated along transformed bands leading to fragmentation under the impact loading. The susceptibility of the adiabatic shear bands to cracking was markedly influenced by strain-rates. The simulation results obtained were compared with experimental results obtained for the AISI 4340 steel under high strain-rate loading in compression using split impact Hopkinson bars. A good agreement between the experimental and simulation results was obtained.

The Behaviour of Fibre-Metal Laminates under High Velocity Impact Loading with Different Stacking Sequences of Al Alloy

The high velocity impact response of composite laminated plates has been experimentally investigated using a nitrogen gas gun. Tests were undertaken on fibre-metal laminate (FML) structures based on Kevlar-29 fiber/epoxy-Alumina resin with different stacking sequences of 6061-T6 Al plates. Impact testing was conducted using a cylindrical shape of 7.62 mm diameter steel projectile at 400m/s velocity, which was investigated to achieve complete perforation of the target. The numerical parametric study of ballistic impacts caused by similar conditions in experimental work is undertaken to predict the ballistic limit velocity, energy absorbed by the target, and comparisons between simulations by using ANSYS AUTODYN 3D v.12.1 software and experimental work to study the effects of the shape of the projectile with different (4, 8, 12, 16 and 20mm) thicknesses on the ballistic limit velocity. While only one thickness was used with 24mm of back stacking sequence, it was not penetrated. The sequence of the Al plate position (front, middle and back) inside laminate plates of the composite specimen was also studied. The Al back stacking sequence plate for the overall results obtained was the optimum structure to resist the impact loading. The simulation results obtained of the residual velocity hereby are in good agreement with the experimental results with an average error of 1.8%. The energy absorption was obtained with 7.3% and 2.7% of the back to front and back to middle of the Al stacking sequence respectively. Hence, the back Al stacking sequence is considered the optimum position for resisting the impact loading. The data showed that these novel sandwich structures exhibit excellent energy-absorbing characteristics under high-velocity impact loading conditions. Hence, it is considered suitable for aerospace applications.

Numerical analysis of CFRP fluid-filled tubes subjected to high-velocity impact

In recent years, vulnerability against high-velocity impact loads has become an increasingly critical issue in the design of composite aerospace structures. The effects of Hydrodynamic Ram (HRAM), a phenomenon that occurs when a high-energy object penetrates a fluid-filled container, are of particular concern in the design of wing fuel tanks for aircraft because it has been identified as one of the important factors in aircraft vulnerability. The projectile transfers its momentum and kinetic energy through the fluid to the surrounding structure, increasing the risk of catastrophic failure. In the present paper, the commercial finite-element code ABAQUS/Explicit has been used to simulate an HRAM event due to the impact of a steel spherical projectile into a water-filled woven CFRP square tube. In order to simulate the fluid–structure interaction, the Coupled Eulerian Lagrangian (CEL) approach is used. Experimental tests which indicate the pressure at different points of the fluid, strains of the walls and cavity evolution for different impact velocities are compared with the numerical results in order to assess the validity and accuracy of CEL technique in reproducing such a complex phenomenon. Also, several numerical impacts at different initial projectile velocities are performed to study its influence in the HRAM phenomenon.

High velocity impact response of Kevlar-29/epoxy and 6061-T6 aluminum laminated panels

The high velocity impact response of composite laminated plates has been experimentally investigated using a nitrogen gas gun. Tests were undertaken on sandwich structures based on Kevlar-29 fiber/epoxy resin with different stacking sequence of 6061-T6 Al plates. Impact testing was conducted using cylindrical shape of 7.62mm diameter steel projectile at a range of velocities (180–400m/s) were investigated to achieve complete perforation of the target. The numerical parametric study of ballistic impact caused by same conditions in experimental work is undertaken to predict the ballistic limit velocity, energy absorbed by the target and comparison between simulation by using ANSYS Autodyn 3D v.12 software and experimental work and study the effects of shape of the projectile with different (4, 8 and 12mm) thicknesses on ballistic limit velocity. The sequence of Al plate position (front, middle and back) inside laminate plates of composite specimen was also studied. The Al back stacking sequence plate for overall results obtained was the optimum structure to resist the impact loading.The results obtained hereby are in good agreement with the experimental (maximum error of 3.64%) data where it has been shown that these novel sandwich structures exhibit excellent energy absorbing characteristics under high velocity impact loading conditions. Hence it is considered suitable for applications of armor system.

Characterising failure in textile-reinforced thermoplastic composites by electromagnetic emission measurements under medium and high velocity impact loading

The presented work focuses on failure in fibre-reinforced thermoplastic composites under medium and high velocity impact loading conditions. The investigated material is a fabric-reinforced polypropylene (PP) made of hybrid E-glass/PP yarns. Small mass impacts with velocities of 40 up to 1600 m/s are used to damage the plane specimens with a thickness of t = 2 mm. Electromagnetic and acoustic emission signals (EME and AE) are measured during the impact and signal interdependencies are elaborated. A relation can be drawn between the signal sources and the according failure modes. The examinations show, that electromagnetic emission phenomena are mainly related to fibre-matrix-debonding (FMD).

Stress-strain behavior of carbon filament yarns under high strain rates

Fiber-reinforced composites used in numerous technical applications have to meet the ever increasing safety requirements. Resistance to extreme stress under high velocity impact loads assumes even greater significance. Previous studies on the behavior of fiber-reinforced composites under impact loads provide little insight about the properties of filament yarns, a basis for many composite applications. Hence this paper focuses on the development of a suitable test method for performing high speed tensile tests on all filament yarn types, and the acquisition and analysis of the test results. This will enable the derivation of material models for their usage in the field of composites applications. Initially, the widely used carbon fiber filament yarns have been tested. The conclusive test results with a reduced yarn clamp mass and high stiffness of the test apparatus indicate that tensile strength and modulus of elasticity of carbon filament yarns increase with higher strain rates.

Impact Behaviour of Composite Materials by Using Low and High Speed Impact Tests

The study presents the need for instrumented testing to optimizing materials against impact forces. The objective of the study is how the impact behaviour of composite materials is investigated by slow and high speed impact tests. Instron Dynatup 9250HV and Instron Dynatup 8150 Impact test machines (Fig.1.) are used which are located in TUBITAK-MRC, Materials Institute , Impact Test Laboratory". The damage process in composite materials under low and high velocity impact loading and the impact energy-displacement properties of the composite materials were investigated. Composite samples were produced by woven fabrics. The results are given as graphs and tables. The Impulse Data Acquisition software is used to send the data to computer.

Impact Behaviour of Composite Materials by Using Low and High Speed Impact Tests

The study presents the need for instrumented testing to optimizing materials against impact forces. The objective of the study is how the impact behaviour of composite materials is investigated by slow and high speed impact tests. Instron Dynatup 9250HV and Instron Dynatup 8150 Impact test machines (Fig.1.) are used which are located in TUBITAK-MRC, Materials Institute , Impact Test Laboratory". The damage process in composite materials under low and high velocity impact loading and the impact energy-displacement properties of the composite materials were investigated. Composite samples were produced by woven fabrics. The results are given as graphs and tables. The Impulse Data Acquisition software is used to send the data to computer.

High-Velocity Impact Behaviour of Prestressed Composite Plates under Bird Strike Loading

An experimental and numerical analysis of the response of laminated composite plates under high-velocity impact loads of soft body gelatine projectiles (artificial birds) is presented. The plates are exposed to tensile and compressive preloads before impact in order to cover realistic loading conditions of representative aeronautic structures under foreign object impact. The modelling methodology for the composite material, delamination interfaces, impact projectile, and preload using the commercial finite element code Abaqus are presented in detail. Finally, the influence of prestress and of different delamination modelling approaches on the impact response is discussed and a comparison to experimental test data is given. Tensile and compressive preloading was found to have an influence on the damage pattern. Although this general behaviour could be predicted well by the simulations, further numerical challenges for improved bird strike simulation accuracy are highlighted.

Numerical study of a new multi-arch double-layered blast-resistance door panel

The blast-resistant structures, such as blast door panels, are traditionally designed and fabricated with solid materials of huge weight to resist blast or high velocity impact loads. This not only increases the material and construction costs, but also undermines the operational performance of the protective structures. To overcome these problems, many researchers have tried to use high-strength materials and different structural forms in structural design to resist the high blast and impact loads. This study introduces a new configuration of a double-layered panel with a structural form of multi-arched-surface. Its blast loading resistance capacity and energy absorption capacity are numerically investigated by using finite element code Ls-Dyna. To calibrate the numerical model, some existing panels are also modeled. The results are compared with existing numerical and testing data. A good agreement between them is obtained. The calibrated numerical model is used to simulate the dynamic responses of the proposed panels with different number of arches subjected to blast loading. The peak and permanent displacements of center point of internal layer, internal energy absorption and boundary reaction force of different panels are calculated and compared. It is found that the proposed new multi-arched panel performs better than other forms of panels in resisting the blast loadings. In order to maximize the structural performance of the multi-arched panels, parametric studies are also carried out to investigate the effects of panel configurations on the blast-resistance capacity of multi-arched panels with the same weight of structure material. The optimal design of the proposed multi-arch double-layered panel is determined. The optimal design of the proposed panel is also compared with the traditional blast door design by considering the damage criteria from the existing codes. It demonstrates that the panel with this new structural form can sustain higher blast loads.