Low-velocity Impact Loading Research Articles

Impact damage assessment of thin defective composite laminates manufactured by Automated Fiber Placement technique

Fiber tow-gap is a composite defect that occurs during fiber deposition using Automated Fiber Placement technique. This defect resulting from the inherent gap between the adjacent fiber tows causes local thickness consolidation. It creates relatively large resin pockets that can affect the impact response and residual compressive strength of composite structures. Although the tow-gap effect on the in-plane response of composite laminates has been widely studied, there is still a lack of knowledge in damage assessment of defective composite plates subjected to Low-Velocity Impact (LVI) loading. In this study, a series of experimental Compression After Impact (CAI) tests are carried out on the standard carbon/epoxy composite plates subjected to impact loads to investigate the combined effect of impact damage and gap defect on the residual compressive strength reduction. Effect of the curing process on the gap formation is evaluated by measuring the Induced Gap Shrinkage Factor (IGSF) using microscopic observation. This factor is used for numerical simulation of the defective composite plates. Digital Image Correlation (DIC) technique is also used to measure the in-plane strains of the specimen under compressive loading. Furthermore, a numerical investigation is carried out by implementing the Induced Defect Layer Method (IDLM) model. The numerical studies contain both LVI and CAI results, including the impact response, impact damage, and final failure of impacted specimens under compressive loading. Results show that the induced gaps play a significant role in the residual strength reduction of thin composite plates, which is significant in low-velocity impact loading. It is also shown that IDLM is a robust meso-macro damage method for damage analysis of the composite laminates with manufacturing-induced gaps.

Experimental and numerical study on multi-impact performance of pre-damaged beams strengthened with CFRP

In this study, low-velocity impact loads were first applied to produce pre-damages in three reinforced concrete (RC) beams, followed by strengthening with carbon fiber reinforced plastic (CFRP) and reloading under multi-impact. Two strengthening types, say, in straight and U-wrapping ways, were considered in the tests to determine which type is more effective to resist impact load. Furthermore, a 3D nonlinear finite element model was established to compare the experimental results and hopefully to investigate deeply in exploring other reinforcement measures. The results show that cracks of beams with U-wrapping CFRP are mainly concentrated in the mid-span area, and the crack distribution range is significantly reduced compared with un-strengthened RC beams or beams strengthened with straight CFRP. It can be concluded that CFRP can help to improve the performance of the pre-damaged beam, decrease the stiffness loss due to damage and enhance energy dissipation. The deformation recovery capacity of CFRP-strengthened beams is also enhanced under multi-impact, particularly in U-wrapping CFRP beams. In addition, the multi-impact performance in terms of the cracking pattern, impact force, and displacement history can be well visualized in the numerical simulation.

Ponašanje betonskih ploča od cementnog morta pri udarnom opterećenju

As structural members can be subjected to impact loading during their service life, impact experiments are applied for various materials and objects by taking different methods into consideration. The effect of impact load on concrete slabs prepared with cement-based mortars is investigated in this study. A total of nine slab specimens measuring 500 x 500 x 50 mm are tested within the experimental program. Test specimens are prepared to provide different strength values and tested under impact load using the drop test setup. The impact resistance values of the specimens are determined based on the acceleration, impact load and displacement values, as obtained by measurement devices. In addition, crack distribution of the specimens is observed during impact tests. It has been noted that mortar types have a significant effect on the impact behaviour of the specimens. The finite element analysis is generated for each specimen to verify experimental results. Time histories of the acceleration, displacement and impact load values of the selected test specimens are compared. The relationship between experimental and numerical analysis results is presented, which reveals that the finite element procedure can be used in the evaluation of dynamic response of test specimens under the low velocity impact loading.

Residual Performance of Reinforced Concrete Beams Damaged by Low-Velocity Impact Loading

With the advancement of material science and infrastructure design technology, there is an increased need to reflect various loading scenarios in the design of civil infrastructures, including extreme loads such as impact and blast. However, structural behavior under extreme loads has been observed to be very different from that under static loads due to the high-rate loading condition. Therefore, extensive experimental and analytical studies have been conducted to explain the structural behavior under impact loading. From these efforts, various discussions have been presented on the impact behavior and the evaluation of the impact resistance of structures. However, despite the fact that the repair or reinforcement of structures damaged by impact loads is a critical issue, studies of the residual performance of impact-damaged structures are insufficient. This paper presents an experimental study to improve the understanding of the residual performance of impact-damaged structures. A total of 29 RC beams were tested under static loads to examine the effect of impact-damage on the residual behavior of RC beams. Of these, 8 RC beams were intact, and the other 21 RC beams suffered impact loading through drop weight impact tests and were damaged prior to the static flexural tests. The residual performance was evaluated by comparing the maximum load capacity, flexural stiffness, and displacement ductility of impact-damaged beams and intact beams. The comparison indicated that the flexural stiffness and displacement ductility of the impact-damaged RC beams decreased significantly, whereas their flexural strengths did not exhibit a significant difference. Predictive models for flexural stiffness and displacement ductility of the impact-damaged beams were suggested from the test results, and through these, the residual performance of the impact-damaged structure could be evaluated.

In-plane compression behavior of a novel 3D auxetic honeycomb

The star-triangular honeycomb (STH) has a unique deformation mode and mechanical properties, especially under low-velocity impact load. This study proposes a novel 3DSTH based on 2DSTH. 3DSTH specimens were manufactured by the 3D printing process, and quasi-static compressive tests were implemented to verify the reliability of the established finite element model. Numerical results show that 3DSTH exhibits a typical negative Poisson’s ratio effect under in-plane compression, and the cells near the platens bend and collapse in the out-of-plane direction. The theoretical analytical formulas of elastic modulus and Poisson's ratio were derived according to the deformation mechanism of the typical cell. The theoretical prediction results match the numerical simulation results accurately. In addition, the parameter analysis shows that the elastic modulus decreases firstly and then increases with the increase in the inclined angle, showing a quadratic parabolic relationship. However, the Poisson’s ratio only increases monotonically with the increase in the inclined angle. Subsequently, the angle and thickness gradients of 3DSTH with different distribution patterns were constructed. The influences of gradient distribution on deformation mode and energy-absorbing characteristics of 3DSTH were studied by performing a comparative analysis. The bidirectional gradient distribution is more conducive to improving the deformation stability and energy absorption characteristics of 3DSTH than the unidirectional gradient distribution. Moreover, 3DSTH exhibits more excellent deformation mode and energy absorption characteristics, approximately 25.45% higher than that of 2DSTH with the same geometric parameters. This study is mainly expected to create some new ideas for the design of the auxetic structure.

Finite element procedure to simulate sandwich structure with an auxetic core under impact loading using ABAQUS/Explicit

<span>A sandwich structure with an auxetic core is promising in improving the performance of a sandwich structure by implying an auxetic core as its core to combine the advantages of the two structures, e.g., sandwich structure’s superior ability in flexural and shear resistance, auxetic structure in localizing damage, and densification phenomena. This paper discusses a finite element modeling procedure to simulate a sandwich structure with a heterogeneous re-entrant auxetic core. The material of the face is a unidirectional carbon fiber reinforced polymer (UD CFRP) and the core is polylactic acid (PLA). The model is subjected to a low-velocity impact loading and is run through the ABAQUS/Explicit software. We found that the model we developed here could simulate up to the elastic region and identify which element had failed. However, it could not fully resemble and represent the model from reference, where fracture or damage does not occur. This model can be further improved in its material modeling strategy, especially in the fracture modeling of the composite face with compatible material properties in all required sectors, especially damaged sections, which are strictly necessary.</span>

Mechanical properties of shear thickening fluid filled warp-knitted spacer fabrics

This paper reports the compressive behavior and low-velocity impact behavior of warp-knitted spacer fabrics (WKSFs) filled with shear thickening fluid (STF) when subjected to quasi-static compression and low-velocity impact loadings, respectively. In the steady rheological test, the STF experiences a shear thickening transition at a critical shear rate. Besides, the critical shear rate decreases with the increase of silica mass fraction. The compression curves of WKSFs filled with STF composites were consistent with those of WKSFs, including initial, elastic and compaction stage. As the compression speed increases, the overall compression load values of WKSF and WKSFs filled with STF composite increase. Moreover, the compression load of composite increases with the increase of silica mass fraction. Compared with WKSFs, WKSFs filled with STF composite composites need more work to deform under the same strain. The impact result reveals that STF-filled WKSF would absorb more energy and keep the peak load at a lower level than WKSF under the same impact loadings. In a certain range, the energy absorption of composites becomes more excellent with the increasing silica mass fraction. However, over a certain range, the energy absorption of the composite decreases with the increase of the mass fraction of silica. It was concluded that the STF-filled WKSF composite could be expected as a damping or energy-absorptive material for human body protection.

Investigation of viscoelastic properties in composite sandwich panels subjected to low-velocity impact: Experimental and numerical approaches

This study investigates the effect of viscoelastic properties in the numerical modeling of foam-filled sandwich panels when their composite facesheets are exposed to low-velocity impact loading. A finite element (FE) model is developed using the software package of ABAQUS and used to investigate the effect of both considering and ignoring the time-dependent behavior of composite laminate facesheets as well as the foam core. Material constitutive equations, damage characteristics, and failure modes are defined in a FORTRAN user-subroutine VUMAT. To characterize the material behavior of both polyurethane foam and glass/epoxy composite material, a set of experimental tests are conducted under compression, tension, and stress relaxation modes. To validate the numerical results, low-velocity impact tests at two energy levels of 9.81 and 17 joules are carried out on sandwich panels with foam core. The results of numerical simulations are found to be in good agreement with the experimental test results. It is shown that ignoring the viscoelastic properties in the composite laminate and the foam core can lead to deviations of up to 7% and 25%, respectively, from experimental results. The analysis reveals that, ignoring the viscoelastic behavior along the fiber-direction in the composite facesheets does no change the results considerably. The viscoelastic properties perpendicular to the fibers, however, has a more noticeable effect on the results due to the prevalence of the properties of the polymeric resin in that direction.

Design, Manufacturing, and Numerical Characterization of Hybrid Fiber Reinforced Polymer under Dynamic Loads

In the present work, the response of composite hybrid laminates made of carbon and glass fibers in different stacking configurations was tested under low-velocity impact loads. Experimental drop impact tests were conducted on three different stacking sequences and three rising impact energy levels. The results from the tests were assumed for the development and validation of a numerical impact model reproducing for each stacking sequence and all the impact energy levels, the laminates impact response. The validated model investigated the occurred damage mechanisms, their distribution in the panel thickness and their extension on the plane of the laminate. Depending on the stacking sequence and the impact energy level, the energy absorption capacity was related to the dominant damage mechanism. The percentage contribution of interlaminar and intralaminar damages was presented and conclusions were drawn about the influence of stacking sequences on energy absorption and damage characteristics.

Study of the Mechanical Behavior of Low-Velocity Impact Loading in the Presence of Stress Concentrators

Study of the Mechanical Behavior of Low-Velocity Impact Loading in the Presence of Stress Concentrators

Tensile behavior of the bolt‐jointed GFRP after low‐velocity impact

AbstractBolt joint is one of the most commonly used methods for fiber‐reinforced polymers (FRP) structures whose mechanical properties are quite sensitive to both the stacking sequence and the low‐velocity impact loading. For this reason, herein, we conducted an experimental method with the self‐designed clamp to evaluate the influence of low‐velocity impact (LVI) at the compressed area nearby the bolt hole on the residual tensile properties of bolt joint GFRP laminates with various stacking sequences. After comparing the displacement‐force curves and failure modes of different bolt joints in tensile tests after LVI loading, the effect of LVI and the failure mechanism subjected to post‐impact loading are discussed in depth. Tensile properties show that the damage caused by impact has a significant effect on the performance of the composite. Under the condition of incident energy of 9 J and distance of 6 mm from the center of the bolt hole to the impact point, the ultimate tensile value of Ply‐A, Ply‐B, and Ply‐C samples are reduced by 32.3%, 34.6%, and 50%, respectively. This influence can also be confirmed by observing the failure morphology, describing the interlaminar behavior of the specimens varying stacking sequence, where a more serious interlaminar damage was identified at Ply‐C specimens. In addition, we also utilized the VIC‐2D device to record and analyze the strain of different ply stacking sequence specimens in the failure process. The result shows that all types of cases have the same initial failure mode of bearing, but various final failure modes.

Energy absorption characteristics of a lightweight auxetic honeycomb under low-velocity impact loading

Auxetic honeycombs have been extensively studied due to their unique mechanical properties. In this paper, the dynamic response of WSH (The windmill-like configuration composed of stars and hexagons) honeycomb is systematically studied by theoretical calculations, numerical simulations, and experimental methods. The static and dynamic plateau stresses of WSH honeycombs are calculated, and critical velocities leading to different deformation modes of WSH honeycombs are derived. Finally, the gradient design of WSH honeycombs is carried out. And the dynamic response of the gradient WSH honeycombs is explored through numerical simulations. Under low-velocity impact loading, WSH honeycombs have better energy absorption capacity than conventional honeycombs. Both the concave angle gradient design and thickness gradient design can further enhance the energy absorption performance of WSH honeycombs.

Dynamic Responses of Sandwich Beams with Polymethacrylimide (PMI) Foam Cores When Subjected to Impact Loading.

This paper focusses on the load-sustaining and transfer mechanisms of sandwich beams with various types of PMI foam cores under low-velocity impact loading. In the case of quasi-static loading, the different failure modes, failure loads, and deflections were obtained, which agreed well with the results predicted by the theory of sandwich structure. In the case of impact loading, the clamped sandwich beams were subjected to the impact of a striker bar with a momentum of 10 kg∙m/s to 20 kg∙m/s. The de-acceleration of the strike bar was measured to analyze the impact force and energy absorption, and the corresponding failure modes were also obtained. The results showed that the impact force and the corresponding duration time increases with the increases in the thickness of the face sheet and the density of the core. In addition, the failure modes of the sandwich beams transferred from the shear failure mode to the tensile failure mode, which was attributed to the strength ratio between the bottom face sheet and the core. In combination with the experimental results and the plastic hinge theory, the deformation mechanisms of the different sandwich beams are also discussed.

TDOF model for UHPC members under lateral low-velocity impact

The dynamic behaviors of ultra-high performance concrete (UHPC) members under lateral low-velocity impact loading have been intensively studied by experiment and numerical simulation, while the corresponding structural design approach is still limited and mainly concerned in this study. Firstly, through comprehensively considering the compression arch action, strain rate effect, and unloading stiffness, the dynamic resistance function of UHPC members, e.g., reinforced UHPC (R-UHPC) and UHPC-filled steel tube (UHPC-FST) beams/columns, are built and verified. Then, a two degrees of freedom (TDOF) model of the drop hammer test on UHPC members is improved by considering the P-δ effect and stress wave propagation effect. Furthermore, the impact force- and mid-span deflection-time histories of the existing five series of drop hammer tests on UHPC members are applied to verify the validity and applicability of the established TDOF model. The influence of considering the stress wave propagation effect, strain rate effect, and steel fibers in UHPC for predictions of the TDOF model are discussed by parametric analyses. Finally, based on the energy balance, the explicit formulae are proposed to predict the maximum and residual mid-span deflections of the flexural critical UHPC members under the lateral low-velocity impact, which can be helpful for the design of UHPC structures under lateral impact loadings.

Multiple mesoscopic hotspot mechanisms model and its applications in drop impact hazard simulations

ABSTRACT Due to the complex stress state (shearing, compression, friction, etc.) of the charged polymer-bonded explosives (PBXs) under accidental drop impact loading, a physical model is needed to describe the hotspot formation and ignition responses to predict the safety of PBXs. A mechanical-thermal-chemical-coupled model incorporating microcrack and microvoid-related damage followed by ignition, which is predicted based on an innovative defect-related ignition criterion of effective inelastic work, is developed to evaluate PBX charge hazards under drop impact loading. The results show that (i) microcrack hotspots play a critical role in the total temperature under low-velocity (<175 m/s) drop impact loading. Under high-velocity (>175 m/s) drop impact loading, microvoid hotspots become the dominant position owing to the timescale of the microvoid hotspot formation being shorter (~10 μs), (ii) the bulk temperature rise can be ignored for its insignificant contribution to the calculation of total temperature, and (iii) a defect-related criterion model is established which is suitable for determining threshold velocity for dropped PBX charge ignition occurrence. The simulated pressure and defect-related ignition response of PBXs are in good agreement with the tests performed by China Academy of Engineering Physics.

Study on hybrid fibre reinforced UHPC beams under single and repeated lateral impact loading

This paper investigates the dynamic response of hybrid fibre reinforced ultra-high performance concrete (UHPC) beams against single and repeated low-velocity impact loads. A brief description of the drop weight impact tests on the UHPC beams was presented first followed by the development of the material and structural model in finite element analysis. A plasticity-based concrete material model with validated compressive and tensile strength surface and damage algorithm was adopted for hybrid fibre reinforced UHPC material. Based on the test results, the bond-slip behaviour between steel rebar and UHPC matrix was developed in an empirical form and incorporated in the model. Compared to the model with the bond-slip definition, the model with perfect bonding was found to underestimate the maximum mid-span deflection, which highlighted the necessity of considering the bond-slip behaviour in dynamic analysis where large deflection occurs. The repeated impact tests were performed numerically, and the results were validated with experimental data. A parametric study was then performed to investigate the effect of key parameters, including different impact energy and the same impact energy but different impact numbers. The results indicated when the total energy increased, the repeated impact loads became more hazardous than the single impact load. With the validated model, the dynamic shear force and bending moment distribution diagrams were compared to study the failure mechanism in single and repeated impact loads.

Stacking design of uniaxial/biaxial braided composite tube under low-velocity impact load

In this paper, the transverse low-velocity impact behavior of uniaxial and biaxial braided composite tubes is investigated to improve the impact resistance of tubular composite structures and to reveal their energy absorption mechanism. Based on the nonlinear progressive damage model, the finite element method is used for experimental verification and numerical analysis. The composite tubes with different stacking sequences and stacking angles were simulated, and their impact responses were evaluated by the peak load Fmax , the maximum indentation depth αmax and the energy absorption EA. The results show that the numerical simulation results are in good agreement with the experimental data, and can accurately capture the failure behavior in the actual scene. The damage mode of the braided composite circular tube under low-velocity impact load is mainly manifested as matrix cracking and interlayer separation, and the expansion direction of the damaged area is basically consistent with the fiber orientation. The impact resistance depends more on the stacking sequence, while the stacking angle is the main factor affecting the energy absorption. Under reasonable design, the uniaxial/biaxial hybrid braided tube can obtain the best low-velocity impact characteristics.

Behavior of two-way RC slab with different reinforcement orientation layouts of tension steel under drop load impact

In this research work, dynamic nonlinear incremental finite element analysis is conducted in ABAQUS software with an inbuilt explicit module to study the impact response of a two-way singly reinforced concrete slab, 1000 mm × 1000 mm × 75 mm, with different flexural tension steel reinforcement orientation layouts subjected to low-velocity falling-weight impact loading by cylindroconical impactor with a flat nose of 40 mm diameter having overall mass 105 kg from 2500 mm drop height. Computational findings are found in consonance with experimental results of Sadraie et al. (2019) on an isotopically reinforced concrete slab of the same dimensions with flexural tension steel in two layers i.e., one at 90-degree and the other at 180-degree orientations, subjected to the same impact load. Local damage in the form of punching occurs in the immediate vicinity of the impacted region. Overall deformation and cracking reflect the global behavior of the slab. The slab reinforcement is improvised by providing the same tension steel (0.88 %) in three layers: the first- and third- layer having orientation layouts of (a) 30- & 150-degree, (b) 45- & 135-degree, (c) 60- & 120-degree, and (d) 75- & 105-degree; the second layer is common with 90-degree orientation, and analyses are carried out under the same impact load. The Concrete Damage Plasticity and Johnson-Cook models, respectively, including the strain rate effects, are chosen to define concrete and steel materials’ response under impulsive impact loading. Computational results are discussed and presented in terms of displacement–time plots, stress distribution in the concrete and reinforcing bars, damage dissipation energy, and damage profiles. The tension steel in three layers and its orientation is found to have a noteworthy influence on the response of the slab and reduces the damage severity under impact loading.

Experimental-numerical-virtual (ENV) modelling technique for composite structure against low velocity impacts

In this paper, an experimental–numerical-virtual (ENV) modelling technique for the FRP composite laminate structures subjected to low-velocity impact loading is presented. The developed system is based on experimental observation, finite element numerical modelling and a damage-visible predictive algorithm relying on the machine learning technique. Both experimental and numerical results are used as training sources for the ENV framework to calibrate then make new predictions. The random damage mechanism is achieved by implementing non-deterministic failure criteria into the FRP composite structure. And the freshly developed clustering based extended support vector regression (C-XSVR) method is adopted to recognize the inherent relations between the variational systematic parameters and the structural progressive damage behaviors by analyzing the modes of induced fiber/matrix damage, energy absorption, and force deflection curves. Stimulating by the continuously changing inputs in real-life environment, different stages of impact process can be predicted by the ENV framework against future forecasted information in a computationally efficient manner. To illustrate the capability and efficiency of the proposed technique, two practical impact applications are fully investigated with both experimental tests and numerical simulations. Predicted impact damage patterns under variational working conditions are then provided and validated against different data sources.

Effects of support type and geometric shape of steel tube on concrete-encased concrete-filled steel tube beam under low velocity impact

Concrete-encased concrete-filled steel tube (CEFST) beams prepared by placing steel tube in RC beam is a beam type that has widespread use. The current study examines the effects of steel tube geometry and support conditions on the behavior of CEFST beams under low velocity impact loading. First, CEFST beam specimens with different lengths and different tube section geometries (circular and square) were produced in the laboratory. These four test specimens with two different support conditions were exposed to low velocity impact load tests. Finite element models of the test specimens were created using Abaqus\\Explicit software. In the next stage, 4 finite element models were verified by the experimentally obtained damage conditions, maximum displacements, and support reaction forces. In the final stage, as parametric study 12 finite element models were prepared in addition to 4 verified models and analyzed under low velocity impact loading. In addition to experimental values, Von Mises stresses, Equivalent plastic strain, Friction energy, and Damage energy values were also obtained by the numerical study and thus the effects of steel tube geometry and support conditions were comparatively examined. The obtained results showed that CEFST beams with a square tube section exhibited better performance under a sudden load such as impact compared to the beams with a circular tube section. Moreover, both numerical and experimental results pointed out that the beam length beyond support has a positive impact on the performance of beam under impact loading.