Drop-weight Impacts Research Articles

Finite elements analysis of drop weight impact loading on GLARE-6A

Fibre Metal Laminate (FML) is a damage-tolerant material being opted in modern aerospace industries. FML impact absorbing properties were studied for determining FML response against impact loading. The author contributed by experimentation of drop weight impacts on FML GLARE 6A samples. In this paper, the author reproduced drop weight impact simulation using Finite Element Analysis (FEA). The author modeled GLARE FML, and a simulation of drop weight impact was performed using the explicit dynamics module of ANSYS. The drop weight impact method in Ansys was used to analyse the impact force effect on GLARE FML. Simulation results depicted drop weight impacts measured impact force, velocity, and energy vs time. Maximum deformation of plots was presented.

Repeated impact resistance of steel fibre-reinforced dry UHPC: Effects of fibre length, mixing method, fly ash content and crumb rubber

Dry concrete exhibits significantly different mechanical properties and failure patterns under dynamic loads from those under static conditions. Considering the practical applications of dry concrete, the flexural impact load renders its units/structures susceptible to unpredictable damage or even failure. In this study, the modified low-velocity drop-weight impact test was conducted on steel fibre-reinforced dry UHPC (FR-DUHPC) beam samples to simulate the frequent and multiple impact loads. The effects of steel fibre length, mixing method, fly ash content and crumb rubber addition on the impact resistance of FR-DUHPC were evaluated based on dynamic responses. Weibull distribution was then conducted to analyse the failure hit number of concrete samples in consideration of the large value variation. Test results indicated that the flexural impact performance was markedly improved with longer steel fibres, but deteriorated with increased fly ash content (especially at early ages) and considerably degraded with the inclusion of rubber aggregate. Hybrid medium/long fibre reinforcement was proposed for DUHPC that were mainly subjected to impact loads during service, while the fly ash content utilized to partially replace cement was not recommended to exceed 40%. Rubber aggregate was not proposed to be added to dry concrete mixtures since the impact load would greatly accentuate the initial matrix defects caused by the weak bonding. Additionally, the two-parameter Weibull distribution was verified to be a desirable statistical method for analysing the failure blow number of FR-DUHPC under repeated low-speed drop-weight impacts. The impact energy for the practical structural design could be preliminarily computed utilizing the derived Weibull parameters at different levels of reliability.

Experimental Research on Dynamic Behavior of Stiffened Plates under Drop-Weight Impacts of a Wedge Impactor.

Deck structures subjected to drop-weight low-velocity impact are critical safety elements for ships and offshore structures. Therefore, the aim of the present study is to propose experimental research on dynamic responses of deck structures composed of stiffened plates subjected to drop-weight impact of a wedge impactor. The first step was to fabricate a conventional stiffened plate specimen and a strengthened stiffened plate specimen, as well as a drop-weight impact tower. Then, drop-weight impact tests were carried out. Test results show that local deformation and fracture occurred in the impact area. A sharp wedge impactor caused premature fracture, even under relative low impact energy; the permanent lateral deformation of the stiffened plate was reduced by 20-26% by the strengthening effect of a strengthening stiffer; residual stress and the stress concentration of the cross-joint caused by welding may cause undesired brittle fracture. The present investigation provides useful insight for improving the crashworthiness design of the deck structure of ships and offshore structures.

Multi-scale modeling and numerical analysis of sandwich beams with FG auxetic 3D lattice cores and GRC face sheets subjected to drop-weight impacts

Auxetic metamaterials possess enhanced impact resistance and hence are the ideal core of sandwich structures. Here we present the enhancement of the drop-weight impact resistance of composite sandwich beams with nanocomposite facesheets by taking advantage of auxetic 3D lattice cores with FG (functionally graded) configurations. 2D and 3D lattice metamaterials with negative Poisson’s ratios (NPRs) were modeled, analyzed, and compared. Furthermore, the 3D lattice cores were designed to possess four FG patterns along the thickness direction. The mechanical properties of graphene reinforced composites (GRCs) were determined by means of micromechanical modeling according to the extended Halpin-Tsai model. In view of shape changes of lattices due to the localized deformation, full-scale finite element modeling is necessary, followed by nonlinear analysis. Compared with a re-entrant honeycomb core having the same relative density, the auxetic 3D lattice core can lead to even better resistance. Moreover, FG configurations are proven to have different effects on the impactor displacement and local transient thickness decrease of sandwich beams subjected to low-velocity impacts. Our results demonstrated the advantages of FG auxetic 3D lattices, and are expected to be beneficial to further studies and instructive for practical applications.

Experimental and Numerical Model of CFRP Retrofitted Concrete Beams with Intermediate Notches Subjected to Drop-Weight Impact

This paper presents an experimental and numerical model of carbon fiber reinforced polymer (CFRP) retrofitted concrete beams with and without intermediate notches, developed to predict the impact load, energy response, cracking pattern and interfacial stress distribution under drop-weight impact. Considering a drop-weight falling from different heights, a CFRP sheet was bonded to the bottom surface of the beam. The specimen’s drop-weight impacts were established for retrofitted and un-retrofitted simply-supported notched concrete beams. A nonlinear finite element model was developed and calibrated to analyze the interfacial stress field, failure modes, cracking patterns and damage sequences. The results obtained numerically in terms of load–time, load–displacement and energy–time curves were compared to those given by the authors’ experimental tests and also to those given in the literature. For un-retrofitted beams, the impact time was very short, with failure at the center of the beam. For notched beams, the impact load decreased as notch depth increased, owing to the local loss of rigidity at the center of the concrete beam. For the retrofitted beam, however, the impact time was quite long, with a very high impact load, because the beam was more resistant, with rebound.

Geometrical and topological evolution of a closed-cell aluminium foam subject to drop-weight impact: An X-ray tomography study

The mechanical properties of closed-cell aluminium foams are governed by their geometrical and topological evolution during impact. Here we non-destructively investigate the deformation mechanisms of a closed-cell aluminium foam sample at the cell scale. The sample has been compressed with 21 interrupted drop-weight impacts at a nominal strain rate of 40 s−1 and the post-impacted sample has been imaged at four pertinent strain states with high-resolution X-ray micro-computed tomography (XCT). Moreover, a number of qualitative and quantitative structural analyses are carried out using advanced 3D image analyses to understand the effect of foam geometry/topology on its mechanical response. Our results show that the deforming microstructure creates strong correlations across a range of geometrical, topological and shape characteristics. Quantitative image analyses of the sample at four different strain states reveal that the regions with large void fractions predominantly undergo collapse and subsequently reduces the structural heterogeneity. Further, we show that the deformation mechanism leaves topological signatures in the evolving microstructure, which can be used to better understand the mechanical response of the sample at various stages of impact-deformation. We demonstrate here, for the first time, how the topological quantities can be used to explore the foam deformation mechanisms and to correlate the deformation with mechanical response during impact.

Experimental investigation of (FRSC) cementitious composite functionally graded slabs under projectile and drop weight impacts

The purpose of this study was to investigate the impact strength of functionally graded reinforce concrete (FGRC) slabs made with self-compacting cementitious composites. To this end, 60 slabs measuring 40 × 40 × 7.5 cm which had been reinforced with 1% fixed volume of steel and nylon fibers were made. Some of them were entirely reinforced concrete slabs, and their constituent fibers were uniformly distributed in them. Others were FGRC slabs, and their constituent fibers were non-uniformly distributed in them. Non-uniform distribution in the present study means that the fibers were increased in the peripheral layers and they were decreased in the inner layers, although the whole volume of fibers were equally used in all the slabs. Projectile impact tests (high speed) and drop weight impact tests (low speed) were performed on the slabs. The results of these tests showed the considerable effects of steel fibers, and insignificant effects of nylon fibers, on the impact strengths of slabs. The non-uniform distribution of fibers in the FGRC slabs led to their behavior improvement. The FGRC slabs had lower penetration depths and destroyed volume in the projectile impact tests, and they had higher failure strength and energy absorption in the drop weight impact tests, as compared to the entirely reinforced slabs.

An experimental investigation on Parallel Bamboo Strand Lumber specimens under quasi static and impact loading

This paper investigates the compression behaviour of 42 Parallel Bamboo Strand Lumber (PBSL) specimens under quasi-static and drop-weight impact loading. 25 mm × 25 mm square specimens with varying heights and fibre orientations were tested to investigate compression and impact resistance of PBSL. Quasi-static results indicated a distinct 5-stage failure pattern for the complete quasi-static loading history with a 45° failure plane in all specimens when the compression load was applied parallel to the fibres. Specimen height did not affect the ultimate load carrying capacities but showed considerable influence on the initial stiffness as well as the post-ultimate loading regime. Experimental results showed that the deformation ratio and the energy absorption ratio for longer specimens were not affected by fibre orientations. Low-velocity drop-weight impacts were conducted under 25.4 J and 101.5 J energy levels. Specimen height did not play any significant factor during impact testing. The initial impact level dominated the maximum deformation and dynamic energy absorption observed in specimens that failed showing a 4-stage deform procedure. Although the deformation patterns between fibre orientations were distinctly different, similar energy absorption capacities were obtained regardless of specimen height. Comparison of experimental evidences obtained from quasi-static and drop-weight impact tests clearly showed that the compression response of PBSL is strain-rate sensitive.

Experimental Study on Hybrid Effect Evaluation of Fiber Reinforced Concrete Subjected to Drop Weight Impacts.

In this paper, the impact energy potential of hybrid fiber reinforced concrete (HFRC) was explored with different fiber mixes manufactured for comparative analyses of hybridization. The uniaxial compression and 3-point bending tests were conducted to determine the compressive strength and flexural strength. The experimental results imply that the steel fiber outperforms the polypropylene fiber and polyvinyl alcohol fiber in improving compressive and flexural strength. The sequent repeated drop weight impact tests for each mixture concrete specimens were performed to study the effect of hybrid fiber reinforcement on the impact energy. It is suggested that the steel fiber incorporation goes moderately ahead of the polypropylene or polyvinyl alcohol fiber reinforcement in terms of the impact energy improvement. Moreover, the impact toughness of steel-polypropylene hybrid fiber reinforced concrete as well as steel-polyvinyl alcohol hybrid fiber reinforced concrete was studied to relate failure and first crack strength by best fitting. The impact toughness is significantly improved due to the positive hybrid effect of steel fiber and polymer fiber incorporated in concrete. Finally, the hybrid effect index is introduced to quantitatively evaluate the hybrid fiber reinforcement effect on the impact energy improvement. When steel fiber content exceeds polyvinyl alcohol fiber content, the corresponding impact energy is found to be simply sum of steel fiber reinforced concrete and polyvinyl alcohol fiber reinforced concrete.

Effects of large-sized granules on ignition and burning of powdery explosives under drop-weight impacts

ABSTRACTDrop-weight impact experiments are performed on a thin layer of cyclotetramethylenete tranitramineand cyclotrimethylene trinitramine (RDX) powdery explosives, which were mixed with a few large-sized explosive or NaCl salt granules. The drop-weight impact machine is equipped with the high-speed photographic systems for observing the impact process. It showed that the mixed explosives are more sensitive than pure powdery explosives. Hard salt particles could greatly enhance the burning probability of powdery explosives because frictional heat between salt particles and target explosive powders, stress concentration, and new generated fragments of large granule jointly contribute to heat accumulation at the junction of dissimilar materials. Burning front always initiates at the interface of powdery explosive and additive granule. Adiabatic shear bands were frequently observed in the mixture of ultra-fine RDX powdery explosive with RDX large granules, which mainly resulted from the softening effect of ultra-fine powders once the temperature arises.

Prediction of the impact force on reinforced concrete beams from a drop weight

It is always a challenge to efficiently and accurately estimate the force on structures from falling objects. This study aims to predict the maximum impact force on reinforced concrete beams subjected to drop-weight impact using artificial neural network. A new empirical model including a comprehensive version and a simplified version is proposed to estimate the maximum impact force. The model was verified against a database collected from the literature including 67 reinforced concrete beams tested under drop-weight impacts. The database covers the concrete strengths ranging from 23 to 47 MPa, the projectile mass from 150 to 500 kg, and the impact velocity up to 9.3 m/s. The prediction of the comprehensive version of the proposed model fits the experimental results very well with an average absolute error of 11.6%. The simplified version of the proposed model is established for easy estimation, with the average error of 23.2% in prediction of the maximum impact force.

Finite element simulation of damage and failure predictions of relatively thick carbon fibre-reinforced laminated composite panels subjected to flat and round noses low velocity drop-weight impact

This work is concerned with the dynamic computational modelling of fibre-reinforced laminated composite panels subjected to low velocity drop-weight impacts. Wings and fuselage of aircraft structures are prone to tool (box) drop impacts during normal shipping and handling of component assembly and maintenance services. Flat nose impacts inflict localised barely visible internal damage that severely reduce compressive residual strength and might result in catastrophic failure during future operations. Hence it is important to have a better understanding of the impact response of composites to mitigate the damage and avert the unexpected failures. Many reported works on the topic are experimental, based on quasi-static indentations that take longer than contact time, and produce global deformations of thin panel where short time effects and through-thickness stresses are neglected. Hence dynamic model is needed to investigate impact behaviour of thick panels for detailed information. The present computational model includes short time effects and utilise through-thickness stresses in mode-based failure criteria to differentiate ply-by-ply failure modes. Cases of laminates up to 7.6mm thick impacted with flat and round (for comparisons) nose impactors were simulated using ABAQUS™/Explicit dynamic routine. High stress concentration regions were meshed with adaptive meshing techniques using reduced integration elements. Selected simulation results were compared against experimental, intra-simulation results, and the data available in the literature and found to be in acceptable agreement.

Prediction of load threshold of fibre-reinforced laminated composite panels subjected to low velocity drop-weight impact using efficient data filtering techniques

This work is concerned with physical testing of carbon fibrous laminated composite panels with low velocity drop-weight impacts from flat and round nose impactors. Eight, sixteen, and twenty-four ply panels were considered. Non-destructive damage inspections of tested specimens were conducted to approximate impact-induced damage. Recorded data were correlated to load–time, load–deflection, and energy–time history plots to interpret impact induced damage. Data filtering techniques were also applied to the noisy data that unavoidably generate due to limitations of testing and logging systems. Built-in, statistical, and numerical filters effectively predicted load thresholds for eight and sixteen ply laminates. However, flat nose impact of twenty-four ply laminates produced clipped data that can only be de-noised involving oscillatory algorithms. Data filtering and extrapolation of such data have received rare attention in the literature that needs to be investigated. The present work demonstrated filtering and extrapolation of the clipped data using Fast Fourier Convolution algorithm to predict load thresholds. Selected results were compared to the damage zones identified with C-scan and acceptable agreements have been observed. Based on the results it is proposed that use of advanced data filtering and analysis methods to data collected by the available resources has effectively enhanced data interpretations without resorting to additional resources. The methodology could be useful for efficient and reliable data analysis and impact-induced damage prediction of similar cases’ data.

Experimental Tests of Steel Fibre Reinforced Concrete Beams under Drop-Weight Impacts

Concrete is a brittle material, especially under tension. Intensive researches have been reported to add various types of fibres into concrete mix to increase its ductility. Recently, the authors proposed a new type of steel fibre with spiral shape to reinforce concrete material. Laboratory tests on concrete cylinder specimens demonstrated that compared to other fibre types such as the hooked-end, deformed and corrugated fibres the new fibres have larger displacement capacity and provide better bonding with the concrete. This study performs drop-weight impact tests to investigate the behaviour of concrete beams reinforced by different types of steel fibres. The quasi-static compressive and split tensile tests were also conducted to obtain the static properties of plain concrete and steel fibre reinforced concrete (FRC) materials. The quasi-static tests were carried out using hydraulic testing machine and the impact tests were conducted using an instrumented drop-weight testing system. Plain concrete and concrete reinforced by the commonly used hooked-end steel fibres and the proposed spiral-shaped steel fibres were tested in this study. The volume dosage of 1% fibre was used to prepare all FRC specimens. Repeated drop-weight impacts were applied to the beam specimens until total collapse. A 15.2 kg hard steel was used as the drop-weight impactor. A drop height of 0.5 m was considered in performing the impact tests. The force-displacement relations and the energy absorption capabilities of plain concrete and FRC beams were obtained, compared and discussed. The advantage and effectiveness of the newly proposed spiral-shaped steel fibres in increasing the performance of FRC beam elements under impact loads were examined.

A fluid structure interaction study of a viscous mechanism for energy absorption in protective structural panels

Sandwich panels designed to resist blasts and high-velocity impacts usually dissipate most of the delivered energy through inelastic deformation of solids. A concept is explored in this study to improve the energy absorption of such structures by the addition of a viscous mechanism. The mechanism relies on the fact that when a viscous liquid is forced through narrow passages at high speeds, it undergoes high shear rates that cause viscous energy dissipation. A simple test specimen in the form of a steel tube with capillaries attached at both ends was chosen for study. Both empty and liquid-filled test specimens were subjected to experimental and simulated drop-weight impact tests and simulated blast load tests. Fluid structure interaction analyses in the form of Coupled-Eulerian–Lagrangian simulations were performed to assess the energy dissipated both by solid plastic deformation and liquid viscous dissipation in the drop-weight and blast simulations. The liquid flow speeds generated by the applied loads were found to be a critical factor in determining the contribution of the viscous mechanism. The moderate liquid flow speeds generated by the drop-weight impacts resulted in negligible viscous energy dissipation. The simulated blast loads generated much higher liquid flow speeds and as a result the viscous energy contribution to the total absorbed energy in the test specimens approached 30%. The viscosity of the liquid has a major effect on the fraction of energy absorbed in the form of viscous dissipation. Results of this study support the viability of the concept of viscous-assist for improving the ability of protective panels and structures to withstand high-speed impact and blast loads.

Experiments and modeling of HMX granular explosives subjected to drop-weight impact

The drop-weight impacts on a thin layer of cyclotetramethylene tetranitramine (HMX) explosive particles have been performed. The drop-weight impact machine is equipped with high-speed photographic techniques. Effects of drop heights and the amount of particles on compaction, deformation, and thermal responses were investigated. Considering the contact plasticity, friction, melting, fracture, as well as chemical reaction, an analytical model based on heating and kinematics equations for a single layer of explosive particles has been developed. The calculated average pressure, loading time and time-to-ignition agree well with experimental measured ones. Threshold conditions for ignition can be derived from the predicted temperature history curves. Individual particles’ pressure–strain exhibited stress–strain-like relationships, from which close relations can be found between experimental phenomena and the material's softening/hardening characteristics.

A microscopic model for predicting hot-spot ignition of granular energetic crystals in response to drop-weight impacts

A micro-mechanics model describing hot-spot formation in the energetic crystal powders cyclotetramethylene tetranitramine (HMX) and pentaerythritol tetranitrate (PETN) subjected to drop-weight impact is developed. Considering contact deformation, friction and chemical reactions at the particle level during the impact loading process, three hot-spot sources are included, namely, the micro-particle contact deformation between two equal-sized particles with no relative sliding, the contact sites between the impacting surface and the particles, and the contacting zone between particles with a sliding velocity along the maximum shear-stress direction. Drop-weight impacts on samples composed of equal-sized particle layers are considered. The temperature rise due to plastic and frictional dissipation is estimated, and melting is included. Hot spot ignition is predicted via thermal explosion, using an Arrhenius thermochemical model. The effects of drop height and particle size on the ignition processes are analyzed. The hot-spot sources at the interface between the particles and the impacting surfaces play the most important role during the early ignition stage. The time-to-ignition of the three sources increases with decreasing drop height. Calculated results show that the HMX and PETN crystals demonstrate monotonously increasing time-to-ignition with a reduction in particle size. The effect of particlesize on the hot spot ignition threshold can be predicted. Samples with a smaller particle size undergo larger localized deformations and lower average pressures.