Finite element modelling of helmeted head impact under frontal loading

Finite element models of the head and helmet were used to study contact forces during frontal impact of the head with a rigid surface. The finite element model of the head consists of skin, skull, cerebro-spinal fluid (CSF), brain, tentorium and falx. The finite element model of the helmet consists of shell and foam liner. The foam is taken as elasto-plastic, the brain is assumed to be viscoelastic and all other components are taken as elastic. The contact forces and coup pressures with helmet on the head are much lower than in the absence of the helmet. A parametric study was performed to investigate the effect of liner thickness and density on the contact forces, pressures and energy absorption during impact. For 4 ms⁻¹ velocity, expanded poly styrene (EPS) foam of density 24 kgm⁻³ gave the lowest contact forces and for the velocities considered, thickness of the foam did not affect the contact forces.     

An experimental study of square tube crushing under impact loading using a modified large scale SHPB

This paper presents an experimental study on square tubes made from a rate insensitive material under static and impact loading. Rate insensitivity of the base material (Cu–Zn alloy) is confirmed by static and dynamic tests on small samples cut from the tubes. A direct impact large scale Hopkinson bar (80 mm diameter, 10 m length) system is used to perform tube crushing tests. A two-point measurement method is applied to extend measuring duration of the pressure bar, which is usually limited by its length. The proposed method permits to monitor the whole tube crushing process.Static and impact tests (7–15 m/s) on these square tubes reveal that there is a significant increase under impact loading of both initial and successive peak loads with respect to quasi-static loading. Such a study is useful for the understanding of strength enhancement under impact loading observed for cellular materials such as honeycombs.     

An experimental study of non-Newtonian properties of water under electroexplosive loading

Non-Newtonian properties of water in the shock-wave (electroexplosive) experiment are studied. Powerlike dependences of the deformation rate depending on the stress amplitude at the wave front and the spall strength depending on the strain rate during unloading are obtained.     

Plastic deformation behaviour of pavement granular materials under low traffic loading

This paper analyses the permanent deformation performance of an unbound granular material for base layers of low traffic roads. The material has been subjected to repeated triaxial loads. Three models were fitted to express the cumulative permanent strain as a function of the number of load cycles. In general, the predictions of two models previously studied by other researchers proved to be good but in the long-term, they tended to underestimate the measured values. In contrast, a third new model-the sum of two well known models-offered excellent predictions, which in the long term did not tend to either underestimate or overestimate the measured values. The granular material did not give satisfactory results to be used in low traffic volume road pavements.     

Numerical and experimental studies of polyurethane foam under impact loading

Proper predictions of the behaviour of shock absorber materials are of utmost importance in safety assessments for licensing casks for transport and storage of highly active waste. After having identified significant discrepancies between numerical results and the actual response of polyurethane foam limiters subjected to accidental scenarios, a new research project ENREA was established by BAM. A major objective is to enhance and to develop advanced material models intended to simulate limiters under impact loading. They should account for all major factors influencing the load–deformation relationship like temperature, strain rate and specific stress state. The corresponding test program, applicable plasticity models, the overall parameter identification strategy based on local and global optimization techniques as well as experimental and numerical results are presented here in particular for closed cell foams.     

A SPH-based numerical study of the crack arrest behaviour of rubber toughened PMMA under impact loading

International Journal of Fracture - The objective of the present work is to propose an engineering-oriented numerical methodology capable of reproducing crack initiation and arrest in semi-brittle...     

Mechanical behaviour of polymer-modified ferrocement under different exposure conditions: An experimental study

This paper, based on laboratory programs, evaluated the load-deflection characteristics, first crack strength, crack width and crack spacing of three commercial polymer-modified ferrocements namely styrene-butadiene rubber (SBR), polyacrylic ester (PAE), and vinyl acetate–ethylene (VAE), and unmodified ferrocement elements cured in air and saltwater exposure conditions. The results indicated that continuous saltwater exposure significantly improved the behaviour of polymer modified ferrocements in flexure by exhibiting higher experimental values of the first crack load and ultimate strength of all the specimens. Irrespective of the exposure conditions, polymer modified ferrocements showed lower average crack width than that of the unmodified ferrocement.     

High cycle fatigue behaviour of impact treated welds under variable amplitude loading conditions

A comprehensive variable amplitude (VA) fatigue testing program and analysis was performed to address a number of concerns raised regarding the use of impact treatments for the fatigue enhancement of welds in the high and ultra-high cycle (up to 100 million cycles) domains. A total of 67 fatigue tests were conducted on two different welded joints representing load-carrying and non-load carrying welds in steel structures. Two different VA loading spectrums, generated using traffic data and influence lines for highway bridge girders, were used. The effects of load cycles with high stress ratios (R > 0.4) and large tensile overloads (greater than the yield strength) were studied. The test results were then used to evaluate a number of previously proposed recommendations for the fatigue design of impact treated welds. The nominal, structural, and effective notch stress approaches were considered. Finite element (FE) analysis was performed to determine the structural and effective notch stresses. A statistical analysis of the fatigue test results was conducted and characteristic S–N curves with slope, m = 5 are proposed for the fatigue design of treated welds under VA loading in the high cycle domain.     

Computational modelling of irregular open‐cell foam behaviour under impact loading

Cellular structures represent an important class of engineering materials. Typical representative of such structures are metallic foams, which are being increasingly used in many advanced engineering applications due to their low specific weight, appropriate mechanical properties and excellent energy absorption capacity. For optimal design of cellular structures it is necessary to develop proper computational models for use in computational simulations of their behaviour under impact loading. The paper studies the effects of open‐cell metallic foam irregularity on deformation behaviour and impact energy absorption during impact loading by means of parametric computational simulations, using the lattice‐type modelling of open‐cell material structure. The 3D Voronoi technique is used for the reproduction of real, irregular open‐cell structure of metallic foams. The method uses as a reference a regular mesh structure and utilises an irregularity parameter to reproduce the irregularity of real open‐cell structure. A smoothing technique is introduced to assure proper stability and accuracy of explicit dynamic simulations using the produced lattice models. The effects of the smoothing technique were determined by comparative simulations of smoothed and unsmoothed lattices subjected to dynamic loading.     

冲击作用下CFRP修复损伤RC叠合梁动力性能试验研究

采用落锤冲击试验机,进行了现浇梁、叠合梁以及碳纤维增强复合材料(carbon fiber reinforced polymer,CFRP)修复的损伤梁的动力性能对比试验。冲击作用下试件破坏形态以及冲击力与变形时程曲线表明,叠合面在一定程度上抑制了跨中裂缝向上开展的趋势,损伤修复后的叠合梁在冲击荷载下的破坏程度明显减轻,初始损伤减弱了梁跨中位移的滞后现象;相比完好梁,现浇与叠合修复梁抗冲击承载力有着同等程度的提高。基于有效应变计算的修复梁承载能力提升与试验结果的对比显示,CFRP能有效改善并增强构件的整体受力性能,在冲击作用下对构件的实际抗力贡献大于原有构件承载力与FRP受拉能力简单叠加时的理论抗力贡献。     

The behaviour of concrete under impact loading: Experimental procedures and method of analysis

The behaviour of concrete beams under impact loading was studied, using an instrumented drop-weight impact apparatus capable of dropping a 345 kg mass from heights of up to 3 m. Both plain and conventionally reinforced beams, with overall dimensions (length x width x depth) of 1,400×100×125 mm, were tested on a 960 mm span, with the impact load applied at mid-span. The load on the instrumented striking tup, and the acceleration of the beam itself, were measured as a function of time; the entire impact events had a duration of 10 to 70 ms. This paper describes the methods used to analyze the impact data, as well as some preliminary results.     

A theoretical and experimental study on metal hexagonal honeycomb crushing under quasi-static and low velocity impact loading

In the study of honeycomb crushing under quasi-static loading, two parameters are important; the mean crushing stress and the wavelength of the folding mode. On the other hand, the kinetic energy absorbed by the honeycomb is investigated in the impact loading. In this paper, through fully considering the cylindrical curvature effects and implementing the energy method, a new theoretical model for the estimation of the mean crushing stress and the wavelength of the folding mode of the metal hexagonal honeycomb is presented. Afterwards, developing this static model to the dynamic state, a theoretical model for study of the behavior of these energy absorbers, in the low velocity impact loading, is proposed and the required initial velocity of the impactor, for creation of the desired folding length in these structures, is determined, analytically. The presented theoretical models have been compared with experimental results obtained from experiments on three kinds of honeycomb with the various minor diameters and thicknesses of the cell walls under quasi-static and low velocity impact loading in the axial direction. Excellent correlation has been observed between the theoretical and experimental results.     

Perforation of aluminium foam core sandwich panels under impact loading—An experimental study

This paper reports an original inverse perforation tests on foam core sandwich panels under impact loading. The key point is the use of an instrumented Hopkinson pressure bar as a perforator and at the same time a measuring device. It aims at a high quality piercing force record during the whole perforation process, which is a weak point of common free-flying projectile-target testing schemes.     

In-situ investigation on the deformation and damage behaviour of diamond-like carbon coated thin films under uniaxial loading

In the present work, the failure behaviour of diamond-like carbon (DLC) coatings on thin steel substrate under uniaxial tensile loading is analyzed in-situ in scanning electron microscopy as well as ex-situ using focused ion beam cross section and transmission electron microscopy. Aim of the work is to find correlations between the failure behaviour of the coating system, the adhesion and the stress-strain behaviour of a DLC coating system under tensile loadings conditions. Therefore thin amorphous DLC films were coated onto thin stainless steel foils using a plasma assisted chemical vapour deposition technique. It is found from the in-situ investigations that at increasing strains cracks were formed in the coating, with decreasing spacing at higher strains. By comparing uncoated steels foils with coated systems the stress-strain behaviour of a DLC coating was determined. The DLC coating, although already strongly cracked, bears loads up to a total strain of 15%. Cross section analyses with a focused ion beam and microscopy techniques supported these investigations. During straining the formation of two deformation bands adjacent to the Cr adhesion layer was observed. This deformation bands also indicate a high interfacial adhesion.     

Method for the experimental investigation of laws governing the deformation of materials under nonisothermal complex loading

A method for the testing of thin-wall tubular specimens formed from metallic materials under an axial force, torsional moment, and temperature effects on an TsDMU-30t testing machine equipped with systems for the measurement and recording of loads and strains, and a heating unit, as well as programmed units for automatic loading and heating in the 273–1273 K range is described.The results of two programs for the complex loading of specimens formed from heat-resistant alloy ÉI437 along paths with intermediate unloading at 973 K, which confirm the correctness of the postulate of isotropy for this class of loading processes, are presented, and the applicability of this method to study of laws governing the deformation of materials under complex loading at high temperatures is demonstrated.Mechanics Institute, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 11, pp. 57–61, November, 1989.