S.R. Reid

Dynamic uniaxial crushing of wood

Experimental results are provided from a series of tests on the uniaxial dynamic crushing of cylindrical specimens of five species of wood selected for the density range they cover and tested up to impact velocities of approximately 300 ms−1. An account of the macro-deformation and micro-deformation modes resulting from quasi-static and dynamic uniaxial compression is given. Measurements of the force pulses generated by the impact of the wood specimens on the end of a Hopkinson bar load cell show that significant enhancements of the initial crushing strengths of the specimens occur under dynamic loading conditions. The deformation mechanisms of wood are localised under quasi-static compression and under dynamic loading conditions they become even more localised and propagate through the material as crushing wave fronts which have some of the characteristics of shock waves. A simple shock model based upon a rate-independent, rigid, perfectly-plastic, locking (r-p-p-l) idealisation of the stress-strain curves for wood is proposed to provide a first order understanding of the dynamic response. This model is particularly successful in predicting the dynamic enhancement of the crushing strength of specimens loaded across the grain as confirmed by comparisons between the experimental data and theoretical results. It is less successful for those compressed along the grain. The source of the discrepancy is discussed and explanations are provided for the fairly constant crushing stress enhancement factor observed at low to moderate impact velocities, for the high impact velocity at which shock-type response is initiated and for the existence of clearly delineated crush fronts which characterise these specimens.

Impact loading of plates and shells by free-flying projectiles: A review

Abstract This paper reviews recent research into the penetration and perforation of plates and cylinders by free-flying projectiles travelling at sub-ordnance velocities. It is shown that over the last few years there has been a significant amount of experimental research into a wide range of projectile-target configurations. Although most of this research has been concerned with the normal impact of monolithic metallic plates by non-deformable projectiles, valuable work has also been carried out on non-normal impact, impact by deformable projectiles, impact of non-homogeneous metallic and non-metallic targets (including laminated targets) and impact of pipes and tubes. Recent analytical developments that enable the important characteristics of the penetration and perforation process to be modelled are reviewed. These include models that predict local deformations and failure, global deformations or both. It is shown that for some impact situations fairly simple analytical models are capable of predicting target response reasonably accurately, but for others, particularly when both local and global mechanisms contribute significantly to overall target response, more complicated models are required. The development of numerical codes that predict target response to projectile impact is briefly reviewed and the capabilities and limitations of current codes are discussed. The review also includes a section on the impact of soils and reinforced concrete structures.

PLASTIC DEFORMATION MECHANISMS IN AXIALLY COMPRESSED METAL TUBES USED AS IMPACT ENERGY ABSORBERS

The characteristics of a number of metal components proposed as impact energy absorbers are reviewed, attention being focussed on to modes of deformation which stem from the axial compression of metal tubes. Progressive buckling, inversion and splitting are discussed and areas for future work identified. The buckling of thin-walled square section tubes filled with polyurethane foam is also described. Reference is also made to recent work on cellular materials which highlights the influence of inertia in axially compressed tubes and tubular arrays.

Static and dynamic axial crushing of foam-filled sheet metal tubes

Abstract Experimental results are provided for the quasi-static and dynamic axial crushing of thin-walled square and rectangular tubes manufactured from sheet metal. The tubes were tested both empty and filled with polyurethane foam of various densities. Both the stability and the energy absorbing characteristics of the tubes are described and discussed. Simple theoretical models are proposed to explain and quantify the interaction between the foam and the sheet metal tubes.

Impact Behaviour of Fibre-reinforced Composite Materials and Structures

Impact behaviour of fibre-reinforced composites Recent developments in impact damage assessment of fibre composites Modelling impact of composite structures using small specimens Impact damage - tolerant composite structural design Damage resistance and tolerance of thick laminated woven roving GFRP plates subjected to low-velocity impact Elastic impact stress analysis of composite plates and cylinders Impact behaviour and analysis of CFRP laminated plates Perforation of FRP laminates and sandwich panels subjected to missile impact High velocity impact damage to polymer matrix composites.

Effect of strain hardening on the lateral compression of tubes between rigid plates

Abstract The lateral compression of circular tubes to large deformations is examined. The discrepancy between the theories and experiments reported previously is attributed in the main to an inadequate modelling of the stationary plastic hinges which are produced in the tube as it deforms. A model which utilises standard elastica theory is proposed which shows good agreement with the experimental data produced by the authors and that already published.

Inertia effects in uniaxial dynamic compression of a closed cell aluminium alloy foam

Abstract The dynamic compressive characteristics of a closed cell aluminium alloy foam (manufactured by Hydro Aluminium AS, Norway) have been studied experimentally by using a direct impact technique for a range of velocities up to 210 m s–1. Experimental data on the dynamic initial crushing and plateau stresses are compared for two average cell sizes of approximately 4 and 14 mm. The data reveal significant dynamic enhancements of the initial crushing strengths throughout the range of velocities used. The dynamic plateau stresses are insensitive to impact velocity below the values of 50 and 100 m s–1 for the large and small cell foams respectively. Beyond a critical velocity value of ~ 100 m s–1, the crushing wave front propagates through the foam with shock like characteristics. The inertia effects associated with the dynamic localisation of crushing and the microinertia of the cell wall/edge material on the dynamic strength enhancement are discussed. A one-dimensional shock model based on a rate independent, rigid, perfectly plastic locking idealisation of the nominal stress–strain curve for foams is employed to provide a first order understanding of the various parameters involved in the crushing process. The results of the analyses are seen to predict well the dynamic strength enhancements that are measured experimentally. The sources of discrepancies are highlighted and discussed, as are the limitations and shortcomings of the shock model.

Static and dynamic crushing of tapered sheet metal tubes of rectangular cross-section

Abstract The use of tapered sheet metal tubes as impact energy absorbers is considered. Their behaviour is compared with tubes of uniform cross-section with regard to the stability of the gross deformation modes. Single-tapered and double-tapered tubes are considered and estimates of the variation of their mean crushing forces with reduction in length under both quasi-static and dynamic axial loading conditions are provided. The practically important question of the response of tapered tubes to oblique impact loads is also considered.

Inertia effects in impact energy absorbing materials and structures

Experimental data and numerical/computational models concerning the internal inversion of metal tubes and the dynamic crushing of aluminium honeycombs are presented and discussed as illustrations of impact energy absorbers whose behaviour is strongly influenced by inertial effects.

Large deformations of thin-walled circular tubes under transverse loading—II: Experimental study of the crushing of circular tubes by centrally applied opposed wedge-shaped indenters

Abstract The behaviour of aluminium and mild steel tubes of 2 in. dia., 0.064 in. thickness and of lengths ranging from 2–24 in. loaded centrally by opposed wedge-shaped indenters is examined. Three modes of deformation are identified and analysed on the basis of plastic work considerations. Reference is also made to the behaviour of a very thin-walled tube of diameter to thickness ratio equal to 190.