S.T.S. Al-Hassani

Static and dynamic penetration of steel tubes by hemispherically nosed punches

Abstract An experimental investigation into the quasi-static and dynamic local loading of steel tubes has been carried out. Emphasis is placed on the energy absorption properties of the tubes when subjected to inward radial penetration from a hemispherically tipped indenter and to impact from hemispherically tipped projectiles travelling at velocities close to the ballistic limit of the target tube. The ballistic limit provides the most sensitive measure of a targets ability to resist impact. The tubes tested all had diameter to wall thickness (D/t) ratios of 31 and the punch/projectile diameter to tube diameter ratio was kept constant at a value of 0.125. Two types of steel tube were used in the tests: cold drawn mild steel tubes with outside diameters of 51, 102 and 153 mm and rolled tubes with a welded seam with outside diameters of 51 mm. The effects of projectile mass, projectile nose radius and the means of target support on the energy required for perforation of the tube wall were investigated as was the effect of the difference in the type of loading, static or dynamic.

Experimental investigation of deformation and jetting during impact spot welding

A test setup is described for investigating flyer plate deformation during impact spot welding. High speed photography is used to study the dynamics of bulge formation, spark growth, and projectile illumination during primary and secondary impact of flat-ended projectiles. It is suggested that fragmentation of the spark is due to inter-metallic jetting. Measurements on collision angle β and collision front velocity are reported for several metal plate combinations.

The influence of inertia and strain-rate on large deformation of plate-structures under impact loading

Abstract The static and dynamic behaviour of plate-structures subjected to in-plane axial and compressive impact loading is investigated using the finite element method (FEM). The material model is linear elastic with nonlinear isotropic work hardening characteristics with strain-rate dependence, inertia (via a consistent mass matrix) and geometrical nonlinearities are retained. The FEM results are in very close agreement with those obtained from experiments but the main advantage of the method is that it clarifies the time history behaviour of the model. The previous theoretical work has identified two distinct phases of deformation. Phase one is the compression of the plate, following the inelastic collision, until the structure becomes plastic. This is followed by a second phase of continuous plastic work dissipation in rotation about the hinges formed. The FEM results indicate the nature of the collision strain rate intensity and the differences that exist between static and dynamic modes of deformation response. The numerical modelling also reveals four phases in the deformation process. A phase of stress wave propagation is identified during elastic loading and the build up of high initial axial forces in the specimen. This is followed by a squashing phase, hinge rotation and elastic recovery (or, when the impact energy is high enough, structural closure) before the striker rebounds. The closure phase resembles a severe forming process followed by elastic recovery of the specimen and rebound of the striker. The differences between the various treatments are discussed and analysed in some detail.

Axisymmetric dynamic buckling of submerged cylindrical shells

Abstract Dynamic buckling response of tubes immersed in water and subjected to an axisymmetric external throughwater pressure pulse is investigated using the finite element method. A loading routine describing the hydrodynamic net pressure, incorporating incident and reflected pressure pulses on the structure was developed. This is interfaced with the general-purpose finite element code ABAQUS to solve for the structural response which has non-linearity in both material properties and displacements. Idealized geometric imperfections in the radius are used in the finite element model to trigger the radial buckling mode. The same numerical procedure was employed to predict the dynamic buckling response of tubes subjected to impulsive loading in air. Comparisons are made with plastic flow buckling theory and experiments reported in literature.

Crack straining-based spall model

Mean void growth-based spall models that avoid the complications of nucleation have been successfully applied to the problem of ductile spallation. However, similar models based on mean crack growth, applicable to brittle spallation, are not promising. This is because it remains to be demonstrated how an appropriate mean crack size is chosen to identify the brittle spall strength as the threshold pressure for crack growth. In the authors view, the solidity evolution is not merely a consequence of the nucleation and growth of cracks but also determined by the crack straining caused by the relaxed tensile pressure. This paper presents a crack straining-based spall model which assumes that the inelastic volumetric strain caused by the relaxed tensile pressure at a critical fragment volume is the main factor governing the solidity evolution during the process of coalescence and fragmentation. Such an approach makes the modelling of brittle spall compatible with that of ductile. The developed model requires only two additional parameters to those required by the chosen constitutive model and equation of state. We present the comparisons between experimental and computational simulation of the free surface velocity history of the target in a plane impact plate experiment. These show very good agreement.

Assessment of bond interface in impact spot welding

Abstract Results of an investigation on weld interfaces of impact-welded dissimilar metal combinations are presented. Polymeric projectiles of various nose shapes are utilized at an average impact speed of 750 m/s. It is found that welded interfaces generally comprise wavy and plane zones. Areas adjacent to waves contain hardened and plastically deformed regions. Quantitative data is presented on the composition of copper/brass interfaces. Results of microhardness measurements on copper/brass, titanium/brass and zirconium/brass weld interfaces are provided. Measurements on the local plastic thinning of the flyer plate (FP) are furnished. Effects of standoff distance (SD) and FP thickness on interface characteristics are investigated.

Resistance of steel-concrete sandwich tubes to penetration

Abstract The ability of steel-concrete-steel sandwich tubes to resist local lateral loads has been investigated experimentally. Two types of loading were employed: quasi-static, by means of a 12.7 mm diameter hemispherically tipped punch, and dynamic, by means of a 12.7 mm diameter hemispherically tipped projectile fired from a compressed air gun. The performance of tubes with skin thicknesses of 1 mm and filler thicknesses of 10, 20 and 38 mm was measured in terms of energy absorption capabilities and compared with the equivalent performance of monolithic steel tubes.

The influence of projectile nose shape on the morphology of interface in impact spot welds

The weld interface is investigated between copper flyer plates and brass parent plates that are impact spot welded by the use of LDPE projectiles of various nose shapes. Projectiles with a flat nose featured plane and wavy interfaces with a central unwelded zone. In contrast, projectiles with conical noses did not display unwelded zones at the spot. Hollow ended projectiles featured shaped-charge effects, and generally resulted in welded regions comprising a central pocket, a concentric toroidal ring and concentric zones of wavy and plain regions at various radial distances. Cases involving entrapped air and jetting are observed.

A simple non-local spallation failure model

A new, simple theoretical model to characterise the average continuum behaviour of the spall process is presented. Unlike all previous models, it is based on a non-local view of spallation and is applicable to both ductile and brittle materials. The model was incorporated into one and two dimensional hydrodynamic code to predict the free surface velocity of ductile and brittle targets in planar plate impact experiments, and can be extended to three dimensional problems. The results compared well with experimental data. The model can be used with common equations of state and constitutive equations.

Simulation of Wave and Jet Formations in Explosive/Impact Welding

Explosively driven impact welding is a process to produce bi-metallic plates and tubes. Whilst well established it has been essentially an empirical process. Recent work to numerically analyse part of the process is described. Using a finite difference engineering package, the oblique impact of a thin flyer plate on a relatively thick base was modelled. The results were validated by data from carefully controlled experiments using a pneumatic gun. Straight and wavy interfaces and jetting phenomena were modelled, and the magnitude of the waves and the velocity of jet predicted. The numerical analysis predicted a hump ahead of the collision point. Wave formation appears to be the result of variations in the velocity distribution at the collision point and periodic disturbances of the materials. Higher values of plastic strain were predicted in wavy interfaces. Bonding was found to be a solid state welding process. Phase changes which occur may be due to high temperatures (but less than the melting temperature) at the collision point.Copyright