Papers by Keyword: Impact

Abstract: Adiabatic shear bands are microstructural features that appear when metals, and some non-metals are subjected to impact loading at strain rates in excess of 10³ s^-1 and large strains. The formation of these bands is generally attributed to several competing mechanisms, among them is an initial strain hardening followed by adiabatic thermal softening that may lead to crack initiation within the bands. The authors have developed a model for formation of adiabatic shear bands in metallic materials as they are formed during testing using a torsional Hopkinson Bar. The model relies on a one dimensional analysis which predicts accurately the two steps of forming adiabatic shear bands in terms of strain hardening followed by thermal softening. In this current research, the model is extended to a two-dimensional analysis which would be suitable for application in either a two bar compression Split Hopkinson Bar or in a direct impact compression system developed by the author (Nabil Bassim) to produce high strain rates and large strains. The algorithm relies on applying the concept of dynamic recrystallization in order to determine the onset or initiation of the adiabatic shear bands.

Abstract: Hydrodynamic Ram (HRAM) is a phenomenon that occurs when a high-energy object penetrates a fluid-filled container. The projectile transfers its momentum and kinetic energy through the fluid to the surrounding structure increasing the risk of catastrophic failure and excessive structural damage. In this work a numerical study of the influence of metallic plates, inside a fluid-filled aluminium tube, in the attenuation of the HRAM phenomenon effects is performed. The numerical results regarding walls displacement and pressure inside the tank will be compared with a case in which there are no barriers inside the tube. The simulations are performed with the software LS-DYNA applying the ALE technique.

Abstract: In this work simulations of high velocity impacts of ice spheres on carbon/epoxy laminates are accomplished. The Drucker-Prager model has been chosen to describe the mechanical behavior of the ice under high velocity impact conditions. Results have been validated by means of experimental tests performed in a wide range of impact velocities. The delaminated area was chosen as comparison variable, and reflects that the model predicts adequately the impact process.

Abstract: The thickness and weight of a bulletproof glass material can be reduced using strengthened glass possessing current protective abilities. In this study, numerical simulations are used to estimate the protective ability of strengthened borosilicate glass used in bulletproof glass systems. High-velocity impacts and perforation behaviors are well described by a dynamic brittle fracture model. A parametric study of the material model of glass is conducted by comparing test results of individual impacts with corresponding numerical estimations; size of back-surface spall, morphology of perforated surface, and fractured areas are compared. Material parameters of strengthened and nonstrengthened borosilicate glasses are determined. Numerical simulations using a material model with these parameters well describe the overall fracture behavior of bulletproof glass. The main parameters that affect the protective ability are initial compressive yield stress and fracture stress. Furthermore, the protective ability of strengthened borosilicate glass is ~20% better than that of nonstrengthened borosilicate glass.

Abstract: In mountain areas, natural hazards, such as snow avalanches, landslides and rockfall threat towns, communication routes and people. It is known that forests have a major capacity to dissipate rockfall energy. Forest maintenance or storms can reduce forest’s protective capacity; after such a reduction, felled trees can be strategically left on the slopes in order to replace live trees. The efficacy of these devices and their optimal position can be analyzed by developing a numerical model describing the rock-wooden device interaction. To develop a relevant model of these wooden devices when impacted, the research was focused, in one hand, on a rigorous characterization of the fresh wood mechanical properties to recreate the real dynamic response of stems after the impact. In the other hand, the local impactor-wood stem interaction at the contact point was analyzed. Laboratory experiments using Charpy pendulum, presented in this text, have been carried out to assess the calibration of the numerical model. Experimental results of the impact force and their relation with stems mass and the impact energy level were treated and commented. A second serie of experiments has enabled to characterize the law describing the contact between the stem and the impactor.

Abstract: The effect of impact compression on age hardening behavior was examined for Meso20 and 6061 aluminum alloys using a single stage gun. The hardness of Meso20 and 6061 aluminum alloy applied with an impact compression (about 5.0GPa) after the solution treatment increased with the aging time. The cluster of point defects like stacking fault tetrahedral (SFT) was observed in the 6061 aluminum alloys with the impact compression (5.3GPa) after the solution treatment. Even after the impact compression, distribution of the aging precipitates was clearly identified.

Abstract: Aircraft structures and wings should endure tyre debris impacts. Therefore, numerical simulations which are able to predict such phenomena are highly appreciated for the aircraft structures design. However these simulations have to be developed, tuned and validated by means of experimental data. Nowadays, full-scale tests are widely used. However, they include too complex mechanisms, which are difficult to consider in a process of model development. Besides, these experiments are extremely expensive. In this work, we are dealing with a simplified experimental set-up with elementary boundary conditions. Namely, we are experimentally investigating rubber balls’ impact on composites panels. The main novelty is the use of 3D-stereo correlation technique.

Abstract: A set of 18 armour steel plates were stacked on top of each other and subjected to shape charges that went through the plates and created a hole that decreased in diameter as it went through consecutive plates. Afterwards, the plates were examined and the hardness near the hole and away from the hole was taken to determine the effect of the passing of the shaped charge through the plates. Also, specimens from the walls of the holes were taken to determine changes in the microstructure due to the shock wave and the resulting excessive heating from the shape charge. It was observed that the shock wave produced significant changes in the microstructure resulting in the appearance adiabatic shear bands (ASBs). These ASBs persisted in holes in plates placed further down the stack (up to 8^th in the stack). More complex microstructural mechanisms are thought to take place as opposed to erosion from the flow of the molten metal through the holes in the plates.

Abstract: The high temperature deformation and dislocation substructure of Ti-6Al-7Nb biomedical alloy are investigated under high strain rate loading conditions using a split-Hopkinson pressure bar. Impact tests are performed at strain rates ranging from 1x10³s^-1 to 3x10³s^-1 and temperatures of 300°Cand 700°C, respectively. The experimental results show that the flow stress, work hardening coefficient and strain rate sensitivity all increase with increasing strain rate, but decrease with increasing temperature. Transmission electron microscopy observations reveal that the dislocation density increases with increasing strain rate, but decreases with increasing temperature. A pronounced thermal softening effect is observed in the specimens deformed at 700°C due to the rapid annihilation of the dislocations. However, a work hardening effect occurs at higher strain rates and lower temperatures due to an enhanced degree of dislocation multiplication and tangling. Finally, a linear relationship is observed between the square root of the dislocation density and the flow stress.

Abstract: The present study is concerned with the development of a fracture criterion for the impact fracture of jointed steel plates of a lap bolted joint used in the suspension parts of a car body. For the accurate prediction of crash characteristics of car bodies by computer-aided engineering (CAE), it is also necessary to examine the behaviour and fracture of the jointed steel plates subjected to impact loads. Although the actual impact fracture of jointed steel plates of a lap bolted joint in cars is complicated, for simplifying it is classified into the shear fracture and the extractive fracture of jointed steel plates. Three kinds of steel plates, i.e., common steel with the tensile strength of 270 MPa and two high tensile strength steels with the tensile strength of 440 and 590 MPa level used for vehicles, are examined. In the impact shear test, the specimens are made of two plates and jointed by a bolt, and in the impact extractive test the specimens are made of a plate and drilled in the centre for a bolt. The impact shear test of jointed steel plates of lap bolted joints is performed using a modified split Hopkinson bar apparatus, while the impact extractive one is performed using one-bar method. Numerical simulations by a FEM code LS-DYNA are also carried out in order to understand the mechanism of shearing and extractive fractures process of jointed steel plates. The obtained results suggest that a stress-based fracture criterion may be developed for the impact shearing and extractive fractures of jointed steel plates of lap bolted joints used in a car body.