Stress Pulse Duration Research Articles

Characteristics of the damage evolution and the free surface velocity profile with dynamic tensile spallation

The spalling behavior of ductile metals is a process involving void nucleation, growth, and coalescence. Limited by diagnostic techniques, spallation experiments only provide the free surface velocity profiles and the information about recovered targets, but some quantitative damage evolution information about the spalling target is lacking. In this research, the damage nucleation seeds are randomly arranged on the grain boundary in the central region of a target with grain geometry, and a two-dimensional mesoscale numerical model of a plate impact spall experiment is established. By analyzing the free surface velocity profile and the stress history, it is demonstrated that the spall strength obtained with the pull-back velocity essentially corresponds to the maximum tensile stress at the target center. The effects of the impact stress and the stress pulse duration on the dynamic characteristics of the void growth and coalescence are analyzed in-depth by using the damage evolution dissipation energy and the plastic strain contours at different times. The dynamic process of the damage evolution determines the characteristics of the oscillation after the pull-back signal. The stress history controls the damage degree and the kinetic process of the target in the spallation damage process. The impact stress has the most important effect in determining the damage evolution rate, while the stress pulse duration only affects the void coalescence process and irrelevant to the void growth. The damage degree of the void growth and the coalescence process are the result of the joint action of the impact stress and the pulse duration.

Considerations for laboratory resilient modulus testing of unbound pavement base materials

This paper explains equipment related considerations on differences between the lab and field conditions that can lead to variations in resilient modulus (Mr) values of unbound materials as determined in accordance with AASHTO T307 standard. Three different granular materials and an elastic polyurethane (control) sample were tested using commercially available laboratory test equipment. A field test bed was also constructed to measure vehicle (Class 3) induced loading conditions at the bottom of an unbound granular layer underlying new hot-mix asphalt pavement to determine stress pulse duration as a function of vehicle speed. Various interpretation issues were identified within the framework of the testing methods and equipment including: (1) insufficient laboratory sensor sampling rate (per standard); (2) the laboratory specified 0.1 s load-pulse duration and haversine shape are not matching the field stress pulse duration and shape; (3) the need for careful tuning of the load system gain settings; (4) the number of LVDTs used in the vertical strain calculation; (5) limiting quality control and quality assurance to deformation ratio values in the preconditioning sequence; and (6) limiting the load step calculations to the last 5 of 100 load cycles. The goal of this paper is to provide equipment users and specification developers with user knowledge concerning laboratory resilient modulus determination.

The structural temporal approach to dynamic and quasi-static strength of rocks and concrete

This work presents results of an experimental and theoretical study on dynamic and quasi-static failure of rocks and concrete. The results of dynamic compression and splitting of rocks (gabbro, granite, marble), as well as dry, water-saturated and frozen limestone and concrete are discussed. The tests were conducted using the Split-Hopkinson pressure bar with the diameter of 20 mm. It is shown that one material (or its condition) can have a lower dynamic strength for a higher static strength compared to the other material (or its condition). Also, it is shown a dependence of the threshold limit stress on the stress pulse duration. An unified interpretation of the experimental results, based on the structural–temporal approach is presented.

Plate impact experiments to investigate shock-induced inelasticity in Westerly granite

In the present study, plate-impact experiments are employed to better understand the stress threshold for inelasticity in Westerly granite. The experiments are designed to obtain the Hugoniot Elastic Limit (HEL) as well as spall strength following shock-induced compression in the Westerly granite samples. The HEL for Westerly granite is estimated to be in the range 3.2 to 3.5GPa, while the spall strength is ∼50MPa for a pulse duration of 3μs, and is nearly independent of the compression stress level in the stress range employed in the experiments (i.e. 0.7–5.0GPa). In addition, soft-recovery plate-impact experiments are employed to better understand the threshold for initiation of micro-cracking in the granite samples. These recovery experiments are conducted at the same stress levels as the normal plate impact experiments (∼0.7GPa), and the duration of the tensile pulse is varied from 20 to 500ns by varying the pre-set gap between the specimen and the momentum trap. The results indicate that the stress pulse duration threshold for initiation of spall is about 120ns at an applied tensile stress level of ∼0.7GPa. In addition, tensile fracture in Westerly granite is observed to be time dependent with the spall strength dependent on the duration of the tensile pulse. For the soft recovery experiments that do not show a clear spall signal, the recovered granite specimens show relatively large micro-cracks (30–40μm in width) near the plane where the unloading waves from the flyer and target free surfaces intersect. The spall plane reveals a relatively rough and a tortuous fracture surface. At higher magnifications, several cracks and pits/holes are detected on the spall plane. The fracture/damage mode is seen to be predominantly brittle, with damage initiating at the weak grain boundaries.

A new split Hopkinson tensile bar design

This work presents a new design for a split Hopkinson tensile bar (SHTB) as well as generated representative experimental results. The new design uses a U shaped striker bar as projectile and addresses several shortcomings of classical SHTB designs using hollow striker bars. The results presented show that the non-symmetrical striker bar is capable of generating a clean and virtually oscillation free square pulse signal five times longer than typically achieved by classical striker tubes, whilst at the same time offering superior signal quality. Due to the longer stress pulse duration, the new SHTB design allows for the characterisation of materials at strain rates that were difficult to achieve for hydraulic testing machines and classical striker tube based SHTB designs. In addition, the developed SHTB is based on a simple and modular design and allows for a wide range of pulse shaping methodologies to be applied. Therefore, materials requiring different input stress pulse shapes, such as square (ductile), trapezoid or triangular (brittle), can be experimentally characterised at a large range of strain rates.

Tunnel oxide degradation under pulsed stress

A key issue for Flash cell scaling down is the reduction of tunnel oxide thickness limited by the higher gate leakage current (Stress Induced Leakage Current, SILC) after cycling. It is possible to reduce the oxide degradation during cycling by reducing the stress pulse duration and increase the time between pulses. This allows the annealing of precursor sites with an overall reduction of stable traps. Aim of this work is the investigation of the SILC induced by pulsed stress and the corresponding charge trapped in the oxide during stress. The impact of the oxidation technology will also be discussed.

Fragmentation of Kidney Stone by Shock Wave-Experimental Approach

To understand fundamentals of kidney stone™s fragmentation by shock wave impingement, experimental approach was carried out. Air gun system was used to transfer a well-controlled stress wave into a thin disk specimen. The specimen was fabricated with plaster and its dimensions are 10 mm in diameter and 1.2 mm in thickness. The obtained results are summarized as follows: As the stress pulse duration is decreased from 12 microseconds to 5 microseconds, the critical stress for fracture increased tremendously. The critical fracture stress for a specimen with a small hole in the specimen center is about one fifth of that for the plain specimen. The fracture takes place along the diameter perpendicular to the stress wave incident direction. Numerical stress analysis suggests that the principal stress criterion can explain the fracture mode.

Damage evolution and damage model validation in spalled tantalum

The dynamic damage resistance of two grades of very clean tantalum is compared. The materials were loaded to incipient failure using an 80 mm gas gun equipped with soft recovery using a parallel flyer plate experimental configuration. Both loading time and applied shock pressure was varied from 1.1-2 μs and 5.6-9.8 GPa, respectively. A VISAR recorded the back surface velocity. The recovered samples were metallographically sectioned, polished, and etched. Image analysis and optical profilometry quantified the resulting damage. Longitudinal porosity distributions of the tests are compared. A calibrated hydrocode based damage model was used to simulate the tests. We compare predicted and experimental measurements of free surface velocity and porosity. The model also produced free surface pullback signals suggesting high spall strength with little evolved porosity. This is further confirmation that in clean, high Peierls energy materials the VISAR pullback signal is associated with the creation of a region of enormous hydrostatic tensile stresses in the material, but is relatively distinct from the void coalescence process. A transition in coalescence mechanism, from primarily void growth impingement to long-range plastic instabilities, occurs with increasing hydrostatic tension and increasing stress pulse duration.

Latent damage investigation on lateral nonuniform charge generation and stress-induced leakage current in silicon dioxide subjected to high-field current impulse stressing

This paper reports on positive lateral nonuniform (LNU) charge generation in silicon dioxide, and its relationship to transient or AC stress-induced leakage current (SILC), for MOS capacitor devices subjected to high-field current impulse stressing using a transmission line pulsing technique. The formation of LNU charge was attributed to the localized injection of avalanche hot carriers from the silicon substrate together with the accompanying impact ionization within the oxide. The short stress pulse duration was identified as an important factor for the generation of LNU charge as a longer duration stress pulse or DC stressing gives rise to more uniform charge trapping. A model, consisting of several similar area and equivalent MOS capacitors connected in parallel, was used to explain the effect of LNU charge generation on the high-frequency capacitance-voltage curves. Electrical annealing results indicate that the positive LNU charge traps are located close to the silicon-oxide interface. The LNU charges are distributed with a certain minimum trap energy level which require a critical fluence or gate voltage to be applied before significant LNU charge annealing is observed. The positive LNU charges, which lower the energy levels of neutral traps, are responsible for the transient SILC observed in the thicker oxide devices.

Relationship between the Duration of Stress Pulse and Dynamic Stress Concentration in a Strip Plate with Fillet of Photoelastic Material.

The purpose of this study is to clarify the dynamic behavior of stress waves in strip plates with fillets experimentally. The reflection coefficient, with corresponds to the changes in the duration of the incident pulse and the fillet shape, has been obtained by the strain gauge method. The relationship between the duration of incident stress waves to the fillet and the dynamic stress concentration factor is obtained by a dynamic photoelastic method. An interaction of the incident wave and the reflected wave takes place at the fillet. Depending on the fillet shape, the dynamic stress concentration factor becomes 10-70% higher than the static stress concentration factor for incident waves. The relationship between the duration of stress pulse and the dynamic stress concentration factor has also been obtained.

単一応力波によるエネルギ反射率の測定

In order to study the propagation characteristics of stress pulses in various structures, the reflection wave reflected from a fillet part is measured. The part is made of same material as the structure. A semiconductor strain gauge is used for the detection of stress waves. The reflection coefficient is obtained as the summation of the transfer function. The reflection coefficient is calculated by changing the duration of stress pulse and the curvature of fillet. As the result, it becomes clear that the reflection coefficient can be obtained from strain energy without using Discrete Fourier Transform if a single pulse is used.

A criterion for the time dependence of dynamic fracture

Dynamic fracture is a time-dependent phenomena in which the spall stress depends on the stress pulse duration. It is shown that the empirical relation between the spall stress and the square root of the stress gradient is equivalent to a simple impulse criterion. A more general criterion based on the concept of cumulative damage is proposed, and good agreement is obtained between calculated and observed values of the spall layer thickness in aluminum.