Stress Wave Loading Research Articles

Dynamic crack growth under stress wave loading

The crack growth process has been analysed on the basis of a fracture criterion of a dynamic stress intensity factor when a crack in an infinite plate was subjected to a pulse type of stress wave. The crack velocity and the amount of crack extension were related to the constant amplitude and the duration of the stress pulse. The calculated amount of crack extension was well in agreement with the experimental one for the polymer material Acrylite (which is similar to polymethylmethacrylate) found by the authors, thus indicating the validity of the present approach.

A split Hopkinson bar study of energy absorption in dynamic rock fragmentation

Fragmentation of unconfined cylindrical specimens of rock due to stress wave loading has been investigated using a Split Hopkinson Bar (SHB). Photographs were taken during the fracture process using a high-speed framing camera, and incident, reflected and transmitted stress pulses were recorded using a digital transient recorder. The energy absorbed by the specimen during the fragmentation process was evaluated from the recorded stress pulses. Two different rocks were studied, namely Bohus granite and Solenhofen limestone. The specimen length was 50 mm, and the diameter was 25 mm. The incident pulse durations were 50, 100 and 200 μs, and the amplitude was in the range of 0·5−4 kbar. The high-speed photographs show that fracture occurs as previously observed for other rocks using SHB-devices. Notably, the main crack orientation is axial, and the degree of fragmentation increases with increasing load. The energy absorption of the specimen increases markedly when the load applied approaches a certain value. For the Bohus granite specimens the critical load was found to be 1·8 times the static compressive strength, and for Solenhofen limestone it was 1·3 times the static compressive strength. Two simple specimen models were analyzed theoretically, namely, one linearly-elastic and one rigid-plastic. The first model gives the best agreement with experimental results if the degree of fragmentaion is low, the second model gives the best agreement if the degree of fragmentation is high. The rigid-plastic model explains the experimental observation that the specimen always absorbs less than half the incident pulse energy.

Microstructural origins of dynamic fracture in 2024 aluminum alloy

The microstructural aspects of void formation leading to the initiation of spallation have been examined for a commercial aluminum alloy. Metallographic examination of specimens subjected to stress-wave loading conditions has shown that brittle second-phase particles constitute nucleation sites for voids under dynamic loading conditions. The probability of void formation depends on the number of dispersed particles. The ability of the matrix to hinder void growth is affected by solution treatment and is a factor determining dynamic fracture strength.

Experiments on critical stress intensity factors resulting from stress wave loading

Experiments on critical stress intensity factors resulting from stress wave loading

Crack propagation in an elastic solid subjected to general loading— IV. Obliquely incident stress pulse

T he stress intensity factors of a half-plane crack extending nonuniformly in an isotropic elastic solid subjected to stress wave loading are considered. A plane stress pulse is obliquely incident on the crack, and the wavefront strikes the crack at some initial time. At some arbitrary later time, the crack begins to extend at a nonuniform rate. It is found that the mode I and mode II stress intensity factors each have the form of the product of a universal function of instantaneous cracktip speed with the stress intensity factor for an equivalent stationary crack. An energy-rate balance fracture criterion is applied to obtain an equation of motion for the crack tip and to determine the actual delay time between the arrival of the incident wave and the onset of fracture as a function of angle of incidence of the loading wave.

Dynamic yield-strength determination at elevated temperatures after nanosecond pulse heating

An experimental method is described that has been used to determine the yield strength of 6061-T6 aluminum after extremely short times at elevated temperature. The method combines electron-beam pulse heating and onedimensional stress-wave loading. A 3.5-MeV pulsed electron-beam source (pulse width of 70 nanoseconds) is used to deposit energy uniformly through the thickness and along a limited region of a slender aluminum rod. An axial compressive stress wave, produced by projectile impact on one end of the rod, propagates into the heated region a few microseconds after energy deposition.

Skew crack propagation under stress-wave loading

Skew crack propagation under stress-wave loading

Crack propagation in an elastic solid subjected to general loading—III. Stress wave loading

T he stress intensity factor of a half-plane crack extending non-uniformly in an isotropic elastic solid subjected to stress wave loading is determined. A plane stress pulse strikes the crack at time t = 0, the wavefront being parallel to the plane of the crack. At some arbitrary later time t = τ, the crack begins to extend at a non-uniform rate. It is found that the stress intensity factor is a universal function of instantaneous crack-tip velocity times the stress intensity factor for an equivalent stationary crack. An energy rate balance fracture criterion is applied to obtain an equation of motion for the crack tip. The delay time between the arrival of the incident pulse and the onset of fracture is also calculated for this fracture criterion.

Mechanical properties of 6061 Al-Mg-Si alloy after very rapid heating

The effects of time, temperature and strain rate on the yield strength determined at elevated temperature have been investigated for 6061-T6 Al-Mg-Si alloy. To achieve very short times-at-temperature, nanosecond pulse heating produced by electron beam energy deposition was used along with one-dimensional stress-wave loading. When a relatively thick sample is heated in this way it cannot expand on the same time scale as the temperature increase. As a result, stress relief waves propagate in the material after energy deposition, and this deformation occurring at high strain-rates and elevated temperatures produces microstructural changes that reduce the strength of the alloy on the time scale of a few μs. This strength reduction occurs in addition to that due to the lowered shear modulus at-temperature, and is an essentially permanent change reflected in the greatly reduced room temperature strength of the material following nanosecond pulse heating. If the material is heated slowly enough so that the sample can expand as the temperature increases, and if the soaking time at temperature is less than required for microstructural changes in the alloy (e.g. approximately 3 s at 260°C), the yield strength measured at-temperature under wave propagation conditions drops in proportion to the shear modulus. In addition, the yield strength measurement is sensitive to the rate of deformation at elevated temperatures even for short times-at-temperature. The nature of this sensitivity is discussed in terms of thermally-activated dislocation motion.

The effect of low-frequency biaxial loads on stress-concentration factors in plates

It is generally recognized that stress-concentration factors under stress-wave loading are lower than those under static loads. In this work, the effect of low-range frequency of biaxial sinusoidally varying alternating stresses on the stress-concentration factors for circular and elliptical holes in Plexiglas plates is investigated. The experiments have been performed on a specially designed and built “biaxial cyclic-stress machine” and the results are presented in the form of curves. In the case of biaxial alternating stresses, the stress-concentration factor is defined as the ratio of amplitude of the maximum alternating stress around the geometrical discontinuity to the larger of the amplitudes of the two principal alternating stresses which would occur at the same point, if the geometrical discontinuity was not present. Both values are considered over a stress cycle. The results indicate a slight decrease in the values of stress-concentration factors with increase in frequency.

Dynamic fracture of a quartz-phenolic composite under stress-wave loading in uniaxial strain

A two-dimensionally reinforced quartz-phenolic composite material is subjected to a high-amplitude stress-wave loading condition in uniaxial strain to determine its dynamic fracture behavior. This loading condition is achieved by using exploding-foil plate-impact techniques. The direction of wave propagation through the thickness of the material is normal to the principal direction of reinforcement. Two failure levels are defined: the visual threshold, and a microscopic threshold represented as the lower limit of connected delamination. These data indicate that the macroscopic and microscopic spallation thresholds of the laminated, 0-degree shingle-angle, quartz-phenolic composite are sensitive to the tensile-pulse duration, amplitude, impulse and shape at the spall-plane location. The time-dependent aspects of the spallation threshold of two-dimensionally reinforced quartz phenolic are found to obey failure theories which are rate-process oriented, and which reflect the effects of tensile-pulse duration, peak tensile stress, tensile impulse and tensile pulse shape.

Time-dependent fracture criteria for 6061-T6 aluminum under stress-wave loading in uniaxial strain

New spallation threshold data for 6061-T6 aluminum were obtained under stress-wave loading conditions in uniaxial strain, covering the range of tensile pulse durations of 60 to 200 nsec. This range of pulse duration was achieved by using exploding-foil techniques to accelerate thin Mylar plates against thin aluninum specimens. A comparison was made between exploding-foil spallation tests on 6061-T6 aluminum in air and vacuum. The data indicate that the spallation threshold of 6061-T6 aluminum is sensitive to the tensile pulse duration, amplitude, and impulse at the spall location. The exploding-foil impact conditions were reduced to stress-pulse loading parameters by using a one-dimensional elastic-plastic hydrodynamic computer code. The time-dependent aspects of the spallation threshold of 6061-T6 aluminum were found to obey failure theories which were rate process oriented, and which combine the effects of tensile-pulse duration, peak tensile stress, tensile impulse, and tensile-pulse shape. The present data have been used to quantitatively establish failure relationships for 6061:T6 aluminum. Where applicable, supplemental information in the literature concerning dynamic fracture of 6061-T6 aluminum was utilized.

Techniques for studying strain rate effects in brittle materials

AbstractTechniques for studying brittle fracture over a strain rate range of 10−7–101 in./in./sec. and at temperatures from 75 to 1800°F are discussed. Techniques for achieving a uniaxial tensile stress in prismatic bars via a reflected stress wave method at strain rates up to 103 in./in./sec. at room and elevated temperatures are presented. Results of the aforementioned effects in flexure, and experimental verification of the stress‐time‐position history for the stress wave loading technique are presented along with applicable theoretical explanations. Applications of the above techniques to a broader spectrum of brittle and semi‐brittle materials are described.