Stress Wave Loading Research Articles

Rupture of fat cells using laser-generated ultra short stress waves.

Ultrasound-assisted liposuction has been investigated as an aid to breakup fat for its less traumatic removal, and minimizing the post-operative recovery period. This research focused on understanding the mechanism of fat interaction with laser-generated ultra short stress waves with high amplitudes. Freshly extracted human fat was secured in an Aluminum (Al) cavity. Sixteen nanoseconds duration stress waves were generated by exfoliating the bottom surface of the Al cavity by focusing 3 nanoseconds-long YAG laser pulses over a 2 mm diameter area at 10 Hz with a maximum pulse energy of 0.95 J. The lipids released due to cell rupture were extracted and measured Four minutes of pulsing released about 0.005 g, which was over 1% of the initial weight of the tissue. In situ temperature rise of only 5 degrees C was measured at the maximum stress wave loading duration of 5 minutes. This was evidenced by histological sections, which showed no burn artifacts. This research shows that ultra short stress waves can mechanically cavitate fat in vitro without significant damage to adjacent structures, and forms the basis for future clinical work.

Push‐Pull Fatigue of Sintered Silicon Nitride Ceramics at Elevated Temperatures

The fatigue tests under push‐pull completely reversed loading and pulsating loading were performed for silicon nitride ceramics at elevated temperatures. Then the effects of stress wave form, stress rate, and cyclic understressing on fatigue strength, and cyclic straining behavior, were examined. The cycle‐number‐based fatigue life is found to be shorter under trapezoidal stress wave loading than under triangular stress wave loading, and to become shorter with increasing hold time under the trapezoidal stress wave loading. Meanwhile, the equivalent time‐based life curve, which is estimated from the concept of slow crack growth, almost agrees with the static fatigue life curve in the short and intermediate life regions, showing the small cyclic stress effect and the dominant stress‐imposing period effect on cyclic fatigue life. The fatigue strength increased in stepwise stress amplitude increasing test, where stress amplitude is increased stepwise every given number of stress cycles, at 1100° and 1200°C. Occurrence of cyclic strengthening was proved through a gradual decrease in strain amplitude during a pulsating loading test at 1200°C in this material, corresponding to the above cyclic understressing effect on fatigue strength.

809 DYNAMIC DEBONDING STRENGTH OF FIBER GLASS COMPOSITE

Debonding strength of a GFRP is investigated under dynamic loading. The specimen is subjected to stress wave loading. A circumferential V notched is machined in the middle section of each specimen. The fracture initiates from the notch root and propagates to the interface. Using data gathered from a finite element analysis the separated dynamic J integral is computed. The result shown the critical dynamic J integral is larger than static J integral.

Strain rate effects on dynamic fracture and strength

An experimental procedure and accompanying theoretical analysis is presented to produce a well-characterized technique for quantifying dynamic fracture properties of quasi-brittle materials. An analytical and experimental investigation of mode I fracture of concrete was conducted under the dynamic loading of a split Hopkinson pressure bar. Fracture specimens in the form of notched-cavity splitting tension cylinders were subjected to stress wave loading that produced strain rates nearing 10/s. Fracture parameters were extracted by the application of the two-parameter fracture model, a nonlinear fracture model for quasi-brittle materials. Finite element analysis verified the experimental configuration and addressed inertial contributions within the dynamic environment. Ultra-high-speed digital photography was synchronized with the fracture process to provide additional validation and insight to the experimental technique. Results show that the effective fracture toughness and specimen strength both increase significantly with loading rate. The numeric and photographic results validate the experimental technique as a new tool in determining rate dependent material properties.

Initiation, propagation and arrest of an interface crack subjected to controlled stress wave loading

An experimental study has been conducted to investigate the initiation, propagation, and arrest of bimaterial interface cracks subjected to controlled stress wave loading in the form of a tensile dilatational stress wave pulse. The tensile pulse is generated by detonating lead azide explosive in a specially designed specimen. Dynamic loading of the bimaterial interface results in crack initiation, propagation, and arrest, all in the same experiment. This failure event is observed using photoelasticity in conjunction with high speed photography. Full field data from the experimentally obtained isochromatic fringe patterns is analyzed to determine time histories of various fracture parameters such as the crack tip speed, the dynamic complex stress intensity factor, the energy release rate, and the mixity. The experimental data is also used to quantify the values of the dynamic initiation and arrest toughness and to evaluate a recently proposed dynamic interface fracture criterion.

Theoretical simulations of a propagating crack subjected to in-plane stress wave loading by caustic method

The optical method of caustics for measuring the dynamic stress intensity factor in a transient process is investigated in this study. The transient full-field solutions of a propagating crack contained in an infinite medium subjected to step-stress wave and ramp-stress wave loadings are used to establish the exact equations of the initial and caustic curves. The results of the stress intensity factor obtained from the caustic method are compared with theoretical predictions and some experiments. The results demonstrate that a significant deviation can occur in the determination of the dynamic stress intensity factor from shadow spot measurements. The factors, such as screen distance, magnitude of loading, crack speed and rising time which can influence the accuracy of the experimental measurements are discussed in detail. In addition, the valid region of the dynamic stress singular field for the propagating crack is discussed in detail and it gives a better understanding of the appropriate region of measurements for investigators.

Identification of a percussive drill rod joint from its response to stress wave loading

In percussive drilling of rock, elastic stress waves are generated in a drill string through repeated axial impacts by the hammer of a rock drill. For holes deeper than a few meters, several drill rods are commonly joined by means of cylindrical coupling sleeves with internal threads which connect drill rods with external threads at their ends. Each coupling sleeve (CS) joint serves to transfer stress wave energy from one drill rod to the next with minimum loss of energy due to reflection and dissipation. This paper deals with the development of an identification procedure for the nonlinear dissipative spring mass (NDSM) model of a CS joint developed by Lundberg et al. Stiffness, friction and mass parameters were determined by minimizing the diffence between simulated and measured responses of the joint, in reflection or transmission, to the same incident stress wave loading. Similar results were obtained as with an existing mixed static and dynamic identification procedure, but with considerably less expenditure of equipment and time. The most reliable results and the smallest deviation between simulated and measured responses were achieved in transmission.

Effects of Stress Waveform on Push-Pull Cyclic Fatigue Properties of Sintered Silicon Nitride Ceramics at Elevated Temperatures.

Push-pull cyclic fatigue tests were performed for silicon nitride ceramics with stress-rate-controlled triangular and trapezoidal stress waves at 1 200°C at stress ratio R=-1.0. Then the effects of stress rate and stress hold time on the cyclic high-temperature fatigue were examined. The cyclic fatigue life is shorter under trapezoidal stress wave loading than under triangular stress wave loading, and becomes shorter with increasing hold time under the trapezoidal stress wave loading. Cyclic fatigue lifetime is successfully estimated through the crack extension law based on a slow crack growth mechanism, showing a small cyclic stress effect and dominant stress-application-period effect on cyclic fatigue lifetime. This was also evident from scanning electron microscope observations of fracture surface. Cyclic fatigue life was influenced little by stress waves which have hold time on both the tension and compression sides or only on the compression side. Thus compressive stress hold seems not to induce cyclic fatigue damage.

Exact elastodynamic analysis of some fracture specimen models involving strip geometries

A procedure for analysing a class of transient elastodynamic crack problems is presented. These problems model certain experimental situations which can be used to infer fracture toughness values for materials under stress-wave loadings. In particular, the present analysis provides exact expressions for the elastodynamic stress intensity factor at the tip of a long external crack in a striplike body whose lateral (upper and lower) boundaries are parallel to the crack line. Plane stress/strain conditions are assumed to prevail. In this class of problems, the crack may be situated asymmetrically with respect to the mid-strip line, and various dynamic loadings are considered, including crack face tractions and lateral face displacements. The loadings considered will have an arbitrary time dependence, but will be spatially uniform. The problem analysis is based on integral transforms and an asymptotic usage of the Wiener-Hopf technique. Two useful cases are considered in detail as examples; the symmetrically cracked strip with traction-free lateral boundaries under sudden crack face pressure, and the symmetrically cracked strip with suddenly displaced shear-free lateral boundaries.

Fracture behaviour of steel 20 MnMoNi 5 5 under stress wave loading

Le comportement d'initiation de la rupture de I'acier a petits grains 20 MnMoNi 55 sous charge par ondes de contrainte fut analyse au moyen de barres de Hopkinson dans la gamme de temperatures de 50 a 20° C. Si les temperatures sont inferieures a 20° C la rupture est provoquee par clivage; K Id descend et tombe jusqu'a la tenacite a rupture. Par contre, sous temperature ambiante une croissance stable de la fissure apparait avec un J i proche de la valeur d'initiation statique de l'integrale de J. L'analyse de la configuration des pointes de fissure permet de faire reconnaitre qu'une croissance stable de fissure est conditionnee par les processus aleatoires suivants declenches simultanement: generation de microfissures locales empechees de se propager, neutralisation de ces pointes de fissures isolees et deformation des liaisons de materiaux le long du front des pointes de fissure

Numerical modelling of elastic-plastic deformation at crack tips in composite material under stress wave loading

L'article traite la propagation des ondes elasto-plastiques dans un materiau anisotrope soumis a un impact. On s'interessera plus particulierement a un materiau composite cubique dans un etat de deformation plane. Les equations differentielles forment un systeme hyperbolique. C'est pourquoi une methode des caracteristiques est proposee afin de calculer la formation de la zone plastique autour de la pointe d'une fissure

The dominance of the K I-field for a crack under stress wave loading

The extent of the dominant singular field is investigated for a finite crack under stress wave loading. Using a boundary integral equation method the complete solution as well as the near field solution is determined. A comparison of the two fields indicates that the singular field dominates within a small domain at the crack tip. The size of the dominance region in the dynamic case may be very different from that in the static case.

Computational analysis of dynamically propagating cracks in axisymmetric solids

Dynamic crack propagation behavior in axisymmetric solids is investigated using an effective computational procedure. First, an accurate method to extract energy release rate of a dynamically propagating crack from finite element solutions is formulated for axisymmetric geometries. The method is applied to an analysis of a radially growing circular crack under remote tension in an infinite medium. The computed dynamic energy release rate shows an excellent agreement with the exact solution. Next, we have developed an iterative technique to propagate a crack whose velocity history is initially unknown. With the iterative procedure, the crack propagates according to a condition specified by a dynamic fracture criterion. At each increment of crack growth, an optimum velocity at which the crack growth condition satisfies is obtained by the iterative scheme. This procedure requires no artificial input or no preset crack tip speed in a simulation study. The iterative scheme is employed in a dynamic fracture analysis of ceramics. The computational analysis is carried out for simulation of fracture experiments using circumferentially notched round bars. In the test, a ceramic specimen is subjected to tensile stress wave loading and after crack growth initiation, the external crack propagates to tear the specimen. The computational simulation is carried out for the entire fracture process including the crack growth initiation and the dynamic propagation. The iterative procedure enables us to predict the crack tip velocity which is unmeasurable in the experiment. Suitabilities of proposed fracture toughness criteria for the ceramics are investigated by comparing measured and computed transmitted pulses through the uncracked ligament. This study proves the usefulness of the computational procedures for dynamic fracture analysis. It is most effective in characterizing dynamic fracture toughness where determination of every relevant parameter is difficult in the experiments.

Measurement of dynamic crack tip displacement field by speckle photography and interferometry

Speckle interferometry has been used to measure the transient displacement field around a crack due to stress wave loading. Double exposure interferograms are recorded with a pulsed ruby laser, and correlation fringes formed by spatial filtering of the developed film. A new technique is presented for reducing the noise level in spatially filtered speckle interferograms. Independent fringe patterns, obtained from one interferogram but with the filtering aperture sampling different regions of the Fourier transform plane, are digitised by a CCD camera and combined numerically. It is demonstrated that accurate analysis of fringe patterns by the Fourier transform method can be carried out, even if no carrier fringes are present. The measurements are compared with those obtained by speckle photography. Least-squares fitting of the theoretical displacement field is used to calculate the stress intensity factor at the crack tip as a function of time.

The interaction of two inclined cracks with dynamic stress wave loadings

To gain insight into the phenomenon of the interaction of stress waves with material defects and the linkage of two cracks, the transient response of two semi-infinite inclined cracks subjected to dynamic loading is examined. The solutions are obtained by the linear superposition of fundamental solutions in the Laplace transform domain. The fundamental solution is the exponentially distributed traction on crack faces proposed by Tsai and Ma [1]. The exact closed form solutions of stress intensity factor histories for these two inclined cracks subjected to incident plane waves and diffracted waves are obtained explicitly. These solutions are valid for the time interval from initial loading until the first wave scattered at one crack tip returns to the same crack tip after being diffracted by another crack tip. The result shows that the contribution of diffracted waves to stress intensity factors is much less than the incident waves. The probable crack propagation direction is predicted from the fracture criterion of maximum circumferential tensile stress. The linkage of these two cracks is also investigated in detail.

Thermal effects in rudimentary crack edge inelastic zone growth under stress wave loading

Inelastic zone growth at the edges of a pre-stressed crack due to stress-wave loading by diffraction is considered. The Dugdale zone model is used, and the surrounding material is a weakly-coupled thermoelastic solid. The growing zone acts as a heat flux site for the solid. A transient analysis for the brief period after zone growth initiation is performed and, for generality, the crack itself is allowed to extend. Expressions for the heat flux and the inelastic zone edge temperature rise are obtained, and show dependence on several important parameters. In particular, the temperature rise is enhanced by the presence of a pre-existing zone, yet varies inversely with the zone growth rate. This rise increases rapidly upon stress wave diffraction, but levels off at values on the order of those found in stress-state analyses.

Probabilistic analysis of an impulsively loaded deep‐based structure

AbstractA deep‐based (i.e. deeply buried) structure subjected to stress wave loading is analyzed probabilistically. The finite element method, coupled with a fast probability integration technique, is used to perform the analysis. Randomness due to uncertainties in loading and stratum material properties is addressed. The results presented, although for an idealized linear elastodynamic problem, clearly demonstrate that more realistic probabilistic analyses of impulsively loaded deep‐base structures are feasible.

Dynamic fracture behavior of SiC whisker-reinforced aluminum alloys

This paper presents a study of dynamic fracture initiation behavior of 2124-T6 aluminum matrix composites containing 0, 5.2, and 13.2 vol pct SiC whiskers. In the experiment, an explosive charge is detonated to produce a tensile stress wave to initiate the fracture in a modified Kolsky bar (split Hopkinson bar). This stress wave loading provided a stress intensity rate, KI,, of about 2 × 106 MPa√m/s. The recorded data are then analyzed to calculate the critical dynamic stress intensity factor,KId, of the composite, and the values obtained are compared with the corresponding quasi-static values. The test temperatures in this experiment ranged from −196 °C to 100°C, within which range the fracture initiation mode was found to be mostly ductile in nature. The micromechanical processes involved in void and microcrack formation were investigated using metallographic techniques. As a general trend, experimental results show a lower toughness as the volume fraction of the SiC whisker reinforcement increases. The results also show a higher toughness under dynamic than under static loading. These results are interpreted using a simple dynamic fracture initiation model based on the basic assumption that crack extension initiates at a certain critical strain developed over some microstructurally significant distance. This model enables us to correlate tensile properties and microstructural parameters, as, for instance, the interspacing of the SiC whiskers with the plane strain fracture toughness.

An Experimental Laboratory Facility for Studying Shock-Induced Liquefaction

Abstract Our paper describes a new experimental laboratory apparatus developed to investigate the transient and long-term porewater pressure response of saturated soils subject to compressional stress wave loading. An overview of the laboratory system, the various components, and the experimental procedure followed are presented. Measurements include porewater pressure-time histories for the specimen and total stress time histories for the applied load. Test results indicate that Monterey No. 0/30 sand can be liquefied even at high initial densities and effective stresses under undrained, one-dimensional, confined compressive shock loadings.

Dynamic behavior of a boron carbide aluminum cermet: experiments and observations

Experimental techniques to study and characterize the dynamic behavior of ceramics and ceramic composites are described. A new technique based on the split Hopkinson pressure bar is used to obtain the deformation and failure characteristics of brittle materials at moderately high strain rates. A normal plate impact recovery technique is used to study the evolution of damage under controlled loading conditions and understand the micromechanisms of damage in materials. The normal plate impact recovery technique has the advantage of applying sufficiently high stress levels to initiate damage and yet providing controlled amounts of energy input to avoid catastrophic failure of the material. Hence, the samples can be recovered intact and evaluated for damage using standard characterization techniques such as microscopy and ultrasonics. The damage assessment in the material together with the real-time interferometric measurements serves as a powerful experimental procedure to study processes such as damage initiation, damage evolution, and the failure modes of very hard and brittle ceramics and ceramic composites under stress wave loading. Results are presented for a boron carbide aluminum cermet, a ceramic composite and the role of its microstructure on the failure mechanisms is studied. The cermet exhibits high strength and large strain to failure, in comparison to monolithic ceramics. The basic failure mechanisms in the cermet appear to be axial microcracking due to compression, and transverse cracking generated by compression release and by tension. The cracks are also found to be bridged by ductile aluminum or intermetallics, which together with the microcracking could possibly increase the toughness of the cermet at high loading rates.