High Strain Rate Conditions Research Articles

High Nitrogen Steels. Effect of Nitride (Cr2N) Precipitation on the Mechanical, Corrosion, and Wear Properties of Austenitic Stainless Steel.

High-nitrogen austenitic stainless steels are thermally unstable and susceptible to nitride precipitation (primarily Cr2N) during elevated temperature exposure. This paper describes the effect of Cr2N precipitation on the mechanical, corrosion, and abrasive wear properties of a high-nitrogen austenitic stainless steel, nominally Fe-19Cr-5Mn-5Ni-3Mo-0.02C-0.7N. In the annealed state, Cr-rich nitrides (Cr2N) precipitate sequentially as intergranular, cellular, and finally, intragranular precipitates. Cold working, prior to aging, increases grain boundary and intragranular precipitation kinetics, but retards cellular phase formation. Nitride precipitation has only a minor influence on the yield and ultimate tensile strength of annealed materials, but causes embrittlement which is enhanced by high strain rate conditions such as impact testing and by the accelerated pricipitation kinetics associated with prior deformation. Nitride precipitation results in sensitization which is also accelerated by prior deformation. The degree of sensitization, as measured by electrochemical potentiokinetic reactivation (EPR) testing, correlates to intergranular, cellular, and intragranular Cr-depletion. Although the degree of sensitization is defined by both the Cr-minimum and width of Cr-depleted region, a Cr level below approximately 14 wt% is required for sensitization. Precipitation also results in a greater corrosion susceptibility as determined by potentiodynamic polarization scans. The wear behavior of the 0.7 wt% N alloy is little affected by the precipitation of Cr2N since the volume fraction and size (compared to the abrasive) of the Cr2N precipitates is insufficient to alter the wear resistance of the alloy.

Processing and Properties of Tungsten Heavy AUoys with Ni48Al12Fe40Inteimetallic Matrix

Abstract The superior ballistic penetration behavior of Depleted Uranium (DU) alloys compared to W-Ni-Fe heavy alloys (WHA) has been attributed to a self-sharpening behavior in DU where failure occurs along adiabatic shear bands. Since adiabatic shear represents a plastic flow instability condition between competing processes of thermal softening and work-hardening, cracking along these bands will occur more readily if the matrix were to embrittle with increased temperature along these shear bands. While conventional materials exhibit decreasing strength and increasing ductility with increasing temperature, certain Ll2+f.c.c intermetaHic compounds such as Ni-12Al-40Fe(at.%) exhibit 'anomalous' behavior where sharply decreased ductility and gradually decreasing strength is observed with increasing temperatures. Hence, tungsten heavy alloys that utilize such intermetaHic matrices have potential to fail by adiabatic shear under high strain rate conditions. Heavy alloys with different weight fractions of tung...

Viscoplastic analysis of plate-impact spallation

A model to predict material damage and spall fracture under high strain-rate conditions is applied to plate impact experiments. The model uses the Perzyna viscoplastic constitutive theory, appropriately modified to include a nonlinear isotropic hardening law that allows for saturation of the hardening with increase of strain. The constitutive equation contains a scalar variable for description of the material damage, expressed as the void volume fraction of the polycrystalline solid with microvoids. Incorporation of the damage parameter into the completely phenomenological elasto-viscoplastic constitutive equations permits description of rate-dependent, compressible, inelastic deformation, and ductile fracture. The evolution equation for the parameter describes microvoid nucleation and growth. The model for microvoid growth is based upon a random distribution of microvoids, idealized as spherical holes of arbitrary size. Microvoid coalescence is considered by incorporation of void interaction functions to describe enhanced nucleation and growth rates. A local spall fracture criterion based upon the attainment of a critical microvoid volume is utilized. The constitutive equations are specialized to uniaxial deformation with multiaxial stress, which is appropriate for the planar impact experiments. A finite-difference wave propagation computer code is used to solve the equation of motion. The computed stress wave profiles demonstrate the effect of using a viscoplastic material description. Calculations predicting the rear-surface velocity-time profile and the stress-time profiles of the OFHC copper target are compared with measured profiles. The damage (void volume) distribution across the plate thickness is also calculated and compared with experimental data.

High‐Speed fracture mechanics by photography of polypropylene copolymers

AbstractThe use of photography applied to stroboscopy in analyzing the processes inherent to the starting and propagation of cracks in materials is a technique which has proved to be of great interest, especially since it enables one to check and “directly” study the evolution of such phenomena. Using fracture mechanics criteria this technique has been applied to the study of the impact behavior of some polypropylene copolymers at different rubber contents obtained either by blending or by synthesis. This technique makes it possible to determine numerous parameters of fracture mechanics, including C.O.D. (crack opening displacement), C.O.A. (crack opening angle), JIc, the plastic work parameter, determined from the resistance curve R, tearing modulus, and crack propagation velocity. Furthermore, under high strain rate conditions, the value taken on by the coefficient “λ” relating the J‐integral to C.O.D. (1, 2) were checked for those materials using the equation J = λ. σy · C.O.D., Hayes and Turner (3) and Boyle (4). From analysis of the materials it was possible to note that the synergetic effect of the EP (ethylenepropylene) rubber increased, especially when present at percentages of more than 10 percent. Annealing the materials, on the other hand, produced an increase in fracture toughness for those products having a low rubber content; however it did not have any effect on those with an elevated rubber content (26 percent).

Impact resistance and energy absorption mechanisms in hybrid composites

The response of hybrid composites to low-velocity impact loading has been investigated. The energy absorbing mechanisms of laminates containing various fibers were studied primarily by means of the instrumented falling dart impact testing technique. Static indentation tests and scanning electron microscopy (SEM) were also employed to assist in the identification of failure mechanisms. The composites containing polyethylene (PE) fibers, which were of high strength and high ductility, were found to be effective in both dissipating impact energy and resisting through penetration. Polyester (PET) fiber reinforced epoxy also exhibited superior impact characteristics even though the PET fabric layers without epoxy did not have good modulus or ductility. Good energy absorbing capability was also observed in epoxy reinforced with woven fabrics made of high-performance Nylon fibers. Nylon, PE and PET fibers were found to enhance the impact resistance of graphite fiber composites. Upon impact loading, the composites containing either PE or PET fibers in general exhibited a great degree of flexural plastic deformation and some level of delamination, thereby dissipating a significant amount of strain energy. Hybrids containing Nylon fabric showed analogous behavior, but to a lesser degree. The stacking sequence in hybrid laminates was found to play a critical role in controlling plastic deformation and delamination. This implies that the stacking sequence is a major factor governing the overall energy sorbing capability of the hybrid structure. The penetration resistance of hybrid composites appeared to be dictated by the toughness (strength plus ductility) of their constituent fibers. The fiber toughness must be measured under high strain rate conditions.

電磁圧接に関する研究 Ｖ 電磁圧接時の変形抵抗ならびに圧接に要する衝突速度と加速度

Electromagnetic welding is classified as a high energy rate welding, and hence, the feasibility of this welding depends, in principle, on the velocity of flying body at the time of collision, as experienced well in the case of explosive welding. The authors pointed out in this paper that its acceleration could be an important factor in this welding, as well as its velocity. The deformation test under a high velocity on the aluminum tube and the welding test on the joint of tube and core brought the following conclusions.(1) Under the high strain rate condition of electromagnetic welding, the increment of flow stress caused by the strain rate hardening should be accounted, other than that by the strain hardening, because the former increased very significantly with increasing the strain rate.(2) Both the velocity and acceleration of flying tube controlled the feasibility of welding; the larger the acceleration was, the smaller became the velocity required for obtaining the welding joint.(3) The result of (2) suggested that the welding was performed when the sum of the kinematic energy and strain energy given by the pressure at collision reached a critical value.

動的直接観察測定法（ＭＩＳＯ法）による溶接凝固割れ現象の解明 ＩＩ 急速引張割れ試験による凝固割れ発生・伝播時の限界歪の測定

The MISO technique, invented by the authors to observe dynamically the behaviors of solidification front and solidification cracking during welding, has been expanded to measure the critical strain required for initiation and propagation of solidification crack. For this purpose, dotty marks in high speed cine photos resulting from ruggedness on macrostructure on weld metal surface have been utilized as the re-ference points of gauge length to measure the dynamical change of the strain. The optimum gauge length has been shown to be 1 to 2 mm on the basis of feasibility, although the critical strain has been increased with shortening of gauge length. This technique has been applied to a tensile hot cracking test under the condition of high strain rate for carbon steels and stainless steels, and the critical strain measured has been shown to be more precise and reasonable than that measured by the Varestraint cracking test.

Application of the IFR portable pendulum for the assessment of the mechanical properties of solid foams

The use of the IFR portable pendulum, with further instrumentation, for assessment of the mechanical properties of solid foam structures is described. The instrument has been used to measure the resistance to penetration of various solid foamed foods produced by extrusion cooking. These data, obtained under high strain rate conditions of impact, provide an alternative to the use of low strain rate texture measuring apparatus based on tensile testing devices. The instrumented pendulum records the transient penetration of a hammer into the sample using an angular displacement transducer. The signal is acquired using a microcomputer and may be differentiated to obtain the instantaneous retardation. The restitution of the pendulum hammer is also recorded as a function of time to the point of initial contact with the specimen.

The dependence of JIC on the temperature and strain rate in structural steels.

Static and dynamic fracture toughness tests were carried out using the convenient JIC test method proposed in the previous paper, to investigate the effect of temperature and strain rate on JIC. It was found that in the upper shelf region, JIC increases with a decrease in temperature and that JId is lower than, or almost equal to, JIC. The reason why J Id is lower than J IC is considered to be that plain strain ductility at the crack tip is reduced with an increase in yield strength under high strain rate conditions.

STRESS AND SURFACE TEMPERATURE OF SHORT FIBER-RUBBER COMPOSITE UNDER DYNAMIC FATIGUE

The change in stress and surface temperature of short fiber-rubber composites under a repeated constant displacement were measured. The fatigue test was carried out in the direction of fiber orientation of composite with 5% of static strain and ±2.5% of sinusoidal strain under 5 and 25Hz. By giving intermission repose at the mid of the fatigue test, the effect of duration of intermission and fatigue cycles until intermission on change in the stress and surface temperature was investigated.Under the condition of high sinusoidal strain rate, the interfacial region between fibers and rubber matrix giving rapid displacement is destroyed and the internal heat generation is large. Consequently, the stress decreases radically and the surface temperature rises till the slip of the short fibers takes place by the destruction of the interfacial region. It seems that the stress decay of composite was caused by the destruction of interfacial region and the internal heat generation from the results of measuring stress and surface temperature before and after duration of intermission under discontinuous fatigue test. The composite shows the recovery of the strain including the interfacial region between fibers and rubber matrix when the duration of intermission is applied before the slippage of the short fibers.

Dynamic plasticity: A comparison between results from mechanical testing and machining

The data from several previously published mechanical tests on steel and copper at high strains and strain rates are reviewed. These are presented in the form of true stress-true strain rate curves at various true strain levels and are corrected for the effects of adiabatic heating where appropriate. It is found that thermal softening cannot always satisfactorily account for the shape of the curves, and hence a dual deformation—heating model, in which the scale or intensity of the deformation is carefully considered, is proposed. Further data from machining tests are then examined and compared with those generated by mechanical testing. It is found that the evidence for the dual model is substantiated. It is concluded that, when strain and heating effects are operating in the bulk sense, adiabatic corrections to the stress—strain rate curves are possible. However, catastrophic shear, which may be induced under certain conditions of high strain, strain rate, hydrostatic stress and microstructure, cannot be accounted for in such a simple fashion.

Influence of modal composition on deformation in garnet Iherzolite nodules

A textural distinction has been drawn, in peridotites derived from kimberlite pipes, between relatively undeformed granular types and strongly deformed sheared types1, that corresponds roughly with the absence and presence of large strained olivine grains (porphyroclasts) surrounded by smaller, recrystallized grains (neoblasts). More detailed classifications have also been proposed2–6. The textural variety is probably caused by both widespread mantle creep and more localized phenomena connected with kimberlite eruption7–12. From consideration of dynamically recrystallized grain size11 and dislocation densities12, it seems that nodules that contain neoblasts have been deformed in high stress and strain rate conditions, over short periods of time, before eruption. The formation of the sheared textures and the eruption of the kimberlite are approximately simultaneous. I report here detailed textural observations on 10 garnet Iherzolite nodules collected from kimberlite pipes in South Africa and Lesotho. They all have sheared textures, as defined above, showing a range from small amounts of grain boundary recrystallization to almost complete consumption of olivine porphyroclasts. Sections were cut parallel to the foliation and modal analyses were accurately determined by point counting one slide from each sample. The number of olivine porphyroclasts on each slide was then independently counted. The fewer the grains, the greater is the accuracy of estimating their number. Average porphyroclast areas were then calculated (Table 1). The data suggest that modal composition controls the extent of shearing.

An intrinsic time to fracture criterion for amorphous polymers

An energy probability theory of global fracture is formulated using the notion of intrinsic time starting with first principles. Comparisons are made between the theory and experiments on Styrene-Butadiene Rubber, thus explaining various aspects of failure. It is proposed that fracture occurs as a result of accumulation of broken carbon-carbon bonds. When a critical concentration of broken bonds is reached, catastrophic failure begins and the specimen fractures. It is assumed that the probability of fracture of a single carbon-carbon bond is determined by the energy content of the bond. Non-uniform distribution of the free energy of the specimen among bonds is taken into account by means of an exponential distribution function. The intrinsic time measure pertinent to the constitutive equation of the material is used as the time measure of fracture. This allows use of the time-temperature shift principle, applicable to fracture of polymers, and enables the prediction of lifetimes in high strain rate conditions. Finally the theory predicts correctly that, at constant amplitudes, the number of cycles to failure at low frequencies is directly proportional to the frequency and is independent of the latter at high frequencies.

The strength of the anterior cruciate ligament in humans and Rhesus monkeys

The mechanical properties of anterior cruciate bone-ligament-bone specimens from humans and rhesus monkeys were determined in tension to failure under high strain-rate conditions. The age range of the human specimens was from sixteen to eighty-six years. The values fro human specimens obtained from young adults with regard to elastic modulus, ultimate tensile stress, and strain energy to failure were approximately two to three times those for specimens from humans in the sixth decade and older. The major mode of failure was ligament disruption in the specimens from young adult humans and avulsion of bone beneath the ligament insertion site in the specimens from older humans. The difference in mode of failure correlated with histological observations of decreased bone mass at the site of ligament attachment in the specimens from older humans. Rhesus monkey specimens had higher values for elastic modulus, failure stress, and strain energy. Significant reductions in strength and stiffness properties of ligament units were shown to occur with advancing age to a greater degree than expected. All experiments in which specimens from older human cadavera are used should be interpreted with caution when the results are applied to mechanisms of ligament failure for younger or athletic individuals.

極低炭素鋼の下降伏応力に及ぼす温度およびひずみ速度の影響について

The relationship between lower yield stress and strain rate has been measured on a low carbon steel at various temperatures.The results obtained are summarized as follows:(1) Deformation twins were observed in Lüders band under the condition of low temperature and high strain rate at which the lower yield stress value was over 50kg/mm2. The slope in the logarithmic plots of lower yield stress against strain rate in the range where the deformation twins were observed during propagating Lüders band was lower than that without a deformation twin.(2) In the range of temperature and strain rate where no deformation twin was observed, the relation between lower yield stress and strain rate in logarithmic plots was represented by a straight line having only one or two slopes over the strain rate of 10-5∼102 1/s at a constant temperature.(3) The fact that the slope of the above straight lines at a constant temperature depends upon the strain rate may be caused by the disagreement between the Lüders band front velocity and the cross head speed.

Shock-induced phase transition in single crystal CdS

The wurtzite-to-rocksalt structural change in CdS as observed under hydrostatic conditions at 27 kbars is also observed to occur under the very high strain rate conditions that accompany a strong shock wave incident on CdS single crystals. Under the influence of a single incident shock wave, the attendant large decrease in specific volume that accompanies the phase change results in a propagated two-wave structure. There are differences in the shape and amplitude of the propagated wave dependent upon the orientation of the anisotropic crystal. The decrease in specific volume was calculated using measurements of the propagated wave velocities, the stress amplitude of the first wave, the input stress amplitude, and the initial specific volume. In the case where the shock propagation is along the c-axis, a volume decrease of 17% is calculated for the phase change, and perpendicular to the c-axis it is calculated to be 18%, as compared to a 19% decrease reported for the hydrostatic experiment. Based on the experimental data, the wurtzite-to-rocksalt phase change in CdS is virtually completed in less than 1 μsec under dynamic conditions imposed by a strong shock.

Shock Wave Compression of ‘Plexiglas’ in the 2.5 to 20 Kilobar Region

PRECISE data concerning the Hugoniot equation of state for polymethyl methacrylate (‘Plexiglas’, ‘Lucite’ polymers, and ‘Perspex’) are lacking at pressures below 20 kbar. Accurate dynamic data in this pressure region are helpful when the polymer is used as an attenuator material for forming shocks in the lower pressure region, and in general are necessary to understand its rheological behaviour under the high strain-rate conditions of shock compression. Solid materials customarily exhibit shock instability at pressures somewhat larger than the yield stress in uniaxial strain. This instability gives rise to an ‘elastic precursor’ wave front preceding the main shock front1. Such a structural wave might be expected in ‘Plexiglas’ since an indication of a yield zone was found in the vicinity of 2 kbar in measurements of the static linear compressibility by the method of Hughes and Maurette2, and it is of interest to know if it does occur.

Yield and fracture in polycrystalline niobium

Tensile tests on specimens prepared from an ingot of electron-bombardment melted niobium have shown that the material undergoes a ductile—brittle transition and can be made to twin. The variation of yield stress σ v with grain-size 2 d has been used to determine the effect of temperature (293°K−20°K at a strain-rate of 2.02 × 10 −4 sec −1) and strain-rate (2.02 × 10 −4 sec −1−6.18 × 10 −2 sec −1 at 77°K) on the values of σ i and k y in a Petch type equation σ y = σ i+ K yd −1 2 , the results indicating that the ductilebrittle transition characteristics are in accordance with Cottrell's transition equation ( σ id 1 2 + k y)k y = βμγ . Niobium shows a greater resistance to brittleness than carburized α-iron because it has a smaller k y value, and the reluctance of the material to twin except under extreme conditions of low temperature or high strain-rate is also attributed to a low value of k y . The mechanical behaviour of niobium relative to that of other body-centred cubic transition metals is discussed.