Quantitative Characterization of the Fracture Surface of Si Single Crystals by Confocal Microscopy

Experiments are conducted to study the dislocation nucleation conditions at the crack tip in {110}〈110〉 oriented Si single crystals. Specimens with surface cracks are first statically loaded at elevated temperatures for a prolonged period of time to initiate and move dislocations away from the crack tip, then cooled down to room temperature and loaded to fracture to measure the fracture toughness. Fractographic analysis of the fracture surfaces is performed. Distinct wavy patterns on the fracture surface at the initial cleavage crack front are observed, which is attributed to the existence of local mixed mode I/mode III stresses resulting from the inhomogeneous dislocation activity. Confocal microscopy is employed to quantify the fracture surface roughness. The results show that the increase of fracture toughness is directly associated with the increased area of the rough surface, which is characterized by the roughness number or the fractal dimension increment. Our results also demonstrate that dislocation nucleation can occur only at discrete sites. The spacing between these dislocation nucleation sources is of the order of 1 μm. A simple model is developed for the relationship between the fracture toughness and the surface roughness parameters, which is in good agreement with the experimental results.     

Formation of a Near‐Surface Fracture Zone in a Contact Explosion

This paper describes optical recording studies of explosive fracture on the surface of colophony specimens. It is shown that the existence of a free surface results in formation of a conical fracture front, which considerably increases the volume of the dynamic fracture zone. On the other hand, the free surface effect is responsible for spallation in the surface layer. Simple estimation of the spall layer thickness using measured blast wave parameters is in good agreement with experimental values. A comparison of the experimental results with data on the structure of meteorite craters shows similarity in structure between the fracture zones.     

Deformation and fracture of urethane–methacrylate resins

The tensile properties of three urethane–methacrylate resins that varied in the soft segment content of the urethane were characterized. The strain birefringence at a circular hole was observed during loading–unloading cycles to progressivley higher displacements. The shear strain distribution at the hole was calculated from the isochromatic fringe contours and compared with results from linear elastic analysis. When the onset of nonlinearity, and the subsequent appearance of residual strain at the root of the hole, were correlated with features of the macroscopic stress-displacement curves, three regions of prefracture deformation were defined. A region of linear elastic behavior was observed at the lowest strains. The maximum shear strain at the linear limit was the same in all the resins, and appeared to correlate with the yield condition at the hole. When the shear strain at the hole exceeded about 2.8%, the fringe patterns started to deviate from the elastic prediction. However, strain was fully recoverable in this region as indicated by the absence of residual birefringence at the hole after unloading. This region of nonlinear, recoverable deformation extended to progressively higher strains as the amount of urethane soft segment increased. This feature was attributed to the network structure of the urethane–methacrylate resins. A region characterized by nonrecoverable deformation at the hole followed at higher strains; the urethane soft segment content had a major effect on the amount of permanent deformation sustained before fracture. The fracture surfaces exhibited features typical of brittle fracture without crazing. © 1995 John Wiley & Sons, Inc.     

Effect of preloading on the formation of adiabatic localized shear in copper

This paper presents the results of a study of the formation of localized shear in M1 copper of two types: as-received and after preloading by a quasi-entropic compression wave. The experiments were performed with hat-shaped samples using the split Hopkinson bar method. For both types of copper, dynamic compression diagrams were obtained at strain rates of 2100–2500 s^?1. The copper structure was subjected to metallographic analysis, and the effect of preliminary shock deformation on the dynamic mechanical properties of the material was estimated. It is shown that preloaded higher-strength metals with a smaller degree of strain hardening are more prone to the formation of adiabatic shear bands.     

On the shock-based determination of the adhesive strength at a substrate-coating interface

Evaluating the bonding strength at the interface between two layers is an issue of considerable practical interest for a wide variety of engineering applications involving coatings, such as thermal protective ceramics coated on engine blades. Spallation under laser driven shock loading is one of the experimental means to test interface debonding. However, numerical simulations are usually needed to infer a quantitative value of the bonding strength from such tests, where the coating free surface velocity is usually the only measurable variable. In this paper, the analysis of the propagation and interactions of compression and release waves leading to spall fracture in a shock-loaded material is detailed, then it is extended to a substrate-coating system. Different cases are considered, depending on the acoustic impedances of the substrate and coating materials and on the duration of the loading pressure pulse with respect to the wave transit time through the coating thickness. In each case, the interfacial strength can be analytically estimated from the velocity variations without resorting to numerical models.     

Notch Strength and Fracture Behavior of 2-D Carbon-Carbon Composites

This study examined Mode I in-plane fracture in two-directional carbon-carbon composites. Linear elastic fracture mechanics (LEFM) provided good predictions for the failure loads of the compact tension specimens, and consistent values of K _IC were calculated. However, values for the strain energy release rate G _IC were inconsistent. Analysis of loading/unloading cycles applied to the specimens showed that significant inelastic behavior occurred prior to fracture. Hence the J -integral method is not thought to be appropriate for fracture prediction. It was also found that the real crack tip stresses in carbon-carbon composites are much lower than theoretical predictions. An interesting discovery is that cracks in carbon-carbon composites appear to be naturally blunt. The lower limit on the crack tip diameter corresponds with the spacing of the woven fiber bundles. A significant conclusion from this study is that the material behavior can be modeled as the sum of two contributions: (i) a linear elastic solid, which fails according to LEFM, and (ii) an independent damage mechanism, which absorbs energy but does not alter the ultimate fracture load. Possible candidates for the damage mode are fiber pullout and inelastic deformations due to shear stresses.     

Simulation of spall fracture of uranium at different temperatures in the region of polymorphic phase transitions

This paper presents the results of numerical simulation of Zaretskii’s experiments on loading of natural uranium in the phase-transition region at temperatures of 27–862°C. Simulation of these experiments is of interest because of the observed features of spall fracture of uranium in the phase-transition region. Spall fracture and compaction was simulated using the DGC-L model of the dynamics of growth and compaction in a liquid medium, which takes into account the effect of strength properties, pressure, surface tension, viscosity, and inertial forces on the growth and collapse of pores. Calculations were carried out according to the UP program— a Lagrangian method for calculating deformation problems of continuum mechanics in a onedimensional approximation.     

Atomic-scale strengthening mechanism of dislocation-obstacle interaction in silicon carbide particle-reinforced copper matrix nanocomposites

Using molecular dynamics (MD) simulations, the strengthening mechanism of silicon carbide (SiC) particle acted as barrier for the motion of edge dislocation in copper (Cu) matrix nanocomposites under shear loading is investigated. The dislocation glide behavior and the dislocation-particle interaction accounting for the effect of the temperature and the particle size are discussed in terms of the stress-strain relationship, the crystal energy change, the stress distribution and the dislocation evaluation. The results show that the critical depinning stress and the critical depinning strain are found to vary significantly depending upon the temperature and the particle size, consistent with previous studies. Higher temperature or smaller particle results in lower critical depinning stress to allow dislocation to bypass particle. Moreover, the critical depinning stress from the current atomic study is lower than the theoretical value due to the presence of cross-slip and thermal activation. At low temperature the mechanical activation controls the depinning stress of the dislocation bypassing particle, while at high temperature the thermal activation dominates that. In addition, the trapping sites of dislocations for the dislocation multiplication are observed at high temperature or large particle, due to the lattice mismatch of particle taken as the dislocation source. Compared to the dislocation interacting with void or metal-particle, the interaction of dislocation and SiC-particle in Cu + SiC nanocomposite reveals much complex factors, such as the multiple glide planes, the nanosize effect, as well as the coupling effect of mechanical and thermal activation, on the strengthening mechanism. It maybe apply these results to a high throughput experimental design which provides the dense and targeted material data, to accelerate material discovery with the large-scale and low-cost fabrication strategies.     

Tests of Unconfined Single Fragment Generators

A single fragment cannot be accelerated very well and undisturbed by an unconfined high explosive charge. The converging shock and release waves eject a higher number of very small spall debris of the fragment in the firing and target direction. This means, the fragment is considerably deformed and looses some mass. In addition, the release waves reduce the high pressure zone after the fragment very fast. Therefore, only moderate velocities are achieved.     

Influence of shear stress state on dynamic fracture of a glass ceramic Macor

The effect of shear stress on the dynamic failure of ceramic materials is not sufficiently investigated in the published literature. With the use of a bespoke split Hopkinson pressure bar, this paper presents an effort to investigate the dynamic shear compressive response of Macor, a model ceramic material with zero porosity and light weight characteristics. A cone specimen and cylindrical specimens with varying inclined angles are used to introduce the shear stress to the Macor ceramic. The dynamic failure initiation and crack propagation are monitored by the high speed photography and Digital Image Correlation techniques. It is found that the equivalent stress of Macor at the initiation of failure decreases nonlinearly with the increase of shear stress. The high speed images show that the crack originates from the minimum cross-section of the cone specimen and the obtuse angle corner of the inclined cylindrical specimens. The cracks propagate parallel to the inclined plane instead of the axial loading direction. The fractographic analysis shows the compacted zone in the shear fracture surfaces of the cone specimen and the inclined cylindrical specimens. This indicates a significant role of shear loading in the dynamic failure process of Macor.     

Dynamic fracture of C/SiC composites under high strain-rate loading: microstructures and mechanisms

We investigate dynamic fracture of C/SiC composites under high strain-rate compression or tension with split Hopkinson pressure bar (SHPB) and gas gun loading. Components of the as-fabricated composites are mapped and quantified with X-ray computed tomography, including C fibers and fiber bundles, SiC matrix, and inter- and intrabundle voids. Compression loading is applied along the out-of- and in-plane directions by SHPB at strain rates of 10²–10³ s⁻¹ along with in situ X-ray phase contrast imaging. Out-of-plane direction compression and tension are examined with gas gun impact at strain rates 10⁴–10⁵ s⁻¹. For the out-of-plane loading, compression induces fracture via void collapse and shear damage banding, while delamination dominates fracture for the in-plane direction compression. With increasing strain rates, the compression failure modes transit from interbundle to intrabundle fracture of SiC, and then to fiber and bundle breaking. Tensile failure involves delamination, fiber pullout and fiber breaking. In contrary to normal solids, dynamic tensile or spall strength decreases with increasing impact velocities, owing to compression-induced predamage before subsequent tensile loading.     

Behavior and properties of shear bands

Some recent observations on shear bands in polymers are reviewed. These include intrinsic properties such as the mechanism of formation, the intersection of shear bands, the rate of propagation with and without obstacles, recovery of shear strain by annealing, and the stored energy in shear bands. Mechanical responses include the shear yielding criteria, reverse shear behavior and the Bauschinger effect, and the fracture of and at shear bands. Environmental effects include methanol transport in shear bands and in deformed poly(methvl methacrylate) in the region of mixed Fickian and case II behavior and methanol crazing of a shear banded material.     

Simple model for calculating shock adiabats of powder mixtures

A model is proposed for calculating the behavior of porous powder mixtures under shock-wave loading in the one-velocity and one-temperature approximations and also under the assumption of identical pressures of all phases. The model takes into account the presence of the gas in pores. The calculated results are compared with available experimental data for solid and porous media (shock adiabats, double compression by shock waves, and adiabatic unloading). The calculated results are in good agreement with the experimental data, including those for two- and three-species (in terms of condensed phases) mixtures.     

On critical criteria for shifting towards plastic strain localization during explosive cladding of Inconel 625 on low-carbon steel

The present study aims at detecting the critical criteria and corresponding critical impact energy for initiation of strain localization during explosive cladding of the Inconel 625 superalloy as a cladding material and low-carbon steel as a substrate. The results do not reveal adiabatic shear bands, which are the main signs of strain localization, within the superalloy in all studied impact energies up to 205 kJ. At impact energies greater than 78–114 kJ, strain localization is observed in low-carbon steel, and microcracks develop within the adiabatic shear bands. The Johnson-Cook model is used to explains the results obtained and to study the thermomechanical behavior of materials.     

Characteristics of exothermic reaction fronts in the gasless combustion system

Gasless combustion model of the self-propagating high-temperature synthesis process was numerically studied in the non-adiabatic cylindrical sample. The model equations, which are very stiff in the dimension of length as well as time, were solved using finite difference method on adaptive meshes. Travelling waves with constant pattern were observed for adiabatic systems. For higher values of heat of reaction and activation energy, combustion fronts started to oscillate for adiabatic and non-adiabatic systems. Simple and complex oscillatory fronts were observed. Multi-peak and irregular oscillations were also detected to presumably result in the gasless chaotic combustion. In oscillatory fronts the temperature can overshoot the adiabatic reaction temperature to result in the complete conversion of solid reactant. In the two dimensional non-adiabatic cylindrical sample in the domain of longitudinal and angular directions, oscillatory piston waves were observed. In addition asymmetrical fingering as well as rotating waves were detected for an asymmetrical perturbation. For the adiabatic annulus cylindrical sample, the velocity of propagating fronts increased with time and the temperature overshooted the adiabatic reaction temperature if the sample were ignited from the inside. If the sample were ignited from the outside, the velocity of propagating fronts decreased with time and the temperature again overshooted the adiabatic reaction temperature. For smaller diameter of sample, the temperature increased very slowly with time for inside ignition. The temperature after ignition increased very fast overshooting the adiabatic reaction temperature for outside ignition. After several oscillations, the reaction rate decreased and the region with very slow reaction was established.     

Sliding Wear of Oxide Ceramics at Elevated Temperatures

Sliding wear tests of sintered alumina and mullite consistently showed that the wear loss significantly decreased at 800°C and above by an order of magnitude. Microscopy of the room-temperature wear surfaces revealed a feature suggesting material removal by brittle fracture. Microscopy of the wear surface at 1000°C revealed that the immediate vicinity of the wear surface consisted of a very fine grain size polycrystalline structure. The zone below this consisted of heavily deformed grains containing dense dislocation networks forming a cellular structure. The results suggest that, at high temperatures, dynamic recrystallization at the wear surface forms the fine grain size structure which suppresses further material removal.     

Micromechanics of Failure Waves in Glass: II, Modeling

In an attempt to elucidate the failure mechanism responsible for the so-called failure waves in glass, numerical simulations of plate and rod impact experiments, with a multiple-plane model, have been performed. These simulations show that the failure wave phenomenon can be modeled by the nucleation and growth of penny-shaped shear defects from the specimen surface to its interior. Lateral stress increase, reduction of spall strength, and progressive attenuation of axial stress behind the failure front are properly predicted by the multiple-plane model. Numerical simulations of high-strain-rate pressure-shear experiments indicate that the model predicts reasonably well the shear resistance of the material at strain rates as high as 1 × 10₆/s. The agreement is believed to be the result of the model capability in simulating damage-induced anisotropy. By examining the kinetics of the failure process in plate experiments, we show that the progressive glass spallation in the vicinity of the failure front and the rate of increase in lateral stress are more consistent with a representation of inelasticity based on shear-activated flow surfaces, inhomogeneous flow, and microcracking, rather than pure microcracking. In the former mechanism, microcracks are likely formed at a later time at the intersection of flow surfaces. in the case of rod-on-rod impact, stress and radial velocity histories predicted by the microcracking model are in agreement with the experimental measurements. Stress attenuation, pulse duration, and release structure are properly simulated. It is shown that failure wave speeds in excess to 3600 m/s are required for adequate prediction in rod radial expansion.     

A mixing model for multicomponent reactions in twin screw extruders applied to the polymerization of urethanes

To understand the performance of multicomponent reactions in twin screw extruders the mixing mechanism in the extruder had to be understood. Therefore, two new relevant mixing parameters are defined; the mixing efficiency, which is the average number of passages of material through a high shear region; and the mixing deficiency, which is the fraction of material that does not pass through a high shear region. With these parameters an analysis can be made of the mixing circumstances in the extruder. The new model was applied to the polymerization of urethanes in a counter-rotating twin screw extruder. The results agreed very well with the theoretical expectations.     

Electrodiffusion detection of the near-wall flow reversal in liquid films at the regime of solitary waves

The liquid film flow down an oscillating plate was used as a suitable flow configuration to study the dynamic behaviour of electrodiffusion friction probes at large fluctuations. The two-segment probe was flush mounted into the plate wall to measure the fluctuating wall shear rate. The hydrodynamics of the experiment made it possible to adjust both the steady and oscillatory component of the wall shear rate through the operation parameters (flow rate, plate inclination, amplitude and frequency of wall oscillations). The approximate model of the probe dynamic response based on the similarity of concentration profiles at the probe surface was verified. This simple model proved to be able to calculate the instantaneous wall shear rate from the measured current signal even at large flow fluctuations. The analysis of the probe dynamic behaviour under reversing flow conditions provided a new method of the detection of short-time flow reversal. Finally, this method was successfully applied to confirm the existence of a small backflow region located in front of the large solitary waves, which were excited on the surface of a liquid film flowing down an inclined stationary plate.