Fast Crack Propagation Research Articles

On the synthesis, characterization and hydrogenation behaviour of Fe 1 − xTi 1 + yNi x ( x = 0.2, y = 0.3) hydrogen storage material

The hydrogen absorption characteristics of TiFe partially substituted with small amount of Ni for Fe and corresponding to Fe 0.8Ti 1.3Ni 0.2 has been synthesised through RF melting. The XRD and TEM characterization of Fe 0.8Ti 1.3Ni 0.2 revealed the occurrence of curious structural phases; namely, a superlattice with “a” periodicity of 5.93 Å (which is double that of the 2.98 Å FeTi 1.3 phase) and a modulated phase having a modulation period of 4 times along [110] as compared to d 110 of the superlattice phase. The Fe 0.8Ti 1.3Ni 0.2 phase has been found to have faster hydrogen desorption kinetics (30–40 cc/min) which is about 2–3 times higher than that of the parent phase FeTi 1.3. It has been suggested that the faster desorption kinetics are due to the Ni substitution leading to the material Fe 0.8Ti 1.3Ni 0.2. This material is biphasic in nature and embodies built-in interfaces and has faster crack propagation on hydrogenation.

Analysis of pipeline fracture by fast crack propagation

An analysis is presented of the influence of wave processes in gases and in a pipeline wall on extended fracture by fast crack propagation. The influence of design and technological parameters of the pipeline and of the pipe metal fracture toughness on the extent of fracture is shown.

Numerical simulation of rapid crack propagation in viscoplastic materials

A finite element model for the simulation of fast crack propagation under dynamic Mode I load conditions is developed. The principal tools are an enhanced strain formulation and a quasi-contact algorithm. Crack propagation is controlled by the implemented fracture criterion. The model accounts for viscoplastic material behavior, dynamic effects and contact in the crack zone. The numerical results demonstrate that these physical effects are of major importance when load speed is increased to a high level and therefore they cannot be neglected.

Tensile properties of as fabricated, aged, and stretched SiC whisker and particle reinforced 2009 aluminium alloy

Tensile tests were carried out using specimens of 2009 aluminium alloy reinforced by either SiC whiskers or particles. The size distributions of the whiskers and particles in the matrix were obtained by image analysis. It was found that failure was a result of uniform void nucleation and coalescence in the as fabricated composites, or a result of fast crack propagation initiated by a flaw developed at clusters of SiC in the aged or stretched and aged composites. The strengths of the as fabricated composites were estimated based on the results of image analysis using continuum mechanics and dislocation theories. The estimation indicated that the tensile strengths are largely contributed to by composite strengthening, supplemented by residual dislocation strengthening and work hardening. Owing to the flaw controlled failure, the tensile strengths of the aged or stretched and aged composites were independent of aging time, aging temperature, and the amount of stretching. The elastic moduli of the composites were estimated using the Halpin–Tsai model and a good correlation was found between the measured and estimated moduli.MST/3438

Integral estimates to the fast brittle fracture mechanics

In the terms of a general approach the problem of the fast crack propagation in elastic bodies is considered. The time dependence of, so called domain of possible fractures due to the fast crack propagation, is obtained. The dynamic domain of possible fractures is the upper estimate of the zone of real fractures and thereby it takes into account such important features of the fast brittle fracture as the instability of the fast crack propagation, branching, waving, the influence of boundary conditions and so on. The problem of dynamic crack initiation is considered in some details. Within the linear fracture mechanics the dynamic criterion of crack initiation is offered.

Comparison of the fast fracture properties of acrylonitrile-butadiene-styrene and styrene-acrylonitrile

The resistance of polymers, with toughening agents, to fast crack propagation under steady load conditions was studied, and the fracture resistance of acrylonitrile-butadiene-styrene (ABS) was compared with styrene-acrylonitrile (SAN). Small laboratory specimens were used which could be precisely loaded and allowed to stress relax before being fractured. It was important for these experiments to achieve good isolation between crack initiation and crack propagation.

Mechanics of failure of plastics by thermal imaging examination of a thermoplastic polypropylene + EPDM blend

AbstractThermal wave imaging combined with stress pattern analysis by measurement of thermal emission (SPATE) is used to study the failure behavior of a polypropylene (PP) + ethylene propylene diene (EPDM) polymer blend. Images corresponding to a propagating crack in a single edge‐notched specimen were measured at three rates of testing: 4 mm/min, 8 mm/min, and 20 mm/min. Conversion to stress values is made through use of a thermoelastic function. It is found that in a 4 mm/min test the crack tip radii blunt rather than propagate with little initiation. Large‐scale necking precedes fracture. At 8 mm/min, some blunting occurs, followed by fast crack propagation. At 20 mm/min, fast crack propagation occurs. The images are digitized to obtain the values of the temperatures at every point in the sample. Data corresponding to the plane of the propagating crack over the span of the test are presented.

Development of a simple dynamic model for fast fracture research

The aim of this study was to devise a simple fracture model, which could be hosted on a personal computer (PC), to assist in researching the dynamic behaviour of materials during fast crack propagation. A model was required which could be easily reconfigured to represent different materials, provide good visibility of the dynamic fracture processes being simulated and generate information to complement the results of experimental research. A version of the devised PC model and its use with a recently developed experimental technique to achieve rapid crack propagation in a small specimen, is presented.

Boundary element study on the optical method of caustic for measuring fast crack propagation toughness K ID

The dynamic stress field near the tip of a crack tip which is accelerated and decelerated in an elastic plate with finite width under impact loading is analyzed by the boundary element method, and a simulation of measuring fast crack propagation toughness K ID by the caustic method is performed. The results of the simulation agree qualitatively with the experimental results by Arakawa and Takahashi, and indicate the dependence of the ‘measured’ K ID not only on crack acceleration but also plate width. The dependence of ‘measured’ K ID on crack acceleration may result from the fact that under the condition of high loading rate or abrupt change in crack velocity, the transient stress field near the initial curve of caustic can not be represented fully by the dynamic stress intensity factor K I(t, v), as suggested by Rosakis et al. The dependence of ‘measured’ K ID on plate width may be attributable to the fact that the transient stress field near the initial curve is affected directly by the reflected stess wave and also indirectly through crack acceleration which depends on the reflected stress wave. The possible dependence of the ‘measured’ K ID on loading rate, loading history, crack propagation history is also discussed.

Nylon-6/rubber blends

The deformation zone of fractured nylon-6/ethylene-propylene rubber specimens was studied using scanning electron microscopy (SEM). In this deformation zone three distinct layers were observed. In the main part of the stress-whitened zone only cavitation was visible. From 150-5 μm below the fracture surface, massive plastic deformation was observed. Directly under the fracture surface there was a layer about 3-5 μm thick where no cavitation or deformation was visible. This zone without cavitation was only visible when the specimen was fractured at high deformation rates. It is proposed that this top layer consists of material which was molten during fast crack propagation under adiabatic conditions. This hypothesis has been confirmed by model calculations and experiments

Transition from small to long fatigue crack in CMn steel

Fracture mechanics is an effective mechanism for characterizing the fatigue crack propagation. Generally, fatigue crack growth rate (da/dN) is presented as a function of stress intensity range ([Delta]K). A conventional fatigue crack growth curve of da/dN versus [Delta]K can be divided into three regions, i.e. region 1 (near threshold), 2 (Paris) and 3 (fast) crack growth regions. Depending upon the region of crack growth, fatigue crack growth behavior can be sensitive to microstructure, environment and loading conditions. In the near threshold region, fatigue crack growth rates are very slow and the influencing of microstructure, environment and load ratio on the rates of crack propagation is very significant. In the Paris region, there is a linear relationship between log(da/dN) and log([Delta]K). In this region the crack growth properties are not so sensitive to microstructure and load ratio. In the fast crack propagation regime, the rate of crack growth is relatively sensitive to microstructure and load ratio. In the present study small fatigue crack behavior in the ferrite-bainite microstructure of C-Mn steel is investigated. The fatigue crack growth threshold is calculated with respect to barrier to crack growth and transition from microstructure sensitive (small crack) to microstructure insensitive (long crack) crack growthmore » is estimated.« less

B.E. Analysis on Caustic Method for Measuring Fast Crack Propagation Toughness.

The acceleration and deceleration behaviors of a semi-infinite crack in an elastic plate with a finite width, which is subjected to time-dependent loading, are analyzed and the caustic method for measuring the fast crack propagation toughness KID is simulated by using the boundary element method. It is shown that the "measured" KID depends on the shape of the specimen when the dynamic stress intensity factor KI ( t, v) and/or the crack speed v changes rapidly. The dependence is explained by the numerical results that the KI(t, v) -controlled field does not develop enough to cover the location of the initial curve of caustics, i. e., the displacement field near the initial curve is influenced not only by KI (t, v) but also by (1) the superposition of stress wave reflected at the boundary and (2) the change of the crack acceleration due to the reflected stress wave

The closure of indentation cracks and strength recovery by low temperature ageing in Y-TZP

This paper reports that the microstructure-mechanical property relationships in yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) have been well established in the advanced ceramic community. These highly toughened ceramic materials suffer, unfortunately, a thermal instability when they are subjected to a hydrothermal environment at temperatures in the range of 100 to 400[degrees]C, due to the tetragonal to monoclinic transformation occurring on the aged surface. It has been observed that a Y-TZP will be completely disintegrated on ageing at the low temperatures for a long enough period. On the other hand, surface toughening is obtainable when the Y-TZP is thermally treated at the low temperatures in a controlled manner, as a result of the development of compressive stresses on the aged surface. Structural ceramic materials are very sensitive to microstructural defects, which include both the processing-related flaws and the structural damage caused during the period of application. Catastrophic failure of ceramic materials is a consequence of fast crack propagation under loading conditions. It is therefore essential to control both the number and sizes of microstructural defects present in an advanced ceramic material in order to improve its engineering reliability. For many engineering applications, such as those in chemical engineering industries, Y-TZP ceramics aremore » often subjected to a combination of mechanical and hydrothermal environments. Apparently, the presence of any microstructural defects such as cracks and pores may have a large impact on the response of Y-TZP to these mechanical and hydrothermal conditions. The aim of the present work is to investigate the effects of a hydrothermal environment on the post-indentation cracks in a 3 mol% yttria stabilized tetragonal zirconia polycrystals.« less

Boundary Element Analysis of Caustic Method. Estmation of Fast Crack Propagation Toughness KID.

An elastodynamic boundary element analysis of a fast crack propagation test is performed. A propagating crack subject to a time-dependent uniform pressure on its surfaces is considered. The crack speed is determined in such a way that the dynamic stress intensity factor K1(t, v) is equal to the fast crack propagation toughness KID. Caustics in reflection are generated by making the assumption of plane stress. The time variation of the measured KID is obtained from the diameter of the generated caustics. The KID thus obtained is found to depend on crack propagation history due to the transient nature of the elastic stress field near a propagating crack tip. This may explain to some extent the physical meaning of the experimental data showing the dependence of KID on crack acceleration.

Crack Propagation Behavior in Gas Pressure Sintered Silicon Nitride

The crack propagation behavior in gas pressure sintered Si3N4 was investigated under cyclic and static loading by 3-point bending for pre-cracked specimens by indentation on the center of the mirror finished tensile surfaces. Crack propagation occurred under cyclic loading with a maximum applied stress below the applied stress at which crack propagation stagnated under static loading, showing that crack propagation velocity under cyclic loading was higher than that under static loading. Although intergranular crack propagation with crack-bridging was observed in both cases under cyclic and static loading, crack-bridging, observed after static loading, was found to be fractured by the subsequent cyclic loading. It was thought that the crack shielding force by crack-bridging, which is effective for restraining crack propagation under static loading, was decreased by cyclic loading, resulting in faster crack propagation under cyclic loading than under static loading.

Mechanics of dynamic debonding

Singular fields around a crack running dynamically along the interface between two anisotropic substrates are examined. Emphasis is placed on extending an established frame work for interface fracture mechanics to include rapidly applied loads, fast crack propagation and strain rate dependent material response. For a crack running at non-uniform speed, the crack tip behaviour is governed by an instantaneous steady-state, two-dimensional singularity. This simplifies the problem, rendering the Stroh techniques applicable. In general, the singularity oscillates, similar to quasi-static cracks. The oscillation index is infinite when the crack runs at the Rayleigh wave speed of the more compliant material, suggesting a large contact zone may exist behind the crack tip at high speeds. In contrast to a crack in homogeneous materials, an interface crack has a finite energy factor at the lower Rayleigh wave speed. Singular fields are presented for isotropic bimaterials, so are the key quantities for orthotropic bimaterials. Implications on crack branching and substrate cracking are discussed. Dynamic stress intensity factors for anisotropic bimaterials are solved for several basic steady state configurations, including the Yoffe, Gol’dshtein and Dugdale problems. Under time-independent loading, the dynamic stress intensity factor can be factorized into its equilibrium counterpart and the universal functions of crack speed.

Relationships between fracture parameters and fracture surface roughness of brittle polymers

Fracture parameters such as crack velocity a, stress intensity factor Kd and a specific crack extension resistance R* were measured for Homalite-100, PMMA and epoxy in the course of fast crack propagation using a Cranz-Schardin type high speed camera. Fracture surface roughness λ was evaluated as a function of crack length a so that it could be correlated with the fracture parameters above. The results showed that none of those parameters could be uniquely related to λ. Instead, there was a good correlation between λ and a product R*a.

Moving Finite-Element Simulation of Fast Curving Fracture Using Automatic Element-Control Method.

To overcome various difficulties in finite element modelling of fast curving crack propagation, an automatic element-control method is developed for moving finite-element simulation. The Present automatic element-control method is based on Lagrangean-element mapping together with a concept of fictitious element-controlling plane. A component separation method of the path independent dynamic J integral is also presented, for the evaluation of mixed-mode dynamic stress intensity factors. Generation phase simulation for a fast curving fracture test is carried out using the present techniques. In the finite element simulation, the formation process of caustic pattern is also simulated for dynamically curving cracks. Shapes of the simulated caustic patterns agree well with the caustic patterns in high-speed photographs, and with those based on a higher-order theory of caustics.

Anti-plane steady state crack propagation in elastic-viscoplastic material

Anti-plane steady-state fast-crack propagation in an elastic-viscoplastic material is considered where the inertia effect is included through the normalized crack velocity parameter in addition to the material parameter. The stress field near the tip of a semi-infinite crack is intensified and can be characterized by the sum of an elastic stress intensity factor K a and plastic stress intensity factor K p both of which are coefficients of the 1 √r singular term with r being the distance measured from the crack tip. A numerical scheme is developed to solve the integral equation that relates the stress to the plastic strain rate. The changes in the size of the plastic zone surrounding the moving crack tip, which decreases with increasing crack velocity, are estimated. Results are also discussed with reference to the ways with which the material and kinematic parameters affect the intensification of the local stresses that are dictated by the external disturbance.

Growth of electrical trees in solid insulation

It is shown that electrical trees in solid dielectrics grow as a result of the explosive effects of localised breakdown at points of electrical stress concentration. Tree growth is sensitive to applied mechanical strain and to internal strain built up owing to prolonged exposure to vibrational electrostatic forces. It is concluded that the ability to withstand fast crack propagation should be added to the list of factors involved in the choice of insulating materials. Also, when designing insulating systems, areas of mechanical stress concentration must be avoided.