Crack resistance behavior of particulate reinforced composites at various test speeds and temperatures

In this study, the fracture behavior and characteristics of particulate-reinforced composite materials were evaluated by performing wedge splitting tests. The crack resistance of the materials was evaluated using the crack tip opening displacement and crack tip opening angle. The composites were tested under various temperatures and test speeds. The digital image correlation method was used to analyze the strain field at the crack tip. The fracture surface under test conditions was observed using a scanning electron microscope. The test results showed that the fracture energy increased with decreasing temperature, and the crack resistance increased with increasing test speed. The crack tip opening angle is divided into an unstable region and stable region. The critical crack tip opening angle can be defined as the fracture mechanics parameter measured in a stable region. The surface strain fields obtained by digital image correlation method are distributed in the range from 1.5 % to 4.5 % at the initiation of the crack. A crack grows with dewetting phenomenon at the temperature range from 60 °C to −40 °C, and the crack propagates with fracture of ammonium perchlorate oxidizer particles at the glass transition temperature of −70 °C.     

Prediction of ductile-brittle transition for ductile crack growth in reactor pressure-vessel steels

We have investigated the local stress-strain state at the crack tip using the finite-element method in a geometrically nonlinear formulation (with account of the variations in the crack tip blunting) for both a stationary crack and a crack growing by a ductile mechanism. Combined with the criterion of brittle fracture, the derived relationships governing the generation of the stress-strain state at the tips of stationary and growing cracks allowed us to explain the ductile-brittle transition for reactor pressure-vessel steels. We propose a technique for predicting the value of the ductile crack extension up to the instant of the ductile-brittle transition depending on the test temperature, and a procedure for calculating fracture toughness taking into account ductile crack extension. The calculations for predicting the ductile-brittle transition are made as applied to reactor pressure-vessel steel 15Kh2MFA. Analysis is made of the results obtained and available experimental data. Various approaches to the interpretation of the ductile-brittle transition are discussed. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 6, pp. 5–22, November–December, 1999.     

The brittle-ductile transition in silicon: The influence of pre-existing dislocation arrangements

It has been shown that the value of the brittle-ductile transition temperature in specimens of silicon containing pre-cracks at the surface is strongly dependent on the pre-existing dislocation arrangement close to the crack tip. In particular, removing dislocations from the vicinity of the crack tip has the effect of raising the transition temperature, while introducing more surface dislocations by grinding reduces the transition temperature. In both cases the transition temperature remains sharp, implying that the crack tip sources have to be nucleated in the test before effective shielding occurs. The differences in the transition temperatures reflect the differences in the distances of the dislocations from the crack tips, and in the source lengths of the pre-existing sources which send dislocations to the crack tip.Specimens which are pre-stressed at the brittle-ductile transition to a K value at which crack tip sources are expected to be nucleated exhibit a gradual “soft” transition over a wide range of temperature. This result is in agreement with the predictions of the computer modelling of dislocation emission from crack tips, developed by Hirsch et.al [3,7], and together with the results on the dependence of T_c on dislocation arrangements, provides strong support for their theory of the brittle-ductile transition.     

Influence of material properties on dynamic fracture toughness of steels

Recent studies of plastic enclave formation at running brittle cracks were extended to account for the influence of crack tip boundary conditions on the temperature at which the enclaves start to develop. The En 2A and three other steels were used in the analysis. It was found that this temperature depends very strongly both on the magnitude and on the distribution of the stresses in the discrete crack tip zone. This suggests that the onset of enclave formation and the rate of their growth are governed by the balance of two sets of material characteristics. The first set consists of at least two parameters describing the microscopic fracture resistance which promotes enclave formation. The second set includes the macroscopic yield and flow properties which may make enclave formation more difficult in higher strength steels.These findings are related to the dynamic or crack arrest fracture toughness which is found to be derived from two different sources. One is connected with the microscopic plastic deformation of the fracturing metal in the crack tip zone and is present at all temperatures. The other is the result of enclave formation, it is present only at higher temperatures and is responsible for the energy transition. In contrast to the case of crack initiation, the dynamic fracture toughness depends not only on the microscopic fracture strength or strain but on the complete stress-displacement relationship of the weakened material which is governed by the microscopic fracture mechanism at the tip of a running crack. It is noted that the present results can be expected to be valid for all steels which fracture in the cleavage or quasi-cleavage modes.     

Atomistic process on hydrogen embrittlement of a single crystal of nickel by the embedded atom method

A molecular dynamics (MD) simulation by the embedded atom method (EAM) was conducted on hydrogen embrittlement (HE) at a crack tip of a single crystal of nickel composed of {1 0 0} planes under uniaxial tension along the [1 0 0] direction perpendicular to the crack plane at room temperature. A three-dimensional (3D) crack-tip model with 16,632 nickel atoms and from 2 to 653 hydrogen atoms segregated in the notched area was designed and the EAM potentials proposed by Baskes and co-workers were used for the simulation. The hydrogen-free specimen showed good ductility associated with a significant blunting of the crack tip. In the hydrogen-charged specimen, the elongation at the fracture decreased with increasing hydrogen content due to the suppressed and localized plastic deformation. Fracture occurred macroscopically on the (1 0 0) plane perpendicular to the tensile direction. Twin deformation associated with the fracture was formed in the specimen with 653 hydrogen atoms, corresponding to a thin layer of hydride at the notched area, while no twin deformation was formed in the other hydrogen-charged specimens or the hydrogen-free specimen. The simulation results were in good agreement with published experimental results.     

Approximate stress analysis near crack tip by non-local elasticity

In this paper the non-local elasticity and stress formulae of the classical fracture mechanics are used to analyze the state of stresses in the core region around the tip of a sharp crack, and one-dimensional stress formulae of mode I, II and III problems have been obtained. The results differ from the classical fracture mechanics in that the stresses around the crack tip are finite by using the non-local elasticity, but they are infinite in the classical fracture mechanics. The present calculated results coincide with Eringen's [1] but our model and calculation process is much simpler. At the same time, the maximum tensile stress of the crack tip and fracture toughness are in close agreement with the original experimental results.     

Anisotropy of the plastic deformation and fracture processes in a dynamically loaded crystallite

The influence of anisotropic dynamic loading on the character of plastic deformation and fracture processes in a crystallite near the free surface was studied by methods of molecular dynamics. It was established that there are threshold deformation levels determining the opening of a seeding crack (notch) and the subsequent crack stop. The simulation results show that loading in the [100] direction leads to the crack stop due to the formation of a disordered material region at the crack tip.     

A model of brittle fracture propagation part I—continuum aspects

The micromechanism of crack propagation in steel is described and analyzed in continuum terms and related to the macroscopic fracture behavior. It is proposed that propagation of cleavage microcracks through favorably oriented grains ahead of the main crack tip is the principal weakening mode in brittle fracture. This easy cleavage process proceeds in the Griffith manner and follows a continuous, multiply connected, nearly planar path with a very irregular front which spreads both forward and laterally and leaves behind disconnected links which span the prospective fracture surface. A discrete crack zone which extends over many grains thus exists at the tip of a running brittle crack. Final separation of the links is preceeded by plastic straining within the crack zone and occurs gradually with the increasing crack opening displacement. It is suggested that in low stress fracture, straining of the links is the only deformation mode. However, it is recognized that under certain conditions plastic enclaves may adjoin the crack zone. This deformation mode is associated with high stress fracture, energy transition and eventually with crack arrest.

Energy dissipation resulting from the two deformation mechanisms is related to crack velocity, applied load and temperature and the crack velocity in a given material is expressed as a function of the external conditions. Fracture initiation and crack arrest are then discussed in terms of the conditions which are necessary to maintain the propagation process. Finally, the dimensions of a small scale crack tip zone for a steady state, plane strain crack are evaluated as functions of material properties and the elastic stress intensity factor.

The microstructural aspects of brittle fracture will be discussed in a separate Part 2 [1].     

Crack growth process in Ni-Single crystal with hydrogen cathodic charging

Notched tensile tests {orientation tensile axis [001] and ${[\bar{1}\bar{1}1]}$ , direction of notching: [010], [011], [112]} were performed to investigate the crack growth process in Ni-single crystal with hydrogen cathodic charging. It was investigated that the crack growth direction tends to be <011> on {001} with any tensile direction. Y-shaped hillocks and striation-like pattern were observed on the fracture surfaces of hydrogen embrittled specimens using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The striation-like patterns were not matched on both fracture surfaces of specimens, though the Y-shaped hillocks were exactly matched. Moreover, it was indicated that the striation-like pattern is perpendicularly formed to the crack growth direction on the fracture surface behind the crack tip and its shape results in an obtuse angle with respect to the fracture surface. The Y-shaped hillock was formed between micro-cracks located at crack tip by shear fracture. Furthermore, we proposed a crack growth model in Ni-single crystal at hydrogen atmosphere from the observation of the fracture surfaces.     

The effect of temperature on the fracture of polycarbonate

The fracture toughness of polycarbonate was obtained over the temperature range 20 to ? 120° C. There is a strong thickness dependence which is described in terms of plane stress and plane strain values which are insensitive to temperatures above ?40° C but the plane stress value increases below this temperature. This change is associated with theβ transition and stable crack growth was observed in this region with accompanying instabilities arising from adiabatic heating at the crack tip.     

Dynamic fracture of a beam or plate under tensile loading

The dynamic fracture response of a long beam of brittle elastic material under tensile loading is studied. If the magnitude of the applied loading is increased to a critical value, a crack is assumed to propagate across the beam cross section. In a parallel analysis to [t] the crack length and applied loading at the fracture face are determined as functions of time measured from fracture initiation. The results of the analysis are shown in graphs of crack length, crack tip speed and fracturing section tensile loading vs time. As found in [1], the crack tip accelerates very quickly to a speed near the characteristic terminal speed for the material, travels at this speed through most of the beam thickness, and then decelerates rapidly in the final stage of the process. Finally, by appropriate change of the elastic modulus, the results may be applied to plane strain fracture of a plate under pure tensile loading.     

An investigation into the crack propagation behaviour of tooth-coloured particulate-filled dental restorative materials

Particulate-filled resins, or dental composites, are being increasingly used to restore the load-bearing surfaces of teeth. If these restorations are not to fracture in service, and if improvements are to be made, an understanding of the fracture behaviour of these materials is essential. The fracture parameter used is the stress intensity factor at crack instability (K _IC). This has been calculated using the double torsion test in an Instron universal testing machine. The fracture behaviour was studied by varying the amount of filler (7, 15, 26 and 41 vol %); the surface treatment of the filler (coated or uncoated); the environment (air or water); and the crosshead rate (0.05, 0.5, 5 and 50 mm min^–1). Fracture was found to occur in either a continuous or stick-slip manner. The stick-slip behaviour was due to blunting of the crack tip, which was controlled by the yield behaviour. If there was no significant blunting, continuous crack growth occurred. A unique fracture criterion was shown to apply, which was that a critical stress of approximately three and a half times the yield stress must be obtained at a critical distance ahead of the crack tip.[/p]     

Measurement of the temperature field induced by dynamic crack growth in Beta-C titanium

The temperature increase at the tip of a dynamically propagating crack may have a significant effect on a material's mechanical properties, and hence on its dynamic fracture toughness. In order to start to understand this phenomena, measurements of the temperature field at the tip of a dynamically growing crack in Beta-C titanium alloy were performed using a linear array of high speed infrared detectors. The results show that a maximum temperature of 450°C is reached at the crack tip. In addition, the dynamic fracture toughness was measured for crack speeds from 0 to 500 m/s. In this speed range, the toughness appears to be constant. Estimates of the crack tip energy release rate and plastic strain rate are made using analysis of the experimental data.     

Change of fracture mode in Charpy toughness transition temperature range

Abstract

A transition layer of width 5 - 10 μm was found on the boundary between ductile and brittle fracture for Charpy V notch specimens in the transition temperature range of a structural steel having a microstructure of polygonal ferrite -pearlite. The fracture mode in the transition layer was shearing with occasional submicrometre dimples. From tensile tests on notched specimens, the cleavage fracture stress and flow stress by ductile decohesion were determined. Based on the experimental data and the assumption that the volume of metal involved in the plastic deformation during fracture was related to the volume of the dimples, it was deduced that the transition layer width represents the size of the plastic zone immediately before cleavage initiation. The crack opening displacement and the crack tip radius for the change of fracture mode were calculated.     

Ratchetting strain as a damage parameter in controlling crack growth at elevated temperature

Progressive increase in tensile strains near a crack tip has been observed from finite element studies of stationary and growing cracks (Zhao, 2004, 2008) [1] and [2] under cyclic loading conditions. In this work, the significance of such a phenomenon was further explored. In particular, stress-controlled experiments were carried out to evaluate the uniaxial ratchetting response of a nickel-based superalloy, and the material parameters were re-calibrated using both strain-controlled and stress-controlled experimental data. An additional kinematic hardening term was introduced in the viscoplastic constitutive model and the models were utilised via a user-defined subroutine to study near crack tip ratchetting behaviour of a single edge notch tension (SENT) model geometry at elevated temperature. Loading modes near the crack tip were examined, together with the influence of particular constitutive models on the mechanistic response of the crack tip. The crack tip deformation was found to be predominantly strain-controlled, where the mean ratchetting strain seems to be more relevant to crack growth than the strain range. The former was used as a measure of crack tip damage to correlate crack growth rates at selected loading conditions.     

Microscale characterization of granular deformation near a crack tip

This paper presents a study of microscale plastic deformation at the crack tip and the effect of microstructure feature on the local deformation of aluminum specimen during fracture test. Three-point bending test of aluminum specimen was conducted inside a scanning electron microscopy (SEM) imaging system. The crack tip deformation was measured in situ utilizing SEM imaging capabilities and the digital image correlation (DIC) full-field deformation measurement technique. The microstructure feature at the crack tip was examined to understand its effect on the local deformation fields. Microscale pattern that was suitable for the DIC technique was generated on the specimen surface using sputter coating through a copper mesh before the fracture test. A series of SEM images of the specimen surface were acquired using in situ backscattered electronic imaging (BEI) mode during the test. The DIC technique was then applied to these SEM images to calculate the full-field deformation around the crack tip. The grain orientation map at the same location was obtained from electron backscattered diffraction (EBSD), which was superimposed on a DIC strain map to study the relationship between the microstructure feature and the evolution of plastic deformation at the crack tip. This approach enables to track the initiation and evolution of plastic deformation in grains adjacent to the crack tip. Furthermore, bifurcation of the crack due to intragranular and intergranular crack growth was observed. There was also localization of strain along a grain boundary ahead of and parallel to the crack after the maximum load was reached, which was a characteristic of Dugdale–Barenblatt strip-yield zone. Thus, it appears that there is a mixture of effects in the fracture process zone at the crack tip where the weaker aspects of the grain boundary controls the growth of the crack and the more ductile aspects of the grains themselves dissipate the energy and the corresponding strain level available for these processes through plastic work.     

Modeling the brittle–ductile transition in ferritic steels: dislocation simulations

We present a model for the brittle–ductile transition in ferritic steels based on two dimensional discrete dislocation simulations of crack-tip plasticity. The sum of elastic fields of the crack and the emitted dislocations defines an elasto–plastic crack field. Effects of crack-tip blunting of the macrocrack are included in the simulations. The plastic zone characteristics are found to be in agreement with continuum models, with the added advantage that the hardening behavior comes out naturally in our model. The present model is composed of a macrocrack with microcracks ahead of it in its crack-plane. These microcracks represent potential fracture sites at internal inhomogeneities, such as brittle precipitates. Dislocations that are emitted from the crack-tip account for plasticity. When the tensile stress along the crack plane attains a critical value σ _F over a distance fracture is assumed to take place. The brittle–ductile transition curve is obtained by determining the fracture toughness at various temperatures. Factors that contribute to the sharp upturn in fracture toughness with increasing temperature are found to be: the increase in dislocations mobility, and the decrease in tensile stress ahead of the macrocrack tip due to increase in blunting, and the slight increase in fracture stress of microcracks due to increase in plasticity at the microcrack. The model not only predicts the sharp increase in fracture toughness near the brittle–ductile transition temperature but also predicts the limiting temperature above which valid fracture toughness values cannot be estimated; which should correspond to the ductile regime. The obtained results are in reasonable agreement when compared with the existing experimental data.     

On the dependence of the dynamic crack tip temperature fields in metals upon crack tip velocity and material parameters

Although various approximations have been used to analytically predict the temperature rise at a dynamic crack tip and its relation to the crack tip velocity or the material properties, few experimental investigations of these effects exist. Here, the method of using a high speed infrared detector array to measure the temperature distribution at the tip of a dynamically propagating crack tip is outlined, and the results from a number of experiments on different metal alloys are reviewed. First the effect of crack tip velocity in 4340 steel is investigated, and it is seen that the maximum temperature increases with increasing velocity, the maximum plastic work rate density increases with velocity and the active plastic zone size decreases with increasing velocity. Also, it is observed that a significant change in the geometry of the temperature distribution occurs at higher velocities in steel due to the opening of the crack faces behind the crack tip. Next, the effect of thermal properties is examined, and it is seen that, due to adiabatic conditions at the crack tip, changes in thermal conductivity do not significantly affect the temperature field. Changes in density and heat capacity (as well as material dynamic fracture toughness) are more likely to produce significant differences in temperature than changes in thermal conductivity. Finally, the effect of heat upon the crack tip deformation is reviewed, and it is seen that the generation of heat at the crack tip in steel leads to the localization of deformation in the shear lip. The shear lip is actualy an adiabatic shear band formed at 45° to the surface of the specimen. In titanium, no conclusive evidence of shear localization in the shear lip is seen.     

Crack propagation in a Tantalum nano-slab

Using molecular dynamics with an accurate many-body potential for metallic Tantalum, we studied crack propagation in a pre-notched nano-slab under uniaxial strain in a [100] direction. We study dislocation emission from the crack tip for various strain rate and temperatures, focusing on the influence of the local temperature at the crack tip on the propagation of the crack. We find a close connection between the local temperature at the crack tip and dislocation emission. This revised version was published online in June 2006 with corrections to the Cover Date.