Variation Of Fracture Toughness Research Articles

Low Temperature Effects on the Fracture Behavior of Cold-Worked STS 304 Stainless Steel for Membrane of LNG Storage Tank

The temperature dependence of the tensile properties and the fracture toughness of the cold-worked STS 304 stainless steel have been examined in the temperature range of 293 K to 111 K. The tensile strength significantly increases with a decrease in temperature, but the 0.2% yield strength is relatively insensitive to temperature. The total elongation at 193 K abruptly decreases by 50% of that at 293 K, and it decreases slightly at 193 K to 111 K. The strain hardening exponents at low temperatures are about four times as high as that at 293K. Initiation fracture toughness (Jc) and tearing modulus (Tmat) tend to decrease with a decrease in temperature. The Jc values exhibit an inverse dependency on the effective yield strength (σflow) at all the test temperatures. Fractographic examination revealed that the critical stretch zone width (SZWc) at room temperature was about three times as large as that at 111 K. This indicates that the variation in fracture toughness according to temperature corresponds to the decrease in SZWc with decreasing temperature.

Plane strain fracture toughness of scandium doped Al–6Mg alloy

The effect of microstructural parameters on the plane strain fracture toughness of scandium doped Al–6Mg alloys has been studied. Al–6Mg alloys with trace addition of scandium are subjected to annealing at different temperatures. Plane strain fracture toughness property ( K IC) of the alloys under different thermal conditions is measured. It is found that fracture of coarse insolubles adversely affect the fracture toughness of binary Al–6Mg alloy. Scandium treated alloys have shown improvement in fracture toughness due to the reduction in size and amount of second phase particles. The isochronal age hardening behaviour of low scandium alloy is found similar to the variation of fracture toughness with ageing temperature. However, high scandium alloy records a minimum in fracture toughness value at the peak age hardening temperature due to extensive formation of planar slip bands.

Simulation of Crack Propagation in Metal Powder Compaction

This paper presents the fracture criterion of metal powder compact and simulation of the crack initiation and propagation during cold compaction process. Based on the fracture criterion of rock in compression, a displacement-based finite element model has been developed to analyze fracture initiation and crack growth in iron powder compact. Estimation of fracture toughness variation with relative density is established in order to provide the fracture parameter as compaction proceeds. A finite element model with adaptive remeshing technique is used to accommodate changes in geometry during the compaction and fracture process. Friction between crack faces is modelled using the six-node isoparametric interface elements. The shear stress and relative density distributions of the iron compact with predicted crack growth are presented, where the effects of different loading conditions are presented for comparison purposes.

The effect of variation of microstructure on fracture mechanics parameters of HSLA-100 steel

The variations in the FCGR curve, the J–R curve, the blunting-line slope and the stretch zone width (SZW) for systematic variation of microstructure in Cu-strengthened HSLA-100 steel has been studied. The microstructural variation in the steel has been introduced through ageing at various temperatures after an initial quenching treatment. This has resulted in progressive tempering of the as-quenched martensitic matrix, accompanied by nanoscale precipitation of coherent Cu particles that gradually coarsen and loose coherency with overageing. It was observed that although there was a systematic trend, the FCGR curves were least sensitive to microstructural changes. The variation of fracture toughness, characterized by JQ in most cases and by KQ for the microstructures displaying highest strengths, correlated well with the inverse relationship between fracture toughness and strength. A systematic trend was also observed for the pre-exponent and exponent of the power-law tearing curve (for cases in which brittle fracture was precluded), the blunting-line slope and the SZW. The effect of coherency of precipitates in restricting plastic flow, as implied from the variation of mechanical properties with ageing temperature is thought to be responsible for the effects observed.

Fracture Toughness Measurements and Acoustic Emission Activity in Brittle Rocks

Fracture toughness measurements under static loading conditions have been carried out in Barre and Lac du Bonnet granites. An advanced AE technique has been adopted to monitor real-time crack initiation and propagation around the principal crack in these tests to understand the processes of brittle failure under tension and related characteristics of the resulting fracture process zone. The anisotropy of Mode I fracture toughness has been investigated along specific directions. Microcrack density and orientation analysis from thin section studies have shown these characteristics to be the primary cause of the observed variation in fracture toughness, which is seen to vary between 1.14 MPa.(m)1/2 and 1.89 MPa.(m)1/2 in Barre granite. The latter value represents the case in which the crack is propagated at right angles to the main set of microcracks. The creation of a significant fracture process zone surrounding the propagating main crack has been confirmed. Real-time imaging of the fracture process and formation of fracture process zone by AE techniques yielded results in very good agreement with those obtained by direct optical analysis.

Fracture in metal powder compaction

This paper presents a preliminary assessment and qualitative analysis on fracture criterion and crack growth in metal powder compact during the cold compaction process. Based on the fracture criterion of granular materials in compression, a displacement based finite element model has been developed to analyse fracture initiation and crack growth in metal powder compact. Approximate estimation of fracture toughness variation with relative density is established in order to provide the fracture parameter as compaction proceed. A single crack initiated from the boundary of a multi-level component made of iron powder is considered in this work. The finite element simulation of the crack propagation indicates that shear crack grows during the compaction process and propagates in the direction of higher shear stress and higher relative density. This also implies that the crack grows in the direction where the compaction pressure is much higher, which is in line with the conclusion made by previous researchers on shear crack growth in materials under compression. In agreement with reported work by previous researchers, high stress concentration and high density gradient at the inner corner in multi-level component results in fracture of the component during preparation.

Material constraint parameters for the assessment of shallow defects in structural components—Part I: Parameter solutions

This Part I paper describes the numerical derivation of a series of material constraint parameters appropriate for the prediction of constraint effects on cleavage fracture toughness in ferritic steels. A series of Lookup Tables and functions are derived which enable parameters to be readily established based on a knowledge of the yield and work hardening behaviour of the material at the temperature of interest and the Beremin cleavage fracture model parameter m. These are provided for use where crack-tip constraint is quantified either in terms of the elastic T-stress normalised by the yield stress or the elastic–plastic Q-stress. Methods for the derivation of these values are described and illustrated with respect to a mild steel plate tested at −50 °C. A comparison of the predicted variation of cleavage fracture toughness with decreasing constraint has been shown to provide a conservative description of the fracture toughness data. The companion Part II paper describes the use of the tables within the context of the R6 defect assessment procedure to assess the behaviour of a cruciform specimen containing a shallow crack under equibiaxial loading.

A Statistical Approach to the Prediction of Brittle Fracture in Heat-Affected Zones of A707 Steel Welds

This article presents a statistical approach to the prediction of the variations in fracture toughness of the heat-affected zones in A707 steel welds. Fractographic analyses were first used to determine the inclusion size distributions from the ductile fracture regions that precede the onset of cleavage fracture. The inclusion size distributions were shown to be well characterized by the three-parameter Weibull distribution. Based on the inclusion size distribution, a modified version of the model by Lin, Evans, and Ritchie was used to estimate the statistical variability of the crack-tip opening displacement (fracture toughness). The modification involves the use of a polynomial function in the determination of the dimensionless stress parameter, in the Hutchinson-Rice-Rasengren (HRR) field solution. This enables a fully analytical solution to be obtained for the critical driving forces associated with specified failure/survival probabilities under elastic-plastic fracture conditions. The comparison showed that the modified model prediction was in good agreement with measured crack-tip opening displacement (fracture toughness) data. Finally, the implications of the results are discussed for the probabilistic design of damage tolerant structures in which cleavage fracture can occur, following an initial stage of ductile tearing by microvoid coalescence.

Crack propagation in graded composites

Recent experimental and theoretical studies of the fracture and fatigue behaviour of composites with graded material composition are reviewed. The effects of compositional and microstructural gradation on crack-tip stress fields, stress intensity factors and crack extension direction are summarised for cracks initially oriented both parallel and perpendicular to the gradient direction. Spatial variation of intrinsic fracture toughness, residual stresses, crack-wake effects, and the scale of the material gradient have all been observed to strongly influence crack propagation behaviour and failure resistance. Pertinent issues such as composite property prediction, crack deflection criteria, stress intensity factors for curved cracks and crack bridging are also discussed.

Erosion of a reaction bonded silicon nitride

Erosion of a reaction bonded silicon nitride

Combined effects of increasing temperature and confining pressure on the fracture toughness of clay bearing rocks

Abstract In order to understand the effects of increasing temperature and confining pressure on the fracture toughness of clay bearing rock, fracture toughness experiments were carried out at temperatures elevated from room temperature up to 200°C using single edge-notched round bar in bending (SENRBB) and semi-circular bend (SCB) specimens of Kimachi sandstone and Tage tuff. This paper describes the methodology for evaluating level 1 fracture toughness and the crack growth resistance curve. The crack growth resistance curve is shown to yield true fracture toughness even when under-sized specimens are employed. The under-sized specimens refer to those which are too small to produce valid fracture toughness value using the standard methods. The fracture toughness of Kimachi sandstone did not vary significantly with temperature up to 125°C, but above that point, it increased with temperature. SENRBB tests showed that the level 1 fracture toughness increased by approximately 40% at 200°C over its value at room temperature. The fracture toughness of sandstone and tuff was found to be significantly affected by increasing confining pressure. For example, in the arrester orientation, the fracture toughness of Kimachi sandstone increased by approximately 470% at 9 MPa confinement over its value at atmospheric pressure. A quite similar variation of fracture toughness is caused by the combined effects of temperature and confining pressure. For example, under a confining pressure of 7 MPa, the fracture toughness of sandstone decreased with temperature up to 75°C, and then increased between 75°C and 100°C.

Fracture behavior of lithia disilicate- and leucite-based ceramics

Objective: This study was designed to characterize the fracture behavior of ceramics and test the hypothesis that variation in strength is associated with a variation in fracture toughness. Methods: The following four groups of 20 bar specimens (25×4×1.2 mm) were fabricated (ISO standard 6872): E1, a hot-pressed leucite-based core ceramic (IPS Empress); E2, a hot-pressed lithia-based core ceramic (IPS Empress 2); ES, a hot-pressed lithia-based core ceramic (Experimental); and GV, a glass veneer (IPS Empress2 body). Specimens were subjected to four-point flexure loading in 37 °C distilled water. Fractographic analysis was performed to determine the fracture origin ( c) for calculation of fracture toughness ( K IC). Weibull analysis of flexure strength ( σ) data was also performed. Results: Differences in mean σ and K IC were statistically significant for E1 and GV ( p<0.05). These differences are associated with processing effects and composition. Significance: The higher mean σ and K IC values of E2 and ES core ceramics suggest potentially improved structural performance compared with E1 although the Weibull moduli of E1 and E2 are the same.

Fracture toughness variability of foamed alumina cements

Preexisting micropores and microcracks within solid cement paste cause a variation in both the cell wall modulus of rupture and fracture toughness of foamed cements. Here, we determine for the variability of cell wall modulus of rupture by assuming that it follows a Weibull statistical distribution. Consequently, the variation of fracture toughness of foamed cements depends on the Weibull modulus of solid cement paste. A series of measurements on the Mode I fracture toughness of foamed alumina cements with various relative densities were conducted. Results indicate that the Weibull moduli of solid cement paste for foamed alumina cements are within the range of 7 to 10 except for the foamed alumina cement with a relative density of 0.167.

Variation of fracture toughness with constraint

`Curving' could occur in Mode I brittle fracture and it can be attributed to the constraint. The variation of the apparent fracture toughness with constraint with or without crack curving is investigated. Using a critical opening or hoop stress as the fracture criterion, it is demonstrated that a fracture event can be quantified by two mechanics parameters, i.e. K and A3 for fracture without curving and K and T for fracture with curving. K, T and A3 are the first, second and third coefficient in the Williams' asymptotic series solution, respectively, and are used here to quantify the constraint level. Test data are used to validate the theory. It is shown that while the apparent fracture toughness increases with increasing T it decreases with T after curving occurs.

Fracture Toughness of Carbon Foam

Mode I fracture toughness of open cell carbon foam was measured using single edge notched four-point bend specimens. A micromechanical model was developed assuming a rectangular prism as the unit-cell. A small region surrounding the crack tip was modeled using finite elements. Displacement boundary conditions were applied to the boundary of the region based on linear elastic fracture mechanics for orthotropic materials. From the finite element results the Mode I stress intensity factor that will cause failure of a crack tip element was determined and it was taken as the predicted fracture toughness of the foam. A simpler model in which the foam consisted of struts of square cross section was also considered. The micromechanical simulations were used to study the variation of fracture toughness as a function of solidity of the foam. The good agreement between the finite element and experimental results for fracture toughness indicates that micromechanics can be an effective tool to study crack propagation in cellular solids.

The assessment of reactor pressure vessel defects allowing for crack tip constraint and its effect on the calculation of the onset of the upper shelf

The paper shows how a specialised application of the R6 method could be used to calculate a pressure–temperature failure envelope for postulated defects in a reactor pressure vessel (RPV), making due allowance for the distribution of constraint around the crack front. As such, the technique provides a means of estimating a defect-specific onset of upper shelf temperature (OUST). A material's constraint-based toughness behaviour can be described using separate curves to represent the variations of ductile fracture toughness with constraint and cleavage fracture toughness with constraint and temperature. Armed with such a constraint-based material response, and the calculated variation of constraint with pressure determined at different points along the crack front of a defect, the pressure–temperature failure envelope can be estimated by solving equations inherent in the R6 method. The method is illustrated for a spherical RPV fabricated from ferritic steel, with radius 10 m and wall thickness 100 mm, by application to three postulated surface-breaking defects: an extended, circumferential defect; and two semi-elliptical defects. Constraint-based fracture toughness curves can be obtained by applying micro-mechanical models to a series of modified boundary layer Finite Element analyses. For each postulated defect an estimate of the OUST is presented and compared against the value which would be obtained from an assessment of high constraint fracture toughness data. For the 5 mm deep extended, circumferential defect, allowance for loss of constraint resulted in a beneficial shift (i.e. to a lower temperature) in the OUST of about −18.8 °C. For the 5 mm deep by 30 mm long surface breaking, semi-elliptical defect, there was a shift in the OUST of −17.8 °C, a value which did not change appreciably when residual stresses were allowed for. For the 25 mm deep by 150 mm long surface-breaking, semi-elliptical defect, the OUST was shifted by −19.7 °C without allowance for residual stresses, and by −18.7 °C allowing for residual stresses. All these values represent significant improvements in the OUST. Because the direction of these shifts in the OUST is opposite to that which would occur due to material degradation by in-service neutron irradiation, it is evident that crack-tip constraint is an important factor that should be taken into account in assessing RPV lifetime.

Dependence of fracture toughness on multiscale second phase particles in high strength Al alloys

An improved model is described to predict variations in fracture toughness of high strength aluminium alloys with volume fraction, size, and characteristics of the contained multiscale second phases, i.e. ellipse shaped constituents, sphere shaped dispersoids, and disc shaped precipitates, in an integrated manner. Results show that predictions are in broad agreement with values measured experimentally for an aged Al-Cu-Mg alloy. Furthermore, the model was employed torelate the anisotropic fracture toughness ofalloy plate to its orientation. A diagramis presented to illustrate the relationship between yield strength and fracture toughness of the aged alloy.

Dynamic Fracture Toughness and Charpy Transition Properties of a Service-Exposed 2.25Cr-1Mo Reheater Header Pipe

Residual life analysis of power plant components like boiler tubes, superheater outlet headers, reheater headers, steam pipes, etc., is important for life extension and avoidance of catastrophic failure. In this context, fracture toughness is very important. The fracture characteristics after prolonged exposure to high temperatures and pressures are likely to be different from that of the virgin material. 2.25Cr-1Mo reheater header pipe exposed at 813 K for 120,000 h was studied by instrumented impact tests (IIT) to evaluate dynamic fracture toughness and Charpy transition properties. The methods presented in this paper for estimating dynamic fracture toughness from IIT of Charpy specimens give reliably conservative results without the need for precracking. For estimating fracture appearance transition temperature (FATT) from IIT load-time traces, the equation for percent shear fracture, PSF3, gives the best 1:1 correlation with measured values from fracture surfaces. The lower bound equation for variation of fracture toughness with temperature derived in the present study is higher than that obtained from the FATT master curve (FATT-MC) approach. Comparison of Charpy indices like FATT and upper-shelf energy for the service exposed steel to results for the virgin material reported in the literature and the compositional J-Factor estimates for temper-embrittlement susceptibility indicate that the present steel, even after 120,000 h exposure to high temperature service, has probably undergone only very little or nil degradation in toughness properties.

Effect of electric fields on fracture behavior of ferroelectric ceramics

The asymptotic problem of a semi-infinite crack perpendicular to the poling direction in a ferroelectric ceramic subjected to combined electric and mechanical loading is analyzed to investigate effect of electric fields on fracture behavior. Electromechanical coupling induced by the piezoelectric effect is neglected in this paper. The shape and size of the switching zone is shown to depend strongly on the relative magnitude between the applied electric field and stress field as well as on the ratio of the coercive electric field to the yield electric field. A universal relation between the crack tip stress intensity factor and the applied intensity factors of stress and electric field under small-scale conditions is obtained from the solution of the switching zone. It is found that the ratio of the coercive electric field to the yield electric field plays a significant role in determining the enhancement or reduction of the crack tip stress intensity factor. The fracture toughness variation of ferroelectrics under combined electric and mechanical loading is also discussed.

Lifetime prediction for ABS pipes subjected to combined pressure and deflection loading

AbstractAs part of an investigation into the performance of acrylonitrile‐butadiene‐styrene (ABS) systems for water transportation, this paper presents a method for predicting the service lifetimes of buried pipes under in‐service loading conditions. A linear fracture mechanics approach was used to analyze brittle failure initiating from adventitious flaws located at the bore surface of pipe. Failure criteria were determined using the time‐dependent, quasistatic, plane strain fracture toughness of the ABS material, combined with empirical parameters that describe slow, steady crack growth. The expected operating conditions of a buried pipe were then separated into static loading contributions from internal pressure, diametrical deflection and residual stress. Idealized stress intensity factors associated with mode‐I crack opening under each of these components were determined using a finite element analysis and superposed to describe the general case in service. The computed nett stress intensity factor was then combined with the previously determined fracture toughness and slow crack growth data in an algorithm to simulate incremental radial crack growth from the pipe bore. Predicted failure times compared well with an experimental model of expected operating conditions, which combined hydrostatic pressure and parallel‐plate deflection loading of an internally notched pipe. The prediction method was also used to identify the factors that control the lifetime of a pipe in service. The influence of material visco‐elasticity was investigated by simulating variations in fracture toughness and slow crack growth resistance. It was proposed that, in practice, these variations are governed by opposing changes in visco‐elasticity. The effect of changing diametrical deflection and residual stress distribution were also simulated, allowing recommendations on pipe manufacture and installation conditions to be made.