Critical Fracture Strain Research Articles

Snow Slab Release, its Mechanism and Conclusion for the Arrangements of Supporting Structures

The paper presents a model of snow slab release based on the concept of the presence of ground-parallel thin super-weak layers. The fact that the same snow type under same state may or may not fracture under a certain stress unless a critical strain-rate and critical fracture-strain is reached highlights the necessity of stress and strain-rate concentrations. Super-weak layer, which cannot or can only insufficiently transmit the shear stress caused by the overlying snow layers, is considered to provide such concentrations. The model establishes that although the existence of weak layers in a snowpack is a necessary condition, yet it is not sufficient for avalanche formation. A minimum length of the super-weak zone is required for the crack to propagate leading to the release of a slab avalanche Critical crack lengths are found to be of the order of 5 m to 10 m. Critical value of crack length has been found to be dependent on slope, thickness and viscosity of the weak layer. The model does not hold good, if the super-weak zone vanishes. The paper finally discusses the arrangement of supporting structures to minimise the development of super-weak zones.

Microstructural analysis of fracture toughness variation in 2XXX-series aluminum alloy composites reinforced with SiC whiskers

The effects of local microstructure on fracture properties in powder-metallurgy (P/M)-processed 2124/SiC/15w and 2009/SiC/15w composites are analyzed in this study. Ductility and fracture toughness of the 2009/SiC/15w, in which dispersoid-forming elements such as manganese and iron were nearly absent, were greater than in the 2124/SiC/15w, while its tensile and yield strengths were somewhat less. Microstructural examination and fracture parameter analysis revealed that the improved fracture toughness of the 2009/SiC/15w compared to the 2124/SiC/15w was due to the increase in the critical microstructural distance,l* when manganese-containing particles are absent. 2009/SiC/15w was also heat-treated in T4P and overaged (OA) conditions. The OA 2009 composite showed lower fracture toughness than the 2009-T4P composite and the critical fracture strain of the OA condition was much lower, too. Detailed fractographic analyses indicated that interface precipitates facilitate premature SiC whisker failure in the OA condition.

Earthquake rupture complexity due to dynamic nucleation and interaction of subsidiary faults

We numerically study the dynamic interaction of propagating cracks. It is assumed that propagating cracks can nucleate and drive subsidiary cracks because of shear strain enhancement near the propagating crack tips. The critical strain fracture criterion is assumed in the analysis. Intense interaction is expected to occur among the cracks. All the cracks are assumed to be parallel and antiplane strain deformation is assumed in the computation.

Effects of prestrain on the fracture properties of pressure vessel steel

In this paper, various methods have been used to measure the effects of prestrain on the fracture toughness of 20 g steel experimentally. Based on elastic-plastic FEM analysis, the relationships between crack tip opening displacement δT and the maximum strain emaxc have been obtained and the effects of prestrain ep on the critical fracture strain emaxc have been examined. At the same time, based on the Rice-Tracey's void growth model [4], the cause of the void formation ahead of the crack tip, the condition of the principal crack propagation and the effects of prestrain ep on the fracture properties of 20 g steel were investigated. For ductile material there was a maximum value of V G ahead of the crack tip that was the cause of the void formation in that place. It is believed that when the maximum value of V G ahead of the crack tip reaches a critical value, it may be the principal crack propagating condition for high ductile material.

Dependence of ductile fracture toughness of a weld metal on notch root radius and inclusion content

Using different fluxes of high and low basicity, two different welds have been produced with low and high inclusion content. Inclusions have been characterized in 2 and 3 dimensions, from measurements on both polished sections and on fracture surfaces. The fracture toughness is evaluated with J0integral, crack tip opening displacement (CTOD) and stretch zone width measurements. Ductile fracture satisfies the criterion of a critical fracture strain over a characteristic distance. These two critical values can be determined from the results obtained with blunt notch specimens. Characteristic distances are an order of magnitude larger than the inclusion spacing. This is attributed to the need to reach conditions of sustained crack growth for initiation of ductile tearing.

On the mechanics of snow slab release

The mechanics of snow slab release is considered from the viewpoint of mechanics of continua. Its result shows that snow cannot fracture until a critical strain rate and a critical fracture strain is reached in a weak layer of the snowpack. To achieve these conditions, the existence of a “super-weak' zone — where shear stresses from the overburden snow can only insufficiently be transmitted — must be stated. Such super-weak zones interrupt the weak layer and cause stress and strain rate concentrations, making failure possible, which is not the case in homogeneous weak layers. In the first part numerical and analytical investigations are made about stress and strain rate distribution in a snowpack on a uniform slope, containing an initial crack expanding infinitely across the slope. In the second part, starting from the above distributions, the conditions and speed of fracture propagation are considered.

Spall due to short high-intensity impulses

Spallation thresholds for (3)/(16) – (3)/(8) -in. 6061-T6, 5456, and 1100 aluminum and 6A14V titanium alloys were determined for impulses from 5 to 30 ktap with a pulse duration of 0.15 μs. These experiments show that for aluminum alloys the spall fracture stress is nearly a constant but varies between 16 and 20 kbar in different batches of material and shows suggestion of a weak dependence on strain rate with strain rates from 0.04 to 0.4 μs−1. The impulse threshold for spall was 7.5 ktap. For 6A14V titanium the spall fracture stress was 60 kbar and the impulse threshold was 15 ktap. The stress pulses were produced by impacting target samples with a 0.012-in. (0.305 mm) thick by 3-in. (76 mm) square Kapton impactor projectile at velocities ranging from 0.13 to 0.44 cm/μs. Data obtained include detailed records of the impactor and rear (spalled) surface velocity versus time measured with a dual-beam Fabry–Perot interferometer, as well as soft recovery of the target residual and spall fragments. From these we are able to determine the partitioning of impulse between the target residual and the spall fragment(s) and the critical spall fracture stress and strain rate. We have also made microphotographic inspections and measured the stress-strain properties of recovered target. The data show that the amount of impulse (momentum per unit area) absorbed by a structural wall by a short duration pulse is limited by the spall threshold. Increases in applied impulse above this spall threshold result in higher spall fragment velocities but no apparent increase in the impulse delivered to the residual structure. Spall is thus an effective damage limiting mechanism which must be considered when estimating the effectiveness of systems whose primary damage is structural collapse due to impulse generated by short duration pulses.

Effects of high temperature austenitization on fracture, fatigue and stress corrosion cracking behavior of cast and forged high strength steels I: Monotonic fracture behavior

Abstract An investigation into the effects of high temperature austenitization (h.t.a.) on the monotonic fracture behavior of cast and forged high strength steels has been carried out. The h.t.a. increased the fracture toughness, the ductility and the impact toughness of the cast steel, whereas the ductility and impact toughness of the forged steel were reduced. Both the critical fracture stress and strain of the cast steel were enhanced by h.t.a., but the fracture strain of the forged steel decreased. Analysis of microstructural differences and fracture micromechanisms revealed that the higher ductility and impact toughness of h.t.a. cast steel than those of h.t.a. forged steel could be related to a smaller austenite grain, finer martensite packet and lath size, and the discontinuous nature of the interlath retained austenite film. The slight improvement in ductility and impact toughness of cast steel after h.t.a. compared with conventional temperature austenitization (c.t.a.) correlates with a reduction in the dendritic and impurity segregation. The fracture toughness of the cast and the forged steels austenitized at different temperatures has been analyzed using stress-controlled and stress-modified strain-controlled fracture models. The actual fracture stress and strain over the range of the fracture process zone are very different from those determined by notch bending or plane strain tensile tests, and the characteristic distances calculated from the above models were found to have no meaningful relation with the microstructural parameters important to fracture initiation. The improved fracture toughness is a result of a higher local fracture strain ahead of the crack tip for strain-controlled fracture and a larger characteristic distance for stress-controlled fracture in both the cast and forged steels.

Evidence of a thin sheet toughening mechanism in Al−Fe−X alloys

A toughening mechanism, dubbed thin sheet toughening, is proposed for improving the fracture resistance (KIC and tearing modulus) of powder-metallurgy alloys of limited ductility. The basis of this approach is the recognition that internal delamination of thick-section components or cracked specimens into thin sheet ligaments in the fracture process zone leads to a drastic reduction in triaxial stresses, with the consequence of enhancing the critical fracture strain, fracture toughness (KIC orJIC), and tearing modulus. Theoretical analyses indicate that a factor of\(\sqrt 3 \), increase in theKIC value, and even greater increases in the tearing modulus are possible for idealized conditions. The predicted results are compared with experimental results of tensile,KIC, andJ tests conducted on four powder-metallurgy Al−Fe−X alloys at 25 and 316°C. The comparison reveals that thin sheet toughening is a contributor to the highKIC value observed in a state-of-the-art Al−Fe−V−Si alloy. Increasing the critical strain to fracture is also shown to be a possible method to improve the fracture toughness of Al−Fe−X alloys, independent of the thin sheet toughening effect.

Applicability of the critical strain fracture criterion to WC-Co hard metals

A generally accepted view that the fracture of hard metals needs to be modeled as a ductile event (their macroscopic brittle behavior notwithstanding) to account for the microplasticity of the binder phase, led to various attempts in the past to apply the Rice-Johnson model for ductile failure to the fracture of hard metals. The present work is an attempt to take these analyses a step further and is based on the concept of a crack tip process zone in which the contiguous carbide, since it is most severely stressed, is broken owing to microcracking and therefore does not contribute to the load-carrying capacity of the process zone. This leads to an expression relating fracture toughness and microstructural parameters of hard metals. The relationship contains one floating parameter for which reasonable values are obtained when the predictions of the present model are fitted to the available experimental data for WC-Co alloys with binder volume fractions approximately in the range 0.10–0.25.

The effect of hydrogen on fracture toughness of the Fe-Ni-Co superalloy IN903

Fracture toughness values and tensile properties were determined in the Fe-Ni-Co superalloy IN903* as a function of hydrogen concentration, loading rate, and grain size to define the effects of hydrogen on fracture toughness and failure modes. Tests using precracked, precharged three point bend samples showed that fracture toughness decreased from 90 to 50 MPa-m1/2 as hydrogen increased from zero to 5000 appm. The decrease in fracture toughness was accompanied by a fracture mode change from microvoid coalescence in the uncharged samples to principally slip band fracture with some twin band and ductile intergranular fracture in the hydrogen charged samples. Fractographic observations and application of ductile fracture toughness models showed that fracture initiated at matrix carbides in all samples. These carbides established the critical fracture distance for all fracture processes observed in the fracture toughness samples. Hydrogen promoted the secondary fracture processes of slip band, twin band, and ductile intergranular fractures which lowered both the critical fracture strain and the fracture toughness of IN903.

Predicting fully plastic mode II crack growth from an asymmetric defect

Asymmetrically cracked specimens fail with considerably less ductility than symmetrically cracked ones, due to the crack progressing along a shear band into pre-damaged material. A formulation for the accumulation of damage ahead of an asymmetric crack is presented, based on strain increments following a power law relation. These results are integrated both numerically and approximately. The crack growth per unit displacement increases approximately as the logarithm of the total crack advance per inclusion spacing р, and varies inversely as the critical fracture strain γc. This provides a basis for predicting large-scale, fully plastic fracture from asymmetric weld defects, using small-scale laboratory specimens.

Some geometrical observations on crack front profiles in PMMA double torsion specimens

The double torsion (DT) technique has proved its usefulness in fracture mechanical testing. Practical considerations make it preferable to other methods in many cases. Nevertheless, it is sometimes considered with reluctance, first because of its curved crack front and secondly because of a misunderstanding of the related problems. Experiments were conducted on polymethylmethacrylate (PMMA) to elucidate the effect of crack velocity and geometry. It was found that the crack front profile does not vary with crack velocity, whereas the geometry of the specimen has a strong influence on it. A simple model is developed, based on a strain fracture criterion. It describes quite well the experimental profiles of PMMA. Crack opening displacement, shear modulus, stress intensity factor and Poisson's ratio are shown to be of importance. The influence of the subcritical crack growth exponent could be effective for some materials but not for PMMA.

Air embrittlement of a cobalt-base superalloy

The stress-rupture and tensile properties of a cast cobalt base superalloy, MM509, are compared after high temperature exposure in air and vacuum. A loss in tensile ductility after air exposure is observed over the entire temperature range from room temperature to 1000 °C with the effect being most severe at 400 ° to 600 °C. This appears to be related to grain boundary oxygen penetration and is compared and contrasted with similar observations in nickel base alloys. The cobalt base alloy has high intrinsic ductility at high temperatures and, even after air exposure, there is no loss in creep life associated with the embrittlement. It is argued that in this alloy the critical strain for fracture of embrittled grain boundaries is much higher than that for high strength nickel base superalloys.

Discussion of “critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels”

Discussion of “critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels”

Fracture resistance of notched specimens

This paper gives a theoretical approach to the general problem of the static fracture resistance in the presence of stress concentrations. First of all, an approaching calculation model is proposed for elastic and plastic stress in the reduced section of a circumferentially notched cylindrical bar. The introduction of a normal stress criterion or of an axial strain fracture criterion on the elastic plastic interface makes it possible to explain the experimental results covering the fracture of various steels above a certain sharpness.

Critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels

Critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels

Critical fracture stress and fracture strain models for the prediction of lower and upper shelf toughness in nuclear pressure vessel steels

Critical fracture stress and stress modified fracture strain models are utilized to describe the variation of lower and upper shelf fracture toughness with temperature and strain rate for two alloy steels used in the manufacture of nuclear pressure vessels, namely SA533B-1 (HSST Plate 02) and SA302B (Surveillance correlation heat). Both steels have been well characterized with regard to static and dynamic fracture toughness over a wide range of temperatures (−190 to 200°C), although validJIc measurements at upper shelf temperatures are still somewhat scarce. The present work utilizes simple models for the relevant fracture micromechanisms and local failure criteria to predict these variations in toughness from uniaxial tensile properties. Procedures are discussed for modelling the influence of neutron fluence on toughness in irradiated steel, and predictions are derived for the effect of increasing fluence on the variation of lower shelf fracture toughness with temperature in SA533B-1.

Further considerations on the inconsistency in toughness evaluation of AISI 4340 steel austenitized at increasing temperatures

A study has been made of the influence of austenitizing temperature on the ambient temperature toughness of commercial AISI 4340 ultrahigh strength steel in the as-quenched (untempered) and quenched and tempered at 200°C conditions. As suggested in previous work, a systematic trend ofincreasing plane strain fracture toughness(K)Ic anddecreasing Charpy V-notch energy is observed as the austenitizing temperature is raised while the yield strength remains unaffected. This effect is seen under both static and dynamic (impact) loading conditions, and is rationalized in terms of a differing response of the microstructure, produced by each austenitizing treatment, to the influence of notch root radius on toughness. Since failure in all microstructures was observed to proceed primarily by a ductile rupture (microvoid coalescence) mechanism, an analysis is presented to explain these results, similar to that reported previously for stress-controlled fracture, based on the assumption that ductile rupture can be considered to be strain-controlled. Under such conditions, the decrease in V-notch Charpy energy is associated with a reduction in critical fracture strain at increasing austenitizing temperatures, consistent with an observed decrease in uniaxial and plane strain ductility. The increase in sharp-crack fracture toughness, on the other hand, is associated with an increase in “characteristic distance” for ductile fracture, resulting from dissolution of void-initiating particles at high austenitizing temperatures. The microstructural factors which affect this behavior are discussed, and in particular the specific role of retained austenite is examined. No evidence was found that the enhancement of fracture toughness at high austenitizing temperatures was due to the presence of films of retained austenite. The significance of this work on commonly-used Charpy/KIc empirical correlations is briefly discussed.

Relation between KIc and microscopic strength for low alloy steels

A simple model has been developed to determine K IC in terms of the microscopic cleavage strength σ f ∗ and the tensile yield strength σ Y for low temperature cleavage fracture in A302B and A533 reactor grade quenched and tempered steels. The model applies at sufliciently low temperatures or in irradiated steels where σ f ∗ ≦ 3.4 σ Y . It was determined that σ f ∗ is independent of temperature below −150°F and then increases with increasing temperature. At this time, it appears that σ ∗ is independent of irradiation. At temperatures above that at which σ f ∗ ≦ 3.4 σ Y , unstable fracture initiates when a critical plastic strain ϵ f is achieved near to the crack tip. The critical local plastic strain for unstable fracture also increases with increasing temperature.