Intergranular fracture in cleavage fracture temperature range

Abstract

The fracture of α iron and C–Mn submerged arc weld metal at temperatures below the brittle–ductile transition temperature is examined. The probability of intergranular fracture occurring in a regime where cleavage fracture is observed is considered with respect to simple theoretical arguments that take into account the geometry of grains and the relative energy of the two fracture processes. The experimental observations are examined in relation to the predictions of these arguments. Simple geometrical arguments demonstrate that a small proportion of intergranular fracture is expected to accompany cleavage fracture in most specimens of polycrystalline α iron and ferritic steels fractured at temperatures below the brittle–ductile transition temperature.

MST/1943     

Mechanism of anisotropy in fracture behaviour and fracture toughness of high strength aluminium alloy plate

Abstract

The fracture behaviour and J_Ic fracture toughness of a commercial Al–Zn–Mg–Cu alloy plate has been investigated. Based on the experimental results and available theoretical analyses, the following results were obtained. Secondary grain boundary cracks appeared ahead of the main crack, which served as a triggering mechanism for small scale shearing. Shear failure facets on the fracture surface of single edge notched bend specimens represent the same type of fracture as the fast shear failure that occurred during tensile tests on notched specimens. The grain boundary cracking–small scale shearing mechanism is essentially a type of shielding event that not only makes the fracture appearance obviously anisotropic, but also, to a considerable extent, accounts for the strong anisotropy of fracture toughness.

MST/1111     

Strain rate effects in fracture of polymer modified cements

Abstract

Because of the role of the polymer in governing the fracture behaviour of polymer modified (ICI ‘NIMS’) cements, it is of considerable importance to investigate the effects of strain rate. Creep rates in polymers are generally higher than those associated with cement or concrete; this is particularly the case when a polymer is plasticised. One of the most effective plasticisers for a water soluble polymer is obviously water. It is known that polymer modified cements suffer a relatively rapid loss of strength on immersion in water. However, this loss will be exacerbated by creep effects, especially in the presence of stress concentrations, resulting in fracture through stress corrosion processes. This paper presents recent data obtained on these problems for both Portland and calcium aluminate cements.

MST/717     

Hydrogen-assisted fracture at notches

Abstract

Although notched specimens have been much used in studies of hydrogen embrittlement, precracked–specimen testing has become more widespread in the last 15 years or so. However, recent work has revived interest in the use of notched specimens for fundamental studies. The background for, and recent results from, the use of notched specimens is described, and current problems in the interpretation of fracture behaviour are discussed. Of particular interest are the questions relating to the appropriateness of assuming a critical, local hydrogen concentration for fracture and to the degree of fracture control by stress, strain, or some combination. An encouraging development is the recent convergence in interests between micromechanism studies by materials scientists and work on the parameters of microfracture by the mechanics community.

MST/74     

Micromechanism of cleavage fracture in fully pearlitic steels

Abstract

The micromechanism of cleavage fracture in a fully pearlitic steel has been investigated. Uniaxial tensile and compression test specimens, together with single notched bend (SNB) and double notched bend (DNB) specimens, were heat treated such that the prior austenite grain size remained constant while the pearlite interlamellar spacing was varied. The SNB specimens were used to determine the cleavage fracture stress σ_fM, over the temperature range ?25 to ?196°C. The DNB specimens were used to study the initial stages of crack nucleation. The results indicate that pearlite can exhibit two different cleavage mechanisms which are dependent on the strength of the steel. For cleavage fracture stresses below about 2100 MN m^?2, fracture is nucleation controlled and involves shear linking of carbide nucleated microcracks before unstable cleavage can occur. Under these conditions, the cleavage fracture stress is dependent on temperature and is proportional to the uniaxial proof stress. For cleavage fracture stresses above 2100 MN m^?2, cracked carbides act directly as cleavage nuclei. Fracture is then propagation controlled and the cleavage fracture stress is independent of temperature. The transition from nucleation–controlled to propagation–controlled cleavage may be achieved by either a reduction in pearlite interlamellar spacing or a reduction in testing temperature.

MST/355     

Microstructural and crystallographic features of hydrogen-related fracture in lath martensitic steels

ABSTRACT

This paper summarised the microstructural and crystallographic features of hydrogen-related fracture under tensile deformation in martensitic steels. The specimens with relatively low strength exhibited quasi-cleavage fracture. Crystallographic orientation analysis demonstrated that the quasi-cleavage fracture occurred on {011} planes. On the other hand, the macroscopic fracture surface morphologies of the specimens with relatively high strength appeared to be intergranular-like. However, nature of the fracture was somewhat different from a typical intergranular fracture, and the fracture surfaces consisted of facets parallel to {011} planes on a microscopic level. Based on the experimental results, we concluded that the crystallographic feature of {011} plane itself had an important role on the hydrogen-related fracture.

This paper is part of a thematic issue on Hydrogen in Metallic Alloys     

Vickers indentation fracture toughness test Part 1 Review of literature and formulation of standardised indentation toughness equations

Abstract

There is considerable interest in determining the fracture toughness of brittle materials by measuring the extent of cracking associated with a Vickers indentation because of the ease of specimen preparation and the simplicity of the test. However, confusion has been engendered by the multitude of models and equations in the literature relating the degree of cracking to the fracture toughness. In Part 1 of this work, nineteen of these equations are reviewed and then modified in a standard manner for both experimental convenience and direct comparison.

MST/1050a     

Vickers indentation fracture toughness test Part 2 Application and critical evaluation of standardised indentation toughness equations

Abstract

The standardised indentation fracture toughness equations formulated in Part 1 have been applied to a range of brittle materials: namely, glass ceramics, aluminas, zirconias, and WC–Co cermets. Analysis of the results has enabled a critical assessment of (i) the ability of the nineteen equations to yield the same fracture toughness values as a conventional fracture toughness test and (ii) their ability to rank materials in order of fracture toughness. Also, specific equations have been recommended as being the most appropriate equations to use.

MST/1050b     

Plane strain fracture toughness of modern ferritic structural steels

Abstract

The temperature dependence of the plane strain fracture toughness of a low carbon, fine grain, ferritic steel for structural applications is investigated. The ductile–brittle transition is found to occur in the interval between 160 and 184 K. The experimental results are interpreted by an analytical model which permits calculation of the plane strain fracture toughness K _1c in the brittle domain as a function of the tensile properties and the cleavage fracture stress, making use of a piecewise approximation for the distribution of tensile stress on the crack axis and applying a deterministic fracture criterion at the stress peak. A similar criterion, which consists of equating the severest strain on the crack axis to a critical strain for cavity nucleation, provides the upper shelf fracture toughness. A relatively simple figure for predicting the transition temperature of steels in this family as a function of material properties can be obtained in this way.     

Crystallography of brittle fracture and deformation twinning in ferritic steels

Abstract

The crystallography of brittle fracture and deformation twinning in ferritic steels is difficult to study experimentally, because of its three-dimensional aspects. The present paper reports the development of methodologies to study the phenomenon via customisation of various electron backscatter diffraction and SEM routes. It is shown that both direct (from the fracture surface) and indirect (from an adjacent polished side) measurements yield valuable information on crystallographic aspects of the fracture processes. Specifically, brittle fracture in three ferritic steels is studied: a C–Mn weld metal, a low alloy Mn–Mo–Ni steel similar to grade A533 and an ultralow carbon (0·002 wt-%C, 0·058 wt-%P) steel plate. The main conclusions resulting from development of the experimental techniques are that cleavage fracture occurs only on {001} planes, and that intergranular accommodation is present at the fracture surface. Further observations suggest that a cleavage side crack, initially deflected by a deformation twin, eventually blunts at the intersection of two deformation twins. This provides a mechanism for limiting the mean length of microcracks during brittle fracture.     

Strain-rate dependent fracture initiation

The theory of linear elastic dynamic fracture mechanics for Heaviside loading of an isolated crack is employed to formulate the response to constant strain-rate loading of a single crack. Numerical integration of the Heaviside solution is shown to lead to fracture initiation stresses that are dependent upon the imposed strain rate. These fracture initiation stresses are also shown to be relatively independent of the crack size and crack shape. The results are used to explain the strain-rate dependent fracture stress observed in some rocks as being a structural response, rather than a basic material property.

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Résumé La théorie de la mécanique de la rupture linéaire élastique dynamique dans le cas d'une mise en charge de Heavyside sur une fissure isolée est utilisée pour la formulation d'une réponse à une sollicitation à vitesse de déformation constante sur une fissure unique. L'équation numérique de la solution de Heavyside est démontrée comme susceptible de conduire aux contraintes d'amorçage d'une fissure qui sont dépendantes de la vitesse de déformation imposée. On montre que les contraintes d'amorçage de la rupture sont relativement indépendantes de la dimension de la fissure et de la forme de la fissure. On utilise les résultats pour expliquer la contrainte à la rupture dépendant de la vitesse de déformation, telle qu'elle est observée dans certains matériaux pierreux, et qui se trouvent être davantage une réponse structurelle qu'une propriété de base du matériau.

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This article sponsored by the U.S. Department of Energy under Contract DE-AC04-76-DP00789

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A U.S. Department of Energy Facility.     

Brittle fracture of solids with arbitrary cracks

One of the problems of fracture mechanics is the prediction of the propagation of cracks in solids. The present paper deals mainly with linear fracture mechanics which owes its origin to the works of A. A. Griffith [1, 2] and studies the development of cracks under sufficiently low loads when the behaviour of the material within a region sufficiently remote from the edges of cracks may be regarded as linearly elastic, At present, linear fracture mechanics [3] is restricted mainly to special kinds of loading geometry, with the crack extending rectilinearly (in a plane case) or in its plane (in a three-dimensional case). The main problem here is to establish a relationship between the dimensions of cracks and the loads applied. Within the framework of linear fracture mechanics the fracture itself and other non-linear phenomena that precede it are assumed to take place only within local regions which are small compared to the dimensions of cracks. The possibility that such a situation exists is associated with the fact that when the crack dimensions are sufficiently large the characteristic dimension of the end region is fully determined by a certain intrinsic dimension of the material structure. Therefore, if the material does not exhibit time dependency, the state of the end region at the moment of rupture becomes fully independent of the loads applied and the geometry of the solid, i.e. autonomous. The notion of autonomy [4] leads to the formulation of this theory as one of limit equilibrium.

If the conditions of rectilinear extension of the crack (or those of the crack extension in its plane) are disturbed, there arises a problem of determining not only the dimensions of the crack, but also the path of the crack extension under such conditions of loading that a slow, quasi-static crack development is possible. This problem can be actually subdivided into two: (1) Criteria for the determination of the dimensions and paths of the crack extension, and (2) Expressions for the characteristics of the stress-strain state which are constituents of these criteria through the geometry of solid with cracks and the loads applied.

As regards (1), there have been many assertions, and the connections between them are not quite clear at present. The first of the suggested criteria, namely that of local symmetry for the plane problem formulated by Barenblatt and Cherepanov [5, 6] and by Erdogan and Sih [7] can be within certain limits substantiated and generalized for the three-dimensional case. The guiding principle here is the treatment of the theory of cracks from the standpoint of the method of inner and outer expansions or that of singular perturbations [8]. The concept of the stress intensity factor which is basic in linear fracture mechanics is decisive in matching inner and outer expansions to find the main term of the asymptotic solution of the complete problem. Actually the construction of the theory of equilibrium cracks [4] implicitly employs this technique for a certain specific model. More explicit indications are given in Ref. 9. In the treatment of the problem of plastic zones in the vicinity of notches, the idea of the boundary layer is employed in Ref. 10. The problem of fracture of a solid is analysed from this standpoint in Ref. 11.

As regards (2), progress has been hampered by the lack of efficient techniques for fording the stress-strain state of a solid having non-rectilinear cuts. A number of investigations have been carried out for cuts of a particular kind an arc of a circumference [12, 13], an arc of a parabola [l4], and a three-link broken line which is close to a straight line to such an extent that the boundary conditions are assumed referable to the direction of the middle portion [15, 16]. The problem of a semi-infinite curvilinear cut slightly deviating from a rectilinear one by expanding complex elastic potentials in the magnitude of deviation of the cut from the rectilinear axis tangent to the line of cut at its end is considered in Ref. 17. An exact solution of the problem of a semi-infinite cut having the form of a two-link broken line is given in Ref. 18.

The present paper is devoted to the investigation of the development of cracks under arbitrary loading conditions.

In Section 1 the criterion of local symmetry is substantiated and generalized for the three-dimensional case. In Section 2 an effective procedure of finding stress intensity factors for the plane case is given, in terms of which the criterion is formulated. Closed first approximation formulas for these magnitudes are presented in the case of a slightly curved crack, numerical calculations showing the applicability of the latter with an error not exceeding 10 to 15 with the angles of deviation of the crack from the straight line coming to 20°. In Section 3 equations of extension of curvilinear cracks are derived on the basis of the first approximation formulas and criterion of local symmetry. In Section 4 some examples are considered.

     

HAZ fracture toughness of titanium containing steels

Abstract

Steels containing various combinations of microalloying elements (Nb, V, and Ti) were welded at heat inputs from 3 to 6 kJ mm^?1. It was shown by detailed crack tip opening displacement fracture toughness testing of coarse grained heat affected zone (HAZ) regions in single pass weld deposits that the poorest toughness properties were exhibited by steel containing a combination of Nb, V, and Ti. Steel microalloyed with only titanium had the best HAZ fracture toughness at all heat input levels. Detailed microstructural analysis, grain size measurement, hardness, and precipitation in HAZ regions were evaluated to explain the fracture toughness properties observed.

MST/887     

Comparisons of deformation and fracture behaviour of PC/ABS blend and ABS copolymer under dynamic shear loading

Abstract

The dynamic shear deformation and fracture characteristics of PC/ABS blend and ABS copolymer with regard to the relation between mechanical properties and strain rate, are studied experimentally using a torsional split Hopkinson bar at room temperature under strain rates ranging from 8 × 10² s^-1 to 3.4 × 10³ s^-1. Fracture phenomena are analysed by scanning electron microscopy and correlated with macroscopic behaviour. The relative properties and fracture mechanism of both polymers are also compared. Results show that strain rate enhances shear strength of both PC/ABS blend and ABS, but fracture shear strain tends to decrease with increasing strain rate. ABS exhibits better ductility and lower shear strength. For both polymers, strain rate sensitivity increases with increasing range of strain rate, while an inverse tendency occurs for activation volume. Higher strain rate sensitivity and lower activation volume are found in PC/ABS blend. PC/ABS blend fracture is dominated by mixed shearing and tearing, but ABS fracture shows only shearing. Due to the increasing deformation heat, fracture surface viscoplastic flow for both polymers increases with increasing strain rate, inducing lower flow resistance and lower fracture strain at higher stain rates. The viscoplastic flow behaviour in ABS is more active.     

Ductility and fracture of copper bicrystals with boundary SiO2 particles

Abstract

High temperature deformation andfracture ofCu-SiO₂ bicrystals with [001] twist boundaries of various misorientation angles were investigated under the condition of non-activation of grain boundary sliding. As the misorientation angle increases, the bicrystals became more susceptible to intergranular brittle fracture. Clear intermediate temperature embrittlement was observed in bicrystals with a random high angle boundary. The boundary segregation of O atoms was found to enhance intergranular fracture. Although the boundary SiO₂ particles provide stress concentration sites which cause early formation of boundary cavities, the boundary dependent deformation and fracture behaviour is essentially determined by inherent boundary strength, which is afunction of the misorientation angle.

MST/1969     

Forming and fracture limits of aluminium alloy

Abstract

Forming and fracture limits of an AA 3104-H19 aluminium alloy sheet were studied by hydraulic bulging and Marciniak type deep drawing and tensile tests. The alloy appeared to be highly anisotropic, exhibiting distinctly different fracture patterns in the rolling and transverse directions. The preferred fracture direction was transverse to the rolling direction. In the tensile test, samples loaded in the rolling direction failed transverse to the rolling direction, but in the transverse direction, the fracture was inclined at ～55° to the tensile axis. In some cases, two such competing fractures in the characteristic directions could be observed. Scanning electron microscopy studies revealed a typical ductile fracture pattern. The fracture occurred by shearing in the through thickness direction, and typical alternating shear lips in a direction inclined at ～45° to the through thickness direction could be observed. Forming limit diagrams for both rolling and transverse directions were determined from the experiments. The measured limit strains in uniaxial tension were predicted well by the modified Rice–Tracey theory, but in equibiaxial tension, the theory overestimated the fracture limit strains.     

On continuum models of ductile fracture

Several fracture criteria are reviewed with respect to ductile fracture. It is suggested that both critical crack-tip displacement, 2V _c ^*, and critical fracture strain,∃ ^*, criteria may describe the fracture of a ductile second phase rod in a ductile matrix. As a first approximation, this is experimentally verified by observations of ductile stainless steel fibres fracturing in an age-hardened aluminium matrix. For 0.05, 0.10 and 0.20 volume fraction composites, the average fracture strains are calculated to be 1.15 as compared to a measured average of 0.93 while the average critical crack-tip displacement is calculated to be 0.50 mm as compared to an “observed” average of 0.40 mm. The statistical variation in the fracture strain was not sufficiently small to allow any choice between these proposed criteria. In fact, both the experimental and theoretical evidence point to the equivalency of these criteria as given by 2V _c ^*=π/^*∃^* where /^* is the microstructural unit in front of the crack over which the strain is greater than or equal to∃ ^*.     

Influence of heat treatment and hot working on fracture toughness of cast aluminium base composites

Abstract

Evaluation of toughness in terms of the fracture energy E*, obtained using Charpy impact testing and the fracture toughness K_Ic obtained from bend tested specimens, has been carried out for various cast particle reinforced aluminium base composites, namely, A356–SiC, A357–SiC, 6061–Al₂O₃, and 2014–Al₂O₃. In practice, the first two are used in the as cast foundry condition and the last two in the cast and extruded condition. Hot extrusion or rolling to reduction ratios between 2 : 1 and 50: 1 was conducted on the 6061 and 2014 composites to characterise the influence of working processes. Heat treatment conditions considered included the as cast (or as worked), solid solution treated, and T6 temper. The results show that extrusion or rolling can markedly improve the toughness, but on thermal aging the toughness is reduced. The increase in total fracture energy by hot working is mainly caused by the increase of initiation energy, whereas the decrease of fracture energy by artificial aging is controlled by the propagation energy. The values of K_Ic obtained for these composites are from 15 to 25 MN m^?3/2. Comparisons and interpretations of the dynamic Charpy fracture energy, quasistatic fracture toughness, and fracture surface of the four composites are also presented.

MST/1806     

Effect of aging on high temperature brittle intergranular fracture in austenitic stainless steels

Abstract

The present paper describes the effect of aging on crack growth at 550–750°C in a series of 316 and 347 based stainless steels. Crack initiation parameters and crack growth rates have been measured, and detailed fractography and microstructural characterisation carried out. The study shows that the high temperature brittle intergranular fracture mechanism operates in these alloys, as expected from incidences of cracking in austenitic stainless steels used in power plant. High temperature brittle intergranular fracture leads to lower crack tip opening displacements at initiation, and slightly higher crack growth rates than ductile intergranular failure. Susceptibility to high temperature brittle intergranular fracture is enhanced by aging. This increased susceptibility is explained in terms of the increased hardness, the reduction in dissolved C, and grain boundary precipitation. The effects of temperature, composition, and loading mode on the behaviour of the aged alloys are determined.

MST/3100     

Cleavage to quasicleavage fracture transition in steels

Abstract

The cleavage behaviour of plain carbon steels containing from 0· 2 to 0·8%C has been investigated. It is observed that each steel displays two characteristic temperatures at which a transition in the mode of fracture occurs. These are the transition temperatures for cleavage T_c and for general yielding T_g. At temperatures below T_c, the steels fail by pure cleavage. This involves the generation of a cleavage crack nucleus in a carbide particle followed by cleavage crack propagation. The cleavage fracture stress σ _f is independent of temperature. Between temperatures T_c and T_g, the steels fail by quasicleavage. This involves the generation of a crack nucleus by a localised fibrous process followed by cleavage crack propagation. The crack nucleation stage is shear stress controlled and therefore the quasicleavage fracture stress σ_q increases with decreasing test temperature. Above temperature T_g, failure occurs at or after general yielding.

MST/1045