ON THE APPLICABILITY OF THE MAGNETIC EMISSION TECHNIQUE FOR DETERMINING DUCTILE CRACK INITIATION IN IMPACT TESTS

Abstract— The magnetic emission (ME) technique is used to determine the onset of crack propagation in brittle and ductile steels under dynamic loading. Brittle fracture is directly indicated by the ME signal; however the onset of stable crack propagation can usually not be seen in this signal. A new result is that the integrated ME signal, which is proportional to the magnetic field at the site where the probe is located, is capable of detecting this event. Critical values of the J -integral derived with this method, in different test series, are compared with conventional R -curve and stretch zone methods. The ME-derived critical dynamic J -integral data fit very well to the stretch zone results which describe physical crack initiation. It is therefore concluded that the easy-to-apply and low-cost ME technique is capable of indicating material failure even with stable crack propagation and can therefore provide critical material parameters.     

THE INFLUENCE OF AGEING AT INTERMEDIATE TEMPERATURES ON THE MONOTONIC STRESS-STRAIN BEHAVIOUR AND FRACTURE TOUGHNESS OF A DUPLEX STAINLESS STEEL

Abstract— The mechanical behaviour of the duplex stainless steel AISI 329 has been investigated for ageing times up to 15,000 h at 475, 425, 375, 325 and 275°C. The study has concentrated on changes in the monotonic stress-strain behaviour and fracture toughness as a function of ageing temperature and time. It is shown that the tensile behaviour of the steel changes strongly due to ageing. A large increase in yield strength and reductions in ductility and fracture toughness are observed. The deformation hardening behaviour of the aged steel is explained by using a model based on a modified rule of mixtures. Finally it is shown that the higher toughness of aged duplex stainless steels, in comparison with ferritic stainless steels aged under the same ageing conditions, may be associated with the increase in crack growth resistance induced by ductile ligaments of austenite which bridge the crack faces.     

Fracture toughness of layered structures: Embrittlement due to confinement of plasticity

The fracture toughness of a layered composite material is analyzed employing a combined two dimensional dislocation dynamics (DD)-cohesive zone (CZ) model. The fracture mechanism of an elastic-plastic (ductile) material sandwiched within purely elastic layers approaches ideally brittle behaviour with decreasing layer thickness. We investigate the influence of different constitutive parameters concerning dislocation plasticity as well as the effect of cohesive strength of the ductile material on the scaling of fracture toughness with layer thickness. For a constant layer thickness, the results of the numerical model are consistent with the expectation that fracture toughness decreases with increasing yield strength, but increases with the cohesive strength of the material. The scaling behaviour of the fracture toughness with layer thickness depends on these material parameters, but also on the dislocation microstructure in the vicinity of the crack tip. Strain localization due to easy dislocation generation right at the crack tip improves toughness in thin layers and leads to a jump-like increase of fracture toughness with layer thickness. However, the fracture toughness for films that are thick enough to exhibit bulk behaviour proves to be higher when the distribution of dislocations is more homogeneous, because in this case the crack grows in a stable fashion over some distance.     

Assessment of effect of ductile tearing on cleavage failure probability in ductile to brittle transition region

The fracture behaviour of ferritic and ferritic martensitic steels in ductile to brittle transition (DBT) region has been extensively studied in recent years and a probabilistic approach of master curve method is generally used to describe the fracture toughness of BCC steels in DBT region as a function of temperature. The assessment of cleavage failure probability however is still untouched in the upper region of ductile to brittle transition, although various extensions of master curve approach and various local approaches has been explored. Additionally the geometry and loading in tension and bending also adds up to the difficulties when cleavage failure is assisted with prior ductile tearing. In this work the cleavage fracture is investigated in upper region of DBT and a modified master curve approach is presented which can satisfactorily describe the fracture toughness as a function of temperature as well as amount of ductile tearing preceded by cleavage.     

A load–deformation formulation with fracture representation based on the J–R curve for tubular joints

This study presents a new fracture formulation to describe the ductile tearing and unstable fracture failure for circular hollow section (CHS) joints under monotonically increasing brace tension. The initiation of the ductile tearing occurs when the crack driving force in an assumed initial shallow crack reaches the material fracture toughness determined from a standard fracture toughness test. The joint behavior prior to the ductile crack initiation follows a previously proposed nonlinear formulation based on the latest strength equations recommended by the International Institute of Welding. The load–deformation characteristics beyond the crack initiation assume that the energy release rate and the amount of crack extension adhere to the experimentally measured J–R curve, prior to the unstable fracture failure. Unstable fracture, which leads to the total loss of the joint capacity, occurs when the crack driving force reaches the maximum fracture resistance determined from the J–R curve test. The proposed load–deformation representation for tubular joints, when implemented in the large-scale K-frame pushover analysis with a material fracture toughness test, predicts successfully the global frame response governed by the joint fracture failure, as observed in the frame test.     

Control of ductile to brittle transition temperature in ferritic pressure vessel steel

Abstract

Controlled amounts of cold work are shown to cause a minimum in the ductile to brittle transition temperature (DBTT) in a ferritic steel at a critical level of ～1·5%. Mechanical property assessments show that the hardness values exhibit the same trend. A theory is advanced for explanation of these effects, based on work hardening and Cottrell–Bilby locking models. Consideration is given to an alternative Ashby–Embury model, but it is concluded that the former approach is most successful in predicting the observed DBTT shift behaviour. Although independent of fracture surface type, the degree of plastic deformation shows some dependency on the grain boundary character. This leads to the conclusion that the matrix yield strength is the primary factor in determining the DBTT in these steels. Discussion focuses on methods for exploiting the effect to give higher toughness steels utilising knowledge of how to control matrix hardening and cleavage fracture strength.     

Effect of pre-strain on fracture toughness of ductile structural steels under static and dynamic loading

The ductile fracture process consists of void nucleation, growth and coalescence. The whole ductile process can be divided into two successive steps: (I) the initial state to void nucleation, followed by (II) void growth up to void coalescence. Based on this suggestion, resistance to ductile fracture could be divided into the resistance to stage I and stage II, and accordingly the whole fracture toughness could be regarded to be due to contributions from stages I and II. The fracture toughness contributed from the two steps is, respectively, denoted as void nucleation-contributed fracture toughness and void growth-contributed fracture toughness. The effect of plastic pre-strain on the fracture toughness of ductile structural steels under static and dynamic loading (4.9 m/s) within the ductile fracture range was evaluated by summing contributions due to void nucleation-contributed and void growth-contributed fracture toughness. The effect of strain rate on fracture toughness was also investigated by the same means. The results show that both plastic pre-strain and high-speed loading decrease the void nucleation-contributed fracture toughness while their effects on the void growth-contributed fracture toughness depend on the variations in strength and ductility. Moreover, fracture toughness of structural steels generally decreases with increasing strain rate.     

Analysis of ductile to cleavage transition in part-through cracks using a cell model incorporating statistics

This paper describes an approach to study ductile/cleavage transition in ferritic steels using the methodology of a cell model for ductile tearing incorporating weakest link statistics. The model takes into account the constraint effects and puts no restriction on the extent of plastic deformation or amount of ductile tearing preceding cleavage failure. The parameters associated with the statistical model are calibrated using experimental cleavage fracture toughness data, and the effect of threshold stress on predicted cleavage fracture probability is investigated. The issue of two approaches to compute Weibull stress, the 'history approach' and the 'current approach', is also addressed. The numerical approach is finally applied to surface-cracked thick plates subject to different histories of bending and tension, and a new parameter, ψ, is introduced to predict the location of cleavage initiation.     

Ductile fracture of two-dimensional cellular structures – Dedicated to Prof. Dr.-Ing. D. Gross on the occasion of his 60th birthday

Crack growth initiation and subsequent resistance to propagation are explored numerically for regular and irregular hexagonal honeycomb structures made from ductile cell walls. The elasto-plastic response of the cell walls is described by a bilinear uniaxial stress-strain law, with fracture of the cell walls characterised by the fracture energy per unit area. Estimates for the macroscopic toughness and the associated plastic zone shape are derived analytically on the basis of simple considerations. Crack propagation is simulated numerically by fracturing elements within a finite element model and K-resistance curves are calculated under the assumption of small-scale yielding. The dependence of the crack growth behaviour upon the cell wall material parameters and geometric imperfections of the structure is investigated.     

Strength and toughness properties of two nitrogen alloyed stainless steels and their submerged arc weldments at cryogenic temperatures

Submerged arc weldments of the two nitrogen-alloyed stainless steels X2 CrNiN 18 10 (similar to AISI 304 LN) and X4 CrNiMoN 18 14 (similar to AISI 316 LN) welded with the fully austenitic filler metal X2 CrNiMnMoN 20 16 were investigated. Tensile, impact toughness and single-specimen J-integral tests at room temperature, 77 K and 4 K were performed. The strength of the materials increased whereas the impact toughness and the fracture toughness decreased with decreasing temperature. The toughness of the steel X4 CrNiMoN 18 14 was superior to that of X2 CrNiN 18 10, and for the austenitic weld a good combination of strength and toughness was also found. On the fracture surface of the compact tension specimens, a stretch zone was found, the width of which was reduced with decreasing test temperature. For all three materials at all three temperatures the critical values, JIc, of the J integral determined according to ASTM E 813 are approximately twice the respective values for the J integral at physical crack initiation determined according to the German specification DVM 002 using the width of the stretch zone. © 1998 Chapman & Hall     

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.     

Influence of polycrystal grain size on fracture toughness of and fatigue threshold in Armco iron

Influence of polycrystal grain size on ductile fracture toughness of and fatigue threshold stress intensity in Armco iron has been studied over a grain size range 40 to 1050 μm. Both ductile fracture toughness and fatigue threshold stress intensity have been found to decrease with increasing grain size and the variation in either case follows a relationship similar to that proposed by Hall-Petch for strength. The variation of toughness with grain size can be understood in terms of plastic zone size whereas the fatigue threshold behaviour in Armco iron appears to be controlled by the critical value of crack tip opening displacement range.     

Effect of strength mis-match and dynamic loading on ductile fracture initiation

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using a two-parameter criterion based on equivalent plastic strain and stress triaxiality.The present study focuses on the effects of geometrical discontinuity, strength mis-match, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on the critical condition for ductile fracture initiation using a two-parameter criterion. Fracture initiation testing has been conducted under static and dynamic loading using circumferentially notched round-bar specimens. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal elastic-plastic dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out.The tensile tests on specimens with an undermatching interlayer showed that the relationship between the critical equivalent plastic strain to initiate ductile fracture and stress triaxiality was equivalent to that obtained on homogeneous specimens under static loading. Moreover, the two-parameter criterion for ductile fracture initiation is shown to be independent of the loading rate. It was demonstrated that the critical global strain to initiate ductile fracture in specimens with strength mis-match under various loading rate can be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mis-match and dynamic loading effects on stress/strain behaviors.     

Modeling ductile to brittle fracture transition in steels—micromechanical and physical challenges

Both scientists and engineers are very much concerned with the study of ductile-to-brittle transition (DBT) in ferritic steels. For historical reasons the Charpy impact test remains widely used in the industry as a quality control tool to determine the DBT temperature. The transition between the two failure modes, i.e. brittle cleavage at low temperature and ductile fracture at the upper shelf occurs also at low loading rate in fracture toughness tests. Recent developments have been made in the understanding of the micromechanisms controlling either cleavage fracture in BCC metals or ductile rupture by cavity nucleation, growth and coalescence. Other developments have also been made in numerical tools such as the finite element (FE) method incorporating sophisticated constitutive equations and damage laws to simulate ductile crack growth (DCG) and cleavage fracture. Both types of development have thus largely contributed to modeling DBT occurring either in impact tests or in fracture toughness tests. This constitutes the basis of a modern methodology to investigate fracture, which is the so-called local approach to fracture. In this study the micromechanisms of brittle cleavage fracture and ductile rupture are firstly shortly reviewed. Then the transition between both modes of failure is investigated. It is shown that the DBT behavior observed in impact tests or in fracture toughness specimens can be reasonably well predicted using modern theories on brittle and ductile fracture in conjunction with FE numerical simulations. The review includes a detailed study of a number of metallurgical parameters contributing to the variation of the DBT temperature. Two main types of steels are considered : (i) quenched and tempered bainitic and martensitic steels used in the fabrication of pressurized water reactors, and (ii) modern high-toughness line-pipe steels obtained by chemical variations and optimized hot-rolling conditions. An attempt is also made to underline the research areas which remain to be explored for improving the strength-toughness compromise in the development of steels.     

Preparation, microstructure and mechanical properties of metal-particulate/glass-matrix composites

Composites with a borosilicate glass matrix containing different concentrations of vanadium particles were fabricated by powder metallurgy and hot-pressing. The mechanical properties and fracture behaviour of the composites were assessed by a range of techniques. Young's modulus, fracture strength in bending, and fracture toughness increased with vanadium content. By virtue of the good interfacial bonding and low residual internal stresses, an effective crack-particle interaction during fracture was achieved. The fracture toughness of composites containing 30 vol. % of vanadium inclusions was approximately 65 % higher than that of the unreinforced glass. Experimental values for the fracture toughness increment were in good qualitative agreement with the predictions of theoretical models in the literature. Extensive plastic deformation of the vanadium inclusions was not found, however. This was attributed mainly to the constraint imposed by the rigid matrix surrounding the particles and to possible embrittlement of the particles during composite fabrication at high temperatures. The brittleness index (B) of the composites was calculated and its relevance for characterisation of the ductile versus brittle behaviour of brittle-matrix composites is discussed.     

STRUCTURAL COLLAPSE, THE FAILURE ASSESSMENT DIAGRAM AND "OFF-THE-SHELF" OPERATION

Abstract— The criteria for determining whether ferritic material exhibits fully ductile behaviour are generally based on the fracture toughness vs temperature relationship determined from standard laboratory test pieces (e.g. Charpy V Impact tests or fracture toughness specimens). This relationship defines a ductile-brittle transition region. When fully ductile, microvoid coalescence behaviour is experienced, and fracture toughness is described as being on the “upper shelf”. At “off the shelf” temperatures brittle, cleavage fracture is experienced. On the lower shelf the material is entirely brittle, exhibiting 100% crystallinity on the fracture face. As the temperature increases, initiation of tearing by microvoid coalescence occurs and some stable tearing can occur prior to the cleavage event. Material toughness increases with temperature until the upper shelf condition is achieved. The characteristics of fracture toughness tests in terms of the toughness level exhibited and the extent of ductile tearing experienced have been used as a guide to whether the structural application (e.g. a pressure vessel) will behave in a brittle or a ductile manner. This paper reports on a feasibility study where various worked examples have been performed to examine the concept of using a “cut off” on the failure assessment diagram, determined from the conditions required to cause plastic collapse of a pressure vessel, as a criterion for defining effective “ductile” operation. Fracture assessment procedures (R6 revisions 2 and 3 and PD6493 levels 2 and 3) have been utilised to determine the influence on pressure vessel performance of the behaviour of fracture toughness test specimens. The procedure of plotting a structural collapse “cut off” on a failure assessment diagram enables the assessment of whether a particular flaw geometry would result in gross deformation of the structure at failure. The use of this procedure provides an unambiguous demarcation between “fracture dominated” and “collapse controlled” conditions. This procedure facilitates judgements on the level of toughness necessary to ensure ductile operation and whether a “tearing plus toughness” requirement is necessary. It is recommended that consideration be given to including structural collapse into fracture assessment procedures carried out using R6 revision 3 or PD 6493:1991 procedures in order to determine the conditions when enhanced toughness no longer influences structural performance (i.e. when effective “upper shelf” conditions are attained).     

A simple damage-accumulation model for constraint effects on ductile fracture

A simple model is presented to account for the effects of void-type damage on crack initiation and propagation in ductile steels under plane strain conditions by virtue of elementary fracture mechanics solutions. Multiple primary voids from large inclusions are uniformly distributed ahead of the crack tip. The growth of these primary voids is followed by nucleation of a large population of secondary voids from second-phase particles. A critical accumulative damage based on the length ratio of the damage zone to the spacing of primary voids, is employed as a failure criterion, including contributions from two populations of voids. Damage accumulation depends much on the strain and stress states such as stress triaxilities, which are extracted from existing results instead of detailed computation. Results show the dependence of fracture toughness on the size of damage zones associated with constraints. Initiation of crack growth is insensitive to the constraints since nucleation of fine voids is determined by local deformation. The model captures the transition in mechanisms from void-by-void growth to multiple void interactions in terms of a decreasing trend in the slopes of fracture resistance curves. At high constraints and large damage zone, a steady-state crack advance is identified with constant toughness. Damage accumulation from the growth of primary voids determines subsequent crack growth resistance and the study demonstrates its dependences on the crack-tip constraints.     

Dynamic fracture toughness at crack initiation, propagation and arrest for two pipe-line steels

The influences of temperature and loading rate on fracture toughness of two pipe-line steels at initiation, K_1d, and at arrest, K_1a, and on stretch zone height were measured using specific techniques. Particular attention was given to the mechanism of delamination, typical for the gas internal pressure pipe fracture.     

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.     

渗碳钢扭转变形时断裂行为和渗层厚度对接触疲劳的影响

本文以20CrMnMo 钢为对象比较全面地研究了不同渗碳层厚度、渗碳层碳含量以及不同心部组织对扭转流变硬化行为和断裂特征的影响。结果表明,渗碳件的屈服强度和流变硬化行为主要取决于心部组织。在心部组织相同的条件下,渗碳层的成分、组织和渗层厚度主要影响渗碳件的断裂应变,从而影响其断裂韧度。此外,在材质相同、强度水平相近的条件下,渗碳件的接触疲劳寿命随渗层厚度增加而增大的现象,从力学性能角度看是由于断裂应变随渗层厚度增加而增大,使断裂韧度增大所致,其原因与残余压力大小分布随渗层厚度变化有关。