An Experimental and Numerical Study of the Effect of Pre-Strain on the Fracture Toughness of Line Pipe Steel

New and existing pipelines can be subjected to high plastic strains. Denting a pipeline causes permanent plastic deformation. Onshore pipelines subject to subsidence, frost heave or earthquake loading can experience significant plastic strain during service. Offshore pipelines that are reeled prior to laying, or are laid in deep water, or are operating at high temperatures and high pressures, can experience significant plastic strain both prior to, and during, service. Experimental studies have indicated that pre-strain (permanent plastic deformation) has a detrimental effect on the fracture toughness of steel; it reduces the resistance to crack initiation, reduces the resistance to crack growth, and increases the transition temperature. Consequently, there is a need for a thorough understanding of the effect of pre-strain on the fracture toughness of line pipe. Accordingly, a theoretical model for predicting the effect of tensile pre-strain on the ductile fracture toughness has been developed using the local approach. The effect of pre-strain is expressed in terms of an equation for the ratio of the fracture toughness of the pre-strained material to that of the virgin (not pre-strained) material. The model indicates that the effect of tensile pre-strain on the material’s fracture toughness can be characterised in terms of the effect of pre-strain on the stress-strain characteristics of the material, the critical fracture strain for a stress state corresponding to that during pre-strain, and several parameters that relate to the conditions for ductile fracture (or cleavage fracture). The implications of the model are that it may be possible to estimate the reduction in toughness caused by pre-strain simply from a full stress-strain curve of the virgin material. The model has been validated against the results of crack tip opening displacement (CTOD) tests conducted by Tokyo Gas on two line pipe steels subject to uniaxial tensile pre-strain. It is shown that the predictions and trends of the theoretical model are in broad agreement with the test results.

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

Prediction of temperature and prestraining effects on fracture toughness of high-strength structural steel by the local approach

Effect of prestrain on fracture toughness of HSLA steels

A simple theoretical model for predicting the effect of tensile pre-strain on fracture toughness has been developed using the local approach. The HRR singularity is assumed to describe the stress-strain field around the crack tip. A stress-modified critical strain-controlled model is assumed to describe ductile fracture (and a critical stress-controlled model for cleavage fracture). The Rice and Tracey void growth model is used to characterize the variation of the critical strain with the stress state. The model further assumes that the fracture process does not change with increasing pre-strain. The effect of pre-strain is expressed in terms of an equation for the ratio of the fracture toughness of the pre-strained material to that of the virgin material. The model indicates that the effect of tensile pre-strain on fracture resistance can be characterized in terms of the effect of pre-strain on the stress-strain characteristics of the material, the critical fracture strain for a stress state corresponding to that during pre-strain, and several parameters that relate to the conditions for ductile fracture (or cleavage fracture). Previous experimental studies of the effect of pre-strain on toughness are summarized and compared with the predictions of the model.

The effects of prestrain on the fracture toughness of steels are examined by using a simple cumulative damage model of microvoid growth and coalescence. First the effects of a shear prestrain on the mode III toughness at a single temperature are calculated in order to obtain guidance on how best to approximate the stress—strain curve after pre-strain. Then the models are used to examine the influence of a uniaxial pre-strain on the values of mode I toughness on the upper shelf. The mode I analyses require crack tip blunting solutions, and it is demonstrated that an approximate blunting solution enables the models to be applied readily. Numerical results for various amounts of pre-strain are calculated by taking material properties typical of an A533B steel. It is found that the effects are generally small for the material properties considered.

Grain refinement by rapid cyclic heating and its effect on cleavage fracture behaviour of an S690 high strength steel

On the occasion of recent great earthquakes, great concern is focused on the prevention of unstable fracture of steel structures against the seismic loading. This paper employs the local approach for the evaluation of prestraining and dynamic loading effects, experienced during an earthquake, on the fracture toughness of structural steels. The prestraining and dynamic loading lead to a similar result: increasing the yield stress and tensile strength and decreasing the fracture toughness. It is shown, however, that the combined effects of prestraining and dynamic loading is not equivalent to the sum of each individual effect. The analysis using the local approach demonstrates that the critical Weibull stress at brittle fracture initiation is independent of prestraining and dynamic loading. Based on the Weibull stress fracture criterion, the prestraining and dynamic loading effects on the fracture toughness can be predicted from static toughness results of the virgin material. As an engineering application, a simplified method is proposed for the estimation of fracture toughness under the seismic condition. This method uses a reference temperature concept: the dynamic fracture toughness at the service temperature T with prestrain is displaced by the static toughness of the virgin material at a lower temperature T−ΔTPD, where ΔTPD is a temperature shift of the fracture toughness caused by prestraining and dynamic loading. The temperature shift ΔTPD is provided as a function of the flow stress elevation in the seismic condition.

The fracture toughness (KIC) of high strength casing steel (1130 MPa) has been determined by a three-point-bend specimen. Three specimens with thicknesses of 5 mm, 8 mm and 10 mm, respectively have been used to observe the effect of thickness on the fracture toughness of steel. The analytical formula of the relationship between the fracture toughness and material thickness of high strength casing (1130 MPa) is proposed based on the energy theory and on linear elastic mechanics. The values of fracture toughness, K, measured by three thicknesses of parent metals are combined with the quantitative model of the relationship between fracture toughness and specimen thickness. The material constant (and κ) are calculated by using the least squares method. Furthermore, an analytic formula for the relationship between fracture toughness and specimen thickness is proposed which allows for the calculation of the KIC value. Plane-strain fracture toughness (KIC = 133.94 MPa × m1/2) is a fixed value only if the thickness of the specimen exceeds 43 mm. Fracture toughness values of different thicknesses of high strength casing are derived by fracture toughness tests of several thicknesses based on the model in question in order to save manpower and material resources. This method is also significant for measuring the fracture toughness of other OCTG, since their thicknesses do not meet the requirement of the standard test methods. For the present, it provides general methods and reference rules for obtaining the KIC of other metallic OCTG.

Fracture toughness (K1C) of medium carbon steel (0.5% C) has been determined by round notched tensile specimen. Two notch diameters (5.6mm and 4.2mm) and three notch angles (α) namely 45°, 60° and 75° have been used to observe the effect of notch diameters and notch angle on fracture toughness of the steel. By heat treatment the microstructure of the steel is also varied and its effect on the fracture toughness is also observed. It has been found that fine grained structure improves fracture toughness. Lower notch diameter and higher notch angle show higher value of K1C. Keywords: Fracture toughness, microstructure, notch, heat treatmentDOI: 10.3329/jname.v3i1.925 Journal of Naval Architecture and Marine Engineering 3(2006) 15-22

The authors show that at high tensile strength fracture toughness is insensitive to steel cleanliness, and reduction of area is very sensitive to steel cleanliness. These conclusions are generally substantiated by work conducted at U. S. Steel. The U. S. Steel work amplifies that reported in the subject paper by examining the effect of residual elements (cleanliness) on the reduction of area and fracture toughness of 18Ni maraging steels and the effect of prior-austenite grain size and residual-element content on the reduction of area and fracture toughness of high-strength quenched and tempered alloy steels. The U. S. Steel work shows that for the maraging and alloy steels studied, reduction-of-area values are very sensitive to residual-element level but not affected much by tensile-strength levels in the range 230–300 ksi. The fracture toughness of the maraging steels shows a sensitivity to residual-element level that decreases as the strength is raised. The steels appear to reach a point of complete insensitivity to residual-element level at a tensile strength of 310 ksi. At this strength, the fracture toughness approaches a KIc value of 50 ksi( in.)12. The fracture toughness of the alloy steels is insensitive to residual-element level throughout the tensile-strength range 230–320 ksi, but is sensitive to prior-austenite grain size. The grain-size sensitivity decreases as the strength level increases and appears to reach a point of complete insensitivity at a tensile strength of about 310 ksi. Again, the fracture toughness of the steels at a tensile strength of 310 ksi approaches a KIc value of approximately 50 ksi in.12. It is concluded that lowering residual-element levels by vacuum melting should improve the fracture toughness of maraging steels at tensile strengths below 310 ksi but will probably not improve the fracture toughness of quenched and tempered alloy steel at tensile strengths in the range 230 ksi and higher. Lowering residual elements by vacuum melting improves the ductility, as measured by reduction of area, of both maraging and alloy steels over the entire range of strengths studied. Reduction of area is not very sensitive to tensile strength in the range studied.

The change of fracture toughness of martensitic steels after irradiation in SINQ target-3

The effect of tensile prestrain on the ductile fracture behavior of an interstitial-free (IF) steel has been studied using primarily (1) the analysis of void density by optical microscopy, (2) characterization of the size of dimples by scanning electron microscope (SEM) and image analyzer, and (3) estimation of strain hardening behavior of a series of prestrained tensile specimens, loaded until fracture. The variation of void density with local plastic strain around the necked region of the specimens indicated the existence of two types of void nucleation pertaining to inclusions and precipitate particles. The critical strain for void nucleation (e{inn}) for the precipitate particles initially increases and then decreases with the increase in percentage prestrain. This phenomenon has been explained using the strain hardening exponent and nature of dislocation-particle interaction. The nature of variation of the average size of the dimples and that of e{inn} with prestrain are found to be similar. The dimple size thus bears a proportional relationship with the void, nucleation strain e{inn} and hence the former can be used to predict (e{inn}) for IF steel.

Effect of pre-strain on the indentation fracture toughness of high strength low alloy steel by means of continuum damage mechanics

The aim of the current study is to design multiaxial forging (MAF) schedules in order to achieve submicron-grained (<1μm) structure in a microalloyed (MA) steel as well as an interstitial-free (IF) steel, which could impart a good combination of yield strength and tensile ductility. At the same time, an effort has been made to evaluate the fracture toughness characteristics by conducting 3-point bend tests and computing the KQ, Kee and J-integral values of ultrafine grained (UFG) samples and correlating them with the microstructure, besides evaluating the other mechanical properties. Fatigue strength in the high cycle fatigue (HCF) regime were also investigated and fracture mechanisms analyzed and comparison established between differently processed samples. The microstructural analysis was performed using transmission electron microscopy (TEM) and Electron backscatter diffraction (EBSD) and results corroborated with the mechanical properties. Superior combinations of yield strength (YS), ductility (% El.), fracture toughness (Kee) and high cycle fatigue strength (σf) were obtained under certain conditions, i.e., i) MA steel: intercritical (α+γ) phase regime (~Ar1) controlled and 15-cycle multiaxially forged (MAFed) (YS=1027MPa, %El.=8.3%, σf=355MPa and Kee=90MPa√m), and ii) IF steel: ferritic region (<Ar1) controlled 18-cycle MAFed (YS=881MPa, %El.=11.2%, σf=255MPa and Kee=97MPa√m). In the case of MA steel, an enhancement of the fatigue and fracture toughness properties can be ascertained following the formation of uniformly distributed nanosized fragmented cementite (Fe3C) particles (~35nm size) present in the submicron sized (average ~280nm size) ferritic microstructure. In contrast, in the case of IF steel, this is ascribed to the development of submicron sized ferrite grains (average ~320nm) along with a high density of dislocation substructures. These fine dislocation cells/substructures along with the nanosized Fe3C particles could effectively block the initiation and propagation of cracks and thereby enhance the fatigue endurance and fracture toughness of the steel. Superior fracture toughness along with high mechanical properties in submicron-grained condition render the two steels highly useful for high-strength structural applications.