Fatigue crack growth retardation following load reductions in a plain C-Mn steel

Transient fatigue crack growth effects following perturbations in applied loading conditions have been studied using a low strength plain C-Mn steel. Reductions in both the alternating and maximum components of the fatigue loading spectrum have been systematically investigated and evaluated using a linear elastic fracture mechanics approach. Results are discussed in terms of the residual stress concept, and a model based on an effective stress intensity concept is propossed to rationalise growth rates within the retardation affected zone. Using constant amplitude fatigue threshold and crack growth data obtained under similar conditions, the model is shown to be in good agreement with experimental data.     

Effect of specimen size and geometry on ductile crack growth resistance in a C-Mn steel

The effect of specimen size and geometry on the ductile crack growth resistance of a C-Mn steel has been investigated. The resistance, expressed in the form of J-R curves, was measured using the conventionally calculated J(J_r);(b/J×dJ/da) values. The results show that for specimens of given thickness the effect of specimen geometry was due to a change in the shear lip and not to the flat fracture contribution. This conclusion is consistent with the geometry invariance found in measurements of the crack opening displacement at the growing crack tip in the flat fracture region. In addition, it has been shown that J _m can characterise crack growth resistance over a wider range of conditions than J _r.

---

Résumé On a étudié les effets de la taille et de la géométrie d'une éprouvette sur la résistance à la croissance d'une fissure ductile dans un acier au C-Mn. Exprimée sous la forme de courbes J-R, la résistance a été mesurée par des valeurs de J calculées par voie conventionnelle (J _r) et par la méthode de Ernst modifiée (J _m), pour une gamme de géométries d'éprouvettes sollicitées en flexion et en traction. La résistance globale d'éprouvette ne présentant pas d'entaille latérale a été divisée en deux contributions, qui s'experiment par les zones de ruptures plates et de lèvres de cisaillement, que l'on trouve sur les surfaces de rupture.Grâce à une telle approche, on a ramené à des grandeurs rationnelles la taille et la géométrie d'une éprouvette, et on a déterminé les limites de croissance d'une fissure régies par J _r et J _m en termes de l'extension maximale admissible d'une fissure, et des valeurs minimales de (b/j×dJ/da).Les résultats indiquent que, pour des éprouvettes d'épaisseur donnée, l'effet de la géométrie d'une éprouvette est associé à une modification de la contribution des lèvres de cisaillement, et non de la portion plate de la rupture. Cette conclusion est compatible avec la constance de la géométrie que l'on observe dans des mesures du déplacement d'ouverture de la fissure à la pointe de la fissure en progrès dans la zone de rupture plate. En outre, on a montré que J _m peut caractériser la résistance à la croissance d'une fissure, sur une gamme plus large de conditions que ne le fait J _r.

     

Effect of niobium on creep and creep crack growth of cast Ni–Cr austenitic steel

Abstract

An investigation of the effect of Nb on creep properties and creep crack growth rate in a 25Cr–35Ni–0·4C (wt-%) cast steel at 871 and 950°C was carried out. Tensile tests were also carried out at room temperature, 871, and 950°C. The tensile strength and elongation increased with an increase in Nb content at high temperatures. There existed an optimum Nb content for the creep properties and creep crack growth rate. Creep crack growth is controlled by creep deformation.

MST/1222     

Mismatch effect in creep properties on creep crack growth behavior in welded joints

The finite element method based on ductility exhaustion model was used to systematically investigate the mismatch effect in creep properties on creep crack growth (CCG) behavior in welded joints. The crack-tip damage, stress states, CCG paths, CCG rate and rupture life were calculated for different configurations of creep properties between weldment constituents under the same load level, and the creep life assessment and design for welded joints were discussed. The results show that when the zone containing the crack is softer than at least one of the other two surrounding materials or both, the creep crack propagates straight along the initial crack plane. Otherwise, it will form a second crack in the soft material near interface. These simulation results were confirmed by the experimental observations in the literature, and the mechanism was analyzed. The harder surrounding materials can lead to higher CCG rate and shorter rupture life due to the higher constraint given from them. The early initiation and propagation of the second cracks increase CCG rate and reduce rupture life, and the incubation time of the second cracks in soft materials near interfaces should be accurately determined in the creep life assessment and design for the welded joints. A proper mismatch design with harder material containing crack and softer surrounding material can improve CCG properties of welded joints (decreasing CCG rate and prolong rupture life).     

Prediction of creep crack growth properties of P91 parent and welded steel using remaining failure strain criteria

Old grades of creep resistant materials such as P11 and P22 have been studied in depth and data and prediction models are available for design and fitness for service assessment of creep rupture, creep crack growth, thermo-mechanical fatigue, etc. However, as the 9%Cr material is relatively new, there is relatively limited data available and understanding with respect to quantifying the effect of variables on life prediction of components fabricated from P91 is more difficult. Since grade P91 steel was introduced in the 1980s as enhanced ferritic steel, it has been used extensively in high temperature headers and steam piping systems in power generating plant. However, evidence from pre-mature weld failures in P91 steel suggests that design standards and guidelines may be non-conservative for P91 welded pressure vessels and piping. Incidences of cracking in P91 welds have been reported in times significantly less than 100,000 h leading to safety and reliability concerns worldwide. This paper provides a review and reanalysis of published information using properties quoted in codes of practice and from recent research data regarding the creep crack growth of P91 steel, and uses existing models to predict its behaviour. Particular areas where existing data are limited in the literature are highlighted. Creep crack growth life is predicted based on short-term uniaxial creep crack growth (CCG) data. Design and assessment challenges that remain in treating P91 weld failures are then addressed in light of the analysis.     

Size effect on creep energy dissipation during stable crack growth

Creep energy dissipation (CED) due to crack tip creep zone is determined under steady state conditions for centre cracked thin specimen of 1.25CrO.5Mo steel, with different widths and crack length to width ratios. The creep energy rate increases when the increment in crack length Δa increases and the slope increases with increase in aspect ratio a/W and decreases with increase in width W.     

Experimental investigation of specimen size effect on creep crack growth behavior in P92 steel welded joint

In order to clarify the effect of constraint induced by specimen size on creep crack growth behavior of P92 steel welded joint, creep crack tests were carried out on the compact tension specimens with thick thickness and thin thickness, the crack tip of which were located at different distinct zones of welded joint. Tested results revealed that even in thin thickness specimens, fine grained heat affected zone specimens exhibited a fast creep crack growth rate compared with other micro-zones specimens due to a low creep crack resistance and a high multistress state. The fractographies of these specimens exhibited an accelerated number of spherical particles that were caused by the coalescence of creep voids. Furthermore, the correlation of C^* with creep crack growth rate was dependent on specimen thickness. As the specimen thickness increased from 10 to 30 mm, the creep crack growth rate increased. This was due to the increase in constraint level ahead of crack tip during creep crack propagation.     

Theoretical and numerical modelling of creep crack growth in a carbon-manganese steel

This paper presents an analytical and numerical study of time dependent crack growth at elevated temperatures. A triaxiality dependent damage model is used to represent the multiaxial creep ductility of the material and an analytical model to predict steady state crack growth in terms of the fracture parameter C^∗, designated the NSW-MOD model, is presented. This model is an enhancement of the earlier NSW model for creep crack growth as it accounts for the dependence of stress and strain on angular position around the crack tip. Elastic-creep and elastic-plastic-creep finite element analyses are performed for a cracked compact tension specimen and the crack propagation rate in the specimen is predicted. It is found that in general the NSW-MOD model gives an accurate estimate of the crack growth rate when compared to the finite element predictions and experimental data for a carbon-manganese steel. However, crack growth rates predicted from the finite element analysis at low values of C^∗ may be higher than those predicted by either the NSW or NSW-MOD model. This enhanced level of crack growth may be associated with the non-steady state conditions experienced at the crack tip.     

Some observations on creep crack growth

Equations for the steady state motion of a crack in a creeping structure are investigated. Although general solutions of these equations are not attempted, it is shown that under certain circumstances particular solutions described by the stress intensity factorK or the stationary creep parameterC* are valid. The crack velocities for which these particular solutions hold are determined. A simple formula for estimatingC* is given. Experimental evidence is examined for its relevance toK-controlled orC*-controlled crack growth.

---

Résumé On étudie des équations décrivant le mouvement quasi statique d'une fissure dans une structure en cours de fluage. Même si l'on ne tente pas d'établir des solutions générales pour ces équations, on peut montrer que, sous certaines circonstances, des solutions particulières décrites par le facteur d'intensité de contrainteK ou par le paramètreC de fluage stationaire sont applicables. Les vitesses de fissuration, pour lesquelles ces solutions particulières conviennent, sont déterminées. On fournit une formule simple pour estimerC. Des observations expérimentales sont analysées en ce qui regarde leur relation avec une propagation de fissure sousK contrôlé on sousC contrôlé.

     

Damage mechanics based predictions of creep crack growth in 316 stainless steel

This paper describes a novel modelling process for creep crack growth prediction of a 316 stainless steel using continuum damage mechanics, in conjunction with finite element (FE) analysis. A damage material behaviour model, proposed by Liu and Murakami [1], was used which is believed to have advantages in modelling components with cracks. The methods used to obtain the material properties in the multiaxial form of the creep damage and creep strain equations are described, based on uniaxial creep and creep crack growth test data obtained at 600 °C. Most of the material constants were obtained from uniaxial creep test data. However, a novel procedure was developed to determine the tri-axial stress state parameter in the damage model by use of creep crack growth data obtained from testing of compact tension (CT) specimens. The full set of material properties derived were then used to model the creep crack growth for a set of thumbnail crack specimen creep tests which were also tested at 600 °C. Excellent predictions have been achieved when comparing the predicted surface profiles to those obtained from experiments. The results obtained clearly show the validity and capability of the continuum damage modelling approach, which has been established, in modelling the creep crack growth for components with complex initial crack shapes.     

Macroscopic creep crack growth in type 316 stainless steel.: II. effect of geometric constraint

The effects of geometrical constraint on notched bar rupture properties of three casts of Type 316 steel at 625°C have been examined by varying the notch shape and depth in Double Edge Notched tension specimens. These three casts represented a wide range of material behaviour, from strongly notch strengthening to notch weakening. It was found that increased geometrical constraint generally produced decreased displacement rates and longer rupture lives in these ductile materials. However, geometrical constraint was found to be detrimental to rupture life in a cast of steel which showed anomalous notch weakening. This has been rationalised by a concept termed “total constraint,” which is interpreted as a combination of geometrical and microstructural constraint.     

Load history effects on creep crack growth

In this paper the effects of load history on the high-temperature creep crack growth process are studied through a combined experimental and computational approach. The general features of constitutive response during cyclic creep are reviewed. Next, fracture parameters for creep crack growth are reviewed, with special emphasis on integral parameters. Finally, examples comparing computational predictions of experiments which experience history dependent load histories are presented. This includes displacement time comparisons and fracture parameter comparisons.     

Application of a mechanism-based creep damage model to creep crack growth in a 12% chromium steel

In creep resistant steels, several mechanisms contribute to the degradation of creep properties under long-time service conditions. Most important are the coarsening of the carbide and subgrain structure and grain boundary cavitation. A mechanism-based creep damage model is developed, including a Chaboche type viscoplastic model¹ for transient creep, the Rodin and Parks model² for cavitation damage and a microstructurally motivated softening equation. The model parameters are adjusted to a set of creep curves for a 12% chromium steel (X20 CrMoV12 1) and to quantitative microstructural measurements. The combined model is implemented in the finite element code ABAQUS. Tests on compact specimens are successfully modeled.     

Effect of microstructure degradation on creep crack growth

Creep crack growth by grain boundary cavitation is analysed numerically for center cracked and edge cracked panels. Creep acceleration induced by microstructure degradation is incorporated in the material model that describes the nucleation and growth of cavities in the grain boundary, including the effect of diffusion, dislocation creep and grain boundary sliding. It is found that the creep acceleration significantly reduces the notch sensitivity of the material.

---

Résumé On analyse par voie numérique la croissance d'une fissure de fluage par cavitations aux joints de grain dans le cas de plaques fissurées en leur centre ou sur leurs bords. Un modèle du matériau, qui incorpore l'accélération du fluage due à sa dégradation, décrit la naissance et la croissance des cavités aux joints de grains, en tenant compte de l'effet de la diffusion, du fluage associé aux dislocations et du glissement des joints de grains.On trouve que l'accélération du fluage réduit de manière significative la sensibilité à l'entaille du matériau.

     

Mean stress effects on fatigue crack growth and failure in a rail steel

Over a limited range, the effect of mean stress has been studied on fatigue crack propagation and on the critical fatigue crack size associated with sudden fast fracture in centre-notched plate specimens of a rail steel under pulsating loading. The results have been presented in terms of the stress intensity factor range $\text{ΔK}$ and the ratio $\text{R}$ of the minimum to maximum stress. Increasing $\text{R}$ was found to both accelerate cracking and reduce the critical crack size at instability. The data have been correlated with three crack growth equations currently used in the literature and it was found that the equation of Forman et al. relating crack growth rate to $\text{ΔK}$ and $\text{R}$ gave the best fit. This equation was used to predict life in the finite range of the S-N curve. Fractographic examination revealed that the fracture surfaces were complex and a number of fracture modes contributed to cracking.     

Specimen geometry effect on creep crack growth in 316L(N)

The ASTM E1457‐98 standard describes the procedure to determine the master curve da/dt versus C* parameter, for creeping solids. However, the methodology is only to be applied to compact tension (CT) specimens. The European collaborative program CRETE aims at extending the application of the ASTM E1457‐98 standard to other types of laboratory specimens. In this paper, an existing database of creep crack growth on 316L(N) stainless steel is utilized, concerning three types of specimens: circumferentially cracked round bars (CCRBs) and double edge notched tensile (DENT) specimens for tensile mechanical loading whereas the classical CT specimen combines tensile and bending loading modes. A modified procedure based on the ASTM E1457‐98 standard has been applied to the database, resulting in a unique master curve of da/dt versus C*. The geometry effect is then investigated by introducing the Q* parameter by analogy to the Q parameter in the elastic–plastic J‐Q approach.     

Analysis of the microstructure near the crack tip of ASME Gr.92 steel after creep crack growth

The microstructures near to and remote from the tip of a crack in ASME Gr.92 steel were investigated after creep crack growth at 873 and 898 K, focusing on the martensitic lath, the dislocation structure, and precipitates. After creep, the mean lath width near the crack tip was obviously larger than that of the virgin material, whereas the lath width remote from the crack tip was only slightly larger than that of the virgin material. The mean dislocation density near the crack tip markedly decreased after creep, whereas only a small change was observed in the dislocation density remote from the crack tip. The mean size of M₂₃C₆ particles near the crack tip after creep was larger than that of the virgin material, whereas their mean size remote from the crack tip was almost the same as that of the virgin material.