Creep crack growth in ductile,creep-resistant steels

Creep crack growth in two of the commonly used creep-resistant ferritic steels was investigated. Both steels were tested in the as-processed condition and after many years of service in electric power plants. The test temerature was 540°C (in one case also 500°C and 565°C) and test durations ranged from a few days to a year. In 1/2Cr-1/2Mo-1/4V steel, crack growth occurred intergranularly by grain boundary cavitation, while 21/4Cr-1Mo steel also exhibited transgranular growth. The deformation behavior of CT specimens was analyzed in detail. It was found that for typically 20 percent of the lifetime, the instantaneous elastic-plastic strains and, more importantly, primary creep determine the deformation response. The evolution of the crack length parallels that of the load-line deflection: Initially, the crack grows relatively fast, decelerates until a steady growth rate is reached, and finally accelerates due to the increasing stress intensity at the longer crack. This behavior can be described reasonably accurately by models for creep crack growth taking into account the elastic-plastic transient and primary creep. It is demonstrated that the C _t parameter correlates crack growth rates during the transients, whereas C ^* becomes the appropriate load parameter during steady-state creep. However, C ^* fails to describe crack growth in single-edge notched tension specimens with shallow cracks. This is tentatively ascribed to excessive crack-tip blunting and other geometry changes. Unloading and reloading frequently leads to accelerated crack growth.

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Résumé On a étudié la croissance de fissures de fluage dans deux des aciers ferritiques résistant au fluage les plus communément utilisés. Les deux matériaux ont été essayés à l'état de livraison et après plusiers années de service en centrale électrique. La température d'essais était de 540°C (avec, dans un cas, des valeurs de 500°C et 565°C) et la durée d'essais variait de quelques jours à une année. On a constaté que dans l'acier au 1/2 Cr-1/2Mo-1/4V, la fissure s'est propagée entre les grains par un phénomène de cavitation aux joints de grain. Par contre, l'acier 2 1/4 Cr 1 Mo a fait également état d'une croissance transgranulaire de la fissure. En analysant dans le détail le comportement à la déformation d'éprouvettes CT, on a trouvé que, sur une période de temps typiquement égale à 20% de la durée de vie, la réponse à la déformation est déterminée par les déformations élasto-plastiques instantanées, et, surtout, par le fluage primaire. La longueur de la fissure suit une évolution parallèle à la courbe d'évolution de la charge. A l'origine, la croissance de la fissure est relativement rapide, puis elle ralentit jusqu'à atteindre une vitesse constante, pour enfin accélérer à nouveau en raison l'accroissement de l'intensité de la contrainte agissant sur une fissure de plus en plus large. Un tel comportement peut être décrit de manière raisonnablement précise par des modèles de croissance des fissures de fluage qui prennent en compte la transitoire élasto-plastique et la fluage primaire. On dèmontre que le paramètre Ct est en correlation avec les vitesses de croissance de la fissure durant les transitoires, et que l'intégrale C^* devient le paramètre de charge le plus approprié au cours du fluage stable. Toutefois, C^* ne peut décrire la croissance d'une fissure dans une éprouvette à simple entaille latérale comportant des fissures peu profondes. On pense que cela est dû à un arrondissement excessif de l'extrémité des fissures et à d'autres modifications géométriques. On constate également que la croissance d'une fissure s'accélère lorsque l'on procède fréquemment à un déchargement suivi d'une remise en charge.

     

Effect of crack depth on the shift of the ductile-brittle transition curve of steels

Nuclear reactor pressure vessel (RPV) steels degrade due to neutron irradiation during normal operation. As a result, the ductile-brittle transition curve of the steel shifts to higher temperature which decreases operation margins in both the temperature and pressure. The loss of these margins however can be offset somewhat by appealing to arguments based on constraint of potential/postulated shallow cracks. In this paper, it is demonstrated that the fracture toughness values for shallow flaws are higher than those determined from standard deep cracked test specimens based on constraint consideration. The J-A₂ three-term solution is used to characterize the crack-tip stress field where J represents the level of loading and A₂ quantifies the level of constraint. Based on the RKR cleavage model, procedures to quantify the temperature shift between specimens with different constraint levels are developed. The experimental data by Sherry et al. [Sherry AH, Lidbury DPG, Beardsmore DW. Validation of constraint based structural integrity assessment methods. Final report, Report No. AEAT/RJCB/RD01329400/R003, AEA Technology, UK, 2001] for the A533B RPV steel are used to demonstrate the procedure and it is shown that the ductile-brittle transition curve shifts to lower temperature from high constraint to low constraint specimens.     

Stable crack growth in ductile polymers

Growth of a crack in ductile polymers (polyarylate, PTFE, and PU/PMMA blends) was studied. The crack growth was described assuming that local yielding in the crack tip is similar to large-scale shear yielding in rigid-plastic materials. Crack growth was stable, and a wedge shaped crack tip was formed. The crack tip opening displacement and the crack extension in the initial stage of the crack growth were proportional to the square of the strain up to 11% elongation. Dugdale’s equation was modified to describe the magnitude of the crack tip opening. In PA, a yield subzone near the crack tip was observed. This revised version was published online in November 2006 with corrections to the Cover Date.     

Usage of the small-punch-test for the characterisation of reactor vessel steels in the brittle–ductile transition region

This paper presents a method for the identification of hardening parameters and Weibull-parameters in the brittle and brittle–ductile transition region. A small-punch-test device is developed for hot cells, where a miniaturised disk-like specimen is manufactured and deformed by a spherical punch until failure. Its load–displacement-curve is analysed regarding its information about the material behaviour. Using neural networks, an identification routine is developed, which avoids time-consuming calculations with FEM during an optimisation algorithm. Identified material properties are compared with data from tensile tests. Using the identified hardening parameters, Weibull-parameters are calculated for temperatures at which cleavage fracture occurs.     

The transition from ductile to slow crack growth failure in a copolymer of polyethylene

The failures of ethylene-hexene copolymer single-edge notch tensile specimens were observed under a constant tensile load. The notch opening was measured against time over a range of stress. Three failure modes were observed: ductile, brittle and transitional. The microscopic changes at the notch tip were correlated with each of the modes of failure. Early in the test the ultimate mode of failure can be predicted from the microstructural changes in the notch. In the transition region, the lifetime increases as the stress increases because the blunting of the notch offsets the effect of the applied stress-stress field. The ductile failure is controlled by the macroscopic creep behaviour and the brittle failure occurs by slow crack growth that starts at a craze.     

Analysis of the effect of biaxial loading on the fracture toughness of reactor pressure-vessel steels

The stress-strain state at the crack tip and its relation to the crack opening displacement and the J-integral under biaxial loading have been studied by solving elastoplastic problems in a geometrically nonlinear formulation by the finite-element method. Numerical investigations have been performed for various cracks and two modes of biaxial loading (tension and bending) under conditions of both small-scale and large-scale yielding. For prediction of the influence of biaxial loading on fracture toughness (at brittle fracture) a procedure has been developed that is based on established laws of stress-strain state formation at the crack tip under biaxial loading and a criterion of brittle fracture proposed earlier. The effect of biaxial loading on fracture toughness is predicted as applied to reactor pressure-vessel steels. Calculated results are compared with avilable experimental data. Alternative approaches to prediction of the effect of biaxial loading on fracture toughness are discussed. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 5, pp. 5–26, September–October, 1999.     

Simulation of crack growth in ductile materials

During crack growth of real materials, the total energy released can be partitioned into elastic and dissipative terms. By analyzing material models with mechanisms for dissipating energy and tracking all energy terms during crack growth, it is proposed that computer simulations of fracture can model crack growth by a total energy balance condition. One approach for developing fracture simulations is illustrated by analysis of elastic-plastic fracture. General equations were derived to predict crack growth and crack stability in terms of global energy release rate and irreversible energy effects. To distinguish plastic fracture from non-linear elastic fracture, it was necessary to imply an extra irreversible energy term. A key component of fracture simulations is to model this extra work. A model used here was to assume that the extra irreversible energy is proportional to the plastic work in a plastic-flow analysis. This idea was used to develop a virtual material based on Dugdale yield zones at the crack tips. A Dugdale virtual material was subjected to computer fracture experiments that showed it has many fracture properties in common with real ductile materials. A Dugdale material can serve as a model material for new simulations with the goal of studying the role of structure in the fracture properties of composites. One sample calculation showed that the toughness of a Dugdale material in an adhesive joint mimics the effect of joint thickness on the toughness of real adhesives. It is expected, however, that better virtual materials will be required before fracture simulations will be a viable approach to studying composite fracture. The approach of this paper is extensible to more advanced plasticity models and therefore to the development of better virtual materials.     

A mechanism for ductile crack growth in epoxy polymers

It has been previously shown that at relatively high test temperatures/slow test rates crack propagation in thermosetting epoxy polymers occurs in a stable ductile manner. The present paper proposes a mechanism for this type of crack growth based upon a meniscus instability model which both qualitatively and quantitatively accounts for the experimental observations.     

Determination of the critical ductile-brittle transition temperature in testing the cracking resistance of steels

Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 1, pp. 7–11, January, 1988.     

Mechanisms of stable crack growth in ductile polycarbonate

An extensive study of the phenomena appearing during crack propagation in ductile polycarbonate during the phase of stable crack growth (SCG) was undertaken. It has been shown that this phase starts at a critical loading step where a blunted crack was established and a damaged ligament in front of the crack tip developed and stabilized. This phase may be divided into two successive steps. The first is characterized by the step by step advancement of the blunted crack by exhausting respective parts of the damaged ligament so that the overall length of crack and ligament remains stationary. In this step the crack tip opening angle is increasing and attains its final value of the order of 55 degrees.This progressive crack growth, at the expense of the damaged ligament, is replaced by the proper steady crack growth where the crack advances steadily under almost constant external load and under constant CTOA up to a limit where catastrophic fracture occurs. The influence of the in-plane and transverse constraint factors was studied and important results concerning the mechanisms of fracture under predominating plane-stress or plane-strain conditions were established.     

Detection and toughness characterisation of ductile crack initiation in 316 stainless steels

Tests on SEN bend specimens of cold worked 316 stainless steels have enabled values of the crack opening displacement at fibrous crack initiation, _v, to be compared with initiation toughness values implied by various crack monitoring techniques and methods of assessing crack tip ductility. The estimation methods available for obtaining values of the J contour integral from a load/displacement curve have been utilised and compared and have enabled relationships between J _Q, ₁ and specimen geometry to be investigated for the test materials. The results indicate that the sensitivity of the crack monitoring techniques to the detection of initiation is not only dependent on experimentation but also on the rate at which ductile crack growth proceeds following initiation. The interrelation between sensitivity, crack growth rate and toughness is discussed.

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Résumé Des essais sur des éprouvettes de flexion à entailles latérales simples en acier inoxydable 316 écroui ont permis d'établir les valeurs du COD lors de l'amorçage de fissure fibreuse, ces valeurs étant comparées avec les valeurs de la ténacité à l'amorçage déduites de diverses techniques de détection de fissuration ainsi que de méthodes pour établir la ductilité à fond d'entaille. On a utilisé les méthodes approximatives qui sont disponibles pour obtenir les valeur de l'intégrale de contour J à partir d'une courbe charge-déplacement; on a comparé et établi des relations entre J _Q, le COD et la géométrie de l'éprouvette utilisée pour les matériaux d'essai. Les résultats montrent que la sensibilité des techniques de détection d'une fissure et plus particulièrement de son initiation ne dépend pas seulement des conditions expérimentales mais également de la vitesse à laquelle la croissance de la fissure se produit après l'initiation. On discute les relations entre la sensibilité, la vitesse de croissance de la fissure et la ténacité.

     

Prediction of the energy dissipation rate in ductile crack propagation

In this paper, energy dissipation rate D vs. Δa curves in ductile fracture are predicted using a ‘conversion’ between loads, load‐point displacements and crack lengths predicted by NLEFM and those found in real ELPL propagation. The NLEFM/ELPL link was recently discovered for the DCB testpiece, and we believe it applies to other cracked geometries. The predictions for D agree with experimental results. The model permits a crack tip toughness R(Δa) which rises from J_c and saturates out when (if) steady state propagation is reached after a transient stage in which all tunnelling, crack tip necking and shear lip formation is established. J_R is always greater than the crack tip R(Δa) and continues to rise even after R(Δa) levels off. The analysis is capable of predicting the usual D vs. Δa curves in the literature which have high initial values and fall monotonically to a plateau at large Δa. It also predicts that D curves for CCT testpieces should be higher than those for SENB/CT, as found in practice. The possibility that D curves at some intermediate Δa may dip to a minimum below the levelled‐off value at large Δa is predicted and confirmed by experiment. Recently reported D curves that have smaller initial D than the D‐values after extensive propagation can also be predicted. The testpiece geometry and crack tip R(Δa) conditions required to produce these different‐shaped D vs. Δa curves are established and confirmed by comparison with experiment. The energy dissipation rate D vs. Δa is not a transferable property as it depends on geometry. The material characteristic R(Δa) may be the ‘transferable property’ for scaling problems in ELPL fracture. How it can be deduced from D vs. Δa curves (and by implication, J_R vs. Δa curves) is established.     

Effect of dynamic strain aging on fatigue crack growth behaviour of reactor pressure vessel steels

Abstract

Fatigue tests under constant amplitude load were conducted on compact tension specimens of SA533B3 steels with four levels of sulphur content at different temperatures. A modified capacitance type crack opening displacement (COD) gauge was shown to be suitable for fatigue crack length measurement at high temperatures. Test results obtained with different measurement techniques show good consistency. The observation that the Young's moduli measured at a strain rate of 4 × 10^?3 s^?1 for the SA533B3 steels at 150 and 300°C do not decrease with increasing temperature seems to be related to the presence of dynamic strain aging. The fatigue crack growth rates at 150 and 300°C are about two and half times slower than those tested at 400°C because dynamic strain aging prevails at 150 and 300°C. Fractographic examination results suggest that inclusions embedded in secondary cracks enhanced the fatigue crack initiation rather than the fatigue crack growth.     

Creep crack growth laws in heat-resistant steels

The conditions of various fundamental fields' domination at the creep crack tip have been considered. The creep crack growth properties and criteria are discussed. The relation between the creep crack growth kinetics and mechanisms have been studied for heat-resistant turbine steels over a wide test temperature and load range. In recent years much attention has been paid to the theoretical analysis of creep crack growth, though experimental investigations were generally performed for a comparatively narrow temperature and test time interval. The paper is concerned with the criteria and basic laws of creep crack growth. The results of our experimental investigation of the crack kinetics and crack growth mechanisms in heat-resistant steels in wide temperature and test time ranges are also presented.     

Fatigue crack growth characteristics of rotor steels

Room temperature fatigue crack growth rate data were generated for Ni-Mo-V (ASTM A469, Cl-4), Cr-Mo-V (ASTM A470, Cl-8) and Ni-Cr-Mo-V (ASTM A471, Cl-4 and a 156,000 psi yield strength grade) rotor forging steels. Testing was conducted with WOL type compact toughness specimens and the results presented in terms of fracture mechanics parameters. Data show that the Ni-Cr-Mo-V steels exhibit slower fatigue crack growth rates at a given stress intensity range (ΔK) than do the Ni-Mo-V steels. In addition, the Cr-Mo-V steel was found to exhibit slower growth rates than the other alloys at ΔK levels below 40 ksi √in but somewhat foster rates at ΔK levels in excess of 45 ksi √in. The fatigue crack growth rate properties of the alloys studied conform to the generalized fracture mechanics crack growth rate law where da/dN = C₀ΔK^R. It was noted that the fatigue crack growth rate parameters n and C₀ tend to decrease and increase, respectively, with increasing material toughness, K_ic.     

Size-scale transition from ductile to brittle failure: structural response vs. crack growth resistance curve

The concept of brittleness number is revised emphasizing the distinction between a stress-intensification treatment and an energy treatment. In the case of power-law hardening materials the relation between plastic stress-intensity factor and J-integral is translated into a relation between stress brittleness number and energy brittleness number. The structural response generally depends on both such numbers and, therefore, is not physically similar when varying the size-scale of the body. A connection is then established between structural response and crack growth resistance curve, the relevant parameters of a scale-invariant J-resistance curve being related to the energy brittleness number.     

Damage mechanics models of ductile crack growth in welded specimens

Two-dimensional, plane strain, finite element analyses of strength-mismatched welded joints have been performed using the modified boundary layer formulation. The welds were idealized as two-material joints with the material interface running parallel to the crack, which was embedded in the weld material. The Rousselier ductile damage model was employed within the weld material to simulate crack extension due to the growth and coalescence of microvoids. By analysing models with different levels of material mismatching, weld dimensions and applied T -stress levels, it was possible to analyse the effects of crack tip constraint due to both material mismatching and specimen geometry on the fracture resistance of the weld material.
The results show that material strength overmatching (where the weld material is stronger than the base material) reduces the level of constraint ahead of the crack, which can increase the resistance to fracture of the weld material. Conversely, material strength undermatching increases crack tip constraint, reducing the fracture resistance of the joint. By employing estimates for the crack tip constraint levels, Q _M , based on the applied load, level of material mismatching and weld region thickness, it has been possible to 'order' the J– resistance curves of overmatched joints by generating a family of J–Q _M loci which describe the effects of constraint on the fracture resistance of the weld material. However, it is shown that the Q _M-stress parameter is not capable of describing the effect of material strength undermatching on the fracture resistance of a joint, which can be much lower than that obtained from a high-constraint homogeneous specimen of weld material.