An elastic–plastic fracture mechanics based methodology to characterize cracking behavior and its application to environmental assisted processes

Cracking processes suffered by new structural and piping steels when used in petroleum or other energy installations have demonstrated the need for a cracking resistance characterization methodology. This methodology, valid for both elastic and elastoplastic regimes, should be able to define crack propagation kinetics as a function of their controlling local parameters. This work summarizes an experimental and analytical methodology that has been shown to be suitable for characterizing cracking processes using compact tensile specimens, especially subcritical environmentally assisted ones, such as those induced by hydrogen in microalloyed steels. The applied and validated methodology has been shown to offer quantitative results of cracking behavior and to correlate these with the existing fracture micromechanisms.     

Coupled phenomenological and fracture mechanics approach to assess the fracture behaviour of TWC piping component

The present study demonstrates the numerical prediction of experimental specimen J-R curve using Gurson-Tvergaard-Needleman phenomenologically based material model. The predicted specimen J-R curve is used to determine the geometric independent initiation fracture toughness (J_SZWc) value that compares well with experimental result. Using the experimentally determined and numerically predicted J_SZWc values and specimen J-R curves, the accuracy of predicting the fracture behaviour of the cracked component is judged. Thus the present study proposed a coupled phenomenological and fracture mechanics approach to predict the crack initiation and instability stages in cracked piping components using numerically predicted specimen J-R curve obtained from tensile specimens testing data.     

Ductile fracture of A 508 Cl 3 steel in relation with inclusion content: The benefit of the local approach of fracture and continuum damage mechanics

A new methodology for ductile fracture analysis based on the local approach of fracture, through constitutive relations that take into account void growth and damage, has been applied to three heats of A 508 Cl 3 steel with different inclusion contents in the region of 10⁻³–10⁻⁴. The ductility of the three heats is well predicted by the ductile fracture model through its parameter f₀: the initial volume fraction of voids. The model, first calibrated with the simple notched tension test, gives a good prediction of crack initiation and growth in a precracked specimen. Finally the statistical aspects in ductile fracture are briefly discussed.     

Failure analysis of a generator rotor with a deep crack detected during operation: fractographic and fracture mechanics approach

This paper reports the results of an activity of failure analysis performed on a 370 MVA generator rotor that was scrapped after about 133 000 operating hours and about 400 start-ups, when the on line monitoring system revealed a progressive increase in vibrations amplitude, due to the presence of a deep crack. It was actually detected by automated ultrasonic equipment using advanced signal processing. Failure analysis and laboratory tests demonstrated that the crack started from a forging defect located at the root of a longitudinal slot, due to the high stress range induced by oil film instability problems. During the following years the crack grew mainly during transients and not during operation.     

Evaluation of SCC crack behavior in zirconium and zircaloy-2 using nonlinear fracture mechanics parameters

Applicability of nonlinear fracture mechanics parameters, i.e. J-integral, crack tip opening displacement (CTOD), and crack tip opening angle (CTOA), to evaluation of stress corrosion crack (SCC) propagation rate was investigated using fully annealed zirconium plates and Zircaloy-2 tubing, both of which produce SCC with comparatively large plastic strain in an iodine environment at high temperatures.Tensile SCC tests were carried out at 300°C for center-notched zirconium plates and internal gas pressurization SCC tests at 350°C, for Zircaloy-2 tubing, to measure the SCC crack propagation rate. The J-integral around semi-elliptical SCC cracks produced in Zircaloy-2 tubing was calculated by a three-dimensional finite element method (FEM) code.The test results revealed that the SCC crack propagation rate dc/dt could be expressed as a function of the J-integral, which is the most frequently used parameter in nonlinear fracture mechanics, by the equation dc/dt = C · Jⁿ, where C and n were experimental constants.Among the other parameters, CTOD and CTOA, the latter appeared to be useful for assessing the crack propagation rate, because it had a tendency to hold a constant value at various crack depths.     

Developments in fracture mechanics and non-destructive examination

The purpose of this paper is to give a brief review of the major developments and trends that have occurred in some of the technical areas covered by Division G of SMiRT since the first conference was held in Berlin in 1971. As with any human activity, time and experience produce organic change so that the technical areas covered by Division G have changed somewhat over the 18 years of its existence. As first conceived by Tom Jaeger, Division G was one of two divisions devoted to Light Water Reactor Components other than fuel and cladding. Division F was conceived with Core Structures and Piping whilst Division G was devoted to the Steel Reactor Pressure Vessels.     

Integration of nondestructive examination reliability and fracture mechanics

A multi-year program at the Pacific Northwest Laboratory is in progress to determine the reliability of ultrasonic ISI that is performed on light-water reactor primary systems, using probabilistic fracture mechanics (FM) analysis to determine the impact of NDE unreliability on system safety, and to evaluate advanced ultrasonic techniques. This paper is a review of the last year's highlights. Emphasis is placed upon the results of a pipe inspection round robin, advanced technique evaluation, joint study with Westinghouse, qualification document, underclad crack detection sizing studies, and a FM analysis using the PRAISE code for studying inspection parameters.     

Integration of nondestructive examination reliability and fracture mechanics

A multi-year program on the Integration of Nondestructive Examination and Fracture Mechanics (NDE/FM) has been funded by the U.S. Nuclear Regulatory Commission at the Pacific Northwest Laboratory. Many activities are being pursued under this program. This paper highlights some of the activities: input to the NRC Pipe Crack Task Group, an evaluation of manual ultrasonic testing of centrifugally cast stainless steel, interaction matrix, advanced UT technique evaluation, qualification document, evaluation of crack characterization techniques, international NDE reliability work, siamese imaging technique for imaging planar-type radial defects in reactor piping, fracture mechanics analysis for PTS-type flaws and piping reliability, and a position paper on piping ISI.     

Evaluation of stainless steel pipe cracking: Causes and fixes

This paper discusses (1) studies of impurity effects on susceptibility to intergranular stress corrosion cracking (IGSCC), (2) intergranular crack growth rate measurements, (3) finite-element studies of the residual stresses produced by induction heating stress improvement (IHSI) and the addition of weld overlays to flawed piping, (4) leak-before-break analyses of piping with 360° part-through cracks, and (5) parametric studies on the effect of through-wall residual stresses on intergranular crack growth behavior in large diameter piping weldments. The studies on the effect of impurities on IGSCC of Type 304 stainless steel show a strong synergistic interaction between dissolved oxygen and impurity concentration of the water. Low carbon stainless steel (Type 316NG) appear resistant to IGSCC even in impurity environments. However, they can become susceptible to transgranular SCC with low levels of sulfate or chloride present in the environment. The finite-element calculations show that IHSI and the weld overlay produce compressive residual stresses on the inner surface, and that the stresses at the crack tip remain compressive under design loads at least for shallow cracks.     

Some recently-developed analytical aspects of fracture mechanics

The paper aims to describe new approaches to fracture mechanics which take into account local damage of the material, with special reference to materials susceptible to sudden damage. The basic idea is that classical stress-strain laws are valid only below a given deformation threshold, _R. Above that threshold, the stresses irreversibly fall towards zero. The classical crack concept is then replaced by that of a quasi-crack with a process or damaged zone of finite, non-zero thickness. On the basis of the energy equation and drawing a distinction between the plastic work and fracture rates, it is demonstrated how the R curve can be obtained. The cases of brittle and ductile fracture are discussed. In order to rejoin the classical theories of fracture mechanics, it is sufficient to extend the deformation threshold _R towards infinity. The ratio between fracture energy and energy dissipated through plastic work tends towards zero at that limit. In the damage fracture theory presented here, where _R is finite, the fracture energy is other than zero.     

The application of elastic-plastic fracture mechanics parameters in fracture safe design

The report summarizes some of the methods which are currently used for assessing the fracture toughness of materials under elastic and elastic-plastic conditions. The main parameters which are considered are (1) plane strain fracture toughness (K_Ic), (2) equivalent energy (K_Icd), (3) contour integral (J) and (4) crack opening displacement (COD). Gross strain crack tolerance and stress concentration methods are also discussed.It is concluded that of these parameters, the contour integral and the crack opening displacement have most potential for future development. These two parameters are shown to be equivalent, however, at the present stage of development the COD concept has several advantages over the J concept. Firstly, the COD concept is able to take into account, secondary stresses, such as welding residual stresses. Because these stresses are in equilibrium, they do not appear in energy measurements to evaluate J. Secondly, the COD value is a physical measure of the crack tip conditions which includes the effect of stress state and thickness. It is, therefore, possible to measure and calculate COD levels for cracks in real structures. It is not possible to evaluate J for real structures since J methods are appropriate only to in-plane problems. This also means that partial wall (thumbnail) flaws are better characterized by the COD concept.The COD concept has been developed to a stage where it is possible to estimate the significance of flaws in welded structures provided the toughness of the material and the acting stresses or strains are known. This development is described and the method used to analyze tests on model pressure vessels with 6″ thick walls. A comparison is made with other methods, and it is concluded that although the COD analysis gives conservative estimates of the flaw size to cause failure, further work is necessary to be able to predict vessel burst conditions when failure is preceded by extensive plasticity and stable ductile tearing. A simple nomogram to determine COD levels to ensure leak before break conditions is also developed.     

Applications of probabilistic fracture mechanics to FBR components

A probabilistic fracture mechanics code which evaluates fracture probability of a plate model with an elliptical surface crack caused by creep-fatigue crack growth has been developed. The code named PCCF (Probabilistic Fracture Mechanics Code for Creep-Fatigue Crack Growth) uses simplified methods of C^* and J-integral for evaluation of creep-fatigue crack growth and a stratified sampling method for two input variables to improve the solution convergency. According to the test analyses focused on an applied stress level using PCCF code, leak probability is sensitive to a stress level and increases rapidly when an applied stress is close to a yield stress level.     

Application of fracture mechanics concepts to the failure of wide plates

An analysis is presented to predict the failure behaviour of wide plates with different crack lengths in a temperature range where brittle, elastic-plastic and fully ductile behaviour is observed. Comparing the characteristic material properties derived from small scale specimens with the corresponding loading conditions in terms of J_appl using two- and three-dimensional finite element analyses, the failure loads can be calculated as a function of temperature. Based on these analyses it is possible to predict the different failure behaviour of the wide plates characterized by the transition temperatures T_gy and T_i.     

Application of fracture mechanics in the design and operation of PWRs in France

Fracture mechanics techniques are used to evaluate the risks associated with material aging, the occurrence of fabrication defects or stress corrosion cracks, and to determine the margins with respect to fast fracture resulting from the design and manufacturing practices. Important programs are underway to improve the accuracy and domain of validity of the fracture mechanics methods.     

Classification and characterization of materials by means of fracture mechanics parameters

Within the scope of the “Integrity of Components” research project a large number of heats of reactor pressure vessel steels representing a broad quality spectrum have been investigated. In this paper the conventional and fracture mechanics parameters of four typical materials are presented. The difficulties in determining the reference temperature for nil ductility transition and the fracture mechanics parameters in the transition and upper shelf region of the Charpy energy are discussed. The technique developed at MPA for the evaluation of a physically meaningful crack initiation parameter based on the size of the stretched zone ahead of the crack tip is described, and values are reported.     

Application and evaluation of fracture mechanics failure concepts to precracked vessels

The elastic-plastic fracture mechanics concepts R-curve method, Two Criteria Approach, COD concept and Battelle formula were applied on three tested vessels made of low and high tough and fine grained structural steel and X 8 Ni 9 with an axial notch on the outer surface. Apart from the R-curve method for the vessel made of low tough material, failure loads were conservatively calculated. The COD concept could not be applied to these vessels and failure geometries using the well known design curve. A modification of the COD concept by means of FE-calculations was made so that a correlation between the displacement at the crack tip and at the vessel surface could be determined. With this procedure, the calculated load at fracture was only 8% below the experimental result.     

Ductile fracture models and their potential in local approach of fracture

Since the pioneering studies conducted by McClintock, Rice and Tracey on the growth of cavities, many studies have covered modelling of ductile fracture; a non-exhaustive list is given of these studies. In the local approach of fracture, the models are used at the tip of cracks, and in the present state, only relatively simple models can be used to solve fracture problems in a complete manner, from material characterization to the analysis of cracked structures.The paper presents two types of models based on a critical ratio of cavity growth or on continuum damage mechanics. In the framework of the local approach of fracture, their possibilities are presented with constant references to experimental validation studies. This allows conclusions to be drawn regarding the effectiveness of the methods proposed and their undeniable contribution to fracture mechanics.     

The significance of applied mechanics in designing the fast test reactor

The discipline of applied-mechanics analysis has had a significant influence in designing the Fast Test Reactor (FTR). The FTR is a sodium-coolod fast flux reactor being constructed to test candidate materials and fuels for the U.S. Liquid Metal Fast Breeder Reactor (LMFBR) program. The influence of applied mechanics is more evident in the design of a liquid metal cooled reactor such as the FTR than it is in the more conventional water-cooled designs, primarily because the combination of environmental conditions in a liquid metal reactor results in an interplay between the mechanics analyst and the reactor designer never before required. Specifically, these environments include a fast neutron spectra, high neutron fluence (flux-time) exposures, an elevated thermal environment, and a high conductivity coolant.     

Environmentally assisted cracking behavior of dissimilar metal weldments in simulated BWR coolant environments

The environmentally assisted cracking behavior of dissimilar metal (DM) welds, including Alloy 52-A 508 and Alloy 82-A508, under simulated BWR coolant conditions was studied. Effects of postweld heat treatment and sulfur content of the base metal on the corrosion fatigue and SCC growth rates of DM welds were evaluated. The crack growth rates for the DM weld heat-treated at 621 °C for 24 h were observed to be faster than those for the as-welded. But the DM weld heat-treated at 621 °C for 8 h + 400 °C for 200 h showed better SCC resistance than the as-welded. The longer the heat treatment at 621 °C, the higher the chromium carbides density along the grain boundary was observed. Sulfur could diffuse out of the base metal and segregate along the grain boundaries of the dilution zone, leading to weakening the grain boundary strength and the SCC resistance of the Alloy 52-A508 weld.