Crack Growth In Components Research Articles

Fatigue crack growth in real structures

Components and structures are usually constructed according to strength principles. Fatigue crack growth in components and fracture of structures consistently occur due to, e.g. high local stresses, material and manufacturing defects. The following article deals with different examples of crack growth in real structures. These examples shall exemplify learning from damages and consequently preventing failure of components and structures in the future. Furthermore a simulation method is presented to simulate fatigue crack growth in any three-dimensional structures.

Effects of cold work and stress on oxidation and SCC behavior of stainless steels in PWR primary water environments

Recently intergranular stress corrosion cracking (IGSCC) in austenitic stainless steel 316L of Primary Loop Recirculation (PLR) piping and also a nozzle, and several core shrouds (CS) have been experienced in BWRs [1-4]. PWSCC incidences in nickel base alloys of Vessel Head Penetrations (VHP), Bottom Mount Instrumentation (BMI) nozzle and Steam Generator (SG) inlet nozzles in PWR were also reported [5-7]. Therefore, a great concern has been focused and an extensive research and development have been done on these SCC phenomena from a point of view of structural integrity of reactor components. In addition to those SCC incidences in BWR and PWR, there is one unique SCC incidence in 316 stainless steel in PWR plants [8-9]. The mode of this cracking is intergranular and cracks were found in weld HAZ of 316 stainless steel welded to a low alloy steel of SG nozzle with nickel base alloy 82 where PWSCC were also observed. Preliminary root cause analysis implied that the cracked 316 HAZ has a highly hardened not only at surface by machining but also at the region of a far distant from an inner surface of 316 pipe, probably due to a weld shrinkage strain as have been observed in PLR piping and Core shroud cracks in BWR. In laboratory tests [10, 11], there are many well-known evidences that strain hardened austenitic stainless steels such as 304, 304L, 316, 316L and 347, show SCC susceptibility without any sensitization of grain boundary when they are hardened by any sources during fabrication or under an operational condition. An extensive metallographic and TEM analysis has been performed on this cracked 316 HAZ sample as a part of failure analysis to characterize the microstructure and oxides morphology, compositional profiles as well as their crystal structures near the cracked surface of 316 HAZ [8, 9]. Typical strain hardening was measured on the sample due to weld shrinkage and also due to surface grinding. In addition to these analyses, an extensive surface oxides analysis has been performed by Raman Spectroscopy (RS) on a break-opened IGSCC surfaces and a cross-section sample where oxides can be observed in the cracks of concerns. In order to identify the oxides by RS, an extensive laboratory analysis of oxides formed in well-known environmental conditions in the laboratory was performed to generate a set of reference database of oxides by Raman spectroscopy. Based upon the observation of these oxides on/in the cracks, the crack growth history during the plant operation will be discussed in connection to a residual stress distribution at HAZ and oxides variation on/in the cracks. Possible time dependence of crack growth rate with crack growth in components will be discussed based upon the evidences observed about oxides. Finally, an importance of surface integrity assessment in SCC initiation and propagation will be emphasized from a point of view of oxidation localization which can be promoted by strain (dislocation density), straining and stress, which play an crucial role in an oxidation due to accelerated mass transfer in oxides as well as underlying metallic materials. Especially, preferential oxidation along slip bands suggests that oxygen diffusion in such a region with a high dislocation density is faster than the other region. This fact implies that grain boundary can also be a preferential path of oxidation as has been observed by TEM, TOFSIMS and 3D-APT. This localization of oxidation and acceleration will be discussed based upon an analysis of an profile development at an oxide/metal interface which is recently proposed by Y. Takeda et al. [12] based upon some statistical analysis of an oxide/metal interface profile and a concept of surface integrity will be emphasized for an quantitative prediction of SCC initiation.

Simulation of fatigue crack growth in components with random defects

The paper presents a probabilistic method for the simulation of fatigue crack growth from crack-like defects in the combined operating and residual stress fields of an arbitrary component. The component geometry and stress distribution are taken from a standard finite element stress analysis. Number, size and location of crack-like defects are ‘drawn’ from probability distributions. The presented fatigue assessment methodology has been implemented in a newly developed finite-element post-processor, P • FAT, and is useful for the reliability assessment of fatigue critical components. General features of the finite element post-processor have been presented. Important features, such as (i) the determination of the life-controlling defect, (ii) growth of short and long cracks, (iii) fatigue strength and fatigue life distribution and (iv) probability of component fatigue failure, have been treated and discussed. Short and long crack growth measurements have been presented and used for verification of the crack growth model presented.

Evaluating creep cracking in welded fracture mechanics specimens

At present there lacks a unified approach in understanding the mechanistic role of weldments cracking in fracture mechanics specimens at elevated temperatures. The effects of residual stress, the development and crack tip damage due to it and the subsequent creep relaxation at high temperatures can be evaluated using relevant test data, numerical modelling and residual stress measurements. These problem areas are considered in this paper and discussed in context under the newly formed collaborative international effort in the Versailles Agreement of Materials and Standards committee, VAMAS TWA31. The plans for this collaborative effort are to evaluate tests at elevated temperatures in a number of high strength steels and also model and measure weldments containing residual stresses. The aim of the four year programme will be to make recommendations and establish pre-standardisation methods for testing, measuring and analysing creep crack initiation (CCI), creep crack growth (CCG), and low frequency creep fatigue crack growth (CFCG) (where creep dominates) characteristics in weldments containing residual stress. The fracture mechanics geometries that will be considered have already been validated in the previous TWA25 collaboration [VAMAS TWA25, Draft Code of Practice, ‘Creep/fatigue Crack Growth in Components’, VAMAS document. Nikbin K, editor. May 2005.] for testing of parent material. Examples of testing and analysis techniques are presented in this paper to highlight the future objectives for this work.

Prediction of the influence of flaws on the fatigue strength under biaxial loading

A method for the prediction of the fatigue strength of flaw-containing components subjected to in-phase biaxial loading is presented based upon a knowledge of the materials fatigue strength in bending. A key aspect of the method is the relationship between stress concentration factors and stress intensity factors. The fatigue strength is taken to be that stress needed to overcome a materials resistance to crack propagation, and a material constant, r e, is used to relate fatigue strength and the threshold for crack propagation. Continuous crack growth in components containing flaws occurs when the driving force for crack propagation exceeds the resistance to crack growth which is created by the need to exceed the threshold level as well as to overcome the effects of crack closure. Good agreement between the predictions and experimental results was obtained.

Finite element analysis of fatigue crack growth with interspersed mode I and mixed mode overloads

The fatigue crack growth in components or structures is influenced by the service loading. In contrast to a uniform fatigue crack growth during a constant amplitude loading, an overload for instance, which is interspersed into this constant amplitude loading, leads to a retardation effect. Within the scope of this paper fatigue crack growth under variable amplitude loading in a real structure is modelled using an elastic–plastic finite element analysis. It can be shown that due to an overload depending on the overload ratio R ol and the mode I/mode II ratio plastic deformations occur, which on the one hand reduce the near-tip closure and cause a far-field closure. On the other hand, the plastic deformations change the stress distribution along the crack flanks as well as along the ligament. A comparison of the numerically determined fatigue crack growth rates with the experimental data shows a good agreement.

Sensitivity in Creep Crack Growth Predictions of Components Due to Variability in Deriving the Fracture Mechanics ParameterC*

AbstractIn this paper the uncertainties in estimating C* creep fracture mechanics parameter, on component defect assessment, is considered. Comparisons of reference stress σref and the stress intensity factor K for a surface cracked plate under tension and bending and a pipe geometry under internal pressure and external bending using formulae from available codes of practices such as the R5/R6 [1-2], BS7901 [3] and A16 [4] are made with three-dimensional Finite Element (3D FE) analysis calculations. It is shown, using the same material properties and specimen dimensions, that the uncertainty associated with calculating σref could be as much as ±40% whereas for the stress intensity K it is about ±10%. The σref values are used to calculate C* using the mean properties of a base 316LN type stainless steel plate tested at 650 °C and a cross-weld P22 circumferentially welded pipe tested at 565 °C. Both the pipe and the plate were tested within a European collaborative programme ‘HIDA’ between (1996–2000) [5]. The tendency is that ‘global’ limit load analysis will give a lower and more conservative C* than the ‘local’ analysis. From the comparisons of the pipe and plate initial cracking rates, using different C* estimations, with standard creep crack growth rate data on compact tensions (CT) specimens, using the ASTME1457-2002 to evaluate C*, it is shown that predictions of cracking rates could vary by as much as two decades in magnitude. This inconsistency yields itself to an analysis based on an empirical model for predicting crack growth in components.

A combined modeling–experimental study of the crack opening displacement fracture criterion for characterization of stable crack growth under mixed mode I/II loading in thin sheet materials

Recent experimental studies have shown that a critical Crack Opening Displacement (COD) has the potential to be a viable parameter for predicting the onset of crack growth in thin sheet 2024-T3 aluminum under combined tension (mode I) and in-plane shear (mode II) loading conditions. To assess the viability of using a critical COD criterion for prediction of crack growth in components experiencing mixed mode loading, the enclosed work presents a complete set of simulation studies and experimental measurements for crack growth under nominally mode I/II conditions. For the first time, finite element crack growth simulations under mixed mode I/II conditions have been performed for the Arcan test specimen using recently measured crack growth path information and the load–crack extension data for the mode I/II experiments. Simulation results indicate that the predicted COD is in excellent agreement with measurements. Specifically, simulation results indicate that (a) the magnitude of COD during simulated crack growth is nearly constant for Δ a>5 mm; (b) for Φ≤60°, the predicted COD is primarily perpendicular to the crack path (i.e., mode I in nature); (c) for Φ>60°, the predicted COD is primarily parallel to the crack path (i.e., mode II in nature); (d) near the transitional angle between mode I and mode II fracture, the COD components are more unstable during the crack growth process and (e) simulation predictions for the strain fields are in quantitative agreement with measurement. Taken together, the results from the combined simulation–experimentation program provides strong justification for the use of a COD-based fracture criterion to predict crack growth in thin-sheet materials.

Local approach applied to creep-fatigue crack initiation and crack growth in circumferentially notched 316L tubes under combined tension and cyclic thermal shocks

This study concerns the application of the local approach to creep-fatigue crack initiation and creep crack growth in components submitted to combined mechanical and thermal loadings leading to intergranular damage. The purpose is to assess the remaining life of a structure with a crack detected during in service inspection or with a geometrical singularity. A new numerical method based on finite element computations and on a damage model relying on quantitative observations of grain boundary damage is proposed. The numerical results obtained are in good agreement with experimental data. They also fit them better than the classical global approach predictions.

Methods of estimatingC

Several methods are available for estimating the creep fracture mechanics parameter C* used for describing creep crack growth in components and test specimens. A procedure is presented for determining C*, from experimental data, which makes use of limit analysis concepts and requires little knowledge of the creep strain behaviour of materials. Transient effects, during the early stages of cracking due to stress redistribution and a gradual build-up of creep damage at the crack tip, are discussed. It is shown that improved correlations of creep crack growth data are achieved when these effects are taken into account.