Effective Modeling of Fatigue Crack Growth in Pipelines

Pipeline operators must rely on fatigue crack growth models to evaluate the effects of operating pressure acting on flaws within the longitudinal seam to set re-assessment intervals. In most cases, many of the critical parameters in these models are unknown and must be assumed. As such, estimated remaining lives can be overly conservative, potentially leading to unrealistic and short reassessment intervals. This paper describes the fatigue crack growth methodology utilized by Det Norske Veritas (USA), Inc. (DNV), which is based on established fracture mechanics principles. DNV uses the fracture mechanics model in CorLAS™ to calculate stress intensity factors using the elastic portion of the J-integral for either an elliptically or rectangularly shaped surface crack profile. Various correction factors are used to account for key variables, such as strain hardening rate and bulging. The validity of the stress intensity factor calculations utilized and the effect of modifying some key parameters are discussed and demonstrated against available data from the published literature.

Effect of Rotational Stiffness Evolution at Crack Part on Critical Crack Size in SFR

This paper describes the efficiency of the deployment of rotational stiffness evolution model in the critical crack size evaluation for Leak Before Break (LBB) assessment of Sodium cooled Fast Reactor (SFR) pipes. The authors have developed a critical crack size evaluation method for the thin-walled large diameter pipe made of modified 9Cr-1Mo steel. In this method, since the SFR pipe is mainly subjected to displacement controlled load caused by thermal expansion, the stress at the crack part is estimated taking stiffness evolution due to crack into account. The stiffness evolution is evaluated by using the rotational spring model. In this study, critical crack sizes for several pipes having some elbows were evaluated and discuss about the effect of the deployment of the stiffness evolution model at the crack part on critical crack size. If there were few elbows in pipe, thermal stress at the crack part was remarkably reduced by considering the stiffness evolution. In contrast, in the case where the compliance of the piping system was small, the critical crack size could be estimated under displacement controlled condition. As a result, the critical crack size increases by employing the model and LBB range may be expected to be enlarged.

The Fatigue Behaviour of a High Strength CrNi-Steel Regarding Fretting Fatigue

In many assemblies of moving components, contact problems under various lubrication conditions are lifetime-limiting. There, relative motion of contacting bodies, combined with high loads transmitted via the contact surface lead to fretting fatigue failure. For a reliable prediction of in service performance load type, different damage and failure mechanisms that may be activated during operation have to be known. In this contribution selected results of a currently conducted research project are presented. The aim of this study was to examine the material behaviour of a surface stressed steel. The influence of the fretting regime on fatigue properties has been investigated.

Status of French Road Map to Improve Environmental Fatigue Rules

During the past 30 years many fatigue tests and fatigue analysis improvements have been developed in France in order to improve Codified Fatigue Rules of RCC-M and ASME Codes [1, 2]. This paper will present the major technical improvements to obtain reasonable evaluation of potential fatigue damage through EDF road map. Recently new results [3] confirm possible un-conservative fatigue material data: - High cycle fatigue in air for stainless steel, - Environmental effects on fatigue S-N curve for all materials - Fatigue Crack Growth law under PWR environment for stainless steel. In front of these new results, EDF has developed a “Fatigue Road Map” to improve the different steps of Codified fatigue rules. A periodic up-dating of proposed rules in the different French Codes: RCC-M, RCC-MRx and RSE-M with research of harmonization with other Code rules developed in USA, Japan and Germany in particular, will be done on a yearly basis. During the past 15 years, many results have been obtained through fatigue tests of stainless steel materials: - mean and design fatigue curve in air, - environmental effects on fatigue curves, - plasticity effects, - bi-axial load effects, - mean stress effects, - stress indices, - transferability from small to large specimen, - weld versus base metal. In parallel, many new developments have been made in non-nuclear pressure equipment industry: like the reference stress of ASME Section VIII or the structural stress of EN 13445. These methods are mainly well adapted to fatigue pressure cycling. In front of that situation, the French nuclear code organization needs to propose reliable rules for new design and for operating plants. Different proposals are under discussion and the status of the EDF proposals are presented in the paper. The consequences could be important for the utilities because a large part of the in-service inspection program is connected to some fatigue usage factor level between 0.5 and 1.

A Cyclic Plasticity Modeling for Uniaxial and Multiaxial Ratcheting Simulation of Austenitic Steel

The first objective of this paper investigates the influence of the previous strain history on ratcheting of the 304 L stainless steel on ambient temperature. The identification is done using the Chaboche constitutive model. New tests were performed where different strain-controlled histories have been applied prior to ratcheting tests. It is demonstrated that under the same conditions, one can observe ratcheting, plastic shakedown or elasticity according to the prior strain-controlled history. The second objective points out the correlation between the experimental data base devoted to the identification of the material parameters and the quality of the predictions in cyclic plasticity. The results suggest that the choice of the tests should be closely linked to the capabilities of the model. In particular, the presence of non proportional strain-controlled tests in the data base may be not a good choice if the model itself is not able to represent explicitly such a character.

Shape Recovery Model of a Curved Beam Using Piezoelectric Actuation

A thin composite curved beam under external loading is used to simulate different structures such as pressurized pipes, O-Rings etc. These products can undergo dynamic shape change and warping. A piezoelectric actuator is to be used to design for thin curved structures. Simple thin quarter circular beam geometry is analyzed with external loading at the free end. The orientation and placement of the piezoelectric material on a composite curved beam directly affects the deflection and stress applied to that beam. The optimal placement for an actuator for shape recovery was estimated.

Microstructural Stability and Lattice Misfit Characterisations of Nimonic 263

Nimonic 263 has been selected as a candidate header/piping material of advanced ultra-supercritical (A-USC) boilers for the next generation of fossil fuel power plant. Experimental assessments on the microstructural stability of this material are presented in this paper. Microstructural evolution has been quantified by high resolution field emission SEM and TEM. Electron diffraction and the combined XRD and Gaussian peak-fitting have been applied to investigate the coherency and lattice misfit between the gamma prime (γ′) precipitates and the gamma (γ) matrix. The micro structure subjected to solution and hardening treatment consists of γ-matrix and a network of carbide precipitates along the grain boundaries. Large quantities of fine γ′-Ni3(Ti,Al) precipitates were observed, with an average size of 17 nm and coherent with the matrix lattice. The overall misfit has been quantified to be 0.28%. After long term aging at 700 and 725 °C for various periods up to 20,000 hours, γ′ was still the predominant precipitate and mostly coherent with the matrix. A few needle-shape η-Ni3Ti intermetallic precipitates were found in the grain boundary regions. The γ′ size has grown progressively to 78 nm, accompanied by the γ′-γ constrained misfit increasing to 0.50%. Moreover, the M23C6-type grain boundary carbides were found to have experienced morphological evolution, including the nucleation of Widmanstatten-type needles and their initial growth towards the matrix.

EPRI Guidelines for Fabrication of Components Manufactured From Grade 91 Steel

Over the last 10 years EPRI has been researching critical information on the factors affecting the performance of creep strength enhanced ferritic steels in general and Grade 91 steel in particular. This work has resulted in a major new report which provides recommended guidelines for fabrication and the associated quality assurance to ensure that component properties meet or exceed the minimum expectations of ASME design approaches. The present paper outlines the recommendations in the report and provides technical background for specific aspects of the guide.

Investigation on the Allowable Temperature During Electrofusion Welding of Polyethylene Pipe

Polyethylene (PE) pipe material may degrade into lower carbon number volatiles quickly during the electrofusion welding process when the welding temperature rises up to more than about 370°C. Meanwhile, PE may also degrade into lower molecular weight (MW) polymer when subject to a lower temperature. As a result, the allowable temperature during electrofusion welding is uncertain. In this paper, a typical PE100 material was chosen to conduct thermogravimetic analysis (TGA) and Gel permeation chromatography (GPC) test. The thermal degradation behavior of PE100 was investigated in Dynamic and isothermal mode. And the composition of the thermal degradation residue was determined through MW and molecular weight distribution (MWD) measurements of the residue. Based on the experimental results, the initial temperature of thermal degradation with volatilization was derived and the thermal degradation process was studied in detail. To limit the thermal degradation degree of PE in a required range in thermal welding process, the preliminary allowable welding temperature for typical commercial PE100 material was determined. In addition, some regular information was obtained, which could promote the ultimate determination of the allowable welding temperature.

Dissimilar Metal Weld Pipe Fracture Testing: Analysis of Results and Their Implications

Typically in flaw evaluation procedures, idealized crack shapes are assumed for both subcritical and critical crack analyses. Past NRC-sponsored research have developed estimation schemes for predicting the load-carrying capacity of idealized cracks in nuclear grade piping and similar metal welds at the operating conditions of nuclear power reactors. However, recent analyses have shown that growth of primary water stress corrosion cracks (PWSCC) in dissimilar metal (DM) welds is not ideal; in fact, very unusual complex crack shapes may form, i.e., a very long surface crack that has a finite length through-wall crack in the same plane. Even though some experimental data on base metals exists to demonstrate that complex shaped cracks in high toughness materials fail under limit load conditions, other experiments demonstrate that the tearing resistance is significantly reduced. At this point, no experimental data exists for complex cracks in DM welds. In addition, it is unclear whether the idealized estimation schemes developed can be used to predict the load-carrying capacity of these complex-shaped cracks, even though they have been used in past analyses by the nuclear industry. Finally, it is unclear what material strength data should be used to assess the stability of a crack in a DM weld. The NRC Office of Nuclear Regulatory Research, with their contractor Battelle Memorial Institute, has concluded an experimental program to confirm the stability behavior of complex shaped circumferential cracks in DM welds. A combination of full-scale pipe experiments and a variety of laboratory experiments were conducted. A description of the pipe test experimental results is given in a companion paper. This paper describes the ongoing analyses of those results, and the prediction of the load-carrying capacity of the circumferential cracked pipe using a variety of J-estimation scheme procedures. Discussions include the effects of constraint, appropriate base metal material properties, effects of crack location relative to the dissimilar base metals, and the limitations of the currently available J-estimation scheme procedures. This paper concludes with plans for further development of J-estimation scheme procedures for circumferential complex cracks in DM welds.