Application of Extended Finite Element Method (XFEM) to Stress Intensity Factor Calculations

The extended finite element method (XFEM) is an extension of the conventional finite element method based on the concept of partition of unity. In this method, the presence of a crack is ensured by the special enriched functions in conjunction with additional degrees of freedom. This approach also removes the requirement for explicitly defining the crack front or specifying the virtual crack extension direction when evaluating the contour integral. In this paper, stress intensity factors (SIF) for various crack types in plates and pipes were calculated using the XFEM embedded in ABAQUS. These results were compared against handbook solutions, results from conventional finite element method, and results obtained from finite element alternating method (FEAM). Based on these results, applicability of the ABAQUS XFEM to stress intensity factor calculations was investigated. Discussions are provided on the advantages and limitations of the XFEM.

Experimentation and Adaptive Modeling for Temperature Effect Quantification in PVP Structural Health Monitoring With PWAS

The thermal effects at elevated temperatures mostly exist for pressure vessel and pipe (PVP) applications. The technologies for diagnosis and prognosis of PVP systems need to take the thermal effect into account and compensate it on sensing and monitoring of PVP structures. One of the extensively employed sensor technologies has been permanently installed piezoelectric wafer active sensor (PWAS) for in-situ continuous structural health monitoring (SHM). Using the transduction of ultrasonic elastic waves into voltage and vice versa, PWAS has been emerged as one of the major SHM sensing technologies. However, the dynamic characteristics of PWAS need to be explored prior its installation for in-situ SHM. Electro-mechanical impedance spectroscopy (EMIS) method has been utilized as a dynamic descriptor of PWAS and as a high frequency local modal sensing technique by applying standing waves to indicate the response of the PWAS resonator by determining the resonance and anti-resonance frequencies. Another SHM technology utilizing PWAS is guided wave propagation (GWP) as a far-field transient sensing technique by transducing the traveling guided ultrasonic waves (GUW) into substrate structure. The paper first presents EMIS method that qualifies and quantifies circular PWAS resonators under traction-free boundary condition and in an ambience with increasing temperature. The piezoelectric material degradation was investigated by introducing the temperature effects on the material parameters that are obtained from experimental observations as well as from related work in literature. GWP technique is also presented by inclusion of the thermal effects on the substrate material. The MATLAB GUI under the name of Wave Form Revealer (WFR) was adapted for prediction of the thermal effects on coupled guided waves and dynamic structural change in the substrate material at elevated temperature. The WFR software allows for the analysis of multimodal guided waves in the structure with affected material parameters in an ambience with elevated temperature.

Development of Flaw Acceptance Criteria for Aging Management of Spent Nuclear Fuel Multiple-Purpose Canisters

A typical multipurpose canister (MPC) is made of austenitic stainless steel and is loaded with spent nuclear fuel assemblies. The canister may be subject to service-induced degradation when it is exposed to aggressive atmospheric environments during a possibly long-term storage period if the permanent repository is yet to be identified and readied. Because heat treatment for stress relief is not required for the construction of an MPC, stress corrosion cracking may be initiated on the canister surface in the welds or in the heat affected zone. An acceptance criteria methodology is being developed for flaw disposition should the crack-like defects be detected by periodic Inservice Inspection. The first-order instability flaw sizes has been determined with bounding flaw configurations, that is, through-wall axial or circumferential cracks, and part-through-wall long axial flaw or 360° circumferential crack. The procedure recommended by the American Petroleum Institute (API) 579 Fitness-for-Service code (Second Edition) is used to estimate the instability crack length or depth by implementing the failure assessment diagram (FAD) methodology. The welding residual stresses are mostly unknown and are therefore estimated with the API 579 procedure. It is demonstrated in this paper that the residual stress has significant impact on the instability length or depth of the crack. The findings will limit the applicability of the flaw tolerance obtained from limit load approach where residual stress is ignored and only ligament yielding is considered.

The Influence of Creep Strain on Crack Length Measurements Using the Potential Drop

One of the most common methods for estimating crack extension in the laboratory is electrical potential drop (PD). A key limitation of this technique is that it is sensitive to strains at the crack tip as well as crack extension. When producing J-R curves the onset of crack growth may be identified from a point of inflection on a plot of PD vs. CMOD. For creep crack growth (CCG) tests however, the effects of strain are often ignored. This paper investigates whether a similar method may be applied to CCG testing. A single CCG test was performed on type 316H stainless steel and a point of inflection, similar to that observed during J-R curve testing was identified. A finite element (FE) based approach was used to investigate this phenomenon further. A 3D sequentially-coupled structural-electrical FE model was used to reproduce the experimental PD vs. CMOD plot up to the point of inflection. The model was capable of predicting the general relationship between strain and PD. It predicted the magnitude of the change in PD to within 30%. A simplified 2D FE model was then used to perform a parametric study to investigate whether a similar trend may be expected for a range of materials. Power law tensile and creep properties were investigated with stress exponents of 1, 3 and 10. The results confirm that a point of inflection should be observable for the range of material properties considered.

Overview of Recent NUGENIA Activities Particularly in Relation to Structural Integrity

NUGENIA, an international non-profit association founded under Belgian legislation and launched in March 2012, is dedicated to nuclear research and development (R&D) with a focus on Generation II and III power plants. NUGENIA is the integrated framework between industry, research and safety organisations for safe, reliable and competitive nuclear power production, and is aimed at running an open innovation marketplace, to promote the emergence of joint research and to facilitate the implementation and dissemination of R&D results. The technical scope of NUGENIA consists of eight technical areas. One of these areas, Technical Area 4, is associated with the structural integrity assessment of systems, structures and components. A brief overview of recent NUGENIA activities in general is provided in this paper and a specific focus is given on developments in relation to Technical Area 4.

Optimization of the Existing Screening Method by Introducing Fen Correlations

In the wake of numerous experimental tests carried out in air and also in a PWR environment, both abroad and in France, an update of the current fatigue codification is underway. Proposals are currently being formulated in France [1] [2] and discussions are taking place in the frame of a French working group involving EDF, AREVA and CEA. In parallel with these worldwide modification efforts, it is necessary to evaluate their impact on the NSSS components. In the USA, many such evaluations have already been implemented for license renewal to operate power plants beyond their initial 40 years of operation. In order to reduce the scope of the calculations to perform, a preliminary screening was carried out on the various areas of the primary loop: this screening is detailed in an EPRI report [3]. The output of this screening process is a list of locations that are most prone to EAF degradation process and it is on these zones only that detailed EAF calculations are carried out. In France, with the approaching fourth decennial inspection of the 900 MWe (VD4 900 MWe) power plants, EDF needs also to map out the impact of these updates to the RCC-M code before initiating detailed calculation efforts. The EPRI report was not applicable as such to the French plants due to domestic specificities and more particularly, a need for a more detailed Fen estimation. A method was therefore developed by EDF, peer-reviewed by SI with the main innovation being the introduction of correlations enabling the calculation of Fen on the basis of the geometrical dimensions and the information available in the transient document. This paper presents how these correlations were built and proposes to benchmark them with some existing sample case problems.

Burst Test on the Steam Generator Tube With Multiple Part-Through-Wall Flaws

This study carried out burst tests on steam generator (SG) tube specimens containing multiple axial part-through-wall (PTW) flaws at room temperature (RT). The specimens were machined from SG tube of Alloy 690TT with an outer diameter of 19.05mm and a thickness of 1.067mm. The flaws were made by electro-discharge machining (EDM) method on the outer surface of specimen. In the experiment, six types of multiple PTW flaws with a constant depth of 50% of wall thickness and single flaw with four different lengths were considered. The results showed that the interaction effect for collinear axial PTW flaws diminished with increasing ligament length between flaws. The ligament length had less influence on the interaction effect for longer flaws than shorter flaws. For non-aligned axial PTW flaws, however, the interaction effect was increased and remained with increase in circumferential ligament length. For parallel axial PTW flaws, the positive interaction effect appeared when the ligament between flaws was less than 2mm. However, the failure pressure decreased with increasing circumferential ligament length between flaws and reached the minimum when they were separated by about 90-degree in circumferential direction. Interaction effect was enhanced as number of flaws for collinear and non-aligned axial PTW flaws increased. Regardless of flaw configurations and ligament lengths, multiple PTW flaws of shorter length were failed by rupture at bottom of flaws followed by coalescence and unstable tearing. But, the flaws of length longer than 25.4mm were failed by rupture at bottom of one of multiple axial flaws.

A Proposed Generalized Model for Forming Limit Diagram

One of the most significant approaches for predicting formability is the use of forming limit diagrams (FLDs). The development of the generalized model integrates other models. The first model is based on Von-Misses yield criterion (traditionally used for isotropic material) and power law constitutive equation considering the strain hardening exponent. The second model is also based on Von-Misses yield criterion but uses a power law constitutive equation that considers the effect of strain rate sensitivity factor. The third model is based on the modified Hill’s yield criterion (for anisotropic materials) and a power law constitutive equation that considers the strain hardening exponent. The current developed model is a generalized model which is formulated on the basis of the modified Hill yield criterion and a power law constitutive equation considering the effect of strain rate. A new controlling parameter (γ) for the limit strains was exploited. This parameter presents the rate of change of strain rate with respect to strain. As γ increases the level of the FLD raises indicating a better formability of the material.

Innovative Repair of a Holed Through Pressure Vessel

A trayed column at an operating facility suffered a loss of containment incident and was shut down. The damage to the steel is thought to have been caused by loose tray components which rattled around and eventually wore through the shell. An initial repair plan involving welded repairs was proposed. This plan would entail a field post weld heat treat (PWHT) due to the process environment of the vessel. Upon further developing the PWHT plan it was determined that this approach was costly and would have excessive lost profit opportunity (LPO) due to the time it would take to execute and the criticality of this vessel to plant operations. Instead, a second approach involving no field welding was devised, vetted, fabricated, and implemented. The facility was able to restart the process, saving several days of production. A permanent repair or replacement will be planned and implemented at the next planned shutdown.

Effect of Cladding Residual Stress Modeling Technique on Shallow Flaw Stress Intensity Factor in a Reactor Pressure Vessel

Prior probabilistic fracture mechanics (PFM) analysis of reactor pressure vessels (RPVs) subjected to normal cool-down transients has shown that shallow, internal surface-breaking flaws dominate the RPV failure probability. This outcome is caused by the additional crack driving force generated near the clad interface due to the mismatch in coefficient of thermal expansion (CTE) between the cladding and base material, which elevates the thermally induced stresses. The CTE contribution decreases rapidly away from the cladding, making this effect negligible for deeper flaws. The probabilistic fracture mechanics code FAVOR (Fracture Analysis of Vessels, Oak Ridge) uses a stress-free temperature model to account for residual stresses in the RPV wall due to the cladding application process. This paper uses finite element analysis to compare the stresses and stress intensity factor during a cool-down transient for two cases: (1) the existing stress-free temperature model adopted for use in FAVOR, and (2) directly applied RPV residual stresses obtained from empirical measurements made at room temperature. It was found that for a linear elastic fracture mechanics analysis, the application of measured room temperature stresses resulted in a 10% decrease in the peak stress intensity factor during a cool-down transient as compared to the stress-free temperature model.