Cyclic Plasticity Analysis of Welded Joint With Welding Residual Stress Using the Direct Method

To meet the growing energy demands, the power sector continuously strives at enhancing the efficiency of its power plants by increasing the operating temperature. Under cyclic loading conditions, this leads to creep-cyclic plasticity driven damage mechanisms such as cyclically enhanced creep, creep enhanced plasticity and creep ratcheting. A detailed understanding of creep and related damages is therefore essential for predicting any potential failure mechanisms and ensuring confidence in the safe-working of the components. This becomes particularly difficult and challenging in the presence of welds due to two main reason; a) presence of different material zones, namely parent metal, weld metal and heat affected zone; b) introduction of residual stress during welding. An extended Direct Steady Cycle Analysis within the Linear Matching Method (LMM) framework has been previously developed to consider the full interaction between creep and cyclic plasticity. This paper presents the basic theory and an overview behind the LMM framework along with a new application of a welded flange, considering for the first time the effect of residual stress due to welding. A 3D finite element model is adopted for the flange, and it is subjected to a constant pressure and cyclic thermal load of varying dwell. The effects of welding residual stress on the creep-cyclic plastic response of the welded flange are investigated. Additional parametric studies considering different levels of the applied load and dwell period are performed. The results reflect the ease of using LMM over conventional inelastic analysis.

European Project ATLAS+: Small and Large Scale Ductile Tearing Experiments on Ferritic Steel WB36 to Study Transferability of Material Ductile Properties

The 4-years European project ATLAS+ (Advanced Structural Integrity Assessment Tools for Safe long Term Operation) has been launched in June 2017. One of its objectives is to study the transferability of material ductile properties from small scale specimens to large scale components and validate some advanced tools for structural integrity assessment. The study of properties transferability is based on a wide experimental programme which includes a full set of fracture experiments conducted on conventional fracture specimens and large scale components (mainly pipes). Three materials are considered in the programme : a ferritic steel WB36 typical from secondary feed water line in German PWR reactors, an aged stainless steel austenitic weld representative of EPR design and a typical VVER austenitic dissimilar weld (DMW). This paper describes the experimental work conducted on the ferritic steel WB 36 (15NiCuMoNb5) and summarizes the experimental results available after 2 years of work. Numerous mechanical tests have been conducted on a wide panel of fracture mechanics specimens for a full characterization of the ferritic steel: Tensile properties, Hardness, Charpy Energy, pre-cracked Charpy PCC, Master curve on CT and SENT specimens, ductile tearing properties on CT and SENT specimens. In parallel, it is planned to test three 4PB large scale tests on pipings (FP1, FP2 and FP3) at room temperature on the EDF test facility with 3 configurations (shape, size and location) of cracks: through wall crack (TWC), internal and external ½ elliptical cracks. Progress of these large scale experiments is described including first results.

Nondestructive Evaluation of Metal Strength, Toughness, and Ductility Through Frictional Sliding

Traditional assessment of mechanical properties requires the removal of a standardized specimen for destructive laboratory testing. A nondestructive in-situ method is a cost-effective and efficient solution for applications where sample cutouts are not feasible. This work describes developments in contact mechanics that use frictional sliding to evaluate the material strength and toughness of steel pressure vessels and pipelines. Hardness, Strength, and Ductility (HSD) testing is a portable implementation of frictional sliding that provides a tensile stress-strain curve for assessment of the yield, ultimate tensile strength (UTS), and strain hardening exponent for power-law hardening metals. HSD testing incorporates four styluses of different geometry that generate grooves on the surface of a material as they travel. The measured geometry of these grooves along with the normal reaction forces on the stylus are correlated to representative tensile stress-strain values through finite element analysis (FEA) simulations. These principles have been extended to account for nonlinear strength behavior through the thickness of seam-welded steel pipes by using a combination of the HSD surface measurement, microstructure grain size, and chemistry. Frictional sliding tests are also used to assess material variation across a welded seam to identify different welding processes and the effectiveness of post-weld-heat-treatments (PWHT). A second implementation of frictional sliding is Nondestructive Toughness Testing (NDTT), which provides an NDE solution for measuring fracture toughness by using a wedge-shaped stylus with an internal stretch passage to generate a Mode I tensile loading condition on the surface of a sample. The test produces a raised fractured surface whose height provides an indication of the materials ability to stretch near a propagating crack and is correlated to the crack-tip-opening-displacement (CTOD) measured from traditional laboratory toughness testing. Experiments on pipeline steel indicate that NDTT can provide an index of fracture toughness to benchmark materials tested under similar conditions. Implementation of these new instruments to gather data for integrity management programs, fitness for service assessments, and quality control of new manufacturing will help to reduce risk and uncertainty in structural applications.

A Simplified Large Thin Plate Model for Modeling Fracture Behavior of a Hydrided Irradiated Zr-2.5Nb Pressure Tube Specimen With an Axial Crack

The crack tip opening displacements (CTODs) and the effective plastic strains ahead of the crack front in a hydrided irradiated Zr-2.5Nb pressure tube specimen with an axial crack are investigated using two 3-D finite element models in this paper. The first model is a pressure tube with 80 split circumferential hydrides distributed through the thickness ahead of the crack front. The second model is a large thin plate with a central crack with four split circumferential hydrides under symmetry/symmetry, free/symmetry and free/free constraint conditions. The results for CTOD indicate that the CTOD of the pressure tube specimen with 80 hydrides is slightly smaller than that for the large thin plate with the free/symmetry constraint condition and larger than that for the large thin plate with the symmetry/symmetry constraint condition. The effective plastic strain of the pressure tube specimen with 80 hydrides is smaller than that for the large thin plate with the free/symmetry constraint condition and larger than that for the large thin plate with the symmetry/symmetry constraint condition at large normalized loads. The computational results show that instead of modeling a full 3-D pressure tube with a larger number of hydrides, a large thin plate model with a limited number of hydrides can be used to efficiently determine the upper and lower bounds of the CTODs and the effective plastic strains ahead of the crack front in a pressure tube specimen.

Role of Constraint in Specimen Geometries When Evaluating Fracture Toughness/Material Fracture Resistance for a Surface-Flawed Elbow

The fracture behavior of a circumferential surface crack in an elbow was evaluated using past data from the International Piping Integrity Research Group (IPIRG-2) Experiment 2-4. The elbow tested was nominal 16-inch diameter Schedule 100 TP304 material, which was solution-annealed after final fabrication. The elbow was loaded with an in-plane-closing bending moment and internal pressure of 15.51 MPa (2,250 psig) at 288 C (550 F). The surface crack was 180-degrees on the ID surface and centered on the extrados, but after fatigue precracking the depth was variable and the greatest was at about 45-degrees from the extrados. FE analysis of the IPIRG-2 elbow test was conducted with a state-of-the-art and precise 3D FE mesh (including variable surface crack depth, variable thickness, and initial elbow ovalization). The flaw depth for the single-edge notch tension (SENT) tests was selected to be equivalent to the deepest point in the elbow specimen crack front that provided the largest J-value in the elbow experiment, i.e., ao/W = 0.68. Comparison of the J-value for initiation (Ji) and crack-tip-opening displacement (CTODi) at crack initiation suggested that there was a slight difference in constraint between an identical depth SENT specimen (a/W = 0.68 with the same L-R orientation as the surface crack in the pipe) and an elbow with a circumferential surface crack (a/t = 0.68) [Ji was 0.368 MN/m, (2.1 ksi-inch) in the SENT tests, while it was 0.490 MN-m (2.8 ksi-inch) in the elbow test]. The more significant finding in this work was that the compact tension (C(T)) test Ji-value was much higher at 1.086 MN/m (6.2 ksi-inch) or ∼3 times higher. The elbow to SENT to C(T) specimen comparison illustrates very large differences in constraint between these geometries. From past work by several researchers it was determined that the constraint in C(T) specimens gives Ji-values that agree well with a circumferential through-wall crack in a straight pipe, but this difference with surface-cracked elbow or pipe is envisaged to be new information to the international research community. Additionally, from state-of-the-art FE analysis of the 180-degree surface-cracked elbow test it was found that the maximum J-value occurs at a position that was about 45-degree away from the extrados location. This trend showed that caution should be exercised when selecting the crack locations for elbow integrity evaluation, since for shorter flaw lengths it may be more critical to consider a crack that is closer to the 45-degrees from the extrados, which could be true for fracture as well as stress corrosion cracking (SCC) elbow evaluations.

Advanced Structural Integrity Assessment Tools for Safe Long Term Operation: ATLAS+ Project — Status of the Activities of the WP3 on Modelling

The main objective and mission of the ATLAS+ project is to develop advanced structural assessment tools to address the remaining technology gaps for the safe and long term operation of nuclear reactor pressure coolant boundary systems. ATLAS+ WP3 focuses mainly on ductile tearing prediction for large defect in components: Several approaches have been developed to accurately model the ductile tearing process and to take into account phenomena such as the triaxiality effect, or the ability to predict large tearing in industrial components. These advanced models include local approach coupled models or advanced energetic approaches. Unfortunately, the application of these tools is today rather limited to R&D expertise. However, because of the continuous progress in the performance of the calculation tools and accumulated knowledge, in particular by members of ATLAS+, these models can now be considered as relevant for application in the context of engineering assessments. WP3 will therefore: • Illustrate the implementation of these models for industrial applications through the interpretation of large scale mock-ups (with cracks in weld joints for some of them), • Make recommendations for the implementation of the advanced models in engineering assessments, • Correct data from the conventional engineering approach by developing a methodology to produce J-Δa curve suitable case by case, based on local approach models, • Improve the tools, guidance and procedures for undertaking leak-before-break (LBB) assessments of piping components, particularly in relation to representing structural representative fracture toughness J-Resistance curves and the influence of weld residual stresses. To achieve these goals, WP3 is divided into 4 sub-WPs and this paper presents the progress of the work performed in each sub-WP after 24 months of activities.

Design of an Intelligent Python Code for Validating Crack Growth Exponent by Monitoring a Crack of Zig-Zag Shape in a Cracked Pipe

When a small crack is detected in a pressure vessel or piping, we can estimate the fatigue life of the vessel or piping by applying the classical law of fracture mechanics for crack growth if we are certain that the crack growth exponent is correct and the crack geometry is a simple plane. Unfortunately, for an ageing vessel or piping, the degradation will, in practice, change not only the crack growth exponent but the crack shape from a simple plane to a zig-zag pattern. To validate the crack growth exponent for an ageing vessel or piping, we present the design of an Intelligent PYTHON (IP) code to convert the information of the growing crack geometry measured by monitoring a small crack that was initially detected and subsequently continuously monitored over a period of time such that the IP-based analysis code will use the realistic zig-zag crack geometry as a series of re-meshed finite-element meshes for finding the correct crack growth exponent. Using a numerical example, we show that such an IP-assisted continuous monitoring program, using PYTHON as the management tool, TRUEGRID as the topological crack meshing tool, and two finite-element analysis codes for verifiable stress analysis, is feasible for predicting more accurately the fatigue life of a cracked vessel or piping because the material model has a field-validated crack growth exponent. Significance and limitations of this IP-assisted approach are discussed.

Inconel X-750 Spacer Fitness-for-Service

The CANDU1 (CANada Deuterium Uranium) reactor core consists of 380–480 horizontal Zr-Nb pressure tubes, which support fuel bundles and provide pressurized heavy water cooling. The pressure tubes are supported by fuel channel annulus spacers, which maintain the gap between the hot pressure tube and colder calandria tube while providing a means of leak detection through the annulus gas system. Research and testing in this area have shown that spacer material degradation in later life operation could impact the ability of the component to meet its design requirements. This paper presents a fitness-for-service strategy that could be utilized in demonstrating continued safe operation of these components. Fitness-for-service is based on analysis of crush tests on ex-service spacers to determine the load carrying capacity projected into the future and endurance tests to determine fatigue life. This paper describes these technical approaches and their application in fitness-for-service evaluation of spacers in CANDU operating plants to satisfy requirements for an annulus spacer surveillance program under Clause 12.5 of the CSA Standard N285.4-14.

Impact of the Harsh Environment on E-Glass Epoxy Composite

Composite materials are being used in many industrial applications such as automobile, aerospace, marine, oil and gas industries due to their high strength to weight ratio. The long-term effect of sustained loads and environmental factors that include exposure to UV light, temperature, and moisture have been under investigation by many researchers. The major objective of this study is to evaluate the effects of harsh environment (e.g. seawater and high temperature) on the structural properties of E-glass epoxy composite materials. These effects were studied in terms of seawater absorption, permeation of salt and contaminants, chemical and physical bonds at the interface and degradation in mechanical properties. Samples were immersed in seawater at room temperature (23°C), 65°C and 90°C for the duration of 6 months. Results show that seawater absorption increased with immersion time at 23°C and 65°C, whereas the weight of the specimens decreased at 90°C. The moisture causes swelling at 23°C and 65°C and breakdown of chemical bonds between fiber and matrix at 90°C. It is observed that high temperature accelerates the degradation of the E-glass epoxy composite. At 90°C, the tensile strength of E-glass epoxy sharply decreased by 72.92% but no significant change was observed in modulus of elasticity of the composite.

Crack Repair Using Hybrid Additive Manufacturing and Friction Stir Processing

A hybrid additive manufacturing (AM) and friction stir processing (FSP) was used to heal a crack in 6 mm thick Al 6061-T6 aluminum alloy. AL-6061 is usually used in H2 high-pressure vessel fabrication as well as aerospace applications. In this work, Al-Si powder was utilized to fill the crack, then FSP was applied to consolidate and stir the powder with the base metal to fill and close the crack zone. Effect of FSP parameters including welding speed and tool rotation speed on the quality of repair was studied. Various mechanical tests, as well as characterization techniques such as hardness test, optical microscopy, scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS), were employed to study the newly developed hybrid process on the quality of the repair. The investigation revealed that low rotational speed of 800 rpm results in minimum variation in microhardness. Moreover, the impact of welding speed on microhardness is smaller as compared to rotational speed.