Component Low Cycle Fatigue Behavior Based on Standard Calculation Procedure and Non-Linear FEA

Components of conventional power plants are subject to potential damage mechanisms such as creep, fatigue and their combination. These mechanisms have to be considered in the mechanical design process. Against this general background — as an example — the paper focusses on the low cycle fatigue behavior of a main steam shut off valve. The first design check based on standard design rules and linear Finite Element Analysis (FEA) identifies fatigue sensitive locations and potentially high fatigue usage. This will often occur in the context of flexible operational modes of combined cycle power plants which are a characteristic of the current demands of energy supply. In such a case a margin analysis constitutes a logical second step. It may comprise the identification of a more realistic description of the real operational loads and load-time histories and a refinement of the (creep-) fatigue assessment methods. This constitutes the basis of an advanced component design and assessment. In this work, nonlinear FEA is applied based on a nonlinear kinematic constitutive material model, in order to simulate the thermo-mechanical behavior of the high-Cr steel component mentioned above. The required material parameters are identified based on data of the accessible reference literature and data from an own test series. The accompanying testing campaign was successfully concluded by a series of uniaxial thermo-mechanical fatigue (TMF) tests simulating the most critical load case of the component. This detailed and hybrid approach proved to be appropriate for ensuring the required lifetime period of the component.

Investigation on Effect of Analysis Variables on Structural Integrity of the Nuclear Piping Under Beyond Design Basis Earthquake

In this study, effect of analysis variables on structural integrity of nuclear piping under beyond design basis earthquake was investigated via performing dynamic time history seismic analysis. A finite element model of the piping system such as shut-down cooling line was developed combining solid and beam elements. Dynamic time history analysis was performed via finite element elastic plastic stress analysis. Validity of the dynamic time history analysis procedure was verified via comparing with the previous study results. Finally, the effect of analysis variables such as finite element characteristics, transition length between elbow and straight line, fluid effect, etc. was investigated via performing parametric dynamic time history seismic analysis. As a result, it was found that use of the 1st incompatible element is recommended, the transition length is the same as curvature of the elbow, and fluid has to be considered.

Recent Developments of Fracture Mechanics Tools for Nozzle Corners

A recent effort was performed within AREVA-NP in order to develop and validate specific tools dedicated to the Fracture Mechanics Assessment of nozzle and penetration corners. This work is motivated by the need to perform accurate defect assessments for this geometry potentially submitted to a high level of stress. It includes: • The development of a specific compendium for the calculation of the Stress Intensity Factor under various types of loading like internal pressure or cold thermal shocks; • The evaluation of the effect of plasticity through simplified analytical schemes; • The validation through the comparison to existing solutions. One important target of those tools being the Fatigue Crack Growth evaluation under cyclic thermal shock loading situations, a validation of the fully analytical evaluation scheme was performed by a comparison to a complex 3D Finite Element Modelling of a defect propagating at a flange corner. This comparison is presented in the paper and illustrates the large benefit in terms of time and simplicity of the analytical scheme for such evaluation.

Difference of Strength Evaluation Approach Between for DBE and for BDBE

Preparation for beyond design basis events (BDBE) becomes important as the lessons learned from the Fukushima Daiichi nuclear accident. The objective of strength evaluation for design basis events (DBE) is a confirmation to prevent structural failure for assumed events. For BDBE, main objectives are weak point survey, deterministic and probabilistic risk assessment, and planning of countermeasures including potable equipment and accident management. According to the above objectives, strength evaluation approach have to be different between for DBE and for BDBE. (1) DBE Conservative approach to prevent of failure. Design by analysis concept is basically adopted Assumption of hypothetical failure modes to prevent actual failure modes Stress criteria to bond actual strength Elastic analyses for conservative loading assumption Design factor to bound uncertainties (2) BDBE Best estimation of failure behavior with uncertainties to plan mitigations Identification of realistic failure modes to identify failure consequences Criteria by dominant parameters of failure phenomena Inelastic analyses for realistic loading prediction Probabilistic evaluation to quantify uncertainties. Strength evaluation concept has not yet been established for BDBE. It is necessary to discuss from basic philosophy to make sharable concepts. Adequate criteria is required to meet above concepts. Instead of stress, strain is one of candidate. New evaluation technics are desired to satisfy above criteria. This paper indicates the direction of strength evaluation for BDBE with same examples proposals. Its aims is to promote international discussions and to implement new technologies to actual countermeasures against BDBE.

Comparisons of CFD and Traditional Solutions for Steam Hammer Events

Sudden pressure changes in the piping system of power plants are inevitable, and thus potential serious damage to large components, piping system, and piping supports is possible. To protect valuable components from such events, abrupt valve closure is employed to restrict the flow and prevent significant incidents and the resulting plant downtime. Unfortunately, when a valve is suddenly closed to prevent damage caused by unexpected events, a pressure wave within the flow is created, which travels upstream and impacts at the pipeline elbows. These events, involving sudden changes in pressure, are known as steam hammer. This steam hammer pressure wave, traveling through the pipe system, is capable of producing significant transient loads and stresses, which can disrupt the piping supports. As such there is a need for further investigation. The pressure wave depends on the characteristics of the flow, valve closure time, the elbow-to-elbow pipe section lengths, and the piping system flexibility. The present study performs a CFD analysis of the fluid experiencing such a sudden pressure change. OpenFOAM is used for this analysis and considers all the flow parameters, valve closure time, and critical length of the straight pipe. The study intends to provide a means of calculating the transient steam hammer loads applied on the pipe elbows, which consequently allows appropriate pipe support selection based upon the resulting peak loads. This computational analysis is compared to analytical methods for peak load determination such as rigid column theory, the Joukowsky method, and the steam hammer method explained by Coccio (1967) and Goodling (1989).

Sufficiency of Reference Stress Solutions for FFS Evaluation of Crack-Like Flaws

In Japan, a new standard of an assessment procedure for crack-like flaws in pressure equipment at elevated temperature is now under development in the High Pressure Institute of Japan (HPI). In this standard development, it is needed to adopt reference stress solutions for crack-like flaws in pressure equipment being subjected to membrane stress and/or bending stress. Such reference stress solutions have been proposed in various references such as ASME FFS-1/API579-1, BS7910, R5, FBR draft guideline, HPIS Z101-2, etc. A comparative study of those reference stress solutions was conducted in order to select appropriate one. As a result, reference stress solutions in HPIS Z101-2 were adopted. The sufficiency of adopted reference stress solutions was introduced in this paper. Also, the reference stress solutions for axially and circumferentially through-wall rectangular flawed cylinders, which were not provided in the HPIS Z101-2 standard but were utilized to derive those solutions adopted in the standard, were introduced in this paper. These solutions should be adopted in a new HPI standard for crack-like flaws in pressure equipment at elevated temperature.

Consideration of Reduction in Stiffness due to Cracking and the Impact on Standard Stress Intensity Factor Solutions

An API 579-1/ASME FFS-1 Failure Assessment Diagram based Fitness-for-Service assessment was carried out on an embedded crack-like flaw found in a nozzle to shell weld in a pressure vessel. Stress intensity factors were initially calculated by utilizing stress results from a Finite Element Analysis (FEA) of an uncracked configuration, with the standard embedded crack stress intensity factor solution given in API 579-1/ASME FFS-1. Due to the complex nozzle geometry and flaw size, a second analysis was carried out, incorporating a crack into the FEA model, to calculate the stress intensity factors and evaluate if the standard solution could be applied to this geometry. A large difference in the resulting stress intensity factors was observed, with those calculated by the FEA with the crack incorporated into the model to be twice as high as those calculated by the standard solutions, indicating the standard embedded crack stress intensity factor solution may be non-conservative in this case. An investigation was carried out involving a number of studies to determine the cause of the difference. Beginning with an elliptical shaped embedded crack in a plate, the stress intensity factor calculated with an idealized 3D crack mesh agreed with the API 579-1/ASME FFS-1 solution. Examining other crack locations, and crack shapes, such as a constant depth embedded crack, revealed how the solution began to differ. The greatest difference was found when considering a crack mesh with a small component height (i.e. the distance measured perpendicular from the crack face to the top of the mesh). A close agreement was then found between the stress intensity factors calculated in the nozzle model and an idealized crack mesh with component heights representative of the true geometry. This revealed that reduced structural stiffness is a key factor in the calculation of the stress intensity factors for this geometry, due to the close proximity of the embedded crack to the inner surface of the nozzle. It was found that this reduction is potentially significant even with relatively small crack sizes. This paper details the investigation, and aims to provide the reader with an awareness of situations when the standard stress intensity factor solutions may no longer be valid, and offers general recommendations to consider when calculating stress intensity factors in these situations.

Nonlinear Buckling Analysis of Cylindrical Shell With Normal Nozzle Subjected to Axial Loads

Design of Large-scale and light-weight pressure vessels is an inexorable trend of industrial development. These large thin-walled vessels are prone to buckling failure when subjected to compression loads and other destabilizing loads. Thus, buckling analysis is a primary and even the most important part of design for these pressure vessels. Local buckling failure will probably occur when cylindrical shells with nozzle subjected to axial loads. In this paper, a FE model of cylindrical shell with a normal nozzle is established in ANSYS Workbench. The bifurcation buckling analysis is performed by using an elastic-plastic stress analysis with the effect of nonlinear geometry, and a collapse analysis is performed with an initial imperfection. The axial buckling loads are obtained by these two types of method. Some issues about nonlinear buckling analysis are discussed through this study case.

Plastic Loads for Cones Subjected to Internal Pressure and Axial Tension

The paper discusses envelopes of combined loading corresponding to: (i) first yield (ii) plastic load, and (iii) plastic instability load. The latter two were researched in the past but for a single load, only. The past idea has been expanded in the paper to two, practically relevant, simultaneously acting loads. Conical shell serves here as an example. It is shown that the ratio of area of plastic load envelope to the area associated with the first-yield envelope is 3.2 whilst the similar ratio of plastic instability to the first-yield envelope amounts to 25.8. This indicates ‘a modest’ (320 %) increase of possible range of loading, and a substantial reserve of strength above the end-of-elastic behavior (approx. 26-times).

Viscoelastic and Damage Model of Polyethylene Pipe Material for Slow Crack Growth Analysis

Polyethylene (PE) pipe is widely used for oil and gas transportation. Slow crack growth (SCG) is the main failure mechanism of PE pipes. Current SCG resistance testing methods for PE pipes have significant drawbacks, including high cost, time-comsuming and uncertain reliability. Alternative method is in need to reduce the testing time and/or cost. In this paper, a numerical model is proposed by taking the viscoelastic and damage effect of PE material into account. The material behavior is described on the basis of linear viscoelastic integral constitutive model, along with damage effect in effective configuraion concept. Three dimensional incremental form of damage viscoelastic model is derived and implemented by ABAQUS UMAT. It is found that the curve of tensile displacement via time, as well as the curve of crack opening displacement via time from numerical results fit well with those from the standard PENT test (ASTM 1473). Based on the proposed model, SCG failure process is analyzed, and the effects of damage parameters on SCG process are furtherly studied and discussed.