Peter Gill

Revised Guidance on Residual Stresses in BS7910

A major revision of the British Standard BS7910 on “Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures” is being planned for issue in 2012. This paper provides an overview of the proposed revised guidance in relation to recommended weld residual stress profiles. As such, the paper is focussed on the proposed revised Annex Q of BS7910 which deals with residual stress distributions in as-welded joints.Copyright

Leakage Rates Through Complex Crack Paths Using an ODE Method

The geometry of a crack is a fundamental consideration when calculating leakage rates for Leak-before-Break assessments. Carrying out fluid mechanics calculations does not give any additional benefit if there is not enough information on the crack shape. To address this issue, work is being carried out under the R6 development programme to derive a model that couples fluid mechanics and solid mechanics. The aim is to combine complex crack shapes with relatively simple fluid mechanics models and compare with experimental data. Then, the model can be extended to examine various stress distributions, and give indications as to how conservative are the current models. The model is a development of the one presented in a previous PVP paper (Reference 1), and a special case of isothermal gas flow is considered, where the equations reduce to an Ordinary Differential Equation (ODE). This is solved using a Runge-Kutta integration scheme in MATHCAD. A test case is presented based on the crack geometries considered in experiments, and upon comparison with numerical results; it is clear that choosing the correct crack shape is crucial in obtaining accurate predictions of leak rate. The assumed crack openings are rectangular, diamond or elliptical. In addition to this, weld residual stress profiles are postulated, based on experience of welds in piping components. Comparing the numerical simulations with the simplified DAFTCAT model indicates that the more precise ODE method can reduce conservatism in calculation of leak rates.Copyright

A Coupled Fluid-Solid Model to Investigate Leak Rates for Leak-Before-Break Assessments

A multiscale model is discussed here which incorporates fluid mechanics into a structural model. This is so that leak rates can be output from a crack in a finite element model without any complex meshing or post processing. The model can be implemented into any standard finite element solver, meaning leak rates can be investigated for cracks in realistic components. Crack opening area is obtained directly from the special elements that surround the crack, and leak rate is output with the solution. By adopting this holistic approach, it makes the calculation of leak rate more efficient, with the added benefit of higher accuracy. Furthermore, this provides a tool to investigate thermal interactions between the fluid and solid. Presented in this paper is a description of the physical model, and an outline of the numerical solution procedure. Leak rates are also discussed for the case of crack in a plate. The numerical tool provides an effective way of coupling a fluid model with microscale effects, to a macroscale solid model. Investigations using this new method have the potential to reduce uncertainty in leak rate evaluation. Also, by reducing the uncertainty, it is suggested that there would be an increase in the number of situations where Leak-before-Break could be applied in Nuclear Power Plants.Copyright

Investigating the Influence of Fluid-Structure Interaction on Leak-Before-Break Analyses With a New Finite Element Method

This paper describes a new finite element method to analyse Leak-before-Break using the extended finite element method (XFEM). Traditionally, a Leak-before-Break analysis is performed using a methodology outlined in a Structural Integrity Assessment Procedure such as R6. This may achieve sufficient accuracy for the application of interest. However, there may be times when finite element (FE) analysis is required. With this in mind, a finite element model has been developed in which all the physics of the crack and leak rate is bundled into a special element, with only the crack size and position, external loads and fluid properties required as an input. All the meshing and leak rate calculation is dealt with by the code. This means that Leak-before-Break procedures can be performed more efficiently, even if FE modelling is required. The element was implemented in MATLAB and then used to investigate thermo-mechanical behaviour of a leaking thermo-fluid in a plate. For bulk fluid temperatures 20°C hotter than the structure, it was found that the leak rate was reduced by 40% due to crack closure.Copyright

A Special Finite Element for Leak-Before-Break Analysis

Leak-before-Break is increasingly being used as part of safety justifications, particularly within the nuclear industry. In order to make a Leak-before-Break case for a pressurised component, it is necessary to determine leak rates through cracks under the operating load conditions. The R6 assessment procedure provides equations to calculate leak rates from a known crack opening area. Leak rates evaluated from this calculational route, however, can be subject to safety factors being applied due to various uncertainties. As such there is a strong motivation to better understand the factors affecting leak rates through cracks in pipes, so that there is less conservatism in the leak rate estimation. To perform the investigations into these factors, a special finite element has been developed. This element uses the extended finite element method to model cracks with boundary conditions on the faces due to the leaking fluid. Previous work has shown that leak rates through a crack are affected by closure of the crack, which is due to convective heat transfer from the leaking fluid to the crack faces. Therefore the new 2-D element is used to investigate this and it shows the effect of heat transfer and pressure acting on the crack faces. The first results of leak rate evaluations, using this new element, are presented in the paper.Copyright

The Evaluation of Crack Opening Areas for Through-Wall Cracks in the Vicinity of Pipe Branch Connections

A number of papers have been presented at previous ASME PVP conferences, which have evaluated the crack opening areas (COA) and stress intensity factors (K), using elastic finite element analysis techniques, for through-wall cracks in a region where an attachment is welded to a plate. This was a simplified geometry aimed at representing a more complicated geometry of a pipe-branch connection. A number of analyses were considered and conclusions made on the estimation of COA and K using simple handbook solutions. More recently the analyses included the application of nonlinear geometry and the addition of crack face contact when applying bending loads. This paper is a continuation of these previous studies, assessing through-wall cracks in a more realistic pipe-branch connection geometry. The calculated COA and K values for the more complex geometry are compared to values from pipe models with no branch connections, in a similar manner to that applied in the previous work on the simplified plate geometry. Judgments are made on the conservatism, or otherwise, of the estimated COA and K for the more complex geometry solutions compared to the simple geometry solutions.Copyright