A Micromechanics Approach to Assess Ductile Crack Initiation in Damaged Pipelines

This study explores the capabilities of a computational cell framework into a 3-D setting to model ductile fracture behavior in tensile specimens and damaged pipelines. The cell methodology provides a convenient approach for ductile crack extension suitable for large scale numerical analyses which includes a damage criterion and a microstructural length scale over which damage occurs. Laboratory testing of a high strength structural steel provides the experimental stress-strain data for round bar and circumferentially notched tensile specimens to calibrate the cell model parameters for the material. The present work applies the cell methodology using two damage criterion to describe ductile fracture in tensile specimens: (1) the Gurson-Tvergaard (GT) constitutive model for the softening of material and (2) the stress-modified, critical strain (SMCS) criterion for void coalescence. These damage criteria are then applied to predict ductile cracking for a pipe specimen tested under cycling bend loading. While the methodology still appears to have limited applicability to predict ductile cracking behavior in pipe specimens, the cell model predictions of the ductile response for the tensile specimens show good agreemeent with experimental measurements.

Recent Advances for J Simplified Assessment in RSE-M Code

The RSE-M Code [1] provides rules and requirements for in-service inspection of French Pressurized Water Reactor power plant components. Non mandatory guidance is given in the Code for analytical flaw evaluation in a wide range of situations. In Appendix 5.4 of the Code, influence coefficients are provided to calculate stress intensity factors in pipes and shells containing semi-elliptical surface defects. The J assessment method is based on the reference stress concept with two options for reference loads evaluation: “CEP elastic plastic stress” and “CLC modified limit load”. In this paper recent advances concerning J assessment under mechanical loading for a crack located in a pipe-elbow junction are presented. Reference stress evaluation with “CLC” option is developed and mechanical foundations of the equation of large scale yielding under complex loading (pressure, in-plane and out-of-plane bending) are presented.

Frequency Transfer Function From Fluid Temperature Fluctuations to Stress Intensity Factors

Temperature fluctuation from incomplete fluid mixing can induce fatigue cracks on structures of nuclear components, which should be prevented. For rational analysis of this phenomenon, the authors have developed a frequency transfer function that translates fluid temperature fluctuation to stress intensity factors. This function is formulated by a product of the effective heat transfer and the stress intensity factor functions, and enables us to quickly calculate stresses intensity factors induced by fluid temperature fluctuations. Furthermore, it can evaluate sensitivities of stress intensity factors to frequencies of temperature fluctuation, Biot number and constraint conditions of structures. Applicability of this function was verified through comparison with stress intensity factors calculated by the finite element method.

Evaluation of Fracture Toughness From Instrumented Charpy Impact Tests for a Reactor Pressure Vessel Steel

The monitoring of neutron induced embrittlement of nuclear power plants is provided using Charpy impact test in the surveillance program. However structural integrity assessments require the fracture toughness. Some empirical formulas have been developed but no direct relationship was found. The aim of our study is to determine the fracture toughness of a Reactor Pressure Vessel steel from instrumented Charpy impact test using local approach to fracture. This non-empirical method has been applied in the brittle domain as well as in the ductile to brittle transition for an A508 C1.3 steel. In the brittle domain, fracture occurs by cleavage and can be modeled with the Beremin model. Fracture toughness has been successfully determined from Charpy impact tests results and the influence of several parameters (mesh design, Beremin model with one or two parameters, number of Charpy impact tests results) on the results was considered. In the ductile to brittle transition, cleavage fracture is preceded by ductile crack growth. Ductile tearing has been accounted for in the simulations with the Rousselier model whereas cleavage fracture is still described with the Beremin model. The determination of fracture toughness from Charpy impact tests gave encouraging results but finite element simulations have to be refined in order to improve predictions.

Elastic-Plastic Fracture Mechanics Assessment of Test Data for Circumferential Cracked Pipes

This paper presents experimental validation of two reference stress based methods for circumferential cracked pipes. One is the R6 method where the reference stress is defined by the plastic limit load. The other is the enhanced reference stress method, recently proposed by the authors, where the reference stress is defined by the optimized reference load. Using thirty-eight published pipe test data, the predicted maximum instability loads according to both methods are compared with the experimental ones for pipes with circumferential through-thickness cracks and with part circumferential surface cracks. It is found that the R6 method gives conservative estimates of the maximum loads for all cases. Ratios of the experimental maximum load to the predicted load range from 0.54 to 0.98. On the other hand, the proposed method gives overall closer maximum loads than R6, compared to the experimental data. However, for part through-thickness surface cracks, the estimated loads were slightly non-conservative for four cases, and possible reasons were fully discussed.

Tests and Calculations of Thermomechanical Shocks in the Brittle/Ductile Transition Zone of a Pressure Vessel Steel

In order to study the effects of a sudden cooling in a thick hot structure, such as the vessel of a pressurised water reactor, a specially-adapted compact tension specimen has been developed. It consists of a CT50 (2T-CT specimen) with holes through the specimen to cool the crack tip locally by liquid nitrogen. Therefore, this new test allows to study in details different loading-temperature histories near the brittle/ductile transition zone which may put the classical crack intiation criteria in the wrong. First, this article describes in details two tests for which a cleavage rupture has been obtained during the thermal shock on this 16MND5 steel. Either the Crack Mouth Opening Displacement was maintained during the test or the applied load. Then, numerical calculations have been realised in order to estimate the local mechanical fields at the crack tip and to evaluate the global fracture mechanics parameters. Thanks to these tests and these thermal and mechanical simulations, a work is done on rupture criteria under thermal shocks by using either the “Master Curve” approach or the Beremin model.

Mechanical Analysis of Underclad Cracks in Reactor Pressure Vessel Nozzles

During the in-service inspection in 1999 in Tricastin 1 Nuclear Plant, underclad cracks had been discovered in the core shell. Following specific justification of these defects, other parts of the reactor vessel also sensitive to this kind of defects have been re-examined, and particularly RPV nozzles, which are classified as “concerned by fast fracture”. Recent assessments with large defects, superior to maximum realistic ones, have been conducted in different sections in the reactor vessel inlet and outlet nozzles: circular cracks of 10 mm in the inner radius, and 20 mm depth in the straight section. Theses assessments highlighted the verification of acceptance criteria in various, normal and emergency conditions with large margins. Fatigue analysis has also been conducted. For underclad cracks located in the inner radius of RPV inlet nozzles, a small propagation in the austenitic cladding is demonstrated but would not lead to any cladding perforation within 40 years.

The Energy Approach of Elastic-Plastic Fracture Mechanics Applied to the Analysis of the Warm Pre-Stress Effect

An experimental program was recently carried out in the context of a MPA-EDF collaboration, in order to study the “Warm Pre-Stress” effect on CT25 side-grooved specimens made of 18MND5 (equivalent to A508) ferritic steel. At EDF, an energy approach has been developed and an energy release rate, called Gp, has been defined. Conversely to the J-approach, this energy approach is, in principle, valid in all situations, and can be used in particular to study the “Warm Pre-Stress” effect in RPV structural integrity assessment. For the WPS cycle called “LCF” (Loading + Cooling + Fracture), the comparison to the experimental results and to the results obtained with the Beremin model is very satisfactory, all the results being in a good agreement. For the WPS cycle called “LUCF” (Loading + Unloading + Cooling + Fracture) the results obtained with the energy method, compared to experimental results, show a conservatism which is not very important, anyway smaller than the conservatism obtained with the Beremin model.

Normalized Stress Intensity Factor Solution of an Inner-Surface Circumferential Crack in Thin- to Thick-Walled Cylinder Under Thermal Striping

In this paper we considered the normalized stress intensity factor (SIF) of an inner-surface circumferential crack in a thin- to thick-walled finite-length cylinder under thermal striping. The edges of the cylinder were rotation-restrained and the outer surface was adiabatically insulated. Inner surface of the cylinder was heated by a fluid with sinusoidal temperature fluctuation. We combined an analytical temperature solution for the problem and our SIF evaluation method for the crack, and as a result, showed that the transient SIF solution can be expressed in a generalized form by dimensionless parameters such as mean radius to wall thickness ratio, Biot number, normalized striping frequency and Fourier number. Finally, normalized SIF ranges for the 1st cycle and steady state were given for these dimensionless parameters in tables for mean radius to wall thickness ratio of 10, 5 and 1.

Stress Intensity Factor Solutions for Part-Throughwall Pipe Flaws

The need to quantify crack tip stress intensity solutions for complex stress states and geometries has forced the development of theoretical stress intensity solutions by numerical methods, such as finite element analysis (FEA). Using FEA, we developed a stress intensity handbook solution to the previously analyzed geometry of part-throughwall pipe flaws. This paper presents the results of this effort for infinitely long, axial internal flaws in cylinders. We also examined the mesh parameters that need to be considered to obtain reasonably accurate elastic mode I stress intensity solutions using the ANSYS finite element program.