Session: CS-16-01 Fatigue and Ratcheting Issues in Pressure Vessel and Piping Design-1

Paper Number: 151762

151762 - Assessment of Ratcheting: A Proposed Rule and Engineering Guideline to Prevent Ratcheting 

Abstract: 

When a structure is subjected to a cyclic loading with non-zero mean stress, an accumulation of plastic strain may occur, and it might lead to a failure of a structure. If that accumulation ceases after a significant number of cycles, the phenomenon is known as “finite” ratcheting or a ratcheting followed by a plastic shakedown. If it never ceases, the phenomenon is known as “infinite” ratcheting or ratcheting.

For nuclear power plants, the combination of an internal pressure and thermal fluctuations, under certain conditions, can cause ratcheting. Because of that, a ratcheting check is required for the components of a nuclear power plant that are submitted to those kinds of loadings.

In these cases, ratcheting is mainly investigated by means of simplified rules. According to nuclear design codes (RCC-M, RCC-MRx, ASME, R5…), an elastic Finite Element Analysis (FEA) must be performed, and then the stresses have to be compared with criteria which ensure that (elastic or plastic) shakedown occurs. As the plasticity is not considered on the elastic FEA, those methods are often overly conservatives. In such cases, alternative methods must be carried out. An alternative method is to perform a direct cycle-by-cycle elastic-plastic FEA in order to evaluate that the component is within shakedown and the accumulated plastic strain is lower than the strain criteria. However, that method is more computationally expensive and complex to use than commonly used ones.

The aim of the paper is to propose a rule and an engineering guideline for the assessment of ratcheting. The latter must be less conservative than simplified commonly used rules and without requiring a full cyclic elastic-plastic computation.

This paper is composed of fifth sections. The first section introduces the ratcheting phenomenon. The second section briefly presents the ratcheting rules typically used in the design codes: 3S_M rule and Bree diagram. The third section proposes a new ratcheting method. An engineering guideline related to the method is also described. Moreover, criteria for the rules are proposed and discussed for an austenitic stainless steel. The fourth section is dedicated to the evaluation of the new method. Experimental tests from the literature are exposed. Their results are used to compare and discuss the performance of the new rule. Numerical tests, using a direct cycle-by-cycle elastic-plastic FEA method, are also performed. It has been demonstrated that there is a good agreement between the new rule and the results from experimental and numerical tests, even though the new rule seems to be more restrictive. Considering the experimental and numerical cases above mentioned, comparisons with commonly used methods are made. It has been shown that the new methods are less restrictive than the typically used methods. The fifth and last section concludes with suggestions for future work.

Presenting Author: Jean Macedo Framatome

Presenting Author Biography: Modeling and Simulation engineer at Framatome

Authors:

Jean Macedo Framatome
Antoine Martin Framatome
Olivier Ancelet Framatome