Multiaxial fatigue studies on carbon steel piping material of Indian PHWRs

The tests studies and analyses have been carried out in the area of “Multiaxial Fatigue” with an objective to improve the damage assessment methodologies and design rules. Nearly 50 numbers of fatigue tests were conducted on solid and tubular specimens of SA333Gr.6 material under pure axial, pure shear and combined axial-torsion in-phase/ out-of-phase loading combinations. A software has been developed for the evaluation of multiaxial fatigue damage for the analyses of tests data using different invariant fatigue models such as ASME Sec.III code procedures, von-Mises etc. The fatigue crack initiation life was predicted using the best fit axial fatigue life curve (without use of safety factors). These tests and their analyses have helped in understanding the fatigue failure behavior of piping material under complex cyclic loadings where the principal directions rotate during a loading cycle. The crack initiation angles have also been measured by analyzing the image of the tested specimens. The measured crack angles will help in validation of the critical plane based models

Reliability Assessment of Failure Assessment Diagram Based Fitness for Service Procedure Including the Effect of Bias in Modeling

This paper presents the estimation of the reliability levels associated with a cracked pipe found acceptable as per the failure assessment diagram (FAD) based acceptance criteria of ASME Section XI, Appendix H. This acceptance criterion is built on the concepts of fracture mechanics. The parameters which enter the acceptance criteria are piping geometry, applied loading, crack size, and the material properties (tensile and fracture). Most of these parameters are known to exhibit uncertainty in their values. The FAD used also has an associated modeling bias. The code addresses these uncertainties by providing a factor of safety on the applied load. The use of a common factor of safety for a variety of pipe sizes, crack configuration, load combination, and materials may not ensure consistent level of safety associated with the piping component being evaluated. This level of safety can be evaluated by using structural reliability concepts. This paper analyzes the reliability level which is achieved if a cracked pipe passes the acceptance criteria prescribed by the code. The reliability is evaluated for a range of pipe and crack geometry, different load combination, and different materials using Monte Carlo method. The realistic assessment of reliability also requires the assessment of modeling bias associated with the FAD. This bias is also evaluated using the results from the published fracture experiments

Uniaxial and biaxial ratchetting study of SA333 Gr.6 steel at room temperature

The phenomenon of ratchetting is defined as constant accumulation of plastic strain or deformation under combined steady state and cyclic loading. It can reduce the fatigue life or can cause failure of piping components or systems subjected to seismic or other cyclic loads. The uniaxial ratchetting characteristics of SA333 Gr.6 steel have been investigated at room temperature in the present paper. The specimens were subjected to cyclic axial stress with a constant mean stress of 200 MPa and a varying amplitude stress of 149, 174, 199 and 224 MPa. Tests were also performed on 203.2 mm, Sch 80, SA333 Gr. 6 carbon steel straight pipe. The pipe was subjected to a constant internal pressure of 18 MPa and a cyclic bending load. The effects of amplitude of load on the rate of ratchetting have also been investigated in the present paper. The uniaxial experiments showed that specimens exhibited shakedown at low stress amplitude after some strain accumulation. However, specimens experienced continuous ratchetting at higher stress amplitudes with no shakedown before failure. Ovalization of the pipe cross-section was observed when the pipe was subjected to constant internal pressure and cyclic point load. Local bulging was observed at higher loading. The pipe did not show any shakedown behaviour for the given cycles of loading and exhibited continuous ratchetting under the varying amplitude loading.© Elsevie

Uniaxial and biaxial ratchetting in piping materials-experiments and analysis

The performance of the Chaboche kinematic hardening model has been evaluated in this paper to predict the ratchetting responses for a broad set of uniaxial and biaxial loading histories. The investigations have been performed with reference to both uniaxial and biaxial experimental data, viz. (a) strain and stress controlled uniaxial tests on tensile specimens; (b) biaxial tests on straight pipes with constant internal pressure and cyclic bending load; and (c) a shake table test on elbow. The parameters of the Chaboche model have been calculated from the uniaxial strain controlled stable hysteresis loop. Amongst the various parameters in the Chaboche model, it has been found that the selection of the value of γ3 plays a crucial role in achieving better simulation. The Chaboche model was observed to predict complete shakedown for γ3=0. On the other hand, the model closely simulated the experimental results for γ3=9. The same parameters have been used to analyze the biaxial loading condition. Ratchetting simulation studies by the Chaboche model have resulted in reasonably good agreement with experiments.© Elsevie