Pipe Test Results Research Articles

Effect of surface crack dimension on very low cycle fatigue crack growth behavior of circumferential cracked pipes

In this study, the effects of surface crack depth and length on the fracture behavior of circumferentially cracked pipes under very low cyclic loading were investigated via finite element (FE) damage analysis using an energy-based damage model. The FE analysis method was first validated by comparison with the SA508 Gr.1a surface-cracked pipe test results. Using FE damage analysis, a parametric study was conducted for a surface-cracked pipe with cracks of different depths and lengths, giving the same limit load. The variations in crack penetration and unstable fracture cycles depending on crack depth and length are discussed.

Flaw Evaluation Procedure for Cast Austenitic Stainless-Steel Materials Using a Newly Developed Statistical Thermal Aging Model

Abstract Thermal embrittlement of some cast austenitic stainless steels (CASS) occurs at reactor operating temperatures leading to very low fracture toughness. Because of their low aged toughness with high variability, flaw evaluations of CASS material need to be established with an understanding of the materials aged condition, especially since most U.S. pressurized water reactor (PWR) nuclear plants have been given plant life extensions for 60-year operation. A flaw evaluation procedure for CASS materials is presented here using a new statistical model developed to predict the toughness of fully aged CASS using the materials' chemical compositions. In this procedure, the dimensionless-plastic-zone-parameter (DPZP) analysis is used to determine when limit-load is applicable and also approximate the elastic-plastic correction factor (Z-factor) to predict the failure stress for CASS pipe/fittings with a circumferential surface crack. The procedure was validated against several CF8m pipe test results which include various pipe diameters, crack sizes, ferrite contents, failure modes. The as-developed flaw evaluation procedure was also used to determine the Z-factors for four different pipe diameters for a database of 274 pipe/elbows in U.S. PWR plants –solving 1096 sample problems to understand what range of Z-factors in U.S. PWR plants (for CF8m CASS materials). Finally, the applicability of the CF8m-based statistical model for use with CF3 and CF8 CASS materials was also verified with available test results. The outcome of this work has been implemented in an ASME boiler and pressure vessel (BPV) code case and has been approved in 2020 as Code Case N-906.

FATIGUE LIFE CHARACTERISTICS OF WATERWORKS PIPE WELDS

The fatigue characteristic of a material or a structure is derived from fatigue tests of standard specimens. However, many test results of standard specimens are very different from those of real structures or components because of geometrical effect, surface condition and so on. In this study, fatigue tests with specimens and pipes were carried out to evaluate the fatigue characteristic of a real waterworks pipe. Standard fatigue specimens and non-standard specimens were extracted from a steel pipe used in waterworks system. Also, fatigue tests of real pipes used in water service were carried out. This result was compared with that of standard specimens and non-standard specimens. To evaluate pipe's fatigue characteristics based on life distribution, the statistical analysis method was introduced. Probability density functions of the specimen based on the normal distribution function were obtained from fatigue tests at particular stress levels. These functions were then transformed to probability density functions based on a specific number of cycles to failure. This procedure was also adapted to the pipe's test results. From these results, the fatigue characteristic of waterworks pipe was evaluated.

Techniques to characterize fatigue behaviour of full size drill pipes and small scale samples

Drill pipe fatigue damage occurs under cyclic loading conditions due to, for instance, rotation in curved sections of an oil well. Failures caused by crack nucleation and propagation are one of the highest risks to the structural integrity of these pipes. Usually, failure mechanisms develop in the transition region of the tool joint. A fatigue simulator was designed and built to test full-scale drill pipes under rotating cyclic bending and constant tension loading. Additionally, the fundamental fatigue mechanisms were investigated via laboratory tests in small-scale samples. These tests were conducted in an opto-mechanical fatigue apparatus that was designed to perform in-situ real time monitoring surface analysis during the experiments, by using differential interference contrast (DIC). Scanning electron microscopy fractography of fatigued samples was also performed. The fatigue-life curve was obtained for the drill pipe API S–135 grade steel from small scale tests. The full size drill pipe test results are coherent with the expected, validating the experimental procedures.

Technical Basis for the Extension of ASME Code Case N-494 for Assessment of Austenitic Piping

In 1990, the ASME Boiler and Pressure Vessel Code for Nuclear Components approved Code Case N-494 as an alternative procedure for evaluating flaws in light water reactor (LWR) ferritic piping. The approach is an alternate to Appendix H of the ASME Code and allows the user to remove some unnecessary conservatism in the existing procedure by allowing the use of pipe specific material properties. The Code case is an implementation of the methodology of the deformation plasticity failure assessment diagram (DPFAD). The key ingredient in the application of DPFAD is that the material stress-strain curve must be in the format of a simple power law hardening stress-strain curve such as the Ramberg-Osgood (R-O) model. Ferritic materials can be accurately fit by the R-O model and, therefore, it was natural to use the DPFAD methodology for the assessment of LWR ferritic piping. An extension of Code Case N-494 to austenitic piping required a modification of the existing DPFAD methodology. Such an extension was made and presented at the ASME Pressure Vessel and Piping (PVP) Conference in Minneapolis (1994). The modified DPFAD approach, coined piecewise failure assessment diagram (PWFAD), extended an approximate engineering approach proposed by Ainsworth in order to consider materials whose stress-strain behavior cannot be fit to the R-O model. The Code Case N-494 approach was revised using the PWFAD procedure in the same manner as in the development of the original N-494 approach for ferritic materials. A lower-bound stress-strain curve (with yield stress comparable to ASME Code specified minimum) was used to generate a PWFAD curve for the geometry of a part-through wall circumferential flaw in a cylinder under tension and bending. Earlier work demonstrated that a cylinder under axial tension with a 50-percent flaw depth, 90 deg in circumference, and radius to thickness of 10, produced a lower-bound FAD curve. Validation of the new proposed Code case procedure for austenitic piping was performed using actual pipe test data. Using the lower-bound PWFAD curve, pipe test results were conservatively predicted (failure stresses were predicted to be 31.5 percent lower than actual on the average). The conservative predictions were attributed to constraint effects where the toughness values used in the predictions were obtained from highly constrained compact test specimens. The resultant development of the PWFAD curve for austenitic piping led to a revision of Code Case N-494 to include a procedure for assessment of flaws in austenitic piping.

Ductile fracture behavior of circumferentially cracked type 304 stainless steel piping under bending load

A ductile pipe fracture test program has been conducted in Japan Atomic Energy Research Institute (JAERI) to investigate the ductile fracture behavior of circumferentially cracked pipes and to demonstrate the validity of the leak before break concept in LWR pipings. In the paper are described the scope of the pipe test program and current test results for 6-inch diameter type 304 stainless steel pipes. Test pipes with a through-wall or a part-through crack in the circumferential direction were bent under low or high compliance condition, and stable or unstable pipe fracture behavior was investigated. J based tearing instability criterion and the net section collapse criterion are compared with the pipe test results, and the validity of these fracture criteria is discussed. Furthermore, geometries of acceptable flaws in pipes are evaluated considering the leak before break condition.