Pipe Fracture Tests Research Articles

Ductile tearing simulation of STS410 pipe fracture test under load-controlled large-amplitude cyclic loading: Part II – effect of load amplitude and sequence

As a companion paper of Part I, this paper presents ductile tearing simulation results of STS410 through-wall cracked pipe test data with different amplitudes of load-controlled fully-reversed cyclic loading. The multiaxial fracture strain energy damage model is used with the Chaboche model. The simulated crack growths are not dependent on the ratcheting term in the Chaboche model, but deformations are slightly sensitive. Simulation results suggest that acceleration and retardation behavior in crack growth rates due to the load sequence may not occur under large-amplitude cyclic loading.

Ductile tearing simulation of STS410 pipe fracture test under load-controlled large-amplitude cyclic loading: Part I—Effect of load ratio

Ductile fracture of circumferential through-wall cracked pipe under large-amplitude load-controlled cyclic loading with three different load ratios was simulated using a fracture strain energy damage model. Damage model parameters were determined from a tensile test and a pipe test under monotonic loading. Comparison of simulated and experimental data reveals that the simulation results depend on the cyclic hardening model. Further investigation confirmed that selection of an appropriate cyclic hardening model is more important under load-controlled cyclic loading then under displacement-controlled one.

Ductile Fracture Behaviour of Class 2 and 3 LWR Piping and Its Implications for Flaw Evaluation Criteria

To achieve a rational maintenance program for aged Light Water Reactor components, it is important to establish and to improve the flaw evaluation criteria. The current flaw evaluation criteria such as ASME Boiler and Pressure Vessel Code Section XI are focused on Class 1 piping which usually shows relatively higher toughness. On the other hand, flaw evaluation criteria suitable for Class 2, 3 piping with moderate-toughness are also required because some Class 2, 3 piping systems are as important to plant safety analysis as Class 1 piping. In this study, both analytical and experimental studies were conducted to provide the evaluation method of fracture loads for acceptance criteria for Class 2, 3 piping. Pipe fracture tests by four-point bending were conducted on circumferentially cracked carbon steel pipes with moderate-toughness. The Net-Section Collapse criterion overpredicted experimental maximum loads for through-wall-cracked pipes, which suggested the necessity of Z-factor. Three-dimensional finite element analysis and simplified analysis based on the reference stress method were conducted to complement the limited pipe fracture tests. It was ascertained that the reference stress method always gave moderately conservative fracture loads compared with the finite element analysis and pipe fracture tests as well. Z-factor for Class 2, 3 piping was then derived and formulated using the reference stress method. Z for Class 2, 3 piping was affected by radius-to-thickness ratio, and was higher than Z for Class 1 piping in the present codes.

Effects of Simulated Seismic Loading on LBB Assessment of High Energy Piping

The current Leak Before Break (LBB) assessment is based primarily on the monotonic fracture tearing instability. In it the maximum design accident load is compared with the fracture-tearing resistance load. The effect of cyclic loading has not been generally considered in the fracture assessment of nuclear power plant piping. It is a well-known fact that reversible cyclic loading decreases the fracture resistance of the material, which leads to increased crack growth. Indian nuclear power reactors consider Operational-Basis-Earthquake (OBE) and Safe-Shutdown-Earthquake (SSE) events in the design of various structures, systems, and components. Keeping this in view a series of cyclic tearing tests have been conducted on straight pipes, made of ASTM SA333 Gr.6 carbon steel. This is the material of primary heat transport (PHT) piping material of Indian Pressurized Heavy Water Reactors (PHWR). In this series 13 tests have been carried out circumferentially through wall cracked seamless and circumferential seam welded straight pipes under reversible cyclic bending loading. All the tests have been conducted under quasistatic, i.e., slow loading rates and dynamic inertia effects are not considered. The cyclic test results have been compared with the corresponding monotonic pipe fracture test results. These test results and its comparison with corresponding monotonic tearing clearly illustrate the need of addressing the reduction in apparent fracture toughness of material under reversible cyclic loading and the safe number of load cycles in the LBB assessment.

Specimen temperature conditioning and drift before an S4 pipe fracture test

The resistance of plastic (primarily polyethylene) pipe to rapid axial crack propagation under pressure is specified using the ISO 13477 ‘S4’ (small-scale steady state) test method. Pipe performance in this test may be very sensitive to temperature under standard test conditions. There has recently been concern that the temperature conditioning of pipe specimens required by ISO 13477 may not be sufficiently precise. In order to compensate for an over-estimated increase in surface temperature between removal from the conditioning fluid and testing, some laboratories condition specimens well below the test temperature. Here, infra-red imaging data are used to calibrate a simple and conservative model for the evolution of surface temperature during this interval. The results broadly support the present specification for testing at 0 °C, and provide a more secure basis for conditioning of specimens to be tested at lower temperatures.

Reference stress based fracture mechanics analysis for circumferential through-wall cracked pipes: experimental validation

This paper presents experimental validation of the reference stress based J estimates for circumferential through-wall cracked (TWC) pipes, recently proposed by the authors. Using the pipe test data for circumferential TWC pipes given in the Pipe Fracture Encyclopedia [Pipe Fracture Test Data, vol. 3, Battelle, 1997], the predicted fracture initiation and instability loads according to the proposed reference stress method are compared with experimental ones as well as predictions according to the reference stress method embedded in R6. The results show that both the R6 method and the proposed method give conservative estimates of initiation and maximum moments for circumferential TWC pipes, compared to experimental data. For longer cracks, the proposed method reduces the conservatism embedded in the estimated J according to the R6 method, and the resulting predictions are less conservative, compared to those from the R6 method. For shorter cracks, on the other hand, the proposed method reduces possible non-conservatism embedded in estimated J according to the R6 method, and the resulting predictions are slightly more conservative.

1216 軽水炉中靱性配管に対する Z 係数の策定

To achieve a rational maintenance program for aged light water reactor components, it is important to establish and to improve the flaw evaluation criteria. Current flaw evaluation criteria are focused on Class 1 piping with high toughness, while flaw evaluation criteria suitable for Class 2 and 3 piping with moderate-toughness are also required from the viewpoints of in-service inspection request, reduction of operating cost, and systematization of consistent code/standard. In this study, cracked pipe fracture tests as well as finite element analysis and simplified fracture analysis by the reference stress method were conducted using typical moderate-toughness pipe materials. The experimental and analytical results were reduced to the Z-factor to predict fracture loads of cracked pipes. Fracture loads for Class 2 and 3 piping were smaller than those for Class 1 piping, consequently Z-factors for Class 2 and 3 piping were about 25% higher than those for Class 1 piping.

Low alloy steel piping test for fracture criteria of leak before break

A total of seven pipe fracture tests were performed to provide the data for establishing fracture criteria of leak before break for the low alloy steel pipe, which is expected to be applied to reactor coolant piping and feedwater piping in advanced PWRs in Japan. Test pipes were 6-inch and 8-inch diameter pipes made of SFVQ1A or STPA24 low alloy steel. A circumferential through-wall crack was introduced at the center of a pipe, and four-point bending load was applied without internal pressure. Stable crack extensions were observed in all of the experiments. The net-section criteria (NSC), R6 method option 2 and option 3 were used to estimate the maximum applied load. The predicted values by three kinds of evaluation methods were compared with the experimental loads. Most of the predicted maximum loads agreed well with the experimental maximum loads within 20% difference. The NSC gave the most accurate prediction but also gave unconservative results in some test cases. The predicted maximum loads by R6 option 2 were conservative in all of the test cases. From the viewpoint of conservativeness R6 method can be used for evaluation of the low alloy steel pipe fracture. Therefore, the leak before break (LBB) concept could be applied to the protective design standard against pipe break for the material.

Approximate evaluation method for ductile fracture analysis of a circumferentially through-wall-cracked pipe subjected to combined bending and tension

To estimate the structural integrity of the Light Water Reactor piping, combined loading consists of a tensile load due to internal pressure and a bending load under seismic conditions should be considered as a basic loading mode. However, theoretical investigation on the methodology to evaluate ductile fracture behavior is not adequate to date. In this study, an approximate evaluation method, ‘LBB.ENGC’, for ductile fracture analysis of a circumferentially through-wall-cracked pipe subjected to combined bending and tension was newly developed. This method can explicitly incorporate the contribution of both tension and bending. The effect of growing crack is also considered in the method. The LBB.ENGC was then applied to the full-scale pipe fracture tests. Based on the comparison with experimental results as well as finite element calculations, it could be ascertained that the LBB.ENGC could well predict ductile fracture behavior under combined loading. The effect of combined loading on ductile fracture was sensitivity-studied using the LBB.ENGC. As a result, it was quantitatively found that the superposition of longitudinal stress reduced the maximum bending load of cracked pipe.

Crack-opening-area analyses for circumferential through-wall cracks in pipes—Part II: model validations

This is the second paper in a series of three papers generated from a recent study on crack-opening-area analysis of circumferentially cracked pipes for leak-before-break applications. This paper (Part II—Model Validations) focuses on the evaluation of current analytical models, discussed in the first paper (Part I—Analytical Models) as well as finite element models for conducting crack-opening-area analyses of pipes with circumferential through-wall cracks. The evaluation was performed by direct comparisons of the predicted results with the test data from full-scale pipe fracture experiments. The results from 25 full-scale pipe fracture experiments, conducted in the Degraded Piping Program, the International Piping Integrity Research Group Program and the Short Cracks in Piping and Piping Welds Program, were used to verify the analytical models. The main objective was the evaluation of engineering analysis procedures (estimation methods) as well as the ability of the finite element method to predict crack-opening displacements and shapes in pipes with circumferential through-wall cracks. Statistics were developed to quantify the accuracy of the current predictive models. A wide variety of pipe fracture tests involving cracks in base metals, weld metals and bimetallic weld metals were analyzed. Pipes containing both simple through-wall cracks and complex cracks were evaluated.

A research program for dynamic fracture evaluation of Japanese carbon steel pipes

A research program was developed to investigate the dynamic load effect on fracture behavior of Japanese carbon steel STS410 pipe. The program comprises material tests, pipe fracture tests and development of estimation scheme. Material property tests showed that the flow stress was nearly constant or slightly increased with strain rate. Pipe tests showed that fracture load was nearly predicted by the net-section collapse criterion for both quasi-static and dynamic loading. Significant dynamic effect was not observed for STS410 carbon steel piping. Crack growth was well formulated by using J-integral parameter for low cycle fatigue with large scale yielding. Combining the crack growth behavior and unstable fracture criterion, an estimation scheme was newly developed and validated for constant amplitude cyclic loading conditions.

Results of reliability test program on light water reactor piping

The Japan Atomic Energy Research Institute has conducted a piping reliability test program to demonstrate the safety and reliability of light water reactor primary piping. In this program, pipe fatigue test, leak-before-break (LBB) verification test and pipe rupture test were carried out to examine the integrity of piping, to verify the LBB and to demonstrate the effectiveness of protective measures against jet impingement and pipe whip loads under a pipe rupture event. In the pipe fatigue test, a procedure to predict the fatigue crack growth was developed, and the integrity of piping during the plant service life was evaluated. In the LBB verification test, the pipe fracture test and the leak rate test were performed to verify the LBB in the primary piping. In the pipe rupture test, the influence of jet impingement on the target disk and the deformation behavior of whipping pipe and restraint were investigated. Using the test results, the jet impingement behavior and the effectiveness of pipe whip restraint were demonstrated.

軽水炉１次冷却系配管信頼性実証試験の成果概要

Upon request by the Science and Technology Agency of Japanese Government, the Japan Atomic Energy Research Institute has conducted Piping Reliability Test Program to demonstrate the safety and reliability of light water reactor primary pipings. In this report, the results of the program are summarized. In the test program, pipe fatigue tests, Leak-Before-Break (LBB) verification tests and pipe rupture tests were carried out to examine the integrity of pipings, to verify the LBB concept and to demonstrate the effectiveness of the protective measures against jet impingement and pipe whip under pipe rupture event, respectively. In the pipe fatigue tests, a procedure to predict the fatigue crack growth was developed and the integrity of piping during plant service life was demonstrated. In the LBB verification tests, pipe fracture tests and leak rate tests were performed using cracked pipes. Based on the test results, LBB in the primary pipings was demonstrated. In the pipe rupture tests, the influence of jet impingement on the target plate and the interaction between whipping pipe and restraint were investigated. Using the test results, the effect of jet impingement and the effectiveness of pipe whip restraints were demonstrated.

A criterion for contact fatigue of ion-nitrided gear: Wu, Y.S., Ma, B.T., HeHo, J.W. and Lyo, B.Z. Wear Jan. 1989 129, (1), 13–21

This is the second paper in a series of three papers generated from a recent study on crack-opening-area analysis of circumferentially cracked pipes for leak-before-break applications. This paper (Part II—Model Validations) focuses on the evaluation of current analytical models, discussed in the first paper (Part I—Analytical Models) as well as finite element models for conducting crack-opening-area analyses of pipes with circumferential through-wall cracks. The evaluation was performed by direct comparisons of the predicted results with the test data from full-scale pipe fracture experiments. The results from 25 full-scale pipe fracture experiments, conducted in the Degraded Piping Program, the International Piping Integrity Research Group Program and the Short Cracks in Piping and Piping Welds Program, were used to verify the analytical models. The main objective was the evaluation of engineering analysis procedures (estimation methods) as well as the ability of the finite element method to predict crack-opening displacements and shapes in pipes with circumferential through-wall cracks. Statistics were developed to quantify the accuracy of the current predictive models. A wide variety of pipe fracture tests involving cracks in base metals, weld metals and bimetallic weld metals were analyzed. Pipes containing both simple through-wall cracks and complex cracks were evaluated.

Evaluation of JAERI's ductile pipe fracture test results on stainless steel and carbon steel piping

At JAERI a ductile pipe fracture test program has been conducted as part of a piping integrity and safety research program on LWRs. In the program the ductile pipe fracture behavior of piping under 4-point bending load without internal pressure has been investigated using 3-, 6-, 12-inch diameter stainless steel and 6-inch diameter carbon steel pipes with a through-wall or a part-through circumferential crack in the inner surface of the test pipes. In this paper are described the evaluation of the pipe test data with respect to the validity of net-section stress approach and the flaw acceptance criteria of ASME code Sec.XI, IWB 3640. Pipe fracture data are also used to evaluate the critical conditions at the onset of an unstable ductile pipe fracture under displacement controlled bending load, and a procedure is presented to evaluate the stability of the piping system against unstable ductile fracture using empirical expressions of pipe fracture data.

Pipe fracture safety analysis using limit load and J-integral techniques

J-integral fracture toughness tests were performed on full scale pipe specimens to assess the fracture safety performance of two reactor piping alloys. The two alloys investigated were ASTM A106 Grade B carbon steel and circumferentially welded Type 304 stainless steel. The full scale pipe fracture tests were performed on 1.2 m long, circumferentially cracked pipes loaded in four-point bending on a variably compliant test bed. Results of the experiments were analyzed using the limit load approach currently being considered for inclusion in Section XI of the ASME Code. The results were also evaluated using two tearing instability approaches. One approach assumed elastic-perfectly plastic material behavior and the other accounted for material hardening by requiring actual load and displacement data. The limit load analysis provided a good prediction of the maximum load carrying capacity of the pipe specimens in most cases. The results were especially good for the ASTM A106 steel pipes when the materials property data was used to calculate the flow stress. The J-integral tearing instability analysis was shown to accurately describe the ductile tearing instability behavior of the ASTM A106 steel pipe providing material hardening was taken into account.

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.

Fracture behavior of cracked Type 304 stainless steel pipes under tensile and thermal loadings

Because of frequent discoveries of stress corrosion cracking in BWR primary piping systems, it became an urgent problem to evaluate the possibility of unstable fracture of flawed stainless steel pipes. To understand the fundamental fracture property as well as the behavior under actual plant conditions, the authors performed the following four types of tests on the cracked Type 304 stainless steel plates or pipes. 1. 1. Plate fracture test under tensile loading. 2. 2. Pipe fracture test under tensile loading. 3. 3. Leak-before-break test under high-compliance loading. 4. 4. Thermal shock test. These test revealed quite high resistance of the flawed stainless steel pipes against unstable fracture.

Stable Growth and Instability of Circumferential Cracks in Type 304 Stainless Steel Pipes Under Tensile Load

Flaws due to stress corrosion cracking have frequently been found in Type 304 stainless steel piping of some boiling water reactor (BWR) plants. In this work, in order to quantitatively examine the integrity under the presence of such cracks, pipe fracture tests and related finite element analyses were conducted. The unstable fractures were observed under the quasi-static and the dynamic tensile loading conditions with a high-compliance test machine. The finite element methods were used to demonstrate the effectiveness of the J-integral and the crack tip opening angle (CTOA) as criteria for stable crack extension, and that of the total displacement stationariness condition as a criterion for the onset of instability.