Full-scale Pipe Tests Research Articles

Complex hydraulic studies of welded pipes with different visco-plastic characteristics

For the first time, complex hydraulic studies of pipes welded according to three different schemes were carried out, which made it possible to combine pipes of different viscosities, which affects the performance of welding joints. It was found that in the experimental temperature range, the zone of thermal influence of the second welding variant has the greatest resistance to the initiation of fractures under shock loading, and the lowest - the first variant. The static strength and ductility of the welded pipe joints of the second and third pipe welding options are approximately equivalent, and the resistance to the initiation of the thermally affected zone in all options is almost the same and not lower than that of the base metal. The results of the performed experimental studies indicate a weak correlation in the area of small values of impact viscosity with the characteristics of resistance to fracture propagation of welding joints in the conditions of full-scale pipe tests. The characteristics obtained from tests of full-thickness DWTT specimens are closer than the Charpy specimens to the actual performance characteristics of the weld joints in the pipe. In particular, it was established that the values of Az (-150C)=0.75 kJ and Ar (-150C)=0.45 kJ provide satisfactory resistance to the initiation and propagation of fractures in welding joints (at the base metal level). These characteristics correspond to KSU-60 = 0.5 MJ/m2, which is close to the impact toughness requirements for the base metal (0.55 MJ/m2). The use of optimal structural materials, that is, materials with high resistance to hydrogen destruction of both the base metal and zones of welding joints subjected to a thermo-deformation cycle of welding. A significant effect can be achieved by technological measures that will reduce residual welding stresses, as well as significantly reduce stress concentrators due to structural improvement of the shape of the welding unit. Application of such heat treatment regimes that restore the stability of metal with a coarse-grained and defective structure. Use improved pipe steels and welding materials, such as 06G2BAand 08 KhMCHA steels, which are characterized by increased resistance to hydrogen embrittlement and high crack resistance in aggressive environments, for the construction of pipelines of responsible purpose.

Novel XFEM variable strain damage model for predicting fracture in small-scale SENT and full-scale pipe tests

The tensile strain capacity (TSC) of a welded pipeline under bending and/or axial loads is critical to pipeline integrity assessment, and the fracture toughness expressed as a J-R curve plays a key role in predicting the TSC. The single edge notched tension (SENT) tests have been recommended for fracture toughness measurement of pipelines due to the similar low constraint geometries with the full-scale pipelines. The extended finite element method (XFEM) has been increasingly implemented in predicting the fracture of pipelines from small-scale to full-scale tests. However, current damage criteria based on a fixed critical stress or strain value have not been well calibrated, and limitations have been found. This paper develops a novel XFEM variable strain-based damage model by adopting a critical varying strain profile that accounts for the crack-tip constraint derived from a modified Mohr-Coulomb (MMC) fracture criterion. The novel damage criterion is calibrated by reproducing results like those measured from small-scale SENT tests of X100 pipe steel and full-scale pressurized tests of X52 circumferentially surface-cracked pipeline. Two sets of optimal damage parameters are calibrated specifically for X100 and X52. The initial crack in each model is simulated by cutting a notch in the XFEM domain resembling the original crack pre-machined on the specimen. Previously, researchers inserted a XFEM planar crack into the finite element mesh. The comparisons between results from notched models and planar cracked models reveal a distinct difference in the crack tip constraint and fracture resistance. Predictions from the novel variable damage criterion demonstrate advantages over those from the simple fixed damage criterion.

Application of cohesive zone model to large scale circumferential through-wall and 360° surface cracked pipes under static and dynamic loadings

This paper presents ductile fracture simulation of full-scale cracked pipe for nuclear piping materials using the cohesive zone model (CZM). The main objective of this study is to investigate the applicability of CZM to predict ductile fracture of cracked pipes with various crack shapes and under quasi-static/dynamic loadings. The transferability of the traction-separation (T-S) curve from a small-scale specimen to a full-scale pipe is demonstrated by simulating small- and full-scale tests. T-S curves are calibrated by comparing experimental data of compact tension specimens with finite element analysis results. The calibrated T-S curves are utilized to predict the fracture behavior of cracked pipes. Three types of full-scale pipe tests are considered: pipe with circumferential through-wall crack under quasi-static/dynamic loadings, and with 360° internal surface crack under quasi-static loading. Computational results using the calibrated T-S curves show a good agreement with experimental data, demonstrating the transferability of the T-S curves from small-scale specimen.

A further study on fracture resistance evaluation of nuclear pipes using CP specimens

The leak-before-break (LBB) concept has been widely used to design the piping system in nuclear power plant, which is based on the fracture resistance (J-R) curves of cracked nuclear pipes. However, accurate fracture resistance evaluation of nuclear pipe is always a tough task. Full-scale nuclear pipe tests are costly and difficulty, and the specimens listed in test standards (e.g. ASTM E1820) are usually too conservative. Thus, compact pipe (CP) specimen has been developed to evaluate the J-R curves of nuclear pipes with high accuracy but much lower cost than full-scale pipe tests. But, the practical application of CP specimen still asks for more theoretical and experimental research. In this study, the assembly of CP specimen was modified to improve the assembly convenience and ensure a more uniform loading condition. Instead of load-line displacement (LLD), crack mouth opening displacement (CMOD) based J-formulation was proposed to determine the J-R curves of modified CP specimens considering more reliable evaluation of CMOD. In addition, the differences of constraint effect among CT specimen, CP specimens and full-scale pipes are distinguished using equivalent plastic strain (εp) isoline regions, which agrees quite well with the results of fracture resistance tests.

Fracture response of X65 pipes containing circumferential flaws in the presence of Lüders plateau

A yield discontinuity or Lüders plateau can be observed in tensile tests conducted on seamless pipe manufactured to API 5 L X65 strength grade steel. Such material behaviour is associated with strain localisation which can significantly affect the fracture behaviour of X65 steel pipe subjected to plastic strain. This study considers the Lüders plateau, using the so-called “up-down-up” (UDU) constitutive model, in finite element (FE) analyses of seamless X65 pipes containing circumferential surface-breaking cracks and subjected to axial plastic straining. The softening modulus of UDU model was found to significantly affect the simulated evolution of plasticity, crack driving force and crack-tip fields of the cracked pipe. The FE analysis results were validated against the full-scale pipe test data. It was found that by correctly selecting the softening modulus, a suitable level of accuracy and conservatism was obtained by using an UDU model in FE analyses for assessing fracture response of flawed pipes which show Lüders plateau behaviour. In contrast, the existing stress- and strain-based fracture assessment solutions generally underestimate the crack driving force in the Lüders plateau phase.

Transformation of fracture resistance curves by using bending modified Q (Qm) factor

The application of the leak-before-break (LBB) concept to design and assess piping systems for nuclear power plants requires fracture resistance curves (J-R curves) of cracked full scale pipes. However, it is expensive and difficult to perform full scale pipe tests. Laboratory test specimens with smaller geometry size were used to obtain the J-R curves for full scale pipes, but as both the specimen geometry and loading configuration have a strong effect on J-R curves, it is difficult to predict the J-R curves of full scale pipes accurately from laboratory test specimens. In this study, based on the bending modified Q (Qm) factor, and the J-R curves of compact tension (CT) specimens and curved wide plate (CWP) specimens, constraint corrected J-R curves of the stainless steel SA-312 TP304L were obtained, which were used to predict the J-R curves of full scale pipes. The predictions show good agreement with full scale pipe test results within the specific crack growth range.

Reference stress based J and COD estimation of circumferential through-wall cracked elbows under in-plane bending

This paper proposes J-integral and crack opening displacement (COD) estimation equations based on the reference stress method for circumferential through-wall cracked elbows under in-plane bending. For best estimates of J and COD, an optimized reference load, used to define the reference stress, is given. The proposed equations are compared with detailed finite element results. They are further validated against eight published full-scale pipe test data. Comparison of crack initiation and maximum loads shows good agreement.

Effect of vibration loading on the fatigue life of part-through notched pipe

A systematic experimental and analytical study has been carried out to investigate the effect of vibration loading on the fatigue life of the piping components. Three Point bend (TPB) specimens machined from the actual pipe have been used for the evaluation of Paris constants by carrying out the experiments under vibration + cyclic and cyclic loading as per the ASTM Standard E647. These constants have been used for the prediction of the fatigue life of the pipe having part-through notch of a/ t = 0.25 and aspect ratio (2c/a) of 10. Predicted results have shown the reduction in fatigue life of the notched pipe subjected to vibration + cyclic loading by 50% compared to that of cyclic loading. Predicted results have been validated by carrying out the full-scale pipe (with part-through notch) tests. Notched pipes were subjected to loading conditions such that the initial stress-intensity factor remains same as that of TPB specimen. Experimental results of the full-scale pipe tests under vibration + cyclic loading has shown the reduction in fatigue life by 70% compared to that of cyclic loading. Fractographic examination of the fracture surface of the tested specimens subjected to vibration + cyclic loading have shown higher presence of brittle phases such as martensite (in the form of isolated planar facets) and secondary micro cracks. This could be the reason for the reduction of fatigue life in pipe subjected to vibration + cyclic loading.

Determination of the elastic-plastic fracture mechanics Z-factor for alloy 182 weld metal flaws

One of the ways that the ASME Section XI code incorporates elastic-plastic fracture mechanics (EPFM) in the Section XI Appendix C flaw evaluation procedures for circumferential cracks is through a parameter called Z-factor. This parameter allows the simpler limit-load (or Net-Section-Collapse) solutions to be used with a multiplier from EPFM analyses. This paper shows how 3-D finite element (FE) analyses were employed to investigate the sensitivity of the crack-driving force as a function of crack location (i.e., crack in the center of weld, or closer to the stainless or low alloy steel sides) in an Alloy 182 dissimilar metal weld (DMW), and how an appropriate (or equivalent) stress-strain curve was determined for use in the J-estimation schemes. The J-estimation schemes are then used to cover a wider range of variables, i.e., pipe diameters, cracks lengths, and also incorporate crack growth by ductile tearing. The Z-factor equations as a function of pipe diameter were calculated using the LBB.ENG2 J-estimation scheme along with the most conservative equivalent stress-strain curve from the FE analyses. The proposed Z-factor approach was then validated against an Alloy 182 DMW full-scale pipe test that had a circumferential through-wall crack in the fusion line. The predicted EPFM maximum load showed excellent agreement with the experimental result. Furthermore, it was shown that the proposed Z-factor equation is not sensitive to the location of the crack.

Effects of local wall thinning on net-section limit loads for pipes under combined pressure and bending

This paper provides plastic limit loads of pipes with local wall thinning under combined pressure and bending, by quantifying effects of the axial extent and the shape of local wall thinning. The effect of the axial extent on plastic limit loads is not so significant for bending but could be more significant for internal pressure. It is also found that the effect of the shape of local wall thinning on limit loads could be significant. Thus idealization of local wall thinning as a rectangular shape rather than a circular one could lead to significantly conservative estimates of maximum loads, which is also supported by comparison with published full-scale pipe test data.

A Study on Estimation of the Pipe Fracture Characteristics

In order to analyze the elastic-plastic fracture behavior of a structure, the fracture resistance curve of the material should be known. However, it is difficult to evaluate the fracture characteristics with an experiment on the piping system. Instead, the fracture toughness obtained from standard specimen tests is used to analyze the structure and assess the fracture characteristics of the total structure. It is known that toughness data from the standard specimen test are conservative to predict fracture behavior of the real piping. Thus the fracture resistance curve by the fracture test of the real scaled pipe specimen would be applied to the integrity evaluation for the piping system. However, it is not only difficult to perform but also very expensive to perform full-scale pipe tests. The objective of this thesis is to propose a method to estimate the fracture resistance curve of a pipe from the result of standard specimen fracture test. To estimate the fracture resistance curve for a pipe specimen, load – load-line displacement records of a standard specimen were transformed to those of the pipe specimen. The load ratio method was proposed in order to calculate the crack length from load – crack mouth opening displacement records for the pipe specimen. To prove the validity of this estimation results, fracture tests for pipe specimens were performed. Consequently the applicability of the proposed method was investigated by comparing estimated results with tested results.

Net-section limit load approach for failure strength estimates of pipes with local wall thinning

The net-section limit load approach is typically used in assessment of pipes with local wall thinning, based on which a maximum load carrying capacity is easily estimated from one equation that includes two terms associated with the effect of the defect geometry and the material's resistance (strength). To better understand the applicability of the net-section limit load approach to pipes with local wall thinning, four different limit load expressions for pipes with local wall thinning under pure bending are considered, together with two different definitions of the material's resistance. Estimated failure moments are then compared with full-scale pipe test data. It is found that the use of an appropriate limit load solution reduces not only the degree of conservatism but also the dependence of the assessment results on the wall thinning geometry, and thus gives the best results. Therefore, finding such solutions for pipes with local wall thinning is an important issue.

Experimental investigation of the failure behavior of notched wall-thinned pipes

The present study performed full-scale pipe tests using 100A Schedule 80 pipe specimens with simulated notched and circular wall thinning to investigate the failure behavior of notched wall-thinned pipes. The tests were conducted under both monotonic and cyclic bending moments at a constant internal pressure of 10 MPa at room temperature. The failure pattern, load carrying capacity, deformation ability, and fatigue strength of the notched wall-thinned pipes were evaluated by comparing results to those of circular wall-thinned pipes. The investigation showed that the effect of the type of thinning on the failure behavior was more sensitive under cyclic loading conditions than under monotonic loading conditions. The load carrying capacity of pipes with notched wall thinning was approximately the same or slightly less than that of pipes with circular wall thinning when the thinning area was subjected to tensile stress. However, when the thinning area was subjected to compressive stress, the load carrying capacity of pipes with notched wall thinning was greater than that of pipes containing circular wall thinning. The deformation ability and fatigue strength increased proportionally with the axial length of the thinning defect, and thus these properties were significantly reduced in notched wall-thinned pipes.

복합하중이 작용하는 국부감육배관 평가법 개발

Recently authors have proposed a new method to estimate failure strength of a pipe with local wall thinning subject to either internal pressure or global bending. The proposed method was based on the equivalent stress averaged over the minimum ligament in the locally wall thinned region, and the simple scheme to estimate the equivalent stress in the minimum ligament was proposed, based on the reference stress concept. This paper extends the new method to combined internal pressure and global bending. The proposed method is validated against FE results for various geometries of local wall thinning under combined loading. The effect of internal pressure is also investigated in the present study. Comparison of maximum moments, predicted according to the proposed method, with published full-scale pipe test data for locally wall-thinned pipes under combined internal pressure and global bending, shows good agreement.

Reference Stress Based Approach to Predict Failure Strength of Pipes With Local Wall Thinning Under Combined Loading

In the previous work carried out by the authors, a new method to estimate failure strength of a pipe with local wall thinning subject to either internal pressure or global bending has been proposed. The proposed method was based on the equivalent stress averaged over the minimum ligament in the locally wall thinned region, and the simple scheme to estimate the equivalent stress in the minimum ligament was proposed, based on the reference stress concept for creep stress analysis. This paper extends the new method to combined internal pressure and global bending. The proposed method is validated against FE results for various geometries of local wall thinning under combined loading. The effect of internal pressure is also investigated in the present study. Comparison of maximum moments, predicted according to the proposed method, with published full-scale pipe test data for locally wall thinned pipes under combined internal pressure and global bending, shows good agreement.

Engineering J Estimation Methods for Leak-Before-Break Analyses of Nuclear Piping

The present work presents a method to estimate elastic-plastic J for circumferential through-wall cracked pipes for the Leak-Before-Break (LBB) analysis of pressurized piping. The proposed method is based on the GE/EPRI approach and the key issue is to propose a robust Ramberg-Osgood fitting procedure. Two different estimation schemes are given, one for the case when full stress-strain data are available, and the other for the case when only yield and ultimate tensile strengths are available. The proposed J estimates are compared with those from extensive elastic-plastic finite element (FE) analyses. Experimental validation is also performed by comparing maximum moments predicted by fracture mechanics analysis using the proposed J estimates with those from full-scale pipe test. Both results show sufficient confidence in the accuracy of the proposed J estimates.

Prediction of Failure Behavior for Nuclear Piping Using Curved Wide-Plate Test

One important element of the Leak-Before-Break analysis of nuclear piping is how to determine relevant fracture toughness (or the J-resistance curve) for nonlinear fracture mechanics analysis. The practice to use fracture toughness from a standard C(T) specimen is known to often give conservative estimates of toughness. To improve the accuracy of predicting piping failure, this paper proposes a new method to determine fracture toughness using a nonstandard testing specimen, curved wide-plate in tension. To show validity of the proposed curved wide-plate test, the J-resistance curve from the full-scale pipe test is compared with that from the curved wide-plate test and that from C(T) specimen. It is shown that the J-resistance curve from the curved wide-plate tension test is similar to, but that from the C(T) specimen is lower than, the J-resistance curve from the full-scale pipe test. Further validation is performed by investigating crack-tip constraint conditions via detailed three-dimensional finite element analyses, which shows that the crack-tip constraint condition in the curved wide-plate tension specimen is indeed similar to that in the full-scale pipe under bending.

Laboratory Investigation of Local Bending in Profiled Thermoplastic Pipes

Full-scale pipe tests were conducted to investigate local bending on profiles of different thermoplastic pipes. Five profiled pipes including two lined corrugated, one box and one tubular profile of HDPE material and one ribbed profile of PVC material were considered. The study demonstrates that a three-dimensional bending mechanism governs the strains on some components of the pipe profiles. The local bending was particularly significant in the lined corrugated and the tubular profiled pipes. The mechanism of the local bending at the crown and the springline was different for lined corrugated pipes under biaxial loading. Ratios of the circumferential strains on the liner to those at the inner wall were higher at the crown/invert than the ratios at the springline. The liner with longer span was found to undergo even lesser strain relative to the liner with shorter span. The local bending did not affect the strain on the other elements of the lined corrugated profile.

Failure Strength Assessment of Pipes With Local Wall Thinning Under Combined Loading Based on Finite Element Analyses

Failure assessment of a pipe with local wall thinning draws increasing attention in the nuclear power plant industry. Although many guidelines have been developed and are used for assessing the integrity of a wall-thinned pipeline, most of these guidelines consider only pressure loading and thus neglect bending loading. As most pipelines in nuclear power plants are subjected to internal pressure and bending moment, an assessment procedure for locally wall-thinned pipeline subjected to combined loading is urgently needed. In this paper, three-dimensional finite element (FE) analyses are carried out to simulate full-scale pipe tests conducted for various shapes of wall-thinned area under internal pressure and bending moment. Maximum moments based on ultimate tensile stress were obtained from FE results to predict the failure of the pipe. These results are compared with test results, showing good agreement. Additional finite element analyses are then performed to investigate the effect of key parameters, such as wall-thinned depth, wall-thinned angle and wall-thinned length, on maximum moment. Moreover, the effect of internal pressure on maximum moment was investigated. Change of internal pressure did not show significant effect on the maximum moment.

흰 광폭평판 시험을 이용한 원자력 배관의 파괴거동예측

One important element of the Leak-Before-Break analysis of nuclear piping is how to determine relevant fracture toughness (or the J-resistance curve) for nonlinear fracture mechanics analysis. The practice to use fracture toughness from a standard C(T) specimen is known to often give conservative estimates of toughness. To improve the accuracy, this paper proposes a new method to determine fracture toughness using a non-standard testing specimen, curved wide-plate in tension. To show validity of the proposed curved wide-plate test, the J-resistance curve from the full-scale pipe test is compared with that from the curved wide-plate test and that from the C(T) specimen. It is shown that the J-resistance curve form the curved wide-plate tension test is similar to, but that from the C(T) specimen is lower than, the J-resistance curve from the full-scale pipe test. Further validation is performed by investigating crack-tip constraint conditions via detailed 3-D FE analyses, which shows that the crack-tip constraint condition in the curved wide-plate tension specimen is indeed similar to that in the full-scale pipe under bending.