Pipe Flaw Research Articles

Applicability investigation on failure assessment diagram of ASME code section XI - Appendix H for ferritic nuclear piping

The failure assessment diagram (FAD) is a widely used tool for evaluating the fracture acceptance of structural components. In addition to linear elastic fracture mechanics, elastic-plastic fracture mechanics, and limit load analysis, the ASME Code incorporated FAD as an alternative safety evaluation method for flaws in nuclear pipes starting from 2004 edition. This paper conducts a series of finite element elastic-plastic fracture mechanics analyses to establish the specific J–based FAD curves considering various loading and geometry conditions for ferritic pipes containing various circumferential flaws. By comparing with the suggested FAD of ASME Code Section XI - Appendix H, it is found that the radius-to-thickness ratio of pipes and the flaw depth influence the applicability of FAD, but with minor effects of crack length. In addition, the effects of material properties between base metal and weld materials on FAD are also discussed. The result of this work could be a reference when applying the FAD to evaluate the flawed piping for nuclear power plants.

Efficient Runge–Kutta solver for stochastic crack growth analysis

Stochastic crack growth analysis by simulation may easily require a significant amount of computational effort. The Initial Value ordinary differential equations appearing in crack propagation problems may be solved by the Runge–Kutta (RK) family of solvers. However, crack propagation considering uncertainties imposes additional challenges for RK solvers: possibility of vectorized and/or parallelized computation of sample realizations; and adaptation of time discretization for each sample, to handle changes and discontinuities in the derivatives of crack growth curves, caused by changes in loads and in crack propagation rates. Existing adaptive RK methods, with change in time step length during crack growth computation, struggle with these challenges. In this setting, the present paper proposes a modified adaptive RK method which allows for simultaneous crack growth computations with different time-step discretizations, and aims at efficiently dealing with cases where discontinuities are present in the derivative of the crack growth curve. Application of the proposed method to three crack propagation examples, including one related to weld flaws in pipes, indicates that it is as accurate as other adaptive methods from the literature, while requiring only a fraction of the computational time.

Pin-loaded SENT specimen for constraint-matched fracture testing of radially propagating longitudinal cracks in thin-walled pipelines

There is considerable interest in the fracture community for representative measurements of the resistance of pipe structures to fatigue crack propagation and fracture. However, to date there has been a lack of standard test methods available for the accurate fracture testing of radially propagating longitudinal flaws in thin-walled piping. A pin-loaded single edge-notched tensile (SENT) specimen was developed, denoted the CSR SENT specimen. This test geometry has been shown to reproduce the plastic constraint for longitudinal cracks in thin-walled piping, resulting in transferable fracture test data to that application beyond the conventional limits of single-parameter J-dominated fracture mechanics. The CSR SENT was simulated with finite element analysis (FEA) to yield the parameters necessary for the experimental determination of the J-integral using standard fracture test methods. The FEA results showed that the stress intensity geometry factor, F, for the CSR SENT matched literature solutions for most crack lengths, but that the CMOD-Eta factor – ηCMOD, used to calculate the plastic component of the J-integral from the applied load and crack mouth opening displacement (CMOD) – differed from literature solutions and is specimen size-dependent. The error associated with the calculation of the J-integral is estimated to be < 5% with the obtained ηCMOD, while the use of literature solutions for ηCMOD result in significant non-conservative overestimates of the experimental J-integral. It is recommended that fracture testing be limited to a specific specimen geometry associated with a FEA-derived ηCMOD solution. The crack growth correction to the J-integral measurement was also obtained, and is recommended to be derived from the ηpin (also referred to as ηLLD, derived from the load-displacement of the pin boundary), which can be used alongside the ηCMOD for experimental calculation of the J-integral in standard single-specimen fracture testing. Friction at the pin-specimen interface results in significant non-conservative bias error in the J-integral, and a rolling pin-clevis contact is recommended with a pin diameter twice that of the specimen width to mitigate pin contact plasticity and damage.

Strain-based fracture response of X80 steel pipe welded girth based on constraint-modified J-R curves: from SENT specimen to full-scale pipe

In the study, a methodology is proposed based on the SENT experiments and crack-tip constraint theory to determine the tensile strain capacity of a flawed pipe welded girth. Firstly, the clamped SENT specimens with a wide range of geometrical variations are conducted to determine the J-R curves. The constraint-quantified J-R curves are then formulated using the out-of-plane extension of bending modified J-Q theory, and show well match with the experimental data. Finally, the extrapolated resistance curves from SENT to full-scale pipes are used for strain-based fracture assessment of a flawed pipeline. A relatively good agreement in terms of tensile strain capacity between the present work and ExxonMobil’s approach can be achieved except that the latter gives conservative predictions when the crack length 2c/t is longer than 3.0..

Secondary Stress Weighting Factor and Plastic Reduction Factor of Surface Cracked Pipes Under Bending

Abstract In piping design analysis, the secondary stresses (displacement controlled) may have different design limits than primary stresses (load-controlled stresses). The current design limits for secondary stresses are based on elastic stress analysis. But realistically, a flaw in the piping system can cause nonlinear behavior due to the plasticity at the crack plane as well as in the adjacent uncracked-piping material. Hence, the actual stresses in a cracked piping system, which are elastic-plastic, are different than the design stresses, which are elastically calculated. To assess margins in the secondary stresses calculated using elastic stress analysis in pipe flaw evaluation, two parameters are defined in this paper. The first one is the secondary stress weighting factor (SSWF) on total stress, which is defined as the ratio of actual elastic-plastic stresses in a system to the elastic design stress. An alternative approach to applying margins on secondary stresses is to use a reduction factor only on stresses above the yield stress called plastic reduction factor (PRF). In this paper, a methodology developed to determine these factors for circumferential surface-cracked TP304 stainless steel pipes subjected to bending loads at ambient temperature is described. Four-point-bend tests are conducted on pipes with varying circumferential surface-crack lengths and depths. The moments and rotations needed for the pipe failure for different crack sizes are determined and compared to elastically calculated moments and rotations to establish margins.

Modeling of magnetic fields and signals of a ferromagnetic pipe flaw detector induced by a through hole defect

A numerical model has been developed for interaction of the magnetizing field of a short solenoid with a ferromagnetic pipe exhibiting a through hole in the wall, which takes into account nonlinear magnetic properties of the test object. This model was used to calculate spatial distribution of magnetic induction in the pipe wall and spatial distribution of the components of the magnetic induction vector near the pipe surface with a through hole. The validity of the numerical model was confirmed by physical modeling results. The results of theoretical and experimental studies were used to develop a simplified analytical model that describes behavior of the magnetic field of the defect and the electrical signal of the induction transducer of a magnetic flaw detector that implements the method of magnetic flux leakage. It is shown that the dependence of the signal of the induction transducer induced by the through hole on the gap is close to a linear one, and its dependence on the hole diameter is close to a quadratic one. The results obtained in the study can be used for creation, numerical modeling and measurement of magnetic fields.

Analysis of Crack Behaviour in Pipeline System Using FAD Diagram Based on Numerical Simulation under XFEM

For a long time, cracked structures have triggered various researchers to develop a structural integrity approach and design models to address the fracture problems. In the present study, a pipeline with an axial semi-elliptical surface defect was examined in detail. Recent works have highlighted the use of the classical finite element method (CFEM) as numerical tools to solve the fracture mechanics; however, this approach comes with a few difficulties in the modelling aspects. To overcome this issue, we proposed the use of the extended finite element method (XFEM), which was implemented in the commercial version of Abaqus software. Moreover, we have used the results based on this technique in the volumetric method to estimate the stress intensity factors (SIFs). Then, this parameter was employed to build the failure assessment diagram (FAD). The FAD curve was used in the current investigation because it is one of the conventional methods for the evaluation of flaws in steel pipes. The XFEM simulations enable us to draw an FAD curve that can be used as a practical reference for defect evaluation in pipeline systems in the industrial world.

Studying the interaction of crack-like flaws using the MATLAB toolbox int_defects

int_defects is a free and open-source toolbox for MATLAB which uses the Abaqus finite element analysis suite to automate the analysis of interacting crack-like flaws in structures. It is designed to aid the development of flaw interaction criteria for structural integrity assessment procedures and to simplify sensitivity analysis of flaw proximity. It can also perform automatic 3D elastic, elastic-plastic or limit load analysis of elliptical embedded flaws and semi-elliptical surface flaws in plates and thick-walled pipes under a wide range of loading conditions. This technical note outlines the capabilities of int_defects and gives some examples of its use.

Failure bending moment of pipes containing multiple circumferential flaws with complex shape

Flaws due to stress corrosion cracking have been detected in stainless steel or nickel-based alloy dissimilar metal weld joints of piping systems in nuclear power plants. Generally, stainless steel and nickel-based alloy are ductile materials with high toughness, and the failure mode of flawed pipes made of ductile materials is expected to be net-section plastic collapse. Failure bending moment of a ductile pipe containing a circumferential flaw is predicted using the net-section stress approach according to ASME Code Section XI as a limit load criterion. However, in the current code, the failure bending moment can only be adopted for a pipe containing a single circumferential flaw with constant depth, although many flaws detected actually are multiple flaws, and the flaw shapes are generally complex. In this study, a failure estimation method for pipes containing multiple circumferential flaws with complex shapes was proposed. Furthermore, failure experiments were performed for 8-inch diameter Schedule 80 stainless steel pipes containing two circular circumferential flaws. The failure bending moments obtained from the experiments were compared with the estimated results. Based on the experimental results, it was concluded that the proposed failure estimation method satisfactorily represents the failure behavior of the pipes and can be applied in engineering application.

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.

Defect mapping in pipes by ultrasonic wavefield cross-correlation: A synthetic verification

This work presents a reverse-time imaging technique by cross-correlating the forward wavefield with the reverse wavefield for the detection, localization, and sizing of defects in pipelines. The presented technique allows to capture the wavefield reflectivity at the places of ultrasonic wave scattering and reflections. Thus, the method is suitable for detecting pipe defects of either point-like or finite-size types using data from a pulse-echo setup. By using synthetic data generated by 3D spectral element pipe models, we show that the 3D wavefield cross-correlation imaging is capable in the case of cylindrical guided ultrasonic waves. With a ring setup of transducers, we analyze the imaging results obtained from the synthetic single-transducer and all-transducer firings. The presented pipe flaw imaging method is straightforward to carry out using a suitable wave equation solver. Also, the method does not suffer from long iterative runs and numerical convergence issues commonly connected with imaging methods based on either deterministic optimization or statistical inference. The imaging procedure can be fully baseline-free by performing data processing to remove direct arrivals from the ultrasound data.

Flaw interaction under bending, residual stress and thermal shock loading

Crack-like surface flaws in pipes, pressure vessels and other structural components sometimes occur near to one another. When this happens, it is often necessary to take the mutual interaction of the flaws into account when performing fracture-mechanics-based fitness-for-service assessment. Integrity assessment procedures including ASME BPVC Section XI, BS 7910 and R6 include interaction criteria which are used during flaw characterisation to ensure that interaction is accounted for conservatively. This paper considers the interaction which occurs when flaws are loaded by a non-uniform through-wall distribution of stress, as may arise due to bending, thermal shock or welding/cladding residual stresses. Using parametric finite element analysis of a large number of crack pairs subjected to different distributions of stress, it is shown that the degree of flaw interaction can be enhanced under non-uniform loading. Therefore, care should be taken when performing integrity assessment using interaction criteria based on uniform tension loading only.

Experimental Verification of Computational Model of Magnetized Segment of Pipeline with Artificially Created Defects

A computational model of a magnetized segment of a pipeline created for the software of a new and promising method of internal pipe flaw detection is presented. Verification of the applicability of the computational model is performed on a test bench containing a control segment of the pipe with artificially created defects of different shapes. The values of the induction of the magnetic field inside the pipe near the defects obtained by means of the computational model are compared to data obtained in the course of measurements of the magnetic field by variable inductance and flux-gate transducers. Recommendations to improve the capabilities of fl aw detection measurement systems are given on the basis of an analysis of the results of a calculation.

Estimation of the sensitivity of acoustic reflectometry to flaws in heat-exchange pipes

A novel approach to calculating the acoustic tract in the method of acoustic reflectometry is suggested for inspection of heat-exchange pipes. Results of modeling and analysis of the acoustic tract are presented. The sensitivity of acoustic reflectometry to flaws in the form of through holes and deposits against the background of acoustic noises due to permissible deviations in the thickness of the pipe wall is estimated.

Constraint Effect on the Ductile Fracture Behavior of High-Strength Pipeline Steels

This work presents an investigation of the ductile tearing properties for a pipe with internal and external circumferential cracks using 2D plane strain and axisymmetric models. Crack growth resistance curves were computed using the complete Gurson model. The pipes with various crack depths and internal pressures were analyzed. The results were compared with those of corresponding SENT and SENB specimens. It clearly indicated a significant effect of constraint on the resistance curves for internal and external cracked pipes. A minor effect of hoop stress induced by internal pressure on the CTOD-resistance curves is expected for deep-cracked pipes. The SENT specimen is a better representation of circumferentially flawed pipes and an alternative to the conventional standard SENB specimen for the fracture mechanics testing in engineering critical assessment of high-strength pipeline steels.

가압기 노즐 이종금속 용접부의 구조적 오버레이 용접부에 대한 위상배열 초음파기법의 현장 적용

Abstract Weld overlay was first used in power plants in the US in the early 1980s as an interim method of repairing the welds of flawed piping joints. Weld overlaid piping joints in nuclear power plants must be examined periodically using ultrasonic examination technology. Portable phased array ultrasonic technology has recently become available. Currently, the application of preemptive weld overlays as a mitigation technique and/as a method to improve the examination surface condition for more complex configurations is becoming more common. These complex geometries may require several focused conventional transducers for adequate inspection of the overlay, the original weld, and the base material. Alternatively, Phased array ultrasonic probes can be used to generate several inspection angles simultaneously at various focal depths to provide better and faster coverage than that possible by conventional methods. Thus, this technology can increase the speed of examinations, save costs, and reduce radiation exposure. In this paper, we explain the general sequence of the inspection of weld overlay and the results of signal analysis for some PAUT (phased array ultrasonic testing) signals detected in on-site inspections.

The sensitivity of the acoustic-emission method during the detection of flaws in pipes

A method for determining the sensitivity of acoustic-emission inspection during the detection of flaws of different sizes is presented. According to the method, the sensitivity is determined as the detection probability of through holes of a certain size during the axial compression of 09Γ2C steel cylindrical pipes. The obtained sensitivity of the acoustic-emission method is compared with the wellknown data on the sensitivity of the ultrasonic, capillary, and radiographic nondestructive-testing methods.

周方向表面欠陥を有する管の欠陥評価に用いる安全率

In order to prevent failure of piping, limitation on flaw depth according to the circumferential flaw angle is prescribed in the Fitness-For-Service Codes (FFS Codes) of the Japan Society of Mechanical Engineers (JSME). First, the applicability of the limit load assessment method was examined when the flaw angle was large. Then, the influence of the crack growth estimation and flaw sizing errors on the reliability of the flaw evaluation was examined. Since the limit load assessment method can predict plastic collapse strength of a flawed pipe appropriately irrespective of a flaw angle, it is applicable to flaw evaluation even if a flaw angle becomes large. However, precision of flaw evaluation much depends on the variation of the material strength (flow stress) and the flaw size measurement error. Probabilistic analyses considering these variations were performed and influence on failure probability of a pipe was evaluated quantitatively. As a result, the simplified method for evaluating the safety factors according to those variations and satisfying target reliability was proposed. The proposed methods give the flaw evaluation result which has equivalent reliability irrespective of a flaw angle.

Engineering approach for circumferential flaws in girth weld pipes subjected to bending load

The current investigation pursues extending the applicability of fully plastic solutions for J-integral and crack-tip opening displacement (CTOD), originally cataloged in a series of handbooks (also known as EPRI handbooks), in defective girth weld pipes subjected to bending load for a wide range of surface crack dimensions and weld strength mismatch levels. The suitability of the given set of solutions in this work is constrained by a certain number of parameters that are derived from the coupled effect of weld strength mismatch and configurational effects upon near-tip stress-strain fields in heterogeneous media. These results reveal a weak dependence of coupled effect of weld strength mismatch and weld groove size upon crack driving force, when the ratio of the mismatch between weld and base normalizing stresses is considered moderate (≤1.3), whereby the proposed estimation method becomes essentially valid for failure assessment procedures and fitness-for service (FFS) evaluations.

The Reflection of Guided Waves from Multiple Flaws in Pipes

Inspection of pipes and pipelines for corrosion is vital for their safe and cost effective operation. Ultrasonic guided wave inspection is a relatively new technique used for this. Recent research has advanced the technique so that it may be possible to characterise individual flaws, making guided waves a potentially viable alternative to direct examination where internal in-line inspection (ILI) is not feasible. However, corrosion often occurs in clusters and the presence of one flaw could affect the signals received from subsequent flaws. Therefore, the issue of multiple flaws must be dealt with if flaws are to be adequately characterised in the field. In order to address this, a semi-analytical modelling methodology has first been developed for the reflection and transmission of guided waves from a single flaw and then, this has been combined with an analytical model for wave propagation to extend it to simulate the reflections from two or more flaws. The semi-analytical model for two flaws has been validated by comparison with experimental data and verified against three-dimensional finite element simulations.