Through-wall Crack Research Articles

Erratum to “Prediction of fracture parameters of circumferential throughwall cracks in the interface between an elbow and a pipe under internal pressure”

Erratum to “Prediction of fracture parameters of circumferential throughwall cracks in the interface between an elbow and a pipe under internal pressure”

Stress analysis of adhesive in a cracked steel plate repaired with CFRP

To repair a cracked steel-plate with carbon fiber reinforced polymer (CFRR) is an effective and simple method to strengthen a cracked region in a steel structure, which can improve security and extend steel structure's lifetime. As the repaired structure is subjected to tension loads, initial damage usually occurs in the adhesive. In this paper, the stress distributions of adhesive in a cracked steel plate repaired with CFRP were obtained based on a theoretical method and the finite element method (FEM) with different crack lengths. The maximum shear stress and peel stress in the adhesive were obtained from products of stress ratios and stresses for the case of a through-wall crack. As stress ratios could be approximated as a curve independent of material combinations, and stresses for a repaired steel plate with a through crack have explicit expressions, we could simply predict adhesive failure or the maximum load using quadratic stress criteria.

Fatigue resonant tests on drill collar rotary shouldered connections and critical thread root identification

A fatigue test series on a drill collar rotary shouldered connection, thread type 5-1/2 FH MOD, was performed with a dedicated alternating bending resonant test rig. Compared to a quasi-static testing approach, the dynamic loading allowed high testing frequency and a relatively light supporting structure. Crack initiation was identified as the onset of a discrepancy between two strain gauge signals which were monitored throughout the test. A through-wall crack condition was assumed as the effective fatigue failure. The stress distribution in the connection was then analysed with a finite element plane harmonic axisymmetric model, taking into account both the initial make-up torque and the alternating bending load during the test. In these simulations, the most critical stress between the pin or box, in terms of fatigue, mainly depended on the preload. The modelled contact behaviour of the incomplete threads of the box played a significant role in predicting the position of the crack initiation thread root, which was then compared with the test results.

Numerical analysis for the thermal ageing effect on fracture behaviors of CF8A pipes and piping systems under monotonic and very low cycle fatigue loading conditions

In this research, the effect of thermal ageing for CF8A is studied under displacement-controlled low cycle fatigue. Virtual tests of pipe and pipe system are considered and they have the same shape and loading conditions as the pipe and pipe systems tested in Battelle. Only material was changed from A106 Gr. B to CF8A. The pipe tests are four-point bending tests with three different load ratio (R= -1, 0, 1) having through-wall crack. The pipe system test also has through wall crack with seismic loading condition (PGA=1g). To simulate the cyclic loading and thermal ageing effect, multi-axial fracture strain energy based FE damage model and thermal ageing constant “C” was applied. From the simulation, it can be known from the virtual pipe test that crack growth rates of unaged and aged CF8A become similar when loading type is changed from monotonic to cyclic. Also, it is can be known from the virtual pipe system test that the crack growths of unaged and aged CF8A are almost same when the size of through-wall crack is 43.2o. However, the crack growth of unaged CF8A is almost 25% smaller that aged CF8A when the size of through-wall crack is 90o.

Elastic T-stress and I-II mixed mode stress intensity factors for a through-wall crack in an inner-pressured pipe

Elastic T-stress and stress intensity factor (SIF) solutions for through-wall-cracked pipes under internal pressures have been investigated by three-dimensional (3D) finite element (FE) calculations. The distribution of normalized SIFs (mode I KI and mode II KII) and T-stresses along the crack front for different crack lengths, crack orientations, thickness ratios and Poisson's ratios has been obtained in detail. Our FE results show that the T-stresses increase with increasing Poisson's ratio and crack angle, respectively. The normalized KI increases while KII decreases with increasing Poisson's ratio, respectively. The normalized KI decreases with increasing crack angle, while KII increases with increasing crack angle from 0° to 45° and then decreases from 45° to 90°. Finally, the empirical formulae of the three-parameter KI, KII and T-stress have been derived by fitting present FE results with the least-squares method for the convenience of engineering applications.

Variable mode-mixity during fatigue cycles – crack tip parameters determined from displacement fields measured by digital image correlation

This paper focusses on discussing equivalent stress intensity factors and kink angles after a change of mode-mixity from one cycle to the next and when the mode-mixity changes continuously during the fatigue cycles. Thin-walled tubes with through-wall cracks have been loaded by proportional and non-proportional tension and torsion. In the experimental investigation, the region of fatigue crack growth was observed by applying the digital image correlation technique. Data on the variations of the displacement and strain fields during the cycles were acquired and used to determine mixed-mode variations of stress intensity factors associated with opening modes I, II and III. For each specific specimen the crack path was observed in order to relate its curvature – both kinks and continuously developing warped cracks – with the variations of the displacement field and associated stress intensity factors.

Determination of critical angle of circumferential throughwall crack for structurally distorted 90° pipe bends under bending moment with and without internal pressure

Throughwall circumferential cracks (TWC) in elbows can considerably minimize its collapse load when subjected to in-plane bending moment. The existing closed-form collapse moment equations do not adequately quantify critical crack angles for structurally distorted cracked pipe bends subjected to external loading. Therefore, the present study has been conducted to examine utilizing elastic-plastic finite element analysis, the influence of structural distortions on the variation of critical TWC of 90° pipe bends under in-plane closing bending moment without and with internal pressure. With a mean radius ( r) of 50 mm, cracked pipe bends were modeled for three different wall thickness, t (for pipe ratios of r/ t = 5,10,20), each with two different bend radius, R (for bend ratios of R/r = 2,3) and with varying degrees of ovality and thinning (0 to 20% with increments of 5%). Finite element analyses were performed for two loading cases namely pure in-plane closing moment and in-plane closing bending with internal pressure. Normalized internal pressures of 0.2, 0.4, and 0.6 were applied. Results indicate the modification in the critical crack angle due to the pronounced effect of ovality compared to thinning on the plastic loads of pipe bends. From the finite element results, improved closed-form equations are proposed to evaluate plastic collapse moment of throughwall circumferential cracked pipe bends under the two loading conditions.

CANDU pressure tube leak detection by annulus gas dew point measurement: a critical review

Abstract In the event of a pressure tube leak from a small through-wall crack during CANDU reactor operations, there is a regulatory requirement – referred to as Leak Before Break (LBB) – for the licensee to demonstrate that there will be sufficient time for the leak to be detected and the reactor shut down before the crack grows to the critical size for fast-uncontrolled rupture. In all currently operating CANDU reactors, worldwide, this LBB requirement is met via continuous dew point measurements of the CO2 gas circulating in the reactor's Annulus Gas System (AGS). In this paper the historical development and current status of this leak detection capability is reviewed and the use of moisture injection tests as a verification procedure is critiqued. It is concluded that these tests do not represent AGS conditions that are to be expected in the event of a real pressure tube leak.

New J and COD estimates for thin-walled pipes with axial through-wall cracks and high strain hardening exponents

In this study, the approximate estimates of elastic-plastic J and COD for thin-walled pipes with axial through-wall cracks (TWCs) and high strain hardening exponents under internal pressure are developed based on the GE/EPRI and enhanced reference stress (ERS) methods. For the estimations based on the GE/EPRI method, the proposed tabulated plastic influence functions for fully plastic J and COD are derived from three-dimensional finite element (FE) analyses. On the basis of these plastic influence functions, an optimized reference load (which plays an important role in the ERS method) is newly suggested. Finally, the present elastic-plastic J and COD estimations are verified by comparing the predicted results with the FE results using actual tensile behavior of SA312 type 304 SAW stainless steel. The estimations based on both GE/EPRI and ERS methods provide better approximations for thin-walled pipes with axial TWC and high strain hardening exponent, than do the existing estimations. The importance of the elastic-plastic fracture mechanics assessment, using the present solutions for thin-walled pipe with axial TWC and high strain hardening exponent, are also discussed.

Evaluation of postulated cross sections with ovality and thinning for 90° pipe bends with circumferential throughwall cracks subjected to in-plane closing bending

Abstract A comparative study of postulated cross sections for failure analysis of 90°structurally deformed pipe bends with critical circumferential throughwall cracks was performed. Elliptical and semi-oval cross sections were assumed to determine the collapse loads under in-plane closing bending moment. Finite element analysis was conducted based on elastic-perfectly-plastic material considering geometric nonlinearity and twice-elastic-slope method was used to obtain collapse loads for each model. The influence of pipe ratio and bend radius in the presence of ovality and thinning were also investigated for the postulated cross sections. The results indicated a pronounced effect of ovality on collapse loads of throughwall circumferentially cracked pipe bends, primarily with elliptic cross sections whereas the thinning effect is significantly lower for both the cross sections. The present study provided a better estimation of the plastic loads and elliptic cross sections were found to be ideal in the analysis of cracked pipe bends for the present geometries and across the boundary conditions considered.

Engineering Approach Based on Reference Stress Concept for Calculating J and Crack Opening Displacement of Complex-Cracked Pipes

In this study, an engineering approach for estimations of fracture mechanics parameters, i.e., J-integral and crack opening displacement (COD), for complex-cracked pipes was suggested based on reference stress concept, where stress-strain data of the material was used to assess structural integrity of complex-cracked pipes. In the present study, new reference loads that can reduce the dependency on strain hardening of the material have been suggested for complex-cracked pipes under each loading mode. By using the proposed optimized reference load for complex-cracked pipes, J-integral and COD estimation procedures have been proposed based on the reference stress concept together with the elastic solutions for complex-cracked pipes. The predicted J-integrals and CODs based on the proposed method have been validated against published experimental data and FE results using actual stress–strain data. Moreover, the predictions using the proposed methods are also compared with those using the existing solutions for simple through-wall cracks (TWCs) based on reduced thickness analogy concept.

Quasi-cyclic fracture studies on stainless steel welded straight pipes with circumferential through-wall crack in the weld

Fracture studies were carried out on 170 mm OD welded straight pipes having circumferential through-wall crack in the weld under quasi-cyclic loading. The specimens were made of Type 304 LN stainless steel and welded using conventional Shielded Metal Arc (SMA) welding or Narrow Gap (NG) welding. The specimens were subjected to different amplitudes of load/displacement, which was decided based on the monotonic load carrying capacity of the specimens. It was found that the failure load reduces to 78.5% and 74.0% of the monotonic failure load for resisting 50 and 100 loading cycles, respectively in the case of NG welded pipes. The NG welded specimens showed more energy absorbing capability and crack growth resistance, when compared with SMA welded specimens. Crack growth was more stable in the case of specimens tested under displacement control; whereas the same was unstable in the case of load-controlled experiments and specimens failed by sudden collapse.

Fretting fatigue on thread root of premium threaded connections

Identification of the fatigue failure mode of the premium threaded connection for Oil Country Tubular Goods pipes was conducted via full-scale fatigue tests. A through-wall crack was found at the imperfect thread root of the male embodiment, but the crack initiation site depended on the stress level. At relatively higher stress amplitude region, the crack originated from the thread rounded corner by stress concentration. At relatively lower stress amplitude region, the crack originated at the middle of the thread root because of fretting fatigue. To investigate the fretting fatigue mechanism in the threaded connection, a fundamental fretting fatigue test was conducted. This test achieved the fretting fatigue failure at the middle of the contact surface under large gross slip condition.

Crack-tip constraints of through-wall cracked pipes and its similarity to curved wide plates

In the present study, the effects of pipe geometries, material properties and loading conditions on crack-tip constraints of pipes with circumferential Through-wall crack (TWC) were investigated via systematic 3-dimensional (3-D) Finite element (FE) analyses. The crack-tip constraints were quantified by Q-stress, and to characterize the elastic-plastic strain hardening material behavior, Ramberg-Osgood (R-O) material was employed. Based on the FE results, it was observed that crack-tip constraints of pipes with TWC were dependent on crack length and thickness of pipe, however, the effects of each variables decreased as either thickness of pipe becomes thinner or crack length becomes longer. Moreover, the effects loading modes on Q-stresses for thin-walled pipes with TWC are negligible. Finally, the present Q-stresses of pipes were compared with those of Curved wide plate (CWP) in tension to address the similarity of crack-tip constraints between pipe and CWP, which could be used to produce the CWP to measure the fracture toughness of pipes accurately.

Prediction of fracture parameters of circumferential through-wall cracks in the interface between an elbow and a pipe under internal pressure

This paper provides plastic influence functions of GE/EPRI method for calculating J and Crack opening displacement (COD) of pipes with a circumferential Through-wall crack (TWC) in the interface between an elbow and a straight pipe by using 3-dimensional (3-D) elastic-plastic finite element analyses for Ramberg-Osgood (R-O) materials, in which internal pressure was considered as a loading condition. The proposed plastic influence functions are tabulated as a function of the pipe geometries, crack length and strain hardening exponent. In order to provide sufficient confidence for the proposed plastic influence functions, the estimation scheme using the proposed plastic influence functions for J and COD of cracked elbows was validated against FE results using R-O parameters for the SA312 TP316 stainless steel. Moreover, the predicted J and COD for elbows with a TWC in the interface between an elbow and a pipe by the proposed scheme were compared with those for cracked straight pipes to investigate the effect of the elbow geometries on crack behavior of elbows. One important point is that crack behaviors in the interface between an elbow and a straight pipe can be significantly different with those in straight pipes according to pipe thickness, crack length and bend radius of elbows. Thus, the proposed plastic influence functions can be useful to predict accurate J and COD for cracked elbows.

Experimental Study on Failure Estimation Method for Circumferentially Cracked Pipes Subjected to Multi-Axial Loads

When a crack is detected in a piping line during in-service inspections, failure estimation method provided in ASME Boiler and Pressure Vessel Code Section XI (ASME Code Section XI) or JSME Rules on Fitness-for-Service for Nuclear Power Plants (JSME FFS Code) can be applied to evaluate the structural integrity of the cracked pipe. The failure estimation method in the current codes accounts for the bending moment and axial force due to pressure. The torsion moment is not considered. Recently, analytical investigation was carried out by the authors on the limit load of cracked pipes considering multi-axial loads including torsion. Two failure estimation methods for multi-axial loads including torsion moment with different ranges were proposed. In this study, to investigate the failure behavior of cracked pipes subjected to multi-axial loads including the torsion moment and to provide experimental support for the failure estimation methods, failure experiments were performed on 20 mm diameter stainless steel pipes with a circumferential surface crack or a through-wall crack under combined axial force, bending moment, and torsion moment. Based on the experimental results, the proposed failure estimation methods were confirmed to be applicable to cracked pipes subjected to multi-axial loads.

내압이 작용하는 직관과 엘보우의 경계면에 존재하는 원주방향 관통균열의 응력확대계수 및 탄성 균열열림변위 예측식

Article history: Fracture mechanics analysis for cracked pipes is essential for applying the leak-before-break (LBB) concept to nuclear piping design. For LBB assessment, crack instability and leak rate should be predicted accurately for through-wall cracked pipes. In a nuclear piping system, elbows are connected with straight pipes by circumferential welding; this weld region is often considered a critical location. Hence, accurate crack assessment is necessary for cracks in the interface between elbows and straight pipes. In this study, the stress intensity factor (SIF) and elastic crack opening displacement (COD) were estimated through detailed 3D elastic finite element (FE) analyses. Based on the results, closed-form solutions of shape factors for calculating the SIFs and elastic CODs were proposed for circumferential through-wall cracks in the abovementioned interfaces under internal pressure. In addition, the effect of the elbow on shape factors was investigated by comparing the results with the existing solutions for a straight pipe.

Evaluation of <i>J</i> Integral for Interacting Twin Collinear Through-Wall Cracks in a Plate under Tension

The interaction behavior of twin collinear through-wall cracks in tension loaded plate under elastic-plastic condition is investigated by the finite element method (FEM). The fracture parameter J integral for interacting cracks is calculated and compared to the J integral for a single crack the same size. In this way, the interaction factor of cracks under elastic-plastic condition is defined. This interaction factor is compared to the results of analytical solution of the interaction factor under linear elastic condition. The results show that interaction factor of cracks under elastic-plastic condition is higher than interaction factor of same cracks under linear elastic condition. Also the interaction effect of cracks under elastic-plastic condition is influenced not only by the crack configurations but also by the material properties, especially the strain hardening exponent n.

Bayesian Approach for the Seismic Fragility Estimation of a Containment Shell Based on the Formation of Through-Wall Cracks

AbstractThe primary containment (PC) in a nuclear power plant provides the last barrier against radiation leakage to the environment. Design specifications fail to assess the performance of a seismically-damaged containment against accidental overpressure. The present work focuses on the seismic fragility estimation of a PC considering formation of through-wall cracks, which may lead to radiation leakage. This is significantly challenging, considering the computation involved in the integration over a multidimensional probability space, detailed numerical modeling of the containment including concrete damage, and nonlinear dynamic analyses of the structure. A Bayesian approach using a Markov chain Monte Carlo (MCMC) integration scheme is adopted, along with more traditional reliability algorithms (point estimate, first-order second moment (FOSM), and response surface), to address the issue of computation while estimating fragility considering both record-to-record variability and structural parameter unce...

Stress Intensity Factor and Elastic Crack Opening Displacement Solutions of Complex Cracks in Pipe Using Elastic Finite-Element Analyses

In the present paper, the closed-form expressions for the stress intensity factors (SIFs) and the elastic crack opening displacements (CODs) of complex-cracked pipes are derived based on the systematic three-dimensional (3D) elastic finite-element (FE) analyses. The loading conditions that are evaluated include global bending moment, axial tension, and internal pressure. In terms of geometries, the geometric variables affecting the SIFs and the elastic CODs of complex-cracked pipes, i.e., the crack angle of through-wall cracks (TWCs), the crack depth of fully circumferential, internal surface cracks in the inner surface of pipe, and the ratio of pipe mean radius to thickness, are systematically considered in the present FE analyses. The FE analysis procedure employed in the present study has been validated against the existing solutions for the circumferential TWC pipes. Using the present FE results, the shape factors of SIF and elastic COD for complex-cracked pipes are tabulated as a function of geometric variables. The results are applied for closed-form expressions of SIF and elastic COD when the pipe is subjected to simple loading conditions of bending, axial tension, or internal pressure. The proposed closed-form expressions can estimate SIF and elastic COD of complex-cracked pipes within maximum differences of 2.4% and 5.9% with FE results, respectively.