Failure Assessment Diagram Research Articles

Design and maintenance of pipe networks transporting hydrogen pure or blended with natural gas

Steel is subject to hydrogen embrittlement (HE). This problem is relatively accurate for pipes transporting hydrogen pure or blended with natural gas. Therefore this problem has to be taken into account for the design and maintenance of pipe networks for this kind of transport. Design needs to modify the design factor for computing maximum working pressure in this case. Defect harmfulness needs specific tools for each type of defect which are the same as for pipe transporting natural gas, but the admissibility criterion is modified when transporting hydrogen. For cracking, harmfulness is determined with a failure assessment diagram with steel fracture toughness under HE. For defect correction, the estimated repair factor (ERF) is changing due to modification of the flow stress. For gouging, the Constraint Modified Failure Assessment Diagram (CMFAD) incorporates the actual material failure master curve. For dents, the criterion proposed by Oyane et al take into account the major reduction of elongation at failure. The influence of HE on fatigue endurance is seen through the fatigue assessment diagram(fAD). Discussion is based on recategorisation of defect, assessment tools, embrittlement and fatigue life duration.

Pipe networks transporting hydrogen pure or blended with natural gas, design and maintenance

Steel is subject to hydrogen embrittlement (HE). This problem is relatively accurate for pipes transporting hydrogen pure or blended with natural gas. Therefore this problem has to be taken into account for the design and maintenance of pipe networks for this kind of transport. Design needs to modify the design factor for computing maximum working pressure in this case. Defect harmfulness needs specific tools for each type of defect which are the same as for pipe transporting natural gas, but the admissibility criterion is modified when transporting hydrogen.For cracking, harmfulness is determined with a failure assessment diagram with steel fracture toughness under HE. For defect correction, the estimated repair factor (ERF) is changing due to modification of the flow stress. For gouging, the Constraint Modified Failure Assessment Diagram (CMFAD) incorporates the actual material failure master curve. For dents, the criterion proposed by Oyane et al. take into account the major reduction of elongation at failure. The influence of HE on fatigue endurance is seen through the fatigue assessment diagram(fAD). Discussion is based on recategorisation of defect, assessment tools, embrittlement and fatigue life duration.

Butt welded joints assessment after fire exposure

The need of in service life of a steel structure after being exposed to fire raises the problem of strengthening or replacement of some structural elements. In many cases, after structural safety assessment, fire exposed steel structures can be reused. Establishing the post-fire residual capacity of the structure has a major importance in overall assessment process. The paper presents a study for fire behavior of the butt-welded joints subjected to low cycle fatigue loading, taken into account the post fire conditions and the possibility of welding strengthening. There is presented a case study – a building structural element subjected to dynamic loading after fire exposure. For the in case study, post fire investigations revealed several welding flaws including welding surface type flaws. An assessment was needed in order to determine the structural integrity and life assessment of some structural elements. From the fracture mechanics point of view, a Failure Assessment Diagrams FAD - level 2 procedure was applied in order to determine the in service safety of the structure. Several flaws types were assessed and conclusions were taken. The results can be used for damage assessment and strengthening technique of post fire steel structures.

Failure Assessments for MQXF Magnet Support Structure With a Graded Approach

The high luminosity large hadron collider (LHC) upgrade requires new quadrupoles, MQXF, to replace the present LHC inner triplet magnets. The MQXFA magnet is the first prototype that has a 150-mm aperture and uses Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting technology in a 4.2-m magnetic length structure. The support structure design of the MQXFA magnet is based on the bladder-and-key technology, where a relatively low pre-stress at room temperature is increased to the final preload targets during the cool-down by the differential thermal contraction of the various components. The magnet support structure components experience different load levels from pre-load to cool-down and excitation. Consequently, a few parts experience high stresses that may cause localized plastic deformations or internal fracture development. The concept presented in this paper for the failure assessment of support structures integrates nonlinear finite-element (FE) analysis with detailed sub-models and fracture mechanics into an advanced engineering tool. The nonlinear FE solutions enable estimations of the structural response to the given loads, and the advanced fracture analysis with failure assessment diagram assesses the structure safety index of results obtained from the FE model. The paper describes how the MQXFA end-shell segments are being optimized based on the failure analyses.

Damage tolerance reliability analysis combining Kriging regression and support vector machine classification

Damage tolerance analysis associates a Fracture Mechanical model with the Failure Assessment Diagram to define the state of a space engine component. The reliability analysis treats the variability of numerical models assessing the probability of failure within Linear Elastic Fracture Mechanics (LEFM) hypotheses. However, these models, while providing quantitative information in the safe domain, give only qualitative information for failed components. This work proposes an original methodology to combine Kriging regression and the Support Vector Machine classification along with transition criteria between both approaches. To accurately describe the limit state, we define a specific enrichment strategy. The efficiency of the proposed methodology is illustrated on reference test cases.

Numerical study of semi-elliptical cracks in the critical position of pipe elbow

Pipelines are considered as a major tool to transport hydrocarbons due to its important role in transporting fluids taking into account the operating conditions. Despite the importance of the elbow which considered as a critical part in the pipelines, the repeated failures and defects became a dangerous and enormously costly issue. In this numerical study, the Fluid-Structure Interaction (FSI) analysis was carried out using Ansys software. This work is divided into four main parts: the first part focused on studying and comparing the effect of bending radius of pipe elbow on the maximum of Von-Mises stress values for each radius with the yield stress of the steel of the pipe. The second part focused on the creation of a semi-elliptical crack for different locations along the elbow angles to show the critical position compared to the stress intensity factors. In the third part, the semi-elliptical crack angle orientation was studied at the critical position to estimate the critical angle. In the last part, the failure assessment diagram (FAD) was used to show the critical crack depth ratios at a critical position and a critical angle.

A Proposal for the Application of Failure Assessment Diagrams to Subcritical Hydrogen Induced Cracking Propagation Processes

In this work, an optimization proposal for a model based on the definition of regions for crack propagation by means of the micromechanical comparison by SEM images and its application to failure assessment diagrams (FADs) is presented. It consists in three approaches. (1) The definition of the crack propagation initiation in the elastic-plastic range. (2) A slight modification of the zones in which the FAD is divided for hydrogen induced cracking (HIC) conditions. (3) The introduction of a simple correction for the definition of the Kr coordinate of the FAD to take into account the fracture toughness reduction caused by an aggressive environment, instead of using a fracture parameter obtained from a test in air. For the experimental work, four medium and high strength steels exposed to a cathodic charge and cathodic protection environments were employed, studying two different loading rates in each case, and testing C(T) samples under slow rates in the environment. The study was completed with a subsequent fractographic analysis by SEM. A good degree of fulfilment was appreciated in both materials and environmental conditions, showing the validity of the predictions supplied by the FAD optimization model proposal, which constitutes an advance in the accuracy of the FAD predictive model.

Hybrid approach for calculation of J-R curve using R6

The fracture property, J-R curve is an essential material property for assessing the cracked structures for unstable ductile tearing. For obtaining this material property, fracture test is conducted and resultant test data namely load, load line displacement (LLD) and relevant crack extension data are post processed to obtain J-R curve. However, in this method, geometry functions η and γ are essential which are available for only limited geometries. To avoid this limitation, recently two approaches based on R6 failure assessment diagram were proposed. First was the load based R6 approach where load vs. crack growth data is converted to fracture property J-R curve using R6 Failure Assessment Diagram (FAD). This method was further modified to displacement based R6 approach where LLD vs. crack extension data is converted to J-R curve using R6 FAD. The predicted J-R curves were in good agreement with conventionally calculated values especially for displacement based approach. However, when these load and displacement based approaches are attempted to be used in case of throughwall cracked elbows, the predicted J-R curve deviated significantly with respect to those obtained using conventional method. On further investigation, it is found that the assumption involved in the displacement based approach is violated for cracked elbows because of severe ovalization of cross section. Hence, a modified approach is proposed in this work where both load and displacement parameter are used for calculation of J-R curves and named as hybrid approach. Further, this approach is adopted for the six through wall cracked pipes which are already investigated by load and displacement based R6 approaches for J-R curves. The predicted J-R curves are found to be in good agreement for all the investigated pipes and elbows. Hence, a simpler yet more accurate hybrid approach based on R6 method is proposed in this paper for post processing the test data to obtain, the material J-R curve.

Investigating the Effectiveness of a Composite Patch on Repairing Pipes Subjected to Circumferential Cracks under Combined Loadings

The purpose of this study is to investigate bending moment and the axial load capacity of a pressurized pipe suffering from a through-wall circumferential crack repaired by a composite sleeve. The three-dimensional finite element method (FEM) was adopted to compute the results, and the failure assessment diagram (FAD) was employed to investigate the failure behavior of the repaired pipe. The findings revealed that, for the investigated range of applied loads and angles of the crack, the interaction of brittle and ductile failure modes is negligible. Additionally, the yield strength of the cracked pipe was considered as reference stress to achieve a conservative design. Two cases of the combined loading state consisting of internal pressure/bending moment and internal pressure/axial tensile force were investigated. Repairing the crack under combined loadings using carbon-epoxy composites was studied where the influences of various parameters, including internal pressure, crack angle, and the composite patch thickness on the capacity of the cracked pipe to withstand bending moment and axial load were included. The results indicated that the bending moment and axial load capacities of the cracked pipe depend on internal pressure, crack angle, and the composite patch thickness; nevertheless, the crack angle is the main parameter. A composite sleeve can increase both bending moment and axial load capacity of the cracked pipe, but bending moment can be increased further than axial load. Using the composite patch to repair the cracked pipe caused the bending moment capacity to improve from 14.28% to 120%. On the other hand, the composite patch raised the axial load capacity from 5.1% to 93.5%. Additionally, an increase in the composite patch thickness caused the axial load capacity to extend more than bending load capacity.

Use of Irradiated Fracture Toughness Values in Nuclear Vessel and Component Design Specifications for Fitness-For-Service Analysis Using the Unified Curve Method

The American Society of Mechanical Engineers (ASME) Section III Rules for Construction of Nuclear Facility Components subsection NB-2331 Material for Vessels requires that the effects of irradiation shall be considered on material toughness properties in the core belt line region of the reactor vessel. The code also states that “the design specifications shall include additional requirements, as necessary, to ensure adequate fracture toughness for the service lifetime of the vessel.” In a design report of nuclear pressure vessel, the design and service loads do not include loads that are affected by fracture toughness of the material. However, in the cases of fitness-for-service assessment for component flaws (prevalent with age of component), irradiated material properties become highly relevant. An example of a fitness-for-service is that of a beyond design basis reactor vessel head drop accident in a pressurized water reactor with a nozzle junction flaw. As a case study, the critical size of a postulated external surface semielliptical circumferential crack in the combustion engineering three-loop pressurized water reactor nozzle–vessel junction is calculated using ansys Workbench (Academic version) with the applied impact load from the vessel head drop accident. Failure assessment diagrams for numerous crack depths and lengths were developed considering the fracture toughness properties of the irradiated reactor vessel steel. The mode I stress intensity results used in the failure assessment diagram were compared with the available finite element and the American Petroleum Institute (API) standard API 579 analytical solutions for validation, showing good agreement. From this case study, it is demonstrated that the effects of irradiation on fracture toughness become prominent at the same postulated crack size in the nozzle–vessel junction dispositioned as “safe” becomes “unsafe” in the fracture assessment diagram. Using the unified curve method, the irradiated fracture toughness data in design specification can be supplied so that it may be used in fitness-for-service analysis to account for component aging.

Failure investigation of liquefied petroleum gas cylinder using FAD and XFEM

To assess the reliability and performance of household liquefied petroleum gas (LPG) cylinder is important for safety standard. Such crack-like flaws in material of cylinder might lead to many serious catastrophes like fluid leakage and gas explosion. In this work, the failure assessment diagram (FAD) was determined according to the standard API 579 for inland produced LPG cylinders. A brand-new and used LPG cylinder of the same steel grade were investigated. Metallographic analyses, tensile tests and fracture mechanics tests using a single-edge tension SE(T) specimen were carried out. The direct current potential drop (DCPD) method was applied by the SE(T) test and the critical contour J-Integral for crack initiation was identified. Then, FE simulations of the cylinders afflicted with different crack configurations were performed under varying internal pressures. The corresponding critical pressure loads were determined and the level 3 FAD for a crack onset could be generated. It was found that the mechanical properties of used cylinder were obviously deteriorated that could therefore lead to an earlier failure when an unexpected certain pressure is given. In addition, FE simulations coupled with extended finite element method (XFEM) were conducted in order to describe the crack propagation in both cylinders. The results were firstly verified by comparing the load vs. crack mouth opening displacement (CMOD) curves. It was shown that the crack propagation in the aged LPG cylinder would become considerably stronger when unintended crack is present.

Effect of corrosion damage on a pipeline burst pressure and repairing methods

On the basis of the calculation of the stress field around the corrosion-damaged areas on the external part of a pipeline used for the transport of hydrocarbons, an analysis of the effect of the level of post-repair burst pressure by composite patches is proposed. This work examines some points of views on the burst pressure standards calculation on real test on pipeline with external corrosion defects. This study divided to three parts. In the first, an inspection is given for a real pipe exhibiting a surface corrosion defect. The corrosion defects were collected and divided into six important defects. The internal pressure has increased to the burst of pipeline. The second part illustrated a numerical study of the defects interaction effect on the safety of pipeline. The third part of this study is showing a finite element analysis of repaired pipe using the composite repair; we will study the effect of the composite sleeve number on the structural integrity of the pipe. The principle of failure assessment (FAD) diagram is described, and particularly, the definition of the non- dimensional applied crack tip loading $${k}_{\mathrm{r}}$$ , the non- dimensional net applied stress (or load) $${S}_{\mathrm{r}}$$ or $${L}_{\mathrm{r}}$$ is given. The interest lays in the fact that many pipes defects are scratch or gouge types. In the FAD, results demonstrated that the composite repair method does not affect the interpolation curve and, at the same time, had a slightly effect on the function point.

Safety Assessment of Damaged Multi-planar Square Hollow Section Welded Joints Using the New BS 910:2013+A1:2015

The safety assessment procedure in the new BS 7910:2013+A1:2015 guide is based on the failure assessment diagram (FAD) method. This paper aims to validate the above procedure for complex geometries such as damaged multi-planar square hollow section (SHS) welded joints and to recommend optimal solutions if necessary. FAD curves are constructed for cracked multi-planar SHS TT-, YT- and KT-joints for the first time and are compared with the Option 1 curve of the BS guide. A robust novel automatic finite element (FE) mesh generator, which is validated using the full-scale experimental test results, is used in this study. The new FE mesh generator addresses the issue of non-convergence by using a key-hole for the modelling the crack tip in elastic-plastic analyses. The new FE mesh generator is capable to model cracks and geometries of arbitrary dimensions and is able to achieve convergence of solutions even at a high plastic deformation. It is shown to be aiding in speedy generation of cracked FE mesh models which is otherwise time consuming to generate using commercial software packages. The results show that the Option 1 curve does not always guarantee safe solutions for multi-planar SHS welded joints. Hence, a penalty factor of 1.1 is recommended to be used to calculate the plastic collapse load. The use of proposed penalty factor gives optimal solutions for cracked multi-planar SHS TT-, YT- and KT-joints.

Some observations on failure of austenitic stainless steel: effects of in- and out of plane constraint

Modern engineering design for safety-critical structures demands meticulous failure assessments. While conventional fracture toughness parameters (e.g. JIC or KIC) are perceived as a reliable tool for characterizing the fracture behaviour of many engineering components, excessive plasticity in low-strength materials with high strain hardening capability, such as austenitic stainless steels, can change the predominant failure mode to plastic collapse. In this case, assessing the component integrity assuming fracture can introduce significant conservatism in its estimated load bearing capacity. Low constraint structures such as thin sections or sections with short cracks can suffer from plastic collapse rather than fracture but there is not a measure that can predict the change of failure mechanism. In this study, the effect of crack tip constraint in terms of the thickness and initial crack size on failure behaviour of AISI Type 316L austenitic stainless steel is studied. To this end, several single edge notched bending (SENB) specimen were manufactured with different initial crack lengths and thicknesses to account for in- (i.e. crack length) and out of (i.e. thickness) plane constraint effect. Fracture tests performed according to ASTM E1820-18 except for the sample thickness and crack lengths. The challenges for measuring the crack extension for resistance curve method as a result of deviating from the standard is discussed. The failure of the samples was also assessed by Failure Assessment Diagram (FAD). The results showed that unloading compliance extremely underestimate the crack extension in the presence of significant plastic deformation of the samples. The results also showed that the plastic collapse occurs in all samples before crack starts to grow, in the blunting regime.

Application of Probabilistic Leak-Before-Break for WWER-1000 Unit

In this paper a probabilistic Leak-Before-Break analysis for WWER-1000 unit was performed based on the Failure Assessment Diagram (FAD), treating strength and crack morphology parameters as stochastic values. In order to perform probabilistic calculations Critical temperature of brittleness and Yield (Ultimate) Stress, were fitted by normal distribution, and based on experimental data taken from the manufacture documentation found at the Ukrainian NPP. The statistical behavior of the leak rate (LR) and Critical crack length was examined. The failure probability was calculated using Monte-Carlo simulation, for different types of cracks. It was proven, then crack morphology parameters highly affects the leak rate, among the mechanical characteristic a Fracture toughness has more influence rather than Ultimate of Yield strength.

Structural Integrity Assessment of a Propellant tank in Presence of Welding Residual Stresses

Upper stage of India’s new generation launch vehicle makes use of cryogenic propellants with liquid hydrogen (LH2) and liquid oxygen (LOX) as fuel and oxidizer respectively. The propellants are stored in tanks fabricated of Aluminum alloy and contain various openings. As carrying out proof test at operating environment is not feasible, the tanks are subjected to proof pressure test at room temperature (RT). During the RT proof pressure test of LOX tank, high strains, greater than 8000με, were recorded by gauges at three locations where weld rework had been undertaken. From structural analysis, the maximum welding residual stress at these locations is evaluated to be 95N/mm2. From stress analysis point of view, positive margins were observed with respect to failure of the tank even in presence of these residual stresses.However, since the tank is operating at cryogenic temperature, though there is an increase in the yield and ultimate strengths of the tank material, there is a reduction in ductility (lower percentage elongation). Therefore, evaluation of structural integrity of the hardware from fracture point of view is of paramount importance. A study was undertaken to investigate the influence of residual stresses on integrity of the tank in presence of minimum detectable crack sizes using conventional NDE techniques. The location where maximum strains were observed, post proof test, is considered for fracture evaluation. Directional stresses at location of interest are predicted through detailed finite element analysis. Subsequently, generalized Failure Assessment Diagram was drawn based on Elasto-Plastic fracture mechanics principles and margin against ultimate load ensured. The tank was used in a launch vehicle which was successfully flown in June 2017.

3-D FE analyses of WRS and simplified elastic-plastic fracture mechanics assessment of a repaired weld

Abstract An effective excavation and re-welding process is often needed to correct the installation slope and geometric misalignment in the piping, where significant local plastic strain damage and complex welding residual stresses (WRS) often occur during the repair process. At the same time, it is complex to perform elastic-plastic fracture assessment for combined loads such as the WRS, bending moment and internal pressure. In this paper, three-dimensional (3-D) finite element (FE) analyses of WRS of the repair welding are performed, and the influence of weld bead sequencing as well as the displacement caused by external loads is simulated. In the study, the plastic strain damage is also assessed to prevent void formation. To avoid the complex elastic-plastic fracture assessment, a simplified elastic-plastic fracture study is carried out based on the failure assessment diagram (FAD) method. The results show that the repair process is effective, and the strain damage caused by the weld repair is acceptable. There are clear differences between WRS caused by initial weld and that of the re-weld, and there is palpable influence of the repair process on the WRS for the unrepaired/unexcavated part of the pipe. Based on the simplified elastic-plastic FAD method, the repair process has obviously influence on the margin of safety. The repair process described in this paper is generic and thus can be applied in similar pipe deflection correction applications.

The determination of the stress intensity factor solutions for the new pipe-ring specimen using FEA

For safe transport and reliable supply of petrochemical substances, it is crucial to ensure the structural integrity of the equipment, pipelines in particular. Regarding the large diameters and high mass per distance, pipelines for natural gas are designed as thin-walled cylindrical structures. To ensure the structural integrity of the unknown material means to measure and therefore empirically test the material fracture properties of the laboratory specimen for giving an assessment of the accepted defect size. Compared to standards and regulations, such as the ASTM E-1820, BS 7448 standards and the GKSS procedure, the production of standard specimens for measuring the fracture toughness is commonly very difficult or even impossible for applications. On the basis of the Slovenian–Russian bilateral project, we investigate and propose a solution for this issue with a new kind of specimen, called the pipe-ring specimen. The specimens were made from a segment of the observed thin-walled pipeline from the construction filed or stored in a warehouse. The measurement procedure is similar as for the standard SENB specimens and extensometer because of the geometry of the specimen, which is cut from the pipe and contains only a machine-made notch. The next step is to test specimens axially on the three-point bending load test on the hydraulic machine. Because the ring as the specimen is not standardized, it is necessary to show and prove how, and if it is possible to use ring specimens as an alternative option to the standard specimens for testing and determining fracture properties of testing material for thin-walled pipelines. In the frame of the three main experimental, analytical and numerical approaches, this publication shows the numerical approach of defining the stress intensity factor (SIF) for crack opening mode I with and without prior fatigue pre-cracking. Besides the limit load, the SIF presents one of two main parameters for developing the failure assessment diagram and estimating the possible accepted defects in a material relating to its’ structural integrity.

Corrosion defect harmfulness by domain failure assessment diagram

WE ARE REPORTING in this study the detection of 1888 corrosion defects using a magnetic pig over 70 km of a pipeline located in Algeria. This large amount of defects has been statistically analysed. The relative defect depth a/t exhibited a large scatter and no correlation was found between corrosion defect depth and length. For the necessity of repairing defect, two tools are available: the first is based on limit analysis and called the estimated repair factor (ERF) while the second is based on the failure assessment diagram. The adopted tool in the current study was the domain failure assessment diagram (DFAD). Analysis made with FAD concerns 66.8% of corrosion defects, with a limit analysis (LA) of 32.5%.After categorizing the corrosion defect according to the used analysis tool, the safety factor or probability of failure of each assessment point was determined and compared to the repairing criteria. It appears that the ERF criterion is more conservative in our case than the probabilistic criterion as a probability of failure of 10-4 or a non-dimensional crack driving force equal to mean minus three standard deviations.

Hydrogen embrittlement effect on the structural integrity of API 5L X52 steel pipeline

Hydrogen plays a decisive role in many future energy systems. For this reason, much research has been performed to see the effect of hydrogen on network canalizations. This paper studied the possibility of hydrogen transportation using the natural gas pipeline. In this paper, the effect of hydrogen embrittlement (HE) on the mechanical properties of pipe steels was examined. It appeared that the yield stress and ultimate strength were few sensitive to HE. However, fracture toughness and elongation at failure exhibited a severe decrease. Failure pressure established by different codes involved a flow stress which is a combination of yield stress and flow stress. Pipe defect assessment involved fracture toughness and flow stress upon using Failure Assessment Diagram (FAD) or Crack Driving Force (CDF) methods. In this paper, the consequence of hydrogen embrittlement on the failure and critical pressures of a pipe with a semi-elliptical defect was examined. The influence of assessment method, relative defect depth and working pressure were examined through the comparison of respective safety factors.