FITNET FFS Research Articles

“Notch sensitivity and heat treatment effect on the fatigue behaviour of AlSi10Mg aluminium alloy processed by additive manufacturing”

This paper studies the fatigue behaviour of smooth and notched specimens made from AlSi10Mg aluminium alloy via LPBF additive manufacturing. Three material conditions were addressed: as-built, stress relief, and hot isostatic pressing. Heat treatments were conducted at a temperature to avoid Al-Si network rupture, resulting in an increased fatigue strength of both the smooth and notched specimens. This beneficial effect of the heat treatments was attributed to the homogenization of residual stresses through the specimen depth. In addition, the introduction of notches had a detrimental effect on fatigue life. Two fatigue life prediction models were successfully applied. The first one accounts for both the initiation and propagation periods using the SWT parameter and the FITNET FFS procedure, while the second one is based on a calibrated curve relating the SWT parameter and the number of cycles to failure generated from smooth specimens. Fatigue crack propagation from subsurface defects had a primary role on fatigue life of the tested alloy.

Effects of depth in external and internal corrosion defects on failure pressure predictions of oil and gas pipelines using finite element models

This study presents, for the first time, the mechanical behavior of API 5L pipeline steels X42, X52, X60, X70, X80, and X100 with external and internal corrosion defects as well as a combination of both defects that has been named external–internal corrosion defects. The conventional methods to predict failure pressure in corroded pipes, such as B31G, RSTRENG-1, SHELL, DNV-99, PCORRC, and FITNET FFS, have also been discussed in this article. In addition, pipeline failure pressure has been estimated using the finite element method, considering that it is the best approach to calculate actual failure pressure. The external and internal corrosion defect investigated in this research manifests as a rectangular shape with spherical ends at the edges. When the external–internal corrosion defect appears, failure pressure data decrease dramatically because of severe damage. This is due to the decrease in the ligament (effective area) caused by the corrosion defect. To have a good estimation of the pipeline failure pressure with an external–internal corrosion defect, DNV-99 method can be used with acceptable certainty.

On the Influence of the Corrosion Defect Size in the Welding Bead, Heat-Affected Zone, and Base Metal in Pipeline Failure Pressure Estimation: A Finite Element Analysis Study

In this study, failure pressure prediction was conducted in a pipeline with localized corrosion in base metal (BM), heat-affected zone (HAZ), and welding bead (WB) by finite element (FE) analysis. In the gas pipeline industry, there are methods (B31G, RESTRENGH, Shell, DNV, PCORR, and Fitnet FFS) and authors' approaches (Choi and Cronin) to determine the failure pressure. However, one disadvantage of these methods is that their equations do not consider damage corrosion at the HAZ or WB. They consider corrosion only in the BM. The corrosion shape is rectangular with a radius at the edges. In this study, the corrosion defect depth (d) was varied. The corrosion defect length (L) and the corrosion defect width (W) were equal. A type of rectangular corrosion defect with a radius at the edges in the longitudinal and circumferential directions was proposed. True stress–strain curves for BM, HAZ, and WB of an API 5 L X52 were introduced in the FE program. The results show that the pressure decreases as d, L, and W increase. This is because the damage corrosion is more severe as it grows, which causes the failure pressure to decrease.

Finite Element Modelling and Retained Life Estimation of Corroded Pipelines in Consideration of Burst Pressures—A Fractural Mechanics Approach

This study used Finite Element Modelling (FEM) to determine the relationship between the burst pressure (Pb) of internally, circumferentially corroded pipelines, with the corrosion defect depth (d), pipe wall thickness (t) and the pipe diameter (D). After modelling X46 and X52 grades of pipes, the Pb estimated was compared with those determined experimentally and with industry standard models—ASME B31G (modified), RSTRENG, DNV F101, SHELL92 and FITNET FFS. The comparison specified a Root Mean Square Percentage Error (RMSPE) that ranged from 7.06% to 20.4% and a coefficient of determination (R2) that varied from 0.7932 to 0.9813. Multivariate regression was also used to compute a general linear relationship between the burst pressure (Pb) and (d/t), (L/D) and (L/√Dt). The resulting FEM burst-pressure model, developed with multivariate regression, was later used to estimate the expected allowable operating pressure of a corroded X46 grade pipeline over the lifecycle duration, for low, mild, high and severe corrosion categories. It was observed that the burst pressure retention ratio (Rr), which is an indicator of the reliability of the pipeline, decreased with the increase in (d) but did not show distinctive changes with the increase in (L). Considering the robustness of the FEM developed in this study, it can be concluded that it will be very vital for flowline design and pipeline integrity management.

Fracture assessment for electron beam welded damage tolerant Ti-6Al-4V alloy by the FITNET procedure

Fracture assessment of the cracked structures is essential to avoiding fracture failure. A number of fracture assessment procedures have been proposed for various steel structures. However, the studies about the application of available procedures for titanium alloy structures are scarcely reported. Fracture assessment for the electron beam(EB) welded thick-walled damage tolerant Ti-6Al-4V(TC4-DT) alloy is performed by the fitness-for-service(FFS) FITNET procedure. Uniaxial tensile tests and fracture assessment tests of the base metal and weld metal are carried out to obtain the input information of assessment. The standard options and advanced options of FITNET FFS procedure are used to the fracture assessment of the present material. Moreover, the predicted maximum loads of compact tensile specimen using FITNET FFS procedure are verified with the experimental data of fracture assessment tests. As a result, it is shown that the mechanical properties of weld metal are inhomogeneous along the weld depth. The mismatch ratio M is less than 10% at the weld top and middle, whereas more than 10% at the weld bottom. Failure assessment lines of standard options are close to that of advanced option, which means that the standard options are suitable for fracture assessment of the present welds. The accurate estimation of the maximum loads has been obtained by fracture assessment of standard options with error less than 6%. Furthermore, there are no potential advantages of applying higher options or mismatch options. Thus, the present welded joints can be treated as homogeneous material during the fracture assessment, and standard option 1 can be used to achieve accurate enough results. This research provides the engineering treatment methods for the fracture assessment of titanium alloy and its EB welds.

Effect of correlated input parameters on the failure probability of pipelines with corrosion defects by using FITNET FFS procedure

The paper presents a probabilistic methodology considering the correlations between the input variables for the failure probability evaluation of corroding pipelines based on the corrosion module of the FITNET FFS procedure. A computer program based on this method is developed to calculate the failure probability of pipelines by considering different numbers of defects and different elapsed times. In case of one defect, the correlation between the initial defect depth and the initial defect length has the most significant impact on the failure probability of the pipeline. If the correlations between these two parameters for an individual defect are not considered, the prediction results are nonconservative when the failure probability is below 40% and conservative when it is above 40%. In case of multiple defects, the independent assumption of variables generally leads to a conservative estimate of the failure probability. The conservatism increases if the elapsed time and/or the actual correlation coefficients of the variables increase. The correlation of the operating pressure, the initial defect depth and material ultimate tensile strength at the location of different defects has a larger impact on the failure probability than the correlation of other parameters at different defects. The upper bound failure probability calculated by engineering method corresponds to that calculated using the presented method without considering the correlation between input parameters. This confirms the validity of the presented model.

Constraint assessment of brittle fracture of steel components, ISO 27306 vs. FITNET FFS

ISO 27306 and FITNET FFS have been developed recently for correction of constraint loss in structural components. Both methods employ the Weibull stress as a driving force of brittle fracture. Nevertheless, the dissimilarity is found between the two methods. This paper compares ISO 27306 with FITNET FFS in terms of the constraint correction ratio provided by these methodologies. Discussion is conducted on 2-dimensional (FITNET) vs. 3-dimensional (IST) approach and input material parameters. Case studies are shown on the fracture assessment of tension wide plates within the context of the failure assessment diagram (FAD).

On the analysis of the causes of cracking in a wind tower

This paper presents an analysis of the cracking causes in a wind tower of a wind farm. The cracks were detected in several towers of the farm, in the welded joint between the lower ring of the towers and the flange connecting the towers to their corresponding foundation. In the extreme case, here analysed, the crack was a through thickness crack. In order to clarify the cracking causes, the component was inspected in situ, non-destructive tests were performed on the base material, the weld bead and the Heat Affected Zone (HAZ), and a Finite Elements (FE) simulation was carried out to determine the stress state in the welded joints and to develop the corresponding fatigue analysis following the Fatigue Module (Chapter 7) of the FITNET FFS Procedure. The analysis has demonstrated that the main cause of the cracking process is an inadequate design of the joint, with high stress concentrations and an insufficient resistant section on the flange. Finally, two tentative solutions (grinding and grinding plus soft transition) have been analysed but none of them have provided satisfactory results.

Probabilistic fracture assessment of piping systems based on FITNET FFS procedure

A probabilistic method based on the fracture module of the FITNET FFS procedure is developed to perform the structural integrity analysis for piping systems. Monte Carlo simulation is used to calculate the failure probability of the whole piping system, as well as that of separate defects by considering the random variables in the method. Both the sensitivity of uncertainties of variables and the model sensitivity are analyzed to identify the most important parameters that affect the failure probability of piping systems, thereby providing an insight into the countermeasures against the failure risk. The results show that the outer diameter of the pipe has the strongest influence on the failure probability of a piping system having a circumferential crack of 0.757rad, followed by the bending moment, the piping wall thickness, the fracture toughness, the crack angle, the axial force, the ultimate tensile strength and the yield stress.

Constraint Assessment of Brittle Fracture of Steel Components, ISO 27306 vs. FITNET FFS

ISO 27306 and FITNET FFS have been developed recently for correction of constraint loss in structural components. Both methods employ the Weibull stress as a driving force of brittle fracture. Nevertheless, the dissimilarity is found between the two methods. This paper compares ISO 27306 with FITNET FFS in terms of the constraint correction ratio. Discussion is conducted on 2-dimensional (FITNET) vs. 3-dimensional (IST) approach.

Probabilistic analysis of pipelines with corrosion defects by using FITNET FFS procedure

A probabilistic method based on the corrosion module of the FITNET FFS procedure is presented to perform the structural integrity analysis for pipelines. The comparison of the failure pressure determined by both deterministic and probabilistic methods shows that even for a high safety factor, the deterministic method may predict nonconservative results. FITNET FFS procedure predicts a similar probability to that given by the Shell-92 model. The sensitivity analysis of both the uncertainty of the variables and the underlying model shows that the corrosion depth has the most significant contribution to the pipeline failure at an inspection interval of 8 years.

Structural integrity assessment of a nuclear vessel with FITNET FFS and Master Curve approach

The structural integrity of a Spanish nuclear vessel was assessed by obtaining the operation pressure–temperature ( P– T) limit curves relevant to the beltline region according to the ASME Code. This method requires the knowledge of the material fracture toughness in the ductile to brittle transition (DBT) region which is conventionally described by the semi-empirical reference temperature RT NDT. Recent development of the Master Curve (MC) methodology, with T 0 as a reference temperature, is an attractive alternative that do not rely on empirical correlations. Thus, the P– T curves were also calculated according to the N-629 and N-631 ASME Code Cases which partially include T 0 through RT T 0 . Finally, to evaluate the conservatism of the different methodologies (ASME Code and Code Cases), the analysis was performed by means of the FITNET FFS procedure that consistently incorporates the MC concepts. The study demonstrates the conservatism of the ASME code which is manly due to the definition of the semi-empirical RT NDT index.

Structural integrity of an X-750 jet pump beam of a BWR by means of FITNET FFS procedure

In this paper, the J– R curves of the X-750 alloy, previously obtained in the rising load test environment, have been implemented in the structural integrity assessment of a jet pump beam of a boiling water reactor manufactured in this material. The analysis was performed with the FITNET FFS procedure considering the in-service loads on the beam. The critical crack lengths for the different scenarios studied have been obtained and, taking into consideration real crack length velocities, the lifespan of the beams were conservatively estimated.

Assessment of local thin areas in a marine pipeline by using the FITNET FFS corrosion module

This paper analyses the structural integrity of the marine stretch of a pipeline which is placed over a natural bay. The pipeline is part of a 30-year-old installation used for the provision of petrochemical products to a nearby chemical plant. Although there have been no relevant leaks in the past, both the visual inspections performed (revealing numerous local thin areas) and the fact that it is located in a highly sensitive place with high ecological and tourist value recommend the assessment of the pipeline in order to ensure that it is working in safe conditions and that there are no risks for the environment or the people living in the surrounding area. The assessment has been performed using the newly developed FITNET FFS procedure, whose local thin areas assessment methodology is also explained and compared to the analyses proposed by other well known procedures.

Assessment of thermal aging embrittlement in a cast stainless steel valve and its effect on the structural integrity

This paper analyzes the thermal aging embrittlement occurred in a cast stainless steel valve, which is part of the reactor water clean-up (RWCU) system of a Spanish boiling water reactor (BWR) nuclear power plant. The aim is to estimate the current and future state of the material and the corresponding structural integrity of the valve. Given that there is no data available for the experimental characterization of the material, the evolution of the mechanical properties (fracture toughness, yield stress, flow stress and Ramberg–Osgood parameters) has been estimated using the ANL procedure. With the obtained estimations, the critical crack size has been calculated using the European procedure FITNET FFS and the ASME Code. This analysis considers not only the evolution of the mechanical properties up to now but also its future evolution in case there is a life extension of the plant until year 2029.

Fracture analysis of strength undermatched Al‐Alloy welds in edge cracked tensile panels using FITNET procedure

ABSTRACTThis paper presents a methodology for the assessment of the remaining load carrying capacity of thin‐walled components under tension containing highly strength undermatched welds and edge cracks. The analysis is based on the strength mismatch option of the fracture module, part of the newly developed European fitness‐for‐service (FFS) procedure FITNET. The mismatch option of the FITNET fracture module allows weld features such as weld tensile properties and weld geometry to be taken into account in the fracture analysis of cracked welded components. The methodology described was verified for centre cracked Al‐alloy large tensile panels containing undermatched welds in Ref. [1] and hence the present work provides validation with experimental results of the single edge cracked (SEC) and double edge cracked (DEC) panels. The material used is an age‐hardening aluminium alloy 6013 in T6 temper condition used in welded airframe components. The welds in the form of butt joints were produced using the CO2 laser beam welding process. The results show that by using the FITNET FFS methodology with an appropriate selection of the input parameters, safe acceptable predictions of the maximum load carrying capacity of the welded panels can be obtained. It should also be noted that one of the main difficulties that engineers encounter when applying mismatch analysis for first time is its apparent complexity. A step‐by‐step analysis is proposed here in order to provide guidance for this kind of assessments.

FITNET FFS procedure: A unified European procedure for structural integrity assessment

The recently completed European fitness for service network has developed a unified European procedure for the structural integrity assessment called FITNET fitness for service procedure, which covers the analysis of components and structures under the four main failure mechanisms: fracture-plastic collapse, fatigue, creep and corrosion. This procedure has been submitted to the European Committee for Standardisation (CEN) for its adoption as a European Standard for structural integrity. This paper describes the origins and the justification of the FITNET FFS procedure, as well as its contents and structure, in specific chapters devoted to the different failure mechanisms and all of the original aspects of this new assessment procedure. The aim is to provide a clear and comprehensive view of the procedure and its potential both as an assessment tool and in various other engineering applications.

Failure analysis of a lift gear shaft: Application of the FITNET FFS procedure fatigue module

This paper analyses the failure of a lift gear shaft, which happened when the lift was carrying three people. Fortunately, the safety systems worked and there were no serious injuries, but the authorities demanded an investigation into the causes of the accident. The analysis was performed using the FITNET FFS fatigue module, and improper working conditions have been identified as the major cause of the accident.

Failure analysis of a bolt in a scaffolding system

This paper analyses the failure of a bolt which sustained a scaffolding during a construction process. The work performed begins with the fractographic analysis of the failure section and continues with a structural integrity assessment using the European FITNET FFS procedure. The main conclusion is that failure occurred due to the superposition of an overloading which the scaffolding was not designed for and an environmental assisted cracking process.

Treatments of Structural Welds using FITNET Fitness-for-Service Assessment Procedure

Recent developments of the advanced welding processes such as laser beam welding (LBW), solid state friction stir welding (FSW) and hybrid welding, numbers of advanced structures are being designed and constructed in industries such as aerospace, power generation, oil and gas transmission and transportation. Development of new structural aluminum and magnesium alloys as well as high strength steels provide further possibilities for the welded structures in similar and dissimilar (material-mix) configurations. Consequently, there is an increasing demand for “Fitness-for-Service” (FFS) assessment of those advanced welded structures by considering the specific features of these weld joints (such as narrow weld width, high strength mis-match, etc.). In year 1999, Structural Integrity Assessment Procedure SINTAP has been developed for analysis of flaws to avoid fracture within the European Commission funded project SINTAP. Recently, the European Community funded project FITNET in the form of a Thematic Network (TN) organisation has completed to review the existing FFS procedures and developed an updated, unified and verified European FITNET FFS Procedure to cover structural integrity analysis to avoid failures due to fracture, fatigue, creep and corrosion. The fracture Module (Section 6) of this new FITNET FFS Procedure, prepared and presented at this conference, FITNET 2006, in three volumes has adopted the SINTAP approach for assessing of the welded structures. This paper describes the FITNET FFS weld assessment route (Option 2 and Option 3) and also aims to demonstrate suitability of weld joint assessment route in prediction of the critical conditions of various advanced welded joints containing flaw. The welded specimens used in this work cover conventional multi-pass welded lnconel-718 turbine blade (T-joint), center cracked wide plates of electron beam welded 13 % Cr supermartensitic stainless steel, laser beam welded shipbuilding C-Mn steel and aluminum alloy as well as welded pipe. The results are showing that the weld strength mismatch analysis option of the FITNET FFS is conservative and degree of conservatism is similar to the analysis options for the homogeneous materials. This provides confidence in the use of the FITNET FFS procedure for assessing of the structural significance of flaws in welded structures.