Failure Assessment Diagram Research Articles

Three-dimensional constraint-based failure assessment diagrams

Current failure assessment diagrams codified in structural integrity assessment standards have been developed to consider in-plane constraint effects through K/J-T or J-Q approaches but neglect the effect of thickness on fracture toughness. Recent developments in the effect of thickness on fracture toughness have demonstrated a unique approach to unifying in-plane and out-of-plane constraint loss through a method proposed as J-Δσ. This method demonstrates that crack tip constraint loss in fully constrained and unconstrained geometries can be described by the secondary parameters T or Q and Δσ. Both secondary terms were based on the change of maximum normal stress from the fully constrained field within the micro-separation distance ahead of a crack front. The present work defined an approach to incorporate in-plane and out-of-plane constraint loss in failure assessment diagrams. The developed approach was successfully applied to a set of established experimental data and demonstrate a competent method for failure assessment diagrams through three-dimensional constraint-based fracture mechanics.

Improvement of fracture assessment method for pipe girth weld based on failure assessment diagram

The increasing demand for oil and gas resources in China has promoted the rapid construction and development of oil and gas pipeline networks. With an increase in the mileage of high-grade pipeline, the intrinsic safety of pipeline girth weld faces serious challenges, and the existing fracture assessment methods have many shortcomings. The failure assessment diagram of the commonly used fracture assessment method in engineering is taken as the research object. Aiming at deficiency that it cannot accurately consider the strength matching characteristics of girth welds. Based on the failure assessment diagram theory and the equivalent stress-strain relationship method, the influence of crack size, pipeline diameter-thickness ratio, and material parameters on the fracture assessment accuracy of the girth weld is studied. The calculation method of optimized limit load and reference stress is established. Moreover, the improved fracture assessment process of pipeline girth weld is proposed, which makes up for the lack of conservative assessment results in traditional methods and improves the accuracy of fracture assessment girth weld under high-stress conditions.

Strain-based fracture analysis for internal surface cracks of X80 pipe girth welds

The girth weld is the weakest region of the entire pipeline. When the pipeline crosses areas with complex terrain or where natural disasters are frequent, the pipeline may be subjected to large displacement effects and large plastic deformations. In these scenarios, the traditional stress-based assessment methods will no longer be applicable. Therefore, it is of great importance to develop strain-based assessment methods for pipelines. This paper investigates the effects of crack size, strength mismatch, heat affected zone (HAZ) width and HAZ softening/hardening coefficient on the crack driving force (CDF) of weld center crack (WCC) and HAZ center crack (HAZCC) on the inner surface of X80 pipes using finite element method. The results show that the CDF is significantly affected by crack size, crack location, strength mismatch, HAZ softening coefficient but slightly affected by HAZ width when HAZ width is greater than 2 mm. The CDF of WCC is greater than that of HAZCC under large strain conditions with HAZ width of 2 mm. The distribution of the J-integral at the crack tip is then investigated and the results show that the maximum value of the J-integral does not necessarily occur at the deepest point of the crack tip. Finally, the factors influencing the failure assessment curve (FAC) of WCC are investigated and the lower bound strain-based failure assessment diagram (SB-FAD) considering strength mismatch and HAZ softening coefficient is developed with varying crack depths. This study can provide a reference for the safety assessment of crack-like defects on the inner surface of X80 pipeline girth welds.

Prediction of corrosion defect critical size and corrosion failure assessment study of SGTs

Alloy 690 is a common material for heat transfer tubes of steam generators (SGTs), and its corrosive properties affect service life. In this paper, a corrosion failure assessment diagram (CFAD) of SGTs is proposed for the first time based on the flow stress model and the "mechanical-electrochemical" (M-E) coupled model, and the critical value of defects for the failure of SGTs is predicted. The results show that the electrochemical reaction at both sides of the corrosion interface transforms from anodic to cathodic with increasing a/t, and the corrosion interface undergoes a transition from elastic deformation to plastic deformation. When a/t > 0.6, the SGTs undergo plastic deformation, the corrosion rate at the corrosion defect becomes more uniform with increasing b/t, and the "M-E" synergistic effect decreases. The critical size of the defect affected by the "M-E" coupling is a/t = 0.676, and the failure mode is transformed from electrochemical to "M-E" coupling failure mode. The anode current density at the corrosion defect was 400 times higher than that at the nondefective site. The "M-E" coupling resulted in a 9.03% increase in defect size for SGT failure. Plastic damage occurs when the depth of cracking under stress reaches 62% of the wall thickness. Corrosion is introduced into the structural integrity assessment of SGTs for the first time, which has a leading role in the prediction of the remaining life of different SGT materials under normal operating/accident conditions.

Numerical simulation of titanium alloy hip replacement implants behaviour under static and dynamic loads

This paper presents development of numerical models for simulation of the behaviour of titanium alloy hip implants under static and dynamic loads, which is a part of extensive research involving the structural integrity of such implants under various loading conditions, as well as under exposure to aggressive environments. Numerical models were created in ABAQUS and ANSYS software packages, in order to determine the stress/strain distribution, the number of cycles to failure and stress intensity factors, for two different hip implant geometries. The ABAQUS models were used for static and ANSYS models for dynamic loads. The next stage in this analysis involved comparing these two cases, to determine the one with the better behaviour, depending on the exploitation conditions, which can greatly vary between different groups of patients. The targeted patient weight for all numerical simulations was 90 kg, while failure assessment diagrams were made for both implant geometries, assuming a crack length of 1 mm.

Burst speed assessment of aero-engine turbine disk based on failure assessment diagram and global stability criterion

Aero-engine turbine disks are safety-relevant components which are operated under high thermal and mechanical stress conditions. The actual part qualification and certification procedures make use of spin-tests conducted on production-similar disks. While these tests provide, on the one hand, a reliable definition of the critical conditions for real components, on the other hand they represent a relevant cost item for engine manufacturers. The aim of this work is to present two alternative burst speed assessment methods under development based on the Failure Assessment Diagram (FAD) and a global stability criterion, respectively. In the scope of the fracture mechanics assessment, the failure modes hoop-burst and rim-peeling are investigated with semi-circular surface cracks modelled at the critical regions on the turbine disk. The comparison of the predicted critical rotational speed shows good agreement between the assessment methods.

Static assessment of flawed thin AlSi10Mg parts produced by Laser Powder Bed Fusion

Several factors must be considered within the assessment of parts produced by Additive Manufacturing (AM) such as, for example, heterogeneous microstructure, process-induced defects, surface quality, residual stresses, and dependence on the material’s properties with the building orientation. All these factors severely affect the resistance to static and fatigue loadings of AMed components. Among the possible failure mechanisms, the failure promoted by static loadings when cracks are present is one of the most important failure conditions, in particular for the as-built parts, which might have reduced fracture toughness and ductility compared to the wrought alloy counterpart. In this work, we present a comprehensive approach to the static assessment of AlSi10Mg parts manufactured by Laser Powder Bed Fusion. Two benchmark fracture geometries were designed to investigate the typical AM geometrical features: i) thin walls in tension and ii) notched components in bending. Finite Element analyses of the benchmark specimens showed that an approach based on elastic–plastic fracture mechanics parameters is needed to correctly predict the experimental failures, despite the quasi-brittle behaviour shown by the AlSi10Mg alloy. In view of these results, this paper explores the applicability of the Failure Assessment Diagram (FAD), a tool used for conventional ductile materials, to static assessment of AMed parts. The results show that the assessment approach based on the FAD makes it possible to properly predict the experimental failures of the benchmark specimens.

Determination of allowable axial flaw sizes in stainless steel pipes based on probabilistic fracture mechanics

The flaw evaluation is required if a flaw is detected in the pipe during the service life. The critical flaw size is the limit size of flaw that will cause the pipe to fail under service conditions, while the allowable flaw size is defined as the limit flaw size determined by evaluation procedures based on the standards. Therefore, the evaluation procedures will be simplified by comparing the detected flaw size with the allowable flaw size. Existing standards and studies have provided the allowable circumferential surface flaw size for pipes under combined various loading conditions and the allowable axial surface flaw size for pipes under pure internal pressure. Consequently, the pipes with axial surface semi-elliptical cracks under combined loading are considered in this paper. Normalized critical flaw sizes (ratio of the flaw depth to flaw half-length, a/t) are obtained by using the deterministic failure assessment diagram (FAD) method, and normalized allowable flaw sizes (ratio of the flaw depth to flaw half-length, a/t) are evaluated by probabilistic analysis. The methods were applied to calculate the numerical results for DN150 304 stainless steel pipes with axial flaws under various loading conditions. The smaller value of the allowable internal flaw size and external flaw size is determined as the final allowable flaw size. For considering the failure probabilities, the results of the allowable flaw size based on the probabilistic analysis are more conservative than the allowable end-of-evaluation-period flaw depth based on the limit load criteria by using the safety factor as provided in the Acceptance Standards of Section XI of the ASME Code (ASME XI).

Structural integrity assessment of additively manufactured ABS, PLA and graphene reinforced PLA notched specimens combining Failure Assessment Diagrams and the Theory of Critical Distances

Failure Assessment Diagrams (FADs) are a widely used engineering tool for the analysis of structural components containing cracks, and are included in recognised structural integrity assessment procedures such as BS7910 and API 579 1/ASME FFS 1. Their consistency and reliability has been demonstrated over the years through numerous laboratory validation tests and industrial applications. Nevertheless, both their theoretical definition and their subsequent validation have been performed in metallic materials and, therefore, their use in other types of materials still requires theoretical support and experimental validation. Moreover, FADs have been initially defined for the analysis of crack-like defects, whereas there are many situations where the defects that are, or might be, responsible for structural failure are not necessarily cracks. This is the case of (non-sharp) defects with a finite radius on their tip, which here will be referred to as notches.Simultaneously, additive manufacturing (AM) is an emergent technology that allows practically any type of geometry to be fabricated through a relatively simple process. One of the main AM techniques is fused deposition modelling (FDM), which consists in the extrusion of heated feedstock plastic filaments through a nozzle tip. The resultant printed materials have rather particular properties that are very dependent on the printing parameters and on the final state of internal defects. Concerning AM polymers and polymer-matrix composites, their use as structural materials, beyond their main current use in prototyping, requires the development of specific structural integrity assessment procedures.This paper provides FAD analyses for three additively manufactured (FDM) materials containing U-shaped notches: ABS, PLA and graphene reinforced PLA. The results show that the FAD methodology may be applied for the estimation of fracture loads in these particular materials, as long as linear-elastic fracture toughness values are used.

A probabilistic model for quantifying uncertainty in the failure assessment diagram

Failure assessment diagrams are an integral component of asset integrity management in a variety of industrial sectors. They allow for the assessment of the significance of cracks in structures, between the domains of brittle fracture and plastic collapse. Numerical modelling, or empirically determined assessment lines across this continuum, are the basis of the guidance in current industrial standards. However, as the requirement of such assessments progresses from demonstrating safety to optimising resilience and resource allocation, it will become necessary to compute more informative (probabilistic) estimates, which are compatible with decision analysis.In this paper, Bayesian regression models are proposed as a suitable method of quantifying (aleatory and epistemic) uncertainty in the limit state on a failure assessment diagram. The data considered in this study consists of laboratory tests completed on wide plate fracture specimens. This work is intended to address the inconsistencies in current editions of industrial standards, and limitations (regarding flexibility and application) of existing scientific literature on the topic.Potential applications are discussed, including the use of fracture mechanics in meaningful reliability analysis, quantitative risk management, and optimising experimental design for future material tests.

An interval failure assessment diagram method for analyzing the reliability of structure containing crack defect

The failure assessment diagram (FAD) is the most widely used method for analyzing the reliability of structure containing crack defect. In practical engineering, the failure assessment point (FAP) and failure assessment curve (FAC) are impossible to be determined accurately. Therefore, an interval failure assessment diagram (IFAD) method is proposed in this paper to analyze the reliability of structure containing crack defect when the FAP and FAC cannot be obtained accurately. Firstly, based on the interval method and FAD method, an interval failure assessment diagram (IFAD) model is established by considering the FAP and the failure critical point (FCP) as interval variables. And then the FAP and FCP intervals are converted into the normalized region to design the IFAD reliability index [Formula: see text] according to the relationship between the limit state function and the normalized region. The structural arbitrary state can be described by the IFAD reliability index [Formula: see text]. In addition, 10 cases are given as examples to clearly expound the IFAD method, and the analysis results are compared with the FAD method, which confirms that the IFAD method is feasible and valid to analyze the structural reliability with the bounds of FAP and FCP. Finally, the IFAD reliability analysis of a certain propellant storage tank ellipsoidal bottom containing surface crack is taken as an application example to verify the practicability of the proposed method. And the results show that the IFAD reliability index can be obtained without the exact value of parameters.

Factors influencing the analytical representation of creep crack development in Alloy 939

Creep crack growth rates originally gathered for the 𝛾′ strengthened cast nickel based superalloy Alloy-939 in terms of apparent stress intensity factor are re-evaluated as a function of the time dependent C* parameter, with crack propagation rates being insensitive to grain size at 750 and 850 °C. However, whereas a˙C(Ka) rates at 750 °C are significantly lower than those at 850 °C, equivalent a˙C(C*) rates are relatively insensitive to temperature, comparing well with the plane strain a˙C(C*) rates predicted using an approximate reference stress based C* model. While overall CT-specimen creep crack growth times for Alloy-939 can be predicted using uniaxial creep-rupture strength data when expressed in terms of reference stress at 850 °C, this appears not to be possible at 750 °C. The observation is explained with respect to evidence provided by a modified time dependent failure assessment diagram. The influence is examined of thermal transient durations at 850 °C (from 750 °C), or at 750 °C (from 850 °C) on a˙C(Ka) creep crack growth rates and tR(𝜎ref) overall lives.

On the failure assessment diagram methodology in polyamide 12

For the first time, the Failure Assessment Diagrams of Polyamide 12, PA12, processed via a conventional technique as injection moulding, IM, and an additive manufacturing technique as Selective Laser Sintering, SLS, were determined via Finite Element Analysis (FEA) using a stress–strain equation more appropriate to describe the mechanical behaviour of thermoplastics. These diagrams were compared with those computed from the options of the BS 7910 standard applied for metals. The diagrams of SLS PA12 were clearly above that of IM PA12, presenting almost constant fracture ratio values for all the load ratios. This indicates the brittle behaviour, with small deviation from linearity, of SLS PA12 in comparison with IM PA12 which exhibited plasticity effects. This trend was supported by the mechanical response shown by PA12 manufactured by the different processing techniques. On the other hand, the diagrams obtained via FEA showed significant differences with the options of the BS 7910 standard, employed for the determination of the failure diagrams in metals, particularly when a Ramberg-Osgood model is used to describe the mechanical behaviour.

Uncertainty quantification of the failure assessment diagram for flawed steel components in BS 7910:2019

The failure assessment line (FAL) describes the interaction between plastic failure and fracture of flawed steel components subjected to tension or bending. This paper quantifies the model uncertainty of the FAL as provided in the internationally used British Standard BS 7910:2019 by comparing the assessment with the actual failure load of 82 wide plate and 4 tubular joint tests. In line with findings of others, it is demonstrated that the accuracy of the assessment is significantly improved if the crack tip constraint is considered in the assessment. Irrespective of this crack tip constraint consideration, a non-negligible number of wide plate tests has a lower failure load than the one predicted by the FAL in BS 7910:2019, if based on three fracture toughness tests. A penalty or safety margin on the FAL is advocated to compensate for this. It appears advantageous to base the assessment on the average instead of the minimum of three equivalent fracture mechanics tests together with an associated (quantified) safety margin.

Leak before break fracture assessment of pressurized unplasticized PVC pipes with axial surface crack: Experimental and analytical analysis

This paper compares Failure Assessment Diagram-based and Stress Intensity Factor-based analytical approaches with experimental results for assessing leak before break in axial finite semi-elliptical cracked Polyvinyl Chloride pipes subjected to internal pressure. Surface crack length of 25, 40 and 60 mm and depth to thickness ratio of 0.25, 0.5 and 0.75 were investigated. Burst pressures are experimentally evaluated for all cracked and uncracked samples. Results illustrated that the Failure Assessment Diagram approach is more reliable than Stress Intensity Factor approach for fracture assessment of unplasticized PVC pipes.

Risks Associated with the Use of Hydrogen as an Energy Carrier or Source

Hydrogen is a dangerous gas due to its low ignition energy, wide flammability range, promotion of the embrittlement of steel, and its high coefficient of permeation for polymers. The fracture toughness and failure elongation of pipe steels are strongly impacted by hydrogen embrittlement, whereas yield stress and ultimate strength are moderately impacted. Specific tools are required for the pipe defect assessment used for the transport of hydrogen, which is pure or blended with natural gas. The safety factors associated with cracks or scratches are obtained through the failure assessment diagram or the notch failure assessment diagram. The corrosion defects are assessed by limit analysis, and the estimated repair factor and dents are evaluated by computing the damage value under service pressure. The design factor in the calculation of the maximum admissible operating pressure (MAOP) is modified depending on the location of the pipes to account for the lethal risks from the torch effect or explosion. It is crucial to monitor the leaks due to hydrogen permeation through domestic hydrogen distribution networks, particularly in closed areas.

Numerical evaluation of Failure Assessment Diagram (FAD) for hydraulic fracture propagation in sandstone

This paper evaluates the degree of inelasticity in hydraulic fracture propagation in weak sandstone by using models obtained by the Discrete Element Method as a basis for quantifying Failure Assessment Diagram (FAD) and J-integral. Although current engineering models use Linear Elastic Fracture Mechanics (LEFM), including improvements like fracture tip damage zone where fracture tip plasticity is small enough to fit into the near-field stress zone, this paper hypothesizes that hydraulic fracture propagates in an inelastic regime in certain conditions typically for underground rock reservoirs. Discrepancies within field or laboratory data, including microcrack clouds identified as acoustic emissions that induce plastic deformations, motivate the investigation of inelasticity and implementation of elastoplastic fracture propagation theories. Discrete Element Method (DEM) has been widely used to approach rock failure problems from the micromechanical level, where explicit modeling of local particle-bond breakage enables insights into propagating fracture branching, damage, microcrack coalescence, stress–strain re-distribution, irreversible deformation, and fracture arrest. Results show that inelastic J-integral increases dramatically with rock confinement, especially its non-elastic portion, and the elastic portion of the total J-integral remains a relatively small part. Higher confinement stresses and higher confinement stress contrast enhance the inelastic fracture propagation. Additionally, shorter fracture lengths and smaller dry tip zones characterize inelasticity. Rock stiffness increase leads to total J-integral increase and decrease of J-elastic, leading to pronounced inelasticity. Therefore, results indicate that LEFM is rarely applicable for describing fracture propagation through rock at higher confinement stresses and stiffness.

Inspection and maintenance planning for offshore wind structural components: integrating fatigue failure criteria with Bayesian networks and Markov decision processes

Exposed to the cyclic action of wind and waves, offshore wind structures are subject to fatigue deterioration processes throughout their operational life, therefore constituting a structural failure risk. In order to control the risk of adverse events, physics-based deterioration models, which often contain significant uncertainties, can be updated with information collected from inspections, thus enabling decision-makers to dictate more optimal and informed maintenance interventions. The identified decision rules are, however, influenced by the deterioration model and failure criterion specified in the formulation of the pre-posterior decision-making problem. In this paper, fatigue failure criteria are integrated with Bayesian networks and Markov decision processes. The proposed methodology is implemented in the numerical experiments, specified with various crack growth models and failure criteria, for the optimal management of an offshore wind structural detail under fatigue deterioration. Within the experiments, the crack propagation, structural reliability estimates, and the optimal policies derived through heuristics and partially observable Markov decision processes (POMDPs) are thoroughly analysed, demonstrating the capability of failure assessment diagram to model the structural redundancy in offshore wind substructures, as well as the adaptability of POMDP policies.

Failure diagrams for subcritical crack growth

Failure diagrams help evaluate safe use, require monitoring or replacement of structural components subjected to service loads based on the material properties. They are commonly used in the pipeline industry as failure assessment diagrams based on the fracture toughness data and Yield Stress. Failure diagrams could help evaluate the fatigue life or crack growth leading to failure of a component subject to fluctuating loads experienced during the service environment. Understanding the subcritical crack growth behavior, its quantification, and application to components in service are essential. In the following, basic understanding and the development of these failure diagrams using Kitagawa-Takahashi plots and the two-parametric approach that we have developed in the past are discussed, taking several examples from the literature.

A simplified formula for calculating the limit load of cracked offshore wind turbine monopile under bending

A simplified methodology is proposed for calculating the plastic collapse (limit) bending moment load of a pipe with a circumferential flaw with an emphasis on its application for use in the assessment of cracked offshore wind turbine monopile using failure assessment diagrams. The proposed methodology is based on the theory of net section collapse (NSC) but differs from existing approaches in that it does not need idealisation and categorisation of the crack before assessment. The proposed methodology is validated against results presented in literature and also finite element analysis results. Although it is possible to obtain limit loads using FE analysis, this is computationally expensive and time consuming. The proposed approach allows for near-instantaneous calculation of limit load for any arbitrary crack configuration and loading direction. This is a significant development for the analysis of offshore wind turbine monopiles as it allows the suitability of the cracked structure to be assessed in pseudo-real time.