Failure Assessment Diagram Research Articles

Water hammer effects on a gray cast iron water network after adding pumps

A numerical model was developed to simulate the propagation of pressure waves in water gray cast iron pipe systems in the presence of pumps. The pressure waves are caused by the water hammer phenomenon due to fast valve maneuvers in the pipe network. The mathematical formulation of the present model is based on a system of two partial differential equations of hyperbolic type. This system was solved by the method of characteristics. The constructed numeric algorithm permits to follow the propagation of pressure waves. It provides the damping of the water hammer waves in the network taking into account the effect of reflection and refraction phenomena to the passage of junctions, bifurcations and change of conducts characteristics. The numerical algorithm sustains the maximum pressure values and; therefore, FEM ABAQUS simulation of gray cast iron pipes with a superficial defect gives the maximum stresses in the different network pipes. The severity of a corrosion crater defect was estimated by calculating the safety factor for the stress distribution at the tip of defects. It allows the acquisition of the applied notch intensity factor. To study the effects of the geometry defects, semi-elliptical defects are deemed to exist up to half the thickness of the pipe wall. To obtain the value of the safety factor, the results were fed into the assessment procedure for the structural integrity (SINTAP) which offers a Failure Assessment Diagram (FAD). Conventionally, it is considered that the risk of failure occurs if the safety factor is less than two. The study allows concluding that the entire network, studied after the addition of pumps, does not work in the security field and, therefore, requires the installation of control and protection systems against sudden pressure variations.

Failure assessment of the shaft of a pumped storage unit

ABSTRACTThe paper is focused on failure assessment of the shaft of a pumped storage unit. The shaft failure occurred during normal operation of the hydropower unit after 35 years of service. Three different methods are applied in order to assess its lifetime. Firstly, a finite element analysis was performed in order to obtain the static stress distribution and to apply a strain‐based crack initiation for the shaft under torsion load. The number of cycles for crack propagation was obtained using the Paris law for mode III. More precisely, the lifetime assessment of the shaft is obtained based on a low‐cycle fatigue analysis for estimating the number of cycles for crack initiation and then followed by that for fatigue crack propagation. Secondly, an analytical estimation based on failure assessment diagram is carried out for circumferential crack in order to determine the safe/unsafe region, where the shaft can operate with a crack. The failure assessment diagram for mode III loading was plotted in order to obtain the critical circumferential crack using the stress intensity factor solution and three analytical solutions for plastic zone. Thirdly, the theory of critical distance is employed for estimating the shaft life under torsional loading. A good agreement is obtained between the estimated results and experimental data for the shaft life and the crack length.

A proposal on cyclic tearing based stability assurance for LBB demonstration of nuclear piping

Currently leak-before-break based design and fracture stability assessment considers the earthquake as once applied non-cyclic load, which monotonically increases up to its maximum magnitude. The cyclic character of earthquake load and associated cyclic tearing failure mode (tearing–fatigue regime) are not explicitly considered while demonstrating stability of a through wall cracked pipe. To single out and quantify the impact of cyclic tearing on fracture stability of circumferential through wall cracked pipes, a systematic programme involving extensive number of tests is carried out. The load reduction factors (β) or safety margins, required to ensure cracked pipe stability for specified number of reversible load cycles, are directly evaluated from cyclic and corresponding monotonic pipe tests. They are presented as Cyclic Tearing Failure Assessment Diagram (CTFAD) and a β-factor equation. The proposed β-factors when applied on the monotonic critical load of a cracked pipe (evaluated using existing procedures/methods) will ensure its fracture integrity for specified number of load cycles. The safety margin required against cyclic fracture instability under different levels of earthquakes is presented.

Structural Integrity Assessment of Thermal Power Plant Superheater Collector

Thermal power plants (TPPs) belong to the important objects, the failure of which can cause accidents with severe consequences. A large portion of Ukrainian TPP has exceeded its design life, which is determined by the main elements of steam power systems. Therefore, the main task is to assess correctly the superheaters collectors limit state using modern approaches and taking into account the service degradation of material.TPP superheater collectors operate in the steamy environment under the pressure of 15.5MPa at temperature 545°C. As a result of combined action of prolonged thermo-mechanical stresses, corrosive and hydrogen environment and slow deformation, fatigue cracks emerge in these components on the inner collector surface.The limit state of boiler steam superheater collector model was assessed using failure assessment diagram (FAD). The FADs were built for Kmat = KIc and Kmat = Kfc taking into account safety factors for plastic collapse and for brittle fracture mechanisms for the collector model with part circumferential internal surface crack for various ratio of crack length l to depth a.The critical sizes of the inner surface defect, that is perpendicular to the axis of the cylinder, considering fracture toughness of 12Cr1MoV steel for static and cyclic loading, were assessed.It was found out, that the increase of l/a ratio decreases the minimum crack depth that is critical for structural element.

J Estimation of Circumferential Through-wall Intrados Center Cracked Elbows under In-plane Bending

This paper provides a J estimation of intrados circumferential through-wall center cracked elbow under in-plane bending. To find the finite element (FE) J-integral, non-linear three-dimensional finite element analyses are performed. The material properties for elastic-plastic FE analysis are assumed to be a Ramberg-Osgood model. Validity of that estimation of circumferential through-wall cracked elbow under given loading condition is evaluated. Next, effects of the elbow geometry and crack lengths are systematically investigated. Results are presented in the failure assessment diagram (FAD) space. The results show that FE J of circumferential through-wall center cracked elbow give more non-conservative results than reference stress based J estimation by R6. If an optimized reference moment was taken into consideration, almost all FE J values in the given range have only slight different of its elbow geometry and crack lengths.

Simulation of Fret for Life Assessment of Feeder Pipes of PHWR

Abstract In case of KAPS and KGS, different ingots were used for manufacturing the coolant channels. Due to difference in the creep related material properties of different ingots, the coolant channels in these reactors experienced different creep rate. Due to the differential creep rates, especially in the neighboring channels, the coolant channels came into contact with each other. This is a serious deviation from as installed conditions wherein the gaps and spacing between closely assembled coolant channel components are decided based on accurate assessment of known loads and forces. Differential creep results in closing of gaps between neighboring feeder pipes or between feeder pipe and gray-loc clamp. Depending on the orientation of the clamp and direction of the feeder pipe at that location, worst contact could be between the first elbow of feeder pipe and the sharp edge of the crown nut on clamp. Several such contacts have been detected in KAPS and KGS. Seriousness of the contact emerges from the fact that the coolant channel and the feeder pipes vibrate due to coolant flow with high velocity. With time the contact conditions like the contact force and location(s) of contact change and this has significant influence on the life and performance of the coolant channel as a whole. Since feeder pipes are constantly vibrating, the contacting locations are likely to be dented or damaged. Whenever and wherever possible, NPCIL has tried to remove the contact locally. Since the process of creep is ongoing, more and more feeder pipes are likely to come in contact and those in which the local corrections for removing the contact are made could also need correction as the time progresses. In order to assess the severity of damage, a full scale setup was erected. Contact was established between feeder pipe and the hard locking nut of gray-loc clamp. Based on previous vibration data collected from feeder pipes and end fittings, and the ASME‘s O&M guidelines on acceptable level of piping vibration in nuclear piping, vibration was induced in the contacting feeder pipes. The test was continued for more than 10 07 cycles. On the basis of damage seen on the feeder pipe due to contact, R6 method has been employed to determine the critical crack size in the feeder pipe with a postulated axial part-through semi-elliptical crack. The failure assessment diagram was constructed in the K-L plane. The lowest critical crack depth was calculated as 4.67 mm as against observed depth of dent of 1.1 mm. The paper deals with the critical assessment of contact between neighboring coolant channels caused by differential creep and assessment of safety margin available before corrective action need to be initiated.

Determination of Shakedown Boundary and Failure-Assessment-Diagrams of Cracked Pipe Bends

Determination of shakedown (SD) boundaries of 90-degree plain smooth pipe bends has recently received substantial attention by several researchers. However, scarce or almost no solid information is found within the literature regarding the determination of the shakedown boundary of cracked pipe bends. The current research presents two additions to the literature, namely, determination of shakedown boundary for a circumferentially cracked 90-degree pipe bend via a simplified technique utilizing the finite element (FE) method and introduction of failure-assessment diagrams (FADs) in compliance with the API 579 failure-for-service assessment of pressure vessel and piping components. The analyzed cracked pipe bend is subjected to the combined effect of steady internal pressure spectrum and cyclic in-plane closing (IPC) and opening (IPO) bending moments. Line spring elements (LSEs) are embedded in quadratic shell elements to model part-through cracks. FAD is obtained through linking the J-integral fracture mechanics parameter with the shakedown limit moments of the analyzed cracked 90-degree pipe bend. The LSE outcomes illustrated satisfactory results in comparison to the results of two verification studies: stress intensity factor (SIF) and limit load. Additionally, full elastic-plastic (ELPL) cyclic loading finite element analyses are conducted and the outcomes revealed very good correlation with the results obtained via the simplified technique. The maximum load carrying capacity (limit moment) and the elastic domain are also computed thereby generating a Bree diagram for the cracked pipe bend. Finally, Crack growth analysis is presented to complement the FAD.

Safety factors in structural integrity assessment of components with defects

Purpose– The purpose of this paper is to develop basic principles of deterministic structural integrity assessment of a component with a crack- or notch-like defect by including safety factors against fracture and plastic collapse in criteria equations of linear and nonlinear fracture mechanics.Design/methodology/approach– The safety factors against fracture are calculated by demanding that the applied critical stress should not be less than the yield stress of the material for a component with a crack or a notch of the acceptable size. Structural integrity assessment of the engineering components damaged by crack- or notch-like defects is discussed from view point of the failure assessment diagram (FAD). The methodology of the FAD has been employed for the structural integrity analysis and assessment of acceptable sizes of throw-thickness notch in a plate under tension and surface longitudinal notch-like defects in a pressure vessel.Findings– Basic equations have been presented to calculate the safety factor against fracture for critical values of the stress intensity factor, crack tip opening displacement (CTOD), the J-integral and the FAD as well as to estimate an acceptable (safe) region for an engineering component with a crack- or notch-like defect of the acceptable size. It was shown that safety factors against fracture depend on both the safety factor against plastic collapse and employed fracture mechanics criterion. The effect of crack/notch tip constraint is incorporated into criteria equations for the calculation of safety factors against fracture.Originality/value– The deterministic method of fracture mechanics is recommended for structural integrity assessment of a component with a crack- or notch-like defect by including safety factors against fracture and plastic collapse in criteria equations of linear and nonlinear fracture mechanics.

Plastic collapse load prediction and failure assessment diagram analysis of cracked circular hollow section T-joint and Y-joint

This paper concerns the validation of standard safety assessment procedure given in BS 7910 for cracked circular hollow section T-joint and Y-joint, using the finite element (FE) results. A robust and efficient FE mesh generator is developed to produce the 3D models of the cracked joints and to calculate the elastic J-integral (Je) and elastic–plastic J-integral (Jep) values of the crack respectively. In order to verify its accuracy and convergence, the plastic collapse loads (Pc) obtained from experimental tests and FE predictions are compared; they agree very well with each other. It is also found from experimental tests that the plastic collapse loads (Pc) predicted using the BS 7910 reduction factor (FAR) are safe and conservative. Subsequently, the failure assessment diagrams (FADs) of five cracked T-joints and three cracked Y-joints are constructed using the FE results, following the J-integral method, which is classified as Level 3C in BS 7910. Thereafter, a comparison between the constructed FAD curves and the standard Level 2A curve is carried out, and it is observed that the safety assessment results using the standard Level 2A curve might be unsafe because some parts of the constructed FAD curves fall inside of the standard one. A penalty factor of 1.15 working on both the elastic–plastic J-integral and plastic collapse load (Pc) is proposed to move all the constructed FAD curves just outside of the standard Level 2A curve.

Assessment of notched structural steel components using failure assessment diagrams and the theory of critical distances

When the structural integrity of notched components is analysed, it is generally assumed that notches behave as cracks, something which generally provides overconservative results. The proposal of this paper consists, on the one hand, in the application of the theory of critical distances for the estimation of the notch fracture toughness and, therefore, for the conversion of the notched situation into an equivalent cracked situation in which the material develops a higher fracture resistance. On the other hand, once the notch fracture toughness has been defined, the assessment is performed using the failure assessment diagram methodology, and assuming that the notch effect on the limit load is negligible. The methodology has been applied to 336 CT notched fracture specimens made of two different structural steels, covering temperatures from the corresponding lower shelf up to the upper shelf, providing satisfactory results and a noticeable reduction in the overconservatism derived from the analyses in which the notch effect is not considered.

On 3-D crack problems in offshore pipeline with large plastic deformation

Offshore pipelines consist of short pipeline segments connected by girth welding method. Surface and embedded elliptical cracks are often observed at welding zone which pose a potential threat to the reliability of the offshore pipelines. In this paper, three-dimensional nonlinear elastic–plastic stress investigation for the fracture responses of such offshore pipeline with surface or embedded elliptical cracks has been carried out. Based on the reference strain method, strain-based estimation formulas for crack tip opening displacement (CTOD) are proposed for the flawed pipelines under pure bending. Furthermore, based on the proposed estimation formulations, two failure assessment diagrams (FADs) for strain-based fracture are established to perform fracture assessment on the specific pipelines.

Structural integrity of nuclear reactor pressure vessels

The paper starts from concerns expressed by Sir Alan Cottrell, in the early 1970s, related to the safety of the pressurized water reactor (PWR) proposed at that time for the next phase of electrical power generation. It proceeds to describe the design and operation of nuclear generation plant and gives details of the manufacture of PWR reactor pressure vessels (RPVs). Attention is paid to stress-relief cracking and under-clad cracking, experienced with early RPVs, explaining the mechanisms for these forms of cracking and the means taken to avoid them. Particular note is made of the contribution of non-destructive inspection to structural integrity. Factors affecting brittle fracture in RPV steels are described: in particular, effects of neutron irradiation. The use of fracture mechanics to assess defect tolerance is explained, together with the failure assessment diagram embodied in the R6 procedure. There is discussion of the Master Curve and how it incorporates effects of irradiation on fracture toughness. Dangers associated with extrapolation of data to low probabilities are illustrated. The treatment of fatigue-crack growth is described, in the context of transients that may be experienced in the operation of PWR plant. Detailed attention is paid to the thermal shock associated with a large loss-of-coolant accident. The final section reviews the arguments advanced to justify ‘Incredibility of Failure’ and how these are incorporated in assessments of the integrity of existing plant and proposed ‘new build’ PWR pressure vessels.

Fatigue strength experiments of corroded small scale steel specimens

The objective of this work is to analyze the fatigue strength of small scale corroded steel specimens. The specimens were cut from a box girder, which was initially corroded in real sea water conditions. The surface of 11 corroded specimens was analyzed applying photogrammetry techniques and a description of an idealized corroded surface was established. The non-corroded specimens are identified as FAT 86 category but the fatigue test demonstrated that due to the severe corrosion degradation the experimental fatigue results of the corroded specimens are located above the fatigue design category FAT 50 and below FAT 100 referring to the nominal stress approach. The regression analysis of fatigue test results leads to m=3.094 and Δσ97.7%,m=3=64.95MPa. Fatigue assessment of crack propagation on a pit like crack flow based on a failure assessment diagram was performed. The admissible initial idealized flow defect has been defined, which matches the fatigue life achieved by the fatigue test for different load categories and corrosion degradation level.

EFFECT OF MECHANICAL PROPERTY MISMATCH ON FAILURE ASSESSMENT CURVE FOR WELDED JOINT WITH A SEMI-ELLIPTICAL CRACK

Elastic–plastic finite element analysis was performed to study the welded joints with a semi-elliptical crack. This research includes the effects of crack depth, strength mismatch ratio, and weld width on the crack driving force J-integral and the constraint parameter A2 at the crack tip. A two-parameter J-A2 fracture criterion based on the present results of crack tip stress field in the welded joints was established. The corresponding failure assessment diagrams were investigated in detail, from which the reliability and safety margin of the welded structures were discussed.

Approximate J estimates for combined primary and secondary stresses with large elastic follow-up

This paper proposes the V-factor for approximate J estimates under combined primary and secondary stresses with large elastic follow-up within the failure assessment diagram approach. The elastic follow-up is characterized by three parameters. The first two are the relative magnitudes of the primary load and of the secondary stress, β. The third parameter is the initial slope of the Vo–β curve where Vo is the value of V for zero primary load. Elastic follow-up effects are more pronounced for contained yielding, smaller β and a larger initial slope of the Vo–β curve. An approximate expression for V/Vo is proposed, showing good agreements with finite element solutions.

Constraint-based fracture mechanics analysis of cylinders with internal circumferential cracks

In this paper, constraint-based fracture mechanics analyses of hollow cylinders with internal circumferential crack under tensile loading are conducted. Finite element analyses of the cracked cylinders are carried out to determine the fracture parameters including elastic T-stresses, and fully-plastic J-integrals. Linear elastic finite element analysis is conducted to obtain the T-stresses, and elastic-plastic analysis is conducted to obtain the fully plastic J-integrals. A wide range of cylinder geometries are studied, with cylinder radius ratios of <TEX>$r_i/r_o$</TEX> = 0.2 to 0.8 and crack depth ratio a/t = 0.2 to 0.8. Fully plastic J-integrals are obtained for Ramberg-Osgood power law hardening material of n = 3, 5 and 10. These fracture parameters are then used to construct conventional and constraint-based failure assessment diagrams (FADs) to determine the maximum load carrying capacity of cracked cylinders. It is demonstrated that these tensile loaded cylinders with circumferential cracks are under low constraint conditions, and the load carrying capacity are higher when the low constraint effects are properly accounted for, using constraint-based FADs, comparing to the predictions from the conventional FADs.

Non-probabilistic defect assessment for structures with cracks based on interval model

Traditional defect assessment methods conservatively treat uncertainty of parameters as safety factors, while the probabilistic method is based on the clear understanding of detailed statistical information of parameters. In this paper, the non-probabilistic approach is introduced to the failure assessment diagram (FAD) to propose a non-probabilistic defect assessment method for structures with cracks. This novel defect assessment method contains three critical processes: establishment of the interval failure assessment curve (IFAC), determination of the assessment rectangle, and solution of the non-probabilistic reliability degree. Based on the interval theory, uncertain parameters such as crack sizes, material properties and loads are considered as interval variables. As a result, the failure assessment curve (FAC) will vary in a certain range, which is defined as IFAC. And the assessment point will vary within a rectangle zone which is defined as an assessment rectangle. Based on the interval model, the establishment of IFAC and the determination of the assessment rectangle are presented. Then according to the interval possibility degree method, the non-probabilistic reliability degree of IFAC can be determined. Meanwhile, in order to clearly introduce the non-probabilistic defect assessment method, a numerical example for the assessment of a pipe with crack is given. In addition, the assessment result of the proposed method is compared with that of the traditional probabilistic method, which confirms that this non-probabilistic defect assessment can reasonably resolve the practical problem with interval variables.

Constraint-based assessment of fracture in welded components

A toughness correction method, developed as ISO 27306, specifies an equivalent crack tip opening displacement (CTOD) ratio, β, for the correction of CTOD toughness for constraint loss in structural components. Using β, the standard fracture toughness specimen and structural components are linked at the same level of the Weibull stress. This paper applies the IST method to the fracture assessment of welded components. Effects of the strength mismatch and residual stress in welds on β are implemented into the toughness correction. The strength mismatch does not have an influence on β, provided that the strength mismatch between the weld and base metals is in the range from 0.9 to 1.2. On the other hand, residual stress exerts a large influence at a low CTOD level below full yielding: β is elevated by the tensile residual stress. It is shown on the failure assessment diagram that the CTOD toughness correction with β eliminates the excessive conservatism often found in the conventional approach and leads to accurate fracture assessments of welded components.

On the application of the theory of critical distances to the structural integrity assessment of stress risers

Purpose – The purpose of this paper is to present and validate a methodology for the structural integrity assessment of components containing a variety of stress risers and subjected to static conditions.Design/methodology/approach – The methodology is based on the use of the apparent fracture toughness prediction provided by the theory of critical distances (in this case, the line method), together with a well‐known, widely‐used engineering tool in structural integrity assessments: failure assessment diagrams. In order to validate the proposed methodology, an experimental programme has been conducted, testing 38 specimens made of aluminium alloy Al7075‐T651, each of them containing a certain stress riser. The comparison between the experimental results and the corresponding predictions provided by the proposed assessment methodology has also allowed the situations for which the theory of critical distances provides accurate predictions to be defined.Findings – The results show that the methodology provid...

Defect assessment of welded specimen considering weld induced residual stresses using SINTAP procedure and FEA

The defect assessment in butt-welded joint of ASTM A36 steel plates and 7075-T7351 aluminum alloy plates containing transverse through thickness crack was analyzed using SINTAP procedure and FEA incorporating weld induced residual stresses. Weld induced longitudinal residual stress profile can be obtained through SINTAP procedure, FEA or experimental analysis. This residual stress profile can be fitted with the trapezoidal residual stress profile available in SINTAP. For three different cases, crack length and residual stress intensity factor (SIF) are calculated and its comparison with the results obtained through FEA is plotted with respect to crack length. The stress intensity factor for mechanical loading is also plotted in the same graph. Using this graphical plot, the total SIF, including residual stress and mechanical loading, can be calculated for any particular crack size. The total SIF can be compared with the fracture toughness of the material for damage tolerance analysis. Also a failure assessment diagram is drawn for welded 7075-T7351 aluminum alloy plates with different crack sizes for as-welded (only residual stress) and mechanical loading along with the existing weld induced residual stresses to show the safety level for a particular crack size and mechanical loading.