Showing papers in "Journal of Pressure Vessel Technology-transactions of The Asme in 1995"

Development of the MPC Omega Method for Life Assessment in the Creep Range

Abstract: A methodology for characterizing and assessing the behavior of materials after service in the creep range has been developed and used on a broad range of materials and components. It incorporates the results of relatively short-term tests and improved databases on materials properties. The essence of the method is the definition of a material performance characteristic which the authors refers to by the symbol {Omega}{sub p}. This coefficient effectively describes the rate at which a material`s ability to resist stress is degraded by strain. While {Omega}{sub p} is a function of stress, temperature, and mode of loading, it is amenable to parametric representation and is, therefore, useful in predicting life and strain accumulation. Time to failure and total accumulated strain are shown to be consequences of a characterizing strain rate, as defined herein, and an appropriate {Omega}{sub p} for the operating conditions and geometry of interest. Accumulated strain, future strain, current creep rate, remaining life, total damage, and damage rate are among the quantities which are easily calculated. The development of the method employs and extends the concepts of Larson-Miller, Monkman-Grant, Robinson, Theta Projection, Kachanov, and Norton.

A Procedure to Predict Solid Particle Erosion in Elbows and Tees

Abstract: A semi-empirical procedure has been developed for predicting erosion rates in pipe geometries, such as elbows and tees. The procedure can be used to estimate safe operating conditions and velocities in oil and gas production where sand is present. In the proposed procedure, a concept is introduced that allows determination of erosion rate for different pipe geometries. In the procedure, based on empirical observations, the erosion rate is related to the impact velocity of sand particles on a pipe fitting wall. A simplified particle tracking model is developed and is used to estimate the impact velocity of sand particles moving in a stagnation region near the pipe wall. A new concept of equivalent stagnation length allows the simplified procedure to be applicable to actual pipe geometries. The equivalent stagnation regions'' of an elbow and a tee geometry of different sizes are obtained from experimental data for small pipe diameters, and a computational model is used to extend the procedure to larger pipe diameters. Currently, the prediction method applies to mild steel and accounts for the effects of sand size, shape, and density; fluid density, viscosity, and flow speed; and pipe size and shape. The proposed method has been verified formore » gas and liquid flows through several comparisons with experimental data reported in the literature. The results of the model accurately predict the effects of sand size and fluid viscosity observed in the experiments. Furthermore, predicted erosion rates showed good agreement with experimental data for gas, liquid, and gas-liquid flows in several 50.8-mm (2-in.) elbows and tees.« less

Constraint in the failure assessment diagram approach for fracture assessment

Abstract: This paper presents a framework for including constraint effects in the failure assessment diagram approach for fracture assessment. As parameters for describing constraint are still the subject of development, the framework is illustrated using both the elastic T-stress and the hydrostatic Q-stress. It is shown that constraint effects can be treated by modifying the shape of the failure assessment curves. In their simplest form, the modifications involve only two parameters: one quantifying the magnitude of structural constraint which depends on geometry and crack size; and the second quantifying the influence of constraint on fracture toughness.

Experimental Fretting-Wear Studies of Steam Generator Materials

Abstract: Flow-induced vibration of steam generator tubes results in fretting-wear damage due to impacting and rubbing of the tubes against their supports. This damage can be predicted by computing tube response to flow-induced excitation forces using analytical techniques, and then relating this response to resultant wear damage using experimentally derived wear coefficients. Fretting-wear of steam generator materials has been studied experimentally at Chalk River Laboratories for two decades. Tests are conducted in machines that simulate steam generator environmental conditions and tube-to-support dynamic interactions. Different tube and support materials, tube-to-support clearances, and tube support geometries have been studied. The effect of environmental conditions, such as temperature, oxygen content, pH and chemistry control additive, have been investigated as well. Early studies showed that damage was related to contact force as long as other parameters, such as geometry and motion, were held constant. Later studies have shown that damage is related to a parameter called work-rate, which combines both contact force and sliding distance. Results of short and long-term fretting-wear tests for CANDU steam generator materials at realistic environmental conditions are presented. These results demonstrate that work-rate is an appropriate correlating parameter for impact-sliding interaction.

Vibration of a Tube Bundle in Two-Phase Freon Cross-Flow

Abstract: Two-phase cross-flow exists in many shell-and-tube heat exchangers. The U-bend region of nuclear steam generators is a prime example. Testing in two-phase flow simulated by air-water provides useful results inexpensively. However, two-phase flow parameters, in particular surface tension and density ratio, are considerably different in air-water than in steam-water. A reasonable compromise is testing in liquid-vapor Freon, which is much closer to steam-water while much simpler experimentally. This paper presents the first results of a series of tests on the vibration behavior of tube bundles subjected to two-phase Freon cross-flow. A rotated triangular tube bundle of tube-to-diameter ratio of 1.5 was tested over a broad range of void fractions and mass fluxes. Fluidelastic instability, random turbulence excitation, and damping were investigated. Well-defined fluidelastic instabilities were observed in continuous two-phase flow regimes. However, intermittent two-phase flow regimes had a dramatic effect on fluidelastic instability. Generally, random turbulence excitation forces are much lower in Freon than in air-water. Damping is very dependent on void fraction, as expected.

Advances in dynamic relaxation techniques for nonlinear finite element analysis

Abstract: Traditionally, the finite element technique has been applied to static and steady-state problems using implicit methods. When nonlinearities exist, equilibrium iterations must be performed using Newton-Raphson or quasi-Newton techniques at each load level. In the presence of complex geometry, nonlinear material behavior, and large relative sliding of material interfaces, solutions using implicit methods often become intractable. A dynamic relaxation algorithm is developed for inclusion in finite element codes. The explicit nature of the method avoids large computer memory requirements and makes possible the solution of large-scale problems. The method described approaches the steady-state solution with no overshoot, a problem which has plagued researchers in the past. The method is included in a general nonlinear finite element code. A description of the method along with a number of new applications involving geometric and material nonlinearities are presented. They include: (1) nonlinear geometric cantilever plate; (2) moment-loaded nonlinear beam; and (3) creep of nuclear fuel channel assemblies.

The Effect of Gasket Creep-Relaxation on the Leakage Tightness of Bolted Flanged Joints

Abstract: The loss of tightness of bolted flanged joints is primarily due to the relaxation of the joint; in particular, the clamping load is affected by the amount of relaxation that a gasket exhibits over time. Test methods are available for evaluating the ability of a gasket to maintain a given compressive stress. It is necessary, however, to evaluate the response of the joint members to a small change in gasket thickness produced by the effect of creep-relaxation. This paper presents an analytical evaluation of the effect of gasket creep-relaxation on the remaining gasket stress taking into account the flexibility of all joint members, thus allowing the evaluation of the required tightening load on the gasket. The results obtained by this approach are shown to agree with those determined on an experimental rig made up of 4-in. class-600-lb pair of flanges. Axisymmetric finite element models of larger flange assemblies are also used for comparison. The results show that creep-relaxation of gaskets has a major influence on the remaining compression load on the gasket.

Calculation of Hydraulically Expanded Tube-to-Tubesheet Joints

Abstract: Tubular heat exchangers are basic components in chemical plant engineering Due to the fundamental importance of heat transfer in chemical processes, fail-safe behavior of the heat exchangers is a decisive prerequisite for reliable performance of plant complexes Most of the nonoperating periods of tube bundle apparatus are induced by damages at tube-to-tubesheet joints In the case of expanded tube connections, irrespective of being rolled or hydraulically fitted, the residual interfacial pressure generated by the expansion process was not sufficiently high and/or additional weakening of the joints by service loadings had happened Therefore, careful design of tube-to-tubesheet joints requires detailed knowledge about the interaction between an expanded tube and the surrounding tubesheet In 1976, Krips and Podhorsky, therefore, used finite circular rings for structural representation of the tubesheets These rings are denoted as equivalent sleeves Comparison with finite element results led to the introduction of a corrective factor by the same authors to adapt the equivalent sleeve diameter This paper discusses a formula for the equivalent sleeve diameter, which is based on an analytical treatment of the mechanical properties of multiholed sheets In the first part of the paper, an elastic-plastic computation of the residual contact pressure is presented In themore » second part, the sleeve diameter is derived by an analytical treatment of finite perforated disks The final part consists of checking the analytical results with nonlinear finite element analyses« less

Stress-Strain and Failure Analyses of Composite Pipe Under Torsion

Abstract: Composite piping systems have been widely used in the petrochemical industry because of their light weight and high corrosion resistance. However, due to the unavailability of torsional testing equipment, very little analytical/experimental research has been conducted for composite pipe under torsion. In this paper, an analytical and experimental study was conducted to investigate the elastic and failure behavior of composite laminated pipes under torsional loading. The analytical model was developed based on the mechanics of composite materials and the classical thin shell theory. The maximum strain failure criterion was applied in the model analysis. Five composite pipe samples were tested on a specially designed pipe torsional tester. A finite element analysis using COSMOS/M software was also performed to compare with the analytical and experimental results. Relationship between applied torque and shear strain, and between torque and torsional angle was obtained by all three methods. Composite pipes with [90/0]{sub n}, [+60/{minus}60]{sub n}, and [+45/{minus}45]{sub n} fiber angles were analyzed in the model and finite element analyses to better understand the effect of fiber orientation on stress, strain and deformation of the pipes. The effect of deviation in fiber angles was also investigated analytically and numerically. It was shown that both analyticalmore » and finite element analysis were in good accord with the experimental results in prediction of elastic shear strain/stress and shear deformation, but failure analysis based on the maximum strain failure criterion deviated significantly with the test results.« less

Ramberg-Osgood Strain-Hardening Characterization of an ASTM A302-B Steel

Abstract: Many elastic-plastic fracture mechanisms analysis procedures require knowledge of the true stress versus true strain response of the material being analyzed. The most common strain-hardening relationship employed is of the form first proposed by Ramberg and Osgood. Here, the Ramberg-Osgood strain-hardening exponents and coefficients are characterized for an unirradiated ASTM A302-B steel over a wide range of temperatures from {minus}129 to 260 C. The strain-hardening exponent increases only slightly with temperature over this range, while the coefficient decreases with increasing temperature. Tensile specimens irradiated to 0.002, 0.029, and 0.046 dpa exhibited significant increases in the strain-hardening exponent with increasing neutron irradiation level.

The Effect of Water Flow Rate Upon the Environmentally Assisted Cracking Response of a Low-Alloy Steel

Abstract: Effect of water flow rate on the environmentally-assisted cracking (EAC) response of a high-sulfur ferritic steel was studied at 243C. In contrast to earlier studies with compact-type specimens, this study employed relatively large tight semi-elliptical surface cracks tested under generally linear-elastic conditions. Flow velocities parallel to the crack as low as 1.68 {minus} 1.84 m/s were effective in mitigating EAC.

Numerical Simulation of Cross-Flow-Induced Fluidelastic Vibration of Tube Arrays and Comparison With Experimental Results

Abstract: Tube arrays exposed to air, gas or liquid cross-flow can vibrate due to vortex-shedding, turbulence, or fluidelastic instability. The major emphasis of this paper is on the phenomenon of fluidelastic instability (or fluidelastic vibration). A numerical model is applied to the simulation of fluidelastic vibration of representative tubes in a tube bundle, based on S. S. Chen's unsteady flow theory. The results are validated against published data based on linear cases. The model is then applied to a nonlinear structure of a U-bend tube bundle with clearances at supports, and the computed results compared to those obtained by experimental testing. The numerical studies were performed using the ABAQUS-EPGEN finite element code using a special subroutine incorporating fluidelastic forces. It is shown that the results of both the linear and nonlinear modeling are in good agreement with experimental data.

Stress-Intensity Factors for an Internal Semi-Elliptical Surface Crack in Cylindrical Pressure Vessels

Abstract: In this paper, the surface crack problem in a cylinder subjected to internal pressure is solved. The analysis is based on the body force method, but it is different from the conventional body force method in the following point. That is, the body forces to be distributed continuously on the assumed boundaries in an infinite body are approximated by some discrete point forces acting on the outside of the assumed boundaries. By using this method combined with the resultant force boundary conditions, solutions with high accuracy are obtained.

Weight Functions and Stress Intensity Factors for Longitudinal Semi-Elliptical Cracks in Thick-Wall Cylinders

Abstract: Semi-elliptical surface cracks are occasionally found in pressure vessels and pipes during service or production. Subsequent fracture and fatigue analysis of such cracks is of great practical interest, and requires the determination of stress intensity factors. Here, weight functions for the surface and the deepest point of an internal longitudinal semi-elliptical crack in a thick-wall cylinder (R{sub i}/t = 1) were derived from a general weight function and two reference stress intensity factors. For several linear and nonlinear crack face stress fields, the weight functions were validated against finite element data. Stress intensity factors were also calculated for the Lame through the thickness stress distribution induced by internal pressure. The weight functions appear to be particularly suitable for fatigue and fracture analysis of surface semi-elliptical cracks in complex stress fields. All stress intensity factor expressions given in the paper are valid for cylinders with the inner-radius-to-wall-thickness ratio, R{sub i}/t = 1.

Stress Analysis in Composite Hollow Cylinders due to an Asymmetric Temperature Distribution

Abstract: A solution is presented for the stresses and displacements in an orthotropic, hollow, circular cylinder subjected to asymmetric temperature distribution at the outer surface and heat convection into a medium at zero reference temperature at the inner surface. Assuming temperature-independent material properties, the heat conduction equation in cylindrical coordinates is solved for a single and multilayer cylinder. Results of temperature analysis along with linear elasticity theory are used to obtain the required thermal stresses and displacements. Numerical results are given for a typical fiber-reinforced composite material where fibers in each layer are oriented axially or circumferentially. The results show that the response of the cylinder is sensitive to changes in thickness, orientation of fibers in each layer, number of layers, and stacking sequence.

Preliminary Design of Tubular Composite Structures Using Netting Theory and Composite Degradation Factors

Abstract: Fiber-reinforced composite materials have been used effectively in applications such as flow lines, downhole tubing for injection and disposal wells, sucker rods, storage tanks, process vessels, and piping for offshore firewater systems. Composite directional drilling systems, and production risers for offshore tension leg platforms are currently being developed and tested for future oilfield use. Here, tubular products and process vessels built using fiber-reinforced composite materials provide significant advantages in applications that require corrosion resistance, high strength, and light weight. A design approach based on netting theory is presented which enables engineers to develop preliminary structural designs for these structures using composite materials. The integration of creep, cyclic loading, and environmental degradation factors into initial design calculations is also discussed and illustrated.

A Technique to Distinguish the Primary and Secondary Stresses

Abstract: A technique is developed which can be used to distinguish the primary and secondary stresses in pressure vessels. The general definition of the two types of stresses stated by the ASME Code is used; a simple viscoelastic model is proposed for each stress category. The proposed model can be extended to elastic as well as plastic regions of strain-hardening materials and can include the mechanical as well as thermal loads. The proposed viscoelastic models are used to judge the nature of elastic stresses and the effective stress-strain curve is used to simulate the state of stress at any stage of loading and the percentage of primary to secondary stresses at any radius of the vessel. It is found that thermal stresses cannot always be categorized as secondary stress, and in the case of thermoplastically loaded vessels they can contribute partly to the primary stress in the vessel.

Alternate Design Charts for Fixed Tubesheet Design Procedure Included in ASME Boiler and Pressure Vessel Code, Section VIII, Division 1

Abstract: Design formulas and calculation procedure for the design of fixed tubesheets of shell and tube heat exchangers are included in Appendix AA--Nonmandatory of ASME Boiler and Pressure Vessel code, Section 8, Division 1. To minimize the number of calculations, charts are provided as part of the design procedure. This article provides alternate charts for certain parameters and the original version of the charts are extended for larger values of tubesheet design parameter. Numerical values are given in tabular form for certain functions used in plotting the design charts. This will help to do design calculations without referring to the charts.

Experiment of Chaotic Vibration of Loosely Supported Tube Rows in Cross-Flow

Abstract: A tube array supported by baffle plates in cross-flow may be subjected to fluidelastic instability in the tube-support-plate-inactive mode. An experimental study is presented to characterize the tube motion. Three series of tests were performed to measure tube displacements as a function of flow velocity for differing clearances. The motion was examined by root-mean-square values of tube displacements, power spectral densities, phase planes, Poincare maps, and Lyapunov exponents. The experimental data agree reasonably well with an analytical model based on the unsteady flow theory.

A Practical Method Based on Stress Evaluation (σd Criterion) to Predict Initiation of Crack Under Creep and Creep-Fatigue Conditions

Abstract: In some parts of primary circuit of fast breeder reactor, where the temperature is higher than 427 C, preservice inspection has revealed indications that were conservatively assumed to be sharp cracks. These pipes are made of 316 SPH material, an austenitic material close to 316L. This material is subjected to creep behavior at this temperature. Here, the behavior of defects like cracks in nuclear components operating at high temperature, where creep is significant, must be under control. There exists the need to have a practical method of analysis, which can be used by engineers, to calculate the time of initiation for defects existing at the start of life of nuclear components. This study presents the background, the development, the application, and results concerning validation work made for a simplified method named {sigma}{sub d} of prediction of initiation for nuclear structures made of 316L austenitic steel and operating at temperature where creep is significant. This method relies on the evaluation of real stress-strain history on a small distance d (d = 0.05 mm) close to the crack front and material characteristics (limiting stresses) that are available in nuclear codes like ASME Code cases or RCC-MR.

Alternative method of RTNDT determination for some reactor vessel weld metals validated by fracture toughness data

Abstract: The fracture toughness curves used for nuclear power plant operation pressure-temperature limits and for pressurized thermal shock evaluations are dependent on the reference temperature for nil-ductility transition (RT{sub NDT}). The original method to determine the RT{sub NDT} was formulated more than 20 yr ago when Section 3 of the ASME Code was adopted. At that time, there were insufficient data to judge whether some of the weld metals used in reactor vessel fabrication were unsuitable for this procedure. Presently, this causes a compliance problem for some weld metals used in nuclear reactor vessels, whereas there is no technical problem in meeting required safety margins. The RT{sub NDT} is a parameter to index degrees of irradiation embrittlement to adjust the Code reference fracture toughness curves to represent the actual degraded fracture toughness at a given fluence of a reactor vessel beltline region. When there is a problem determining RT{sub NDT} value for unirradiated material where Charpy transition temperature is the dominating criterion, an alternative RT{sub NDT} based solely on a drop-weight test was investigated for some of the weld metals. Using a new test method for fracture toughness in the transition range (ASTM, 1993), a fracture toughness curve was directly generatedmore » from a set of compact tension test data and used for validating the nil-ductility temperature (T{sub NDT}) from drop-weight test data as the sole mean for determining initial RT{sub NDT} value.« less

Sloshing Analysis Method Using Existing FEM Structural Analysis Code

Abstract: A fluid analysis method using an analogy relating the pressure wave equation of fluid to elasticity equations is applied to sloshing analysis, where existing FEM structural analysis codes are available. It is seen from theoretical consideration that the present method is equivalent to the classical FEM formulation of linear sloshing analysis. The numerical analyses of liquid sloshing in a rigid cubic tank and of vibration of tubulous fluid under gravitational force are performed by using the present method. The results are shown to be in excellent agreement with the theoretical values.

A constitutive model for high damping rubber bearings

Abstract: In this work, a simple constitutive model simulating the dynamic behavior of elastomeric isolation devices is proposed. This work is part of research activities undertaken at ENEA (Italian National Commission for the Nuclear and Alternative Energies Sources) to analyze the behavior of isolation systems and isolated structures, in view of possible application to the industrial plants, including the innovative nuclear reactors. The research includes both experimental tests and theoretical studies. The activities in this latter field are the following: implementation of high damping elastomeric material constitutive law and its modeling with refined finite element computer codes; simple modeling (1-degree-of-freedom) of the single isolating element; and modeling of isolated structures. In this report, the first results of 2nd and 3rd activities are summarized.

Yield-before-break fracture mechanics analysis of high-strength steel pressure vessels

Abstract: : Case study examples of fracture mechanics testing and analysis of Ni- Cr-Mo high strength steel cannon tubes are presented. The testing and analysis include significant plastic deformation accompanying fracture, which often occurs when high pressure is applied to high toughness steel pressure vessels. The analysis is based on a comparison of the size of the Irwin crack-tip plastic zone with the remaining ligament of the tube in the critical fatigue crack area that cause final failure. The results of the study show that the type of final failure can be predicted as either a relatively safe yield-before-break failure or a less safe running-crack type of failure for a variety of material, configuration, and loading conditions. Pressure vessels, Fracture mechanics, High strength steel, Plastic yielding, Fatigue failure.

J estimation method for a semi-elliptical surface flaw in a cylinder

Abstract: With the emergence of readily available simple J solutions for various types of structures through J estimation techniques, J-integral-based elastic-plastic fracture mechanics has become a common tool for analyzing ductile materials. This paper presents elastic-plastic solutions for semi-elliptical surface flaws of four different crack depths. Solutions are developed from a series of finite element analyses using the ABAQUS computer program with a deformation plasticity material constitutive law. Although the solutions are directly applicable to a single set of Ramberg-Osgood material parameters, they may be extended to include various amounts of strain hardening by a ratioing technique utilizing calibration functions from the EPRI handbook for continuous flaws. This paper addresses cylindrical structures loaded by internal pressure, and excludes any consideration of thermal loadings. Elastic-plastic J solutions, determined for pressures up to 5,000 psi (13,700 Pa), or twice the design pressure, depart from plastic-zone-corrected linear elastic predictions at approximately 3,000 psi (7,620 Pa) pressure. The h[sub 1] plastic calibration functions derived in this paper are limited to the point of greatest crack depth in 6:1 aspect ratio semi-elliptical inside surface flaws in cylindrical reactor pressure vessels.

An Analysis of Pipe Flange Connections Using Epoxy Adhesives/Anaerobic Sealant Instead of Gaskets

Abstract: This paper deals with the strength and the sealing performance of pipe flange connections combining the bonding force of adhesives with the clamping force of bolts. The epoxy adhesives or anaerobic sealants are bonded at the interface partially instead of gaskets in pipe flange connections. The stress distribution in the epoxy adhesives (anaerobic sealant), which governs the sealing performance, and the variations in axial bolt force are analyzed, using an axisymmetrical theory of elasticity, when an internal pressure is applied to a connection in which two pipe flanges are clamped together buy bolts and nuts with an initial clamping force after being joined by epoxy adhesives or anaerobic sealant. In addition, a method for estimating the strength of the combination connection is demonstrated. Experiments are performed and the analytical results are consistent with the experimental results concerning the variation in axial bolt force and the strength of combination connections. It can be seen that the strength of connections increases with a decrease in the bolt pitch circle diameter. Furthermore, it is seen that the sealing performance of such combination connections in which the interface is bonded partially is improved over that of pipe flange connections with metallic gaskets.

Failure Investigation of a Low Chrome Long-Seam Weld in a High-Temperature Refinery Piping System

Abstract: The results of an investigation of a long-seam welded low chrome pipe that failed in a high-temperature refinery piping system are presented in this paper. Based upon the results of a metallurgical investigation, which included a creep testing program and a detailed finite element stress analysis, the cause of the failure has been attributed to creep damage at the weld seam. The metallurgical investigation and creep testing program indicated that the 1-1/4 Cr-1/2 Mo pipe material was normalized and exhibited greater than average creep strength and creep ductility. The results of a piping stress analysis indicated that all pressure, weight, and thermal stresses were in compliance with the ASME B31.3 Piping Code (ASME, 1993a). Nonetheless, the pipe failed after only 100,000 h at a nominal hoop stress of 6 ksi (41.4 MPa) with an operating temperature range of 970°F (521°C) to 1000°F (538°C). Results from subsequent detailed finite element stress analyses of the failed pipe indicated that very high localized bending stresses were present in the pipe due to peaking at the long-seam weld. These stresses partially relax by creep, but after 100,000 h they were still approximately 38 percent higher than the nominal hoop stress. The creep strains resulting from stress relaxation and those associated with the long-term value of the sustained stresses cause severe creep damage at the weld seam. As a result of this damage, cracks initiated at the inside of the pipe and primarily grew through the HAZ/fusion line until an 18-in. through-wall crack developed. The pipe was produced to ASTM A691, Grade 1-1/4 Cr, Class 41 (ASTM, 1989), and the peaked geometry was found to satisfy the fabrication tolerances of this standard. The need for the development of an acceptable tolerance for peaking in addition to the outside diameter and out-of-roundness fabrication tolerances currently included in this standard is highlighted for long-seam welded pipe that is to operate in the creep range.

Deformation Plasticity Failure Assessment Diagram (DPFAD) for Materials With Non-Ramberg-Osgood Stress-Strain Curves

Abstract: This paper presents a brief history of the evolution of the Central Electricity Generating Board`s (CEGB) R-6 failure assessment diagram (FAD) procedure used in assessing defects in structural components. The reader is taken from the original CEGB R-6 FAD strip yield model to the deformation plastic failure assessment diagram (DPFAD), which is dependent on Ramberg-Osgood (R-O) materials to general stress-strain curves. An extension of the DPFAD approach is given which allows the use of material stress-strain data which do not follow the R-O equation such as stainless steel or carbon manganese steel. The validity of the new approach coined piecewise failure assessment diagram (PWFAD) is demonstrated through comparisons with the J-integral responses (expressed in terms of failure assessment diagram curves) for several cracked configurations of non-R-O materials. The examples were taken from both finite element and experimental results. The comparisons with these test cases demonstrate the accuracy of PWFAD. The use of PWFAD requires the availability of deformation plasticity J-integral solutions for several values of the strain-hardening exponent as well as uniaxial tensile stress-strain data at the temperature of interest. Lacking this information, the original R-O DPFAD approach using known engineering yield and ultimate strengths would give the best availablemore » approximation. However, it is strongly recommended that actual uniaxial tensile stress-strain data be used when available.« less

Plastic Deformation and Burst of Pressurized Multilayered Cylinders

Abstract: A large strain analysis is presented for internally pressurized multilayered tubes, in generalized plane strain. Material behavior is modeled by an elastoplastic deformation theory with an orthotropic yield function, introduced by Hill, and arbitrary hardening. Elastic compressibility is neglected. An exact solution is given, in terms of quadratures, along with a general condition for burst. Simple yet useful relations are derived for thin-walled cylinders with the neglect of elastic strains. For rigid/nonlinear-hardening response, we obtain an expression for the onset of burst in terms of overall effective moduli. A few numerical examples are given and the possibility of locating an optimal two-layer configuration is discussed. It appears that optimization with respect to weight is attainable provided that appropriate materials are selected

Life Prediction in High-Temperature Structural Materials

Abstract: Life prediction methods are required to assess the performance and safety of the structural materials used in engineering systems and components that operate at high temperatures. At high temperatures, materials are subject to time-dependent creep and environmental degradation, as well as cycle-dependent fatigue degradation. The life prediction methods must account for all of these degradation mechanisms and their possible interactions. The purpose of this paper is to review the methods that are used to predict the creep and fatigue life of structural materials. Traditional methods that have been used to predict the life of structural materials are based on the initiation of a significant crack; whereas more recent methods employ fracture mechanics to predict life based on the growth of crack from some initial size to a critical size. Crack-growth-based life prediction methods are emphasized in this review.