bluuebunny/crossref_metadata_2025 huggingface dataset:
Assessment of a Damaged Piping System Utilizing Laser Surface Scanning and Finite Element Analysis
Authors: Livingston Hilliard L. , Brodzinski Richard P.
Published: August 2008

The application of laser surface scanning technology in conjunction with finite element analysis to evaluate the fitness for service of a damaged section of piping is presented in this paper. The difference between the results obtained using a simplified dimensional model produced using hand measurements and the laser scan is also discussed. A section of NPS 18 (DN 450) diameter piping that carries vapors generated by a coke drum in an oil refinery delayed coking unit had been damaged in service from contact with a nearby structural steel member. The damaged area was deformed nearly 1.2 in. (30 mm) deep over an area measuring approximately 10 in. × 12 in. (250 mm × 300 mm) and included a pronounced crease along the line of contact with the steel. The piping was also slightly out of round outside of the area of gross distortion. Laser scanning was employed to obtain an accurate dimensional representation of the surface geometry and to develop a finite element analysis model. The decision to allow the piping system to continue to operate until the next scheduled outage was then based upon the operating stresses, maximum strain in the deformed area and fatigue life determined for the damaged section.
Stress Classification and Assessment of Locally Thinned Class 1 Piping Components
Authors: Iyer San
Published: August 2008

Carbon steel piping components, such as elbows, and bends are extensively used in the primary heat transport systems of nuclear power plants. During the design stage, a corrosion allowance of 1/32” to 1/8” is normally assumed for wall thickness calculations. The corrosion is assumed to occur uniformly around the circumference of the piping component. The Class 1 piping components are designed to ASME Section III rules. Flow Accelerated Corrosion (FAC) affects ‘local’ areas creating ‘patches’ or ‘pits’ of thinned wall in critical piping components. If a single thickness value corresponding to the thinnest section is chosen to analyze these components, then the analysis would be too conservative. In addition, the analysis would not reflect the true state of the stress and displacement field in the localized thin wall area. To address these issues, a comprehensive scheme utilizing Finite Element Analysis is developed in this paper. This scheme employs the NB-3200 approach in calculating and classifying the stresses in irregularly or non-uniformly thinned nuclear piping components. The allowable are derived based on the ASME Section III rules. The stresses are classified as primary, secondary, membrane, bending, and peak. The combined primary and secondary stresses, primary and secondary and peak stresses are evaluated and compared to established Code allowable to assure that the requirements are met.. It is shown that the stress classification approach developed in this paper is robust and ensures adequate safety of the structure to withstand design and operational conditions.
Reference Stress Values for Toroidal Vessels Subjected to Uniform Internal Pressure
Authors: Zarrabi Khosrow , Basu Abheek
Published: August 2008

Using the finite element method, the paper presents an explicit expression for the reference stress of tube/pipe bends with various degrees of ovality that are subjected to uniform internal pressures. No such task has been accomplished before. The presented reference stress may be used in conjunction with the uniaxial creep rupture data to obtain the creep life of the bend.
Design and Fabrication of Reactor Pressure Vessel for High Temperature Engineering Test Reactor (HTTR)
Authors: Tachibana Yukio , Nakagawa Shigeaki , Iyoku Tatsuo
Published: August 2008

The reactor pressure vessel (RPV) of the HTTR is 5.5 m in inside diameter, 13.2 m in inside height, and 122 mm and 160 mm in wall thickness of the body and the top head dome, respectively. Because the reactor inlet temperature of the HTTR is higher than that of LWRs, 2 1/4Cr-1Mo steel is chosen for the RPV material. Fluence of the RPV is estimated to be less than 1×1017 n/cm2 (E>1 MeV), and so irradiation embrittlement is presumed to be negligible, but temper embrittlement is not. For the purpose of reducing embrittlement, content of some elements is limited on 2 1/4 Cr-1 Mo steel for the RPV using embrittlement parameters, J-factor and X. In this paper design, fabrication procedure, and in-service inspection technique of the RPV for the HTTR are described.
Sophisticated Creep-Fatigue Life Estimation Scheme for Pressure Vessel Components Based on Stress Redistribution Locus Concept
Authors: Shimakawa Takashi , Nakamura Kyotada , Kobayashi Ken-Ichi
Published: August 2008

High temperature components are operated under cyclic thermal transient. Creep-Fatigue is the most dominant failure mode to be considered in Elevated Temperature Design of these components. Design limit for computed thermal stress is allowed to exceed yielding, because thermal stress is generally regarded as a displacement controlled one. Since creep deformation is considered as additional inelastic behavior, methodology to estimate inelastic strain concentration should be prepared in a design standard. Though inelastic FEM analyses can be applied to calculate inelastic strain concentration magnitude, it is well known that prediction is affected by applied constitutive model. Current design codes recommend to apply elastic FEM and to estimate inelastic strain behavior by simplified method. This paper presents sophisticated technique to estimate inelastic strain behavior based on Stress Redistribution Locus (SRL) method. Applicability of SRL concept is discussed with a help of FEM results for representative components of pressure vessel components such as nozzle, skirt and tube sheet.
Type IV Creep Damage Analysis for Full Size Component Test on Welded P91 Boiler Hot Reheat Piping
Authors: Takemasa Fumio , Nonaka Isamu , Ito Takuya , Saitou Kensuke , Miyachi Yoshikazu , Kagiya Yukio
Published: August 2008

In order to establish the creep life assessment method for modified 9Cr-1Mo steel welded joints on hot reheat piping of generating power plants, internal pressure creep tests conducted with full size components are analysed using the finite element method (FEM). As a result, it is shown that the creep damage distribution in weld heat affected zone (HAZ) can be predicted by stress distribution in HAZ, and that average stress in HAZ can be a representative value for creep strength of welded joints.
Experimental Investigation Into the Fracture Toughness of Polyethylene Pipe Material
Authors: Graice Ihab Mamdouh , Younan Maher Y. A. , Naga Soheir Ahmed Radwan
Published: August 2008

The mechanical behavior of the recently produced gas pipes material PE100 is investigated and compared to the commonly used material PE80 to determine their relative advantages. The two materials show plastic behavior at room temperature. The fracture toughness of the two materials is experimentally determined using the two common elastic plastic fracture mechanics methods: the ASTM multiple specimen test method for determining the J-R curve of the materials, and the crack opening displacement (COD) method. The investigation of the fracture behavior of the two materials includes the effect of the specimen thickness as well as specimen configuration. The experimental tests were carried on the compact tension (CT) specimens and the single edge notch bending (SENB) specimens. At −70°C, the materials show elastic behavior, the ASTM test method for determining fracture toughness is applied to SENB specimens to determine KIC of both materials. PE80 shows greater resistance to fracture than PE100.
Optimum Design of Net Reinforced Composite Pressure Vessel
Authors: Yoshikawa Nobuhiro
Published: August 2008

A new breed of structural reinforcement for high pressure vessels is proposed by netted filament, whereas the conventional composite containers are fabricated with continuous filament winding. An enhancement of design variation is aimed at for light-weight together with adequate structural reliability ensured by means of structural optimization technique in the present study. Modeling the net segments as bar finite elements contacting with the surface of the metallic liner, a shape-finding problem raised by the geometrical non-linearity with the finite rotation of bar element is posed. The problem is solved on the basis of principle of minimum strain energy with respect to nodal position of the element. A formulation is derived to search for the optimum configuration of net reinforcing spherical dome. The optimum length of net segment is determined so as to realize uniform strain in net. Numerical examples validate the proposed formulation.
High Pressure Fatigue Testing of 160 KSI Yield Strength Stainless Steel Tubing
Authors: Koerner Peter , Hiller Waldemar , Wink Rolf , Strackerjahn Henrich , Goeken Matthias
Published: August 2008

High pressure processes like water-jet cutting, hydroforming or LDPE production require high pressure tubing with nominal sizes of 2 to 90 mm for pressures of up to 4,000 bar. The most common materials for these applications are either cold drawn austenitic steels type 304 / 316 or low alloy martensitic steels from the A723 series. Recent developments in material technology made it possible to increase the yield strength of the cold drawn austenitic steel from 700 MPa to 1100 MPa maintaining high toughness values, even for relatively large tube sizes. This steel grade, referred to as “HP160” (nitrogen strengthened 21Cr 10Ni 3Mn 2.5Mo stainless steel), is now increasingly being utilised in the high pressure industry. In this paper the fatigue performance of HP160 under pulsating internal pressure will be compared to fatigue results from conventional steels. The results will also be compared with the requirements of the ASME high pressure code Section VIII Division 3.
Mean Stress Correction Procedure in Fatigue Evaluation for High Pressure Vessels
Authors: Terada Susumu , Yamamoto Kenji
Published: August 2008

One important issue in the fatigue evaluation that must be addressed is the mean stress correction procedure. ASME Div.3 is provided with the mean stress correction procedure. The predicted equivalent alternating stress intensities Seq are compared with the experimental results for push-pull fatigue tests with mean stress for JIS SCM440 and JIS SUS630 H1075. Seq per ASME Div.3 are not coincident with the experimental results. In case of high ratio of tensile mean stress to tensile strength, the difference becomes large and it is un-conservative. On the other hand Seq per KHK Standard are coincident with experimental results. The fatigue test results for pressurized cylinder tube by Davidson and Findley are also compared with predicted values per ASME Div.3 and KHK Standard. The results of comparison are same as that for above mentioned push-pull fatigue tests. Based on the experimental verification, the authors recommend that the mean stress correction procedure per KHK Standard be implemented in ASME Div.3.
Stresses Within Compound Tubes Comprising a Steel Liner and an External Carbon-Fiber Wrapped Laminate
Authors: Parker Anthony P. , Troiano Edward , Underwood John H.
Published: August 2008

There is increasing interest in techniques that permit weight reduction of thick cylinders, particularly gun barrels. The method examined within this paper involves the external wrapping of a carbon fiber-based laminate jacket around a steel liner. Various design options are examined, e.g. prior autofrettage of a steel liner followed by external tension wrapping and benign wrapping of a stress-free steel liner followed by autofrettage of the assembly. Outcomes include optimum overstrain and machining of the steel liner, identification of a potential ‘crushing’ failure mode due to radial compressive stresses and techniques for application of tension wrapping which optimize hoop stress within the composite jacket. Finally, possible combinations of fiber orientations are discussed.
Comparison of Different Codes and Standards Applicable for Design and Calculation of High Pressure Equipment
Authors: Schedelmaier Johannes , Po¨lzl Manfred
Published: August 2008

Depending on the plant location for a new project different Codes and Standards are applied for design and calculation of high pressure equipment. Many countries do not have a specific code for high pressure vessels and components, respectively the high pressure used in petrochemical plant is out of the code range. In this case the purchaser or the process licensor determines the code to be applied on the equipment for the new plant. This paper provides a technical analysis and comparison for high pressure components calculated according to the following codes: ASME VIII, AD 2000 and CODAP 95. Design and calculation results according to the different codes are verified on an example of a double pipe heat exchanger using identical mechanical properties of materials and the same operating conditions. The results are illustrated on figures for different low pressure and high pressure components. The literature does not define a limit between low pressure, high pressure and ultra high pressure. As the difference between steam pressure in the jacket (3,3MPa), pressure in the process tube (50 MPa, 160 MPa, 360 MPa) and autofrettage pressure (1000MPa) is very significant, this sequence is applied in the paper. Economic aspects are more and more important for new projects in order to minimize costs for high sophisticated equipment. For this reason also a cost analysis was performed for the calculation examples and the economic impact of application of different codes is indicated on a diagram. The technical–economical evaluation leads to an optimised product.
Safety Aspects of Design of Cylinders and Hyper-Compressors for LDPE
Authors: Giacomelli Enzo , Battagli Paolo , Lumachi Fabrizio , Gimignani Luciano
Published: August 2008

The cylinders of reciprocating compressors for low-density polyethylene are compound pressure vessels, although generally excluded by Pressure Vessels design codes. Safety is the most important design aspect to be taken into account, because of the very high loads, the extreme internal pressures and the nature of the process gas. The design must evaluate from the safety standpoint all possible failure modes of each component, so that the machine is absolutely safe, notwithstanding the highly severe working conditions. The means to allow a smooth and safe operation of the compressor is the control of overpressure and overloading. The use of innovative methods of simulation and modelling, together with new diagnostic systems with special features, allow very high safety levels, high availability and optimised maintenance procedures.
Fitness-for-Service Methodology Based on Variational Principles in Plasticity
Authors: Indermohan H. , Seshadri R.
Published: August 2008

A variational formulation in plasticity has been used to develop improved limit load estimation technique, such as the m-alpha method. Lower bound limit load estimates are especially germane to design and fitness-for-service assessments. The concept of “integral mean of yield” has been applied to problems involving locally thin areas (LTA) and local hot spots in the context of industrial pressure vessels and piping. Simplified procedures for “fitness-for-service” assessment, suitable for use by plant engineers, have been developed. The results are compared with the corresponding inelastic finite elastic analyses.
Development of a Web-Based Interface for the Automatic Finite Element Analysis of Pressure Vessels
Authors: Russell Colin , Nash David H.
Published: August 2008

The Internet presents an opportunity to facilitate the design of pressure equipment in a new and different way. Current industrial design practice employs computer programmes that perform design-by-formula (DBF) calculations in accordance with ASME VIII Division 1 or other international codes and standards. Design-by-analysis (DBA), however is only undertaken by experienced vessel engineers or general finite element analysis (FEA) consultancy specialists. The present work has established an interface between the Internet and a commercial FEA program for use by designers in the pressure vessel industry. The interface allows users to input variables for a pre-delivered model, obtained from a library of verified models, which may be analysed automatically and the results returned for review. The outcome of the work has been that an interface has been fully established in the form of an interactive dynamically operating web site. It has extensive error checking facilities for user input variables, and is fully operational for the available models, which, for example, includes a reinforced nozzle located in an elliptical end with multiple loadings. The system has been tested by industry and new opportunities have resulted for the training of engineers by allowing access to complex models only after suitable training has been undertaken and levels of competence have been achieved.
Fracture Toughness and Crack Path Deviations in Laser Welded Joints
Authors: Bezensek Bostjan , Hancock John W.
Published: August 2008

The resistance of laser welded joints to cleavage failure has been examined using fatigue cracked fracture toughness specimens and Charpy tests. The apparent toughness of a range of weld microstructures was determined, the lowest being for a crack located in the weld metal. Sharp cracks deviate into the microstructure with the lowest apparent toughness adjacent to the tip and propagate down the toughness gradient. Charpy tests differ in that the crack consistently propagates into the softer material, and extends down the yield strength gradient adjacent to the tip. Charpy tests can thus give misleading information about the fracture behaviour of mismatched weldments, as crack path deviations may mask a potentially dangerously low toughness weld metal. Recommendations are given on supplementing Charpy weld characterisation with the fracture mechanics tests to ensure conservatism in mismatched weldments.
Mechanical Properties and Fracture Toughness of Aluminum Vessels by Friction Stir Welding
Authors: Mochizuki Masahito , Toyoda Masao , Inuzuka Masayuki , Nishida Hidehito
Published: August 2008

Mechanical properties and fracture toughness in friction stir welded joint of vessels of structural aluminum alloy type A5083-O are investigated. Welded joint from 25 mm-thick plate is fabricated by one-side one-pass friction stir. Charpy impact energy and critical crack-tip opening displacement (CTOD) in friction stir weld are much higher than those of base metal or heat-affected zone, whereas mechanical properties such as stress-strain curve and Vickers hardness do not have a conspicuous difference. Effects of microstructure on crack initiation and propagation are studied in order to clarify the difference of fracture toughness between stir zone and base metal. Both tensile test and bending test show that the fine-grained microstructure in stir zone induces to increase ductile crack initiation and propagation resistance by analyzing fracture resistance curves and diameter of dimples in fracture surface. It is found that high fracture toughness value in stir zone is affected fine-grained microstructure by friction stirring.
Stress Analysis of a Elastic Cracked Layer Bonded to a Viscoelastic Substrate
Authors: Chao C. K. , Hsiao C. C.
Published: August 2008

The effect of a viscoelastic substrate on an elastic cracked layer under an in-plane concentrated load is solved and discussed in this study. Based on a correspondence principle, the viscoelastic solution is directly obtained from the corresponding elastic one. The elastic solution in an anisotropic trimaterial is solved as a rapidly convergent series in terms of complex potentials via the successive iterations of the alternating technique in order to satisfy the continuity condition along the interfaces between dissimilar media. This trimaterial solution is then applied to a problem of a thin layer bonded to a half-plane substrate. Using the standard solid model to formulate the viscoelastic constitutive equation, the real time stress intensity factors can be directly obtained by performing the numerical calculations. The results obtained in this paper are useful in studying the problem with defects where a crack is assumed to exist in an elastic body that is bonded to a viscoelastic substrate.
Delayed Hydride Cracking Initiation at Simulated Secondary Flaws in Zr-2.5 Nb Pressure Tube Material
Authors: Cui Jun , Shek Gordon K. , Scarth Douglas A. , Lee William K.
Published: August 2008

Flaws in Zr-2.5 Nb alloy pressure tubes of CANDU nuclear reactors are susceptible to a crack initiation and growth mechanism called Delayed Hydride Cracking (DHC), which is a repetitive process that involves hydrogen diffusion, hydride precipitation, growth of the hydrided region and fracture of the hydrided region at the flaw-tip. The presence of small surface irregularities, or secondary flaws, at the bottom of service-induced fretting flaws in pressure tubes requires an integrity assessment in terms of DHC initiation. Experimental data and analytical modeling are required to predict whether DHC initiation can occur from the secondary flaws. In the present work, an experimental program was carried out to examine the impact of small secondary flaws with sharp radii on DHC initiation from simulated fretting flaws. Groups of cantilever beam specimens containing blunt notches with and without secondary flaws were prepared from unirradiated pressure tube materials hydrided to a nominal concentration of 50 wt ppm hydrogen. The specimens were subjected to multiple thermal cycles to form hydrides at the flaw-tip at different applied stress levels, which straddled the threshold value for DHC initiation. The threshold conditions for DHC initiation were established for different simulated fretting and secondary flaws. The experimental results are compared with predictions from the engineering process-zone DHC initiation model.
Hot Isostatic Pressing of Austenitic Stainless Steel Powders for Pressure Retaining Applications
Authors: Burdett W. Barry , Hurrell Paul , Gilleland Alan
Published: August 2008

Hot Isostatic Pressing (HIP) has been used for many years to consolidate porosity in cast metal shapes to improve mechanical properties. When the technique is applied to fine metal powders, it becomes possible to produce Near Net Shape (NNS) items and more complex geometry components that are fully dense and offer an attractive set of properties and reduced cost. Manufacture of NNS items from powder delivers cost savings by reducing initial material usage and subsequent machining costs. Powder production and HIP processing are automated methods, which also provide protection against forging route obsolescence. Setup costs are lower and smaller batch sizes possible. HIPped powder microstructures are isotropic and equi-axed, with uniformly fine grain sizes not normally achieved in heavy section components. In austenitic stainless steel materials, this provides significant improvements in ultrasonic NDE (Non-Destructive Examination) in thick sections. Use of the technology has grown, particularly in the off-shore oil industry where it is already established in high integrity applications, but take-up in the more conservative nuclear industry has been slow. In a broad programme of testing, Rolls-Royce has established that HIPped powder 316L components, in items up to several tons in weight, have equivalent or slightly better strength, toughness and corrosion properties across a wide range of test environments. A methodology for developing robust safety justifications for use has been developed. Manufacture of pressure seal components is now in progress and the economics of other applications such as pump bowls are being considered. The quality of HIPped powder items can provide through life cost savings since there is greater assurance of structural integrity compared to welded or wrought components.
Design Optimization of Magnetic Coupling Using Genetic Algorithm and 2D FEM Model
Authors: Krejci Petr , Ondrusek Cestmir
Published: August 2008

Magnetic couplings (Figure 1) are widely used to torque transmission between two shafts without any mechanical contact. They are especially well suited for used in hazardous environments, to transmit torque through a separation wall. An additional advantage of a magnetic coupling is that slipping occurs when excessive torque is applied, this can be used to prevent mechanical failure due to torque overloads. This paper deals with influence of temperature on behavior of magnetic coupling and magnetic coupling design optimization. The permanent magnets that are used for torque transmission cannot be used close to Currie point, which is a point of loss of magnetic characteristics. We intend to use the magnetic coupling for pump of radioactive liquid materials for transmutation devices, where the temperature is close to four hundred centigrade. Because of we suggest the design changes for elimination of temperature influence. This paper presents the finite element (FE) parametric model of magnetic coupling, experimental verification of FE model and optimization of the inner part of magnetic coupling in order to increase the maximal torque. The genetic algorithm method in connection with FEM model of magnetic coupling was used for the design optimization procedure.
Properties and Workability of New Creep Strength Enhanced Steels as Known Grades 23, 24, 911 and 92
Authors: Bendick Walter , Gabrel Jean , Vaillant Jean-Claude , Vandenberghe Bruno
Published: August 2008

The improvement of thermal efficiency by increasing the operating temperature and pressure of boilers has recently led to the development of new creep-resistant steels. For 9–12% Cr steels, grade T/P91, developed at the end of the seventies by CE (Combustion Engineering) and ORNL (Oak Ridge National Laboratories) [1] in the USA, marked the starting point for these developments. With its excellent mechanical characteristics at elevated temperatures and good workability, it was rapidly adopted worldwide for erecting new power generators. The tube design temperature is limited to around 610°C (1130°F) inside the combustion chamber. This limit depends on such factors as heat flow and corrosion. Steam lines in P91 are generally operated at temperatures below or slightly above 600°C (1112°F). More recent developments to produce new grades such as T/P911 and T/P92, have improved mechanical properties at high temperatures, in particular an increase in creep strength of 10 to 30% in 100,000 hrs at 600°C (1112°F). This makes it possible to reduce the wall thickness of the pipes and consequently improve their behavior to thermal fatigue. The new grades T/P23 and T/P24 are well suited for boiler components working at lower temperatures. While these grades were initially developed for manufacturing the water wall panels of Ultra Super Critical Boilers (USCB), they are also used for the superheaters and reheaters of conventional boilers and Heat Recovery Steam Generators (HRSG) in combined cycles. In addition to their excellent workability, they have the advantage to be used without post-weld heat treatment in case of thin wall tubes. Furthermore, owing to the good creep properties, they can be used to replace P22 and for some applications even P91, with the advantage of lower costs. This paper sets out the characteristics of these new grades and gives the main recommendations for processing them.
10-Year Experience With T23(2.25Cr-1.6W) and T122(12Cr-0.4Mo-2W) in a Power Boiler
Authors: Komai N. , Masuyama F. , Yamamoto S. , Igarashi M.
Published: August 2008

Tungsten strengthened ferritic steels, 2.25Cr-1.6W-V-Nb and 12Cr-0.4Mo-2W-V-Nb-Cu have been developed and approved by the ASME Boiler and Pressure Vessels Code Committee for use in Section I construction, designated as T23 and T122 respectively. A field exposure test installing both steel tubes in service along with comparative materials in the tertiary superheater and secondary reheater of a 156MW utility power boiler has been conducted since April 1993. The tubes were removed to confirm their material properties and corrosion/steam oxidation behaviors after one-year, 3-year, 6-year and 10-year periods of service. The tensile and creep rupture strengths of both steels showed no remarkable change during service. Examination of steam oxidation scale on the inner surface of the tubes indicated that the scale growth rate of T122 was extremely small following one-year service. The growth rate and morphology of steam oxidation scale is discussed, as compared with conventional materials.
Steam Oxidation of Creep-Strength Enhanced 9Cr Ferritic Steel: Void Formation in Oxide Scale and Its Relation to Scale Exfoliation
Authors: Watanabe Yutaka , Kikkawa Hiroaki
Published: August 2008

The purposes of this research are (1) evaluating the importance of voids existing in oxide scales to exfoliation of oxide scales formed in superheated steam, (2) discussing possible role of Cr-rich layer in void formation in the oxide scales. First, superheater tubes, which are made of 9Cr-1MoVNb ferritic steel and were experienced with scale exfoliation, were removed from a fossil power plant and oxidation interfaces of the boiler tubes were examined in terms of structure, void distribution, elemental profile of the oxide scales and exfoliation characteristics. Then, exfoliation behaviors of oxide scales were evaluated on actual superheater tubes and on the steam oxidation specimens experimentally oxidized in autoclaves by tensile tests in terms of exfoliation strength and of position of exfoliation and its relation with void distribution in the scales. The results indicated an important role of void formation and growth in exfoliation of scale exfoliation. Void formation mechanism was also discussed based on the observed characteristics of the oxide scales.
Application of a Shooting Method to Regenerative Chatter During Turning
Authors: Chandiramani N. K. , Pothala T.
Published: August 2008

A two-degree-of-freedom (2-DOF) model comprising nonlinear delay differential equations (DDEs) is analyzed for self-excited oscillations during orthogonal turning. The model includes multiple time delays, possibility of tool leaving cut, additional process damping (due to flank interference), ploughing force, and shear-angle/friction-angle variation. An algorithm, based on an existing shooting method for DDEs, is developed to simulate tool dynamics and seek periodic solutions. The multiple-regenerative and tool-leaving-cut effects are simulated via an equivalent 1-DOF system by introducing a time shift. While the limit cycle amplitude and minimum-period obtained via shooting and via direct numerical integration compare well, the latter method converges very slowly, thus establishing the efficiency of the former. Numerical studies involving the machining parameters are presented. Only period-1 motion was observed for the range of cutting parameters considered here. Features of a subcritical Hopf bifurcation appear in the amplitude versus width-of-cut plane. This implies the possibility of subcritical instability characterized by sudden onset of finite-amplitude chatter. Additional process damping causes a reduction in chatter amplitudes as well as the subcritical instability to occur at a larger width of cut. An increase in width of cut causes frequent tool-leaving-cut events and increased chatter amplitudes. The frequency of tool disengagement increases with cutting velocity, despite cutting force in the shank direction remaining constant over a certain velocity range. The chatter amplitude at first increases and then decreases when the cutting velocity or the uncut chip thickness is increased. The present plant model and dynamics could be useful for real time active control of tool chatter.
A Continuum Shape Sensitivity Method for Fracture Analysis of Isotropic Functionally Graded Materials
Authors: Rao B. N. , Rahman S.
Published: August 2008

This paper presents a new continuum shape sensitivity method for calculating mixed-mode stress-intensity factors for a stationary crack in two-dimensional, linear-elastic, isotropic FGMs with arbitrary geometry. The method involves the material derivative concept taken from continuum mechanics, the mutual potential energy release rate, and direct differentiation. Since the governing variational equation is differentiated prior to discretization, resulting sensitivity equations are independent of approximate numerical techniques, such as the finite element method, boundary element method, mesh-free method, or others. The discrete form of the mutual potential energy release rate is simple and easy to calculate, as it only requires multiplication of displacement vectors and stiffness sensitivity matrices. By judiciously selecting the velocity field, the method only requires displacement response in a subdomain close to the crack tip, thus making the method computationally efficient. Seven finite-element based numerical examples, which comprise mode-I and mixed-mode deformations and/or single or multiple interacting cracks, are presented to evaluate the accuracy of the fracture parameters calculated by the proposed method. Comparisons have been made between stress-intensity factors predicted by the proposed method and available reference solutions in the literature, generated either analytically or numerically using various other fracture integrals or analyses. Excellent agreement is obtained between the results of the proposed method and previously obtained solutions. Therefore, shape sensitivity analysis provides an attractive alternative to fracture analysis of cracks in homogeneous and non-homogeneous materials.
Application of an Implicit Redundant Genetic Algorithm for Structural Damage Identification of Flexible Structures
Authors: Liszkai T. R.
Published: August 2008

Detecting structural damage is critical in assessing current condition, calculating remaining life, and developing rehabilitation strategies for existing structures. Many structural damage identification methods (SDIM) use vibration data to localize and identify deterioration of structural members. Due to practical constraints, such as cost, number of input channels of the measuring device, or lack of access of parts of the structure, the actual number of sensors used to collect measurement data is much smaller then the number of possible sensor locations. Therefore, the inverse problem associated with structural damage identification is ill formulated and often difficult to solve explicitly. This research addresses the problem of structural damage detection using the linear vibration information contained in frequency response functions (FRF). A structural damage identification method (SDIM) is proposed, which minimizes the error between the analytically computed and measured vibration signatures of structures. The SDIM is formulated as an unconstrained optimization problem, which is solved using genetic algorithms (GA). The implicit redundant representation (IRR) of genes allows the formulation of unstructured optimization problems in which the number of unknown variables is indefinite. The IRR GA efficiently exploits the unstructured nature of structural damage detection by allowing the number of assumed damaged elements to change throughout the optimization. The accuracy and efficiency of SDIM is increased when the IRR GA is used instead of the simple fixed representation GA. The procedure is applied to flexible structures to show that the proposed SDIM is capable of identifying damages in structures often used in the nuclear industry. Noisy measurements are also considered in the simulations to investigate their effect on the proposed SDIM accuracy. Test case results using different measurement noise levels show that the IRR GA has superior performance over the standard fixed representation GA in correctly identifying both the location and extent of damages.
The Effect of Reduced Design Margin on the Fire Survivability of ASME Code Propane Tanks
Authors: Birk A. M.
Published: August 2008

The design margin on certain unfired pressure vessels has recently been reduced from 4.5 to 4.0 to 3.5. This has resulted in the manufacture of propane and LPG tanks with thinner walls. For example, some 500 gallon ASME code propane tanks have had the wall thickness reduced from 7.7 mm in 2001 to 7.1 mm in 2002 and now to 6.5 mm in 2004. This change significantly affects the fire survivability of these tanks. This paper presents both experimental and computational results that show the effect of this design change on tank fire survivability to fire impingement. The results show that for the same pressure relief valve setting, the thinner wall tanks are more likely to fail in a given fire situation. In severe fires, the thinner walled tanks will fail earlier. An earlier failure usually means the tank will fail with a higher fill level, because the pressure relief system has had less time to vent material from the tank. A higher liquid fill level at failure also means more energy is in the tank and this means the failure will be more violent. The worst failure scenario is known as a boiling liquid expanding vapour explosion (BLEVE) and this mode of failure is also more likely with the thinner walled tanks. The results of this work suggest that certain applications of pressure vessels such as propane transport and storage may require higher design margins than required by the ASME.
Using Finite Element Analysis Combined With Strain Gage Testing to Do Proof Test to Establish MAWP
Authors: Florizone Donald J.
Published: August 2008

An amine reboiler was constructed with very large openings in one semi-elliptical head. The openings extended beyond the “spherical” portion of the head into the knuckle region. The vessel was designed to 1998 ASME Section VIII Division 1 (VIII-1). Initially the manufacturer of the amine reboiler vessel chose the proof test after the calculations submitted to the approval agency were not accepted. Non-destructive strain gage proof testing per VIII-1 UG-101(n) was planned, but the minimum proof test pressure to achieve the desired MAWP exceeded the maximum firetube flange test pressure therefore an alternate method was chosen. Finite element analysis (FEA) was done in addition to the strain gage testing. The strain gage results at the maximum hydrotest pressure were used to verify the FEA calculations. The FEA calculated strains were higher than the measured strains. This indicated that the assumptions made in the computer model were conservative. By combining FEA with strain gauge testing, the design was proven to meet Code requirements.
A Thin Shell Theoretical Solution for Two Intersecting Cylindrical Shells Due to External Branch Pipe Moments
Authors: Xue M. D. , Li D. F. , Hwang K. C.
Published: August 2008

Two intersecting cylindrical shells subjected to internal pressure and external moment are of common occurrence in pressure vessel and piping industry. The highest stress intensity occurring in the vicinity of junction, which is a complex space curve when the diameter ratio d/D increases. As the new process of theoretical solution and design criteria research developed by the authors, the stress analysis based on the theory of thin shell is carried out for cylindrical shells with normally intersecting nozzles subjected to three kinds of external branch pipe moments. The thin shell theoretical solution for the main shell with cutout, on which a moment is applied, is obtained by superposing a particular solution on the homogeneous solution. The double trigonometric series solution of cylindrical shell subjected to arbitrary distributed normal and tangential forces based on Timoshenko equation is used for the particular solution and the Xue et al.’s solution, for the homogeneous solution based on the modified Morley equation instead of the Donnell shallow shell equation. The displacement function solution for the nozzle with a nonplanar end is obtained on the basis of the Goldenveizer equation instead of Timoshenko’s. The presented results are in good agreement with those obtained by experiments and by three-dimensional finite element method. The present analytical results are in good agreement with WRC Bulletin 297 when d/D is small. The theoretical solution can be applied to d/D ≤ 0.8, λ = d/DT ≤ 8 and d/D ≤ t/T ≤ 2 successfully.
An Investigation of a Heat Exchanger Tubesheet Crack Caused by a Process Upset
Authors: Seipp Trevor G. , George Shiju V. P.
Published: August 2008

A heat exchanger tubesheet experienced cracks in the ligaments between tube holes. This paper describes the analyses that were performed in order to determine the cause of the crack. The first analysis examined the residual stresses in the tubesheet caused by the tube-rolling. It showed that the residual stresses caused by a typical rolling operation would be negligible. The second analysis examined the steady-state operating condition. It indicated that a tensile stress field existed on the surface of the tubesheet — a potential crack propagation driving force. The third analysis examined a thermal transient caused by a process upset. This transient created high peak stress intensities. However, a fatigue analysis indicated that the stress intensities were inadequate to initiate a crack in the low-cycle fatigue regime. The results of these analyses pinpoint the locations of the cracks accurately, and indicated that while a crack propagation mechanism existed, a crack initiation mechanism did not. Therefore, it was concluded that the crack may have been caused in the manufacturing process.
Assessment of Local Decreases in Wall Thickness at the Connection Straight-Pipe to 0°–90°-Bends
Authors: Kauer Robert , Holzer Wieland
Published: August 2008

Welds are ground during manufacturing to free them from offset edges and notches and thus to obtain a more favorable stress curve. Apart from the above, welds are also ground to prepare them for periodic testing and inspection by improving the conditions for these in-service inspection measures. The latter is one reason for a local decrease in wall thickness in the weld area at the connection between straight pipe and bend, so that there may be local deviations from the original design-based minimum required wall thickness. In order to fulfill the task of evaluating the strength of such material-loss regions, this paper determines appropriate stress indices for typical wall-thickness deviations and various wall-thickness/diameter ratios. For the beginning and the end of the bends, the factors B* and C* for common pipe bend dimensions have been determined. In [1], the factors B* and C* for 90°-bends are compared to the stress indices B and C given in the literature for the crown of the bend. These factors B and C are commonly used in piping calculations, which are based on the transverse beam theory. In this paper the factors B* and C* are presented for both ends of the bends with bend angles in the range of 0° to 90° degree. These factors for the beginning and the end of 0°–90°-bends will also be compared with factors given in the literature. The correct combination of both factors — wall-thickness reduction and B* or C* — allows to decide, whether a detected deviation from the minimum value is permissible.
Thermal Stresses at Dissimilar Pipe-Stanchion Interfaces
Authors: Basavaraju Chakrapani
Published: August 2008

High temperature steam lines in power plant piping systems are often supported by the use of pipe support stanchions welded to the steam pipe. The end of the pipe stanchion has a steel plate welded to it, which typically slides on rack steel. The temperature of the stanchion drops from the process pipe interface along the length of the stanchion. The material for the process pipe carrying high temperature steam can be stainless steel, alloy steel, or carbon steel. The material for the stanchion can also be stainless steel, alloy steel, or carbon steel. It is of course cheaper to use carbon or low alloy steel for the stanchion as there is no steam flow in to the stanchion, when the process pipe is made of stainless steel, or other high alloy steel such A335 Gr. P91. In this paper, finite element thermal analysis is utilized first to obtain steady state temperature distribution due to decay or attenuation from the steam line surface along the stanchion. Conduction of heat from process pipe to stanchion, and convection from stanchion surface are considered. Then finite element structural analysis was performed to obtain steady state thermal stresses at the pipe-stanchion interface utilizing the temperature distribution obtained from thermal analysis as an input. The current industrial practice is to use similar materials for both process pipe and stanchion materials conservatively. Normally encountered pipe materials were considered. The materials studied include 304 & 316 Grade stainless steels, A335 Grades P91, P22, & P11 alloy steels, and A106 Grade B carbon steel. The temperature and stress results are presented. Guidelines are provided for the acceptability of pipe-stanchion dissimilar interfaces.
Evaluations of the Tightening Process of Bolted Joint With Elastic Angle Control Method
Authors: Fukuoka Toshimichi , Takaki Tomohiro
Published: August 2008

Various clamping methods are used to tighten bolted joints and the selection is made according to its configuration, bolt size, etc. Angle control method is commonly used among them and it is usually applied when tightening bolts to the plastic region. However, elastic angle control method is sometimes used to tighten important structures, e.g., for the case where the clamping bolts are to be disassembled in a periodical inspection. In this paper, the tightening process of elastic angle control method is studied and the expression relating axial bolt force and nut rotation angle is proposed, in which the effects of surface roughness of contact surfaces and the inclined angle existing around nut loaded surface are incorporated. The validity of the proposed equation is demonstrated by experiment. It is shown that the elastic angle control method is preferably applied to the case of a bolted joint with large grip length being tightened with high bolt stress.
Finite Element Modeling of Self-Loosening of Bolted Joints
Authors: Zhang Ming , Jiang Yanyao , Lee Chu-Hwa
Published: August 2008

A three-dimensional finite element (FE) model with the consideration of the helix angle of the threads was developed to simulate the second stage self-loosening of a bolted joint. The second stage self-loosening refers to the graduate reduction in clamping force due to the back-off of the nut. The simulations were conducted for two plates jointed by a bolt and a nut and the joint was subjected to transverse or shear loading. An M12×1.75 bolt was used. The application of the preload was simulated by using an orthogonal temperature expansion method. FE simulations were conducted for several loading conditions with different preloads and relative displacements between the two clamped plates. It was found that due to the application of the cyclic transverse load, micro-slip occurred between the contacting surfaces of the engaged threads of the bolt and the nut. In addition, a cyclic bending moment was introduced on the bolted joint. The cyclic bending moment resulted in an oscillation of the contact pressure on the contacting surfaces of the engaged threads. The micro-slip between the engaged threads and the variation of the contact pressure were identified to be the major mechanisms responsible for the self-loosening of a bolted joint. Simplified finite element models were developed that confirmed the mechanisms discovered. The major self-loosening behavior of a bolted joint can be properly reproduced with the FE model developed. The results obtained agree quantitatively with the experimental observations.