Limit Load Method Research Articles

Profile tolerances influence on cryostat base section

The ITER cryostat—the largest stainless steel vacuum pressure chamber ever built which provides the vacuum environment for components operating in the range from 4.5 K to 80 K like ITER vacuum vessel and the superconducting magnets. The cryostat is divided into four sections, of which, base section is most complex because of its web shaped structure sandwiched between two 60 mm thick plates with stringent requirements in manufacturing tolerances. The required profile tolerance is 30(+10/−20) mm at weld locations and 20(+/−10) mm at other locations but during manufacturing, the tolerances are observed to be 60 mm (+10/−50) mm at weld location and 50 mm (+10/−40) at other locations. These increased profile tolerances are expected to affect the structural behavior of Cryostat. The present paper discusses the assessment of these tolerances on cryostat base section using FEM software, ANSYS. The increased profile tolerances on base section are applied using two different methods. Maximum tolerance value was considered and five cases were identified to assess the complete effect of increased profile tolerances on base section. Limit load method is used to analyze structural impact of these increased tolerances cases on cryostat base section. The impacts of critical results were discussed in the end.

Application of elastic-plastic analysis method in high pressure heater tubesheet design

The design method of pressure vessel consists of the elastic analysis method and the elastic-plastic analysis method. The design method based on elastic-plastic theory with more complex computation process contributes to lightweight design, and avoids uncertainty of stress classification in the complex structures. Tubesheet is the main part of high pressure heater, which is very thick based on Chinese code GB151 for the design of heat exchangers. Increased tubesheet with large thermal stress are not conducive to manufacture, heat transmission and detection. The application of limit load method and elastic-plastic method in the analysis and design of high pressure heater tube is to provide direction for the lightweight research of tubesheet. It is shown that high stress appears on the inner wall of jointing of tubesheet with the head due to the original tubesheet owning enough rigidity related to thickness, which should be noted in the design. The limit load method and elastoplastic method could be assessed in the plastic collapse assessment after optimizing the thickness of the tube plate by 5% on the basis of the existing rule design. Compared with the limit load method, the elastic-plastic method could obtain higher limit load due to its higher computational precision. On the other hand, the design method of elastic-plastic analysis method is not required to consider the determination of limit load per ASME VIII-2 judged by its convergence. The elastic-plastic analysis method is worth further study, which is helpful to the revision and improvement of relevant standards in China.

ステンレス鋳鋼配管弾塑性破壊力学評価法のJSME維持規格とASME Section XIの比較

A revision of flaw evaluation of cast stainless steel pipe of JSME rules on fitness for service is planned. The points are to apply the limit load method to unaged or low SC pipe, introduce prediction models for S-S and J-R curve due to thermal aging embrittlement and revised Z-factor equations proposed by other authors. To confirm the applicability of the degradation model and the current and proposed Z-factors of JSME rules, EPFM analysis was performed and Z-factor was directly deduced from the analysis. Input conditions were provided at the ASME Section XI meeting. Directly deduced Z-factors were compared with those of JSME rules and ASME Section XI. It is revealed that the draft prediction models of JSME has good accuracy, Z-factors of JSME have a very large conservativeness, and those of ASME Section XI have proper values.

Mechanical model for controlling floor heave in deep roadways with U-shaped steel closed support

Open U-shaped steel arch supports are commonly used in large-section static-pressure roadways in coal mines that are more than 900m deep; however, it is very difficult to control floor heave of roadways. In this paper, a U-shaped steel closed support with an inverted U-shaped steel arch in the floor is proposed as a method for improving the support effect of the surrounding rock during the process of floor heaving. This research established a mechanical model for the U-shaped steel closed support, and determined the reaction forces at the connection of a camber angle. Using the limit load method calculated the critical buckling load of the inverted U-shaped steel arch, and use of a strength check method tested the strength of the U-shaped steel material. A numerical simulation was conducted using the finite difference software FLAC3D. The simulation results show that the U-shaped steel closed support is able to control the floor heave of roadways, which is successfully used in the West 11-2 development roadway of the Zhuji Mine in the Huainan mining area in China. The cumulative floor heave over two years was less than 50mm.

대형배관의 Curved CT 시편을 이용한 파괴저항특성평가에 관한 연구

The LBB (Leak Before Break) concept is based on evaluating the fracture toughness. NUREG 1061, Vol.3 announced that the specimen for evaluating fracture resistance needs to have same thickness or thicker than pipe. Therefore, it is difficult to collect specimen from pipe which has same thickness as a pipe. So, ASTM standard suggested the use of standard specimen with 1 inch thickness. However, it has been known that an application of LBB by test results of standard specimen is conservative compare with that by real pipe. In this study, to supplement such conservatism, the evaluation of fracture resistance characteristics was performed with curved CT specimen, which has same thickness and curvature as a pipe. In addition, fracture resistance characteristics of curved CT specimen were compared with those of CT specimen. For this, shape factor F, hpl and g were obtained from FEM analysis using the limit load method.

Comparison of Elastic and Plastic Reference Volume Approaches

For finding out reliable limit load multipliers in pressure vessel components or structures using simplified limit load methods, proper estimation of reference volume is important. In this paper, two empirical methods namely elastic reference volume method (ERVM) and plastic reference volume method (PRVM) for reference volume correction are presented and compared. These reference volume correction concepts are used in combination with mα-tangent method and elastic modulus adjustment procedure to achieve converged limit load multiplier solution. These multipliers are compared with nonlinear finite element analysis results and are found to be lower bounded. Elastic reference volume method is the simplest method for reference volume correction when compared to plastic reference volume method.

A Novel Comparison of Design-by-Analysis Methods

There exist some atypical loads on pressure vessels during transportation. This is particularly true when the pressure vessel weighs over 500 tonnes. In this example vessel, the transportation was via rail on a “Schnabel car,” in which the vessel is suspended horizontally between the top nozzle and the skirt, and a significant axial compressive load is applied. During the evaluation of the stresses in the top head, a particularly novel situation was encountered which brought about some interesting issues with regards to the three design-by-analysis methods: elastic, limit load, and elastic-plastic. This paper discusses the comparison between all three of these design-by-analysis methods, and provides recommendations for which is most appropriate for this type of evaluation. Additional recommendations and warnings are provided for the use of the elastic and limit load methods as well.

Development of an Integrity Evaluation System for Steam Generator Tubes in a Nuclear Power Plant

Steam generators working in nuclear power plants convert water into steam from heat produced in the reactor core and each of them contains from 3,000 to 16,000 tubes. Since these tubes constitute one of primary barriers under radioactive and high pressure condition, the integrity should be maintained carefully during the operation. The objective of this research is to introduce an integrity evaluation system for steam generator tubes as a substitute of well-trained engineers or experts. For this purpose, a couplet examination has been carried out on the complicated evaluation procedure and an efficient system named as STiES was developed employing three representative integrity evaluation methods: fracture mechanics analysis (crack driving force diagram and J-integral/Tearing modulus method) and limit load method. Exemplary analyses for steam generator tubes with various types of flaws showed good applicability of the proposed integrity evaluation system. So, it is anticipated that the system can be used for the calculation of reference pressure to decide either the continued operation or repair until next outage.

Nickel-boron coated tungsten and vanadium carbide powders for liquid phase sintering: M.Vélez et al. (Simon Bolivar University, Caracas, Venezeula.) Int. J. Refractory Metals Hard Mater., vol 17, no 1–3, 1999, 99–102

The ITER cryostat—the largest stainless steel vacuum pressure chamber ever built which provides the vacuum environment for components operating in the range from 4.5 K to 80 K like ITER vacuum vessel and the superconducting magnets. The cryostat is divided into four sections, of which, base section is most complex because of its web shaped structure sandwiched between two 60 mm thick plates with stringent requirements in manufacturing tolerances. The required profile tolerance is 30(+10/−20) mm at weld locations and 20(+/−10) mm at other locations but during manufacturing, the tolerances are observed to be 60 mm (+10/−50) mm at weld location and 50 mm (+10/−40) at other locations. These increased profile tolerances are expected to affect the structural behavior of Cryostat. The present paper discusses the assessment of these tolerances on cryostat base section using FEM software, ANSYS. The increased profile tolerances on base section are applied using two different methods. Maximum tolerance value was considered and five cases were identified to assess the complete effect of increased profile tolerances on base section. Limit load method is used to analyze structural impact of these increased tolerances cases on cryostat base section. The impacts of critical results were discussed in the end.

Replacement of the feedwater piping system in the reactor building outside the containment at Philippsburg I nuclear power station

At Philippsburg/Nuclear Power Station the replacement of the feedwater lines (WB35) from their point of entry into the reactor building up to the first external isolation valve of each line brought the quality level of the whole of the feedwater pipe system in the reactor building with basic safety requirements. The design of the new piping was based on a state-of-the-art concept in the mechanical and fluid-dynamics behavior. The extensive analyses focused on the optimization of the closing behavior of the third damped check valves and the integrity analysis of the RPV nozzle. The integrity analysis was performed on the basis of a postulated reliably detectable defect, using the plastic limit load method with the specified yield strength as yield stress. The planning period for preparation of the licensing and approval documents lasted approximately 30 months, following a preliminary concept phase. Replacement was performed in 43 days.

Critical circumferential through-wall cracks according to the unloading of the cracked section under displacement-controlled bending load

The evaluation of critical circumferential through-wall crack lengths in piping is usually performed by the flow stress concept, plastic limit load method or GE-EPRI procedures. Most of these methods treat the secondary stresses, especially those caused by bending moments resulting from restrained thermal expansion, as force-controlled loads. In reality, there is a movement of the piping into the direction of the prescribed displacement and, therefore, a relaxation of the cracked section, which is due to the local rotation of the cracked section. Instead of the bending moment originating from the elastic analysis of the piping system there will be a reduced bending moment, the load decreases, the real critical through-wall crack lengths due to this displacement-controlled loading are larger than those predicted by the load controlled methods. A corresponding analytical procedure taking into account this relaxation was developed and validated by a comparison with experiments as well as finite element calculations. The procedure can be used for the evaluation of the safety of piping systems (e.g. leak-before-break analyses), if the usual methods based on force-controlled loads give unrealistic, conservative results.

Ductility minimum for application of plastic limit load concept to failure analysis of structures with imperfections

When using the plastic limit load concept for structure failure analysis, the stress analyst must confirm that the material of the structure possesses adequate ductility to deform in the plastic domain without instability, even in the presence of known or assumed manufacturing defects. The work reported in this paper has determined that, for use with a lower-bound plastic limit load method, this minimum adequate ductility is 45 J, expressed in terms of Charpy-V-notch energy ( C v-energy ) as measured by standard material tests. The lower-bound method uses plane stress formulae and the yield strength in simple tension σ y instead of the flow stress σ∗ (i.e. no accounting for strain hardening) to yield results on the safe side. This determination is based on an evaluation of more than 400 burst and fracture mechanics tests on components (shell type structures) and specimens (various tensile and bend specimens) made of ferritic and austenitic steels of nuclear grades as well as conventional structural steels, and including some intentionally degraded melts. Another proposed criterion, namely that the C v-energy to yield strength ratio should be equal to or greater than 0·12 J MPa −1 for the application of the plastic limit load concept, was found to result in an unnecessarily stringent limit for materials above 375 MPa yield strength. In addition, the reduction in area measured in tensile tests has been investigated as an alternative index of toughness. This property, however, was found to be unsuitable for defining a ductility limit for use with the plastic limit load concept. Therefore, the aforementioned easy-to-use engineering rule, which requires only geometrical data and two material properties, C v-energy and the yield strength in simple tension, is recommended for failure analysis of cracked structures.