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- 2014 (10) (entfernen)
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- Analytical solution (1)
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- Pressure vessel (1)
- Steam generator tube inspection (1)
Eingeladener Vortrag
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Die Vorhersage der Beanspruchung bei plastischem Versagen von zylindrischen Druckbehältern wird häufig mittels Finite Elemente Berechnungen durchgeführt. Die Berechnung der zulässigen Beanspruchung erfordert ein elastisch-plastisches Werkstoffmodell und die Berücksichtigung von nichtlinearen geometrischen Effekten. Das plastische Versagen verursacht eine gesamte Strukturinstabilität und kann nicht direkt aus der Finite Elemente Analyse bestimmt werden. In dem vorliegenden Beitrag wird die Beanspruchung für plastisches Versagen eines zylindrischen Druckbehälters mittels eines analytischen Verfahrens basierend auf einem linear elastischen ideal-plastischen Materialmodell bestimmt. Wenn Plastizität auftritt, wird zum einen der Werkstoff als inkompressibel und zum anderen der Tensor der plastischen Dehnungen als parallel zum Spannungsabweichungstensor angenommen. In diesem Fall kann die Spannungs-Dehnungs-Beziehung von Henkel angewendet werden, um den Druck zu berechnen, bei dem plastisches Fließen und Versagen eintritt. Die analytischen Ergebnisse werden von den Finite Elemente Berechnungen vollkommen bestätigt.
Nondestructive Testing (NDT) and fracture mechanics are two different disciplines in applied engineering science. Both areas are very important for the evaluation of the component integrity. Unfortunately the technical understanding in both disciplines is not identically, what lead to misunderstandings. In the present paper, the basic ideas about fracture mechanics and the input parameters coming from the NDT side are explained, concerning the integrity evaluation of the components. The widely used NDT methods are magnetic particle, dye penetrant, electrical eddy currents, radiography and ultrasonics. But not all of these common NDT methods can deliver parameters satisfy the fracture mechanics requirements. The first two can detect only surface-breaking or immediately sub-surface defects. Eddy current can also find defects in deeper sub-surface layers, while radiography and ultrasonics can also find embedded, remote defects. These three methods have furthermore a high potential for defect sizing and are therefore predestined to provide fracture mechanics input parameters. At the example of steam generator tube inspection the interplay of eddy current and fracture mechanics as related to tube integrity will be explained.
Pressure Vessel Fire Test
(2014)
Limit load analysis is a well known method to calculate the allowable design pressure of container components. A limit load of a pressurized container is achieved, when the stress of a wall and the flow stress are equal. In the following paper the transferability of limit load analysis from small scale tank containers up to large scale containers (railway tank) are investigated. Finite element calculations are carried out and compared with experimental results. It can be concluded that the limit load analysis works very well. Furthermore, the yield strength of the material should be used as flow stress.
The prediction of the plastic collapse load of cylindrical pressure vessels is very often made by using expensive Finite Element Computations. The calculation of the collapse load requires an elastic-plastic material model and the consideration of non-linear geometry effects. The plastic collapse load causes overalls structural instability and cannot be determined directly from a finite element analysis. The ASME (2007) code recommends that the collapse load should be the load for which the numerical solution does not converge. This load can be only determined approximately if a expensive nonlinear analysis consisting of a very large number of sub steps is done. The last load sub step leading to a convergent solution will be taken as the critical load for the structure. In the instability regime no standard finite element solution can be found because of the lack of convergence of the numerical procedure. Other methods for the calculation of the allowable pressure proposed by the ASME code are the elastic stress analysis and the limit load analysis. In the present paper the plastic collapse load for a cylindrical pressure vessel is determined by an analytical method based on a linear elastic perfectly plastic material model. When plasticity occurs the material is considered as incompressible and the tensor of plastic strains is parallel to the stress deviator tensor. In that case the finite stress-strain relationships of Henkel can be used for calculating the pressure for which plastic flow occurs at the inside of the vessel wall or in the case of full plasticity in the wall. The analytical results are fully confirmed by finite element predictions both for axisymmetric and high costs three dimensional models. The analytical model can be used for fast predictions of the allowable load for the design of a large variety of pressure vessels under safety considerations. The accuracy of the predicted collapse load largely depends on the quality of the temperature dependent wall material data used both in the analytical and numerical calculations.