PSI - Issue 26

Vladimír Chmelko et al. / Procedia Structural Integrity 26 (2020) 417–421 Chmelko et al. / Structural Integrity Procedia 00 (2020) 000 – 000

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Fig. 1. Local defects of pipe: on the left – unfused root of the weld; on the right – off-set of the pipeline foreheads due to reason of geometric variance from ideal circle shape

Nomenclature  con

E E

conventional stress

Young modulus

´

true stress

modulus of hardening

 true

R m,con conventional ultimate stress R m,true true ultimate stress

strain



R e

Yield stress

2. The simulation of pipeline with defect

In contrast to projected state, the real pipeline does not have the ideal geometry of the coat and in some cases even the weld geometry. As a result, stress values in some locations may exceed the yield stress. In order to assess the safety of operation of such pipeline sections, it is necessary to know the burst pressure and the reserve of operating pressure against its value. Pressure pipeline simulations in the FEM environment are a suitable tool for this assessment. Computational simulations of stress-strain state of pipelines with defects resp. geometric deviations must be verified. In numerical simulations to determine the burst pressure value, among other parameters, it is necessary to select the appropriate material model and set the correct destruction criterion. When choosing a material model, it is necessary to start from the material stress-strain curve. It is best to derive a material model based on directly measured stress-strain diagram. The confrontation of the numerical models with the results of the destruction tests shows that it is not appropriate to directly use the multilinear model entered using the table of experimentally obtained points  -  . The destruction of the pipeline under internal pressure occurs by the same mechanism as the fracture in the tensile test. The destruction starts at the point of the coat, where the plastic deformation by gradual flow reaches a value of the strength limit R m . This value corresponds to the conventional value of the tensile test. Furthermore, the local development of plastic collapse continues by bulging the shell until its integrity is compromised (Fig. 2). The tear of the coat corresponds to the point marked as “fracture” on the tensile curve (Fig. 3). The actual value of the stress is then fundamentally different from the stress related to the original cross-section in the tensile diagram. It is experimentally difficult to obtain the dependence of the actual (instantaneous) stress on the deformation during the tensile test. It is also difficult to use a material model in the form of a table of measured values for numerical simulation purposes. Such a model leads to a time-consuming calculation. From an engineering point of view, a good bilinear model is a good choice. There are many ways to create such a model. With respect to the objective of calculating the burst pressure using numerical simulation, it is sufficient to know two points of the conventional material curve, i.e. the yield stress Re and the strength limit Rm. 2.1. Material model