The article presents the results of computer simulation of the conditions for the formation of a local collapse of the pipeline wall, depending on the ratio of internal forces, which are implemented in the cross section of the pipeline under various operating conditions. The values of the maximum bending moment of the pipeline with the loss of local stability of the wall depending on the internal pressure and axial force associated for the underground pipeline with the internal pressure and temperature differential are obtained. The pipeline bending was simulated using the LS-DYNA software for engineering calculations in a three-dimensional shell formulation using finite elements of the first order of accuracy. The plastic properties of the material of the pipe wall were set by a power-stress-strain diagram. The axial force was set in a wide range of values, overlapping tension and compression. Moment loading was set by turning the pipe ends at a constant speed for fixed values of the internal pressure and axial force. The limiting bending moment was determined at the maximum of the moment dependence on the angle of rotation of the ends. According to the results of a series of calculations, the dependences of the limiting bending moment on the axial force for two values of internal pressure 0 and 3.8 MPa are obtained. Fr om the dependencies obtained, it was concluded that the internal pressure with tensile axial loads increases the resistance of the pipe wall to local collapse, while with compressive pressure it decreases. The resulting effect of internal pressure with compressive axial forces differs fr om the results of P. Schaumann, Ch. Keindorf, H. Brьggemann (2005), wh ere a conclusion was made about the supporting effect of internal pressure on the local collapse of the wall with both tensile and compressive axial forces.
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