Ratcheting of Elbow Pipes Subjected to Internal Pressure and External Bending Loads

In petroleum and power plant industries, pipelines connect complex structural network to navigate source of fuel and gas energies. Over service operation, pipelines are subjected to inevitable damages at the positions where designs include geometry change such as an elbow resulting in high localized stresses/strains. Severe damage in pipes and pressurized vessels is induced when pipes are subjected to external vibration or cyclic loads such as seismic loads/ earthquake and internal pressure as well as of hot steam in pipes. Interaction of severe repeated such loads as well as materials deformation leads to progression of plastic strain over service loads referred as ratcheting.Lack of sufficient theoretical approaches and availability of experimental data particularly when applied repeated service mechanical or thermal loads are beyond of elastic domain has motivated the authors to further investigate ratcheting response of austenitic stainless steel alloy subjected to external bending under load-controlled conditions by means of the Ahmadzadeh-Varvani (A-V) hardening rule. The present study intends to predict ratcheting of elbow pipes through use of kinematic hardening rules. Stress-strain hysteresis loops generated by the kinematic hardening models were employed to control ratcheting progress over stress cycles. A finite element model was employed to evaluate stress components at various positions on the elbow midsection. Experimental ratcheting data measured by strain gauges at different elbow positions of crown, intrados, and extrados closely agreed with those of ratcheting strains predicted based on the A-V model.

Keywords:

Repeated loads , ratcheting , stress raisers , kinematic hardening rule , elbow pipe , FE analysis.