G.R Rashed - Ph.D. candidate, Department of Mechanical Engineering, K.N.Toosi University of Technology, Tehran, Iran
R Ghajar - Assistant professor, Department of Mechanical Engineering, K.N.Toosi University of Technology, Tehran,Iran.
S.J Hashemi - Associate professor, Faculty of Petroleum Engineering, Petroleum University of Technology, Ahwaz, Iran

This paper deals with the study of a damage mechanics model for the fatigue life assessment of the drillpipe. The applicability of this model for multiaxial low cycle fatigue is discussed. The first part of this paper summarizes the basic equation of this model, emphasizing on the coupling between plasticity and damage. The identification of the model parameters is discussed in the second part of this paper with the application to drillpipe API S-135. In the last part, the numerical procedure used to implement the model is summarized and the simulation predictions are compared with the experimental results.

Drillpipe, Elasto-plastic damage, Failure, Crack initiation