Digital Repository, ECF14, Cracow 2002

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A Creep Fatigue Crack Propagation Study of the PM Nickel Based Superalloy UDIMET 720 Li

Last modified: 2013-02-10


In this paper, the fatigue and creep crack growth behaviour was studied attemperatures of 650 and 725oC in compact tension specimens, and the results werecompared. Fatigue tests were carried out with a constant stress ratio (R=0.1) at twodifferent frequencies (5 and 0.25Hz) in order to study the effect of frequency in crackpropagation. The results obtained in these tests were converted, based on time, andcompared with the creep results. For fatigue, da/dN vs. ĢK curves were used, and forcreep, da/dt vs. Kmax curves, since, for this material, the stress intensity factor has shown tobe the most appropriate parameter for the study of crack propagation. The FCGR resultshave shown that the effect of frequency is predominant over the temperature influence, thatis, da/dN has a more significant increase when the frequency is lowered from 5 to 0.25Hzthan when the temperature is increased from 650 to 725oC. An analysis of the crackingmechanisms will be presented in the paper, specially for creep, where mixed load typecrack propagation was found (I+II) for loading under mode I only. The main failuremechanism detected in the material was mixed mode transgranular . intergranular.However, in creep intergranular and secondary cracking was predominant when the crackpropagated at inclined angles (I+II) with the crack propagation direction of mode I. Thecrack growth rates, da/dN and da/dt, where mainly related with the stress intensity factorcomponent in mode I.

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