Application of the dc potential drop technique in investigating crack initiation and propagation under sustained load in
- PDF / 1,137,167 Bytes
- 9 Pages / 612 x 828 pts Page_size
- 79 Downloads / 142 Views
I.
INTRODUCTION
THEtechnical
relevance of high temperature fracture under sustained load conditions became apparent with the increase in the operating temperatures of electricity generating plants, petrochemical installations, and aircraft jet engines. These engineering developments focused attention on the need for improved materials having a satisfactory combination of high temperature properties and a good resistance to degradation in hostile service environments. The progressive deterioration of an alloy by a combination of creep damage, microstructural degradation, and environmentally induced damage I usually results in the initiation and propagation of macroscopic cracks. The relative contributions of each of the damaging mechanisms, which are strongly dependent on the material, the service parameters, and the environment, are not necessarily the same during the initiation and propagation stages. Thus, it is only natural, when trying to interpret and predict creep rupture life, to divide the total life into two stages: the time to crack initiation and the crack propagation life. The time to crack initiation is identified with the rupture time of a smooth test bar. Taira and Ohtani 2 were among the first to differentiate between time to crack initiation and time to rupture, as they calculated the creep deformation of axisymmetric notched bars in order to explain quantitatively notch strengthening and/or notch weakening. In more detailed experiments Hayhurst and Leckie 3'4'5 have investigated the evolution of creep damage in axisymmetrically notched bars. These experiments were guided and interpreted by detailed computations of stress redistributions,6 which incorporated not only the usual steady state creep relationships, but also the local accelerating creep rates due to increased cavitation in the tertiary creep range. The time to crack initiation and the damage associated with the 1. P. VASATIS is Staff Scientist, General Electric Corporate Research and Development, P.O. Box 8, Schenectady, NY 12301. R. M. PELLOUX is Professorof Materials Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139. This paper is based on a presentation made in the symposium "Crack Propagation under Creep and Creep-Fatigue" presented at the TMS/AIME fall meeting in Orlando, FL, in October 1986, under the auspices of the ASM Flow and Fracture Committee. METALLURGICALTRANSACTIONS A
formation of a macroscopic crack were shown to depend on the loading conditions, the geometry of the notched body, and the alloy properties. Thus it is recognized that an accurate estimation of creep rupture lives requires consideration of both crack initiation and propagation times. Investigations of sustained load crack propagation rates and micromechanisms in structural alloys have been extensive. The suitability of fracture mechanics parameters to correlate crack growth rates 7'8 has also been investigated. On the other hand, experimental data on sustained load crack initiation times are scarce. The time to crack initiation is usu
Data Loading...
