Creep-fatigue behaviour and life prediction

This thesis describes an investigation into the creep-fatigue behaviour and life prediction for high temperature materials. The methodology adapted in this research was not experimental, but, analytical using data compiled from several sources. High temperature low cycle fatigue (HTLCF) data generated internationally on 0.5Cr-Mo-V, lCr-Mo-V, 1.25Cr-Mo, 2.25CrlMo, 2.25Cr-lMo-V and 9Cr-lMo low alloy steels were compiled and analysed to identify trends in creep-fatigue behaviour and life prediction for those steels. Effects of alloying elements such as chromium and vanadium were investigated and it was shown that with increase in chromium content the life improved, but with vanadium addition to a 2.25Cr-Mo steel the life was lowered. For the annealed condition, in which the material tensile properties were nearly half the value for the normalized and tempered condition, the 2.25Cr-lMo steel had higher life. Phenomenological methods of life prediction such as the damage summation approach (DSA), the frequency modified approach (FMA), the strain range partitioning (SRP), the damage rate approach (DRA), the hysteresis energy approach (HEA), the damage parameter approach (DPA) and the assessment procedure R-5 are all in the developmental stage when examined with the data bank compiled no one method was found to be better than other. The phenomenological methods require a number of material and test parameters determined from complex tests, as a result, alternate methods in the creep-fatigue life prediction are explored. A statistical method, known as Diercks equation has been proposed in the literature as a better method that was modified and its applicability was extended and assessed with the creepfatigue data for low alloy steels compiled in this investigation. The reliability of modified Diercks equation was found to be higher than other methods. Microstructural damage produced under HTLCF was documented optically for a titanium alloy IMI 829 and a nickel based superalloy MAR M 002 under different test conditions. The alloy IMI 829 contained interfacial cracks, cavitation and oxide banding resulting into intrusions and multiple cracking at 600°C. However, wedge type of cracking and oxidation damage by depletion of y' phase were observed for MAR M 002. The HTLCF damage documented is described by a five stage model developed in this investigation and an empirical oxidation life prediction method is developed for MAR M 002. A reasonable prediction was observed at all the temperatures only under unaged condition, however, data were over-predicted under ageing heat treatment which produced material microstructure amenable to cracking. Further work is needed to apply this method in the creep-fatigue life prediction of high temperature materials. LIST OF PUBLICATIONS The following chapters from this thesis became individual papers in International Journals as follows: Chapters 4 and 5 Goswami, T. (1995) Creep Fatigue : Paper I Compilation of data and trends in the behavior of low alloy steels, High Temperature Materials and Processes, Vol. 14, No. 1, pp 1-20. Chapter 6 Goswami, T. (1995) Creep Fatigue : Paper II Life prediction methods and trends, High Temperature Materials and Processes, Vol. 14, No. 1, pp. 21-33. Chapter 7 Goswami, T. (1995) Damage development under creep-fatigue in a titanium and a superalloy, High Temperature Materials and Processes, Vol. 14, No. 2, pp. 47-55. Chapter 8 Goswami, T. (1995) Creep-Fatigue : Paper III Diercks equation : modification and applicability, High Temperature Materials and Processes, Vol. 14, No. 1, pp. 35-45. Following publication of these papers, National Research Institute for Metals, Tokyo, Japan, provided creep-fatigue data for further analysis which are published as follows: Paper 1 Goswami, T. (1995) Applicability of modified Diercks equation with NRIM data, High Temperature Materials and Processes, Vol. 14, No. 2, pp. 81-90. Paper 2 Goswami, T. (1996) Prediction of low cycle fatigue lives of low alloy steels, Iron and Steel Institute of Japan, ISIJ International, Vol. 36, No. 3, pp. 354-360. Paper 3 Goswami, T. and Plumbridge, W. J. (1996) Applicability of new creep-fatigue life prediction models with low alloy steels, Paper No. C494/095/96.1. Mech. E. London, pp. 175-192.