The entering behavior of environmental gases into the plastic zone around fatigue crack tips in titanium
- PDF / 555,642 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 21 Downloads / 169 Views
I.
INTRODUCTION
GASEOUS environments greatly affect fatigue behavior of metallic materials. It has been reported that fatigue life is markedly improved and the crack growth rate is decreased in vacuum as compared to laboratory air.[1,2,3] The effects of hydrogen gases on fatigue life and other mechanical properties have been studied extensively.[3,4] The fatigue crack growth rate increases and the fatigue strength decreases in hydrogen gases as compared to air. The plausible mechanism which has been proposed to date is as follows: the physisorption of hydrogen on the freshly created slip steps or fatigue crack surfaces. The dissociation of the hydrogen molecule into atomic hydrogen occurs. Hydrogen is transported by random walk diffusion through the lattice, by pipe diffusion along grain boundaries, or by mobile dislocations.[3] The hydrogen transported to regions ahead of the crack tip causes hydrogen embrittlement, and this results in an increase in fatigue crack growth rate and/or a reduction in fatigue strength. It is known that, in a gaseous medium which contains oxygen, a chemisorbed monolayer forms very rapidly on a fresh metal surface.[5] The chemisorption of oxygen or the formation of an oxide layer on the slip step, which is created during the tensile portion of a fatigue cycle, makes reverse slip difficult on the same slip plane upon load reversal.[3] It can be thought that a similar mechanism may occur in other gaseous media. Two reasons can be considered for the reduction in fatigue life and for the increase in fatigue crack growth rate in a gaseous medium as compared to M. SHIMOJO, Research Associate, T.H. MYEONG, Graduate School Student, and Y. HIGO, Professor, are with the Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 226, Japan. R. IGUCHI, formerly Graduate Student, Precision and Intelligence Laboratory, Tokyo Institute of Technology, is with Steel Sheet Section, Itochu Corporation, Osaka 541-77, Japan. Manuscript submitted August 14, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
vacuum. One is the reduction in slip displacement on the same plane during unloading due to the adsorption of the gas, and this prevents the fresh surface from rewelding. In this case, reverse slip could occur on some neighboring planes. The other is the embrittlement of the material caused by the absorbed gas which is drawn into the material due to a limited amount of reverse slip. However, a limited number of studies have been carried out on the effects of gaseous media except air and hydrogen on reverse slip behavior during the unloading portion of a fatigue cycle. In addition, it is unclear whether an adsorbed gas, especially a chemisorbed gas, completely prevents reverse slip from occurring. If not, it is also unknown how much and how far the adsorbed gas is transported into the material. In this study, the entering behavior of environmental gases into the plastic zone around the fatigue crack tip in pure titanium specimens and the effects of gases on fatigue crack growth rate ar
Data Loading...
