The influence of gaseous environments on fatigue crack growth in a nickel-copper alloy
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EXPERIMENTAL
PROCEDURE
The material used for this investigation was a commercially-prepared plate of composition: 64.87 pct Ni, 32.86 pct Cu, 0.195 pct Fe, 0.97pctMn, 0.18 pct C, 0.135 pct Si, 0.013 pct AI, and 0.005 pct S (Monel 400*). The plate was ground from an original *Registeredtrademark,The InternationalNickelCompany,Inc. thickness of 0.375 in. to a final thickness of 0.250 in. J. D. FRANDSEN is Staff Associate, N. E. PATON and H. L. MARCUS are Group Leaders at Science Center, Rockwell International, Thousand Oaks, Cal. 91360. Manuscript submitted August 10, 1973. METALLURGICAL TRANSACTIONS
The alloy is non-heat-treatable and all testing was done using material in a fully annealed condition. The plate stock was annealed at 1800~ for 15 rain. and air-cooled prior to specimen machining. The microstructure was essentially equiaxed grains but did contain some inclusion stringers. All test samples were oriented such that crack propagation was perpendicular to the rolling direction (LT) to minimize the effect of the stringers on the fracture process. The material had yield and ultimate strengths of 283 MN/ma (41.0 ksi), and 555 MN/m~ (80.5 ksi), respectively, at ambient conditions. The specimen geometry used for all testing was a tapered double cantilever beam (DCB) specimen of the type initially proposed by Mostovoy.6 The advantage of the DCB geometry is that crack tip stress intensity is independent of crack length over a considerable portion of the sample as a result of the linear relationship between compliance and crack length. 7 This allows changes to be made in environmental conditions while maintaining constant cyclic stress intensity. The specimens contained side grooves to keep the crack propagation direction perpendicular to the loading axis--a very prominent problem in environmental testing. Maximum cyclic load, and, therefore, maximum stress intensity was limited to avoid general yielding of the material ahead of the crack tip. The specimens were tested in an all-metal bakeable vacuum system capable of pressure less than 1.3/~Pa ( l x I0 -s torr). The sample was in contact with copper plates that could be heated and cooled by flowing gas or liquid through metal bellows from outside the chamber. Test specimen temperature was monitored by a thermocouple welded to the sample. Hydrogen, nitrogen, and oxygen gases were introduced into the chamber from "Airco" research-grade "Pyrex" bottles through a bakeable train. The vacuum system was baked at 250~ for 8 h subsequent to opening any portion to the atmosphere. Prior to introduction of either hydrogen, nitrogen, or oxygen, the chamber was evacuated to less than 6.5 /iPa (5• 10-8 torr) to reduce the presence of background VOLUME 5, JULY 1974-1655
g a s e s . The i n i t i a l t o t a l i m p u r i t y l e v e l s f o r the h y d r o gen, n i t r o g e n , and oxygen a s s u p p l i e d w e r e l e s s than 5 p p m , 10 p p m , and 50 ppm, r e s p e c t i v e l y . F u r t h e r d e t a i l s of the v a c u u m s y s t e m a r e r e p o r t e d e l s e
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