Fatigue behaviour of stainless steels under simultaneous corrosion attack: II. Corrosion fatigue behaviour

Fatigue behaviour of stainless steels under simultaneous corrosion attack: II. Corrosion fatigue behaviour

recognized that corrosion damage in the form of pits and the absence of humidity in the ambient environment have a deleterious effect on the fatigue p...

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recognized that corrosion damage in the form of pits and the absence of humidity in the ambient environment have a deleterious effect on the fatigue properties of AI alloys. However, the mechanisms involved in this damage are poorly understood. The effects of localized corrosion and ambient environment on corrosion fatigue processes in 7005-T53 AI alloy were characterized by investigating crack initiation from naturally occurring pits and electrode discharge machine (EDM) notches and the subsequent crack growth of these cracks in environments containing humid (ambient laboratory) and dry (< 5 ppm water vapour) air. Corrosion pits and EDM notches were found to exert an equal effect on fatigue life of this alloy, but the mechanisms of crack initiation for the two types of stress-raisers differ. Crack initiation from pits is controlled by the presence of small tunnels in the pits, while crack initiation from EOM notches is governed by the presence of transformed regions formed during the EDM process. Crack initiation lives and growth of small cracks are not affected by water vapour in the environment, but environment has a pronounced effect on crack growth rates and macroscopic fracture mode. Relatively low overall crack growth rates in dry air result in fatigue lives in dry air that are typically an order of magnitude higher than those in humid air. Crack initiation lives in humid air can account for a relatively large fraction of fatigue lives, while in dry air crack growth processes dominate fatigue life. The effect of humidity on fatigue of this alloy is to inhibit slip processes after cracks grow to some length. In uniaxial fatigue in dry air, predominant mode II crack growth predominates, while in humid air, mode I crack growth is dominant. In uniaxial fatigue in dry air, a critical stress intensity factor ( A K - 6 MPa m 1/2) is associated with the transition from macroscopic mode I to macroscopic mode II crack growth.

Fretting d a m a g e o f h i g h carbon chromium bearing steel. (Dissertation),

Kuno, M. Dies. Abstr. Int. 50, (4), 240 pp, Oct. 1989 The fretting wear properties of high carbon-Cr bearing steel, the effect of debris during fretting wear, an introduction of a new fretting wear test apparatus, and the effects of fretting damage parameters on rolling bearings are discussed. The testa were operated under unlubricated conditions. Using a crossed cylinder contact arrangement, the tests were carried out with the normal load of 3 N, slip amplitude of 50 ~,m, and frequency of 30 Hz at room temperature. In the fretting wear tests after tempering at 200, 230, 260 and 350°C in air, the high C-Cr bearing steel showed low coefficients of friction due to a glaze type oxide film. In the fretting wear tests at 200 °C, a very low coefficient of friction was obtained. Consequently, the oxide films on high C-Cr bearing steel tempered at 200, 230, 260 and 350°C were thought to be protective in fretting damage. Fretting wear volumes were measured using different specimen combinations and fretting oscillatory directions relative to the axes of the cylindrical specimens, although the same material couples. In the study of fracture induced by fretting wear, a critical slip amplitude which led to the shortest fracture life was identified. With the critical slip amplitude (35 i~m), a higher coefficient of friction was obtained, and this result suggested a significant effect of coefficient of friction on fracture induced by fretting wear (or fretting fatigue). The mechanisms of fretting wear and fretting fatigue are also discussed.

Effect o f f u i d flow on corrosion f a t i g u e initiation o f e l u m i n i u m alloy

2017-T4. Huang, Y.H. and Alkire, R.C. O x i d a t i o n and its influence on t h e mechanical b e h a v i o u r o f Ni3AI. (Dissertation). Dowling Jr, W.E. Diss. Abstr. Int. 50, (6), 250 pp, Dec. 1989 Oxidation of Ni3AI at 1000 =C and two potential stress-oxidation interactions were examined. Nickel-rich Ni3AI containing 0.5 st.% Hf was isothermally oxidized in air or 500ppm 02 in Ar. The kinetics of oxidation approached a parabolic time dependence after 30-40 h. The parabolic growth constants had a positive dependence on FOz, thus indicating p-type semiconducting behaviour for the (~-AI203 film. Prior to parabolic kinetics the primary oxide was a transition AI203 in the form of whiskers which transformed to (~-AI203 with concomitant surface smoothing. Oxidation also produced an AI depleted zone of -/-Ni containing 12-14 at.% AI. The depleted zone depth was calculated based on weight gain data and compared to measured values. One of the potential stress-oxidation interactions examined was the flow stress reduction of Ni3 AI at elevated temperatures from the introduction of mobile edge dislocations. Thin oxide films or sample prestrains have been found to significantly reduce the flow stress of BCC metals and NiAI. The influence of both thermal oxide films and -196 °C prestrains on 550-700 °C flow behaviour was examined. Low temperature prestrains produced an initial flow stress reduction of up to 50% in (123) oriented Ni3AI single crystals tested in compression at 550 °C. TEM dislocation analysis showed that the prestrain introduced a random array of dislocations which was subsequently exhausted into the screw orientations with further deformation at 550 °C. The initial 550 °C flow stress was controlled by the edge component of the existing dislocation structure. However, the surface oxide films examined had no effect on the deformation bahaviour of Ni3AI at 550 °C. Another stress-oxidation interaction evaluated was the influence of oxygen on the low cycle fatigue behaviour of polycrystalline Ni3AI at 600 °C. Recently, O was found to cause severe embrittlement of polycrystalline Ni3AI during tensile experiments at elevated temperatures. Ni3AI alloyed with Hf and boron was tested in plastic-strain-controlled low cycle fatigue at 600 °C in either air or vacuum (10 -3 Pa). The accumulated plastic strain to failure was reduced by nearly two orders of magnitude by the exposure to air, with a concomitant change in failure mode from transgranular in vacuum to intergranular in vacuum. An increase in strain rate from 0.001-0.01 s -1 doubled the fatigue life in air and a thin oxide coating (approx. 350 nm) quadrupled fatigue life in air. The cyclic deformation produced compressive peak stresses that were 10% greater than the tensile peak stress for the duration of the experiment. Evidence of strain Iocalizations similar to persistent slip bands was found on the sample surface after cyclic saturation. However, the dislocation substructre appeared homogeneous.

Corrosion and c o m p a t i b i l i t y . Berry, W.E.

CAB Curt. Aware. Bull. (195), 5 - 6 , Nov. 1989 A review of corrosion and compatibility covers: stainless steels for seawater service; AI corrosion in glycol/water; dehumidification during blasting and coating; corrosion fatigue under cathodic potentials of 1Cr-5Ni-lMo steel specimens; prediction of tube-tubseheat galvanic corrosion of Monel alloy 400 tubeshset, 90/10 Cu-Ni tube, and Ni-AI-bronze tubsheat; and a guide to using newer ferritic stainless steels such as alloys 409, 439, 444, E-Brita, Sea Cure, AL 29-4C, 29-4, and 29-4-2. 12 ref.

Effect o f t h e o x i d i z e d f i l m on t h e mechanical

aluminium alloy f o r g i n g . Bei, S.

properties

J. Electrochem. Soc. 136 (9), 2763-2764, Sept. 1989

The presence of corrosive environments, particularly chloride-containing solutions, usually lowers the fatigue resistance of metals by speeding the process of crack initiation. Such cracking phenomena generally initiate at corrosion pits which, for many AI alloys, serve as stress concentrators once they reach a critical size of some 40-50 p,m. A growing body of empirical information is being reported on the effect of convective mass transport on early stages of pitting corrosion. The effect of fluid flow on corrosion fatigue crack initiation of 2017 AI alloy is presented. 7 refs.

Engineering components and structures S o m e aspects o f structural reliability assurance for random excitation processes. Ishikawa, H., Tsurui, A. and Ishikawa, H. JSME Int. J. I 32, 463-471, Oct. 1989 Certain service loads, when applied to practical machines and structures, could be regarded as approximately deterministic. However, most of them would be, to a degree, of a random nature with complicated temporal variation. Hence, in order to assure the desired level of reliability of critical structural components under random loading, it is of crucial importance to clarify, in particular, the effect of the randomness of the applied load on the life distributions or probabilities of failure of the components, in addition to the effects of other various uncertainty factors usually taken into account in the reliability assurance of structural components subjected to constant amplitude Ioadings. In this respect the fundamental concept has been discussed and a state-of-the-art survey has been made to ascertain the structural reliability for stationary random loads from the viewpoint of whether the effect of the applied random load on a critical structural component is cumulative or not. 52 refs.

N e w life t h e o r y o f rolling bearings. Stewart-Lord, M.

S. Afr. Mech. Eng. 39, (10), 4 6 1 , 4 6 3 - 4 6 6 , Oct. 1989

The Lundberg-Palmgren (L-P) fatigue theory is described followed by an introduction to the New Life Theory of Ioannides at SKF. The emphasis is on fundamentals of the theoretical models and a description of their consequences rather than derivation of formulae. The L-P theory presumes that a fatigue crack is initiated at a distance below the surface in rolling contact at which a large orthogonal shear stress and a weak point in the material coincide. Satisfactory predictions were obtained with bearings made in the late 1930s and early 1940s when the theory was introduced, but gives very low estimates of life for modern materials and accuracies. Ioannides postulated in 1984 that the probability of a volume element surviving N load cycles was a function of the difference between the orthogonal shear stress and a critical stress. SKF have selected experimentally the value 350 N/ram -2 for this and will present suitable nomograms and formulae in their new catalogue which take account of contamination, dynamic bearing load and safety factor required. A warning is given that input data need to be accurate as errors can be magnified in the new life theory calculation. 4 refs.

o f LD10

Mater. Mech. Eng. (China) 13, (3), 13-16, J u n e 1989 (in Chinese) Experimental results show that the tensile strength of LD10 AI alloy forging decreased 20%, ductility 79% and tensile fatigue life 26-88% with a change of area percentage of oxidized film from 0.26-1.6%. The higher the percentage, the lower the fatigue life. The artificial crack and oxidized film are initial sources for fatigue cracks. Graphs, photomicrographs.

Fatigue b e h a v i o u r o f stainless steels u n d e r s i m u l t a n e o u s corrosion attack: II. Corrosion f a t i g u e beheviour. Roeder, E. and Schumacher, G. Steel Res. 60, (9), 4 1 2 - 4 1 6 , 1989 (in G e r m a n ) Experimental investigations of the corrosion fatigue behaviour of stainless steels carried out up to now at the material science department of Kaiserslautarn University are presented. Test equipment is described, which enables the performance of fatigue tests under controlled electrochemical corrosion conditions. The dynamic strength of stainless steel X5CrTi12 (1.4512, adequate to AISI 409) in corrosive environment is essentially lower than in air. A fatigue limit cannot be determined. The influence of the electrolyte temperature up to 80 °C on fatigue strength is insignificant under the chosen test conditions. Measurement of the variation in time of the free corrosion potential enables the assertion of a correlation between a fatigue specimen's rupture time and the length of appearing potential levels. The corrosion fatigue strength can be considerably increased by imprinting suitable constant electrochemical potentials. 18 refs.

Int J Fatigue September 1990

General Mechanical interactions b e t w e e n coatings and superalioys under conditions o f fatigue. Wood, M.I. Surf. Coat. Technol. 39-40, ( 1 - 3 ) , 2 9 - 4 2 , 1 Dec. 1989 The increasing mechanical demand being placed on high-temperature gas turbine components has generated interest in establishing a sound understanding of the mechanical behaviour of coatings and coated components. Their bulk and in situ properties and behaviour, primarily under cyclic conditions, are covered. The close interrelationship between bulk coating properties, the presence and nature of structural defects within the coating and the interaction between coating and substrata are emphasized. The role of deformation modelling and fracture mechanics in advancing the understanding and design capabilities is highlighted. The coating of single crystals of PWA 1480 is described. 35 refs.

M e t a l f l o w o f hot form-roiled helical gears and t h e i r f a t i g u e strength after carbonizing. Tanaka, T., Matsui, M., Sawamura, M. and Danno, A. J. Jpn. Soc. TechnoL Plast. 30 (7), 1038-1043, J u l y 1989 (in Japanese) Hut form-rolling and carburizing temperature conditions necessary to produce defect-free helical gears (module 1.25-3.25) with high fatigue strength have been

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