Cyclic intergranular crack growth in bicrystals at high temperatures

On the effest o f persistent slip bend (PSB) parameters on f a t i g u e life.

Cao,W.-D. and Conrad, H. Fatigue Fract. Eng. Mater. Struct. June 1992 15, (6), 573-583 An examination of PS9 data pertaining to the effects of en air-excluding environment end electric current on the fatigue life of polycryatatline Cu and 316 m i n i m steel in rotating bending reveals that the splicing and width of the PSBs ere smaller throughout the fatigue teet for those conditions which lead to increased fedgue life. Moreover, the life correlates directly with the square of the product of the surface eree fraction of PSB patches and the density of the PSBs contained therein. These reeuite suggest that in these cases slip-homogenization is responsible for the increased fatigue life end not a consequence of it. A model is proposed for the observed relationship between slip band parameters and fatigue life, which is based on the strain concentration that a x i m st or near the PSB-matrix interface. The mechanisms by which an air-excluding environment or an electric current can increase slip homogenization are considered. Graphs. 35 refs. A study o f shear fatigue crack mechenlsms. Hu, Z. Me, L. and Cao, S. Fatigue Fract, Eng. Mater. Struct. June 1992 15, (6), 563-573 Studies have been made of the micro- and macroscopic features of a transverse shear fracture and a longitudinal shear fracture. The fatigue crack propagation mechanisms of mode II and mode III are described, which can be used to explain both fracture features end crack shape. Finally, a series of comparisons is made of mode I, II end III fatigue crack propagation mechanisms. 40Cr steel is discussed. Photomicrographs, graphs. 4 refs. Constant empiitucie and p o s t - o v e d o a d f a t i g u e crack g r o w t h behavior in PM aluminlum alloy A A 8009. Reynolds, A.P. Fatigue Fract. Eng. Mater. Struct. June 1992 15, (6), 551-562 The fatigue crack growth behaviour of a recently developed, rapidly solidified, powder metallurgy, dispersion-strengthened AI alloy, AA 8009, wee studied at room temperature in laboratory air. Constant amplitude/constant &K and single spike overload conditions were examined. Alloy 8009 exhibited high fatigue crack growth rates and low closure levels in comparison with typical ingot-proceesed AI alloys. It was proposed that minimal crack roughness and crack path deflection, along with limited slip reversibility, resulting from the uitre-fine microstructure, were responsible for the relatively poor (versus ingot-proceseed AI alloys including 2090 end 2124) da/dN versus AK performance of AA 8009. Photomicrographs, graphs. 21 refs. Retardation and erroat o f fatigue crack g r o w t h in AISI ~ steel by i n t r o d u d n g rest periods end overloads. Kim, S.-H. and Tal, W.-P. Fatigue Fract. Eng. Mater. Struct. June 1992 15, (6), 519-530 The aim is to investigate ths effects of an intermittent rest period at 300°C, an overload in tension, end the combination of an overload and a subsequent rest period on fatigue crack growth in AlSl 4340 steel. The intermittent rest period was found to atop fatigue crack growth completely near the threshold level of AK. The alleviation effect of a rest period on crack growth wss more distinct st lower levels of ~K. With overload, the higher overload ratio caused s greater effect on crack growth rate. The reduced stress intensiW factor caused by crack branching and an enhanced roughness of crack surface probably contributed to the retardation and arrest of fatigue crack growth. The most distinct retardation of fatigue crack growth was found after the combined treatment of an overload and a subsequent rest period. Compressive residual stresses following an overload and strein-age hardening during the rest period st 300 °C are suggested as assisting the srrest or retardation of fatigue crack propagation. Photomicrographs, graphs. 18 refs. Fatigue end fracture toughness properties of T i - g A I - 4 V w i t h ecicular t, microstructures Matsumoto, T. and Muranaka, S. Kobe Res. Day. Apr. 1992 42, (2), 9 1 - 8 4 (in Japanese) High-cycle fatigue and fracture toughness properties of large size Ti-gAI-4V forgings with ecicular ¢= microatructures were investigated. According to forging and heattreatment conditions, ecicular (x microstructures showed higher fatigue strengths than equisxed- a microstructures. With s thinner a phase in the i~ grain boundary, or finer acicular a microstructures, both the fatigue strength snd fracture toughness were improved. Graphs, photomicrographs. 1 ref. Cyclic intergranuler crack g r o w t h in bicrystals at high temperatures.

Lombard, R., Vehoff, H. and Neumann, P. Z. Metallkd. June 1992 83, (6), 463-467 The growth of intergrenular cracks were examined as s function of the loading rate in Fe-2.gsi bicrystals. The crystals had defined boundaries oriented perpendicular to the applied load. They were alloyed with Nb to delay the formation of cavities st the boundary. Fatigue and creep-fatigue experiments st 600 °C were conducted to clarify the influence of oxygen on the intergrenular crack growth rate. The effects of 0 on the crack growth rate could be partitioned in two frequency ranges. (a) At medium frequencies intergrenulsr crack growth was observed and the growth rate increased with decreasing frequency. In this case, the growth rate could be scaled with the penetration depth of O along the boundary. (b) At low frequencies, the plastic crack tip opening rate was lower than the lateral growth rate of the oxide film. In this case, 0 could not reach the crack tip and transgrenular crack growth with a drastically reduced rate was observed. This explains why, under long-term service conditions, cracks sometimes grow slower than predicted from short-term laboratory tests. Photomicrographs, graphs. 19 refs. On the constitutive l a w of e n v i r o n m e n t assisted fatigue: t h e physical meaning of the Perle t y p e equations. II. The conatitutive l a w o f the full crack g r o w t h range. Krausz, A.S., Wu, X., Krausz, K. and Lien, Z. Z. Metallkd. M a y 1992 83, (5), 356-363 The physically based constitutive law of corrosion fatigue, derived in part I from the principles of thermally activated processes and fracture kinetics, is applied for the representation of the crack growth rate over the whole stress intensity range. The behsviour is expressed in terms of design and environmental factors and microstructurel quantities. The constitutive law of fracture kinetics defines explicitly the effects of stress and temperature. Similarly, the role of the stress ratio R, the frequency and microstructure follow rigorously. The influence of these factors on the crack growth rate and threshold behaviour is discussed extensively. It is also

Int J Fatigue July 1993

demonstrated that fracture kinetics provides the framework for the detailed incorporation of corrcoion chemical reaction and the lmmchlwd diffusion proceese¢ Materials dbcumed include AI 2124, 7075, 2L93, Nimonlc 106, 31g mlnlmm, carbon steel, HSLA steel and SMA19. Graphs. 32 rare. Fatigue m i c k t i p d e f o r m a t i o n ~

es ~

bY t h e environ-

merit. McEvily, A.J. and Gonzalez vemzquez, J. . Metall. Trans. A Aug. 1992 23A, (8), 2211-2221 The shape of a fatigue crack tip as influenced by an air or a vacuum environment has been investigated in two m i n i m steele (304, 316) end an AI alloy (2219). Under plane strain conditions end at crack growth rates in the Paris region, the crack tip opening displacement (CTOD) is much larger in vacuum than in air, e circumstance attributed to strain localization in sir due to the presence of moisture and the absence of strain localization in vacuum. In type 304 m i n i m steel, a strain-induced transformation from auatenhe to mertenlite occurs st the crack tip, and the extent of this strein-induced transformation in type 304 stainless steel is consistent with the degree of blunting taking place at the crack tip as influenced by the environment. In sir, the extent of transformation is a function of the AK level, and as a result, the crack opening level is found to differ in e &K decreasing test as compared with a &K increasing test. Fatigue striations ere observed in air but are absent in vacuum. It is proposed that the greater extent of blunting in vacuum is responsible for the absence of striations in vacuum. Photomicrographs, graphs, diffraction patterns. 25 refs. S u r f m chemical effects on near-surfaea slip end fatigue crack Initiation in silver single crystals. Subramanian, S.T. Diss. Abstr. Int. June 1992 52, (12), 117 pp The initiation of fatigue cracks in silver single crystals was studied using scanning tunneging microscopy. A particular location in a specimen was monitored at 400 cycle intervals until pit-shaped cracks were observed. These features formed over a narrow range of cycles, leading to the conclusion that pit-shaped initial cracks form by s nucleation process. This finding allowed the use of the number of cycles to crack initiation as an indicator of the effect of environment on the fatigue process. Fatigue tests up to crack initiation were carried out on pure Ag single crystal specimens and specimens surface-alloyed with Au. The ambient used was 0 at pressures ranging from 10-s Pa to 2.6 kPs. Concurrent investigations of the effect of Au on 0 adsorption on Ag( 111 ) surfaces were carried out using X-rey photoelectron spectroscopy. A strong correlation was observed between the number of cycles to crack initiation and the reactivity of slip steps exposed by the fatigue process and the rats of 0 adsorption on the exposed slip steps. It was also found that 0 was transported into subsurface regions during fatigue. When fatigue testing was done in bromine, subsurface transport of Sr was not observed; yet the fatigue life up to crack initiation was found to be the same as that in O. The absence of Br transport is probably due to its larger atomic size. Based on these studies, it is proposed that the dominant mechanism for accelerating fatigue crack initiation in an active environment is reduction of dislocatio n reversibility due to gas adsorption on exposed slip steps. A rendom-slip-baeed model was developed for the evolution of slip bands. From this model, an expression was obtained for the average height of the slip bands which contained a term for slip irreversibility. The dependence of average height on the number of cycles for air and vacuum was determined using STM observations as a function of cycling. These data were used in conjunction with the model to determine the ratio of slip irreversibility in air to that in vacuum. it was found that the ratio of irreversibility (defined here as the difference in probabilities for forward and reverse slip) in air to that in vacuum was approximately 4. The growth exponents for the slip band heights were determined. The exponent was closer to 0.5 for the vacuum test indicating a greater degree of randomness in the slip. An independent estimate of irreversibility ratios was obtained from estimates of stored energy. These ratios were found to be close to those obtained from the previous technique. Finally the STM observations indicate that the extent of slip is much larger in vacuum, which is consistent with our findings about environmental effects. Microstresses and macrostresses in the fatigue of steel. Winholtz, R.A. Diss. Abstr. Int. June 1992 52, (12), 226 pp Triaxial stress measurement techniques with diffraction have been improved and applied to the study of ferrite and cementlte in steel. It is shown that solving the strain transformation equation by least squares gives superior performance compared with the Dolle-Hauk method. Difficulty in measuring the unstressed lattice spacing and uncertainty in the effect of the associated error have been s drawback to the use of triaxisl stress measurement techniques. It is shown that errors in the unstressed lattice spacing lead to an error only in the hydrostatic component of the stress tensor and that useful analysis can be done without knowing the unstressed lattice spacing. Triaxial stress measurements were made in both the ferrite and cementile phases of a 1080 steel best-treated to have baariitic, spheroidal, and tempered martensitic microstructures. The stresses were measured during both high- and low-cycle fatigue. In low-cycle fatigue where the steel is plastically deformed cyclically, the applied load is not shared evenly by the two phases due to their different plastic beheviour during deformation. Since the ferrhe deforms preferentially to the cementite, the cementlte takes a greater share of the load, and the stress range during fatigue is much higher than in the ferrite. The stresses measured in the spheroidite were accurately predicted by an Eshelby model. Modelling also qualitatively explains the differences in the microstrems and work hardening rates of the spheroidite and peerlite. However, the microstresses grow far larger during cyclic softening in the tempered msrtensita than can be accounted for by the model. In high-cycle fatigue, initially present residual macrostresses and microstresses faded with fatigue, but no stresses developed in initially unstressed specimens. The rate of fading of the microstresses varied inversely with the strength of the microstructure. The rate of fading did not depend upon which phase was in compression and which phase was in tension. Reliability engineering approach t o fatigue crack g r o w t h r i t e in high strength steel HTg0 by de electrical potential . . . ~ . . M a t s u m u r a , i,

Takamatsu, T., Ichikawa, M., Konno, T. and Nisnijima, ~. J. Soc. Mater. ScL Jpn M a y 1992 41, (464), 637-642 (in Japanese) Fatigue crack growth tests under tension-tension loading were performed on centrenotched specimens of s high-strength steel HT60, and the statistical property of fatigue crack growth rate da/dN was studied by testing 36 specimens under the same condition. The fatigue crack length was measured automatically using the dc

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