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Crack growth behaviour and failure micromechanisms in three heat resistant materials at elevated temperature
Subsurface c l a c k i n i t i a t i o n in high-cycle f a t i g u e o f T i - - g A I - 4 V s l a y s a t
cryogenic temperatures. Umezawa, 0., Nagai, K. and Ishikawa, K. Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) J u n e 1990 76, (6), 9 2 4 - 9 3 1 (in Japanese) In Ti alloys subsurface fatigue crack initiation apparently occurs without the existence of any defects such as inclusions or pores. Subsurface crack initiation and subcrecks for "ri-6AI-4V alloys at cryogenic temperatures, and discussed the growth mechanism of microcreck. As the maximum stress was reduced the morphology of the subsurface crack initiation sites was changed from consisting of one facet to more than one facet. The facet was identified as a cracked (=-phase by comparing the chemical composition and morphology. Suberecks were also produced in the (z-phsee. The subsurface crack initiation sites and the suberacks were not perpendicular to the applied stress. Hence it is concluded that a microcreck initiates in an a grain, grows into neighbouring j~-pletes and ¢¢-greins, and finally forms an initiation site for the main fatigue crack. The lower the maximum stress, the greater the size of the subsurface crack initiation sites. The size of the projection of the subsurface crack initiation site on the main crack propagation plane was adopted as a shape parameter of the three-dimensional cracks. Using this parameter, the dependence of the initiation site size on the maximum stress range can be accounted for by assuming that the microcreck growth is controlled by a threshold condition. Photomicrographs, graphs. 16 refs.
S a t u r a t i o n o f d a m a g e in f i ~ t i n g f a t i g u e o f h i g h - s t r e n g t h steels in seawater. Nakazawa, K., Sumite, M. and Maruyama, N. Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) J u n e 1990 76, (6), 9 1 7 - 9 2 3 (in Japanese) The saturation behaviours of damage in fretting fatigue at frequencies of 1 and 20 Hz in seawater under freely corroding conditions were studied using highstrength steels having tensile strengths of 490, 690 and 880 MPa. Fretting damage saturated beyond a certain number of fretting cycles, and this number varied drastically with the steel and the frequency. The saturation behaviours of damage could be divided into two groups, that is the smallest numbers of fretting cycles to cause the saturation were less than 0.1% of the fretting fatigue life (group 1) and more than 40-60% (group 2). They had a strong relationship to crack initiation and growth behaviours. In group 1, cracks propagated normal to the alternating stress axis from the early stage of crack propagation. However, in group 2 the cracks propagated at angles less than 90° to the alternating stress axis, but changed direction to become normal as they propagated beyond a certain length. This relationship could be explained from the mechanism that the decrease in the fretting fatigue lives of group 1 resulted from the decrease in crack initiation life caused by the acceleration of corrosion pit formation, and that of group 2 from crack initiation and growth caused by fretting. Graphs, photomicrographs. 13 refs.
Rnlta-element
s t u d y o f residual stresses in a plate T-joint f a t i g u e
r~oc.mee. Mok, D.H.B. and Pick, R.J. Inst. Mech. Eng. C, J. Mech. Eng. Sci. 1990 204, (C2), 1 2 7 - 1 3 4 Using the finite-element code Abequs to make numerical predictions, this paper studies the crack shape development and thickness effects on a series of welded plate T-joint fatigue specimens. It was found that redistribution of the residual stress only occurs in a small region near the weld toe, the extent depending greatly on the magnitude of the applied load. Graphs. 18 refs.
Environmentally i n d u c e d cracking. Scott, P.M.
Constr. Build. Mater. J u n e 1990 4, (2), 9 8 - 1 0 6
A review of current research topics in the UK on environmentally induced or assisted cracking is given with particular emphasis on how these research interests are related to current individual applications. Although the review cannot be completely comprehensive, some particularly topical items are described in greater detail including corrosion fatigue of offshore structural materials, pipeline cracking, highstrength fasteners end deserator cracking. Several key generic issues ere highlighted such as the relevance of short-term test methods to long-term service experience and the increasing reliance on fracture mechanics analyses with their strengths and weaknesses for facilitating run/repair decisions. 25 refs. Fatigue s t r e n g t h characteristics o f h i g h - s t r a n g t h steel. Asami, K. and
Emura, H. JSME Int. J. J u l y 1990 I 33, (3), 3 6 7 - 3 7 4 In order to clarify the fatigue characteristics of low-alloy steels tempered at low temperature (150-200 °C) Or carburized in generated gas, rotating-bending fatigue tests were conducted in laboratory air and dry air. The influence of the moisture in air was only observed for the cerburized steel. The low-temperature-tempered steel revealed two knees in the S-N diagram, and no fatigue limit was found up to 10s cycles (9 months). The cerburized steel revealed three knees and the fatigue limit was found to be over 108 cycles. When the fatigue test on carburized steel was carried out in dry air or the surface structure anomalies were removed, the fatigue strengths were improved, but neither knees nor fatigue limits were found up to 10s cycles. The reasons for this were discovered from the results of fractogrephy and the equations proposed by Murekami et al for the prediction of the fatigue limit of steel with microdefects. Graphs, photomicrographs. 6 refs. Prediction o f p r o p a g a t i o n and n o n - p r o p a g a t i o n o f s h o r t f a t i g u e cracks at notches u n d e r m e a n s U m . Akiniwa, Y., Tanaka, K. and Taniguchi,
N. JSME Int. J. J u l y 1990 I 33, (3), 2 8 8 - 2 9 6 A resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks emanating from the notch root under an arbitrary stress ratio. The resistance-curve was determined from the experimental result obtained from the fatigue tests of single-edge-notched plates of low-carbon steel under several stress ratios. The relation between the crack opening stress intensity factor at threshold, Kopth, and the non-propagating crack length, Cnp, was approximated by Kopth = K;pth[(Cnp-Cr)/(C2-C1)]1/2(CI=~Cnp~C2)and Kopth = K;pth (C2'~Cnp), where K~oth Wee the value for long cracks, C2 was a linear function of K;pth, and C1 was constant. Under compressive mean stresses, the effect of notch plasticity on crack closure was superposed on the above-mentioned closure. Good agreement between prediction and experiment was obtained for the fatigue limit for fracture and the non-propagating crack length. The fatigue limit for crack initiation of a stage-II crack
Int J Fatigue May 1991
wee almost constant and equal to the value predicted for the case of R = -1. Graphs. 16 refs. resistance in e i u m i n i u m alloys. Jono, M. and Sugete, A. J. Jpn. Inst. Light Met. J u l y 1990 40, (7), 5 4 3 - 5 5 3 (in Japanese) Fatigue crack g r o w t h
During metal fatigue, crack initiation and propagation are very different. When designing high-rigidity machinery components, damage tolerance is to be considered. Aluminium alloy fatigue crack growth beheviour based on the theory of fracture mechanics is reviewed. Data and theory from the literature are provided and discussed. Fatigue failure of AI alloys is divided to two stages: microcreck and macrocreck propagation until fracture. A mathematical analysis of the stress concentration in crack tips is given. The crack propagation beheviours of ingot metallurgy alloys and powder metallurgy alloys are studied, followed by an emphasis on the alloys AI-Si and AI-Li. 7075 and ZK141 alloy fatigue crack propagation under dynamic loading is also discussed. Graphs, photomicrographs. 32 refs. T h e r m a l f a t i g u e u n d e r m u l t i a x i a i stresses. Feng, J., Bian, M. and Dang, Z. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 5 2 5 - 5 3 4 Thermal fatigue under multiaxiel stresses has been investigated. Circular cylindrical specimens were tested under thermal fatigue which was in-phase with the axial mechanical fatigue loading. The axial forces on the specimens (e.g., 3Cr2W8V) were varied throughout the test programme, but all the temperature cycles were identical, so various biaxial stress and strain ratios were obtained. Straight thermal fatigue cracks occurred in different directions and also net-like crazes in various patterns were observed on the surfaces of the specimens. The transient temperature, stress and strain fields have been calculated with a thermal elastoplastic finite-elemeot method. Comparing test results with calculations, it appears that the patterns of thermal fatigue cracks are dependent on the stress state and the plastic strain state, not on the strain state. The direction of the cracks is perpendicular to the maximum principal stress and the maximum plastic strain. Net-like thermal fatigue crazes will occur when one principal stress is about the same as the other one and one plastic strain component is approximately equal to another. Graphs, photomicrographs. 6 refs.
Fatigue microcracks in t y p e 304 stainless steel at elevated t e m p e r a t u r e .
Suh, C.M., Lee, J.J. and Kang, Y.G. Fatigue Frect. Eng. Mater. Struct. M a y 1990 13, (5), 4 8 7 - 4 9 6 Microcrecking of type 304 stainless steel at 538°C has been studied, in particular, the initiation, growth and coalescence of fatigue microcrecks on smooth specimens via surface replicas and photomicrographs. Quantitative information, such as the initiation period, growth and coalescence behaviour, statistical distributions of crack length, density of cracks, distribution patterns and crack growth properties, was obtained. Knowledge of these parameters is critical for the application of fracture mechanics to fatigue life assessment and the evaluation of damage in structures at elevated temperatures, Graphs. 22 refs.
L i f e t i m e p r e d i c t i o n o n C r - M o - V and 316L steels u n d e r t h e r m a l and mechanical cycling. Degallaix, G., Korn, C. and Pluvinage, G. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 4 7 3 - 4 8 5 The thermal-mechanical fatigue resistance of a Cr-Mo-V steel and a 316L steel under combined complex mechanical strain and temperature cycling has been evaluated. The applicability to thermal-mechanical lifetime predictions of several isothermal fatigue damage concepts is investigated. From a designer viewpoint, some predictions are quite acceptable, in particular, with the Degellaix thermal activation model, for which a recent development is reported. Graphs. 27 refs. M e r k o v a p p r o x i m a t i o n t o f a t i g u e crack size distribution. Solomos, G.P.
and Lucia, A.C. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 457-471 The distribution of the crack size during the fatigue crack propagation phenomenon is investigated. A Markovian modelling of the crack is introduced through the fatigue crack growth law, and the associated Fokker-Planck equation is written while special care is devoted to specifying its boundary conditions. This equation is solved by the method of separation of variables and the required distribution function is obtained in the form of a convergent infinite series, An illustrative example, demonstrating the applicability of the approach, shows satisfactory agreement between the theoretical results and the experimental data. The test material was 7475-T7351 AI. Graphs. 25 refs. Fractographic analysis o f crack g r o w t h and shear lip d e v e l o p m e n t under s i m p l e v a r i a b l e - a m p l i t u d e loading. Ling, M. R. and Schijve, J. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 4 4 3 - 4 5 6 Fatigue crack growth experiments were carried out on 2024-T3 sheet specimens, loaded under simple variable-amplitude load sequences, including blocks of overload cycles or underload cycles and blocks of small-amplitude cycles. The purpose was to study interaction effects by analysing striation measurements and shear lip developments. The striations clearly demonstrate delayed retardation. Shear lip measurements indicate significantly slower crack growth rates if the percentage shear lip increases from zero (no shear lips) to 100% (fully in the shear mode). The significance of incompatible shear lips during variable-amplitude loading is discussed. The test results are evaluated with relation to interaction mechanisms and the interpretation given to them by cycle-by-cycle prediction models. Graphs. 19 refs. Crack g r o w t h b e h a v i o u r a n d failure m i c r o m e c h a n i s m s in three h e a t resistant materials s t elevated t e m p e r a t u r e . Hour, K.Y. and Stubbins, J.F. Acts Metell. Mater. Aug. 1990 38, (8), 1463-1474 The crack growth behaviour for three high-temperature materials, alloy 80OH, type310 stainless steel (SS) and Hastelloy X, have been investigated under creep, fatigue and hold time test conditions at 650 +C. All alloys show a predominantly transgrenular crack advance mode when cycled at frequencies greater than 0.5 Hz. At lower frequencies, the failure in 310 SS transitions takes place to a predominantly
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intergrenular mode while Hastelloy X remains transgrenular. The alloy 800H shows a mixed failure mode at intermediate and low frequencies. Creep crack growth rate correlates well with C* for all alloys, and can be extended to correlate fatigue with hold time loading conditions for alloy 800H and 310 SS, but not for Hastelloy X under the conditions studied here. These crack growth observations were supplemented by microhardnass, microstructurel and load response measurements. Crack growth rate predictions based on a local critical strain criterion were also examined and found to agree with the experimental results. Graphs, photomicrographs. 33 refs.
air cooling which arc obtained in practice for components having various thicknesses (i.e. blade foil versus blade root). The material was then ann(~Sled for 8 h at 590 °C. In addition to tensile teats at room temperature, 450 and 510 °C, creep experiments at 450°C (100 h, 390 MPa) and 510°C (100 h, 210 MPa) and load-controlled LCF experiments (R = 0.1, 0.3 Hz) at 450°C were carried out. The significant influence of the cooling rate on the mechanical beheviour is discussedin terms of TEM investigations of tha phase dimensions (lamellse width) and Ti3AI precipitation in the lamellar and equiaxed o-phase. Grepha, photomicrographs. 9 refs.
influence o f t h e degree and mode o f f o r m i n g strain on t h e cyclic m-strain a n d s t r a i n - l i f e b e h e v i o u r o f h i g h - s t r o n g t h gauge f o r m a b l e steels. Godwin, M.J. Comm. Eur. Communities Rep. No 12749 1990 117 pp
The nature o f t h e t w o o p e n i n g levels f o l l o w i n g an o v e r l o a d in f a t i g u e crack g r o w t h . McEvily, A.J. and Yang, Z. Metall. Trans. A Oct. 1990 21A, (10), 2 7 1 7 - 2 7 2 7
The research programme reported here has concentrated on examining the stress-strain fatigue properties of various high-strength sheet steel grades before and after cold working under a range of deformation modes. The materials selected were dual-phase, HSLA, rephosphorized and beke-hardenable steels and, for comparison, a low-carbon mild steel. The different modes of prestrain were introduced using laboratory equipment and included uniaxial, balanced biaxial, plane strain and pure shear strain. Cyclic properties were determined on small test pieces cut from appropriately deformed areas using servohydreulic test equipment operating under strain control. Testing was carried out on all the materials in both the notched and unnotched conditions. On the basis of the results obtained from the test programme, it has been concluded that the cyclic stress-strain beheviour of all the steels examined differed considerably from the monotonic stress-strain beheviour. In general, the cyclic strength of the prestrained steels was greater than that of the corresponding unstrained, as-received material. However, the notch fatigue strength varied with the degree and mode of strain at formation and, in certain cases, no improvement over the as-received notch fatigue strength was detected. Therefore, for design purposes, it will be safer to assume that no contribution to the fatigue strength at notches or discontinuities can be expected from increases in monotonic strength achieved by the strain at formation in pressed panels. It has been shown that a Neuber analysis can give an acceptably conservative estimate of the notch fatigue strength from smooth specimen data for all the material covered in this investigation. Graphs. 11 refs.
E n v i r o n m e n t a l effects in f a t i g u e crack initiation and propagation. Comm. Eur. Communities Rap. No 12205 1990, 65 pp The effect of environment (seawater with or without cathodic protection) on crack initiation and incipient slow growth is examined. The steel used is BS 4360-50D. The results obtained so far show that the number of cycles to initiation can be increased, compared to the values in air, by cathodic protection. The number of cycles to initiation is defined as the number of cycles to grow a crack 0.5 mm long. The prolonged initiation period is explained by a plugging effect from deposition products of calcium and magnesium. After initiation, the crack growth can be retarded or even stopped by Ca and Mg deposition products. The lower the stress ratio, the greater the ability of deposition products to slow down growing cracks. Both the initiation and the subsequent growth of cracks are influenced by the applied potential. The results show that the longest lives are obtained by potentials of -100 mV SCE. EDAX-analysis of the depoaition products shows that, at potentials higher than -900 mV SCE, the deposition products are rich in Ca, while the products are rich in Mg at a potential of -1000 mV SCE or lower, The change in the composition of the deposition products together with the increasing hydrogen uptake at decreasing potentials can explain why the longest lives are found at potentials of -1000 mV SCE. Tests performed at naturally occurring corrosion rates (0.1 mm/year) show that, under this condition, the initiation is facilitated and the growth rate is increased compared to the values in air. All the above-mentioned results have been obtained during tests performed at a frequency of 0.1 Hz. The results of tests performed at 1.0 Hz show that, at potentials of at least 900 mV SCE, one gets conservative results. On the other hand, at potentials of at most 1000 mV, longer lives are measured in tests with the higher frequency. Measurements of the precipitation rate of CaCO3 and Mg(OH)2 show that the precipitation rate is governed by the cathodic current density and is thereby strongly dependent on the potential. This result can explain the difference observed in the results of the 0.1 and the 1.0 Hz tests. Graphs, photomicrographs. 25 refs. Fracture m o d e transitions d u r i n g f a t i g u e crack growth in T i - 6 A I - 4 V alloy. Ravichandran, K.S. Scr. Metall. Meter. J u l y 1990 24, (7), 1275-1280 Fatigue fracture surface morphology has a pronounced effect on crack growth behaviour under cyclic loading conditions. By quantitative estimations of the fracture morphology, a transition from a flat transgranular to a crystallographic fracture at low values of ~,K occurs when the monotonic plastic zone size (MPZS) becomes equal to the colony size in a fine colony microstructure consisting of a thin and discontinuous ~-phase. If the ~-phase is present in thick and continuous form, as in coarse colony structures, (~-Ieths can induce crystallographic fracture when the CPZS approaches the size of ~x-leths. The asperities resulting from crystallographic fracture influence the crack closure levels and a quantitative relationship between the size scale and the magnitude of the crystallographic fracture and crack closure might exist. Ti-6AI-4V was heat treated in a vacuum at 1040 °C for 30 rain producing two different colony sizes by varying the cooling rate. One set of specimens was quenched in Ar while the other was cooled at a rate of 6 °C/rain up to 500 °C and then quenched in Ar. Fatigue crack growth (FCG) tests were performed by a manual load shedding technique. Crack closure measurements were made from unloading elastic compliance curves recorded using crack mouth opening displacement gauges. Crack paths and fracture surfaces were examined in a scanning electron microscope (SEM). The percentage crystallographic fracture at each AK level was estimated by the point count method on SEM micrographs. Graphs, photomicrographs. 10 refs.
Effect o f c o o l i n g r s t e
on creep and Iow-c~cle f a t i g u e resistance in Ti-
6242. Seal, S., Wagner, L., Lutjering, G., Pillhofer, H, and Daeubler, A. Z. Metallkd. Aug. 1990 81, (8), 5 3 5 - 5 3 9 The creep and low-cycle fatigue (LCF) beheviour of the Ti alloy Ti-6242 with a bimodal microstructure (equiaxed primary ~ in a lamellar c~+L3 matrix) was investigated for applications in gas turbine engines. The alloy was cooled from the 'bi-modal' annealing temperature (980 °C) to 600 °C at rates ranging from 2000 K/rain to 1 K/min. This wide variation simulates the cooling rates during an oil quench or
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During the period of retarded fatigue crack growth following a single tensile overload, two crack opening levels are observed. Different interpretations have been put forth concerning the nature of these opening events. In one proposal, the two opening events are conaidered to occur in plane strain, first behind the crack tip and then at the crack tip. In another proposal, the first opening event occurs in the plane strain portion of the crack front and the second in the plane stress portion of the crack front. Critical experiments designed to clarify this issue are described. In most of the experiments, the AI alloy 6061-T6 was used. Additional teats were made using the AI alloy 7090-T6 and the Ti alloy IM1829. The results of these experiments show that over the entire range of &K, including the near-threshold range, the two opening events are associated with a plane strain followed by a plane stress opening process. The results are important in that they not only shed light on the mechanism of crack retardation following an overload, but they also provide a basis for the development of a rational method of analysis for the calculation of the number of delay cycles following an overload, which includes the effects of thickness and AK level, Based upon the findings of this study, a samiempirical method for the determination of the number of delay cycles following an overload is presented. The number of delay cycles predicted by this method is found to be in good agreement with the results of the present investigation and with other results from the literature. Graphs. 14 rafs. A theoretical m o d e l for roughness induced crack closure. Ravichandran,
K.S. Int. J. Fract. J u l y 1990 44, (2), 9 7 - 1 1 0
A theoretical model for the effects of grain size on the magnitude of roughnessinduced crack closure (RICC} at a fatigue crack growth threshold has been proposed. With the basic configuration of a crack propagating incrementally along planar slip bands and being deflected at grain boundaries, an idealized zig-zag crack path is considered. The effective slip band length is considered to be equal to the grain size. It is assumed that the dislocations emitted from the crack tip upon loading, which form the pile-up, are completely irreversible and produce a combined model and mode-II displacement at the crack tip. The assumption of continuously distributed dislocations in the pile-up facilitated the calculation of crack tip sliding displacement (CTSD) along the the slip plane from Which the mode-I closure disregistry just behind the crack tip can be calculated. The closure stress intensity factor at threshold, Kcl,th could then be expressed as a function of critical resolved shear stress, average macroscopic yield stress, angle subtended by the slip plane with the crack plane and the length of the slip band. Comparisons of the predicted trends with experimental data from various alloy (e.g., steels, MAR M200, Ti-6AI-4V, Ti-6AI-5Zr-0.5Mo-0.25Si) systems indicate good agreement. Graphs. 47 refs. Inelastic d e f o r m a t i o n and f a t i g u e d a m a g e in m e t a l s at h i g h homologous temperatures. (Dissertation). Wilcox, J.R. Diss. Abstr. Int. Aug. 1990 51, (2), 319 pp The phenomenology of inelastic deformation and fatigue damage in metals at high homologous temperatures is investigated using experimental and theoretical approaches. Several examples are presented to illustrate the systematic beheviour among the stress ((~) and inelastic strain rate (E) relations for different alloy systems at high homologous temperatures. It is proposed that, at high homologous temperatures, inelastic deformation of metals and alloys at typical laboratory strain rates is controlled by grain boundary sliding in which the accommodation of flow incompatibilities is accomplished through grain boundary migration. The Sn-Pb alloy system is used as a model system for experimental study. Load relaxation experiments and constant-load creep tests are used to determine the (~-e relations at homologous temperatures as high as 0.87. Data are included for four different compositions in the Sn-Pb system: 15, 50, 62, and 70 wt.% Sn. As postulated by state variable concepts of inelastic deformation, it is found that load relaxation and creep give essentially the same ~ - ( result in those regimes where strain hardening is negligible. The results further indicate that the ¢ - ( curves for these alloys are fundamentally similar. A damage integral approach is developed to describe the fatigue of metals at high homologous temperatures. Such an approach requires a precise knowledge of the stress-time profile to provide a basis for the time integration of fatigue damage, which is quantified using fracture mechanics COncepts. Constitutive equations based on state variables are shown to be adequate for stress cycle calculations. This damage integral method is demonstrated for the case of thermal displacement fatigue in 60Sn-Pb solder material in which fatigue damage is accrued through crack propagation. The method is shown to simulate experimentally observed effects of the time-tempereture weveform on the fatigue life. The effect o f processing and m i c r o s t r u s t u r e on t h e mechanical properties o f a l u m i n i u m alloy 339-T5. (Dissertation). Crepeau, P.N. Diss. Abstr. Int. J u l y 1990 51, (1), 243 pp The influence of six microstructural variations of 339-T5 on the tensile behaviour, high-cycle fatigue crack initiation (HCF) and fatigue crack propagation (FCP) was determined. Specimens were tested at room temperature, 177, and 343°C, representing the extremes of the service conditions of diesel pistons. Specimens were cast: (1) conventionally in permanent moulds, (2) in permanent moulds plus water quenching following solidification, and (3) in a squeeze-csst press. One aeries of the above processing conditions was inoculated with P and the other, with Sr, both Si modifying agents. The phosphorus-inoculated specimens surpassed the Sr inoculation ones in tensile strength and crack initiation resistance. Water quenching after casting improved the tensile properties but the effects on crack initiation were trivial. Squeeze casting produced ambiguous results, attributed to a modest microstructural refinement (as opposed to the other conditions) and an inconsistent thermal history. Fatigue crack propagation was not sensitive to the microstructural differences. Tensile failure was controlled by the strength of the interdendritic
Int J Fatigue May 1991
cryogenic temperatures. Umezawa, 0., Nagai, K. and Ishikawa, K. Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) J u n e 1990 76, (6), 9 2 4 - 9 3 1 (in Japanese) In Ti alloys subsurface fatigue crack initiation apparently occurs without the existence of any defects such as inclusions or pores. Subsurface crack initiation and subcrecks for "ri-6AI-4V alloys at cryogenic temperatures, and discussed the growth mechanism of microcreck. As the maximum stress was reduced the morphology of the subsurface crack initiation sites was changed from consisting of one facet to more than one facet. The facet was identified as a cracked (=-phase by comparing the chemical composition and morphology. Suberecks were also produced in the (z-phsee. The subsurface crack initiation sites and the suberacks were not perpendicular to the applied stress. Hence it is concluded that a microcreck initiates in an a grain, grows into neighbouring j~-pletes and ¢¢-greins, and finally forms an initiation site for the main fatigue crack. The lower the maximum stress, the greater the size of the subsurface crack initiation sites. The size of the projection of the subsurface crack initiation site on the main crack propagation plane was adopted as a shape parameter of the three-dimensional cracks. Using this parameter, the dependence of the initiation site size on the maximum stress range can be accounted for by assuming that the microcreck growth is controlled by a threshold condition. Photomicrographs, graphs. 16 refs.
S a t u r a t i o n o f d a m a g e in f i ~ t i n g f a t i g u e o f h i g h - s t r e n g t h steels in seawater. Nakazawa, K., Sumite, M. and Maruyama, N. Tetsu-to-Hagane (J. Iron Steel Inst. Jpn.) J u n e 1990 76, (6), 9 1 7 - 9 2 3 (in Japanese) The saturation behaviours of damage in fretting fatigue at frequencies of 1 and 20 Hz in seawater under freely corroding conditions were studied using highstrength steels having tensile strengths of 490, 690 and 880 MPa. Fretting damage saturated beyond a certain number of fretting cycles, and this number varied drastically with the steel and the frequency. The saturation behaviours of damage could be divided into two groups, that is the smallest numbers of fretting cycles to cause the saturation were less than 0.1% of the fretting fatigue life (group 1) and more than 40-60% (group 2). They had a strong relationship to crack initiation and growth behaviours. In group 1, cracks propagated normal to the alternating stress axis from the early stage of crack propagation. However, in group 2 the cracks propagated at angles less than 90° to the alternating stress axis, but changed direction to become normal as they propagated beyond a certain length. This relationship could be explained from the mechanism that the decrease in the fretting fatigue lives of group 1 resulted from the decrease in crack initiation life caused by the acceleration of corrosion pit formation, and that of group 2 from crack initiation and growth caused by fretting. Graphs, photomicrographs. 13 refs.
Rnlta-element
s t u d y o f residual stresses in a plate T-joint f a t i g u e
r~oc.mee. Mok, D.H.B. and Pick, R.J. Inst. Mech. Eng. C, J. Mech. Eng. Sci. 1990 204, (C2), 1 2 7 - 1 3 4 Using the finite-element code Abequs to make numerical predictions, this paper studies the crack shape development and thickness effects on a series of welded plate T-joint fatigue specimens. It was found that redistribution of the residual stress only occurs in a small region near the weld toe, the extent depending greatly on the magnitude of the applied load. Graphs. 18 refs.
Environmentally i n d u c e d cracking. Scott, P.M.
Constr. Build. Mater. J u n e 1990 4, (2), 9 8 - 1 0 6
A review of current research topics in the UK on environmentally induced or assisted cracking is given with particular emphasis on how these research interests are related to current individual applications. Although the review cannot be completely comprehensive, some particularly topical items are described in greater detail including corrosion fatigue of offshore structural materials, pipeline cracking, highstrength fasteners end deserator cracking. Several key generic issues ere highlighted such as the relevance of short-term test methods to long-term service experience and the increasing reliance on fracture mechanics analyses with their strengths and weaknesses for facilitating run/repair decisions. 25 refs. Fatigue s t r e n g t h characteristics o f h i g h - s t r a n g t h steel. Asami, K. and
Emura, H. JSME Int. J. J u l y 1990 I 33, (3), 3 6 7 - 3 7 4 In order to clarify the fatigue characteristics of low-alloy steels tempered at low temperature (150-200 °C) Or carburized in generated gas, rotating-bending fatigue tests were conducted in laboratory air and dry air. The influence of the moisture in air was only observed for the cerburized steel. The low-temperature-tempered steel revealed two knees in the S-N diagram, and no fatigue limit was found up to 10s cycles (9 months). The cerburized steel revealed three knees and the fatigue limit was found to be over 108 cycles. When the fatigue test on carburized steel was carried out in dry air or the surface structure anomalies were removed, the fatigue strengths were improved, but neither knees nor fatigue limits were found up to 10s cycles. The reasons for this were discovered from the results of fractogrephy and the equations proposed by Murekami et al for the prediction of the fatigue limit of steel with microdefects. Graphs, photomicrographs. 6 refs. Prediction o f p r o p a g a t i o n and n o n - p r o p a g a t i o n o f s h o r t f a t i g u e cracks at notches u n d e r m e a n s U m . Akiniwa, Y., Tanaka, K. and Taniguchi,
N. JSME Int. J. J u l y 1990 I 33, (3), 2 8 8 - 2 9 6 A resistance-curve method was proposed for predicting the growth threshold of short fatigue cracks emanating from the notch root under an arbitrary stress ratio. The resistance-curve was determined from the experimental result obtained from the fatigue tests of single-edge-notched plates of low-carbon steel under several stress ratios. The relation between the crack opening stress intensity factor at threshold, Kopth, and the non-propagating crack length, Cnp, was approximated by Kopth = K;pth[(Cnp-Cr)/(C2-C1)]1/2(CI=~Cnp~C2)and Kopth = K;pth (C2'~Cnp), where K~oth Wee the value for long cracks, C2 was a linear function of K;pth, and C1 was constant. Under compressive mean stresses, the effect of notch plasticity on crack closure was superposed on the above-mentioned closure. Good agreement between prediction and experiment was obtained for the fatigue limit for fracture and the non-propagating crack length. The fatigue limit for crack initiation of a stage-II crack
Int J Fatigue May 1991
wee almost constant and equal to the value predicted for the case of R = -1. Graphs. 16 refs. resistance in e i u m i n i u m alloys. Jono, M. and Sugete, A. J. Jpn. Inst. Light Met. J u l y 1990 40, (7), 5 4 3 - 5 5 3 (in Japanese) Fatigue crack g r o w t h
During metal fatigue, crack initiation and propagation are very different. When designing high-rigidity machinery components, damage tolerance is to be considered. Aluminium alloy fatigue crack growth beheviour based on the theory of fracture mechanics is reviewed. Data and theory from the literature are provided and discussed. Fatigue failure of AI alloys is divided to two stages: microcreck and macrocreck propagation until fracture. A mathematical analysis of the stress concentration in crack tips is given. The crack propagation beheviours of ingot metallurgy alloys and powder metallurgy alloys are studied, followed by an emphasis on the alloys AI-Si and AI-Li. 7075 and ZK141 alloy fatigue crack propagation under dynamic loading is also discussed. Graphs, photomicrographs. 32 refs. T h e r m a l f a t i g u e u n d e r m u l t i a x i a i stresses. Feng, J., Bian, M. and Dang, Z. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 5 2 5 - 5 3 4 Thermal fatigue under multiaxiel stresses has been investigated. Circular cylindrical specimens were tested under thermal fatigue which was in-phase with the axial mechanical fatigue loading. The axial forces on the specimens (e.g., 3Cr2W8V) were varied throughout the test programme, but all the temperature cycles were identical, so various biaxial stress and strain ratios were obtained. Straight thermal fatigue cracks occurred in different directions and also net-like crazes in various patterns were observed on the surfaces of the specimens. The transient temperature, stress and strain fields have been calculated with a thermal elastoplastic finite-elemeot method. Comparing test results with calculations, it appears that the patterns of thermal fatigue cracks are dependent on the stress state and the plastic strain state, not on the strain state. The direction of the cracks is perpendicular to the maximum principal stress and the maximum plastic strain. Net-like thermal fatigue crazes will occur when one principal stress is about the same as the other one and one plastic strain component is approximately equal to another. Graphs, photomicrographs. 6 refs.
Fatigue microcracks in t y p e 304 stainless steel at elevated t e m p e r a t u r e .
Suh, C.M., Lee, J.J. and Kang, Y.G. Fatigue Frect. Eng. Mater. Struct. M a y 1990 13, (5), 4 8 7 - 4 9 6 Microcrecking of type 304 stainless steel at 538°C has been studied, in particular, the initiation, growth and coalescence of fatigue microcrecks on smooth specimens via surface replicas and photomicrographs. Quantitative information, such as the initiation period, growth and coalescence behaviour, statistical distributions of crack length, density of cracks, distribution patterns and crack growth properties, was obtained. Knowledge of these parameters is critical for the application of fracture mechanics to fatigue life assessment and the evaluation of damage in structures at elevated temperatures, Graphs. 22 refs.
L i f e t i m e p r e d i c t i o n o n C r - M o - V and 316L steels u n d e r t h e r m a l and mechanical cycling. Degallaix, G., Korn, C. and Pluvinage, G. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 4 7 3 - 4 8 5 The thermal-mechanical fatigue resistance of a Cr-Mo-V steel and a 316L steel under combined complex mechanical strain and temperature cycling has been evaluated. The applicability to thermal-mechanical lifetime predictions of several isothermal fatigue damage concepts is investigated. From a designer viewpoint, some predictions are quite acceptable, in particular, with the Degellaix thermal activation model, for which a recent development is reported. Graphs. 27 refs. M e r k o v a p p r o x i m a t i o n t o f a t i g u e crack size distribution. Solomos, G.P.
and Lucia, A.C. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 457-471 The distribution of the crack size during the fatigue crack propagation phenomenon is investigated. A Markovian modelling of the crack is introduced through the fatigue crack growth law, and the associated Fokker-Planck equation is written while special care is devoted to specifying its boundary conditions. This equation is solved by the method of separation of variables and the required distribution function is obtained in the form of a convergent infinite series, An illustrative example, demonstrating the applicability of the approach, shows satisfactory agreement between the theoretical results and the experimental data. The test material was 7475-T7351 AI. Graphs. 25 refs. Fractographic analysis o f crack g r o w t h and shear lip d e v e l o p m e n t under s i m p l e v a r i a b l e - a m p l i t u d e loading. Ling, M. R. and Schijve, J. Fatigue Fract. Eng. Mater. Struct. M a y 1990 13, (5), 4 4 3 - 4 5 6 Fatigue crack growth experiments were carried out on 2024-T3 sheet specimens, loaded under simple variable-amplitude load sequences, including blocks of overload cycles or underload cycles and blocks of small-amplitude cycles. The purpose was to study interaction effects by analysing striation measurements and shear lip developments. The striations clearly demonstrate delayed retardation. Shear lip measurements indicate significantly slower crack growth rates if the percentage shear lip increases from zero (no shear lips) to 100% (fully in the shear mode). The significance of incompatible shear lips during variable-amplitude loading is discussed. The test results are evaluated with relation to interaction mechanisms and the interpretation given to them by cycle-by-cycle prediction models. Graphs. 19 refs. Crack g r o w t h b e h a v i o u r a n d failure m i c r o m e c h a n i s m s in three h e a t resistant materials s t elevated t e m p e r a t u r e . Hour, K.Y. and Stubbins, J.F. Acts Metell. Mater. Aug. 1990 38, (8), 1463-1474 The crack growth behaviour for three high-temperature materials, alloy 80OH, type310 stainless steel (SS) and Hastelloy X, have been investigated under creep, fatigue and hold time test conditions at 650 +C. All alloys show a predominantly transgrenular crack advance mode when cycled at frequencies greater than 0.5 Hz. At lower frequencies, the failure in 310 SS transitions takes place to a predominantly
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intergrenular mode while Hastelloy X remains transgrenular. The alloy 800H shows a mixed failure mode at intermediate and low frequencies. Creep crack growth rate correlates well with C* for all alloys, and can be extended to correlate fatigue with hold time loading conditions for alloy 800H and 310 SS, but not for Hastelloy X under the conditions studied here. These crack growth observations were supplemented by microhardnass, microstructurel and load response measurements. Crack growth rate predictions based on a local critical strain criterion were also examined and found to agree with the experimental results. Graphs, photomicrographs. 33 refs.
air cooling which arc obtained in practice for components having various thicknesses (i.e. blade foil versus blade root). The material was then ann(~Sled for 8 h at 590 °C. In addition to tensile teats at room temperature, 450 and 510 °C, creep experiments at 450°C (100 h, 390 MPa) and 510°C (100 h, 210 MPa) and load-controlled LCF experiments (R = 0.1, 0.3 Hz) at 450°C were carried out. The significant influence of the cooling rate on the mechanical beheviour is discussedin terms of TEM investigations of tha phase dimensions (lamellse width) and Ti3AI precipitation in the lamellar and equiaxed o-phase. Grepha, photomicrographs. 9 refs.
influence o f t h e degree and mode o f f o r m i n g strain on t h e cyclic m-strain a n d s t r a i n - l i f e b e h e v i o u r o f h i g h - s t r o n g t h gauge f o r m a b l e steels. Godwin, M.J. Comm. Eur. Communities Rep. No 12749 1990 117 pp
The nature o f t h e t w o o p e n i n g levels f o l l o w i n g an o v e r l o a d in f a t i g u e crack g r o w t h . McEvily, A.J. and Yang, Z. Metall. Trans. A Oct. 1990 21A, (10), 2 7 1 7 - 2 7 2 7
The research programme reported here has concentrated on examining the stress-strain fatigue properties of various high-strength sheet steel grades before and after cold working under a range of deformation modes. The materials selected were dual-phase, HSLA, rephosphorized and beke-hardenable steels and, for comparison, a low-carbon mild steel. The different modes of prestrain were introduced using laboratory equipment and included uniaxial, balanced biaxial, plane strain and pure shear strain. Cyclic properties were determined on small test pieces cut from appropriately deformed areas using servohydreulic test equipment operating under strain control. Testing was carried out on all the materials in both the notched and unnotched conditions. On the basis of the results obtained from the test programme, it has been concluded that the cyclic stress-strain beheviour of all the steels examined differed considerably from the monotonic stress-strain beheviour. In general, the cyclic strength of the prestrained steels was greater than that of the corresponding unstrained, as-received material. However, the notch fatigue strength varied with the degree and mode of strain at formation and, in certain cases, no improvement over the as-received notch fatigue strength was detected. Therefore, for design purposes, it will be safer to assume that no contribution to the fatigue strength at notches or discontinuities can be expected from increases in monotonic strength achieved by the strain at formation in pressed panels. It has been shown that a Neuber analysis can give an acceptably conservative estimate of the notch fatigue strength from smooth specimen data for all the material covered in this investigation. Graphs. 11 refs.
E n v i r o n m e n t a l effects in f a t i g u e crack initiation and propagation. Comm. Eur. Communities Rap. No 12205 1990, 65 pp The effect of environment (seawater with or without cathodic protection) on crack initiation and incipient slow growth is examined. The steel used is BS 4360-50D. The results obtained so far show that the number of cycles to initiation can be increased, compared to the values in air, by cathodic protection. The number of cycles to initiation is defined as the number of cycles to grow a crack 0.5 mm long. The prolonged initiation period is explained by a plugging effect from deposition products of calcium and magnesium. After initiation, the crack growth can be retarded or even stopped by Ca and Mg deposition products. The lower the stress ratio, the greater the ability of deposition products to slow down growing cracks. Both the initiation and the subsequent growth of cracks are influenced by the applied potential. The results show that the longest lives are obtained by potentials of -100 mV SCE. EDAX-analysis of the depoaition products shows that, at potentials higher than -900 mV SCE, the deposition products are rich in Ca, while the products are rich in Mg at a potential of -1000 mV SCE or lower, The change in the composition of the deposition products together with the increasing hydrogen uptake at decreasing potentials can explain why the longest lives are found at potentials of -1000 mV SCE. Tests performed at naturally occurring corrosion rates (0.1 mm/year) show that, under this condition, the initiation is facilitated and the growth rate is increased compared to the values in air. All the above-mentioned results have been obtained during tests performed at a frequency of 0.1 Hz. The results of tests performed at 1.0 Hz show that, at potentials of at least 900 mV SCE, one gets conservative results. On the other hand, at potentials of at most 1000 mV, longer lives are measured in tests with the higher frequency. Measurements of the precipitation rate of CaCO3 and Mg(OH)2 show that the precipitation rate is governed by the cathodic current density and is thereby strongly dependent on the potential. This result can explain the difference observed in the results of the 0.1 and the 1.0 Hz tests. Graphs, photomicrographs. 25 refs. Fracture m o d e transitions d u r i n g f a t i g u e crack growth in T i - 6 A I - 4 V alloy. Ravichandran, K.S. Scr. Metall. Meter. J u l y 1990 24, (7), 1275-1280 Fatigue fracture surface morphology has a pronounced effect on crack growth behaviour under cyclic loading conditions. By quantitative estimations of the fracture morphology, a transition from a flat transgranular to a crystallographic fracture at low values of ~,K occurs when the monotonic plastic zone size (MPZS) becomes equal to the colony size in a fine colony microstructure consisting of a thin and discontinuous ~-phase. If the ~-phase is present in thick and continuous form, as in coarse colony structures, (~-Ieths can induce crystallographic fracture when the CPZS approaches the size of ~x-leths. The asperities resulting from crystallographic fracture influence the crack closure levels and a quantitative relationship between the size scale and the magnitude of the crystallographic fracture and crack closure might exist. Ti-6AI-4V was heat treated in a vacuum at 1040 °C for 30 rain producing two different colony sizes by varying the cooling rate. One set of specimens was quenched in Ar while the other was cooled at a rate of 6 °C/rain up to 500 °C and then quenched in Ar. Fatigue crack growth (FCG) tests were performed by a manual load shedding technique. Crack closure measurements were made from unloading elastic compliance curves recorded using crack mouth opening displacement gauges. Crack paths and fracture surfaces were examined in a scanning electron microscope (SEM). The percentage crystallographic fracture at each AK level was estimated by the point count method on SEM micrographs. Graphs, photomicrographs. 10 refs.
Effect o f c o o l i n g r s t e
on creep and Iow-c~cle f a t i g u e resistance in Ti-
6242. Seal, S., Wagner, L., Lutjering, G., Pillhofer, H, and Daeubler, A. Z. Metallkd. Aug. 1990 81, (8), 5 3 5 - 5 3 9 The creep and low-cycle fatigue (LCF) beheviour of the Ti alloy Ti-6242 with a bimodal microstructure (equiaxed primary ~ in a lamellar c~+L3 matrix) was investigated for applications in gas turbine engines. The alloy was cooled from the 'bi-modal' annealing temperature (980 °C) to 600 °C at rates ranging from 2000 K/rain to 1 K/min. This wide variation simulates the cooling rates during an oil quench or
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During the period of retarded fatigue crack growth following a single tensile overload, two crack opening levels are observed. Different interpretations have been put forth concerning the nature of these opening events. In one proposal, the two opening events are conaidered to occur in plane strain, first behind the crack tip and then at the crack tip. In another proposal, the first opening event occurs in the plane strain portion of the crack front and the second in the plane stress portion of the crack front. Critical experiments designed to clarify this issue are described. In most of the experiments, the AI alloy 6061-T6 was used. Additional teats were made using the AI alloy 7090-T6 and the Ti alloy IM1829. The results of these experiments show that over the entire range of &K, including the near-threshold range, the two opening events are associated with a plane strain followed by a plane stress opening process. The results are important in that they not only shed light on the mechanism of crack retardation following an overload, but they also provide a basis for the development of a rational method of analysis for the calculation of the number of delay cycles following an overload, which includes the effects of thickness and AK level, Based upon the findings of this study, a samiempirical method for the determination of the number of delay cycles following an overload is presented. The number of delay cycles predicted by this method is found to be in good agreement with the results of the present investigation and with other results from the literature. Graphs. 14 rafs. A theoretical m o d e l for roughness induced crack closure. Ravichandran,
K.S. Int. J. Fract. J u l y 1990 44, (2), 9 7 - 1 1 0
A theoretical model for the effects of grain size on the magnitude of roughnessinduced crack closure (RICC} at a fatigue crack growth threshold has been proposed. With the basic configuration of a crack propagating incrementally along planar slip bands and being deflected at grain boundaries, an idealized zig-zag crack path is considered. The effective slip band length is considered to be equal to the grain size. It is assumed that the dislocations emitted from the crack tip upon loading, which form the pile-up, are completely irreversible and produce a combined model and mode-II displacement at the crack tip. The assumption of continuously distributed dislocations in the pile-up facilitated the calculation of crack tip sliding displacement (CTSD) along the the slip plane from Which the mode-I closure disregistry just behind the crack tip can be calculated. The closure stress intensity factor at threshold, Kcl,th could then be expressed as a function of critical resolved shear stress, average macroscopic yield stress, angle subtended by the slip plane with the crack plane and the length of the slip band. Comparisons of the predicted trends with experimental data from various alloy (e.g., steels, MAR M200, Ti-6AI-4V, Ti-6AI-5Zr-0.5Mo-0.25Si) systems indicate good agreement. Graphs. 47 refs. Inelastic d e f o r m a t i o n and f a t i g u e d a m a g e in m e t a l s at h i g h homologous temperatures. (Dissertation). Wilcox, J.R. Diss. Abstr. Int. Aug. 1990 51, (2), 319 pp The phenomenology of inelastic deformation and fatigue damage in metals at high homologous temperatures is investigated using experimental and theoretical approaches. Several examples are presented to illustrate the systematic beheviour among the stress ((~) and inelastic strain rate (E) relations for different alloy systems at high homologous temperatures. It is proposed that, at high homologous temperatures, inelastic deformation of metals and alloys at typical laboratory strain rates is controlled by grain boundary sliding in which the accommodation of flow incompatibilities is accomplished through grain boundary migration. The Sn-Pb alloy system is used as a model system for experimental study. Load relaxation experiments and constant-load creep tests are used to determine the (~-e relations at homologous temperatures as high as 0.87. Data are included for four different compositions in the Sn-Pb system: 15, 50, 62, and 70 wt.% Sn. As postulated by state variable concepts of inelastic deformation, it is found that load relaxation and creep give essentially the same ~ - ( result in those regimes where strain hardening is negligible. The results further indicate that the ¢ - ( curves for these alloys are fundamentally similar. A damage integral approach is developed to describe the fatigue of metals at high homologous temperatures. Such an approach requires a precise knowledge of the stress-time profile to provide a basis for the time integration of fatigue damage, which is quantified using fracture mechanics COncepts. Constitutive equations based on state variables are shown to be adequate for stress cycle calculations. This damage integral method is demonstrated for the case of thermal displacement fatigue in 60Sn-Pb solder material in which fatigue damage is accrued through crack propagation. The method is shown to simulate experimentally observed effects of the time-tempereture weveform on the fatigue life. The effect o f processing and m i c r o s t r u s t u r e on t h e mechanical properties o f a l u m i n i u m alloy 339-T5. (Dissertation). Crepeau, P.N. Diss. Abstr. Int. J u l y 1990 51, (1), 243 pp The influence of six microstructural variations of 339-T5 on the tensile behaviour, high-cycle fatigue crack initiation (HCF) and fatigue crack propagation (FCP) was determined. Specimens were tested at room temperature, 177, and 343°C, representing the extremes of the service conditions of diesel pistons. Specimens were cast: (1) conventionally in permanent moulds, (2) in permanent moulds plus water quenching following solidification, and (3) in a squeeze-csst press. One aeries of the above processing conditions was inoculated with P and the other, with Sr, both Si modifying agents. The phosphorus-inoculated specimens surpassed the Sr inoculation ones in tensile strength and crack initiation resistance. Water quenching after casting improved the tensile properties but the effects on crack initiation were trivial. Squeeze casting produced ambiguous results, attributed to a modest microstructural refinement (as opposed to the other conditions) and an inconsistent thermal history. Fatigue crack propagation was not sensitive to the microstructural differences. Tensile failure was controlled by the strength of the interdendritic
Int J Fatigue May 1991