A note on short-crack generation techniques

the procedure composed of cutting, polashlng by emery paper and buff polishing was utdlzed previously. Here, an improved method using Mlcrotome to reveal sharp striation sections is proposed and the improvement of measurement accuracy is shown. In addition to 2017-T4 AI alloy, type 304 stainless steel was investigated to study the effect of mechamcal properties on the relationship between His and R The experimental results gave almost the same relationship for type 304 stainless steel and 2017-T4 AI alloy This result Imphes that the H/s-R relationship is independent of material properties, suggesting that one master curve of H/s-R and striation spacing s gives crucial information about the service road at the occurrence of fatigue fracture accidents Graphs, photomicrographs. 16 refs

A statistical study on fatigue life distribution of plate specimens having three defects through thickness. Okada, K., Takagi, S. and Tanaka, T. J. Soc. Mater. Sci. Jpn Oct. 1991 40, (457), 1317-1323 (in Japanese) Fatigue tests were carried out under axial loading by using A2017-T3 AI alloy plate specimens having three through-thickness defects to investigate statistically the crack propagation behavlour and their coalescence leading to a failure of the specimen. The important findings are summarized as follows. There was little difference between the number of cycles to crack coalescence and that to failure. Based on the d~strlbutlon characteristics of crack length at an arbitrary number of cycles, probebilistlc evaluations were made on the coalescence of fatigue cracks, and the distribution of the numbers of cycles to crack coalescence has been theoretically derived on the basis of crack coalescence probability. The analytical results obtained were In good agreement with the experimental trends Graphs, numeric data. 8 refs.

Characterization of the topography of turned surfaces and its influence on the f a t i g u e life o f AI-7075. Wiesner, C,, Kunzi, H.-U. and Ilschner, B. Mater. Sci. Eng. A Oct 1991 A145, (2), 1 5 1 - 1 5 8 The surface topography of conventionally turned specimens (e.g. 7075AI) has been measured using optical profilometry and characterized by (a) a combination of the height d:strrbution parameter Rq and the wavelength-sensitive parameter (I) and (b) the profile length ratio,/e the ratio of real profile length to measuring length. Fatigue experiments have been conducted on specimens with different surface topographies, and the numbers of cycles to rupture can be related to these surface topography parameters. Graphs, photomicrographs 29 refs

A note on short-creck generation techniques. James, M.N. Mater. ScL Eng. A Oct. 1991 A417, (1), L 1 3 - L 1 6 Two commonly used techmques for generating small cracks revolve either machining away of the wake of long through-thickness cracks to leave a so-called twodimensional short crack, or rnltiatmg cracks at high stresses (at or above yield) followed by anneahng to restore nominally elastic conditions. However, artefacts of these crack-generation schemes appear to affect the apphcabdl W of such short-crack data to the case of naturally occurring defects in engineering hardware. This letter considers this problem for short cracks m Q1N (HY80) steel Graphs. 9 refs

Case histories on surgical implants and their causes. Zitter, H. Werkst. Korros. Sept. 1991 42, (9), 4 5 5 - 4 6 6 (in G e r m a n ) The third decade of an evaluation of Inadequate =mplants reveals that the kinds of critical cases have not changed essentially. These cases are corrosion fatigue or fatigue caused by structural grooves and local corrosion. Pitting and crevice corrosion were exclusively observed on implants of Cr-Ni-Mo steels with insufficient pitting res*stance equivalents (% Cr + 3× % Mo). Fatigue is also brought about by the fact that the mechanical strength of implant alloys IS often not optimally adjusted to the specific functmn of the implants. On these requirements national and international standards are not sufftcient. Graphs, photomicrographs. 06 refs O p t i m i s a t i o n o f f a t i g u e resistance of a hydraulic cylinder housing.

Samuelaon, J., Holm, D. and Esping, B. Mater. Des, June 1991 12, (3), 1 4 7 - 1 5 7 The optimtzatlon of the fatlgue-crittcal part of a hydraulic cylinder ps discussed. The Investigation Included optimization, fatigue testing, residual stress measurements, life predictions and rellabihty computations. The optimization reduced the critical parameter, the stress intensity factor in the end weld root, by 40%. The fatigue tests verified this reduction. Life predictions based on available da/dN data, combined with computed and tabulated stress intensity factor solutions, were in good agreement with the fatigue test data. Fractography of the fatigue sites revealed the existence of initial cracks of size 0.02-0.06 mm m all the welded joints examined Graphs, photomicrographs. 22 refs

In situ fracture techniques for studying transient reactions with bare steel surfaces. Wei, R.P. and Alavi, A. J. E/ectrochem. Soc. Oct. 1991 138, (10), 2 9 0 7 - 2 9 1 2 To assist In understanding corrosion fatigue and stress corrosion growth m aqueous envrronments,/n sltu fracture techniques have been developed to study the transient reacttons wtth bare steel surfaces (304, 4340). The techniques utdlze the conventional three-electrode half-cell configuration, and two-electrode and four-electrode analogues of the local electrochemical cell at the crack tip. The measurement techniques and some initial test results are described and are discussed m relation to corrosion fatigue crack growth response in steels Graphs, 16 refs.

An application of incremental plasticity theory to fatigue life prediction o f steels. Chen, W.R. and Keer, L.M. J. Eng. Mater. TechnoL (Trans. ASME) Oct. 1991 113, (4), 4 0 4 - 4 1 0 An incremental plasticity model is proposed based on the yon Mises ymld condttlon, associated flow rule, and non-hnear kinematic hardening rule In the present model, fatigue hfe prediction requires only the unlax=al cycle stress-strain curve and the umaxial fatigue test results on smooth specimens. Experimental data of 304 stainless steel and 1045 carbon steel were used to validate this analytical model. It ~s shown that a reasonable description of steady-state hysteresis stress-strata loops and prediction of fatigue lives under various combined axial-torsional Ioadings are given by this model Graphs 31 refs.

70

Application o f m i n i m u m e n e r g y f o r m a l i s m in a m u l t i p l e slip band model for fatigue. I1. Crack nucleation and d e r i v a t i o n o f a generalized C o f f i n - M a n s o n law. Venkataraman, G., Chung, Y.W. and Mura, 7". Acta Metal/. Mater Nov. 1991 39, (11), 2 6 3 1 - 2 6 3 8 A situation of multiple parallel slip bands ~s considered Each slip band is modelled as an accumulation of edge dipoles. The dislocation dipole density within the sl~p bands continually increases with fatigue cycling The strain energy density within the s)tp bands consequently builds up wtth cychng until a critical cycle number when it becomes energatlcally favourable to nucleate a mlcrocrack within one of the shp bands. This is proposed to be the crack nucleation cycle number The above crpterlon based on the strain energy density is used to derive an exphclt expression for the number of cycles for crack nucleabon. It =s shown that there FS a parallel between the criterion for crack nucleation and other crltena that are based on the accumulation of a certain critical amount of damage A generalized Coffin-Manson law for crack Initiation is also derived. Copper is used for companson purposes. The size and most likely site of the lust-nucleated crack is discussed Graphs. 46 refs

Application of minimum energy formalism in a multiple slip bend model for fatigue. I. Calculation o f slip band spacings. Venkataraman, G., Chung, Y.W. and Mura, T. Acta Metal/, Mater. Nov. 1991 39, (11), 2 6 2 1 - 2 6 2 9 A theory of shp band spacing m fatigued materials has been developed based on a criterion of minimum strain energy accumulation within shp bands. A twodimensional, quasl-monotomc, dipole pde-up model is employed for this purpose. Multiple parallel, equally spaced, mutually interacting slip bands are considered. Each slip band is modelled as an accumulation of dipoles Partially Irreversible slip processes with stochastic fluctuations are allowed m the model, it is shown that for a given Imposed plastic work, there exists a unique configuration (number and spacing) of the bands that has the minimum internal energy stored within all bands. An expression for the opttmum slip band spacing in potycrystalhne materials has been derived based on all experpmentally determinable parameters. The effects of plastic strain amphtude, temperature and environment on this optimum slip band spacing are assessed. There is reasonably good agreement between theory and experiment Graphs, photomicrographs. 28 refs.

Surface-initiated spalling fatigue in M-50 and M-50 NiL bearings. Averbach, B.L., Van Pelt, S.G., Pearson, P.K. and Bamberger, E.N. Lubr. Eng. Oct. 1991 47, (10), 8 3 7 - 8 4 3 The initiation and development of spelling fatigue in M-50 and M-50 NIL thrust bearings running under partial EHD lubrication condltmns have been studied, The rnner and outer rings of the M-50 NIL bearings were plated with thin hard dense Cr (TDC), and the bearing was assembled w~th M-50 balls. A spectrum of initial surface flaws was introduced during a prescribed run in a separate test machine in which the oit had been contaminated with 2.5 ppm of 20 p,m alumina. The inibal flaws propagated by a mlcropeehng mechanism, and these developed into macroscopic spells when the peeled regton reached a critical size. The L 10 life of the surface-damaged M-50 bearings was 87 h, compared with a calculated AFBMA L 10 of 24 h. The M-50 NIL, TDC rings had an L 10 of 480 h, under the same test conditions. The enhancement of the life of M-50 NIL, TDC over M-50 by a factor of about six Is ascribed In part to the presence of residual compressive stresses in the subsurface regions of the M-50 NIL rings Graphs, photomicrographs. 7 refs

Fatigue studies in AI-Mg-Si alloys. Gungor, S. Diss. Abstr. /nt. Oct. 1991 52, (4), 157 pp An investigation into the effect of mlcrostructure and forming process on both fatigue life and small crack growth behavlour of AI-Mg-Si alloys Is presented. The influence of slip dtstnbution on initiation and propagation of small fatigue cracks has been examined m a series of wrought AI-Mg-SI alloys which contain differing volume fraction of dispersolds. The slip homogemzation caused by Mn-bearing d;spersolds profoundly increases the alloy's resistance to the propagation of long fatigue cracks owing to the decrease tn intergranular failure. But the extent of this effect on the propagation of small cracks has been found to be less pronounced because of the frequent interruption of the growth of small cracks at the microstructural features and the decrease in effective driving force for crack growth by crack deflection, so that the increase in growth rates due to intergranular failure will be less in small cracks. Thus the increase in fatigue life seen with increasing homogenization of slip is shown to be due to the relative ease of fatigue crack initiation in alloys exhibiting mhomogeneous slip. The fatigue crack initiation and small crack growth behaviour of wrought, forged and squeeze-cast AI-Mg-Si alloys have been examined to explain the effect of manufacturing process on fatigue life. The squeeze-cast alloys shows the lowest fatigue limit because of its lower tensile strength compared with the wrought and forged alloys. It also has the worst ratio of crack imtlatmn life to the total fatigue life owing to the presence of mlcroporosity which ease the format=on of cracks. However, the d;fference in their small crack growth rates controls the Iow-hfe high-stress region of the S-N curve, where the wrought alloy shows the lowest lives whereas the forged alloy shows the highest Fatpgue tests on the as-formed surface specimens of the forged and squeeze-cast alloys showed that surface texture has little effect on small fatigue crack growth rates. However, coarse particles and large defects introduced into the surface during the forming process of the forged and squeeze-cast alloys eased the formation of cracks and consequently reduced their fatigue bruits by 20 and 25%, respectively. The low-cycle htgh-stress region of the S- N curves of the alloys has been successfully predicted usrng the small crack growth data However, at stress levels near the fatigue limit, calculations from the small crack growth data underestimate the fatigue hves because a significant fraction of the hfe is spent in temporary crack arrests. Elastic stresses, twin boundaries and fatigue cracking. Diss. Abstr. Int. Oct. 199t 52, (4), 254 pp

Gopa/an, P.

Prior flmte element method (FEM) calculations had shown that high-cycle fatigue cracks would start at twin boundaries, when the stress axis made an angle other than 90° to the twin boundaries, because elastic interactions would produce highest stresses there. This investigation was undertaken to test this idea. Finite element method analysis was carried out for twin orientations of 70-30 ~-brass, with varying twin boundary angles with respect to stress ax~s,to determine the stress distribution in the vicinity of the boundary The stresses were resolved on to 12 possible slip systems From these data the orientations which develop highest stresses at the boundary, due to elastic interaction, on slip systems parallel to the twin boundary

Int J Fatigue January 1993