Residual stress concentration and its effects on fatigue limit and short crack growth

removed almost all the residual stresses at the weld toe, improving the fatigue strength drastically. The values of stress concentration factor K~ were estimated by Meahida's method and it was concluded that the thickness effect resulted from a combination of both stress concentration end residual stresses, with the contribution of the latter being particularly significant for N~ 106 cycles. Graphs, 6 ref. Fatigue crack p r o p a g a t i o n b e h a v i o r o f w e l d e d material S M 5 3 A under negative stress ratio condition. Makabe, C., Sakihama, H., Kaneshiro,

H. and Nishida, S.-I. J. Soc. Mater. Sci. Jpn (Nov. 1992) 41 (470), 1642-1647 (in Japanese)

The crack propagation behaviour of welded materiel SMSOA under • negative stress ratio condition was evaluated with the maximum stress intensity factor /
Wakai, T., Sakane, M., Ohnami, M., OkRa, K. and Fukuchi, Y. J. Eng. Mater. TechnoL (Trans. ASME) (Jan. 1993) 115 (1), 109-115 The high-temperature low-cycle fatigue of type 304 stainless steel and alloy 718 auperalloy friction-welded joints is assessed. Strain-controlled low-cycle fatigue tests for 304-304 and 718-718 friction-welded specimens were carried out at 923 K in air to obtain the fatigue strength of the joints. These materials were selected as the cyclic hardening and softening materiels, respectively. The 304-304 welded specimens showed inferior fatigue strength in comparison with the base metal while the 718-718 specimens exhibited fatigue strength equivalent to that of the base metal. The difference in the fatigue strength between the two materials is discussed from the viewpoint of the cyclic deformation beheviour and strain reduction at the weld interface. Graphs, photomicrographs, 6 ref. Fatigue life prediction at t h e stress concentrated region st t h e m i l l housing. Im, S.W., Kweon, Y.G. and Chang, R.W.

Research Institute of Industrial Science and Technology (Pohang City) Technical Research Report (Dec. 1992) 14 (48), 6 7 2 - 6 7 8

Fatigue lifetime of a mill housing was estimated for the stress-concentrated region of the nut hole shoulder by means of calculation of the local stress by FEM. It was found that the local stress would be above the allowable stress for the nut shoulder and a fatigue crack could be initiated. Using a modified Miner's rule, the fatigue crack initiation lifetime of the mill housing was predicted. Surface cracking was found to be more dangerous than internal cracking. It is emphasized for the safety of the rolling equipment that fatigue cracking should be detected before the crack size reaches the critical size. Graphs, 7 ref. E n v i r o n m e n t a l f a t i g u e in a l u m i n u m - l i t h i u m alloys. Piascik, R.S. N A S A Report N92-32423/5/XAB (1992) 27 pp AI-Li alloys exhibit similar environmental fatigue crack growth characteristics compared with conventional 2000 series alloys and are more resistant to environmental fatigue than 7000 series alloys.The superior fatigue crack growth bebeviour of AI-Li alloys 2090, 2091, 8090 end 8091 is due to crack closure caused by tortuous creak path morphology and crack surface corrosion products. At high R and reduced closure, chemical environment effects are pronounced, resulting in accelerated nser-thrsehold da/dN. The beneficial effects of crack closure are minimized for small cracks, resulting in rapid growth rates. Limited date suggest that the 'chemically small crack' effect, observed in other alloy systems, is not pronounced in AI-Li alloys. Modelling of environmental fatigue in AI-Li-Cu alloys related accelerated fatigue crack growth in moist air and salt water to hydrogen embrittlement. Advances in f a t i g u e life prediction m e t h o d o l o g y f o r metallic materials.

Newman, J.C. N A S A Report N92-34191/6/XAB (1992) 46 pp The capabilities of a plasticity-induced creak-closure model to predict small- and large-creak growth rates, and in some cases total fatigue life, for four AI alloys and three Ti alloys under constant-amplitude, variable-amplitude and spectrum loading are described. Equations to calculate a cyclic-plastic-zone corrected effective stressintensity factor range from a cyclic J-integral and creak-closure analysis of large cracks are reviewed. The effective stress-intensity factor range against crack growth rate relations are used in the closure model to predict small- and large-crsck growth under variable-amplitude and spectrum loading. Using the closure model and microstructurel features, a total fatigue life prediction method is demonstrated for three AI alloys under various load histories.

it is found that the wall-defined matrix structure inherited from the very efficient cyclic hardening during the ramp-treatment promotse very uniform and homogeneous structures of primary dislocations, i.e. PSB/and wall structure from grain to grain. These structures favour strain localization, and thus lower hardening rates in the cyclic strsss-strsin curves (CSSC) of ramlPtreeted Cu polycrystsls ere obtained. However, plateau-like beheviour was not obsorved. The presence of a weak but noticeable <111>-<100>- 'herd" texture in the ramp-treated specimens studied suggests that the observation of a plateau in the CSSC of polycryetala may be very sensitive to texture. Microstructure, expressed in terms of a complex factor of combined grain size and texture, showed a very significant effect in the cyclic response of Cu at amplitudes where structures which localize deformation are expected to be present. Initial homogeneous multiple slip promotes faster substructure evolution into ceil structure, which accounts for the low levels of localization of deformation observed in the CSSC of ccerse-grained Cu. Finally, a polycrystalline model based on treating differently oriented grains as composite material is shown to account qualitatively for most of the results. A n investigation into t h e m u l t i a x i a l f a t i g u e b e h a v i o r o f IMI 829 t i t a n i u m alloy.

Gerbec, D.E. Diss. Abstr. Int. (Jan. 1993) 53, (7)

This document studies the effect that multiaxiel stress fields have on the fatigue behaviour of IMI 829 titanium alloy, a material often used for compressor d i ~ in gas turbine engines. Experimentation has included the cyclic testing of rotating disc specimens of varying geometries as well es ASTM E-647 crack growth testing employing standard compact tension specimens. Frectographic and microstructursi examinations were also conducted. It was observed that the disc specimens demonstrated lower crack growth rates for a given stress intensity range than did the compact tension specimens. This beheviour was explained by considering the effect of the multiaxisi stress field on the crack tip opening displacement. Any loads parallel to a given crack will effectively reduce the crack tip opening displacement owing to the Poisson effect, resulting in a smaller crack tip plastic zone, which has been previously related to crack growth rate. Frsctogrephic examination of the disc and compact tension specimens indicated that the disc specimens were much more likely to form striations then were the compact tension specimens. This was explained by the fact that the test conditions differed between the disc and compact tension specimens. Owing to mechanical limitations, the cyclic frequency of the disc specimens was considerably lower than the cyclic frequency of the compact tension tests. Differences in test temperature may also have influenced the change in fracture surface between the specimens. The effect that microstructure had on the fatigue crack growth in the disc specimens was investigated as wall. It was observed that the crack generally folJowsd a tranegrenular path. It was also noted that, when possible, the crack would attempt to cross prior beta grain boundaries at right angles. The existence of grain-boundary-aligned alpha was found to influence this behaviour. Propagation o f f a t i g u e s h o r t cracks in steel 16Mn. Wang, W.M., Huang, Y.H. and Xu, H. Acts Metafl. Sinica (China) (Sept. 1992) 28 (9), A 4 1 6 - A 4 2 0 Small bar specimens of steel 16Mn with four blind-holes were fatigued under constant amplitude loading on a rotating bending fatigue machine. Short creak propagation emanating from the blind-holes was obtained. Based on the expedmentel results, an expression of average propagation rate of short cracks is proposed. The statistical beheviour of short crack propagation is also discussed and the evolution of surface sizes of short cracks with fatigue cycle number is given. Graphs, 5 ref. In situ o b s e r v a t i o n o f d e f o r m a t i o n and fracture f o r superalloy GH169 under c o m b i n e d f s t i g u e - c r c o p action. Tian, C.S., Wang, M.-S., Hou,

T.C. and Dan, X.A. Acts Metall. Sinica (China) (Sept. 1992) 28 (9), A 4 2 9 - A 4 1 5 In-situ high-temperature metalloscopic observation was made of deformation and

fracture for superalloy GH169 under the combined action of fatigue and creep. The mode of deformation is revealed as slipping, twinning and grain boundary sliding. The mode of failure is dependent upon microstructure. The intergranuler cracks arise from W-type voids produced by stress concentration at the triple point which would not be relaxed by interior deformation of grains and local deformed region along grain boundaries, and its propagation mechanism is nucleation, growth and linkage of cavities at grain boundaries. Transgranular cracks result from deformation damage of grains, and their propagation mechanism is shear rupture along the slip plane. Photomicrographs, 8 ref. Residual stress concentration and its effects o n f a t i g u e l i m i t and s h o r t

creek growth. He, J.W., Hu, N.A. and Zhang, D.Q. Acts. Metall. Sinice (China) (Sept. 1992) 28 (9), A 4 0 4 - A 4 0 8 Residual stress concentration at a notch depends on both the notch geometry and the yield strength of the material. It varies through the depth, and its magnitude may be higher than the theoretical one. Compressive residual stress concentration at the notch of a shot-peened specimen of soft matedal (e.g. 40Cr) can easily be relaxed, with the surface damage during shot-peening resulting in a minor contribution to the fatigue limit. Compressive residual stress increases the crack closure effect st the notch and may lead to a non-propagating crack. Graphs, 6 rat.

Cyclic response and s u b s t r u c t u r e e v o l u t i o n o f c o p p e r polycrystals.

Lianas, L.M. Diss. Abstr. Int. (Jan. 1993) 53 (7)

T w o critical stress intensities f o r threshold f a t i g u e crack propagation.

Studies on the cyclic response and the substructure evolution of copper polycrystsis have been carried out in the high-cycle fatigue regime. The effects of ramp-loading and microstructure have been investigated using various mechanical tests and electron microscopy. Investigations on the influence of loading mode on the cyclic response of fine-grained polycrystalline Cu and the associated dislocation structures showed that the saturation behavJour under constant load control, for two sets of specimens, with and without initial ramp-loading, exhibits strong differences in the 'intermediate" range of stress amplitudes, i.e. 70-98 MPa. Within this range, the remp-losding mode promotes a gradual substructure evolution, which leads to localization of slip in primary systems and the formation of persistent slip bends (PSBs), whereas conventional loading leads to the formation of elongated cells and multiple sets of wall structures (e.g. labyrinth structure), both intimately associated with multiple slip conditions. In studying ramp-loading as a mechanical pratreatment

Analysis shows that two stress intensity parameters are required to define uniquely the mechanical driving forces for a crack advance under cyclic loading conditions.The concept suggests that two threshold criteria, zlKth end /(max, rather than one, have to be fulfilled simultaneously for fatigue crack growth. Literature results on a wide range of materials support this rationale. Several fatigue phenomena - the effects of microstructure, environment and load ratio (including negative R-retio} - can be rationalized in terms of these two parameters without involving crack closure. The concepts presented are intrinsic to fatigue damage and hence are independent of any other superimposed factors. Finally, it is shown that &Kth and Km=x vary proportionately and the proportionality constant depends on the nature of the crack tip environment. The existence of these two thresholds is independent of specific

156 Fatigue, 1994, Vol 16, February

Vasudevan, A.K., Sadananda, K. and Louat, N. Scr. Metall. Mater. (1 Jan. 1993) 28 (1), 6 5 - 7 0