The action of repeated loading, especially of repeated compression for the crystalline state of 0.41% carbon steel was investigated by X-ray. As a result it was established that the crystal fragmentation occurs when the maximum stress exceeds the lower yield point of that material but has no direct relation with fatigue failure. In the case of alternate tension and compression with equal amplitude (mean stress equals zero), the specimens break down far below the yield point, of course, but is not necessarily preceeded by the crystal fragmentation. Then, what is the true nature of fatigue of metals is our problem. According to the authors' view, failure begins when the accumulation of crystal distorsion exceeds a certain limit proper to the individual grain

Kobayashi, Atsuro

Nishihara, Toshio

English

Kyoto University Research Information Repository

RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:Repeated Stressing and CrystallineState of Annealed SteelNishihara, Toshio; Kobayashi, AtsuroNishihara, Toshio ...[et al]. Repeated Stressing and Crystalline State of Annealed Steel.Memoirs of the College of Engineering, Kyoto Imperial University 1944, 11(4): 61-641944-07-20http://hdl.handle.net/2433/280190Repeated Stressing and Crystalline State of Annealed Steel. By Toshio Nishihara and Atsuro Kobayashi. Abstract. The action of repeated loading, especially of repeated compression for the crystalline state of 0-41 % carbon steel was investigated by X-ray. As a result it was established that the crystal fragmen-tation occurs when the maximum stress exceeds the lower yield point of that material but has no direct relation with fatigue failure. In the case of alternate tension and compression with equal amplitude (mean stress equals zero), the specimens break down far_ below the yield point, of course, but is not neces-sarily preceeded by the crystal fragmentation. Then, what is the true nature of fatigue of metals is our problem. According to the authors' view, failure begins when the accumulation of crystal distorsion exceeds a certain limit proper to the individual grain. 1. Introduction. Many reports have already been published about the fatigue of metals, but we can hardly acknow-ledge that the true nature of it has been investi-gated throughout, besides, there are some literatures which contradict to each other. On the other hand, according to the writers' experiment,<0 in the case when two steel rings were rolled on each other compressed with external load crystal fragmentation was observed at the stress where no pitting occurred. Now the principal cause of pitting (Grubchenbildung) is thought to be the fatigue of that material and so it is expected that the crystal break-up will happen below the en -durance limit for repeated compression. To ascertain this point directly and to make clear the nature of , fatigue of metals the present experiment has been carried out. 2. E:xperimental Procedure. As the material was used 0.41 ~-~ carbon steel the composition of which was denoted in the table 1. The results of static tensile test were showed in the table 2. In most cases test pieces were machined to the dimension as in the figure 1, and after it in order to equalize the quality and to obtain as large grains as possible they were annealed in a vacuum furnace at 86o°C during 4 hours. Testing machine was of Haigh's electromagnetic (1) not yet published. Table 1. Composition of the specimens. C Si Mn p 0.32 Table 2. Results of tensile lest of annealed 0.41% carbon steel. Diameter before test Diameter after lest Elongation Contraction Breaking stress Tensile strength Upper yield point Lower yield point Young's modulus I t:i:: I.I') ..... d =14.175 mm d'=Io.400 ,, if, =36.7% "' =46.2,. Or=84.1 kg/mm2 OB=52.7 " Os =29•5 u a; =242 ,, E =20370,. @·~ .I. K l JJ 40 Fig. 1. type and the experiments were performed with three sorts of stress as follows : i) mean stress 1s compression, ii) mean stress is tension, iii) mean stress is zero. Stress frequency was 1800 cycles per minute. After a proper rei1etition of stresses, test piece being removed from the machine, X-ray photograph was taken from the narrowest portion of it by the back reflection method and was compared with the initial unloaded state. To aim at the same part of the test specimen each time, scratch lines which were marked on it and on the test piece holder were made to coincide at each photographing. X-ray tube was of sealed hot cathode type and the radiation CoK,1. was used. Tube current 3~_4 mA, applied voltage 30 KV, and the exposure time was between 5 and 6 hours. Diameters of the first and second slits were I mm and the distance between them was about 6o mm (fig. 2 ). T. Niskiltara and A, Kobayashi, Table 3. Test results of 0.41% carbon steel under repeated stressing. No. of Stress Mean stress test piece Diameter amplitude tla tlm mm kg/mm2 kg/mm2 HI 6.13 ±20 0 H2 6.15 ±18 0 H3 6.o6 ±IO -IO H4 6.10 ±18 -18 H5 6.05 ±14 -14 H6 6.07 ±16 -16 H7 6.09 ±IO -20 HS 6.og ±IO +20 Hu 6.05 ±12 -12 H12 6.07 ±20 0 H13 6.08 ±5 -16 H14 6.06 ±13.5 +13.5 1151 5.70 ±16 +16 H52 5.72 ±24 -24 H53 5.74 ±12 +12 ,--!l test pitzt! "' e X - ~----- ~ --; ~30_j ) .,,-. slit' --l \ 1: 1: " II :I 1: 1: !! ,: ---- incident X•ray ---60 \m , ... __ ..., :t.,.~----test piece holdef Fig. 2. . ,_.,,_,. } ~~_after e ,·twp n11D1ber of repetilionli not GI Q'Jlt.Jldillol'tld frateurecl MI-O no ~ diarllt Fig. 3. Diagram of endurance limit for repealed tension and compression. 3. Results of the experiment. I Experimental results were summarized m the table 3. Taking the stress amplitude and the mean stress as ordinate and abscissa respectively the diagram of endurance limit will be obtained as in fig1.1re 3. No. of repetitions X-rayed al Remarks N X 106 X 106 5.968 o, 3.0 fractured 12.400 o, 5.3, 7.28, 12-4 not fractured 9.020 o, 2.o6, 4.15, 6.44, 9.02 nql fractured o.58o o, 0.58 not fractured 10.860 O, 0.56, J.346, 3.08, 4.62, 6.42, Io.86 not fractured 4.000 o, 4.0 not fractured o.48o o, 0.48, I.86 not fractured 0.240 o, 0.24 not fractured 13.36o o, 13.36 not fractured 4.16o o, 0.76, 1.3, 3.22, 3.8 fractured JI.28o o, 1.97, 6.28, 11.28 not fractured 10.080 o, 10.08 not fractured 4.958 o, 0.19 fractured 4.940 fractured 10.050 o, 10.05 not fractured Two or three points representing the endurance limits •for certain mean stresses and the point cor-responding to the breaking stress nearly lie on a straight line. If the diameter of test piece is too small it yields as soon as it is set in the testing machine and the nuts are fastened as shown in photogrbphs I and 2. So in most cases test pieces of 6 mm dia-meter were used for safety and those of 5.5 mm diameter were employed only at high load. a) Case whrn the mean .ctress is compression. Static compression test was not carried out, but if it is allowed that the yield point for compression is nearly equal to that of tension, when t.he maxi-mum stress exceeds the yield point of the material there occurs a marked change on the X-ray photo-graph. That is, in the case of specimens H4 and H6 after N =0.58 x 106 and N = 1.86 x 1a6 cycles of stress respectively, the interference spots elongated along the circle and formed continuous rings (photo. 3-7). This indicates the crystal fragmentation due to slip and is supposed to occur still more early. In the case of the specimen H5 which was stressed a little above the lower yield point, similar change occurred after a large number of repetitions (photo. 8-1 I). Stress a little below the lower yield point was applied to Hr I and one can hardly perceive similar change after N = I 3.36 x 1a6, but slight blurring seems to appear in many spots (photo.. I 2-I 3 ). Specimen H3 was stressed still lower than the yield point and after N =9.02 x l06 on the photo-graph we can read no conspicuous change mentioned above except faint blurring of interference spots Repeated Stressing and Crpta!li11e State of Annealed Steel. (photo. 14-16). Test piece H 7 was subjected to the repeated loading whose amplitude was small but whose maximum value exceeded the yield point of the material and already at N = 0.48 x l06 there was noticed a marked change as shown in photo. I 8. After all, a remarkable change such as the crystal smashing happens when the maximum stress goes over the lower yield point of the material, and it was decidedly confirmed by the specimen H13 that the mean stress has no direct relation with crystal break-up. Equal mean stress to H6 was applied to it and at N = I 1.28 x 1d there was observed no sign of crystal fragmentation. (photo. 19-21). b) Case when the mean stnss i's lensi', 111, Specimen H 5 I was subjected to the stress whose maximum value passes over the yield point for tension. After only N =O. 19 x 1d cycles con-tinuation of the interference rings was observed corresponding to the crystal break-up (photo. 22-23) and failure occurred at N =4.96 x 1a6. Similarly H8 yielded obviously at N =0.24 x l06 (photo. 24-25), but the applied stress lies far below the en-durance limit. Maximum stress slightly above the lower yield point was applied to the specimen H14 which showed the yielding of the material at N = 10.08 x 106 but not fractured. H53 was tested at aa ;:, ± I 2 kg/mm2, a,,. = + 12 kg/mm2, that is, slightly below the yield point, and as the result it showed a faint continuation of .interference rings after N = 10.05 x 1c6 still leaving many clear spots on the film (photo. 26-27). In short, at the tension side also, crystal fragmentation happens whenever the maximum stress exceeds the lower yield point and this agrees with Gough's experiment. <2J c) Cal'c when the mean stress is zero. According to the above experiments it was ascertained that the crystal fragmentation has no direct relation with fatigue failure. To inquire into the true nature of fatigue ex-cluding unnecessary conditions is our next problem. Test piece H I 2 was stressed just above the endurance limit and was X-rayed at N =0.76, 1 .3, 3.22, 3.8 X 1d. Photographs show gradual increase of blurred spots (photo. 28-3 I). At N = 3.8 x 1d, that is, 0.36 x 1c6 cycles before fracture clearness of spots is observed. This indicates the dissolution of lattice distortion in most crystal grains. To Hz was ayplied the stress below the fatigue limit. After N = I 2.4 x 106 no mar_ked- change is noticeable, but careful observation exposes a slight increase of blurred spots suggesting lattice deforma-tion (photo. 32-35). 4. General Consideration of Results. The authors reported formerly the experimental research on the fatigue of .0.34 % carbon steel sub-jected to rotary bending .. Pli; In that case also, clear interference spots decreased gradually before fracture and blurred spots indicating crystal distortion in-creased as a whole. This agrees with the result of the present experiment, however, whether the crystal distortions decrease or not just befor fracture it was unable to confirm fully owing to grain fineness. Below the endurance limit also a slight increase of crystal distortion was noticed. And from the present experiment it was established that when the maximum stress exceeds the lower • yield point of the material crystal split takes place but at the lower stress it does not. These facts are very im-portant on considering the mechanism of fatigue rupture. According to Gough and Wood,<4> crystal frag-mentation must occur before fatigue failure, however, as Moller's experiment<•l indicates it does not neces-sarily so. In the present exr:,eriment the sr,ecimen H12 which fractured at N=4.16x 1d showed no sign of continuation of interference rings at N = 3.8 x 1a6. On one side crystal break-up does not necessarily arouse fatigue fracture, on the other side fracture happens without crystal fragmentation. In other words, at least the crystal break-up detectable by X-ray has no direct relation with fatigue failure. What is the principal factor whkh governs fatigue phenomenon ? From the former and present experiments it is considered to be the crystal distor-tion. As Timoshenko, Dehlinger and others state<6l, the sort of stress and its magnitude which acts on individual grain constituting the metal material are all remarkably inhomogeneous. Different grains have d1fferent shapes, sizes and orientations of slip planes relative to the external force, and from thc;:se it is possible that even when one grain produces slipping adjascent grain remains in the range of elastic deformation. Besides the simple elongation, compression or rotation, especially owing to the complexity of grain forms and to the interference of neighbouring , grains and perhaps accompanying (2) II. J. Gough, W. A. Woocl, Proc. Inst. Mech. Eng., 141 (19J9), 175. (3) T. Nishihara, A. Kobayashi, Trans. Soc. Mech. Eng. Japan, Vol. 8 (1942), I-194. (4) II. J. Gough, W. A. Wood, Proc. Roy. Soc., 154 (1936), 510; 165 (1938), 358. (5) II. Moller, M. Hempel, Mitt. K. W. I. Eisenf., 20 (1938), 15. · (6) Timoshenko, Strength of Material,, II, (1930), 668; U. Dehlinger, z. Physik, 115 (1940), 625; F. Wever, W. E. Schmid, Mitt. K. W. I. Eisenf., lI (1929), II4. T. Nishihara and A. Kobayashi. with slip itself inhomogeneous lattice deformations such as in torsion or in bending will occur in greater majority and on account of the interferences from neighbouring crystals etc. these deformation does not necessarily die away even if the external load was removed. During the repetitions of stress such lattice deformation reaches a certain limit causing finally crystal bursting. This is not same as an ideal slip and is supposed to generate a sort of weak point, a submicroscopic crack so to speak. And with the occurrence of these submicroscopic cracks here and there, after a certain period there comes the final rupture. Accordingly, the physical meaning of the en-durance limit is interpreted as follows : Above the endurance limit the number of crystal grains with unrestorable lattice distortion increases gradually, the degree of distortion finally exceeding a certain limit and here the submicros-copic cracks are produced. Then a macroscopic crack will develop rapidly connecting those cracks. Below the endurance limit, the lattice deforma-tion mentioned above does not exceed a definite limit proper to that crystal or, if not so, very few crystals pass over that limit. Crystal lattice distortion is unable to detect through the microscopic examination and even by X-ray it is attended by considerable difficulties. Gough states<7> there is no structural change below the endurance limit, however, as he did not com-pared with the initial state of the same s.i:;ecimen, perhaps the slight change such as crystal distortion was overlooked. About this roint our experimental result agrees with Barrett's experiment. cs> Moller and others say in their reportsC9> that change in the X-ray photograph is very little even above the endurance limit and they do not toutch the problem of crystal d_istortion. But there is a doubt about the generality of their assertion, for they used 0.02 % carbon steel. 5. Summary. An experimental investigation on annealed 0.41 ~,G carbon steel subjected to repeated tension and compression was carried out. As the results the following facts were confirmed : i) When the maximum stress exceeds the lower (7) Joe. cit. in (4). yield point of the material crystal fragmentation due to slip occurs producing a remarkable change on the X-ray photopraph. When the maximum stress is below the lower yield point, above mentioned change of crystalline state cannot be observed. Mean stress has no direct relation with this sort of phenomenon. ii) Crystal fragmentation has no direct relation with fatigue fracture, that is, crystal break-up into smaller crystallites does not necessarily result in fatigue failure, while rupture occurs irrespective of grain size. iii) Increase of crystal distortion was recognized before fatigue breakdown. vi) Slight . increase of crystal strain was also per-ceived below the endurance limit, in other words, there is no conspicuous distinction above and below the endurance limit. Referring the authors' experiment already re-ported and other literatures the nature of fatigue fracture is considered to be as follows : a) Owing to the inequality of sort and magnitude of stress for individual grain and accom-panying with slip within the crystal, there occurs inhomogeneous lattice deformation such as in torsion or in bending and this does not vanish even if the external force is removed. b) Under the stresses above the endurance limit such a lattice deformation goes over a certain limit permissible from the grain characteristic gen-erating a submicroscopic crack. Majority of these cracks arouses a macroscopic crack which ultimately leads to fracture. c) Below the endurance limit the lattice dis-tortion in the individual crystal does not pass over the definite limit or at least very few grains are considered to pass over that limit. According to the authors' view remaining pro-blem is to demonstrate the growth or the change of crystal distortion directly or indirectly and to explain various facts accompanying fatigue phe-nomena. Acknowledgment. Necessary funds for, the pressent research were defrayed from "Hattori Hokokai ". Also the authors express their gratitude to Dr. K. Kojima for his kind advice. · (8) C. S. Barrett, Metals anJ Alloys, 8 (1937), 13; Trans. A. S. M., 25 (1937), n15. (9) Joe. cit., also II. Moller, M. Hempel, Z. V. D. I., 83 (1939), 1183; II. Moller, St. u. E., 58 (193.8), 499; F. Wever, M. Hempel, H. Moller, Arch. Eisenhiittenw., IJ (1938), 315; F. Wever, Z. V. D. I., 82 (19,8), 28; F. Wever, II. Moller, Naturwiss., 25 ( 1937), 449· Photo. 1. Diameter 4 mm, initial state. Photo. 2. Diameter 4 mm, set in the testing machine and removed immediately, shows the yielding of the specimen. Photo. 3. Specimen H4, initial state. Photo. 4. Specimen H4, 11"=±18, 11,n=-I8, N=o.58. Photo. 5. Specimen 1I6, initial state. Photo. 6. Specimem H6, 11a=±I6, 11,,.=-16, N=I.86. Photo. 7. Specimen. H6, da=±l6, d,n=-16, N=4. Photo. 8. Specimen II5, in itial state. Photo. 9. Spebimen H5, da=±14, d-,,,=-14, N=o.58. Photo. 10. Specimen H5, da=±I4, dm=-14, N=3.08. Photo. I 1. Specimen H5, da=±l4, d,,.= -14, N=Io.86. Photo. 12. • Specimen HI 1, initial state. Photo. 13. Specimen I-III, <1t1=±12, <1m=-I2, N=13.36. Photo. 14. Specimen H3, initial slate. Photo. 15. Specimen H3, <1a=±Io, <1m=-Io, N=6.44. Photo. 16. Specimen H3, Photo. 17. Specimen H7, initial state. Photo. 18. Specimen H7, <1a=±io, <1m=-10, N=o.48. Photo. 19. Specimen H13, initial state. Photo. 20. Specimen H13, <1a=±5, dm=-16, N=I.97-Photo. 21. Specimen H13, da=±s, <1,n=-16, N=u.28. Photo. 22. Specimen H51, initial state. Photo. 23. Specimen H51, <10 =±16, <1-,,,=+16, N=o.19. Photo. 24. Specimen HS, initial state. Photo. 25. Specimen H8, On=±IO, Om =+zo, N = o.54. Photo. 26. Specimen H53, initial state. Photo. 27. Specimen H 53, Oa=±l2, Om= +12, N = Io.05. Photo. 28. Specimen II12, initial state. Photo. 29. Specimen H12, 0'0 = ±20, thn= O, N = I.J. Photo. 30. Specimen H12, Oc, =±20, Om =O, N=3.22. Photo. 31. Specimen H1z, dn=±20, d m, =O, N=3.8, clearness of spols noliceable. Photo. 32. Specimen Hz, initial state. Photo. 33. Specimen Hz, a0 =±I8, a.,,.=o, N=5.3. Photo. 34. Specimen Hz, <1a =± I8, d n,=O, N=7.28. Photo. 35. Specimen Hz, <1a=±I8, d n,=O, N=IZ.4. 

Repeated Stressing and Crystalline State of Annealed Steel

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Repeated Stressing and Crystalline State of Annealed Steel

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