SUBSURFACE CRACK INITIATION DURING FATIGUE AS A RESULT OF RESIDUAL STRESSES. (Received 1 I May 1979)

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1 Ftigue of Engineering Mterils nd Sfrucrures Vol. 1, pp. 31%327 Pergmon Press. Printed in Gret Britin. Ftigue of Engineering Mterils Ltd SUBSURFACE CRACK INITIATION DURING FATIGUE AS A RESULT OF RESIDUAL STRESSES P. STARKER, H. WOHLFAHRT nd E. MACHERAUCH Institut flir Werkstotlkunde I, Universitlt Krlsruhe (TH), Krlsruhe, West Germny (Received 1 I My 1979) Abstrct-The influence of shot peening on the bending ftigue strength of hrdened specimens of crbon steel is reported. Erects of residul compressive stresses fter shot peening, s function of distnce from the surfce, re discussed long with the evidence of scnning electron microgrphs from frctured specimens. Subsurfce crck initition is reported t ll stress mplitudes below threshold vlue of 1100 N/mm2. Assuming tht the ftigue strength is enhnced loclly due to compressive residul stresses the experimentl results cn be explined with the id of the Goodmn reltionship. INTRODUCTION A NUMBER of investigtions hve shown tht s consequence of ftigue, subsurfce crck initition is possible in thermochemicl treted steel specimens [l-31. Recently, even in hrdened steel specimens with ground surfces, indictions of subsurfce crcking due to ftigue-bending were found [4]. In both cses mcroscopic residul stresses were ssumed to be one reson for the shift of crck initition from the surfce to the interior of the investigted specimens. On the other hnd, in very comprehensive pper, the influence of surfce roughness, surfce hrdening nd surfce residul stresses on the ftigue strength of crbon steel (0-45 wt-%c) in different het-treted nd mchined conditions ws successfully determined [4, 51. It ws shown tht the influence of residul stresses on the ftigue strength strongly depends on the the ultimte tensile strength of the mteril. In the cse of soft nneled specimens, no influence of residul stresses on the ftigue limit ws estblished. In the cse of hrdened specimens, however, the reltionship ACT^ = -0.4A0, (1) ws fulfilled relting the incrementl difference in reversed bending ftigue strength Ace nd ACT,, the surfce residul stress component prllel to the pplied stresses. All experiments supporting the vlidity of this eqution hve been performed on ground specimens with surfce residul stresses between N/mm2 nd N/mm2. As shot peening provides residul stress distributions below the surfce of the specimens s compred to grinding, it seemed pproprite to (1) verify whether the bove-mentioned eqution is lso vlid for high mgnitudes of residul compressive stresses, nd (2) look for the strting point of ftigue crcks in hrdened steels subjected to residul stresses due to surfce cold working. EXPERIMENTAL DETAILS Crbon steel (Germn specifiction Ck 45) hving chemicl composition 0.47 C, 0.21 Si, 069 Mn, P, S, 0-14 Cr, 0.01 Mo, 002 Ni, Al, 007 Cu (% wt) ws used for the experiments. Shpe nd dimensions of the specimens re shown in Fig

2 320 P. STARKER et 1 I I I t I I ::: Fig. 1. Shpe nd dimensions of the specimens in mm. The smples were ustenized in slt bth for 15 min t 800 C nd quenched in oil t 25 C. After this het tretment the mteril reveled Vickers hrdness of HV 830 in tests performed with lod of 1 N. Figure 2 shows the structure of this mteril. The following shot peening of the specimens ws crried out using n irless blst mchine. In pilot tests, vritions of shot velocity, shot size nd coverge were crried out in order to estblish the best peening conditions for improving ftigue strength. For ll series of differently shot-peened specimens SIN curves were determined under reversed bending conditions. All tests were performed t frequency of 25 Hz. The ftigue strength ws determined t tht stress level t which 59% of the specimens broke within lo7 cycles. This stress level ws evluted in the wy proposed by Mennig [6]. The surfce topogrphy of the specimens ws exmined with stylus-type trcer instrument. Surfce roughness nd rithmeticl centre line verge height were derived from the profile plots. Residul stresses were mesured with the sin $-method of X-ry stress nlysis [7]. Using CrK,-rdition the (21 1 )-interference lines in suitble directions were registered with R- diffrctometer. Line shifts due to homogeneous residul stresses nd line brodenings due to inhomogeneous residul stresses were determined. The hlf width of the interference lines served s mesure of work-hrdening in lyers ner the surfce of the specimens. In ddition to the hlf width the Vickers hrdness ws lso considered s mesure of workhrdening. The frcture plnes of the ftigued specimens were exmined using scnning electron microscope in order to get some informtion bout the crck initition points. 1 EXPERIMENTAL RESULTS In Fig. 3 the full line represents the longitudinl residul stresses t different distnces from the surfce of the hrdened specimens. At the surfce, tensile residul stresses pper ; these decrese with incresing distnce from the surfce. As could be shown by hrdness nd hlf width mesurements surfce dmge hs occurred up to depth of bout 80 pm s consequence of the het tretment. A surfce lyer of 100pm thickness ws therefore electrolyticlly removed from both sides of the specimens. After this procedure, shot peening (shot velocity 83 m/s, coverge 98%) of the specimens with shot size of 2 = 0.3 mm nd J = 0.6 mm leds to the distributions of residul stresses in the surfce lyers of the specimens shown by the dshldotted nd dshed line respectively. As cn be

3 Subsurfce crck initition during ftigue s result of residul stresses 321 Fig. 2. Microgrph of the mrtensitic structure.

4 322 P. STARKER et l. Q = 1300 N/mm 2 Q 2 = 1250 N/mm u = 1200 N/mmL U 200 pn 2 u = 1100 N/mm +----I 200 pm u = 1000 N/mm 2 H 200 prn 2 = 950 N/mm U 200 p m CJ = 900 N/mm2 w 200 p m Fig. 5. Scnning electron microgrphs of frctured specimens t different stress mplitudes.

5 Subsurfce crck initition during ftigue s result of residul stresses ,2 0, v I I I I Number of cycles Fig. 4. SIN-curves of () hrdened but unpeened nd (b) hrdened nd shot-peened specimens.

6 324 P. STARKER er l. seen, nerly the sme compressive residul stresses of bout N/mm2 rise due to the different shot peening conditions t the surfce of the specimen. The distributions nd the mximum mgnitudes of the residul stresses, however, depend upon the peening conditions. In the cses of 2 = 0.3 mm nd 2 = 0.6 mm, mximum vlues of nd N/mm2 respectively re mesured. As consequence of shot peening, surfce hrdness nd surfce roughness were slightly rised compred with the hrdened unpeened stte. Figure 4 revels the results of the reversed bending tests with the vrious series of differently treted specimens. The S/N curves show tht the ftigue strengths of the hrdened unpeened smples re respectively 41 nd 54% smller thn the ftigue strength of the hrdened nd shot-peened specimens. These improvements cn only be explined s being consequence of the high mgnitudes of residul compressive stresses induced by shot peening. But obviously no distinct correltion exists between surfce residul stresses nd the improvement of the ftigue strength. Despite the fct tht the mgnitudes of residul surfce stresses re not very different, the ftigue strength vlues differ considerbly. The improvement of the ftigue strength is in some cses lower, in other cses higher, thn predicted if one uses the mgnitudes of surfce residul stresses in connection with eqution (1). In this context it is of interest tht recent investigtions [8,9] on cse-hrdened nd shot-peened specimens of the Germn steel grde 16 MnCr 5 lso resulted in smller gin of reversed bending ftigue strength thn nticipted from the mgnitude of the compressive surfce residul stress. It hs to be stted tht high mgnitudes of surfce compressive residul stresses my be less efficient in improving the ftigue strength of hrdened steel specimens thn high mgnitudes of tensile residul stresses in reducing the ftigue strength. Explntions for this reltive inefficiency of high compressive residul surfce stresses in improving the ftigue strength my be offered by thorough investigtions of the frctured surfces of the ftigued specimens. In the cse of hrdened unpeened smples, observtions with scnning electron microscope lwys reveled crck initition point on the surfce of the specimens. In the csehrdened nd shot-peened sm'ples, however, crck initition occurred t the surfce only under specil loding conditions. Under reversed bending ftigue crcks strted in mny cses t distinct distnce below the surfce in lyers where smller pplied stresses were present. In Fig. 5 severl scnning electron microgrphs of frctured specimens re shown. On ech microgrph the surfce stress mplitude is noted. As cn be seen, the ftigue crcks strt t the surfce if the smples re loded with very high stress mplitudes between 1200 nd 1300N/mm2. However, lowering the stress mplitude beneth vlue of round 1100 N/mm2 cuses subsurfce crck initition. If the centres of the rosettes on the microgrphs re considered to be the crck initition points, it cn be seen tht the crck initition depth increses with decresing stress mplitude. At stress mplitudes resulting in ftigue life of lo7 cycles or more, crck initition is locted t lest 0-3 mm below the surfce. In Fig. 6 the results of the observtions with the scnning electron microscope re summrized. Independent of the shot size, surfce crcks occur t high stress mplitudes nd subsurfce crcks occur t low stress mplitudes. At low stress mplitudes the men distnce of the subsurfce crck initition points from the surfce of the specimen is lrger the lrger the grin size of the shot. Obviously, the reltionship between crck initition depth nd stress mplitude depends on the peening conditions, nd it is to be expected tht the combined effect of peening residul stresses nd pplied stresses is responsible for these findings.

7 Subsurfce crck initition during ftigue s result of residul stresses 325 Fig. 6. Distnce from the surfce of ftigue crck initition points s observed in the scnning electron microscope. DISCUSSION Figure 7 illustrtes the existing sitution. The thin stright lines specify the bending stress distributions ner the surfce lyers of the specimens, when surfce stress mplitudes between 900 nd 1300 N/mm2 re effective. The open circles 0 nd the closed circles 0 mrk the observed crck initition points for shots d = 0-3, nd d = 0.6 mm respectively. In ddition, the signs A nd A describe the observed loctions of non-propgting crcks in the two differently shot-peened groups of specimens. To interpret the subsurfce crck initition, the simplest ssumption is tht the ftigue strength of the specimens is enhnced loclly ccording to the compressive residul stress cting there. To first pproximtion it is possible to clculte the ftigue strength of ech subsurfce lyer. If one ssumes tht locl ftigue strength (e,,ocl) nd locl residul stress cr(r, re connected ccording to the well-known Goodmn reltion [lo] : (, = ftigue strength of the specimens free of residul stresses, 6, = ultimte tensile strength). The thick curves in Fig. 7 represent the subsurfce ftigue strength distributions in both shot-peening sttes investigted. Compring the clculted curves with the observed crck initition points, some noteworthy fetures pper. Obviously, ftigue crcks only develop in lyers where the locl lod stress is higher thn the locl ftigue strength. Furthermore, ftigue crcks only propgte if no increse of ftigue strength due to compressive residul stresses occurs in the direction of propgtion. As cn be seen from smples loded with medium stress mplitudes, crck initition is present t the surfce s well s below the surfce. In these cses, the stress lines intercept the clculted curves of locl ftigue strength. Thus the surfce crcks re not ble to expnd rpidly to deeper lyers of the specimens. Only the subsurfce crcks expnd in such cses nd led to the

8 326 P. STARKER et l. filure of the smples. In further creful investigtions it hs turned out tht the exct positions of the crck initition points re influenced by non-metllic inclusions. An exmple is shown in Fig. 8. At the centre of the rosette n luminium-silicte-inclusion is present. Obviously, the stress concentrtions due to such inclusions superimpose the effects of the locl residul nd pplied stresses nd influence the crck initition processes. REFERENCES [l] Brodrick, R. F. (1957) Protective shot peening of propellers. Prt 3. WADC Technicl Report [2] Vodopivec, F. nd Kosec, L. (1967) Investigtions on the ftigue strength nd the influence of non-metllic-inclusions on ftigue crcking of cse-hrdened steels (in Germn). Hrterei- Techn. Mitt. 22, [3] Kloos, K.-H. (1976) The influence of surfce conditions nd size of specimens on the ftigue behviour (in Germn). VDI-Berichte Nr. 268, [4] Syren, B., Wohlfhrt, H. nd Mcheruch, E. (1976) The influence of residul stresses nd surfce topogrphy on bending ftigue strength of mchined Ck 45 in different het tretment conditions. Proc. 2nd Int. Con$ on Mech. Behvior of Mterils, Boston, [5] Syren, B. (1975) The influence of mchining on the bending ftigue strength of steel Ck 45 in different het tretment conditions (in Germn). Dr.-1ng.-Diss. Univ. Krlsruhe. [6] Mennig, W.-W. (1970). Remrks on the evlution of the ftigue strength of steel nd some investigtions for the determintion of the rnge of infinite life (in Germn). Muter.-Priifung. 12, [;1 Mcheruch, E. nd Miiller, P. (1951) The sin +-method of X-ry stress nlysis (in Germn). 2. ngew. Phys. 3, [8] Schreiber, R. (1976). Investigtions of the ftigue behviour of the shot-peened cse hrdening steel 16 MnCr 5 in different het tretment conditions (in Germn). Dr.-hg.-Diss. Univ. Krlsruhe. [9] Schreiber, R., Wohlfhrt, H. nd Mcheruch, E. (1978) The effect of shot-peening on the bending ftigue strength of the cse-hrdened steel 16 MnCr 5 (in Germn). Arch. Eisenhiittenwesen 49, [lo] Goodmn, J. (1899) Mechnics Applied to Engineering. Longmn, Green, London.

9 Subsurfce crck initition during ftigue s result of residul stress 327 Distnce from surfce, mm Fig. 7. Bending stress distributions, ftigue strength clculted by the Goodmn reltion nd crck initition points of the two shot-peened test series. Fig. 8. Aluminium silicte-inclusion t the centre of the rosette.