Metal Fatigue Part 9 docx

224 (e) N=3.6 X IO6 Influence of Si-Phase on Fatigue Properties of Aluminium Alloys 225 JSi + Specimen surface "r J Figure 12.6 Fracture surface near fracture origin which is at Si phase, extrusion: 20Al1, u = 147 MPa, Nf = 1.2 x lo', rotating bending. Fig. 12.1 1 shows the fatigue life percentage for crack initiation and propagation. It can be seen that shear-type cracks grow very quickly. 12.2.3 Fatigue Behaviour of Specimens Containing an Artificial Hole In order to clarify the difference in the fatigue mechanisms for the continuously cast material and for the extruded material, fatigue tests were carried out on specimens containing an artificial hole. Fig. 12.12 shows the S-N curves obtained. Fig. 12.13 shows the crack growth behaviour for a specimen which survived for N = lo9 cycles. S-N curve 3 in Fig. 12.12a shows a fatigue limit with a clear knee point. Fig. 12.13 implies that a plasticity induced crack closure mechanism caused non-propagation (or at least tendency towards non-propagation) of cracks emanating from the artificial hole. Comparing the S-N curves 1, 2 and 3 in Fig. 12.12a, we can interpret the S-N curve for unnotched specimens to be a combination of curves 1 and 2, namely curve 1 which is for failure from Si phase and has a hypothetical clear fatigue limit, and curve 2, which is for failure by shear-type cracking, but has no clear fatigue limit, even at N 2 10'. The 226 Chapter 12 A II t. f , '1 - , I. II II , r r- Influence of Si-Phase on Fatigue Properties of Aluminium Alloys 227 / Specimen surface Figure 12.8 Fracture surface near fracture origin showing shear type fracture, extrusion: 20Al1, u = 128 MPa, Nr = 4.5 x lo8, rotating bending. fatigue limit in the curve 3, for specimens containing a small hole, is much lower than the stress which leads unnotched specimens to failure at N = 108-109, and accordingly a curve like curve 2 does not appear explicitly. On the other hand, the specimens of continuously cast materials containing an artificial hole do not have a clear fatigue limit. This is because shear-type cracks initiate at a hole edge and grow to large size, say -150 pm, so plasticity induced crack closure is unlikely to occur, resulting in fatigue failure at extremely high numbers of cycles. 12.3 Mechanisms of Ultralong Fatigue Life If we compare the fatigue mechanisms of steels (see Chapter 6) and aluminium alloys in ultralong fatigue life, it seems there are both common and different mechanisms. The common mechanism is fatigue crack growth in Mode I, which occurs in crack growth from inclusions or Si phase. In this case, more or less plasticity-induced crack closure should be present, though oxide-induced crack closure would be absent in crack growth from internal inclusions. However, we cannot determine the relative degrees of influence 228 Chapter 12 ca 0 3 n s m 2 X 9 4 $ n e 0 2 n cd W Infuence of Si-Phase on Fatigue Properties of Aluminium Alloys 229 r- 2 X v! + & n 3 IC, , . : *. 9. .I . .I . - ,:t ' - h W d 230 400 350 300 m 3250 artificial hole : d=h=I(W11 rn h v. .200' 3150' 2: %o Chapter 12 : X' Si obSeNed at fracN& on&: ' ' ' "I ' ' ' ' ' 4 0 Shear type fracture. 0 6B17(T-C) io 6B17(T-C,Smdl artifitid hole : d=h=500s m){ .A 6B17(R-B) :A 6B17(R-B,Small artifitid hole : d=h=500s m): ro !% A mo : 0 r" &O A A0 A 0 1 50 T-CTension-compression R-B:Rotating-bending 150 L ' "" (j N 9, Axial direction oooomm 0 A 20A17 el A 20~17 e2 0 15C17 e 1 0 15C17 e2 0 15A17 8 1 15A17 e2 Influence of Si-Phase on Fatigue Properties of Aluminium Alloys 23 1 - of plasticity induced crack closure and oxide induced crack closure. This problem still remains unsolved. In the case of fatigue, failure from inclusions in steels in the gigacycle regime, the effect of hydrogen must be considered, as described in Chapter 15. The different mechanism in A1 alloys is shear-type crack initiation and growth beyond N = IO8. Since cracks emanating from Si phase tend to show non-propagating behaviour for N 2: lo*, the initiation and growth of shear-type crack may be regarded as a main cause of fatigue failure at N 3 lo8. Thus, the fatigue failure mechanisms of AI alloys in superlong high-cycle fatigue are different from those of steels. Shear-type cracks initiate in aluminium microstructure at N 2 lo*, and continue to grow, without crack closure mechanisms, until specimen failure. However, cracks emanating from inhomogeneities, such as Si phase, or small de- fects, behave like a non-propagating cracks in steels, in which crack closure mechanisms are thought to prevail. \ b m 2 - N H d - -8 -8 -8 / \ t, t H -~ 50 23 .a.s*. 96 __ 12.5 121 ~- c( 12.4 Low-Cycle Fatigue (see also ref. [3]) The specimen geometry used is shown in Fig. 12.14. Specimen preparation entailed polishing with #2000 emery paper, buff finishing, and 2 wm chemical etching (600 ml phosphoric acid, 10 ml H2S04, 1400 ml distilled water) followed by neutralisation in a 5% NaOH solution. Fatigue tests were performed in a servo-hydraulic system, under strain control, at cyclic frequencies between 0.1 and 0.5 Hz. Crack development was monitored by means of plastic surface replicas, and fracture surfaces were examined using scanning electron microscopy in order to ascertain the details of operative fatigue mechanisms . 12.4.1 Fatigue Mechanism Extensive observations of specimens revealed two basic fatigue failure mechanisms: (1) fracture origin in the Si phase, or at the interface between Si and the matrix; and (2) shear crack initiation and growth in the matrix. Details of each mechanism, as influenced by material processing, are discussed in the following sections. 232 Chapter 12 Figure 12.15 Fracture surface near fracture origin, continuous casting: 3A17, AE/~ = 0.01, Nf = 52. 12.4.2 Continuously Cast Material A representative fatigue fracture surface for the 3A17 material is shown in Fig. 12.15. Here a single shear-type crack initiated in the A1 matrix, and propagated to a critical size in a shear mode, that is inclined at -45" to the surface. No other cracking was observed. Such shear cracks are found to form before interfacial separation between the Si phase and the matrix. 12.4.3 Extruded Material In contrast to the above behaviour, the fracture surface for the 20A17 material is shown in Fig. 12.16, along with surface observations of crack growth in Fig. 12.17. Here it can be seen that cracks form early in the life, invariably in the Si phase, and the low-cycle fatigue process is essentially one of crack growth. This behaviour is similar to that observed in medium carbon steel (0.46 C) where cracks form in the pearlite phase in the early stages of low-cycle fatigue, leading to final fracture [4]. 12.4.4 Comparison with High-Cycle Fatigue Stress life plots, incorporating results from this study, and from the previous high-cycle fatigue study, are shown in Fig. 12.18. The low-cycle data represent the steady-state stress response at the half life. As indicated, the continuously cast material exhibits a shear-type failure mechanism throughout the life regimes; the fracture process is essentially the same for low-cycle and high-cycle tests. Fracture topography for a high-cycle test is shown in Fig. 12.4a; the similarities with Fig. 12.15 are noteworthy. By way of comparison, cracks for the two extruded conditions, shown in Fig. 12.18, Influence of Si-Phase on Fatigue Properties of Aluminium Alloys 233 Figure 12.16 Fracture surface near fracture origin which is at Si phase, extrusion: 20A17, AE/~ = 0.0075, Nt = 421. (a) N = 0 (b) N = 1 (c) N = 10 - (h) N = 421 10011 m U (d) N = 50 (e)N=100 -Axial direction Figure 12.17 Crack initiation and growth, fracture origin at Si phase, extrusion: 20A17, AE/~ = 0.0075, Nf = 421. tend to initiate in the silicon phase, as evidenced by the presence of silicon at the fracture origins. A typical fracture surface for this condition, shown in Fig. 12.19, exhibits features very much like those found in the low-cycle regime (Fig. 12.16). Again, similar mechanisms appear to be operative in both the low- and high-cycle regimes, except at extremely long lives where shear-type failure, similar to the continuously cast material, may be observed. [...]... Communication ( 199 2) 3 D.F Neal and P.A Blenkinsop: Internal fatigue origins in a-B titanium alloys, Acta Met., 24 ( 197 6), 59- 63 Ti Alloys 245 4 D Eylon and J.A Hall: Fatigue behavior of beta processed titanium alloy IMI 685, Met Trans A, 8 ( 197 7), 98 1 -99 0 5 R Chait and T.S DeSisto: The influence of grain size on the high cycle fatigue crack initiation of a metastable beta Ti alloy, Met Trans A, 8 ( 197 7),... subsurface fatigue crack initiation Mech Eng Ser A, 64(626) ( 199 8), 2528-2535 behavior of beta-type titanium alloy, Trans Jpn SOC 14 T Nakamura, M Kaneko, T Noguchi and T Teraguchi, hoc JSME 199 9 Annual Meeting, Vol 111, 199 9, pp 79- 80 247 Chapter 14 Torsional Fatigue 14.1 Introduction So-called classical studies on biaxial (combined stress) fatigue ranging from tensioncompression to pure torsion under... temperatures, Tetsu to Hagane, 76(6) ( 199 0), 92 4 -93 1 9 K Nagai, T Ogata, T Yuri, K Ishikawa, T Nishimura, T Mizoguchi and Y Ito: Fatigue fracture of 26 Ti-5A1-2.5Sn ELI alloy at liquid helium temperature, Tetsu to Hagane, 7 ( ) ( 198 6), 641-648 10 K Taka0 and K Kusukawa: Fatigue crack initiation behavior in notched member of commercially pure titanium, J Soc.Mater Sci Jpn., 40(458) ( 199 1), 1422-1427 I I H Kobayashi,... Nakazawa: Initial stage of fatigue cracking in pure titanium under bending and torsion, J SOC.Mater Sci Jpn., 27(300) ( 197 8), 8 59- 864 12 H Kitagawa and S Takahashi: Fracture mechanics approach to very small fatigue crack growth and to the threshold condition, Trans Jpn Soc.Mech Eng A, 45( 399 ) ( 197 9), 12 89- 1303 13 K Shiozawa, Y Kuroda and S Nishino: Effects of stress ratio on subsurface fatigue crack initiation... applicability of Miner’s rule, Eng Fract Mech., 18(5) ( 198 3), 90 9 -92 4 5 Fatigue Design Handbook, AE-IO, SOC Auto Eng., Warrendale, PA, 198 8 6 Durability by Design: Integrated Approaches to Mechanical Durability Assurance, SP 730, Soc Auto Eng., Warrendale, PA, 198 7 7 S.S Manson: Fatigue: A Complex Subject-Some Simple Approximations, Exp Mech 5-7 ( 196 5) 193 -226 24 1 Chapter 13 Ti Alloys Ti alloys have high... 0.35% C 0. 59 steel [ 141 +- Fable 14.2 Fatigue limits of specimens containing holes of various diameters Diameter of hole d w I Fatigue limit k @ m 2 Torsion zwD=lo - 14.5 40 50 Plain specimen zwD-10 Bending U ~ D , , ~ uwD-IO Fatigue limit k @ m 2 Bending uwD, zwD-10 OwD- 6 24.5 0. 59 25.0 0.58 14.5 24.0 0.60 24.5 0. 59 14.5 23 O 0.63 24.5 0. 59 21.5 0.67 22.0 0.66 20.5 0.71 21.0 0. 69 0.73 19. 5 0. 69 0.75... rotating-bending and tension-compression fatigue tests, Trans Jpn Soc Mech Eng Ser A, 62( 594 ) (I 99 6), 347-355 3 Y Murakami, H Kobayashi, H Ikeda and R.W Landgraf: SAE Paper No 97 0704, Low Cycle Fatigue Properties of AI-Si Eutectic Alloys, 199 7, pp 1-6 4 Y Murakami, S Harada, T Endo, H Tani-Ishi and Y Fukushima: Correlations among growth law of small cracks, low-cycle fatigue law and applicability of Miner’s... Murakami Torsion Murakami 0.1 2.78 3.57 0.2 2.41 2 .99 0.4 1 .97 2.3 1 0.6 1.73 1 .94 0.8 1.57 1.72 1.0 1.47 1.58 Non-propagation of cracks at a hole is the threshold condition for the fatigue limit under both rotating bending and reversed torsion [8-11,13 1 Therefore, strictly speaking, the ratio (rw/aw) the torsional fatigue limit (t,) to the rotating bending fatigue limit of (, for a specimen containing... Internal crack initiation in high cycle fatigue for Ti-5 At-2.5 Sn ELI alloy at cryogenic temperatures, Tetsu to Hagane, 75(1) ( 198 9), 1 59- 166 7 K Nagai and K Ishikawa: Deformation and fracture characteristics of titanium alloys at low temperatures, Tetsu to Hagane, 75(5) ( 198 9) 707-715 8 0 Umezawa, K Nagai and K Ishikawa: Subsurface crack initiation in high cycle fatigue of Ti-6AI4V alloys at cryogenic... 14.2 Effect of Small Artificial Defects on Torsional Fatigue Strength 14.2.1 Ratio of Torsional Fatigue Strength to Bending Fatigue Strength Several studies have been performed in order to investigate the effect of small artificial defects on torsional fatigue strength [8-11,131 Endo and Murakami [9] conducted both rotating bending and torsional fatigue tests on annealed 0.46% C steel (S45C) specimens, . Auto. Eng., Warrendale, PA, 198 7. 7. S.S. Manson: Fatigue: A Complex Subject-Some Simple Approximations, Exp. Mech. 5-7 ( 196 5) (I 99 6), 347-355. 90 9 -92 4. 193 -226. 24 1 Chapter 13. eo Low cycle fatigue I High cycle fatigue e" gauge mark) ex Qt e ex e eo Low cycle fatigue High cycle fatigue io1 io2 10) io4 io5 ios io7 ion io9 Number of cycles. conditions. 3A17 20A17 Table 12.5 Mechanical properties of materials 402 4 79 522 13.1 1 69 I20 396 456 508 15.1 161 1 19 15C17 378 408 423 3.6 164 118 A comparison of the strain cycling