D~-Zeng, Deng Zeng-Jie and Zhou Hui- Jiu 1

D~-Zeng,Deng Zeng-Jie and Zhou Hui- Jiu

%efailure and the prevention of failure of elevator links for oil well drilling have been ,yestigated.After presenting a failure analysis of elevator links, a fracture mechanics approach $4for computing the fracture strength and residual life is described, and the results are omparedwith those obtained by fatigue tests of actual links. Finally, the effect of shot-peening ,the fatigue lives of elevator links is discussed.

lay words: fatigue; fatigue life prediction; shot-peening; fracture analysis; elevator links

elevator link is an important implement in oil drilling: ran elevator link breaks a catastrophic accident may ccur. It is necessary to reduce the weight of elevator links nd to prolong their service life to facilitate drilling opera- ons and to ensure safety. We have developed a low-carbon lartensitic steel, 0. 2C-Si-Mn-Mo-V,with high strength, uctility and fracture toughness, which has been used ltisfactorily for making all kinds of light-weight elevator nks in the People's Republic of China. The weight of these evator links is much less than that of conventional links, but in recent years there have been occasional reports of fracture of these light-weight links after a period of service, apparently due to fatigue. On the basis of failure analysis we have adopted a / fracture mechanics approach dealing with the link as a cracked body and have compared the results of calculations , of critical crack length and residual life with those deter- mined from monitoring the links during service. Finally we have applied shot-peening to overcome the effect of surface defects, further ensuring safety and prolonging the service life of the elevator links. As a result, the quality of the links has been improved considerably and th'ey have been authorized by the API to use its official monogram.

Service conditions and failure analysis of elevator links The geometry of the 500 kN elevator link for our experi- ment is shown in Fig. 1. The straight part of elevator link is subjected to simple tensile stress while the ring part is subjected to combined tensile and bending stresses. Photo-

L Fig. 2 Stress distribution of loaded elevator link

elastic tests indicate that the maximum tensile stress is located at the ring part as shown in Fig. 2. There are three dangerous sections in it (sections 1-1and 2-2 in Fig. 1). The stress in these sections is 4.66-5.66 times as high as that of the straight part. Fracture of elevator links usually L 1 occurs in these sections. The main reason for fracture is a 1 Elevator link dimensions (mm) low-frequency highcycle fatigue with occasional over-

0142-1 123/83/010043-5 $03.00 0 1983 Butterworth & Co (Publishers) Ltd INT. J. FAT1GUE January 1983 43 Fig. 3 Fracture surface of elevator link

Tempering temperature PC)

Fig. 5 Conventional mechanical properties and K1, of O.2C-Si- Mn-Mo-V steel

Fig. 4 Electron fractograph of fatigue fracture area loading. Fig. 3 shows a fractured surface in which the fatigue fracture area and the static fracture area can be seen clearly. Electron fractographic analysis has shown that striation morphology predominates in the fracture area (see Fig. 4), but intermingled features of striations and static fracture mechanisms, such as dimples, which are characteristic of high-strain low-cycle fatigue failure112 have not been observed. The ring part has been redesigned to reduce the stress in the critical sections.

Investigation of mechanical behaviour of elevator link steel The conventional mechanical properties and KlC for the elevator link steel 0.2C-Si-Mn-Mo-Vare given in Fig. 5. ~nvesti~ations~-~have shown that the steel, quenched into a low-carbon martensite and tempered at low temperature, possesses high strength, ductility and fracture toughness under static loading conditions. It has a longer fatigue crack initiation period NoSl(the number of cycles for the appearance of a 0.1 mm deep crack) and a lower rate of crack propagation dalcLN than medium carbon alloy steels similarly quenched and tempered. Fig. 6 plots the fatigue crack propagation rate (da/dN) vs AK. Constants n and c in the Paris equation for stage I1 of the dalavs AK curve will be used to compute the residual life of the elevator links.

44 INT. J. FATIGUE January 1983 order to avoid the effect of differences in diameter, N/2R and a/2R coordinates are used in these curves. With the aid of these curves, and knowing the original crack length a0 and the critical crack length a, (from Fig. 7), we can obtain life values No and Nc and the residual life NR = N, - No.

Results of fatigue tests on elevator links Fatigue tests of actual links were carried out to verify the reliability of fracture strength and residual life computations, as well as to investigate the effect of shot-peening on the fatigue life of elevator links, in order to confirm that it prolonged their service life. The tested elevator links were made of 0.2C-Si-Mn- Mo-V steel. The heat treatment and mechanical properties were the same as those of the specimens. Surface and internal defects of the test links were closely examined with ultrasonic flaw detectors, and a crescent notch was machined on the outside end of one of the rings on some of the test o/2R Fig. 7 K1/P vs aI2R curve for a 500 kN elevator link links. Some were shot-peened after heat treatment. All the tests were performed using a 2000 kN hydraulic pulsating fatigue testing machine. Experimental verification of the reliability of fracture strength and residual life computations Measured and computed fracture strengths and residual lives for the links are listed in Tables 1 and 2. The testing load adopted is the maximum safe working load guaranteed by the manufacturer. In order to reduce the effects of loading frequency and nominal stress level on the residual life computations, the constants n and c used were obtained from specimens which had been tested in a way as close as possible to the test links (see Table 3). Different values of n and c were selected for different maximum loads (P,,) applied to the test links. It can be seen from the tables that the computed critical crack lengths tally well with those measured. The maximum error in critical crack length among the five links is only about 10%.This proves the reliability of calculations of K1 for elevator links. The residual life computation is also precise enough for engineering applications. More precise residual life predictions could be achieved by considering the effect of load spectrum etc.

Table 1. Measured and calculated critical crack sizes for elevator links

a/ 2R Crack size Fig. 8 N12R vs a/2R life computation curve for elevator links

',T Elevator link Measured Calculated Error Residual life computation for elevator links identification number (mm) (mm) (% The stress intensity factors K1 for dangerous sections of the elevator link have been given in Reference 6. Using the relation between stress intensity factor K1 and the strain- energy release rate (d~/da),~the twodimensional finite element method has been used to calculate K1. The Table 2. Measured and calculated residual lives of elevator relationship between K1 and the corresponding crack length links for the 500 kN elevator links is plotted with KIIP and a/2R -- as coordinates in Fig. 7, where K1 is the stress intensity Elevator Fatigue Residual life factory (~~a4x-n)~P is the applied load (kN), a is the crack link load length (mm) and 2R is the diameter of the ring part (mm). identification (P,,,IPi) Measured Calculated By putting the material constants KlC, n and c number (kN) (cycles) (cycles) Obtained from our experiments, and values from the K1/P vs a12R curve; into the Paris equation the life curves for the 22 I4711 3.7 7.05~104 5.31 x lo4 links may be obtained by integration (Fig. 8). In 33 245.2113.7 1.54~1O4 1.72~1O4

INT. J. FATIGUE January 1983 45 /I 11 Table 3. Test conditions of specimen and actual elevator The effect of shot-peening on the fatigue strength links of elevator links Maximum The elevator links are used in drop-forged heat treated nominal condition. Their surface quality is naturally inferior to a stress machined surface and surface defects such as decarburiza. Loading Maximum in the tion are very probable. Consequently we have adopted frequency pulsating ring part shot-peening as a way of reducing the weaknesses on the (cyclelmin) load (kN) (MN/m2) surface of the elevator links. Specimen 500 58.8 282.4 The positions where residual stresses were detected Link 300 147.1 294.2 by X-ray diffraction are shown in Fig. 9. Res'ults of Specimen 500 78.5 372.7 measurement before and after shot-peening are given in Link 300 245.2 392.3

1 Fig. 9 Points of residual stress measurement (arrow indicates stress direction) Table 4. Surface residual stress of test links

Residual stress (MN/m2)

Quenched and Quenched and tempered, Position tempered shot peened

Table 5. Fatigue lives of shot-peened and non-peened links

- - Elevator Fatigue Fracture link load cycles identification Strengthening (Pmx1Pmi) N, number regime (kN) (xi04)

11 Quenched 245.211 3.7 9.02 and tempered 88 Quenched 245.2113.7 32.00 and tempered, shot peened crack origin for:

Table 6. Fatigue lives of shot-peened and non-peened elevator links

d Precracking Crack propagation to fracture

Critical Total Link Load Crack Load crack fatigue life identification PmaxlPmin NI length PmaxIPmi N2 length NI +N2 number Condition (kN) (XIO4 cycles) (mm) (kN) (XIo4 cycles) (mm) (~l0~~y~'~~~ I 33 Non-peened 294.2113.7 0.7 4.75 245.2113.7 1.54 21.3 2.24 55 Shot-peened 294.2113.7 2.5 6.85 245.2113.7 3.14 21.15 5.64 22 Non-peened 245.2113.7 1.0 6.6 147.1113.7 7.05 39.75 8.05 6 6 Shot-peened 245.2113.7 1.0 0.0 147.1113.7 49 Not 50 fractured /

46 INT. J. FATIGUE January 1983 ,-fable4. It is clear that the effect of shot-peening is con- References dderable. The original residual stress on the surface was +22 to +91 MPa, but becomes -392 to -480 MPa after 1. 'Fractography and atlas of fractographs' Metals Handbook (ASM) 9 8th edition ,pt.peening. Fatigue tests show that the fatigue lives of Low, John R. Engng Fracture Mech 1 No 1 (1968) p 47 the shot-peened elevator links are more than twice those of 2. (see Table 5). The shot-peening increases the 3. 'Constructional alloy steel 2OSiMnMoV with high strength of cycles to fatigue crack initiation and to propaga- and high ductility' Xian Jiaotong Univ Research Report (Nov 1974) tion, ~hichsignifies the appearance of a crack length of 0.1 4. The Research Institute for Strength of Metals, Xi'an Jiaotong mm.819 This is also shown by the test Univ 'Relationship between fracture toughness and other remits of some notched elevator links, as given in Table 6. mechanical properties of some constructional alloy steels' The shot-peening prolongs the period of crack initiation Scientia Sinica (in Chinese) No 1 (1977) pp 11-19 ,,,, more than it reduces the rate of the second stage crack 5. Institute for Strength of Metals, Xian Jiaotong Univ 'The propagation,and electron fractographic observations show fatigue crack initiation and propagation in constructional bat the main difference between shot-peened and non- steels' Mechanical Strength (in Chinese) No I I (1979) peenedelevator links is in the region of fatigue origin. For 6. Jia Feng-he 'The finite element computation of stress and the shot-peened links the fatigue origin area shows cyclic stress intensity factor for elevator links' Journal of Xi'an with very fine striations (Fig. lOa), while that of Jiaotong Univ No 3 (1979) ,on-peened links has rather coarse striations (Fig. lob). 7. Miyamoto, H. 'The Finite Element Method and Fracture Mechanics' (in Japanese) (1 972) p 25 8. Deng Zeng-Jie, Jin Da-Zeng and Zhou Hui-Jiu 'Effect of Conclusions shot-peening on the fatigue crack initiation and propagation in some steels' Proceedings of the First international Con- 1) The fracture strength and residual life of elevator ference on Shot-peening 1981, Paris, p 389 links may be forecast sufficiently precisely for 9. Hornbogen, E., Thurnann, M. and Verpoort, C. ibid p 381 engineering applications by use of specimen testing Authors and fracture mechanics calculations, thus providing a scientific basis for safe operation. Drs Jin and Deng and Professor Zhou are with The Research 2) Shot-peening is an effective way of delaying crack Institute for Strength of Metals, Xian Jiaotong University. initiation and retarding early stage crack propagation, Inquiries should be addressed to them at The Research and can thus be used to prolong the fatigue life and Institute for Strength of Metals, Xian Jiaotong University, ensure the safety of elevator links. Xian, Shaanxi Province, The People's Republic of China.

INT. J'. FATIGUE January 1983 47