Effect of Drilling Process on Fatigue Life of Open Holes

TSINGHUA SCIENCE AND TECHNOLOGYISSN 1007-0214 11/21 pp54-57Volume 14, Number S2, December 2009Effect of Drilling Process on Fatigue Life of Open Holes*GU Weiping (顾卫平)", XU Honglu (徐红炉) LIU Jun(刘军)。YUE Zhufeng (岳珠峰)School of Mechanics, Civil Engineering and Architecture,Northwestern Polytechnical University, Xi'an 710072, ChinaAbstract: In this paper, a series of fatigue tests were performed to study the effect of dilling process on fa-tigue behavior of open holes in dog-bone specimens dilled by the traditional drlling process and advancedWinslow diling process. The evaluations of inital fatigue qualities of the two types of holes were carried outbased on the equivalent initial faw size method and coincidence criterion. Fatigue fracture surfaces wereobserved by stereomicroscope. The results show that the location where the fatigue crack initiates is un-changed when these two diferent driling processes are used. A longer fatigue life shows a more severedispersibility. When the Winslow diling process is adopted, the equivalent initial faw size is smallr than0.125 mm. The Winslow dilig process meets the requirement of coincidence criterion.Key words: fatigue life; diling process; fracture surface; initial fatigue qualitythe traditional multi-step drilling hole process is unableIntroductionto ensure fatigue life of fastener holes to meet the re-In the moder aircraft industry, the main objective ofquirements of aiplane design, and due to this, theWinslow Dilling Hole Process was introduced intoaircraft design is weight saving and reliability of air-China.craft. The fatigue life of crucial components are mainlyThe effect of drilling processes on fatigue perform-determined by the geometrical detail design such asance of fastener hole for 7050 aluminum alloy hasfasten holes and illeting, where stress concentrationoccurs. According to the statistic, fatigue fracture ofrarely been reported48. Ralph et al. studied holesquality with different drilling processes9,10. Dong etfastener holes account for 50%-90% of fracture of ag-al."u and Cao et al1.12] studied the equivalent initialing plane'". The decrease of effect of fasten hole'sflaw size for different drilling processes, their resultsstress concentration could improve the anti-fatigueshowed that the equivalent initial flaw size should beperformance and ensure the reliability and security.smaller than 0.125 mm.Traditional multi-step diling hole process'-) has beenThe purpose of this paper is to analyze the effect ofwidely used in anti-fatigue structure in airplane manu-traditional multi-step drilling process and Winslowfacture. With the increasing demand of aircraft design,drilling process on fatigue behavior of open holes.Received: 2009-05-08; revised: 2009-06-22Specimens containing open holes were fatigued under* Supported by the National High-Tech Rremote constant amplitude tensile load. Investigations(863) Program of China (No. 2007AA04Z404), the National Naturalof fracture surface were performed to locate the crackScience Foundation of China (No. 10472094), and the Specializedinitiationtn nhtoin initialfatigue qualitiesResearch Fund for the Doctoral Program of Higher Education ofof hol中国煤化工1aw size method.MOE, P.RC. No. N6C0001)FinallyHCNMHGwasusedtoana-# To whom correspondence should be addressed.E-mail: guweipingl001@126.com; Tel: 86-29-84636398lyze the distribution of stress around the holes whenGU Weiping (顾卫平) et al: Efect ofDrilling Process on Fatigue Life of Open Holes5load was applied. The FEM results revealed the possi-Half of them are drilled using traditional multistepble mechanism for crack initiation.drilling process, the other half are drilled using ad-vanced Winslow drilling process. After testing, com-1 Material Propertyparing with the traditional drilling technology, theWinslow drilling process is much more precise on theThe material used for this study is aluminum alloycontrol of hole size and a lower surface roughness val-7050，with chemical composition, 91.63% of Al,ue can be obtained. However, there is no change in1.12% of Cu, 1.9% of Mg, 0.02% of Mn, 0.08% of Si,geometry.0.19% of Fe, and 0.05% of Ti Typical mechanicalproperties of the material along the rolling direction2.3 Fatigue testare obtained from tensile tests. The stress-strain be-havior of the material is shown in Fig. 1, which showsAll the specimens are tested on a servo-hydraulic fa-that Young modulus is 79 GPa, yield strength is 503tigue testing machine (Instron8802). During fatiguetests, load spectrum is combined by two types of sineMPa. The Poisson ratio is measured to be v= 0.33.constant load amplitudes blocks, which is used to ob-500tain the relationship between the crack length and fa-tigue life. For these two blocks, the max stresses are400the same. The stress ratio R (σmin/σmx) is 0.06 and00frequency is 8 Hz in first block. The stress ratio is 0.53and frequency is 16 Hz in second block. During thetest, the first block loads was applied for 3 min andthen the second block loads was applied for 9 min. Thesequence is repeated until the specimen is fractured.0.020.04 0.060.080.103 Finite Element ModelStrainIn this study, a 3-D FE model is established. The AB-Fig 1 Stress-strain diagram for alay 7050AQUS/standard finite element package was used to2 Experimentcarry out the analysishi3!In the ABAQUS element library, ABAQUS linear2.1 Dimension of specimenshexahedron reduced integration elements C3D8RThe 7050 aluminum alloy is supplied as a sbeet with a(three- dimensional eight nodded continuum elements)is used to mesh the model (as shown in Fig. 3). In athickness of 6 mm and cut into dog bone shape withfinite element analysis, selection of mesh size and lay-centre holes. The form and the final dimensions areout is very critical. It is desirable to use as many ele-shown in Fig. 2. It is noted that the axis of loading isments as possible in the analysis. However, such anparallel to the rolling axis of sheet.analysis will require excessive computational efforts.230100In order to obtain an optimal result, the mesh aroundthe hole was further refined (as sbown in Fig. 4). In16哆this FEM analysis, the number of elements is 36 582.图才The boundary conditions are given as follows: one0°11676_」JR5O>end of model is fixed, the other end of model is uni-6(m互formly tensile loaded as 80 MPa,中国煤化工Fig.2 Geometry and size of specimen (mm)HCNMHG2.2 Drilling processesThere are 42 specimens concerned in the present study.Fig. 3 Finite element model56Tsinghua Science and Technology, December 2009, 14(S2): 54-57edge, which is the same for the two types of drillingprocesses.Fig.4 Refiner mesh near hole4 Results and Discussion4.1 Fatigue results(a) Fatigue fracture under traditional processAfter tests, the mean fatigue lives, coefficient of varia-tion, and the improved multiples of fatigue life arelisted in Table 1.Table 1 Fatigue life of specimensDrllinStress level Mean fatigue Coefficient_process(MPa)__ life (cycles) of variationMultiple8065 9120.154 7221.36896110.148 465)046 3170.509 5501.31(b) Fatigue fracture under W inslow process0.170 9707ig. 5 Fatigue fracture surface of holes with different10029 4620.159 799drilling process1.420041 8770.154 653Due to the effect of two types of sine block loads onTable 1 shows that: contrasted to traditional drilingcrack propagation, bright and dark strips altemativelyprocess, the fatigue life of open holes is longer whenoccurred at fracture surface. Based on the boundary ofWinslow drilling process is used and the improvedbright and dark strips, the relationship between crackmultiple of fatigue life is about 1.36 times. Meanwhile,length and fatigue cycles was retrieved. For aluminumthe dispersion of fatigue life is smaller. The main rea-alloy, the crack initiation life corresponds to the crackson is that the Winslow drilling process confines thelength of 0.076 mm-0.1 mm!416!. Therefore, accordingsurface roughness within a small value. According toto the number of strips on the fracture surface and fa-the fracture mechanics theory, the fatigue performancetigue cycles, crack initiation life accounts for 80% ofis associated with surface roughness which means thattotal fatigue life of fastener holes.a lower surface roughness exhibits a better fatigue per-Using the aforementioned method, the initial fatigueformance. Moreover, after the improvement of thequality of the two types of drilling process is deter-process, the feed impact on the residual stress is alsomined and listed in Table 2.very strong. Feed has become relatively smaller thanTable 2 Initial fatigue qualitythe traditional system, and this increases the residualTraditional process (mm)Winslow process (mm)compressive stress and the fatigue life.0.134 7970.08664.2 Comparison analysis of initial fatigue qualityTable 2 shows that the equivalent initial flaw size isAfter the fatigue test, all fracture surfaces are observedsmaller than 0.125 mm when the Winslow process isused,. which meets the reauirement of coincidenceby optical microscope, and the typical fracture surfaceof fasten hole is shown in Fig. 5, containing the load-crter中国煤化工ss, which yields aninitialYHCNMHGisnotgoodenoughing axis of the specimen normal to the plane. It canto meet the requrement ot coincidence criterion.be seen that the crack initiates at the counter holesGU Weiping (顾卫平) et al: Effect of Drilling Process on Fatigue Life ofOpen Holes54.3 FEM results2007, 47: 35. 45. (in Chinese)[2] Zbang D C, Pei X M. Effects of machining processes onThe main purpose of the finite element analysis onsurface roughness and fatigue life. China Mechanical En-open hole is to find the probable locations of crackgineering, 2003, 16: 1374-1377. (in Chinese)initiation, mechanism of crack initiation, and compari-[3] Swift K G Booker J D. Process Selection. 2nd Ed. Elsevierson of the fatigue results.Science & Technology Books, 2003: 131-159.The stress counter of open hole is shown in Fig. 6. It[4] PeiX M, Chen W Y, Ren B Y, et al. Effect of dilling proc-can be seen that the max stress occurs at the counter-esses on surface integrity of 7075 aluminum alloy holes.sunk holes edge. This is the stress concentration zone,Journal of Beijing University of Aeronautics and Astro-and the stress concentration factor is about 3.9.nautics, 2002, 3: 319-323. (in Chinese)According to the fatigue results (shown in Fig. 6),[5] Zhang D C, Pei X M, Chen W Y, et al. Infuence of ma-the crack initiates at this location. Therefore, thechining processes of fastening holes on its surface residualmechanism of the crack initiation of countersunkstress. Aeronautical Manufacturing Technology, 2003, 5:fastener holes is that the crack is induced by stress40-42. (in Chinese)concentration.[6] PeiX M, Chen W Y, Ren B Y, et al. Infuence of machiningprocesses on surface microphotograph and microstructureof fastening holes. The Chinese Journal of NonferrousMetals, 2001, 11(4): 655-659. (in Chinese)[7] Pei X M. The influence of machining processes of fasten-ing holes on surface work hardening. Machinery Mamu-facturing Engineer, 2004, (); 97-98. (in Chinese)[8] Zhang H, Tang Y x, Yu C Y. Efects of laser shock proc-essing on the fatigue life of fastener holes. Chinese JournalofLasers, 1996, 23(12): 112-1116. (in Chinese)[9] Ralph W C, Johnson W s, Toivonen P, et al. Effect of vari-Fig6 tress counter of open holeous aircraft production drlling procedures on hole quality.Intermational Journal of Fatigue, 2006, 28: 943-950.5 Conclusions[10] Ralph W C, Johnson W s, Makeev A, et al. Fatigue per-formance of production-quality aircraft fastener boles. In-In this study, a series of tests were conducted in alu-ternational Joumal of Fatigue, 2007, 29: 1319-1327.minum alloy 7050 to investigate the effect of drilling[11] Dong D K, Wang J Y, Kong F J. Research on fastener boleprocess on the fatigue behavior of countersunk holes.initial fatigue quality and manufacter technique. Jourmal ofThe following can be concluded:Mechanical Strength, 2000, 22(3): 214-216. (in Chinese)(1) The crack initiates at the counter holes edge. The[12] CaoC N, Wang Z z, Zhao X M. Evaluation and coinci-fatigue crack's nucleation position is the same for thedence check for initial fatigue quality of fastener hole.two types of processes.Journal of Northwestern Poytechnical University, 1999, 1:(2) When Winslow diling process is used, the fa-15-17. (in Chinese)tigue performance is enhanced with a reduced[13] ABAQUS/STANDARD User's Manual Version 6.5. Hibit,dispersion. The initial fatigue quality is smaller thanKarlsson and Sorensen, Inc, 2000.0.125 mm. Winslow drilling process meets the re-[14] Baidurya B, Eligwood B. Continwun damage mechanicsquirement of coincidence criterion.analysis of fatigue crack initiation. International Journal of(3) The mechanism of the crack initiation is that theFatigue, 1998, 20(9): 631-639.crack is induced by stress concentration. Crack initia-[15] Shang D G Wang D J, Han N L. The definition and deter-tion life accounts for 80% of total fatigue life of fas-mining method of fatigue crack initiation size. Journal of .tener holes.62. (in Chinese)[16] L中国煤化工mehod 1o copueReferencesYHC N M H Gurnal of MechanicalStrength, 2000, 22(3): 203 -205. (in Chinese)[1] LiuJ, Yue Z F, Liu Y s. Surface finish of open holes onfatigue life. Theoretical and Applied Fracture Mechanic,