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 diling process on fa-tigue behavior of open holes in dog-bone specimens dilled by the traditinal diling process and advancedWinslow dilling process. The evaluations of initial 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 stereomiroscope. The results show that the location where the fatigue crack initiates is un-changed when these two different dilling processes are used. A longer fatigue life shows a more severedispersibility. When the Winslow dilling process is adopted, the equivalent initial flaw size is smaller than0.125 mm. The Winslow diling process meets the requirement of coincidence criterion.Key words: fatigue life; dilling process; fracture surface; itial fatigue qualitythe traditional multi-step drilling hole process is unableIntroductionto ensure fatigue life of fastener holes to meet the re-In the modem aircraft industry, the main objctive ofquirements of airplane design, and due to this, theWinslow Drilling Hole Process was introduced intoaircraft design is weight saving and reliability of air-China.craf. The fatigue life of crucial components are mainlyThe effect of diling processes on fatigue perform-determined by the geometrical detail design such asance of fastener hole for 7050 aluminum alloy hasfasten holes and fllting, where stress concentrationrarely been reported. Ralph et al. studied holesoccurs. According to the statistic, fatigue fracture ofquality with different drilling processesl9.10. Dong etfastener holes account for 50%-90% of fracture of ag-al."n and Cao et al.!2] studied the equivalent initialing plane". The decrease of effect of fasten hole'sflaw size for different driling 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 drilling hole processl23) 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 Research and Developmentremote constant amplitude tensile load. Investigations(863) Program of China (No.2007AA042404), the National Naturalof fracture surface were performed to locate the crackScience Foundation of China (No. 10472094), and the Specializedinitiation region and to obtain initial fatigue qualitiesResearch Fund for the Doctoral Program of Higher Education ofof h中国煤化工flaw size method.MOE, P.R.C. (No. N6C000)* To whom correspondence sbould be adressed.FinalEI) was used to ana-E-mail: guweiping1001@l26.com; Tel: 86 29 84636398lyzeCH_CNMHGnd the hols WhenGU Weiping (顾卫平) et al: Effet ofDriling Process on Fatigue Life of Open Holes5load was applied. The FEM results revealed the possi-Half of them are drilledl 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 alloy .control 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 behavior of the material is shown in Fig. 1, which showsAll the specimens are tested on a servo-hydraulic fa-hat Young modulus is 79 GPa, yield strength is 503tigue testing machine (nstron8802). 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-tain the relationship between the crack length and fa-tigue life. For these two blocks, the max stresses arethe same. The stress ratio R (σmin/σmx) is 0.06 and: 300frequency is 8 Hz in first block. The stress ratio is 0.53and frequency is 16 Hz in second block. During the200-test, the first block loads was applied for 3 min andthen the second block loads was applied for 9 min. The100sequence is repeated until the specimen is fractured.0.02 0.04 0.06 0.08 0.103 Finite Element ModelStrainIn this study, a 3-D FE model is established. The AB-Fig1 tres-strain diagram for alay 7050AQUS/standard finite element package was used to2 Experimentcarry out the analysishis.In the ABAQUS element library, ABAQUS linear2.1 Dimension of specimenshexahedron reduced integration elements C3D8RThe 7050 aluminum alloy is supplied as a sheet with a(three-dimensional eight nodded continwum 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.，10230In order to obtain an optimal result, the mesh aroundthe hole was further refined (as shown in Fig. 4). In1.6电this FEM analysis, the number of elements is 36 582.The boundary conditions are given as follows: one9001676」R50end of model is fixed, the other end of model is uni-6(12formly tensile loaded as 80 MPa.Fig.2 Geometry and size of specimen (mm)中国煤化工2.2 Diriling processesMHCNMHGThere 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 dilingprocesses.Fig.4 Refner 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.Table1 Fatigue life of specimensDriling Stress level Mean fatigue CoefficientMultiple_process(MPa)__ life (cycles) of variationT865912 .0.154 7228(89 6110.148 4651.369463170.509 5501.31(b) Fatigeue fracture under W inslow Drocess60 6050.170 970rig,5 Fatigue fracture surface of holes with different10029 4620.159 7991.42drilling process_W41 8770.154 653Due to the effect of two types of sine block loads onTable 1 shows that: contrasted to traditional drillingcrack propagation, bright and dark strips alternativelyprocess, the fatigue life of open holes is longer whenoccurred at fracture suface. 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 drlling process confines thelength of 0.076 mm-0.1 mm' 14 16. 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 tbat the equivalent initial flaw size isAfter the fatigue test, all fracture surfaces are observedsmaller than 0.125 mm when the Winslow process isby optical microscope, and the typical fracture surfaceused, which meets the remirement of coincidencecritel中国煤化工ss, which yields anof fasten hole is shown in Fig.5, containing the load-initial|YHCNMHGisnotgoodenoughing axis of the specimen normal to the plane. It canto meet the requirement of coincidence criterion.be seen that the crack initiates at the counter holesGU Weiping (顾卫平)et al: Effect of Drlling Process on Fatigue Ljfe of Open Holes574.3 FEM results2007, 47: 35-45. (in Chinese)[2] Zhang 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] Pei X M, Chen W Y, Ren B Y, et al. Effect of diling 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,Joumnal of Bejing 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 DC, Pei X M, Chen W Y, et al. Infuence of ma-the crack initiates at this location. Therefore, thechining processes of fastening boles 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] Pei X M, Chen W Y, Ren B Y, et al. Infuence of machiningprocesses on surface microphotograph and microstructureof fastening boles. The Chinese Journal of NonferrousMetals, 2001, 11(4): 655 659. (in Chinese)[7] Pei X M. The infuence of machining processes of fasten-ing holes on surface work hardening. Machinery Manu-facturing Engineer, 2004, (5): 97-98. (in Chinese)8] Zhang H, Tang Y X, Yu C Y. Effects 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-Fig,6 Stress counter of open boleous aircraft production diling procedures on bole quality.International Journal ofFaigue, 2006, 28: 943-950.5 Conclusions[10] Ralpb 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 Journal 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 holeprocess on the fatigue behavior of countersunk holes.initial fatigue quality and manufacter technique. Journal ofThe following can be concluded:Mechanical Strength, 2000, 22(3): 214-216. (in Chinese)(1) The crack initiates at the counter boles 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 bole.two types of processes.Journal of Northwestern Polytechnical University, 1999, 1:(2) When Winslow driling 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, Elingwood B. Continum damage mechanicsquirement of coincidence criterion.analysis of fatigue crack initiation. Intermational 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 metbod of fatigue crack initiation size. 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