VHCF-6论文集AM17

Study on the Fatigue Properties of the TIG Weld Joint with the Stainless SteelConduit in AircraftYing Liu, Dongjie Li*, Xiaohong LiAvic Beijing Aeronautical Manufacturing Technology Research Institute,10024, China* Corresponding author: djli@Abstract The research focus on the material of the stainless steel thin conduit in aircraft, named 1Cr18Ni9Ti , and the TIG weld joint of which was investigated to analysis the fatigue properties. The fracture mechanics was used to analysis the crack initiation life and crack propagation life, and the fatigue surface was characterized with scanning electron microscope (SEM). The experimental and analytical results show that, the origin position of fatigue crack is the surface of the conduit. The stress concentration at the weld toe, the crystal structure is not uniform and Stress concentration in the heat affected zone (HAZ) and fusion line, so the fatigue cracks are easily generated in these locations. Delta K increases to a certain value, the HAZ has become one of the most dangerous position. The crack initiation life of HAZ in the total fatigue life is far higher than the proportion of crack propagation life.Keywords1Cr18Ni9Ti stainless steel, Fracture mechanics, Fatigue life, Fatigue crack1. IntroductionThe fatigue fracture is the major form of the metal structure failing, and which may be in the total percentage of 80%. The fatigue fracture of the welding structure is mainly because of the fatigue cumulative damage of the weld joint. Therefore, the fatigue performance of the weld joint directly affects the ability of anti fatigue fracture of weld structure[1].Aero-engine conduit system is an important part of the engine accessory system, which is mainly used for the transportation of fuel, hydraulic, lubricating oil and air. The conduit system is working under the complex stress state, contents the internal pressure of a liquid or gas, the assembly stress and the dynamic stress caused by engine vibration. Conduit state directly affects the safety of the engine, and the connecting method of the conduit is mainly used by the welding technology. According to reports, in the process of the development of new aircraft, defects in the weld zone (WZ) and the crack of the HAZ causing the conduit fracture or joint leakage fault is very frequent. As one type of aero-engine appeared continuously two fuel leak, causing the engine parking accident in the air. To analyze the accident indicated that, the relatively bigger assembly stress caused the conduit fatigue fracture, and in this process the welding defects play an important role in the promoting of the fatigue crack initiation.Aero-engine conduit material 1Cr18Ni9Ti belongs to the austenitic stainless steel, it has the characteristics of good corrosion resistance, flexibility, vibration resistance and wide temperature using range, at the same time, this steel has good mechanical properties and oxidation resistance, which is widely used in aircraft hydraulic, fuel, gas conduit system. In accordance with the conventional TIG welding of 1Cr18Ni9Ti weld joint of the typical of aero-engine conduit structure, the bending fatigue test was carried out to analysis the fatigue life of the welding structure. Fatigue crack growth rate testing of 1Cr18Ni9Ti TIG welding joint was carried out to analysis the crack initiation life and the crack propagation life. The SEM, optical microscope (OM) methods was used to observation and analysis the fatigue fracture.2. ExperimentalFigure 1. Bending fatigue specimenTest material is 1Cr18Ni9Ti austenitic stainless steel and the chemical composition is 0.10C, 1.56Mn, 0.70Si, 17.75Cr, 9.84Ni, 0.45Ti, the other is Fe. As service, the stress ratio of vibration fatigue load of the 1Cr18Ni9Ti conduit during is -1. Because the test object for the conduit is the thin wall structure, test of the vibration fatigue has certain difficulty. So the rotary bending fatigue of the weld joint was used to simulate the service load conditions. The TIG welding structure for the fatigue life test sample is shown in Fig. 1. 1Cr18Ni9Ti conduit and the processed butt was welded under the current 27 A, filled with diameter of 1 mm welding wire in the same chemical composition of base metal. The stress ratio R of the test is -1, the average str ess σm is 0, the stress peak value is between 200-300 MPa. For the butt welding of 10×1 mm conduit, the mechanical properties of weld joint is shown in table 1.Table 1. mechanical properties of base metal and weld jointR p0.2(MPa) R m(MPa) A(%)Base metal 196 600 66.5Weld joint 279 567 -Figure 2. Three point bending specimenThe fatigue crack growth rate tests using the same welding process welding as the conduit welding. Welding test plate is processed into a three point bend specimen standard, the thickness of the sample is 4 mm, and the sample shape is shown in Fig. 2. The test used the INSTRON 8801 50KN testing machine. The change of crack length obtained by the extensometer gap mouth displacement record, the calculation process is automatically completed by the testing machine. After the test, metallographic examination and fracture analysis of microstructure were performed in the OLYMBUS BX51M OM and CS-3400 type SEM.3. Results and discussion3.1. Analysis of fatigue life of welded conduitFigure 3. Fatigue test dataThe actual vibration stress value of the application welded conduit is less than 100 MPa, the purpose in this research of improving the stress peak value to 200-300 MPa is to shorten the test cycle. Fig. 3 are shown the 1Cr18Ni9Ti welding test results of fatigue life under the different stress, and from which we calculate the median fatigue life of N50 according to the original data. In double logarithmic coordinates, N50 data points into a linear distribution; linear fitting by least square method to the S-N relation is obtained:lgS=537.37253-52.9539lgN (1) From Eq. 1, when the peak stress in the 100 MPa, the fatigue life of welded conduit is 1.28×1010, which performance good fatigue property.Figure 4a. The SEM for the fracture morphologyFigure 4b. The SEM for the fatigue fracture zoneFigure 4c. The SEM for the ductile fracture zoneFigure 4. The SEM for the fracture morphologyFig. 4 is the bending fatigue specimen SEM photograph (Fig. 4A) and fracture detail picture (Fig. 4B and Fig. 4C). Fatigue fracture zone exist obvious bending fatigue specimen fracture (region I) and ductile fracture zone (II), as shown in Fig. 4a. Fatigue fracture zone detail pictures as shown in Fig. 4b, the fracture is with no obvious plastic deformation and the inter wall of the conduit can be observed fatigue crack source. The fracture exist obvious local tearing ridge, since the beginning of the specimen surface within a radial divergence state. The crack originated from the surface of the sample.Fig. 4C shows a ductile fracture zone picture. For ductile fracture zone, the fracture morphology is full of dimples. Combined with the Fig. 4a, the plastic deformation is obvious in this area, which is due to fatigue test loading way. This process is accompanied with torsion and shear stress, which increased the deformation of conduit joint.From fracture analysis of Fig. 4, fatigue crack originated in the catheter surface defects. The fatigue crack is on the one hand along the radial extension to the surface, on the other hand along the circumferential expansion. Continued expansion of cracks, the stress on the remaining section continues to increase, while the remaining area is not enough to support the load, i.e. instantaneousoccurrence of fracture[2-4].Figure 5. OM for the rotating bending fatigue specimenRotating bending fatigue specimen is shown in Fig. 5. From Fig. 5, the fracture position is the weld toe. Because the height position of weld fusion line is between the base metal and weld joint, the stress concentration lead the conditions for the generation of fatigue crack, reduces the fatigue life of welded joints.3.2. The crack initiation life and propagation lifeFigure 6a. OM for the crack of WZFigure 6b. OM for the crack of HAZFigure 6. OM for the crack of WZ and HAZFig. 6 shows the fracture of crack growth rate test (Fig. 6a) and heat affected zone (Fig. 6b) morphology of fracture surface. As shown in Fig. 6a, weld crack is perpendicular to the weld columnar crystal, and interface of the crack is smooth, showing uniform microstructure and residual stress state. As shown in Fig. 6B, the heat affected zone interface is undulating, from the heat affected zone extends to the fusion line position, showing the organization and the regional stress state is fluctuation. The grain of the HAZ grew up during thermal cycling, at the same time, the HAZ go through tissue expansion and contraction process, remaining larger residual stress[5]. The fusion line of the welding thermal cycle process of melting, solidification process experience part of the organization, the organization condition and the residual stress nonuniformity are large. The crystal structure of the fusion line is not uniform and Stress concentration in this zone. So the HAZ and adjacent to the fusion line easily become the weak link of the weld joint[6].Figure 7. Curves of crack growth rateFig. 7 shows the test of crack growth rate curves of the base metal, the WZ and HAZ. From Fig. 7, the slope of fatigue crack growth rate of HAZ is relatively high. When the stress intensity factor range Delta K reaches a certain value, the crack growth rate (da/dn) of HAZ is bigger than which of the WZ and base metal, becomes to the most dangerous area. Therefore, the following calculation focuses on the HAZ of crack initiation and propagation characteristics.The relationship between crack growth rate of fatigue crack propagation stage and the stress intensity factor range could represent by Paris law:da/dn=CΔK m(2) Combined with the crack growth rate curve of Fig. 7, the parameter C in the Paris law is 4.599×10-13, and m is 3.85183.Based on the BS7910:ΔK=YΔσ(πa) 1/2(3) In which, Y is the shape factor, can be calculated according to the BS7910. Delta sigma is the stress amplitude, using the peak value stress. Assuming that the crack length is from 0.1 extended to 0.5 mm, and then the Eq. 2 only has a unknown value, the life of fatigue crack growth of delta n. To obtain the fatigue crack propagation life, combined with the Eq. 1, the crack initiation life could be calculated.Figure 8. Curves of the crack propagation lifeThe crack initiation life and crack propagation life are shown in Fig. 8.As Fig. 8 shows, the fatigue crack initiation life accounted for most of the total fatigue life, and crack initiation life is negligible. Therefore, as the fatigue crack initiation, it may be quickly extended until fracture. Fatigue crack that once found, it should be immediately stoped using the welded conduit.4. ConclusionIn view of stainless steel 1Cr18Ni9Ti conduit, the crack growth rate test and fatigue life test were taken off. The characteristics of fatigue crack and fracture morphology were investigated. The conclusions are as follows:(1) The fatigue life of weld joints is able to meet the use requirements, the fatigue crack source in the surface of the conduit position, and the fault location is the weld toe. The weld toe position concentrated stress reduces the fatigue life of welded joints.(2) The stress state of HAZ and fusion line are not uniform, and the slope the crack growth rate curve is relatively bigger, which is considered to be the dangerous position of weld joint.(3) The HAZ of fatigue crack initiation life is accounted for most of the total fatigue life. As the fatigue crack initiation, it quickly expanded until rupture. Therefore, the results showed that the fatigue crack after welding conduit once be seen, it should immediately stop using.References[1] Y. Liu, H. Zhang, Z. zhao, Study on Damage tolerance of Stainless Steel Conduit in Aircraft.Aero Manu Tech, 13 (2012) 94–97.[2] P. J. Haagensen, Fatigue improvement techniques advantages and limitations. Weld World,47(2003) 43–63.[3] X. Gang, Effect of compressive surface stress on the fatigue of 10Ni5CrMoV steel. Devel AppMate,2( 2009) 1–3.[4] X. H. Cheng, J. W. Fisher, H. J. Prask, Residual stress modification by post-weld treatment andits beneficial effect on fatigue strength of welded structures. Inter Journal Fatigue, 25(2003) 1259–1269.[5] M. H. Kim, S. W. Kang, J. M. Lee, Evaluation of fatigue strength improvement of shipstructural details by weld toe grinding. Key Eng Mater, 324 (2006) 1079–1082.[6] C. M. Sonsino, Fatigue life improvement of welded structures by post-weld treatments andsome limitations by geometry and loading mode. Revue de Metallurgie, 104(2007) 51–57.。