自冲铆接Al-5052接头在不同样品结构下的疲劳强度评估【中文7500字】
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自冲铆接Al-5052接头在不同样品结构下的疲劳强度评估【中文7500字】,铆接,al,接头,不同,样品,结构,疲劳强度,评估,中文
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International Journal of Fatigue 80 (2015) 58 68Fatigue strength evaluation of self-piercing riveted Al-5052 joints under different specimen congurationsSe-Hyung Kang a, Ho-Kyung Kim b,a Graduate School, Seoul National University of Science and Technology, Seoul 139-743, Republic of Koreab Dept. of Mechanical and Automotive Engineering, Seoul National University of Science and Technology, Seoul 139-743, Republic of Koreaa r t i c l e i n f oArticle history:Received 21 January 2015Received in revised form 7 May 2015 Accepted 10 May 2015Available online 5 June 2015Keywords:SPR joints Fatigue lifetime Coach-peel Tensile shearEquivalent stress intensity factora b s t r a c t In this study, static and fatigue tests were conducted using coach-peel, cross-tension and tensile shear specimens with Al-5052 plates for evaluation of the fatigue strength of the SPR joints. For the coach-peel, cross-tension and tensile shear geometries, the ratios of the fatigue endurance limit to static strength were 11%, 14% and 34%, respectively, assuming fatigue cycles of 10 6 for an innite lifetime. The equivalent stress intensity factor range can properly predict the current experimental fatigue lifetime. Fatigue crack initiation occurred due to fretting damage between the upper and lower sheets and between the rivet and these sheets.。 2015 Elsevier Ltd. All rights reserved.1. IntroductionOne of the main objectives in the automotive industry currently is to reduce the weights of automobiles. To achieve this goal, a new joining technology as a replacement for spot welding in light- weight metals, such as aluminum and magnesium alloys, is required in the automotive industry. Riveting methods are often considered as substitutes for spot welding. Among the several types of riveting methods available, the self-piercing riveting (henceforth SPR) process is gaining in popularity due to its many advantages. SPR does not require a pre-drilled hole, and this method can be used to join a wide range of materials, including combinations of similar or dissimilar materials.SPR is essentially a cold-forming joining process. During the SPR process, a semi-tubular rivet is pressed by a punch into the sheets. The rivet pierces the upper sheet and ares into the bottom sheet under the inuence of an upset die. A mechanical interlock is formed between the two sheets, which is key to the structural strength of the joints.The fatigue strength of the SPR joints has been investigated by a number of authors for a number of materials 16. For example, Mori et al. 2 examined the static and fatigue strengths of spot-welded and self-piercing rivet joints in aluminum alloy sheets Corresponding author. Tel.: +82 02 970 6348; fax: +82 02 979 7032.E-mail address: kimhkseoultech.ac.kr (H.-K. Kim).under tensile shear and cross-tension congurations. They observed that while the static strength of the self-piercing rivet joint was about 1.5 times as large as that of the spot-welded joints, the fatigue strength was increased by about three times in the ten- sile shear conguration. He et al. 3 investigated the strength, stiffness, impact resistance, failure modes and failure mechanisms of SPR joints with similar and dissimilar metal sheets consisting of an aluminum alloy and a copper alloy. They reported that the fati- gue strength of SPR joints was largely affected by the properties of the sheets and that both the static and fatigue strength of SPR joints increased with an enhancement of the joint stiffness. Xing et al. 4 investigated the static and fatigue strength of multiple-rivet SPR joints. They reported that these levels are inu- enced by the rivet number and rivet distribution pattern. Franco et al. 5 investigated the possibility of joining aluminum alloys and carbon ber composites using SPR. They reported large values of the fatigue resistance of SPR joints, even for load amplitudes close to the maximum static resistance of the joint and a fairly large range of fatigue strengths. Su et al. 6 investigated the fati- gue behavior of SPR and clinch joints in tensile shear specimens of aluminum sheets. They reported that the experimental fatigue lives of these joints can be estimated using structural stress solutions.However, fatigue lifetime data of a SPR joint is normally reported as a function of the applied load range 79. The reported fatigue strength data are not high enough to apply the other types of specimens due to the obscurity of the various factors that govern/10.1016/j.ijfatigue.2015.05.003 0142-1123/ 。 2015 Elsevier Ltd. All rights reserved.Contents lists available at ScienceDirectInternational Journal of Fatiguejournal homepage: /locate/ijfatigue S.-H. Kang, H.-K. Kim / International Journal of Fatigue 80 (2015) 58 68 59Fig. 1. Stress strain curves of the Al5052-H32 alloy.Table 1Mechanical properties of the Al5052-H32.Material ru (MPa) ry (MPa) E (GPa) Elong. (%)Al5052-H32 251.7 186.7 78.3 10their fatigue strengths. The fatigue lifetime of a SPR joint specimen is generally dependent on the load magnitude, the loading type, the dimensions and conguration of the specimen, the sheet mate- rial, and other factors. Even with the same rivet diameter, sheet material, and sheet thickness, the load range amplitude represent- ing the fatigue strength can differ from one specimen type to another due to different loading types. Therefore, the fatigue strength data for the SPR joints under several types of loading are needed in order to design a structure with SPR joints. To solve this problem, it is desirable to adopt general structural parameters, such as the stress, strain, and multiaxial fatigue criteria, to assess the fatigue lifetimes of these joints. Thus far, there has not been any report on appropriate fatigue strength parameters to correlate the fatigue lifetimes of SPR joints with different specimen congurations.Therefore, in this study, fatigue tests under constant amplitude loads are conducted using coach-peel, cross-tension and tensile shear specimens of Al-5052 aluminum alloy sheets to evaluate the fatigue strength of SPR joints under different specimen cong- urations. The experimental fatigue lifetimes of SPR joint specimens are also estimated using fatigue strength parameters. Finally, appropriate parameters for evaluating the fatigue lifetimes of three types of specimens are proposed.2. Experimental procedure2.1. Specimen preparation and fatigue testAl5052-H32 aluminum alloy sheets with a thickness of 1.5 mm were joined by SPR. Tensile tests on the sheet material were con- ducted in order to obtain the tensile stress strain curve for a FEM structural analysis. The tensile specimen was machined to a uniform gage length and width of 70 mm and 12.5 mm,(b) cross-Fig. 3. Cross-section of the SPR joint after riveting.respectively. Fig. 1 shows the engineering stress strain curve for the Al5052-H32 alloy. The mechanical properties of the material are summarized in Table 1.60 S.-H. Kang, H.-K. Kim / International Journal of Fatigue 80 (2015) 58 68Fig. 4. 2-D mesh about cross section from the center of the SPR joint.SPRFig. 7. Comparison of the applied load versus the displacement curves for the three specimen types.Fig. 5. 3-D FEA models of SPR joints: (a) coach-peel, (b) cross-tension and (c) tensile shear specimens.Coach-peel, cross-tension and tensile shear specimens, as shown in Fig. 2, were utilized to evaluate the static and fatigue strengths of the SPR joint. Steel rivets of 0.35 wt.% carbon steel with an aluminum surface coating (Almac) were used. Rivets with a diameter of 5.0 mm and a length of 5.0 mm were supplied by Henrob Ltd. A servo-hydraulic universal testing machine (Instron 8516) with a capacity of 100 kN was used for the SPR joining, static and fatigue tests. A special xture was used for the SPR joining pro- cess. The xture consists of a punch, a die and a blank holder. The xture is mounted into a universal testing machine by xing the die and punch with hydraulic grips. During the SPR process, the punch pushes the rivet through the hole in the blank holder, while, the die moves toward the blank holder to clamp the upper and lower sheets which are positioned between the die and the blankS.-H. Kang, H.-K. Kim / International Journal of Fatigue 80 (2015) 58 68 61holder. Fatigue tests were conducted at a load ratio R = (Pmin /Pmax ) of 0.1 at a frequency of 12 Hz for the tensile shear specimen and 2 Hz for the coach-peel and cross-tension specimens.2.2. Structural analysis of the SPR joint specimenA three-dimensional nite element analysis (FEA) of a single self-piercing rivet was performed. The SPR joint, as shown in Fig. 3, is not perfectly axisymmetrical. Therefore, the shape and dimensions of the rivet after SPR joining were determined, as shown in Fig. 4, after averaging the dimensions of the rivet from its center. Finally, FEA models with a single SPR joint were com- pleted for the coach-peel, cross-tension and tensile shear speci- men geometries, as shown in Fig. 5. FEA analyses were carried out using ABAQUS (version 6.6) for the solver and HyperMesh (ver- sion 7.0) as the pre- and post-processor.Table 2Summarized fatigue testing results for the (a) coach-peel, (b) cross-tension and (c) tensile shear specimens. Note: 1 and 2 depict cracking failure in the upper and lower sheet, respectively.Pamp (N) N f (cycles) Failure type(a)A joint specimen was modeled using solid elements of C3D6 and C3D8. The models for the coach-peel, cross-tension and ten- sile shear specimen were composed of 53,522 nodes with 44,480 elements, 57,176 nodes with 47,376 elements, 57,161 nodes with 47,576 elements, respectively. Contact between the rivet and sheet and between the upper and lower sheet faces was introduced by means of the master slave technique. The friction coefcients between the rivet and sheet and between the upper and lower sheet faces were assumed to be 0.2 and 0.15, respectively 10 . True stress strain curve data with a non-linear kinematic harden- ing elastic plastic material model, as shown in Fig. 1, was adopted in the structural analysis.3. Experimental results and discussion3.1. Optimal punching force for SPR joiningFor SPR specimens, the joint strength is dependent on the sheet thickness, rivet diameter, die geometry, joining force, and other factors. In this study, a series of monotonic tensile tests was con- ducted on tensile shear specimens with different amounts of punching force in an effort to determine the optimal punching force.Fig. 6 shows the punching force against the maximum tensile shear force for the SPR specimens in this study. Each data point is the average value from two specimens. As the punching force increases, the maximum tensile shear force increases continu- ously. The peak value is reached at a punching force of 21 kN with subsequent uctuation, as shown in Fig. 6. Therefore, the optimal punching force was determined to be 21 kN for the current SPR joining condition. All of the coach-peel, cross-tension and ten- sile shear SPR joint fatigue specimens were manufactured at the optimal punching force of 21 kN.3.2. Evaluation of the monotonic strength of SPR jointsFig. 7 shows the monotonic test results after testing the force against the displacement for the coach-peel, cross-tension and ten- sile shear SPR specimens produced with punching force of 21 kN. The coach-peel, cross-tension and tensile shear specimens exhibitFig. 8. Load amplitude against the number of failure cycles for the three types ofspecimens.166.1 14,646 1135.9 28,393 1117.8 40,836 199.6 71,030 190.6 121,321 187.6 429,880 181.5 732,905 175.5 399,278 169.4 1,785,200 1(b)340.7 51,232 1327.6 71,473 1288.2 97,449 1275.1 191,206 1262.0 299,691 1248.9 334,617 1235.8 617,116 1222.7 673,989 1216.2 662,414 1203.1 973,297 1209.6 1,061,163 Non(c)1389.6 139,714 11327.8 239,369 11312.4 199,629 11296.9 340,824 11281.5 474,155 21273.8 554,891 11266.0 110,889 11250.6 463,045 1,21242.9 542,731 1,21235.2 432,344 21219.7 364,706 21212.0 744,908 21204.3 495,545 11204.3 1,177,302 11188.8 1,317,694 21173.4 912,297 21158.0 774,896 21158.0 640,458 11142.5 1,580,590 21142.5 984,742 21127.1 2,004,214 21080.8 1,909,862 21065.3 1,467,037 21034.4 1,944,137 21003.6 2,208,391 2972.7 3,575,872 Non62 S.-H. Kang, H.-K. Kim / International Journal of Fatigue 80 (2015) 58 68maximum forces of approximately 700 N, 1650 N and 3450 N, respectively. The upper and lower sheet partially separated and failed after they reached the peak amount of force. The magnitudes of displacement for the coach-peel and cross-tension specimens were greater than that for the tensile shear specimen under the same applied load. This implies that SPR joints are vulnerable to coach-peel and cross-tension loading as compared to tensile shear loading. This also occurs in spot-welded and mechanical pressed joints 11 .3.3. Evaluation of the fatigue lifetimes of SPR jointsFatigue tests were conducted on SPR joint specimens, with three types of geometries under a controlled cyclic load. Thefatigue lifetimes and failure modes are summarized in Table 2. Fig. 8 shows the applied load amplitude against the fatigue life- times for the coach-peel, cross-tension and tensile shear speci- mens. The fatigue failure time was dened as a visible failure of the specimen. The fatigue strength of the tensile shear specimens was found to be much higher than those of the coach-peel and cross-tension specimens. The difference is primarily due to the loading conditions on the SPR joint. The load amplitudes, corre- sponding to the fatigue strength at 10 6 cycles for the coach-peel, cross-tension and tensile shear specimens, are 71 N, 210 N and 1150 N, respectively. These values are approximately 11%, 14% and 34% of the corresponding static strengths. The coach-peel and cross-tension specimen geometries have very low fatigue ratios, compared to that of the tensile shear geometry, similar toFig. 9. Maximum principal stress distribution of SPR joints for fatigue lifetimes of 1.0 10 6: (a) coach-peel, (b) cross-tension and (c) tensile shear.S.-H. Kang, H.-K. Kim / International Journal of Fatigue 80 (2015) 58 68 63Fig. 10. Fatigue failure specimens: (a) coach-peel, (b) cross-tension, (c) tensile shear in the high-loading range and (d) tensile shear in the low-loading range.Fig. 11. Fatigue-fractured surface of the coach-peel experiment specimen in the low-loading condition (Pmax = 154.3 N); (a) fracture surface of the front view, (b) enlarged local area marked in (a), (c) enlarged local area of the crack initiation location, (d) enlarged local view of the area around the rivet, and (e) enlarged local area of the at fracture surface.the behavior of spot-welded and clinched joints 11 . The load amplitude as a function of the number of failure cycles can beexpressed as: Pamp 715:5 N 0:166 , Pamp 1967 :3 N 0:162 andfrom the FE analyses. The von Mises stress was found to be not as good as the maximum principal stress for identifying potential crack initiation positions. Fig. 9 shows the maximum principal6Pamp 3395 :5 N 0:078 for the coach-peel, cross-tension and ten- stress distribution around the rivet at fatigue lifetimes of 10 forsile shear specimens, respectively.3.4. Structural analysis resultsThe fatigue crack initiation sites observed in the experiments are close to the locations with the maximum principal stressesthe three specimen geometries. Fig. 9(a) shows the stress distribu-tion of the coach-peel specimen at P = 129 N with a maximum stress of approximately 203 MPa. Fig. 9(b) shows the stress distri- bution of the cross-tension specimen at P = 408 N with the maxi- mum stress of approximately 217 MPa. These maximum stresses for the coach-peel and cross-tension specimens are located at fay- ing (bottom) surface o
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