Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A001 3-Dimensional Observation of the Interior Fatigue Fracture Mechanism on Friction Stir Spot Welded AISI 1012 Cold Rolled-steel Sutep Joy-A-Ka1,2, Hiroyuki Akebono2, Masahiko Kato2, Atsushi Sugeta2, Yufeng Sun3 and Hidetoshi Fujii3 1 Materials Properties Analysis and Development Centre, TISTR 35 Moo 3 Khlong Ha, Khlong Luang, Pathum Thani 12120, Thailand, [email protected] 2 Department of Mechanical Science and Engineering, Graduate School of Engineering, Hiroshima University 1-4-1 Kagamiyama, Higashi Hiroshima, Hiroshima 739-8527, Japan 3 Joining and Welding Research Institute, Osaka University 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ABSTRACT In this study, the authors describe an approach to evaluate the fatigue fracture mechanism of cross-tension specimen using low carbon steel by friction stir spot welding. The results show that the fatigue limit of FSSW specimens was very low compared with the static strength of the welded joint. Fatigue fracture modes were independent on the force amplitude. The main crack initiated at the slit tip. The fatigue crack initiation life was dependent on the force amplitude. Base on the 3-dimensional observation, it is clarified that the fatigue crack in the welded joint used in this study initiated at the slit tip then propagated with the complex behavior regardless of the force amplitude level. The macroscopic fracture modes were independent on the force amplitude. The fatigue initiation life was dependent on the force amplitude. In other words, the fatigue crack initiation life under low force amplitude accounted for a comparatively large proportion of the entire fatigue life; whereas the fatigue cracks initiation life occurred in a relatively early stage under high force amplitude Key Words: Friction stir spot welding, Fatigue fracture mechanism, 3-Dimensional observation, AISI 1012 1. INTRODUCTION Friction stir welding (FSW) was developed and patented by The Welding Institute of the UK in 1991 and was initially applied to aluminum and its alloys (Ma, 2008; Mishra & Ma, 2005). This relatively new solid-state joining process is energy efficient, environmentally friendly, and versatile. A new application of the FSW technique, which is called friction stir spot welding (FSSW), has been developed as a superior alternative to resistance spot welding and riveting for the fabrication of automotive and railway body components (Feng et al., 2005; Pathak, Bandyopadhyay, Sarangi, & Panda, 2012). FSSW was invented at Kawasaki Heavy Industries Ltd. in 2000 as a variant of the linear FSW method (Sun, Fujii, Takaki, & Okitsu, 2012); furthermore, in 2003, FSSW was first used to fabricate the rear door panel of the Mazda RX-8 (Wang & Lee, 2007). With FSSW, energy consumption and operational cost are significantly lower than other welding methods such as arc, laser, and gas welding techniques. In the automotive industry, the fabrication of each vehicle currently requires 2000– 5000 spot welds (Tanegashima, Akebono, Kato, & Sugeta, 2013) . Therefore, the abovementioned successful application of FSSW technology has attracted considerable attention from the automotive industry. Previous research of FSSW has focused on microstructure analysis, mechanical properties, and how to produce a joint that optimizes the corresponding parameter sets. However, to this end, most previous studies of mechanical properties have employed only static tensile and hardness test (Shiraly, Shamanian, Toroghinejad, & Ahmadi Jazani, 2014). Few authors have performed fatigue tests on alloy series to analyze the strain behavior (Boroński, Sołtysiak, & Lutowski, 2015) or determine fatigue life, failure modes, and microstructure of the failed specimens (Lin, Pan, & Pan, 2008). Some studies have Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A001 reported on the FSSW of carbon steels (Cui, Fujii, Tsuji, & Nogi, 2007). However, few reports exist on the fatigue fracture mechanism and fatigue crack propagation behavior in FSSW-joined carbon steel, particularly for cross-tension FSSW specimens (Uematsu, Tokaji, Tozaki, Nakashima, & Shimizu, 2011). Moreover, some stress concentration parts remain at the area welded by FSSW: a hollow called a keyhole caused by the tool shape, a discontinuity in shape called a lip, and a slit between the upper and lower sheet. Furthermore, the area welded by FSSW has several microstructures: the heat-affected zone (HAZ), thermo-mechanically affected zone (TMAZ), and stir zone (SZ). It is highly advantageous to identify the relationship between the shape of the welded area unique to FSSW, the microstructure, and the fatigue crack behavior; however, details are not definitively known. Therefore, the purpose of this study is to investigate the fatigue properties and fracture mechanism of friction stir spot welded AISI 1012 cold rolled-steel sheets. Furthermore, fatigue crack initiation and fatigue crack propagation were observed by using the 3-dimensional observation. 2. EXPERIMENTAL PROCEDURES AISI 1012 cold-rolled steel sheet 0.8 mm in thickness (0.12%C, 0.50%Mn, 0.04%P, 0.045%S and bal. Fe) were used in this study. AISI 1012 cold-rolled steel sheet is characterized by its good machinability, formability, and hardenability. It can be used in general applications, particularly in manufacturing automobile parts. The mechanical properties of this steel are 0.2YS of 172 MPa, UTS of 314 MPa, Elongation of 48%, and Hardness value of 115 HV. Figure 1(a) shows the shape and dimensions of the FSSW specimens after the steel sheet was machined into rectangular pieces measuring 150 mm in length, 50 mm in width, and 8.1 mm in punch diameter. Two rectangular steel plates were used to prepare the cross-tension FSSW specimens at the Joining and Welding Research Institute (JWRI), Osaka University. The two rectangular sheets were placed one on top of the other to form a cross shape and welded in the center of the overlapping square region. The FSSW tools were manufactured from tungsten carbide (WC) with a shoulder diameter of 12 mm, probe length of 1 mm, and diameter of 4 mm. The peak temperature of the welded zone (approximately 973 K) could be achieved when FSSW was conducted in air with a force of 15.0 kN, rotational speed of 550 rpm, and weld holding time of 2 s. Figure 1: (a) Shape and dimensions of specimen, (b) Installed to testing machine. Fatigue tests were performed at room temperature with the fatigue testing machine operating at a sinusoidal wave frequency of 5–10 Hz at a force ratio R of approximately 0.01. The specimen was set up in the machine with the upper and lower sheets clamped with rectangular jigs where the plates do Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A001 not overlap, as shown in Figure 1(b). The load was applied to the specimen in the direction perpendicular to the face of the plate. The failure criterion was specified as the point of complete separation of the FSSW joints between the upper and lower sheet. The waveform for the constant force amplitude P is shown in Figure 2. a Figure 2: Illustration of constant force amplitude waveform. 3. RESULTS AND DISCUSSION 3.1. Macrostructure and Microstructure Observation The microstructure of the cross section of the welded area was observed using an optical microscope. The welded area was divided into four primary regions, as indicated in Fig. 5. BM represents the base metal, which was unaffected by heat and deformation. HAZ represents the heat- affected zone which was affected by heat only with the largest area on the welded joint. TMAZ represents the recrystallized structure, termed the thermomechanically affected zone which was affected by both heat and deformation. The TMAZ was small area, corresponding to the width of the shoulder on the upper surface and to the probe diameter at the root. The recrystallized structure was within SZ, termed the stir zone, which corresponded to the smallest area of the probe diameter. Figure 6 shows the microstructures of the four zones. The results show that the base metal and the HAZ contain ferrite in their band structures. The welded zones (the SZ and TMAZ) had a fine ferrite–pearlite dual-phase structure, resulting from the prevention of martensite formation. Therefore, the microstructures can be explained in terms of the peak temperatures of the welding process, which are below the A point 1 (eutectic temperature) on the Fe–Fe C phase diagram. Additionally, the FSSW specimen was tested 3 without inducing a phase transformation(Uematsu et al., 2011). Figure 3: (a) Optical macroscopic image showing the cross-sectional macrostructure of the welded zone, (b) Optical microscopic image showing the cross-sectional microstructure of 4 zones on the welded joint. Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A001 3.2. Static Tensile and Fatigue Test Results The result of tensile test for the welded joint is shown in Figure 4(a). The static strength of the FSSW specimen used herein was observed to be approximately 8 kN. The welded joint fractured at the region between the base metal and the HAZ around the diameter of the FSSW tool shoulder on the upper sheet, as shown in Figure 4(b). Figure 4: (a) Force-displacement curve of FSSW, (b) The fracture of welded joint after tensile test. Fatigue test results under constant force amplitude are shown in Figure 5(a) as a plot of the force amplitude versus the number of cycles to failure. The fatigue limit of the welded joint was obtained by the JSMS standard ‘‘Standard Evaluation Method of Fatigue Reliability for Metallic Materials: Standard Regression Method of S–N Curves’’. The results show that the specimen had a fatigue limit of approximately 0.1 kN. The FSSW joints can be seen to possess significantly lower fatigue resistance, as their fatigue limit was very low compared to the static strength of the welded joint (approximately 8 kN), as mentioned above. Figure 5: (a) P-N curve, (b) Schematic illustration of the FSSW specimen with x-y-z direction incorporated with loading direction, (c) Fracture morphologies of FSSW under constant low force amplitude (P = 0.19 kN), a (d) Fracture morphologies of FSSW under constant high force amplitude (P = 0.50 kN). a Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A001 The fractured specimen was then observed macroscopically. Figure 5(b) shows a schematic of the FSSW specimen, along with the loading direction in the x–y–z coordinate system. Figures 5(c) and (d) show the macroscopic observation results of the fractured specimens near the welded zone under constant low (P = 0.19 kN) and high force (P = 0.50 kN) amplitudes, respectively. The fracture mode a a corresponding to the high force amplitude was observed to be similar to that corresponding to the low force amplitude. The fatigue crack of the upper sheet first propagated around the welded zone and then propagated further into the base metal until the specimen was broken. The fatigue crack in the lower sheet propagated similarly. To specify the crack initiation site, the cross section of the welded joint was fatigued under the constant low force amplitude P = 0.19 kN and interrupted before the final fracture was observed by an a optical microscope. The interrupted cycle N was 7.88 × 104 cycles, and the complete fatigue life N stop f was 1.28 × 105 cycles, which was obtained by the regression equation from P-N curve, as expressed by log P=–0.325logN+3.956 (N /N was approximately 62%). The observation results are shown in a stop f Figure 6. The crack was observed to initiate at the boundary between the edge of the welding interface zone and the noninterface zone, which is located in the HAZ. The fatigue crack on the upper sheet was found to start at the distal slit and continue to the surface of the sheet up to the concave zone. In addition, we confirmed that the crack initiation sites of all welded joints were the same, irrespective of the force amplitude level. These observations revealed that the microstructure near the welded zone hardly affected the fatigue crack initiation and propagation behavior because the fatigue crack initiated at the slit tip which is located in HAZ regardless of the microstructure. Figure 6: Macrograph of cross-sectional welded zone under constant low force amplitude. 3.3. Fatigue Behavior under Constant Force Amplitude Conditions To investigate fatigue crack propagation behavior, a unique three-dimensional observation was performed for the FSSW specimen under low and high force amplitudes. This research mainly investigates about the fatigue crack propagation conducting in the macro and microscopic observation of the fracture surface. Throughout analysis, the fatigue crack initiation and propagation should be observed 3-dimensionally for the detailed consideration, because their behavior appeared at the interface between two thin steels sheet. Therefore, 3-dimensional observation of the small fatigue crack initiated on FSSW was carried out. At first, the specimen was cut out so that the area included welded joint, after the fatigue test was interrupted at the x% cyclic number for the whole fracture life called as N/N. We obtained an observed result at the cross-section using the optical microscope after specimen f was polished and etched. The specimen was polished in each step by approximately 250 µm towards the width direction; observation image was taken using the same procedure mentioned above. Using about 30-100 observation images taken at the cross-section a specimen up to the %N/N, the 3- f dimensional fatigue crack propagation was produced by the 3-dimensional graphics software. According to schematic illustration in Figure 5(b), Figure is shown the FSSW specimen along with x-y-z directions and loading direction shown in three-dimensional morphologies of the fatigue crack obtained in each observational result at the cross-sectional area. Figure 7 shows the fatigue crack propagation behavior under constant low force amplitude. On 61.5%N/N, fatigue crack propagated f toward thickness direction of upper and lower sheets. Moreover, there was no fatigue crack near the Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A001 welded on 55.9%N/N. This fact implies that specimen used in this study requires many cyclic loading f to initiate the fatigue crack in the low force amplitude level. Figures 7(c) and (d) shown that the fatigue cracks grow to the full thickness of upper and lower sheet, and that the fatigue cracks tend to grow around the welded joint as equal to the diameter of shoulder of FSSW tool. Finally, on 83.3%N/N the f fatigue crack propagation of upper sheet grows to the both side of base metal, and two sides of fatigue crack propagation of lower sheet come across nearly circular and propagated to base metal. After the initiation, as the number of cyclic increases, the fatigue crack grew gradually the thickness direction. It is likely for the fatigue crack propagate from the center of spot welded toward loading direction. Figure 7: Crack propagation of FSSW at low force amplitude level (P = 0.19 kN); (i) 3-dimension, (ii) 2- a dimension, (a) 61.5%N/N, (b) 67.1%N/N, (c) 78.3%N/N, (d) 83.3%N/N. f f f f Figure 8 shows the fatigue crack propagation behavior under constant high force amplitude. On 23.4%N/N, Figure 8(a) shown that the fatigue crack propagated with 0.7 mm towards thickness f direction of upper sheet. This indicates that the fatigue crack has grown to the full thickness of the steel. Furthermore, there was no fatigue crack near the welded spot on 15.6%N/N. This fact implies that cross f tension specimen used in this study requires few cyclic loading to initiate the fatigue crack in the high force amplitude level. On 45.2%N/N, Figure 8(b) shows two fatigue cracks on upper sheet. In this case, f fatigue crack grows to the full thickness of sheet. And, the fatigue cracks tend to grows around the welded joint as equal to the diameter of shoulder of FSSW tool and further it grows to the one side on Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A001 base metal. Another fatigue crack was occurred opposite side of the first crack with propagation of 0.4 mm toward thickness direction and the crack length 1.5 mm near the welded joint. Finally, Figures 8(c) and (d) show two fatigue cracks on upper sheet those fatigue cracks grows to the full thickness of sheet and tend to grows around the welded joint. Further, first fatigue crack grows to the one side on base metal and another fatigue crack was occurred opposite side of the first fatigue crack around the welded joint. As cracks on lower sheet, two cracks were occurred and grow around the welded joint. Figure 8: Crack propagation of FSSW at high force amplitude level (P = 0.50 kN); (i) 3-dimension, (ii) 2- a dimension, (a) 23.4%N/N, (b) 45.2%N/N, (c) 58.5%N/N, (d) 70.2%N/N. f f f f Based on the three-dimensional observation, it is clarified that the fatigue crack in the welded joint used in this study initiated at the slit tip then propagated with the complex behavior regardless of the force amplitude level. However, the fatigue crack initiation life against the entire fatigue life was dependent on the force amplitude; the higher force amplitude, the faster fatigue crack initiation life. 4. CONCLUSION In this study, the fatigue behavior of the FSSW specimens under constant force amplitude Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A001 conditions was investigated. The conclusions obtained are as follows. The specimen used for this study had a fatigue limit of 0.1 kN. This value is very low compared with the maximum tensile force of the base metal and FSSW joint itself. The crack initiation occurred as a boundary between the welding interface zone and non-interface zone or slit tip regardless of amplitude level. In addition, the slit tip is located in the heat affected zones. The fatigue crack was found on the upper sheet at the distal slit through to the surface of sheet up to the concave zone. The fracture morphology is the mixed mode fracture. Therefore, fatigue fracture modes were independent on force amplitude level. Base on the 3- dimensional observation, the macroscopic fracture modes were independent on the force amplitude. The fatigue initiation life was dependent on the force amplitude. In other words, the fatigue crack initiation life under low force amplitude accounted for a comparatively large proportion of the entire fatigue life; whereas the fatigue cracks initiation life occurred in a relatively early stage under high force amplitude REFERENCES Boroński, D., Sołtysiak, R., & Lutowski, Z. (2015). Analysis of strain distribution in notch zone in aluminium FSW joints for irregular fatigue loading conditions. Solid State Phenomena, 224, 27– 32. doi:10.4028/www.scientific.net/SSP.224.27 Cui, L., Fujii, H., Tsuji, N., & Nogi, K. (2007). Friction stir welding of a high carbon steel. Scripta Materialia, 56(7), 637–640. doi:10.1016/j.scriptamat.2006.12.004 Feng, Z., Santella, M. L., David, S. a., Steel, R. J., Packer, S. M., Pan, T., Bhatnagar, R. S. (2005). Friction stir spot welding of advanced high-strength steels - A feasibility study. SAE International, 2005–01–1248. doi:10.4271/2005-01-1248 Lin, P., Pan, J., & Pan, T. (2008). Failure modes and fatigue life estimations of spot friction welds in lap-shear specimens of aluminum 6111-T4 sheets. Part 2: Welds made by a flat tool. International Journal of Fatigue, 30, 90–105. doi:10.1016/j.ijfatigue.2007.02.017 Ma, Z. Y. (2008). Friction Stir Processing Technology: A Review. Metallurgical and Materials Transactions A, 39(3), 642–658. doi:10.1007/s11661-007-9459-0 Mishra, R. S., & Ma, Z. Y. (2005). Friction stir welding and processing. Materials Science and Engineering: R: Reports, 50(1-2), 1–78. doi:10.1016/j.mser.2005.07.001 Pathak, N., Bandyopadhyay, K., Sarangi, M., & Panda, S. K. (2012). Microstructure and mechanical performance of friction stir spot-welded aluminum-5754 sheets. Journal of Materials Engineering and Performance, 22(1), 131–144. doi:10.1007/s11665-012-0244-x Shiraly, M., Shamanian, M., Toroghinejad, M. R., & Ahmadi Jazani, M. (2014). Effect of tool rotation rate on microstructure and mechanical behavior of friction stir spot-welded Al/Cu composite. Journal of Materials Engineering and Performance, 23(2), 413–420. doi:10.1007/s11665-013- 0768-8 Sun, Y. F., Fujii, H., Takaki, N., & Okitsu, Y. (2012). Microstructure and mechanical properties of mild steel joints prepared by a flat friction stir spot welding technique. Materials & Design, 37, 384– 392. doi:10.1016/j.matdes.2012.01.027 Tanegashima, R., Akebono, H., Kato, M., & Sugeta, A. (2013). 3-Dimensional observation of the interior fracture mechanism and establishment of cumulative fatigue damage evaluation on spot welded joints using 590MPa-class steel. International Journal of Fatigue, 51, 121–131. doi:10.1016/j.ijfatigue.2012.12.014 Uematsu, Y., Tokaji, K., Tozaki, Y., Nakashima, Y., & Shimizu, T. (2011). Fatigue behaviour of dissimilar friction stir spot welds between A6061-T6 and low carbon steel sheets welded by a scroll grooved tool without probe. Fatigue and Fracture of Engineering Materials and Structures, 34(8), 581–591. doi:10.1111/j.1460-2695.2010.01549.x Wang, D.A., & Lee, S.C. (2007). Microstructures and failure mechanisms of friction stir spot welds of aluminum 6061-T6 sheets. Journal of Materials Processing Technology, 186(1-3), 291–297. doi:10.1016/j.jmatprotec.2006.12.045 Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A002 Fatigue Damage Evaluation of Friction Stir Spot Welded AISI 1012 Cold Rolled-steel under Repeated Two-step Force Amplitudes Sutep Joy-A-Ka1,2, Hiroyuki Akebono2, Masahiko Kato2, Atsushi Sugeta2, Yufeng Sun3 and Hidetoshi Fujii3 1 Materials Properties Analysis and Development Centre, TISTR 35 Moo 3 Khlong Ha, Khlong Luang, Pathum Thani 12120, Thailand, [email protected] 2 Department of Mechanical Science and Engineering, Graduate School of Engineering, Hiroshima University 1-4-1 Kagamiyama, Higashi Hiroshima, Hiroshima 739-8527, Japan 3 Joining and Welding Research Institute, Osaka University 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan ABSTRACT This paper investigates an approach to evaluate the fatigue damage of FSSW cross-tension specimens under two-step force amplitude conditions. In fatigue tests with repeated two-step force amplitude, the fatigue limit of the welded joint disappeared. However, the fatigue damage evaluation using the modified Miner’s rule erred too much on the side of safety, as the modified Miner’s rule tends to overestimate the damage by applied forces below the fatigue limit. Thus, it was determined that, within the testing conditions used in this study, the fatigue damage evaluation using Haibach’s method yielded an accurate evaluation. In the case where significant plastic deformation caused by the applied force occurred near the welded zone, the cumulative fatigue damage value based on Miner’s rule was often larger than unity. Therefore, it is important to consider a cumulative damage estimation that takes into account the effect of pre-strain from the high force amplitude Key Words: Variable force amplitude, Fatigue damage, Miner’s rule, Haibach’s method 1. INTRODUCTION Recently, the majority of fatigue tests were carried out under constant amplitude loading (Edwards & Ramulu, 2015; Jata, Sankaran, & Ruschau, 2000). Engineer are faced with the problem of how to use constant amplitude fatigue data in the prediction of the fatigue lives under the wide range of variable amplitude histories encountered service. The investigated of fatigue properties under variable amplitude loading came to known as the cumulative damage study (Beden, Abdullah, & Ariffin, 2009; Zhao-feng, De-jun, & Hao, 1992). As applied in fatigue test, a block loading is one in which the loading parameters vary stepwise with the time. Interaction effects affecting fatigue crack propagation rates in metallic materials can occur when load levels change during a block fatigue loading. Within a block fatigue loading each block consists of a constant amplitude fatigue loading. At the new load level the fatigue crack propagation rate may not be the same as it would be for the same fatigue load under constant amplitude fatigue loading. Numerous tests have been carried in attempts to quantify interaction effects, including tests using the simple two levels loading. The previous study (Joy-A-Ka, Hirano, Akebono, Kato, & Sugeta, 2013; Joy-A-Ka, Hirano, Akebono, Kato, Sugeta, et al., 2013), the authors proposed the interior fatigue crack mechanism of friction stir spot welding (FSSW) using AISI 1012 cold-rolled steel sheet in detailed of fracture mode, 3-dimensional observation and crack initiation site. In addition, the experiment of FSSW has focused on microstructure, static tensile test, and hardness test. Moreover, most studies on the fatigue properties of FSSW joints were conducted under constant force amplitude conditions; although some studies have reported the fatigue properties under variable force amplitude conditions (Costa, Ferreira, & Borrego, 2011; Costa, Ferreira, Borrego, & Abreu, 2012; Lee, Tjhung, & Jordan, 2007). However, automobile loads randomly vary in service and very little is known about fatigue properties under random force Thailand Welding and Inspection Technology 2015 (TWIT 2015) การประชุมวิชาการดานเทคโนโลยีการเชื่อมและการตรวจสอบ ประจําป 2558 มหาวิทยาลัยเทคโนโลยีพระจอมเกลาธนบุรี รวมกับ มหาวิทยาลัยมหิดล A002 amplitude conditions, which are assumed to occur in actual force situations (Braccesi, Cianetti, & Tomassini, 2015; Zhang & Maddox, 2009) Herein, fatigue tests were performed under repeated two-step force amplitudes to investigate the fatigue properties of FSSW AISI 1012 cold-rolled steel sheet. Welded components are often subjected to repeated two-step force amplitudes, which call for fatigue life prediction methods that consider fatigue damage accumulation. Therefore, this study proposes and demonstrates the effectiveness of a method for evaluating cumulative fatigue damage under repeated two-step force amplitude conditions. 2. EXPERIMENTAL PROCEDURES AISI 1012 cold-rolled steel sheet 0.8 mm in thickness (0.12%C, 0.50%Mn, 0.04%P, 0.045%S and bal. Fe) were used in this study. The mechanical properties of this steel are 0.2YS of 172 MPa, UTS of 314 MPa, Elongation of 48%, and Hardness value of 115 HV Figure 1 shows the shape and dimensions of the FSSW specimens after the steel sheet was machined into rectangular pieces measuring 150 mm in length, 50 mm in width, and 8.1 mm in punch diameter. Two rectangular steel plates were used to prepare the cross-tension FSSW specimens at the Joining and Welding Research Institute (JWRI), Osaka University. The two rectangular sheets were placed one on top of the other to form a cross shape and welded in the center of the overlapping square region. The FSSW tools were manufactured from tungsten carbide (WC) with a shoulder diameter of 12 mm, probe length of 1 mm, and diameter of 4 mm. The peak temperature of the welded zone (approximately 973 K) could be achieved when FSSW was conducted in air with a force of 15.0 kN, rotational speed of 550 rpm, and weld holding time of 2 s. Figure 1: Shape and dimensions of specimen Fatigue tests were performed under the repeated two-step force amplitude. The specimen was set up in the machine with the upper and lower sheets clamped with rectangular jigs where the plates do not overlap, as shown in Figure 2(a). The load was applied to the specimen in the direction perpendicular to the face of the plate. The failure criterion was specified as the point of complete separation of the FSSW joints between the upper and lower sheet. These experiments led us to an investigation on the effect of variable force amplitudes on the fatigue characteristics of the FSSW joints. Based on the illustration of the repeated two-step force amplitude waveform in Figure. 2(b), fatigue tests were performed using the number of cyclic loading with ΔP before the specimen was loaded with ΔP 200 L H and 500 cycles. The force ratio R for ΔP was 0.01and for ΔP was 0.01–0.04, because the minimum H L force of ΔP was set following to the minimum force of ΔP . L H Figure 2: (a) Installed to testing machine, (b) Illustration of repeated two-step force amplitude waveform.