DESIGNING OF A TESTING MACHINE FOR SHEAR-MODE FATIGUE CRACK GROWTH

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1 Advanced Materials Development and Performance (AMDP2011) International Journal of Modern Physics: Conference Series Vol. 6 (2012) World Scientific Publishing Company DOI: /S DESIGNING OF A TESTING MACHINE FOR SHEAR-MODE FATIGUE CRACK GROWTH A. KUSABA 1 *, S. OKAZAKI 1, M. ENDO 1,2, K. YANASE 1,2 1 Department of Mechanical Engineering, Fukuoka University 2 Institute of Materials Science and Technology, Fukuoka University Nanakuma, Jonan-ku, Fukuoka City, Fukuoka , Japan * td100004@cis.fukuoka-u.ac.jp As recognized, flaking-type failure is one of the serious problems for railroad tracks and bearings. In essence, flaking-type failure is closely related to the growth of the shear-mode (Mode-II and Mode-III) fatigue crack. In our research group, it is demonstrated that a shear-mode fatigue crack can be reproduced for cylindrical specimens by applying the cyclic torsion in the presence of the static axial compressive stress. However, a biaxial servo-hydraulic fatigue testing machine is quite expensive to purchase and costly to maintain. The low testing speed (about 10Hz) of the testing machine further aggravates the situation. As a result, study on shear-mode fatigue crack growth is still in the nascent stage. To overcome the difficulties mentioned above, in this research activity, we developed a high-performance and cost-effective testing machine to reproduce the shear-mode fatigue crack growth by improving the available resonance-type torsion fatigue testing machine. The primary advantage of using the resonance-type torsion fatigue testing machine is costefficiency. In addition, the testing speed effectively can be improved, in comparison with that of a biaxial servo-hydraulic fatigue testing machine. By utilizing the newly-designed testing machine, we have demonstrated that we can successfully reproduce the shear-mode fatigue crack. Keywords: Shear mode fatigue crack, testing device, cost efficiency, high performance. 1. Introduction Rolling-contact fatigue failures (e.g., flaking-type failure in bearings and shelling-type failure in railroad rails) have been a major concern associated with long life expectancy for various machine components. In essence, rolling contact fatigue failure is intimately related to the shear-mode (Mode-II and Mode-III) fatigue crack growth 1-3. Therefore, to understand the problem qualitatively and quantitatively, it is necessary to investigate the shear crack-growth behavior. Recently, it has been demonstrated that a shear-mode fatigue crack can be reproduced in cylindrical specimens by applying the cyclic torsion in the presence of the static axial compressive stress 1. However, available biaxial servohydraulic fatigue testing machines are quite expensive to purchase and costly to maintain. Further, regarding the experiment, one of major drawbacks to those machines is their low testing speed. Correspondingly, the objective of this research is to develop a novel, highperformance and cost-effective testing machine to reproduce the shear-mode fatigue 318

2 Designing of a Testing Machine for Shear-Mode Fatigue Crack Growth 319 crack growth by improving the available resonance-type torsion fatigue testing machine. It has been demonstrated that the shear-mode fatigue crack can be successfully reproduced by using the newly-designed testing machine. 2. Background of Research In the literature, a number of groups have investigated the reproduction of shear crack growth and shear fatigue crack threshold. For instance, Fujii et al. 2 examined the shearmode fatigue crack growth of SUJ2 by using their own original testing machine. Otsuka et al. 3 applied static crack opening load, which is equivalent to K Ⅰ = 5~7 MPa m 1/2, to avoid the crack face interference. In addition, they applied the compressive stress parallel to the crack to suppress a branching of Mode-I crack by using a special testing machine and a plate specimen with slit and pre-crack. On the other hand, Murakami et al. 4-7 devised a method of the experiment and used the double cantilever specimen. They performed the K Ⅱ decreasing test by using a shevron-notched SUJ2 specimen. They measured the Mode-II threshold stress intensity factor range by assuming f c (coefficient of crack face friction) to be unity and conducted FEM analysis to calculate K Ⅱ in the case of R = 1. Various testing machines and test methods to investigate the shear-mode fatigue crack growth have been proposed 8. However, those crack lengths are relatively long. Therefore, it is not suitable to study the behavior of small-fatigue crack growth under shear mode. In contrast, Matsunaga et al. 1 proposed a method in which fully-reversed torsion was coupled with static axial compressive stress to obtain a stable shear-mode crack growth in the longitudinal direction of cylindrical specimens. By using this method, the behavior of a small shear-mode fatigue crack from the inclusion and the artificial defect of bearing steels was observed successfully. This experiment used the biaxial servo-hydraulic fatigue testing machine. In practice, this testing machine can control the twisting moment, axial load, torsional angle, and displacement, respectively, by using the servomechanical system. However, a large amount of electric energy is required to operate the hydraulic system by securing a high degree of flexibility for the experiment. Moreover, the presence of axial compression lowers the frequency of torsional loading (About 10Hz), and accordingly, this results in the low testing speed. In addition, financially, the testing machine is costly to purchase and maintain. In the near future, the development of a novel testing machine is necessary to perform the experiment efficiently for a wide range of materials under different test conditions. 3. Designing Concept As previously mentioned, the research on shear-mode fatigue crack is still in the nascent stage. In our laboratory, we have tested the shear-mode fatigue crack by using the biaxial servo-hydraulic fatigue testing machine. However, the frequency of torsional loading tends to decrease associated with an increase of axial compressive loading. To conduct the research efficiently, it is necessary to increase the testing speed. Moreover, the servo-

3 320 A. Kusaba et al. hydraulic testing machine consumes a lot of electric energy. In essence, a large number of cycles is required until one can observe the fatigue crack growth. Accordingly, a high running cost to perform the fatigue test becomes a serious problem. To design and develop a new testing machine, the principal design concept is set as follows: The testing machine must be inexpensive. The testing frequency must be greater than 20Hz. The running cost must be low. The maintenance must be easy. 4. Developed Testing Machine 4.1. Specs and features The specs and features of the testing machine developed in this study are given below: Base testing machine: Shimadzu TB-10 Number of examination axes: Biaxial testing Maximum load capacity: Torsional moment ± 50 Nμm; Compressive load 20 kn Testing frequency: 2000 cycles per minute Driving force: Electric-driven single-phase motor 100V Power supply: AC100V 10A 50/60Hz Weight: Approximately350kg (The concrete foundation is not included) Size : (mm) 4.2. Basic structure In this study, by using the resonance type torsional fatigue testing machine, a new testing machine was developed. To apply the cyclic twisting load in the presence of the static compressive load, the grip part of the testing machine was newly designed. The primary advantage of using the resonance-type torsion fatigue testing machine is cost-efficiency. In addition, the testing speed (33.3Hz) is much faster in comparison with that of a biaxial servo-hydraulic fatigue testing machine. Fig. 1 shows the developed testing machine. The newly-designed grip part can apply the compressive load for the specimen. Further, the torque-load cell and the compression load-cell are attached to it. As shown in Fig. 1, the test specimen is placed between the two steel plates. Thus, the compressive load can be easily applied to the specimen by tightening the two plates. To transmit the cyclic twisting load to the specimen, a thrust bearing is inserted between the plate and the chuck of the specimen at the driving side. It is noted that when a specimen was tested, the amount of bending stress was very small.

Designing of a Testing Machine for Shear-Mode Fatigue Crack Growth 32

Principle of bend stress removal in examination part In general, one of the specimen s ends should be fixed to apply the torsional loading, and in the past, the test piece used to be completely fixed

4 Designing of a Testing Machine for Shear-Mode Fatigue Crack Growth Principle of bend stress removal in examination part In general, one of the specimen s ends should be fixed to apply the torsional loading, and in the past, the test piece used to be completely fixed at both ends. However, when the axis of grip-ends and the axis of the specimen itself are not aligned adequately, the bending moment is developed in the specimen when the specimen is completely fixed. In other words, in the absence of axial compressive loading, the bending moment has a negligible effect during the test. On the other hand, when one applies the twisting moment in the presence of the compressive loading through a thrust bearing, the thrust bearing tends to hit the test specimen unevenly due to the bending moment. As a result of the rubbing and wearing of the thrust bearing, the twisting moment cannot be transmitted properly to the specimen, and the life of the bearing can be decreased significantly. To overcome this problem, by using the plate spring, the specimen fixation chuck was developed to remove the adverse bending moment in the horizontal direction (y-axis) and the vertical direction (x-axis) (cf. Fig. 2). In principle, when the specimen is axially loaded in the x- and y-directions, these adverse loadings can be removed effectively by taking advantage of the flexibility of the plate spring. The twisting moment can be supported adequately at the same time. Additionally, the grip part at the fixed side is adjustable, because it is difficult to adjust the grip part at the driving side. Fig.1 Newly designed shear-mode fatigue crack growth testing machine. Fig.2 A schematic of specimen support mechanism. 5. The Shear-Mode (Mode II) Fatigue Crack Growth Test 5.1. Material and experiment Table 1 gives the chemical composition of SCM435 (JIS) that was used in this study. The steel was heat-treated at 680 C for 100 min and 825 C for 110 min in vacuum condition, followed by oil-quenching at 190 C for two hours. The average Vickers hardness (HV),

5 322 A. Kusaba et al. measured with a load of 9.8 N, was 530. Fig. 3 shows the shape and dimensions of the specimen. The specimen s surface was polished with an abrasive paper and then it was buffed with an alumina paste. As shown by Fig. 4, a penetrating slit was introduced by using the electrical discharging in conjunction with the FIB slits at both ends. Cyclic torsion tests were carried out by using the new testing machine with the testing frequency of 33.3 Hz and a stress ratio of R = 1. To suppress any tendency for Mode-I cracking, the axial static compressive stress of s = 1200 MPa was superposed on the cyclic torsional loading. Table 1 Chemical composition of SCM435 in wt.% C Si Mn P S Cu Ni Cr Mo Fig. 3 Shape and dimensions of specimen (unit: mm) Results and discussion Fig. 4 Shapes and dimensions of defects having crack-like thin slits at the both ends, in μm. This experiment was conducted to obtain the fundamental data for the shear-type fatigue crack growth in SCM435. Fatigue tests were carried out at τ a of 320 ~ 350 MPa under a static compressive stress, σ s, of 1200 MPa, in which τ a is the shear stress amplitude at the specimen surface with FIB slit. Fig. 5 shows the shear-type crack that propagated in the axial direction from the defect. In the beginning, a Mode-II crack of about 30 m long was observed from the point of FIB at τ a = 350 MPa. Because the crack propagation was fast, τ a was decreased by 30 MPa. The Mode-II crack propagated in the axial direction with the development of Mode-I crack branching. Finally, when the crack propagated drastically at N = cycles with τ a = 320 MPa, the experiment was terminated. Moreover, we examined the specimen without the FIB slit under the same condition. The Mode-II crack didn t propagate, and only Mode-I crack propagation was observed. According to the present experimental study, the shear-mode crack was observed from A B B A

6 Designing of a Testing Machine for Shear-Mode Fatigue Crack Growth 323 the FIB of the specimen by using the new testing machine. However, it is necessary to study the effects of the artificial defect and the testing condition in the future. Shear-mode fatigue crack growth Initial defect 2a = 1050 m 6. Conclusions Axial direction 400 m Fig. 5 Shear-mode fatigue crack in axial direction and Mode I branched crack. To study the shear-mode fatigue crack growth efficiently, the testing machine was newly developed based on the resonance type torsional fatigue machine. The shear-mode fatigue crack was observed successfully by using the new testing machine. Acknowledgments This research was in part sponsored by Japanese Ministry of Education, Science, Sports and Culture, Grant-in-Aid for Scientific Research (C) (Fund Number , ). References 1. H. Matsunaga, S. Muramoto, N. Shomura, and M. Endo, J. Soc. Mater. Sci., Japan, 58(9), 773 (2009). 2. Y. Fujii, K. Maeda, and A. Otsuka, J. Soc. Mater. Sci., Japan, 50, 1108 (2001). 3. A. Otsuka, Y. Fujii, and K. Maeda, Fatigue Fract. Engng. Mater. Struct., 27, 203 (2004). 4. Y. Murakami, T. Fukuhara, and S. Hamada, J. Soc. Mater. Sci. Japan, 51, 918 (2002). 5. Y. Murakami, and S. Hamada, Fatigue Fract. Engng. Mater. Struct., 20(6),863 (1997). 6. Y. Murakami, K. Takahashi, and R. Kusumoto, Fatigue Fract. Engng. Mater. Struct., 26, 523 (2003). 7. K. Toyama, Y. Fukushima, and Y. Murakami, J. Soc. Mater. Sci. Japan, 55, 719 (2006). 8. D.F. Socie and G.B. Marquis, Multiaxial Fatigue (Society of Automotive Engineers, Inc, Warrendale, PA, USA, 2000).

Effect of Occasional Shear Loading on Fatigue Crack Growth in 7075 Aluminum Alloy M. Makizaki 1, a, H. Matsunaga 2, 4, b, K. Yanase 3, 4, c 3, 4, d

Materials Science Forum Online: 213-3-11 ISSN: 1662-9752, Vol. 75, pp 264-267 doi:1.428/www.scientific.net/msf.75.264 213 Trans Tech Publications, Switzerland Effect of Occasional Shear Loading on Fatigue