ICF13B

13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- Holmes et al [5, 6] studied the mode I fatigue crack growth behavior of 250 �m thick center-notched Ni-base specimens, which were subjected to tension-tension fatigue at a loading frequency of 2 Hz. Using the Paris relationship, it has been found that the stress intensity factor exponent, m was significantly higher than commonly reported for bulk Ni-base specimens [7, 8]. A similar trend was also observed in Ti-6Al-4V foils. The high cyclic crack growth rate in thin foils was attributed to the lower Mode I lower fracture toughness commonly observed for thin metallic foils [5, 6, 9]. In a very informative study, Meirom et al [9] observed that ~500 nm thin films exhibit low fracture toughness and inferior resistance to fatigue crack growth when compared to conventional micro-scale-grained bulk forms of the materials. Their studies showed that thickness had a substantial effect on the Paris power law exponent and the estimation of fracture toughness. At first glance, the high crack growth rates found during Mode I testing of thin metallic foil [5, 6] is at odds with observations of higher fatigue strength found from standard (S-N) tests with un-notched thin metallic foils [2,3]. Standard fatigue life (S-N) tests include cycles to crack initiation, which may be strongly influenced by yield strength and residual stresses as well as surface finish and microstructural features such as grain size. However, fracture toughness can play an important role in the cyclic crack growth behavior of materials. For certain metals, fracture toughness exhibits a “bell-shaped” dependence on thickness, with fracture toughness reaching a maximum at intermediate thickness. In experiments with Cu, Wang et al [10] observed that Jc initially increased with increasing thickness, reached a maximum at a thickness of about 0.3 mm, and decreased at larger thicknesses. Because of the very low applied loads, the cyclic crack growth behavior of metallic foils requires specialized testing approaches. For lower frequency testing (up to 1000 Hz with specially designed load frames) servo hydraulic and electro-mechanical load frames can be used for tension-tension fatigue loading histories [11, 12]. However, because of the low load levels required, the use of servo hydraulic load frames for fatigue testing is generally limited to foil thicknesses above 50 �m. Moreover, because of high cost, servo hydraulic and electro-mechanical load frames are generally not used for long duration testing of materials. The cost to perform long duration fatigue and fatigue crack growth experiments is of particular concern for low loading frequencies in the range of 0.01 to 5 Hz, which is a frequency range of general interest for many practical devices which utilize foil-thickness materials, including MEMS, fuel cells with metallic interconnects, and printer drives. The objective of the present investigation was to develop a reliable low-cost experimental apparatus to study the low frequency high cycle Mode I cyclic crack propagation behavior of thin foils under tension-tension loading. The approach, which involves use of magnetic coupling between a clamped rectangular specimen and a rotating steel disk, is well suited for studies of Mode I fatigue crack growth. In order to illustrate the test apparatus, the cyclic crack growth rate of 30 �m thick edge-notched high-purity (99.6%) annealed titanium (Ti) foils was investigated at RT. 2. Development of Testing Apparatus The overall design goal was to develop fatigue test apparatus using commercially available components and conventional machining. As shown in Figures 1, this was accomplished by magnetic coupling between a magnet attached to a sliding grip and a rotating eccentric steel disk mounted to a DC motor. As shown in Fig. 1, the specimen is clamped in face-loaded (friction) grips. One of the specimen grips is attached to a linear bearing slide, which is free to translate parallel to the specimen centerline; motion perpendicular to the centerline is constrained by the linear rail. The grip at the other end of the specimen was fixed to a load cell (Honeywell Model 41) which was rigidly fixed to the test frame. A ceramic magnet attached to the end of the translating specimen grip provides magnetic coupling with a rotating eccentric steel rotor. The strength of the couple, and

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