Fatigue Test Methods

Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses. Under these conditions it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for static load. The term

“fatigue” is used because this type of failure normally occurs after a lengthy period of repeated stress or strain cycling. Fatigue is important inasmuch as it is the single largest cause of failure in metals, estimated to comprise approximately 90% of all metallic failures;

polymers and ceramics (except for glasses) are also susceptible to this type of failure.

Furthermore, it is catastrophic and insidious, occurring very suddenly and without warning [21]. Therefore, the fatigue property of the structure material is an important criterion of device reliability. If we can know the fatigue property of the structural material in advance, the device lifetime can be predicted. That is helpful to design practical micromachines and MEMS devices because the moving components involved in these devices are subjected to cyclic loading.

In the conventional fatigue test of bulk materials, a rotating-bending test apparatus, commonly used for fatigue test, is shown in Figure 1.2; the compression and tensile stresses are imposed on the specimen as it is simultaneously bent and rotated [21]. Tests are also frequently conducted using an alternating uniaxial tension-compression stress cycle.

With the device miniaturization, the conventional fatigue test method is no longer suitable for micrometer-scale specimen. Considering the size, grip, alignment, and loading of specimen, a new fatigue test method is necessary including testing techniques,

specimen design, and equipment. However, there are no standard test methods so far.

Among these test methods of approaches, tensile and bending tests are two common schemes for the fatigue characterization on micrometer-scale specimens. And the microsized cantilever-beam is utilized frequently in specimen design. As shown in Figure 1.3, according to loading methods on the free end of the cantilever-beam, three fatigue test types can be identified: (1) in-plan tensile method, (2) in-plane bending method, and (3) out-of-plane bending method.

For the in-plan tensile method, in 2003, Cho et al. [22] used microsample tensile machine and non-contact interferometer strain displacement gage to study the fatigue properties of electrodeposited LIGA Ni microsamples (~400 μm thick) with dog-bone shape under the test frequency of 200 Hz, as shown in Figure 1.4. The test results indicated that the fatigue lifetime of the LIGA Ni microsamples increased with decreasing stress amplitude, and the measured fatigue limit and endurance ratio were 195 MPa and 0.35, respectively. In 2004, Son et al. [23] evaluated and discussed the fatigue properties and notch effect of LIGA Ni film (10 μm thick) by micro-tensile and fatigue test methods under the test frequency of 20 Hz, as shown in Figure 1.5. The test results indicated that the fatigue strength of LIGA Ni was very sensitive to stress concentration, and the measured fatigue limit and endurance ratio were 180 MPa and 0.21 for unnotched specimens, and 143 MPa and 0.17 for notched specimens, respectively. In 2007, Yang et al. [24] also used the micro-tensile testing system to study the fatigue mechanisms of LIGA Ni thin films (70 μm and 270 μm thick) with micro-scale and nano-scale grains under the test frequency of 10 Hz, as shown in Figure 1.6. The test results indicated that films with the nano-scale grains (15 nm average grain size) were shown to have higher strength and fatigue resistance than those with micro-scale columnar grain structures (5 μm wide and 5

~ 25 μm long). The thinner films (70 μm thick with a columnar microstructure) were also

shown to have higher strength and fatigue resistance than those of thicker films (270 μm thick with a columnar microstructure).

For the in-plan bending method, in 2003, Larsen et al. [15] proposed an in situ bending test device with integrated electrostatic actuator and test beam, made of electroplated nanocrystalline Ni (7 μm thick), for fatigue investigation under the test frequency of 200 Hz, as shown in Figure 1.7. The feature of this test device was approximately pure in-plane bending, and the maximum stresses were calculated using finite element method. The test results indicated that the nanocrystalline Ni had good fatigue properties due to the high strength and toughness of nanocrystalline material.

However, the clear fatigue limit and endurance ratio were not obtained in this study.

For the out-of-plan bending method, in 1999, Maekawa [25] et al. performed fatigue lifetime and fatigue crack propagation tests on electroless plated Ni-P amorphous alloy specimen (12 μm thick) to investigate the material fatigue properties under the test frequency of 10 Hz, as shown in Figure 1.8. The microsized specimens were cantilever-beam-type prepared by focused ion beam (FIB) machining, and notches with a depth of 3 μm were introduced in some specimens. The fatigue lifetime curve was obtained for unnotched specimens, and the fatigue crack propagation tests were performed using notched specimens. The test results indicated that the crack was deduced to propagate by cyclic plastic deformation at the crack tip even in microsized amorphous alloys, and the measured fatigue limit and endurance ratio were 20 mN and 0.43, respectively.

From above reviews, we can find that once the sample becomes very small only with several micrometers, the setup of tensile method become stringent to grip, align, and pull a tested sample. In comparison with the tensile method, the bending method can be free of the issues raised by sample gripping and alignment. Furthermore, bending method

requires smaller loading force than that of tensile tests to yield a tested sample with a deformation that is large enough for accurate measurement, which makes the method suitable for thin film characterization. Thus, in this dissertation, a fatigue characterization scheme based on the design of out-of-plane bending method is proposed and utilized for the property investigations on the electroplated Ni and Ni-diamond nanocomposite.