Fretting is a cyclic small amplitude relative slip motion between contacting surfaces of joined structures subjected to a vibration or fatigue load. Fretting is induced by the difference in the elastic deformation between the contacting bodies. Therefore, the amount of the relative slip is quite small. In fact, there are reports that the fretting with a 1-m relative slip range creates fretting fatigue cracks [1, 2].
Fretting fatigue properties are strongly affected by the relative slip range between the contacting surfaces [2-4]. Physical meaning of “slip” associated with fretting fatigue crack initiation is still not fully understood, however, the role of the slip can be interpreted in association with friction. At the small relative slip range, the tangential force acting on the contacting surfaces increases with an increase in the relative slip range. As a result, the fretting fatigue strength decreases with the increase in the relative slip range. At the middle relative slip range, the fretting fatigue strength becomes constant independently of the relative slip range because the increase in the tangential force is saturated and takes a constant value. At the large relative slip range, the fretting fatigue strength changes to increase with an increase in the relative slip range because a large amount of fretting wear relieves the stress concentration at the contacting part and removes small cracks.
Measurement of the relative slip range is important to characterize fretting fatigue properties and understand the mechanisms, however, there are some difficulties for the measurement of the relative slip during fretting fatigue test. Since the relative slip range during the fretting fatigue is at most several tens m, and sometimes less than 1 m, a high resolution is required for the measurement. In addition, the relative slip range is equivalent to an elastic deformation around the contact edge. Therefore, the elastic deformation included in the measured relative slip range should not be ignored as follows. When the relative slip between contacting bodies is measured, reference points are needed to each body. Figure 2.1 shows a schematic of the measurement error due to elastic deformation. When the reference point on the specimen is placed just at the contact edge so that two reference points aligned with the contact edge (points O and A), the measured value is in
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agreement with the true relative slip at the contact edge (O-A’). On the other hand, when the reference point on the specimen is placed with some distance from the contact edge (point B), the measured relative slip includes an elastic deformation of the specimen between the points A’ and B’.
However, sensors have a certain dimension, the setting of the specimen side reference point tends to be set at B. Again, the relative slip range is the same level as the amount of elastic deformation. Thus, the error due to the elastic deformation should not be ignored. Therefore, the reference points for the relative slip measurement should be placed as close as possible to the contact edge in order to measure the relative slip accurately.
Fig. 2.1 Error in the measurement of relative slip range of fretting due to elastic deformation around the reference points
Tensile load O
O A
A’
B
B’
Elastic deformation between A’ and B’
True relative slip Measured value
= true relative slip
Measured value
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For an accurate measurement of the relative slip range during fretting fatigue, a variety of measurement methods have been developed. For example, small displacement sensors [5-7], synchronized laser displacement sensors [8] and a digital image correlation method [9] and so on are found in the literature.
For the fretting fatigue in hydrogen environment, the elucidation of the mechanisms why the fretting fatigue strength is significantly lower than that in air [10-16] is necessary from the viewpoint of relative slip behavior. It is considered that one of the reasons for the reduction in the fretting fatigue strength in hydrogen is local adhesion between the contacting surfaces and subsequent many small crack initiations at the adhered spot [14]. When considering the local adhesion between the contacting surfaces during the fretting fatigue in hydrogen, the relative slip range must be reduced, and it could result in changes in the stress condition at the contacting surface.
Therefore, identification of the slip condition during fretting fatigue in hydrogen environment is necessary to achieve the deeper understanding of the mechanism of the reduced fretting fatigue strength in hydrogen.
In addition to the accuracy of the measurement depending on the sensor setting, there are some barriers for the measurement in hydrogen environment. For example, the output of the sensor element drifts due to hydrogen absorption into the sensor material [17], and the space inside the hydrogen chamber is too narrow for commercial sensors.
Incidentally, micro electro mechanical system (MEMS) technology has been developed this 50 years [18], and it is applied to a variety of devices, such as sensors, actuators etc. Prof.
Sawada in Kyushu University studies and develops small sensors fabricated by MEMS technology.
There is an optical micro encoder in one of the sensors Prof. Sawada developed. Since this optical micro encoder has a high accuracy and the size is small, it was considered that the relative slip range during fretting fatigue in hydrogen can be measured accurately by using the optical micro encoder.
In this context, a new method for the measurement of the relative slip range during the fretting fatigue test in hydrogen was developed in this study by appling the MEMS optical micro-encoder in collaboration with Prof. Sawada’s laboratory. Especially, this study was carried out with the great help of Mr. Morita who is a member of Prof. Swada.
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