Early fatigue damage detecting sensors—A review and prospects

Sensors and Actuators A 198 (2013) 46–60

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Review

Early fatigue damage detecting sensors—A review and prospects Pengfei Wang a,∗ , Toshiyuki Takagi a , Takanori Takeno b , Hiroyuki Miki c a

Institute of Fluid Science, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Graduate School of Engineering, Tohoku University, Aoba 6-6-1, Aramaki, Aoba-ku, Sendai 980-8579, Japan c Center for Interdisciplinary Research, Tohoku University, Aoba 6-3, Aramaki, Aoba-ku, Sendai 980-8578, Japan b

a r t i c l e

i n f o

Article history: Received 19 November 2012 Received in revised form 12 March 2013 Accepted 20 March 2013 Available online 28 March 2013 Keywords: Cyclic loading Cumulative fatigue damage Fatigue damage detecting Fatigue life prediction Fatigue sensor Carbon film

a b s t r a c t Fatigue damage detection and fatigue life prediction are important technological issues in both academic and industrial fields. Numerous techniques have been proposed and developed for the detection of early fatigue damage in structural members. This paper provides a brief review of the current state-of-the-art on fatigue damage sensors. Eight types of existing fatigue damage sensors based on different detecting principles are introduced and discussed, with the emphasize on the two commercially available fatigue damage sensors, the fatigue fuse with a slit and the electrical resistance fatigue gauge. Moreover, a novel concept of utilizing carbon films as a fatigue monitoring sensor is proposed. The preliminary experimental results suggest that metal-containing diamond-like carbon (Me-DLC) films with optimum composition and micro-structure will be good candidate materials for fatigue monitoring sensors. © 2013 Elsevier B.V. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fatigue damage sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Fatigue fuse . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Fatigue fuse for fatigue failure detection (notch shape) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Fatigue fuse for fatigue life prediction (slit shape) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Electrical resistance fatigue gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. Electrical resistance fatigue gauge for crack initiation detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. Electrical resistance fatigue gauge for fatigue life prediction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Surface roughness fatigue gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Laser speckle fatigue sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Specular reflection fatigue gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Martensitic transformation fatigue gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Mean stress fatigue gauge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8. Oxide film fatigue sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9. Carbon film fatigue monitoring sensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biographies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

46 47 47 47 47 48 49 50 51 52 53 53 53 54 55 57 58 58 60

1. Introduction

∗ Corresponding author. Tel.: +81 22 217 5298; fax: +81 22 217 5298. E-mail address: [email protected] (P. Wang). 0924-4247/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.sna.2013.03.025

Fatigue failures of structural members which are subjected to known and unknown stresses are a serious problem for the operation security of these structures in industrial fields. Therefore, fatigue damage detection and fatigue life prediction have long been

P. Wang et al. / Sensors and Actuators A 198 (2013) 46–60 Table 1 Methods of fatigue damage detection. Direct inspection methods

Fatigue damage sensors

Electrical resistance Eddy current sensor Electrochemical fatigue sensor Laser speckle sensor Alternating current potential drop Barkhausen noise Positron annihilation Ultrasonic Rayleigh wave Ultrasonic attenuation Non-linear ultrasonic Acoustic microscopy Acoustic emission Tritium autoradiography

Fatigue fuse Electrical resistance fatigue gauge Surface roughness fatigue gauge Laser speckle sensor Specular reflection fatigue gauge Martensitic transformation fatigue gauge Mean stress fatigue gauge Oxide film fatigue sensor

great challenges to scientists in mechanical engineering practice. A method of detecting and measuring the development of the early stages of fatigue could provide the basis of a testing technique for predicting fatigue life. Numerous methods have been proposed and developed for the detection of early fatigue damage in the structures of industrial products. In a previous technical note, these methods were divided into two categories, direct inspection methods, which involve the direct inspection of the structure of interest, and fatigue damage sensors, which require the application of a fatigue device bonded to the structure [1]. By including the latest development of fatigue damage detection techniques [2–7], the existing methods of fatigue damage detection are summarized and listed in Table 1. Fatigue damage sensors, which have been named fatigue fuses, gauges, indicators, monitors, predictors, testers, detectors, and transducers in previous research, can not only detect the cumulative fatigue damage but also predict the remaining fatigue life of the test structures. A fatigue damage sensor can be further classified into three types, such as crack initiation detection, fatigue failure detection, and fatigue life prediction (or cumulative fatigue damage detection) based on the functions of the sensors. This paper first presents a brief review of eight types of existing fatigue damage sensors developed for the detection of early fatigue damage in structures, and a novel concept of utilizing carbon films as a fatigue monitoring sensor is then proposed and discussed. 2. Fatigue damage sensors 2.1. Fatigue fuse One of the earliest fatigue detecting apparatuses was invented by De Forest in 1948 [8]. The Fatigue Indicator was proposed for determining the fatigue failure or fatigue life of tested materials. This apparatus consisted of a thin conductive filament in the shape of a wire or ribbon, which had a fatigue life slightly below that of the tested material. The integrity of this apparatus was assessed by continually measuring its electrical resistance, with failure indicated by a sudden increase in electrical resistance because of the loss of electrical continuity. This apparatus worked in a similar manner to an electrical fuse, as it interrupted an overloaded circuit, and was therefore named a “Fatigue Fuse”. Later, this type of fatigue monitoring method was further improved and two types of fatigue fuses, for fatigue failure detection and fatigue life prediction, have been proposed and investigated. 2.1.1. Fatigue fuse for fatigue failure detection (notch shape) The commercial Fatigue Fuse device, produced by Tensiodyne Scientific Corporation (recently known as Material Technology, Inc.) in USA, is a modern adaption of De Forest’s idea. The fuse is made of a thin sheet of the same material as that of the structure

47

being monitored, and comprises a number of coupons, each providing a stress concentration of different but known severity. When subjected to the same strain history as the structure being monitored, the coupons fail progressively prior to the structure, therefore providing a warning of its impending failure. By having several coupons in the fuse, each with a different stress concentration, it is possible to estimate the remaining fatigue life of the structure. Based on this concept, a number of coupons with different shapes have been proposed [9–13], and representative outlines of the fuses are shown in Fig. 1(a) and (b). These fatigue fuses are typically attached to the structure being monitored so that the test elements are aligned with the direction of maximum principal stress applied to the test structure. However, these fatigue fuses are limited to the materials of the test structures and sophisticated shape of the coupons. Therefore, Lee et al. proposed a multidirectional fatigue fuse to monitor the actual progress of fatigue damage in any direction [14]. This fatigue fuse consists of a series of fuse elements in a circumferential direction to cover the changing characteristics of the directions of the principal stresses. The fuse elements in each row have the same fatigue lifetime with the same crack lengths, but the lifetimes vary from row to row with different crack lengths. The schematic illustration of the multidirectional fatigue fuse is shown in Fig. 2. Three rows of fuse elements, with fatigue lifetimes of around 10, 30, and 50 percentage of the estimated fatigue lifetime of the test structure, are employed. The fuse elements are arranged at every incremental 15◦ angle. Moreover, a specially designed fatigue gauge, which contains breakable ligaments of either a variable length to measure the fatigue damage of metallic, polymeric and composite materials, or the same length but with different compositions to measure the fatigue strength and fatigue damage of certain composite materials, was proposed by Kwon [15]. Furthermore, this fatigue fuse does not require artificial wakening (e.g. a notch) of the test elements. Fig. 3 presents the schematic illustration of the simple coupon fatigue fuse with ligaments varying in length and surface area. 2.1.2. Fatigue fuse for fatigue life prediction (slit shape) In 1976, Smith developed and patented another type of fatigue fuse, which has a small slit instead of a notch, as shown in Fig. 4 [16]. A thin, rectangular metal base of uniform thickness is employed. It has a narrow crack-like slit cut in one side and a Teflon parting strip attached to the base underlying the slit. During fatigue testing, a fatigue crack begins immediately at the inner end of the slit and increases in length as an approximately linear function of the accumulative fatigue damage strains incurred by the structure. Therefore, this fatigue fuse permits accurate measurement of the accumulated fatigue damage and the remaining fatigue life of a structure. Because of the small, simple, and inexpensive nature of this fuse, it may be possible to monitor most or all of a specific type of structure in actual service (e.g. aircraft and bridge) and hence favors the detection of excessive fatigue damage before unexpected failures occur. In 1983, Geissler and Gallagher [17] and Grandt and DumanisModan [18] employed this type of fatigue fuse for an individual aircraft tracking device. Unfortunately, it was concluded that this type of fatigue fuse is not suitable for a stand-alone tracking device because of its variability in response to spectrum loading. The cause of this variability is not clearly understood. Late in 1998, Fujimoto proposed a new type of fatigue fuse with double slits around the circular hole in the center of the gauge [19]. However, a practically applicable fuse has not yet been developed. By 2003, researchers in Kawasaki Heavy Industries, Ltd. in Japan successfully developed and patented an improved fatigue fuse, which performed well in estimating the degree of fatigue damage and the remaining fatigue life of products [20]. The schematic illustration of the fatigue fuse is shown in Fig. 5 [21]. The principle

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P. Wang et al. / Sensors and Actuators A 198 (2013) 46–60

Fig. 1. Schematic illustrations of the Tensiodyne fatigue fuse. (a) Individual coupon with different shape [9]. (b) A combination of different shaped coupons [11].

fuse has several merits, such as small scale, low cost, high reliability, and long working time (max. 2 years). However, it cannot work in temperatures of more than 60 ◦ C and can only detect the fatigue damage of metal structures made of steel and aluminum. 2.2. Electrical resistance fatigue gauge

Fig. 2. Schematic illustration of the multidirectional fatigue fuse [14].

of this fatigue fuse is based on the crack growth property of metal (e.g. nickel). The crack growth length is measured once or a few times over a certain period after installation. This crack growth length represents the fatigue damage accumulated in the fatigue fuse under service load. As the exponent of the S–N curve is consistent both in the fatigue fuse and in the test structure, the degree of fatigue damage of the test structure can be determined from the degree of fatigue damage of the fuse. Most importantly, this fatigue fuse is already commercially available [22] and has been applied to various actual products, such as railways [23], steel bridges [24,25], ships [26–28] and other structural applications [29,30]. This fatigue

The residual electrical resistivity of metal is strongly related to electron scattering from imperfections in the crystalline structure, such as point (vacancies and inclusions), line (dislocations), and surface defects (grain boundaries and stacking faults). The former two types generally occur at a relatively early stage of fatigue, and consequently result in changes in electrical resistance in a metal undergoing cyclic loading well before crack initiation. In 1966, Harting proposed an electrical-resistance fatigue-life gauge, consisting of a grid of conductive material in the form of a foil, film or wire, mounted on a tested structure in which the change in electrical resistance of the grid material can be used in measuring the cumulative fatigue damage in the structure [31,32]. A commercial fatigue gauge made of constantan (Cu–45% Ni alloy), which is based on the patent of Haring, is supplied by Micro Measurements Inc., USA and named the S/N Fatigue-Life Gauge [31]. The relationship between the change in electrical resistance of the gauge and fatigue loading parameters can be expressed by the following equation: R = K(εp − ε0 )N h , R

(1)

where R is the change in electrical resistance R, K and h are constants for the gauge, N is the number of applied cycles, εp is the

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Fig. 3. Schematic illustration of the simple coupon fatigue fuse with ligaments varying in length and surface area [15].

Fig. 4. Schematic illustration of the fatigue fuse with a small slit [16]. Fig. 6. Typical resistance change in polycrystalline copper due to fatigue cycling [33]. Copyright © 1974, Elsevier.

peak value of the cyclic strain, and ε0 is the endurance limit strain, below which no permanent resistance change occurs. The threshold of sensitivity is the smallest value of cyclic deformation (threshold strain) causing a considerable change in the electrical resistance of the sensor which may be measured with the specified accuracy after the designated number of load cycles. The electrical resistance of the materials chosen for construction of the gauge should be strongly related to the amount of accumulated fatigue-damage but be insensitive to temperature over a wide range. Electrical resistance fatigue gauges can also be divided into two categories. The fatigue gauge for detecting fatigue-induced crack initiation according to a sudden change in electrical resistance, and the fatigue gauge for predicting fatigue life according to a comparison of the S–N curves of the structure material and curves of constant R/R of the gauge material. Details of the research on

these two types of electrical resistance fatigue gauges will be presented in the following sections.

2.2.1. Electrical resistance fatigue gauge for crack initiation detection In 1974, Charsley and Robins investigated the effect of fatigue damage on the electrical resistance of polycrystalline copper; a typical result is shown in Fig. 6 [33]. The gradual increase in electrical resistance in the first region is because of the formation of dislocation and vacancies. Whereas, the much higher rate of change in electrical resistance in the second region is attributed to the inter-granular cracking. Therefore, this kind of fatigue gauge will be useful for the detection of fatigue-induced cracks in structures.

Fig. 5. Schematic illustration of the Kawasaki fatigue fuse [21]. Copyright 2010, Offshore Technology Conference. Reproduced with permission of OTC. Further reproduction prohibited without permission.

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Fig. 7. Grid pattern film for fatigue-induced crack length measurement [34].

From 2006 to 2009, Deng et al. developed a simple and practical method for fatigue crack initiation detection using an ion-sputtered gold film [34–36]. The mechanism of this method is similar to that in polycrystalline copper. The tendency is for the electrical resistance of the film to suddenly change with the appearance of a crack in the film. Thin ion-sputtered pure gold films with a thickness of several tens of nanometers and different shapes, such as a grid pattern film and a one-piece rectangular film, have been developed for crack detection. The schematic illustration of the grid pattern film is shown in Fig. 7. Since the electrical resistance of the film changes only at the point when a grid is entirely snapped by a crack, the calculation of the electrical resistance is not necessary. The crack length can be obtained by counting the number of increasing steps in the recorded curves of electrical resistance and measuring the positions of the grids before and after an experiment. Obvious staircase changes in the electrical resistance of the film were detected and are presented in Fig. 8. The measurement error was 0.2 mm and crack initiation was detected after the crack length reached 1 mm. Although the grid pattern ion-sputtered film can measure a crack length with high precision, the crack length cannot be determined continuously. Additionally, an isolation film is necessary for the application of the grid pattern film onto a conductive surface and the formation of the grid pattern without destroying the isolation film would be very difficult. Therefore, to measure the crack length continuously (i.e. achieving higher measurement precision) and apply the ion-sputtered film to a conductive surface, such as a metal surface, a one-piece rectangular ion-sputtered film was developed [35,36]. This film has been successfully applied to detecting fatigue crack initiation in several types of conductive and non-conductive materials, such as carbon steel, acrylic, soda-lime glass and alumina ceramics. The typical change in the electrical resistance of the ion-sputtered gold film (50 nm thick) during crack

Fig. 9. Change in electrical resistance of the ion-sputtered gold film during crack initiation on a steel test piece [36]. Copyright © 2009, ASME.

Fig. 10. Typical resistance change in Cu–Ni alloy due to fatigue cycling [37]. Copyright © 1975, Elsevier.

initiation on a steel test piece is shown in Fig. 9. The tendency of change in electrical resistance allows us to judge the onset of crack initiation. From the optical observation of the crack, it was found that crack lengths identified in the acrylic and steel test pieces were about 0.35 mm and 1.2 mm, respectively. The measurement error in steel was drastically decreased to 0.068 mm. Therefore, this method is convenient and practicable for the evaluation of the fatigue crack initiation. 2.2.2. Electrical resistance fatigue gauge for fatigue life prediction Based on the pioneer work of Harting, Charsley et al. have further investigated the effect of fatigue damage on the electrical resistance of Cu–Ni alloys [37–39]. The typical electrical resistance change in Cu–Ni alloys due to fatigue cycling is shown in Fig. 10. The increase in electrical resistance during the fatigue of high Ni concentration Cu–Ni alloys, which is of the order of 50 times that for pure cop-

Fig. 8. Electrical resistance of a grid pattern film during crack growth on an acrylic test piece [34].

P. Wang et al. / Sensors and Actuators A 198 (2013) 46–60

Fig. 11. Lines of constant change in electrical resistance and fatigue curves (solid lines) [41]. Copyright © 1985, Plenum Publishing Corporation.

per, can be explained in terms of the break-up of Ni clusters, the growth of which decreases the resistance. The decrease in resistance is explained by a similar mechanism, with the difference that in this alloy the presence of Ni clusters increases the electrical resistance. In the 1980s, Troshchenko and co-workers systematically studied the change in electrical resistance under the influence of constant and variable cyclic strains for a fatigue gauge made of annealed constantan foil [40–44]. The initial electrical resistance of the gauge was 95–105 , and the working range of the change in electrical resistance was 0.5–6.0 . Fig. 11 presents the fatigue curves for specimens of different materials and the limiting lines of constant change in electrical resistance of the sensor with Rmin = 0.5  and Rmax = 6.0 . When the fatigue curve of the specimen is located above the limiting line Rmax = 6.0 , the sensor will fail earlier than the specimen. On the other hand, when the fatigue curve of the specimen is located below the limiting line Rmin = 0.5 , failure of the specimen will occur earlier than when the sensor changes its electrical resistance by a value which can be measured with sufficient accuracy. Therefore, to use the sensor for the prediction of the life of specimens or parts, the fatigue curves of which lie below or above the lines of constant change in electrical resistance, special strain multipliers (also called deformation multipliers) are used to increase or decrease the deformations of the sensor by a specified deformation of the part surface. The application of these deformation multipliers could broaden the working range of the fatigue damage sensors, although the size of the sensors will increase. Some studies on the electrical resistance accumulation behavior of polycrystalline constantan under cyclic loading have been conducted in recent years. In 2004, Zhou et al. pointed out that there are two main factors affecting the electrical resistance accumulation performance of polycrystalline constantan. One is the proportions of the components of raw materials, and the other is the technology of cold rolling and annealing [45]. In 2011, Ren et al. further examined the effects of alloy contents (percentages of Ni, Mn and Fe), cold rolling, and annealing treatments on the electrical resistance of polycrystalline constantan foil under cyclic loading, with the objective of fabricating a polycrystalline constantan foil gauge that has a higher electrical resistance under cyclic loading [46,47].

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It was found that electrical resistance accumulation of the foil improved with an increase in the percentage of Mn, but increases in the percentages of Ni and Fe have a contrary effect in the range of alloy contents studied. The optimum alloy content of polycrystalline constantan foil is 43% Ni, 1.8% Mn, and 0.1% Fe. Besides, the electrical resistance accumulation rate of polycrystalline constantan foils decreases when the cold rolling rate increases. Moreover, the electrical resistance of the foil is enhanced with an increase in the annealing temperature but a prolonged annealing time has a contrary impact. The maximum electrical resistance changes of constantan due to fatigue are generally quite small (e.g. less than 3%) and in some cases are of the same order of magnitude as the electrical resistance changes from ambient temperature variations, and hence are difficult to measure reliably. One of the simple solutions is to increase changes in electrical resistance due to cyclic loading, which can greatly increase the accuracy of measurement. Much larger changes in fatigue-induced electrical resistance (e.g. 40%) have been found in conductive polymers by Dally et al. [48–50]. The polymer was composed of many small, randomly distributed highly conductive graphite particles suspended in an insulating epoxy matrix. The electrical resistance of the polymer was highly controlled by the volume fraction of graphite particles, especially in the contact area between them. During a fatigue test, the cyclic deformation caused the graphite particles to rub against each other and thus increased the contact area. Finally, permanent changes in electrical resistance of the polymer were easily observed. The polymer fatigue gauge was found to be an excellent indicator for the fatigue damage of certain metals, such as aluminum alloy 6061-T6, 1018, and 4130 steel. Although a suitable composition of the materials in the fatigue gauge varied according to the host structure, it was suggested that these electrical particle filled matrix composites are promising candidate materials for the development of highly reliable and practically applicable fatigue gauges. To make fatigue damage gauges with high electrical sensitivity, Karabaev et al. proposed polycrystalline film of the semiconductor compound (Bix Sb1−x )2 Te3 (60 at.% Te, 30 at.% Sb, and 10 at.% Bi) for use as a cumulative fatigue damage gauge [51]. The fatigue-damage indicator produced has an extremely high strain gauge factor of K ≈ 103 –104 (the strain gauge factor K in metallic resistance strain gauges has a low value of 0.4 and 5.5 in manganin (Cu–Mn–Ni) and platinorhodium). Preliminary tests confirmed the possibility of using this semiconducted fatigue-damage indicator. Fatigue gauges with higher electrical sensitivity to cyclic loading can be fabricated with a conductive polymer and semiconductor compound. However, the stability of these fatigue gauges with respect to temperature is significant for large temperature changes. Therefore, in addition to the electrical sensitivity, the temperature coefficient of resistance is also a key factor in the development of new materials for electrical resistance fatigue gauges. 2.3. Surface roughness fatigue gauge When steel specimens are subjected to cyclic loading, fatigue causes slip bands to appear on the surface of the specimen. These slip bands lead to the roughening of the specimen surface as measured by surface roughness parameters such as height deviation and correlation length. Because the evolution of surface roughness is an irreversible process, the surface roughening phenomenon due to slip deformation has been proposed as a useful technique for fatigue characterization. Since 1989, Nagase et al. have reported that the surface roughness of aluminum foil, bonded on a specimen and subjected to a constant or variable amplitude stress, is dominated by both the stress amplitude and number of cycles [52–61]. The

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stress condition, is employed for the evaluation. The equivalent stress ( eq ) can be expressed by the following equation:

  ˛ 1/˛  N i i eq = ,

(3)

Ni

where  i and Ni are the stress amplitude and the number of cycles counted by the range-pair method [60]. When the effect of loading history on the evolution of surface roughness is not significant or does not exist, the equivalent stress also can be expressed by the following equation:

 eq =

Fig. 12. Isoroughness curves of the aluminum foil [52].

surface roughening behavior due to a constant amplitude stress is described by the isoroughness curve, as shown in Fig. 12. The increase in surface roughness with number of cycles under constant stress amplitudes is shown in Fig. 13. Under constant stress-amplitude loading, the relationship between the measured surface roughness, Ra, stress amplitude, , and number of cycles, N, is represented by the following equation: Ra =

1 ˛/2 1/2  N + m, k0

(2)

where k0 , ˛, and m are the material constants appropriate to the aluminum foil used. Surface roughness was measured by a noncontact-type surface roughness tester using the correlation between surface roughness and the characteristics of the distribution of reflected infrared light. On the other hand, under varying stress-amplitude loading, an “equivalent stress”, which is defined as a constant stress inducing the same surface roughness as that under a variable amplitude

Fig. 13. Surface roughness increases at constant amplitude stresses [52].

(Ran − m)2 k02



Ni

1/˛ ,

(4)



where Ran is the roughness measured after Ni cycles. Consequently, the measuring accuracy of the equivalent stress in the proposed method is regarded as being sufficiently high (i.e. less than five percent). Similar conclusions have also been obtained on the surface roughness fatigue gauge made of electroplated copper foil by Nagase and co-workers [62–66]. It should be pointed out that this metal foil fatigue gauge does not need lead wires, which is essential for the electrical resistance strain gauge, and can easily be used in the rotating machine elements. 2.4. Laser speckle fatigue sensor As mentioned in Section 2.3, the fatigue process could roughen the specimen surface by increasing the surface height deviation and decreasing the surface correlation length. When a fatigued specimen surface is illuminated by a laser beam, the reflected and scattered light intensity distribution, in the form of a speckle pattern, reveals surface roughness and hence fatigue damage related information. Therefore, a method based on the laser speckle pattern is proposed by Dai et al. for studying the cumulative fatigue damage of a specimen undergoing cyclic loading [67]. In 1995, Kato et al. introduced the laser speckle technique for the detection of surface change in the aluminum foil gauge which was attached to a specimen undergoing cyclic loading [68]. The spectrum width extracted from the laser speckle pattern increases as a function of the number of loading cycles, as shown in Fig. 14, indicating the possibility that it can be utilized for detecting cumulative fatigue damage.

Fig. 14. Relationship between spectrum width and number of fatigue cycles under different plastic deformation [68].

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Fig. 16. Schematic illustration of the martensitic transformation fatigue gauge [72]. Fig. 15. Degree of reflection of various metallic fatigue gauges as a function of the number of load cycles [70]. Copyright © 1979, Verlag GmbH & Co. KGaA, Weinheim.

2.5. Specular reflection fatigue gauge In 1970s, Sahm et al. proposed and patented a method for detecting fatigue in structural elements by measuring the loss in specular reflection from a polished metal foil secured to an exposed surface of the specimen under test [69]. High reflectance losses were observed when the surface of the test specimen was repeatedly subjected to shearing stresses within its elastic limit but exceeded the elastic limit of the foil material. The loss of specular reflectance from the surface can be correlated with the known fatigue property of the test specimen to predict the remaining fatigue life. The metal foils consisted of either a single crystal or a polycrystalline material of uniform small grain size. The selection of the metal material is controlled by the ability of the foil material to undergo plastic deformation at the prevailing operating temperature within the range of strains occurring in the test specimen. The candidate metal materials for room temperature application are aluminum, tin, indium, zinc, gold, silver, copper, lead, nickel, and titanium. In 1979, the same group developed a new type of optical fatigue gauge based on irreversible changes of optical reflectivity of some types of metal foils subjected to cyclic loading [70]. A portable measurement system was fabricated for detecting the degree of reflection from the optical fatigue gauges. The results showing the relationship between the degree of reflection of various metallic fatigue gauges (i.e. Au, AlSi6, Al, Sn, and In with high purity) and number of load cycles are presented in Fig. 15. In 1983, Meyer and Schott also performed conducted reflectance measurements on metal foils during cyclic stress [71]. A drop in the degree of reflection of Al and Sn foils with the number of fatigue cycles under different cyclic stresses was clearly identified. The observed curves of optical reflectivity of the high purity metal foils are similar to the S–N curves of the tested materials. Therefore, a specular reflection fatigue gauge can be a potential candidate for the prediction of the fatigue life of structures.

Fig. 17. Expected color change of a silver–zinc film in response to a number of cycles of applied stress [72].

change a qualitative indication of the amount of fatigue suffered by the host structure is obtained. The measurement of the amount of cumulative fatigue damage can be rendered quantitative by using a conventional reflectometer. Later, fundamental research on color change behavior and its relationship to phase transformations in Ag–Zn alloys was conducted by Minemura and co-workers [73–75]. The color change temperatures from pink to silver and from silver to pink are equivalent to 425 K and 553 K in Ag50 Zn50 alloy. The reversible color change is attributed to the solid-state phase transformations between the ␤ phase (CsCl-type ordered structure) and ␰ equilibrium phase (hexagonal structure). The spectral reflectivity changes are reversible with respect to color change. The change in reflectivity at wavelengths of 700 nm and 500 nm with aging temperatures, together with the identified phases in Ag50 Zn50 alloy ribbons, are shown in Fig. 18. Furthermore, the reversible color change between copper and gold in Cu–Al–Ni alloy (e.g. Cu–13% A1–4% Ni) has also been observed [76,77]. Therefore, it is suggested that these alloys should be good candidate materials for the martensitic transformation fatigue gauge.

2.6. Martensitic transformation fatigue gauge

2.7. Mean stress fatigue gauge

In 1977, Scott proposed a fatigue gauge that exploited the tendency of silver–zinc (e.g. Ag54 Zn46 ) alloy to change color in response to fatigue loading [72]. As shown in Fig. 16, a thin film or foil of silver zinc is attached to a thin flexible substrate and overlaid with a transparent protective coating. As the host structure experiences cyclic loading, the silver zinc film is subjected to flexing, i.e. cold working. Upon suffering a predetermined level of cold working the silver zinc film undergoes a martensitic transformation which is accompanied by a color change, pink to silver, as shown in Fig. 17. By visually observing the extent of the color

The superposition of a tensile mean stress on a fluctuating load has a detrimental effect on fatigue life. Moreover, it is possible to estimate the reduction in life from S–N curves which include R-ratio (mean stress) variations. Therefore, Rajic and Tsoi proposed a new type of fatigue gauge, the mean stress fatigue gauge [1]. A pretensioned foil made of the same material as the detected structure is attached to the structure. This foil will break at a predetermined (from S–N curves) number of fatigue cycles before the structure, and hence give warning of its impending failure. The fatigue state of the foil could be tracked by continuously measuring its electrical

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P. Wang et al. / Sensors and Actuators A 198 (2013) 46–60

Fig. 19. Schematic illustration of the gel electrode method [78].

Fig. 18. Change in reflectivity at (a) 700 nm and (b) 500 nm with aging temperatures together with the identified phases in Ag50 Zn50 alloy ribbons [75]. Copyright © 1988, Chapman and Hall Ltd.

resistance. Failure would obviously be indicated by a great increase in its electrical resistance. 2.8. Oxide film fatigue sensor In 1984, Baxter proposed a simple electrochemical technique, thereafter referred to as a gel electrode, for the early assessment of fatigue damage in a selected region of a metal structure by detecting fatigue-induced micro cracks in a surface oxide film [78]. Since then, systematic studies have been conducted on the application of this method to detect the fatigue damage of structures [79–89]. The schematic illustration of the gel electrode method is shown in Fig. 19. The surface oxide film is formed prior to the application of fatigue loading. Surface preparation procedures for the oxide films on different metals can be found in [89]. The fatigue life was not influenced by the preparation of these films. During the early stages of a fatigue test, “damage” accumulates in the underlying metal and produces micro cracks in the oxide film. The micro cracks in the

oxide are detected by placing the specimen in contact with a dried surface film on a gel-electrolyte, containing potassium iodide (KI) and starch, and applying a voltage pulse to stimulate a corrosion current which flows in a preferential manner to the micro cracks. This current anodically oxidizes the KI to release iodine ions, which react with the starch to form a black complex. These complexes are retained in the skin of the gel and provide a visual display showing the number, location and size of micro cracks in the oxide film. In addition to providing a visible record, the pulse current may be readily measured and is also directly related to the extent of fatigue-induced oxide cracking. Both visual examination and current measurement provide a basis for predicting the life of the part prior to fatigue failure. The gel electrode has been successfully applied in the fatigue testing of aluminum [79–81,83–85], titanium [82], iron [86,87,89], and magnesium [88] alloys. Typical results for the fatigue detection of 6061-T6 aluminum with the gel electrode technique are shown in Figs. 20 and 21. The visual display of the gel tips, as shown in Fig. 20, indicates that the development of the black spots is clearly related to the number of fatigue cycles, and even after only 1000 cycles (∼0.7% fatigue life) there is an appreciable effect. On the other hand, the quantitative relationship between the density of charge flow and fatigue cycle, as shown in Fig. 21, provides a basis for measuring the severity of the fatigue damage and for estimating the remaining fatigue life. The ability to detect fatigue damage earlier than 1% of the fatigue life, and fatigue cracks as small as 10 ␮m long, demonstrates that

Fig. 20. A series of gel electrode tips after imaging at the locations of maximum charge flow on a series of specimens of 6061-T6 aluminum fatigued by different amounts [83]. Reprinted, with permission, from STP 811 Fatigue Mechanisms: Advances in Quantitative Measurement of Physical Damage, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.

P. Wang et al. / Sensors and Actuators A 198 (2013) 46–60

Fig. 21. Effect of the number of fatigue cycles on the maximum value of charge density which flow to specimens of 6061-T6 aluminum. NF indicates the range of fatigue lives observed for similar specimens tested under the same conditions [83]. Reprinted, with permission, from STP 811 Fatigue Mechanisms: Advances in Quantitative Measurement of Physical Damage, copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428.

the development of micro cracks in a surface oxide film together with gel electrode imaging provides the basis for a very sensitive and quantitative tool for the evaluation of fatigue damage. 2.9. Carbon film fatigue monitoring sensor Amorphous carbon films, such as diamond-like carbon (DLC), have been widely applied in industrial fields because of their excellent mechanical, tribological and chemical properties, such as high hardness, low friction coefficient, high wear resistance, and chemical stability in corrosive conditions [90]. Moreover, the incorporation of metal into amorphous carbon films can not only increase the electrical conductivity of the films by several orders of magnitude but also improve the mechanical (e.g. residual stress and adhesion strength) and tribological behaviors (e.g. low friction coefficient in ambient air) of the films. The exceptional mechanical and electrical properties of the metal-containing diamond-like carbon (Me-DLC) films make them good candidate materials for smart sensor applications. Extensive and systematic studies have been conducted on the potential application of these films in strain sensors [91–99]. One of the key issues in the development of a strain sensor as well as a fatigue monitoring sensor using carbon film is to find suitable sensor material, which can exhibit both a high gauge factor and a low or even zero temperature coefficient of resistance (TCR). Comparison of the gauge factor and TCR in various strain sensors is summarized in Table 2. Schultes et al. have studied the strain sensitivity and TCR of nickel-containing diamond-like carbon (Ni-DLC) films prepared

55

under different parameters [92,93,95]. The optimum Ni-DLC film, with a nickel concentration of approximately 55 at.%, exhibits a gauge factor of about 20 (ten times higher than standard NiCr films in commercial strain sensors) and a TCR below ±50 ppm/K over the wide temperature range of 100–400 K (the TCR of pure Ni is around +6500 ppm/K as reported by Robinson [100]). The combination of these unique behaviors makes Ni-DLC film an attractive material for strain sensor applications. Similarly, Heckman et al. investigated the strain sensitivity and TCR of several Me-DLC films, such as W-DLC, Ti-DLC, Ag-DLC, and Ni-DLC, with various metal concentrations [94]. It was found that the optimum Ni-DLC films show a gauge factor of higher than 15 in combination with a TCR close to zero. This further confirmed that well-designed Me-DLC films can satisfy the requirements of high gauge factor and zero TCR for strain sensor applications. However, the electrical conduction mechanism of these Me-DLC films is still not clearly understood. Over the last few years, Takeno et al. evaluated the electrical properties of several types of Me-DLC films, such as W-DLC [96–98,101,102], Mo-DLC [102], and Cu-DLC [103]. In particular, a systematic investigation on the strain sensitivity of W-DLC films with various tungsten concentrations was conducted using a fourpoint bending test [96–98,101,102]. The electrical resistance of the W-DLC films was proportional to the applied strain varying from −1000 ␮␧ to 1000 ␮␧. The gauge factor depended strongly on the tungsten concentration in the films, and increased with decreasing tungsten concentration. The TCR increased from a negative value to a positive value with increasing tungsten concentration. Therefore, a sample with almost no temperature dependence can be obtained at a certain tungsten concentration. It was clarified that W-DLC films have the potential to be used as a sensor with high sensitivity to strain and low sensitivity to temperature under an optimum tungsten concentration. A model based on a composite insulator–metal cluster structure was also proposed by Takeno et al. to elucidate the electrical conduction mechanism of the amorphous carbon films containing nanocluster metal (e.g. W-DLC and Mo-DLC). The gauge factor (K) and TCR of these films were calculated using the following two equations [101,102]: K=

2d εx



(1 − )

 ıx d + 2 d



ıy d ız d + d d



,

(5)

where d is the distance between the adjacent metal clusters,  is the localization length, εx is the applied strain along x (longitudinal direction),  is the tortuosity factor and ıx d, ıy d and ız d are the variations in tunneling distances along the x, y and z directions. 1 TCR = T2 − T1



e2 2rεd



1−

 1/3  1 0.64

1 − T1 T2



,

(6)

where T is the temperature, e is the elementary charge, r is the radius of the metal cluster, εd is the relative dielectric constant of DLC, and  is the metal volume fraction. The simulated results from

Table 2 Comparison of gauge factor and TCR in various strain sensors. Strain sensor DLC on Si Ni-DLC on alumina Ni-DLC on alumina Ni-DLC on ceramic Ni-DLC on alumina Me-DLC on ceramic (Me = Ag, Ti, and W) W-DLC on Si W-DLC on Si Kyowa stainless strain gauge

Gauge factor K 36–46 1.5–9.0 2–14 1–15 0–22 0.5–2.0 1.4–6.1 1.2–5.2 2.0–2.1

Temperature coefficient of resistance TCR, ppm/K

Optimum K and TCR

Optimum metal concentration

Reference

−6000 to −8000 −5000 to +3000 −4500 to +3000 −2000 to +500 −8000 to +4000 Around +500 −1098 to −5298 −2460 to −310 50–100

– 8, +300 ∼12, 0 ± 50 15, ∼0 20, 0 ± 50 – 6.1, −1098 5.2, −2460 2.0, +80

– ∼55 at.% 52 at.% 45–55 at.% 55 at.% – – 15.2 vol.% –

Peiner et al. [91] Schultes et al. [92] Koppert et al. [93] Petersen et al. [94] Koppert et al. [95] Petersen et al. [94] Takeno et al. [96] Ohno et al. [97,98] Kyowa Co. [99]

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Table 3 Summary of fatigue damage detecting sensors. Fatigue sensor

Detecting principle

Characterization parameter

Detecting apparatus

Sensor shape

Candidate sensor materials

Reference

Fatigue failure detection

Failure of coupon before tested structure with cyclic loading Growing of the crack from slit with cyclic loading

Broken of coupon

Human eye Optical microscope

Same material of the test structure Nickel

[9–15]

Length of crack

Thin metal sheet with coupon(s) Thin metal sheet with small slit(s)

Electrical resistance measurement system Electrical resistance measurement system

Foil, film, and wire

Polycrystalline copper, ion sputtered Au film Cu–45% Ni alloy, conductive polymer

[31–36]

Non-contact type surface roughness tester Laser speckle sensor system

Foil

Aluminum, electroplated copper Aluminum

[52–66]

Reflectometer

Polished metal foil

[69–71]

Color or degree of reflection Electrical resistance

Human eye or reflectometer Ohmmeter

Foil, film Pretensioned foil

Number, location, and size of micro cracks in the oxide film and pulse current E.g. electrical resistance, gauge factor

Gel electrode system

Film

Electrical properties measurement system

Film (single layer or double layer)

High purity Al, Au, Zn, and Ag Ag–Zn alloy, Cu–Al–Ni alloy Same material of the test structure Oxide film of the test material (Al, Ti, Fe, and Mg) Ni-DLC, W-DLC, Mo-DLC

Fatigue life prediction Electrical resistance fatigue gauge

Surface roughness fatigue gauge Laser speckle sensor

Specular reflection fatigue gauge Martensitic transformation fatigue gauge Mean stress fatigue gauge Oxide film fatigue sensor

Carbon film fatigue monitoring sensor

Crack initiation detection Fatigue life prediction

Sudden change of electrical resistance with cyclic loading Accumulation of electrical resistance with cyclic loading

Electrical resistance

Fatigue life prediction

Roughening of test specimen with cyclic loading Change of laser speckle pattern with cyclic loading

Surface roughness

Fatigue life prediction

Fatigue life prediction Fatigue life prediction Fatigue failure detection Crack initiation detection Fatigue life prediction Fatigue life prediction

Loss in specular reflection with cyclic loading Color change of material with cyclic loading Large increase in electrical resistance with cyclic loading Formation of micro cracks in the surface oxide film with cyclic loading Change of electrical properties with cyclic loading

Electrical resistance

Laser speckle pattern (reflected and scattered light intensity distribution) Degree of reflection

Foil

Metal foil

[16–30]

[37–51]

[67,68]

[72–77] [1] [78–89]

[90–108]

P. Wang et al. / Sensors and Actuators A 198 (2013) 46–60

Fatigue fuse

Function

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Fig. 22. (a) Photo of Mo-DLC sensor, (b) experimental setup for cyclic bending test and (c) variation of electrical resistance with number of fatigue cycles [104].

these two equations agree well with the experimental results, and a high gauge factor can be obtained with a low metal concentration or a large metal cluster. According to the above studies on the electrical properties of Me-DLC films, it is clearly indicated that Me-DLC films with optimized composition and structure can possess high gauge factor and near zero TCR. Moreover, it has been mentioned in Section 2.2 that the key requirements for developing new materials for electrical resistance fatigue gauge are high electrical sensitivity and low TCR. Thus, it is assumed that Me-DLC films could be potential candidates for the electrical resistance fatigue gauge. On the other hand, MeDLC film is composed of conductive metal clusters embedded into an insulating amorphous carbon matrix, which is identical to the structure of the conductive polymer regardless of the scale. It can be easily imaged that a permanent electrical resistance change could also be observed in the Me-DLC film after long time cyclic loading. Therefore, the concept of utilizing carbon film as a fatigue monitoring sensor is proposed. It is expected the carbon film fatigue monitoring sensor can be applied for fatigue life prediction. Recently, Takahashi et al. have investigated the possibility of applying molybdenum-containing DLC (Mo-DLC) film as a fatigue monitoring sensor by focusing on the electrical resistance of the Mo-DLC film [104]. As shown in Fig. 22(a), Mo-DLC film with a thickness of 1 ␮m is fabricated on the ZrO2 substrate with the size of 7.0 mm × 4.0 mm × 0.1 mm using a hybrid deposition apparatus consisted of chemical vapor deposition and physical vapor deposition. A customized experimental setup is applied for the vibration test with a dynamic strain in the range of −500 ␮␧ to +500 ␮␧ and a resonance frequency from 60 to 80 Hz (the exact value depends on the resonance frequency of the AISI 1045 steel beam) (Fig. 22(b)). The change of electrical resistance of Mo-DLC film with number of fatigue cycles is shown in Fig. 22(c). The electrical resistance of the Mo-DLC film keeps constant value up to 103 fatigue cycles, and then it increases gradually with increasing fatigue cycles from 103 to 107 . The change of electrical resistance is derived from the structural

changes of Mo-DLC films during the vibration test. Specifically, the increase of carbon sp3 bonds in the Mo-DLC films with fatigue cycles is clearly identified from the Raman analysis of the tested Mo-DLC films. These results strongly suggest that Mo-DLC film has a possibility as a fatigue monitoring sensor. Further studies are required to establish a clear relationship between the electrical properties (e.g. electrical resistance, gauge factor, and TCR) of the Me-DLC films and fatigue cycles. Besides, characterization of the structural changes in the Mo-DLC films is necessary for better understanding of the conduction mechanism as well as the construction of the conduction model. The concept of double-layered structures comprising DLC and Me-DLC layers was proposed by Takagi et al. for the fabrication of smart films for smart materials [105,106]. The top DLC layer provides mechanical functionality such as high hardness, chemical inertness and low friction coefficient. Whereas, the Me-DLC layer is the sensor layer, which can measure temperature, strain or the magnetic field if proper metals are selected and the structure is optimized. Such double-layered structure can realize multifunctionality. For example, a temperature sensor with low friction coefficient, an anticorrosive strain sensor or a magnetic sensor with high hardness. The fabricated DLC/W-DLC double layered film exhibits good electrical and tribological performance, and is a good candidate as a strain sensor with low friction coefficient [107,108]. Consequently, it is expected that the application of this doublelayered multifunctional film in the proposed carbon film fatigue monitoring sensor will greatly extend the application fields of the fatigue sensor (e.g. anticorrosion fatigue monitoring sensor). 3. Summary Table 3 provides a brief summary of the nine types of fatigue damage detecting sensors, which have been discussed in this study. The primary advantages of these fatigue damage detecting sensors are generally summarized as being small in size, with simple

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design, ease of preparation, low cost, good metrological characteristics, and a lack of necessity for the permanent connection of complex measuring and recording instruments. On the other hand, the common drawbacks for the application of these fatigue damage sensors in actual products are high environmental sensitivity, low stability, low reliability as well as low repeatability. Owing to the stringent requirements of industrial products, such as repeatability, reliability, and stability, the fatigue fuse with a small slit and the electrical resistance fatigue gauge are the two most promising and commercially available fatigue sensors for industrial applications. However, until now, only a few of these two types of fatigue damage sensors have been developed commercially and applied in selected industrial areas (e.g. steel bridges). The new concept of utilizing carbon film as a fatigue monitoring sensor is proposed and the preliminary experimental results strongly suggest that Me-DLC films are good candidates for fatigue monitoring sensors. Further study is still required to develop advanced fatigue damage sensors for demanding industrial applications. Acknowledgements This work was partly supported by the Grant-in-Aid for Scientific Research (A) (23246038) of the Japan Society for the Promotion of Science (JSPS), and the Global COE Program, “World Center of Education and Research for Trans-disciplinary Flow Dynamics”, by the Ministry of Education, Culture, Sports, Science and Technology (MEXT) in Japan. This research was partly performed by the Japan Society for the Promotion of Science (JSPS) Core-to-Core Program “International research core on smart layered materials and structures for energy saving”. References [1] N. Rajic, K. Tsoi, Methods of Early Fatigue Detection, DSTO Aeronautical and Maritime Research Laboratory (DSTO-NT-0059), Melbourne, 1996. [2] S.P. Sagar, S. Das, N. Parida, D.K. Bhattacharya, Non-linear ultrasonic technique to assess fatigue damage in structural steel, Scripta Materialia 55 (2006) 199–202. [3] J.M. Papazian, J. Nardiello, R.P. Silberstein, G. Welsh, D. Grundy, C. Craven, L. Evans, N. Goldfine, J.E. Michaels, T.E. Michaels, Y. Li, C. Laird, Sensors for monitoring early stage fatigue cracking, International Journal of Fatigue 29 (2007) 1668–1680. [4] F. Walther, D. Eifler, Fatigue life calculation of SAE 1050 and SAE 1065 steel under random loading, International Journal of Fatigue 29 (2007) 1885–1892. [5] A. Kato, T.A. Moe, S. Kohmura, Full-field visualization and evaluation of fatigue damage using laser, Journal of the Japanese Society for Experimental Mechanics 9 (2009) 369–375 (in Japanese). [6] I. Pitropakis, H. Pfeiffer, M. Wevers, Crack detection in aluminium plates for aerospace applications by electromagnetic impedance spectroscopy using flat coil sensors, Sensors and Actuators A: Physical 176 (2012) 57–63. [7] G. Shui, Y. Wang, Ultrasonic evaluation of early damage of a coating by using second-harmonic generation technique, Journal of Applied Physics 111 (2012) 124902. [8] A.V. De Forest, Fatigue indicator, US Patent 2,449,883 (1948). [9] M.A. Brull, Device for monitoring fatigue life, US Patent 4,590,804 (1986). [10] M.A. Brull, Method of making a device for monitoring fatigue life, US Patent 4,639,997 (1987). [11] R.C. De La Veaux, Metal fatigue detector, US Patent 5,237,875 (1993). [12] M. Creager, Device for monitoring the fatigue life of a structural member and a method of making same, US Patent 5,319,982 (1994). [13] M. Creager, Device for monitoring the fatigue life of a structural member and a method of making same, US Patent 5,425,274 (1995). [14] H.Y. Lee, J.B. Kim, Y. Bong, Multidirectional fatigue damage indicator, US Patent 6,443,018 (2002). [15] Y.W. Kwon, Fatigue measurement device and method, US Patent 6,983,660 (2006). [16] H.W. Smith, Fatigue damage indicator, US Patent 3,979,949 (1976). [17] F.J. Geissler, J.P. Gallagher, Evaluation of the Crack Growth Gage Concept as an Individual Aircraft Tracking Device, vol. 1, Air Force Wright Aeronautical Laboratory (AFWAL-TR-83-3082), Ohio, 1983. [18] A.F. Grandt Jr., A. Dumanis-Modan, Evaluation of the Crack Growth Gage Concept as an Individual Aircraft Tracking Device, vol. 2, Air Force Wright Aeronautical Laboratory (AFWAL-TR-83-3082), Ohio, 1983. [19] Y. Fujimoto, Sacrificial specimen for use in structural monitoring for predicting fatigue damage, US Patent 5,789,680 (1998).

[20] K. Nihei, T. Kobayashi, H. Ono, S. Koe, A. Murakami, G. Nishiyama, Y. Dake, Crack-type fatigue detecting sensor, method for fabricating crack-type fatigue detecting sensor, and method for estimating damage using crack-type fatigue detecting sensor, US Patent 6,520,024 (2003). [21] Y. Takaoka, K. Nihei, P. Vargas, P. Aalberts, M. L. Kaminski, Application of fatigue damage sensors in the Monitas system, 2010 Offshore Technology Conference, Houston, USA, 2010, OTC-20870. [22] Kawasaki Technology Co., Ltd. Webpage: http://www.kawaju.co.jp/ fs/index.html (accessed 22.10.12). [23] T. Kobayashi, O. Muragishi, K. Nihei, Y. Oku, Development of small fatigue detecting sensor and its application to products, in: Symposium on Evaluation and Diagnosis, vol. 2, 2003, pp. 134–138 (in Japanese). [24] T. Kobayashi, O. Muragishi, K. Nihei, Development of fatigue detecting sensor, Key Engineering Materials 261–263 (2004) 1313–1318. [25] O. Muragishi, Fatigue life diagnosis of steel structures with fatigue damage sensor, Journal of the Japan Welding Society 74 (2005) 209–212 (in Japanese). [26] T. Kobayashi, O. Muragishi, K. Nihei, G. Nishiyama, Application of fatigue damage sensor to ship, in: Conference Proceedings of the Japan Society of Naval Architects and Ocean Engineers, vol. 1, 2005, pp. 119–120 (in Japanese). [27] Y. Takaoka, K. Nihei, T. Kobayashi, T. Koiwa, N. Yamamoto, A. Usami, H. Dobashi, Improvement of prediction accuracy of hull fatigue life using a fatigue damage sensor, Journal of the Japan Society of Naval Architects and Ocean Engineers 9 (2009) 183–190 (in Japanese). [28] T. Kobayashi, K. Nihei, Y. Takaoka, T. Shimoda, T. Koiwa, N. Yamamoto, A. Usami, H. Dobashi, Improvement of prediction accuracy of hull fatigue life using a fatigue damage sensor: influences on storm patterns for estimation of fatigue life, Journal of the Japan Society of Naval Architects and Ocean Engineers 13 (2011) 157–164 (in Japanese). [29] T. Kobayashi, K. Nihei, Fatigue life estimation of welded structures with fatigue damage sensor, Journal of the Japan Welding Society 76 (2007) 221–225 (in Japanese). [30] T. Kobayashi, K. Nihei, Fatigue life estimation of welded structures with a fatigue damage sensor, Welding International 22 (2008) 212–217. [31] D.R. Harting, The S/N fatigue-life gage: a direct means of measuring cumulative fatigue damage, Experimental Mechanics 6 (1966) 19A–24A. [32] D.R. Harting, Fatigue life gaging methods, US Patent 3,272,003 (1966). [33] P. Charsley, B.A. Robins, Electrical resistance changes of cyclically deformed copper, Materials Science and Engineering 14 (1974) 189–196. [34] G. Deng, H. Tokunaga, T.D. Redda, K. Ikeda, T. Nakanishi, K. Kaizu, Crack length measurement with an ion sputtered film, Transactions of the Japan Society of Mechanical Engineers, Series C 72 (2006) 997–1002 (in Japanese). [35] G. Deng, K. Sakamoto, K. Ikeda, T. Akanishi, Application of the crack length measurement using an ion-sputtered film to the evaluation of the microcrack growth characteristics of soda-lime glass and alumina ceramics, Transactions of the Japan Society of Mechanical Engineers, Series A 74 (2008) 1373–1378 (in Japanese). [36] G. Deng, Y. Sakanashi, T. Nakanishi, A practical method for fatigue crack initiation detection using an ion-sputtered film, Journal of Engineering Materials and Technology, Transactions of the ASME 131 (2009) 011007. [37] P. Charsley, B.A. Robins, Electrical resistance changes of cyclically deformed copper–nickel alloys, Materials Science and Engineering 17 (1975) 117–123. [38] B.A. Robins, P. Charsley, The effect of strain amplitude and grain size on the electrical resistance changes of cyclically deformed constantan, Materials Science and Engineering 22 (1976) 109–112. [39] P. Charsley, A.F. Shimmin, Electrical resistance changes of cyclically deformed Cu–10% Ni and Cu–30% Ni, Materials Science and Engineering 38 (1979) 23–31. [40] V.T. Troshchenko, Y.I. Koval’, V.I. Boiko, Designing fatigue-damage gauges, Strength of Materials 13 (1981) 1235–1240. [41] V.T. Troshchenko, V.I. Boiko, A fatigue damage sensor and the basis of its use. Report 1, Strength of Materials 17 (1985) 1–7. [42] V.T. Troshchenko, V.I. Boiko, A fatigue damage sensor and the basis of its use. Report 2, Strength of Materials 17 (1985) 8–16. [43] V.T. Troshchenko, S.G. Orlov, V.I. Boiko, A.N. Kuksin, A strain multiplier for the fatigue damage sensor, Strength of Materials 19 (1987) 1171–1177. [44] S.G. Orlov, A method of calibrating a fatigue damage gauge, Strength of Materials 21 (1989) 958–963. [45] Z. Zhou, Z.D. Duan, Z.H. Jia, J.P. Ou, Smart Structures and Materials: Smart Sensor Technology and Measurement Systems, SPIE, Bellingham, 2004, pp. 324–331. [46] W.X. Ren, D. Xiao, G.W. Chen, The effect of alloy contents on electrical resistance accumulation of polycrystalline constantan under cycle loading, Materials Science and Engineering A 529 (2011) 270–274. [47] W.X. Ren, D. Xiao, G.W. Chen, Effect of annealing treatments on electrical resistivity of polycrystalline constantan foil under cycle loading, Experimental Mechanics 52 (2012) 1223–1228. [48] J.W. Dally, G.A. Panizza, Conductive polymers as fatigue-damage indicators, Experimental Mechanics 12 (1972) 124–129. [49] G.A. Panizza, J.W. Dally, Predicting failures with conducting-polymer fatiguedamage indicators, Experimental Mechanics 13 (1973) 7–13. [50] J.W. Dally, G.A. Panizza, Fatigue damage indicator, US Patent 3,738,162 (1973). [51] M.K. Karabaev, K.E. Onarkulov, M.M. Akhmedov, D.A. Yusupova, Semiconductor fatigue-damage indicator, Journal of Engineering Physics and Thermophysics 75 (2002) 1227–1228. [52] Y. Nagase, T. Nakamura, Y. Nakamura, Fatigue gauge utilizing slip deformation of aluminum foil: 1st report, slip initiation and surface roughening

P. Wang et al. / Sensors and Actuators A 198 (2013) 46–60

[53]

[54]

[55]

[56]

[57]

[58]

[59]

[60] [61]

[62]

[63]

[64]

[65]

[66]

[67] [68]

[69]

[70]

[71] [72] [73]

[74]

[75] [76]

[77]

[78]

phenomena under uniaxial stressing, Transactions of the Japan Society of Mechanical Engineers, Series A 55 (1989) 2388–2396 (in Japanese). Y. Nagase, Y. Nakamura, Fatigue gauge utilizing slip deformation of aluminum foil: 2nd report, effect of material factor on the evolution of roughness, Transactions of the Japan Society of Mechanical Engineers, Series A 57 (1991) 1313–1319 (in Japanese). Y. Nagase, Y. Nakamura, Y. Sawaki, Fatigue gauge utilizing slip deformation of aluminum foil: 3rd report, effect of temperature and cyclic frequency on surface roughness, Transactions of the Japan Society of Mechanical Engineers, Series A 57 (1991) 1481–1487 (in Japanese). Y. Nagase, Y. Sawaki, F. Moritake, H. Yoshida, Fatigue gauge utilizing slip deformation of aluminum foil: 4th report, surface roughening under variable amplitude stress, Transactions of the Japan Society of Mechanical Engineers, Series A 57 (1991) 2279–2286 (in Japanese). Y. Nagase, Y. Sawaki, H. Yoshida, Fatigue gauge utilizing slip deformation of aluminum foil: 5th report, controlling parameter of surface roughening under variable stress, Transactions of the Japan Society of Mechanical Engineers, Series A 57 (1991) 2287–2292 (in Japanese). Y. Nagase, T. Nakamura, Y. Nakamura, Fatigue gauge utilizing slip deformation of aluminum foil – slip initiation and surface roughening phenomena under uniaxial stressing, JSME International Journal Series 1-Solid Mechanics Strength of Materials 33 (1990) 506–513. Y. Nagase, Y. Nakamura, Fatigue gauge utilizing slip deformation of aluminum foil: effect of material factor on the evolution of roughness, JSME International Journal Series 1-Solid Mechanics Strength of Materials 35 (1992) 247–252. Y. Nagase, Y. Sawaki, H. Yoshida, The formation of slip bands in aluminum foil subject to variable stress loading and the application of this technique to stress measurement, International Journal of Fatigue 15 (1993) 205–211. Y. Nagase, Y. Sawaki, H. Yoshida, Range-pair-counting fatigue gage, Experimental Mechanics 33 (1993) 242–248. Y. Nagase, Y. Sawaki, F. Moritake, H. Yoshida, Fatigue gage utilizing surface roughening of aluminum foil, Journal of Engineering Materials and Technology, Transactions of the ASME 117 (1995) 56–61. Y. Nagase, T. Yoshizaki, A basic study on fatigue gauge made of copper electroplating: 1st report, effects of understress on slip initiation, Transactions of the Japan Society of Mechanical Engineers, Series A 56 (1990) 231–238 (in Japanese). Y. Nagase, T. Yoshizaki, A basic study on fatigue gauge made of copper electroplating: 2nd report, effects of understress on initiation of grown grain, Transactions of the Japan Society of Mechanical Engineers, Series A 56 (1990) 1058–1066 (in Japanese). Y. Nagase, Y. Sawaki, K. Omura, H. Misu, A basic study on fatigue gauge made of copper electroplating: 3rd report, evaluation of equivalent stress from the density of recrystallized grains under the variable stress condition, Transactions of the Japan Society of Mechanical Engineers, Series A 58 (1992) 153–159 (in Japanese). Y. Nagase, Y. Yoshizaki, A basic study on fatigue gauge made of electroplated copper foil: effects of understress, JSME International Journal Series 1-Solid Mechanics Strength of Materials 34 (1991) 56–63. Y. Nagase, Y. Yoshizaki, Fatigue gage utilizing slip-initiation phenomenon in electroplated copper foil, Experimental Mechanics 33 (1993) 49–54. Y.Z. Dai, A. Kato, F.P. Chiang, Fatigue monitoring by laser speckle, International Journal of Fatigue 13 (1991) 227–232. A. Kato, H. Nakai, K. Tanabe, Y. Nagase, Non-contact strain measurement using metal foil gauges and the laser speckle technique: 1st report, change of speckle intensity distribution by cyclic loading, Transactions of the Japan Society of Mechanical Engineers, Series A 61 (1995) 423–428 (in Japanese). K.F. Sahm, E. Rensen, H. Rieger, F. Jaeger, Non-destructive testing of structural elements for fatigue by measurement of specular reflectance, US Patent 4,255,049 (1981). H. Rieger, E.V. Rensen, K.F. Sahm, A new technique for lifetime monitoring of cyclic loaded components, Zeitschrift Fur Werkstofftechnik 10 (1979) 353–360 (in German). G. Meyer, G. Schott, Reflection measurement on metal foils under cyclic loading, Technische Mechanik 4 (1983) 39–43 (in German). W.R. Scott, Color displaying fatigue sensor, US Patent 4,015,465 (1977). T. Kaneko, Y. Maeda, T. Minemura, I. Ikuta, H. Andoh, Reversible color-change in Ag–Zn alloys, Journal of the Japan Institute of Metals 51 (1987) 911–915 (in Japanese). T. Minemura, H. Andoh, Y. Maeda, Reversible color changes in sputterdeposited Ag–Zn alloy films, Journal of Applied Physics 63 (1988) 4632–4636. T. Minemura, H. Andoh, I. Ikuta, Reversible colour change in Ag–Zn alloy ribbon, Journal of Materials Science 23 (1988) 1735–1738. T. Minemura, H. Andoh, I. Ikuta, Reversible colour change in Cu–Al–Ni alloy ribbon associated with phase transformation, Journal of Materials Science 22 (1987) 932–936. T. Minemura, H. Andoh, M. Nagai, R. Watanabe, S. Shimizu, I. Ikuta, Reversible colour change in sputter-deposited Cu–Al–Ni film, Journal of Materials Science Letters 6 (1987) 1267–1269. W.J. Baxter, Method of manufacturing gel electrode for early detection of metal fatigue, US Patent 4,445,999 (1984).

59

[79] W.J. Baxter, Gel electrode imaging of metal fatigue: Part I. Cracks in 6061-T6 aluminum, Metallurgical and Materials Transactions A 13 (1982) 1413–1419. [80] W.J. Baxter, Gel electrode imaging of metal fatigue: Part II. Deformation in 1100 aluminum, Metallurgical and Materials Transactions A 13 (1982) 1421–1427. [81] W.J. Baxter, Gel electrode imaging of fatigue cracks in aluminium alloys, International Journal of Fatigue 5 (1983) 37–42. [82] W.J. Baxter, Gel electrode imaging of fatigue cracks in titanium, Scripta Metallurgica 17 (1983) 1023–1026. [83] W.J. Baxter, Oxide films: quantitative sensors of metal fatigue, in: J. Lankford, D.L. Davidson, W.L. Morris, R.P. Wei (Eds.), Fatigue Mechanisms: Advances in Quantitative Measurement of Physical Damage. ASTM STP 811, ASTM International, Baltimore, USA, 1983, pp. 115–136. [84] W.J. Baxter, Repeatability of gel electrode measurements of fatigue deformation in 6061-T6 aluminium, International Journal of Fatigue 6 (1984) 243–252. [85] W.J. Baxter, Optimization of the gel electrode for repeatable imaging of fatigue cracks in aluminum alloys, Journal of Testing and Evaluation 13 (1985) 245–256. [86] W.J. Baxter, D.W. Gorkiewicz, Gel electrode imaging of a fatigue crack in anodized steel, Scripta Metallurgica 20 (1986) 1169–1172. [87] W.J. Baxter, D.W. Gorkiewicz, Gel electrode imaging of fatigue cracks in steel, International Journal of Fatigue 11 (1989) 347–352. [88] W.J. Baxter, D.W. Gorkiewicz, Gel electrode imaging of fatigue cracks in magnesium alloys, Fatigue and Fracture of Engineering Materials and Structures 13 (1990) 109–117. [89] W.J. Baxter, Imaging short fatigue cracks with the gel electrode, Journal of Testing and Evaluation 18 (1990) 430–438. [90] J. Robertson, Diamond-like amorphous carbon, Materials Science and Engineering R-Reports 37 (2002) 129–281. [91] E. Peiner, A. Tibrewala, R. Bandorf, S. Biehl, H. Lüthje, L. Doering, Micro force sensor with piezoresistive amorphous carbon strain gauge, Sensors and Actuators A 130–131 (2006) 75–82. [92] G. Schultes, P. Frey, D. Goettel, O. Freitag-Weber, Strain sensitivity of nickelcontaining amorphous hydrogenated carbon (Ni:a-C:H) thin films prepared by r.f. sputtering using substrate bias conditions, Diamond and Related Materials 15 (2006) 80–89. [93] R. Koppert, D. Goettel, O. Freitag-Weber, G. Schultes, Nickel containing diamond like carbon thin films, Solid State Sciences 11 (2009) 1797–1800. [94] M. Petersen, U. Heckmann, R. Bandorf, V. Gwozdz, S. Schnabel, G. Bräuer, C. Klages, Me-DLC films as material for highly sensitive temperature compensated strain gauges, Diamond and Related Materials 20 (2011) 814–818. [95] R. Koppert, S. Uhlig, H. Schmid-Engel, D. Göttel, A. Probst, G. Schultes, U. Werner, Structural and physical properties of highly piezoresistive nickel containing hydrogenated carbon thin films, Diamond and Related Materials 25 (2012) 50–58. [96] T. Takeno, H. Miki, T. Takagi, Strain sensitivity in tungsten-containing diamond-like carbon films for strain sensor applications, International Journal of Applied Electromagnetics and Mechanics 28 (2008) 211–217. [97] T. Ohno, T. Takeno, H. Miki, T. Takagi, Evaluation of electrical properties in WDLC coating for strain sensor applications, Transactions of the Japan Society of Mechanical Engineers, Series C 76 (2010) 267–274 (in Japanese). [98] T. Ohno, T. Takeno, H. Miki, T. Takagi, Evaluation of electrical properties of metal-containing amorphous carbon coatings for strain sensor application, International Journal of Applied Electromagnetics and Mechanics 33 (2010) 665–671. [99] Kyowa Electronic Instruments Co., Ltd. Webpage: http://www.kyowa-ei. co.jp/eng (accessed 22.10.12). [100] M. Robinson, Strain gage materials processing, metallurgy and manufacture, Experimental Techniques 30 (2006) 42–46. [101] T. Ohno, T. Takeno, H. Miki, T. Takagi, Microstructural design for fabrication of strain sensor utilizing tungsten-doped amorphous carbon coatings, Diamond and Related Materials 20 (2011) 651–654. [102] T. Takeno, T. Ohno, H. Miki, T. Takagi, Strain sensor utilizing amorphous carbon coating containing nanocluster metal, in: Proceedings of the 25th Diamond Symposium, Tsukuba, Japan, 2011, pp. 190–191 (in Japanese). [103] T. Takeno, T. Ohno, H. Miki, T. Takagi, Fabrication of copper-nanoparticle embedded in amorphous carbon films and their electrical conductive properties, International Journal of Applied Electromagnetics and Mechanics 33 (2010) 935–940. [104] M. Takahashi, H. Miki, T. Takeno, T. Takagi, Evaluation of electrical properties of Mo-DLC coatings for fatigue sensor application, in: Proceedings of the 26th Diamond Symposium, Tokyo, Japan, 2012, pp. 222–223 (in Japanese). [105] T. Takagi, T. Takeno, H. Miki, Metal-containing diamond-like carbon coating as a smart sensor, Material Science Forum 638–642 (2010) 2103–2108. [106] T. Takagi, T. Takeno, H. Miki, Y. Luo, Metal-containing DLC: toward a smart coating on smart materials, Material Science Forum 706–709 (2012) 2014–2019. [107] H. Miki, T. Takeno, T. Takagi, Tribological properties of multilayer DLC/W-DLC films on Si, Thin Solid Films 516 (2008) 5414–5418. [108] T. Takeno, H. Miki, T. Sugawara, Y. Hoshi, T. Takagi, A DLC/W-DLC multilayered structure for strain sensing applications, Diamond and Related Materials 17 (2008) 713–716.

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Biographies

coatings and their application, and adaptive structural systems using shape memory alloys.

Pengfei Wang received his B.Eng., M.Eng. degrees in mechanical engineering from Xi’an Jiaotong University, China, in 2004, 2007, and D.Eng. degree in mechanical engineering from Tohoku University, Japan, in 2011, respectively. He has been a postdoc at Tohoku University since 2011. His research interests are on the topics of low friction of carbon-based solid lubrication coatings and carbon film monitoring sensor.

Takanori Takeno is an assistant professor in the Department of Nanomechanics, Tohoku University, Japan. He received his B.E., M.E. and Ph.D. degrees in Mechanical Engineering from Tohoku University, Japan in 2002, 2004 and 2007, respectively. His interest includes mechanical, electrical and tribological properties of carbon-based nanocomposite coatings.

Toshiyuki Takagi received his B.Eng., M.Eng., and D.Eng. degrees in nuclear engineering from the University of Tokyo, Japan, in 1977, 1979, and 1982, respectively. He was a researcher at the Energy Research Laboratory of Hitachi Ltd. from 1982 to 1987 and an Associate Professor at the Nuclear Engineering Research Laboratory of the University of Tokyo from 1987 to 1989. From 1989 to 1998 he was an Associate Professor, and since 1998 has been a Professor, at the Institute of Fluid Science, Tohoku University. His current research interests include nondestructive evaluation of materials using electromagnetic phenomena, diamond and diamond-like carbon

Hiroyuki Miki is an associate professor in the Center for Interdisciplinary Research, Tohoku University, Japan. He received his Ph.D. degree in physics from Tohoku University in 1997. His main research area of interests is in the design of the highly efficient machine which is excellent in reliability and durability. He is working on actuators and sensors for functional hard carbon film required for a design of “conductivity and contact surface”, magnetic shape memory alloy which has two electro-magnetic functionalities as “sensing and actuating”, technique which crystallizes powder dynamically by the simultaneous operation of “compression and shearing force”.