Yoshinobu Shimamura

Measurement of orthotropic electric conductance of CFRP laminates and analysis of the effect on delamination monitoring with an electric resistance change method

Since delaminations of composite laminates are usually invisible or difficult to detect by visual inspections, delamination causes low reliability for primary structures. Automatic systems for delamination identifications in-service are desired in order to improve this low reliability. The present study employs an electric resistance change method for detection of delaminations. Since the method adopts reinforcement carbon fibre itself as sensors for delamination detections, this method does not cause reduction of static strength or fatigue strength; also, this method is applicable to existing structures. In the present study, a relationship between fibre volume fraction and orthotropic electric conductivities is confirmed by experimentation and the effect of measured orthotropic electric conductance on delamination monitoring is discussed analytically with FEM analyses. Two types of cross-ply laminates are prepared for delamination monitoring analyses: [0/90]s and [90/0]s. Electric resistance changes due to delamination creation are discussed for both specimen types with results of electric current density diagrams. As a result, it can be concluded that the fibre volume fraction has a large effect on electric conductance of the transverse and thickness directions, and electric conductance of the thickness direction has significant effects on delamination detection with the electric resistance change method.

Delamination monitoring of graphite/epoxy laminated composite plate of electric resistance change method

The present paper employs the electric resistance change method for monitoring of location and size of a delamination crack of graphite/epoxy composite laminates. The method is applied to a plate-type specimen with an embedded delamination of cross-ply and quasi-isotropic laminates. Ten electrodes made from copper foil are mounted on the specimen top surfaces. An embedded delamination crack is created by a static indentation test, and the electric resistance changes are measured using a conventional strain gage amplifier. Response surfaces are adopted as a tool for solving inverse problems to estimate location and size of delamination crack from the measured electric resistance changes of all segments between electrodes. As a result, the present method successfully provides estimations of location and size of the embedded delamination for graphite/epoxy laminated composites.

High performance estimations of delamination of graphite/epoxy laminates with electric resistance change method

Delamination of laminated composites is usually invisible or difficult to detect visually. Delamination causes low reliability for primary structures. Automatic systems for in-service delamination identifications are desired to improve low reliability. The present study employs an electric resistance change method for delamination detection. Since the method adopts reinforcement carbon fiber itself as sensors for delamination detection, this method does not reduce static or fatigue strength; also, the method is applicable to existing structures. Authors have found that the electric resistance change method with response surfaces is very effective experimentally and analytically. However, a large error of estimation remains for estimation of delamination location. In the present study, a new data processing procedure is proposed to improve performance of estimations of delamination location. The new method is applied to laminated composite beams. A delamination crack of a laminated composite beam is monitored with the new method using FEM analyses. As a result, the method reveals excellent performance of estimations of delamination location even for new data not used in regression equations.

Wireless strain monitoring using electrical capacitance change of tire: part I—with oscillating circuit

Strain monitoring of tires of automobiles in service is quite effective to improve the reliability of tires and design tools. Since conventional strain gages have high stiffness and require lead wires, the conventional strain gages are cumbersome for the strain measurement of tires. Sensors of micro-electro-mechanical systems are also usually of high stiffness themselves, and those are not applied to tires. The background requires a new low cost wireless sensor for tires. In the present study, a new strain measurement system utilizing the electric capacitance change of steel wire reinforced tires is proposed and experimentally investigated. A small oscillating circuit is embedded in the tire; deformation of the tire induces a capacitance change of the tire comprising steel wire and rubber; the change of the capacitance makes a change in oscillating frequency of the oscillating circuit. Measurement of the frequency of the oscillating circuit enables us to measure the strain of the tire wirelessly. A rectangular specimen cut from a commercially available tire is adopted as a specimen. A tension test is performed and the frequency of the oscillating circuit is measured during the test. As a result, the method is experimentally proved to be effective for the wireless strain monitoring of tires.

Effects with a matrix crack on monitoring by electrical resistance method

Delamination engenders low reliability for primary structures because delaminations of composite laminates are usually invisible or difficult to detect by visual inspections. Automatic systems for in-service delamination identifications are desired to improve this low reliability. The present study employs an electric resistance change method for detection of delaminations. Although the method is effective in monitoring delamination cracks, it requires many experiments to solve inverse problems. An analytical method for preparation of data sets of the electric resistance changes is desired because the experimental cost is high. However, actual delamination cracks usually include matrix cracking. Therefore, this study uses FEM analyses to investigate the effect of matrix cracking on electric resistance changes between electrodes. Results show that simple calculations using a straight delamination crack model are sufficient to obtain the data set of electric resistance changes to calculate response surfaces.

Wireless strain monitoring using electrical capacitance change of tire: part II—passive

In-service strain monitoring of tires of automobiles is quite effective for improving the reliability of tires and design tools. In a previous study, the authors proposed a new wireless strain monitoring method that adopts the tire itself as a sensor, with an oscillator circuit. In the method, steel wires are employed as electrodes to measure electrical capacitance changes of tires. A specimen cut from a commercial tire is connected to an oscillator circuit, and the oscillation frequency changes with the capacitance changes of the tire. This method is very simple and useful, but it requires a battery to activate the oscillator circuit. In the present study, a new passive strain measurement system utilizing electrical capacitance changes of steel-wire-reinforced tires is proposed and experimentally investigated. The passive wireless strain monitoring method makes use of the specimen cut from the tire as a condenser of a passive filter circuit. Deformation of the tire causes capacitance changes of the tire comprised of steel wire and rubber; the change of the capacitance causes a change of the filtering frequency of a radio wave. Measurement of the frequency of a radio wave passed through the filter circuit enables us to measure the strain of the tire wirelessly. A rectangular specimen cut from a commercially available tire is adopted as a specimen. Tension testing is performed and the change of the filtering frequency is measured during the test. As a result, the method is experimentally proved to be effective for the passive wireless strain monitoring of tires.

Monitoring delamination of laminated CFRP using the electric potential change method (two-stage monitoring for robust estimation)

Detecting delaminations of carbon fiber reinforced plastic (CFRP) laminates is a difficult task for visual inspection. Delaminations cause large reductions in strength and stiffness of CFRP laminates, bringing deterioration of the structural reliability of a CFRP. Monitoring for delamination is, therefore, indispensable to maintain the reliability of a CFRP structure. In a previous study, we adopted the electric potential change method to detect delamination. This method shows good estimation performance for delamination cracks located near the edges of a specimen, but poor performance near the center where large errors that depend on the delamination shapes are created. A zigzag delamination caused by matrix cracking has a large effect on estimation performance; so the electric potential change method was not applicable to monitoring for delamination. In this paper, a mechanism that brings large errors of estimation due to the shape of the delamination is detailed. FEM analyses show a small electric current in the thickness direction in the center segment of a specimen causes large effects on the estimation performance. The problem is overcome by means of a newly proposed concept, a two-stage method. The effectiveness of the method is demonstrated using FEM analyses.

Impact behavior and energy transfer efficiency of pulse-driven bent-beam electrothermal actuators

This paper investigates the dynamics of bent-beam electrothermal actuators and their use in impact actuation of other micromechanical elements, and in particular the issue of energy efficiency achieved by temporal variations in electrical drive signals. A transient thermal model of an actuator beam shows that the uniformity of temperature profile is greater when activating with short electrical pulses, which results in larger achievable displacements and forces. A dynamic force analysis reveals that using a train of pulses, referred to as a burst pulse, for activation achieves significant impact forces due to high velocities at the point of impact. The analytical trends are confirmed through experimental observations of microfabricated metal test structures in which actuators work against bistable mechanisms. Measurements of 2 mm and 3mm long actuators show that pulsed activation results in >5/spl times/ reduction in energy consumption, with the activation energy falling from over 1000 /spl mu/J at dc activation, to less than 200 /spl mu/J using a 0.2-ms voltage pulse. The actuators however consume higher instantaneous power levels at shorter pulses, which may inhibit the use of pulses less than 1 ms in width. Further, the energy consumption through burst activation is 70% that of a single pulse, if sufficient impact forces are generated.

Passive wireless strain monitoring of a tire using capacitance and electromagnetic induction change

Strain monitoring of tires in-service of automobiles is quite effective for improving reliability of tires and an anti-lock braking system (ABS). A previous study by the authors presented a new wireless strain monitoring method that adopts the tire itself as a sensor with an oscillator circuit. This method is simple and useful, but it requires a battery to activate the oscillator circuit. The present study proposes and investigates a new passive wireless strain measurement system using capacitance change of tires. The system consists of external antennas and strain sensor with an inductance capacitance (LC) resonant circuit. This wireless system uses electromagnetic coupling between two inductors of the antenna and the sensor. This method allows use of a part of an actual tire as a capacitor of the LC circuit. Tire deformation changes the sensors resonant frequency. This resonant frequency change is measured as a change in the phase angle of the antenna using electromagnetic induction. Tensile tests are performed and the antenna phase angles are measured during the tests. Consequently, experiments show that this method is effective for passive wireless strain monitoring of tires.

Fatigue Behavior of Unidirectional Jute Spun Yarn Reinforced PLA

Abstract Natural fiber reinforced composites, which can be carbon-neutral materials, have been investigated for use as alternative materials to glass-fiber reinforced plastics (GFRP). The fatigue properties of natural fiber reinforced plastics are, however, not well known. In this study, uniaxial tensile fatigue tests of unidirectional jute spun yarn reinforced polylactic acid (PLA) were conducted in order to clarify the fatigue strength. The damage and fracture morphology of composite specimens were observed to elucidate the fatigue mechanism. Results show that the fatigue strength decreases concomitantly with increasing number of cycles. The fatigue strength at 106 cycles was 55% of the ultimate strength, which is an almost identical percentage to that of GFRP. The fatigue failure of composite specimens was probably caused by the breakage of jute filaments at the tips of fatigue cracks in PLA. This implies that the fatigue strength of the composite was strongly affected by the fatigue properties of PLA.