In this study, the influence of the working distance (WD) on strain measurement under a laser scanning microscope and a way to achieve precise focus were investigated by the scanning moiré method. Experimental results showed that the strain measurement has a good repeatability at a fixed WD. Scanning moiré fringes were clearly observable when the WD variation range was within 0.9% of the given WD of the used objective lens. The relationship of the measured strain error and the WD difference was approximately linear, and the greatest strain error was near 700 με. Fortunately, 2D moiré fringes were distinct only in a very narrow range, i.e., the WD difference was less than 0.1% of the given WD, and the greatest strain error was less than 100 με. 1D moiré fringes in the y direction, 2D moiré fringes in the both x and y directions, and 1D moiré fringes in the x direction became distinct alternately along with the WD change. Consequently, we suggest to use 2D moiré fringes for microscale strain measurement in each focusing process to reduce the errors caused by the WD variation. Moreover, a single-shot 2D moiré image is useful to measure the strain distributions in both two directions quickly and simply, and there is no need to rotate the sample or scanning lines and scan twice as in the conventional way.
Phase analysis techniques of fringe patterns have been widely used for noncontact three-dimensional shape and deformation measurement by the fringe projection method. Recently, we developed two novel accurate phase analysis methods. One is the two-dimensional sampling moiré method to perform robust phase analysis for a single-shot fringe pattern. The other is the two-dimensional spatiotemporal phase-shifting method to analyze phase distribution accurately for multi-step phase-shifted fringe patterns. To perform accurate phase analysis under low signal-to-noise ratio conditions, both the above two methods use the two-dimensional discrete Fourier transform or fast Fourier transform. Therefore, these algorithms are computationally expensive compared with the conventional one-dimensional sampling moiré and phase-shifting methods. In this study, a fast parallelization implementation for two-dimensional phase-shifting methods, including the two-dimensional sampling moiré method and the spatiotemporal phase-shifting method, are presented by utilizing multi-core CPU. Simulation and experimental results demonstrate that phase analysis can reach 7.5 and 5.9 times faster by use of a 12-core CPU compared with a single CPU.
The deformation distributions of carbon fiber reinforced plastics (CFRP) under a three-point bending load were nondestructively investigated using the phase shifting scanning electron microscope (SEM) moiré method. The complex fast Fourier transform (FFT) and the discrete Fourier transform (DFT) were used to filter the useless moiré fringes in the case of bidirectional moiré fringes. The SEM moiré fringes under different magnifications and the deformation results measured by the direct, complex FFT- and the DFT- phase shifting moiré methods as well as the moiré fringe centering method were compared and analyzed. Experiments demonstrate that the deformation measurement is a bit influenced by the useless moiré fringes in the phase shifting moiré methods and complex FFT processing works better for nondense moiré fringes. The relative strain changes gradually and the specimen grating pitch increases gradually from top to bottom along the loading direction, suggesting that the real compressive strain is greater in the upper side. The micro/nano-scale deformation distribution characteristic is helpful for better understanding of the mechanical properties of the CFRP specimen.
Optical methods providing full-field deformation data have potentially enormous interest for mechanical engineers. In this study, an in-plane and out-of-plane displacement measurement method based on a dual-camera imaging system is proposed. The in-plane and out-of-plane displacements are determined simultaneously using two measured in-plane displacement data observed from two digital cameras at different view angles. The fundamental measurement principle and experimental results of accuracy confirmation are presented. In addition, we applied this method to the displacement measurement in a static loading and bending test of a solid rocket motor case (CFRP material; 2.2 m diameter and 2.3 m long) for an up-to-date Epsilon rocket developed by JAXA. The effectiveness and measurement accuracy is confirmed by comparing with conventional displacement sensor. This method could be useful to diagnose the reliability of large-scale space structures in the rocket development.
Phase analysis plays a role in optical science and technology. For instance, phase analysis technique has been widely used for 3-D shape and deformation measurement by fringe projection profilometry. To analyze the phase distribution of a single fringe pattern, various fringe pattern analysis methods such as a Fourier transform, a wavelet transform, and the windowed Fourier transform have been developed. In this study, a fast phase analysis technique, i.e., two-dimensional sampling moiré method, is proposed to determine accurately the phase distribution of a single fringe pattern by using two-dimensional intensity information. In this method, we record diagonally a single fringe pattern image by using a CCD camera, and perform the image processing of down-sampling with a sampling pitch and intensity interpolation in both x- and y-directions to generate a two-dimensional phase-shifted moiré fringe. Then, the phase distribution of the moiré fringe can be determined by using phase-shifting method and a two-dimensional discrete Fourier transform (DFT) algorithm. Finally, the desired phase distribution of the original fringe pattern can be obtained by adding the phase of the sampling point to the phase of the moiré pattern. By the proposed method, the phase error caused by the random noise of the camera can be dramatically decreased because the intensity information is much richer than one-dimensional intensity data, which utilizes a two-dimensional DFT algorithm. The fundamental principle and primary simulation and experimental results are presented. Theses results show that phase analysis can be performed under extremely low signal-to-noise ratio measurement condition.
Imaging based nondestructive monitoring systems are critical for evaluation of large-scale infrastructures. In this study, an accurate and fast in-plane displacement measurement method based imaging technique is developed for the purpose of health monitoring of large-scale infrastructures such as high building, long bridge, etc. The build-in repeated patterns on infrastructure facade, such as tile, checker, and brick wall pattern is used to measure the in-plane displacement distribution accurately. By performing down-sampling and intensity interpolation image processing to the images captured before and after deformations, multiple phase-shifted moiré fringe can be obtained simultaneously. The phase distribution of the moiré fringe is calculated using the phase shifting method and discrete Fourier transform technique. In the present study, both the fundamental and high frequency components are considered to analyze the repeated patterns. The in-plane displacement distribution can be obtained from the phase differences of the moiré fringe before and after deformations. Compared with conventional displacement methods and sensors, the main advantages of the method developed herein are high-resolution, accurate, fast, low-cost, and easy to implement. The principle of the proposed inplane displacement measurement is presented. The effectiveness of our method is confirmed by a simple displacement measurement experiment. Experimental result showed that a sub-millimeter displacement could be successfully detected for the field of view with meter-scale.
A fiber Bragg grating (FBG) ultrasound detection system incorporating a fiber ring laser was developed. In the
system, an FBG was used not only as a sensor but also as the ring cavity mirror. The fiber ring laser emitted
lasing light at the Bragg wavelength of the FBG sensor and the intensity of the lasing light varied with
ultrasonic vibration applied to the FBG sensor. The system proved to detect ultrasonic response consistent with
the characteristics of the ultrasound impinging on the FBG and to have ultrasound sensitivity enough to detect
acoustic emission. This system is quite simple configuration and small in size. Moreover, this system can work
irrespective of the Bragg wavelength of the FBG sensor.
We have developed a generation pulsed-laser scanning method for visualizing the propagation of ultrasonic waves.
While scanning a target object with a pulsed-laser beam to generate thermal-exited ultrasonic waves, we detected the
propagated waves with a fixed PZT transducer. Although the detected waves were generated from different irradiation
points, we were able to produce moving images of the ultrasound generated at the reception-transducer position by
reconstructing the measured waveform data. This method has the following features that make it superior to the
conventional visualization methods such as photo-elasticity method, reception probe scanning method and computer
simulation. (1) it enables us to visualize ultrasonic waves propagating on a complex-shaped object with curved surfaces,
steps, and dents. (2) it provides excellent working efficiency by eliminating the need for adjustments to the laser
incidence angle and the focal distance. For these reasons, we believe that this new method can be effectively applied to
the inspection of defects in the field. In this study, we examined the applicability of this method to CFRP materials, and
the results demonstrate the validity of this method for nondestructive flaw inspection in CFRP-structures.
We introduce a simultaneous multipoint acousto-ultrasonic (AU) sensing system using a tunable laser and fiber wave
Bragg grating (FWBG) sensors. Although the demodulation technique is same as the existing method for a fiber Bragg
grating (FBG), the sensor head is changed to the FWBG sensor for which the FBG is installed in a strain-free
configuration and detects the AU wave not directly but in the form of a fiber-guided wave. Therefore since the strain
cannot make the FBG spectrum move, multiple FBGs with an identical spectrum can be connected with multiple optical
paths realized by equal laser intensity dividers. Temperature difference among the multiple FWBG sensors is passively
resolved by using a short grating, which provides a wider temperature-operating region. Consequently, we can solve the
problem that the FBG spectrum is easily deviated from the lasing wavelength owing to the strain. Also, the simultaneous
multipoint sensing capability based on the single laser improves cost-performance ratio, reduces inspection time, and
enables in-situ monitoring of a real structure exposed to large and dynamic strain. The system feasibility is demonstrated
in the health monitoring examples like acoustic source localization and ultrasonic waves detection burst by a
piezoelectric transducer and a pulsed laser.
Impact damage in CFRP was monitored by ultrasonic inspection method using small-diameter fiber Bragg grating (FBG) sensors. The FBG ultrasound detection system consisted of broadband light source, FBG sensor and tunable optical filter. Broadband light was launched into the FBG sensor. Light reflected from the FBG sensor was transmitted through the tunable optical filter whose transmissive wavelength range is comparable to the reflected wavelength range of the FBG sensor. The operating wavelength of tunable filter was set to optimize the sensitivity of ultrasound detection. Ultrasound vibration was converted into change in intensity of light transmitted through the filter. A cross-ply carbon fiber-reinforced plastic (CFRP) plate was used as a test specimen for impact damage monitoring. A 6.3 X 9mm2 impact damage was introduced by ball dropping. Both FBG ultrasound sensor and piezoelectric ultrasound transmitter were attached on the CFRP surface. The change in responses to ultrasound excited by either spike signal or toneburst signal before and after impact damage was investigated. In response to ultrasound excited by spike signal, the response after impact damage showed a scattered behavior where the period of response signal got longer. In response to ultrasound excited by toneburst signal, damage signal features scattered and distorted waveform. Experimental results proved that the FBG inspection system could monitor a 6.3 X 9mm2 impact damage in CFRP.
An ultrasonic sensing system in which a fiber Bragg grating was used as an ultrasonic detector was constructed. The system utilizes wavelength-optical intensity modulation technique and includes a broadband light source, fiber Bragg gratings for sensing and filtering, as well as a photodetector. Feasibility of active sensing diagnosis by the system was examined. Ultrasonic Lamb wave generated by a piezoelectric device was propagated in a cross-ply CFRP with visible impact damage. Response of fiber Bragg grating sensor to Lamb wave propagated through damage area was compared with the reference response in intact area. In the active sensing diagnosis, the ultrasonic transmitter was excited by three kinds of signal to determine the optimal transmitter drive signal for impact damage detection. Compared with response in intact area, response to Lamb wave propagated through damage area demonstrated distinctive features. A spike signal proved to be optimal transmitter drive signal for impact damage detection of CFRP.
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