Digital shearography (DS) is a noncontact, whole-field, and laser-based optical interferometric method. It is a valuable tool in the field of nondestructive testing (NDT) and received considerable acceptance in the automotive and aerospace industries. Conventional DS only detects the flaws in one or two shearing/sensitive directions, which leads to the missed inspection and leaves quite a lot of suspected defects underdefined. A trishearing DS for NDT is proposed to solve the mentioned issue. The presented shearography is designed based on a multiplexed Mach–Zehnder interferometer embedded with the spatial phase shift technique. It measures the defects in three sensitive directions simultaneously and has balanced optical paths with high light efficiency. Moreover, the measurement sensitivity of each direction can be adjusted during the test, without affecting the carrier frequencies, so it meets various needs for NDT applications. The principle and the experimental validation of the proposed system are introduced in detail.
This paper presents the use of the high-speed digital image correlation (DIC) method for measurement of the head injury of a pedestrian in a vehicle–pedestrian collision experiment. Two simultaneous high-speed cameras captured the images of the impact process between the head-simulator and the car hood. The DIC method can achieve comprehensive analysis of the impact and help to reconstruct the impact process, from which not only the velocity, acceleration of the headform impactor, and the head injury criterion (HIC) but also the angular velocity and acceleration can be calculated. The possibility of using DIC to calculate of the neck injury criterion (NIC) is also discussed in this paper. Furthermore, the quantitative calculation of the full-field deformation of the car hood during the collision can also be achieved with the DIC method, which is of great practical value in optimizing automobile design and collision protection. A vehicle–pedestrian collision experiment is carried out, and the comprehensive results show that the DIC method can obtain accurate and comprehensive data in the calculation of human and vehicle collision.
It is difficult to measure absolute three-dimensional deformation using traditional digital speckle pattern interferometry (DSPI) when the boundary condition of an object being tested is not exactly given. In practical applications, the boundary condition cannot always be specifically provided, limiting the use of DSPI in real-world applications. To tackle this problem, a DSPI system that is integrated by the spatial carrier method and a color camera has been established. Four phase maps are obtained simultaneously by spatial carrier color-digital speckle pattern interferometry using four speckle interferometers with different illumination directions. One out-of-plane and two in-plane absolute deformations can be acquired simultaneously without knowing the boundary conditions using the absolute deformation extraction algorithm based on four phase maps. Finally, the system is proved by experimental results through measurement of the deformation of a flat aluminum plate with a groove.
We present a spatial phase-shift digital speckle patterns interferometry (SPS-DSPI) system with the capability of measuring three-dimensional (3-D) deformations under a dynamic loading condition simultaneously using multiple cameras. The Fourier transform method is utilized to calculate the phase difference. The consistency of different cameras is achieved using digital image correlation (DIC) technology. Calibration and calculation programs are compiled to make sure each subset on the measuring surface is uniform. SPS-DSPI and DIC techniques are combined to provide a direct measurement of the 3-D deformation of the entire surface area simultaneously. The theory, application result, and validation experiment are presented.
This investigation applied the digital image correlation technique (DIC) on a pressure vessel that contained several surface pit defects under high pressure. Data on the deformation of the defects and peripheral area is obtained by this method. The results show that the stress and strain increase with the depth among different pits and are the largest at the bottom of any given pit. This method has proven to be a good choice for this type of experiment, where elastic and plastic surface strains need to be measured. The DIC can satisfy the requirements of being in situ, in real time, full-field and make non-contact measurements with more accurate and obvious experimental results compared with traditional measurement methods and pressure vessel test regulations. Also, it is a new, effective way for monitoring defects in online pressure vessels as well as a reliable basis for pressure vessels’ safety evaluation.
Digital Speckle Pattern Interferometry (DSPI) is an optical method for measuring small displacement and deformation. It allows whole field, non-contacting measurement of micro deformation. Traditional Temporal phase shifting has been used for quantitative analyses in DSPI. The technique requires the recording of at least three phase-shifted interferograms, which must be taken sequentially. This can lead to disturbances by thermal and mechanical fluctuations during the required recording time. In addition, fast object deformations cannot be detected. In this paper a DSPI system using Spatial Carrier Phase Shifting (SCPS) technique is introduced, which is useful for extracting quantitative displacement data from the system with only two interferograms. The sensitive direction of this system refers to the illumination direction and observation direction. The frequencies of the spatial carrier relates to the angle between reference light and observation direction. Fourier transform is adopted in the digital evaluation to filter out the frequencies links to the deformation of testing object. The phase is obtained from the complex matrix formed by inverse Fourier transform, and the phase difference and deformation are calculated subsequently. Comparing with conventional temporal phase shifting, the technique can achieve measuring the vibration and transient deformation of testing object. Experiment set-ups and results are presented in this paper, and the experiment results have shown the effectiveness and advantages of the SCPS technique.
The phase fringe patterns obtained by the phase shifting technique are inherently full of speckle noise in Digital Speckle Pattern Interferometry (DSPI),. The phase fringe patterns filtering method is very important to obtain the precisely deformation information. In this paper a effective method for filtering the speckle noise of phase fringe patterns is applied, the method filter the discrete images which generated from phase fringe patterns instead of classical filtering phase fringe patterns directly. Comparing with existed filters method, it has a better performance on phase jump information preservation and does not have any blurring effect on the phase distribution providing the filtering is implemented on the equal-phase window. Moreover, its capability of noise reduction is more powerful. An optical measurement system based on DSPI is built to demonstrate the filtering method, and the experiment results have shown the effectiveness and advantages of the filter method.
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