Two-wave-plate compensator method for single-point retardation measurements

The two-wave-plate compensator (TWC) technique is introduced for single-point retardation measurements. The TWC method uses a known wave plate together with a wave plate of unknown retardation and produces a linearly polarized output that allows a null of intensity to be detected. The TWC method is compared both theoretically and experimentally with the existing Brace-K?hler and Sénarmont methods. The resolution of the TWC is shown to be 0.02 nm. TWC enables the measurement of a sample retardation with as little as 0.13% error and thus is more accurate than either the Brace-Kohler or the Sénarmont method.     

Analytical phase-measuring technique for retardation measurements

An analytical method for phase detection in retardation measurements is proposed. The experimental setup is based on a simple linear polariscope with a λ plate. The intensity modulation at the output of the polariscope is measured when the wavelength is changed and a grid of phase-shifted intensity values is recorded. The phase difference between the components of the light propagating along the principal axes of the birefringent sample is determined with Greivenkamp's algorithm employed in phase-stepping interferometry. Error analysis for the new method is performed. Simplified algorithms for faster data analysis are proposed. The accuracy attained is comparable with the accuracy of known phase-detection methods used in retardation measurements.     

Parameters identification of polymer concrete using a fracture mechanics test method and full-field measurements

In the present work it is proposed an easy-to-implement alternative procedure to identify not only the fracture properties but also the mean elasticity modulus E and Poisson’s ratio ν of arbitrary polymer concretes. Only one test using a standard single-edge-cracked three-point bend specimen is necessary. Digital image correlation (DIC) method is used to obtain the full-field displacement close to the crack tip. The mean properties (E, ν) are determined by fitting analytical expressions for the displacement field to the experimental data. The adequacy of the proposed methodology is checked by comparing the material parameters obtained using the proposed procedure with the ones obtained through standard procedures.     

Neural network modelling for damage behaviour of composites using full-field strain measurements

This paper presents a neural network modelling method for damage behaviour of composite materials in conjunction with full-field strain measurements. The proposed method utilises the overall structural response of a laminate composite specimen to develop the constitutive model of a single ply unidirectional laminate. Based on an energy principle, a performance function for training the neural networks is derived in terms of the applied external work and the induced strain energy. This allows the proposed method to develop the neural networks without the presence of stress information that is not necessarily obtainable in experiments with non-uniform deformation. The use of neural networks also enables the proposed method to model the damage behaviour without the constraints on the parameter space, such that a more representative model is developed for the actual material behaviour. An example of tailoring the proposed method to model the in-plane shear damage behaviour of a carbon fibre reinforced plastic (CFRP) laminate is demonstrated as well as its numerical validation. The practical application of the proposed method to multi-axial damage-related nonlinear behaviour of composite is presented using the experimental data obtained from a tensile test with an open-hole specimen.     

Full-field heterodyne polariscope with an image signal processing method for principal axis and phase retardation measurements

We develop a heterodyne polariscope for measuring the two-dimensional principal axis and phase retardation in a linear birefringence material using novel three-frame and two-frame integrating-bucket methods and a CCD. By using a complex programmable logic device to provide an external trigger to the CCD, integrating buckets with multiple frames are achieved. The advantages of the proposed three-frame and two-frame integrating-bucket methods include a simpler signal processing algorithm based on fewer frames and the elimination of flyback error caused by a sawtooth modulation signal at higher frequencies.     

Local stiffness reduction in impacted composite plates from full-field measurements

The virtual fields method (VFM) is used as an inverse procedure to process curvatures obtained from full-field measurements for the identification of the local loss of stiffness in composite plates. The VFM has been adapted to the case of a plate with stiffnesses varying spatially. Two parameterization methods for the stiffness reduction were developed: continuous (polynomial) and discrete (piecewise). In this study, the procedure is described, validated on simulated data and several experimental results are given. The method not only picks up the location of the damage but also provides a fairly good estimate of the stiffness reduction in the damaged area when the through-thickness displacement distribution in the damaged region is reasonably linear.     

Application of 3D image correlation for full-field transient plate deformation measurements during blast loading

A high-speed stereo-vision system is employed to quantify dynamic material response during buried blast loading. Deformation measurements obtained using 3D image correlation of synchronized, patterned stereo-vision images obtained with an inter-frame time in the range 16 μs ≤ t ≤ 40 μs indicate that (a) buried blast loading initially induces highly localized material response directly under the buried blast location, with severity of the blast event a strong function of depth of explosive burial, (b) for relatively shallow (deep) depth of explosive burial, plate surface velocities and accelerations exceed 220 m s⁻¹ (100 m s⁻¹) and 6 × 10⁶ m s⁻² (1.5 × 10⁶ m s⁻¹) during the first 30 μs (80 μs) after detonation, respectively.     

Image restoration method based on Hilbert transform for full-field optical coherence tomography

A full-field optical coherence tomography (FF-OCT) system utilizing a simple but novel image restoration method suitable for a high-speed system is demonstrated. An en-face image is retrieved from only two phase-shifted interference fringe images through using the mathematical Hilbert transform. With a thermal light source, a high-resolution FF-OCT system having axial and transverse resolutions of 1 and 2.2 microm, respectively, was implemented. The feasibility of the proposed scheme is confirmed by presenting the obtained en-face images of biological samples such as a piece of garlic and a gold beetle. The proposed method is robust to the error in the amount of the phase shift and does not leave residual fringes. The use of just two interference images and the strong immunity to phase errors provide great advantages in the imaging speed and the system design flexibility of a high-speed high-resolution FF-OCT system.     

Improved Newton-Raphson digital image correlation method for full-field displacement and strain calculation

The two-dimensional in-plane displacement and strain calculation problem through digital image processing methods has been studied extensively in the past three decades. Out of the various algorithms developed, the Newton-Raphson partial differential correction method performs the best quality wise and is the most widely used in practical applications despite its higher computational cost. The work presented in this paper improves the original algorithm by including adaptive spatial regularization in the minimization process used to obtain the motion data. Results indicate improvements in the strain accuracy for both small and large strains. The improvements become even more significant when employing small displacement and strain window sizes, making the new method highly suitable for situations where the underlying strain data presents both slow and fast spatial variations or contains highly localized discontinuities.     

Simple design method for third-order dispersion compensation with a thin-film dispersion compensator

A novel and simple numerical method for finding the best thin-film structures for third-order dispersion compensation has been achieved. A target third-order dispersion value is specified first; then the multilayer thin-film structure is optimized to have a second-order dispersion spectrum, which has the least deviation from linearity and has a slope that equals the specified third-order dispersion value. Numerical examples are presented for reflection-type and transmission-type thin-film compensators. Both types can achieve required phase compensation, but the reflection type has a flatter amplitude response than the transmission type.     

基于DIC的斯特林制冷机振动非接触三维全场测试方法

为快速准确测试斯特林制冷机在工作时的振动状态,建立一种基于数字图像相关(DIC)的非接触三维全场测试新方法.该方法通过对制冷机表面的自然或人工标识进行图像相关分析,获得制冷机表面的外形(或标识)位移和应变的非接触三维全场测量,对测量的三维位移场进行傅里叶分析,进一步计算制冷机表面(或标识)的三维全场振动,实现制冷机振动振幅与频率的高精度无接触测量.通过斯特林制冷机的三维振动测试结果表明:该方法不仅能作为斯特林制冷机振动输出测量的新方法,还能有效应用于其他运动物体的振动输出测量.     

Unidirectional phase-Doppler method for particle-size measurements

We propose what we believe to be a novel unidirectional phase-Doppler method for sizing moving spherical particles on the basis of the phase difference of two polarized Doppler beat signals in a single scattering direction. Light scattered by a moving particle is divided into two rays, which are detected with different polarization angles to transmit dominantly reflected or refracted rays. The phase difference between two signals is linearly proportional to the particle diameter with a given particle refractive index and geometrical parameters of the optical system. To explore the optimum polarization condition, we numerically investigated the phase-diameter properties in relation to polarization angles by using the geometrical optics approximation and generalized Lorenz-Mie theory. We also performed experiments with polystyrene and glass particles to verify the usefulness of the proposed method.     

Experimental method for high-accuracy reflectivity-spectrum measurements

An experimental method for accurate measurements of the reflectivity spectrum of mirrors is presented. It combines the noise reduction obtained with multiple beam reflections on two identical mirrors; high-beam quality, owing to the use of single-mode optical fibers; and high immunity against intensity variations of the beam. This method is demonstrated for characterizing a 30-period GaAs/Al(0.65)Ga(0.35)As distributed Bragg reflector designed for long-wavelength vertical-cavity surface-emitting lasers. Its peak reflectivity is found to be 99.43 ? 0.04% at 1.562 mum, and an optical absorption coefficient of alpha = 36 ? 6 cm(-1) is derived. The peak internal reflectivity of this distributed Bragg reflector used as the top mirror in a wafer-fused vertical-cavity surface-emitting laser is calculated to be 98.87 ? 0.12%, and the transmission is 0.28%.     

Calibration method for multiangle lidar measurements

A new method based on a two-angle approach is developed to determine the lidar solution constant from scanning elastic lidar data, hence providing a relative calibration for each lidar scan. Once the solution constant is determined, the vertical profiles of atmospheric extinction can be calculated. With this calibration method a minimization technique is used that replaces the linear regression used in a known two-angle approach that requires only local atmospheric homogeneity over a restricted altitude calibration range rather than overall horizontal homogeneity. Lidar signals from at least one pair of elevation angles are used, averaged in time when the system is operated in a permanent two-angle mode, or an arbitrary number of signal pairs is used, when a two-dimensional lidar scan is being processed. The method is tested extensively with synthetic data. The calibration method is a robust tool for determining the solution constant to the lidar equation and for obtaining vertical profiles of atmospheric extinction.     

Photoelastic investigation of mixed-mode fracture parameters with full-field selected line method

The mixed-mode SIF (stress intensity factor) and other fracture parameters, such as additional non-singular terms in the algorithm, for bending specimens with biased straight cracks and 60° inclined edge cracks have been measured using a full-field selected line method, which may simplify calculations and improve data analysis. The modified Westergaard expressions derived from only stress symmetry have been provided to increase the accuracy of measurements and amplify the data analysis region.