On the Industrial Applications of Moiré and Fringe Projection Techniques

Abstract: This paper describes a set of tools and procedures based on the moiré and fringe projection techniques, which were developed to suit the requirements of real industrial situations. Particular attention was paid to flexibility, robustness and reliability, ease of use and calibration, and cost efficiency. Three case studies are being discussed in the paper. The first case describes a prototype shadow moiré instrument, which was developed for the detection of surface defects such as barely visible impact damage or corrosion inside lap-joints, during maintenance inspections of aeronautical structures. The second case presents a portable instrument based on the fringe projection technique for the automatic, full-field measurement of the shape and deformation of an arbitrary object. The system has applications ranging from deformation measurement of bioprostheses to correction of shape effects in the photoelastic analysis of complex aerospace components. Finally, a portable instrument was developed for the measurement of the location, orientation, shape and deformation of an arbitrary object, by combining the fringe projection technique with the laser spot technique for distance measurement. The system was used to assist in the integration of non-destructive evaluation techniques throughout the life cycle of an aircraft.     

Applicability of Optical Geometric Differentiation for Time-average Geometric Moiré

Abstract:  Optical experimental method for analysis of derivatives of dynamic displacements from patterns of time-average moiré fringes is proposed in this paper. Derived analytical relationships for one-dimensional optical model are validated for two-dimensional elastic structures performing in-plane vibrations. The fact that digital image filtering techniques are normally required for the identification of time-averaged super-fringes does not lessen the practical value of the proposed method.     

Thermal deformation measurement of the solder joints in electronic packages using electron moiré method

In this paper, electron moiré method is used to measure the thermal deformation of electronic packages. In order to observe the electron moiré fringe, a holographic grid was replicated on the cross section of the BGA type package at 150°C. This grid was fabricated on a glass plate using a moving point holographic system, and was replicated to measured area at high temperature. Under SEM, the holographic grid(specimen grid) and a programmed electron beam scan(master grid) interfere and form electron moiré patterns. The shear strain measurement technique using electron moiré method is described. Using the electron moiré method, the shear strains in the different solder joints were measured and analysed. Some useful results were obtained.     

Deformation Data from Thermal Marking

Abstract: A novel experimental technique for strain measurement, named thermal moiré, and based on a combination of thermography and moiré concepts, is presented in this paper. The proposed technique essentially involves the temporary imprinting of a thermal pattern onto the surface of the object of interest as a substitute for conventional gratings, to remove the need for physical printing, etching or bonding. In materials with low diffusivity, the thermal pattern can be observed with an infrared camera and analysed using conventional techniques. Work on the development of suitable laboratory apparatus is described, and preliminary experimental results are presented to illustrate the feasibility of the method.     

The measurement of deformation of a bridge by moiré photography and photogrammetry

Optical techniques offer advantages over mechanical methods for displacement measurement. A comparative study has been made between photogrammetry and moiré photography during a destructive test applied to a bridge. The first provided accurate values for point displacements whereas the second revealed the whole field movements and indicated the region of eventual failure.     

Thermal Properties Measurement of Micro-electromechanical System Sensors by Digital Moire Method

Abstract:  The thermal properties of a micro-electromechanical system sensor were analysed by a novel digital moiré method. A double-layer micro-cantilever sensor (60  μ m long, 10  μ m width and 2  μ m thick) was prepared by focused ion beam milling. A grating with frequency of 5000 lines mm⁻¹ was etched on the cantilever. The sensor was placed into a scanning electron microscope system with a high temperature device. The observation and recording of the thermal deformation of the grating were realised in real-time as the temperature rose from room temperature to 300 °C at intervals of 50 °C. Digital moiré was generated by interference of the deformed grating and a digital virtual grating. The thermal properties including strain distribution of the sensor and the linear expansion coefficient of polysilicon were accurately measured by the phase-shifted moiré patterns.     

A single exposure reflection moiré interferometric technique for complete curvature analysis of bent plates

Most known moiré methods need to be repeated three times for getting curvature information in three different in plane directions, for the complete analysis. To overcome this difficulty, a new single exposure moiré interferometric technique is proposed that maps completely partial curvature contours in three different directions of laterally loaded specularly reflecting plate models. Converging HeNe laser light is diffracted by a lens plane cross grating. The 00, + 10 and the +01 diffraction orders are permitted through the Fourier transform (FT) plane and collimated to illuminate the plate model. The reflected light from the loaded model records the model with the inherent deformed interferometric cross grating superimposed on it. A posteriori Fourier filtering of the developed recording (moirégram) gives partial curvature contours pertaining to three different in plane directions from which the principal curvatures and the principal directions are obtained. Also, this technique, being a single exposure technique, has the potential for complete bending analysis of plates subjected to lateral impact loading and destructive loading applications, where the repeatability of the event may not be possible.     

A study on the production of 950°C moiré gratings in metal materials

In this paper, a new method for producing high temperature gratings is put forward. With two deposited metal layers on the specimen, the oxidation resistant ability of this grating was improved, and its applications were enlarged, to cover almost all metals. The procedure for producing a grating and the controlling factors of grating quality are discussed in detail.     

The use of strippable photographic film for moire grating production

Practical details are presented concerning the use of strippable film for the production of moiré gratings. Strippable film is produced primarily for application in graphic arts. The influence of the use of the film on the accuracy of measurement in the direct moiré method is discussed.     

Curvature contours of flexed plates by a multiple illumination moiré shearing interferometer

A new moiré method is proposed to map partial slope, partial curvature and twist curvature contours of laterally loaded specularly reflecting plate models. Converging HeNe laser light is diffracted by a lens plane cross grating, different combinations of orders are selectively permitted through the Fourier transform plane and collimated to illuminate the plate model. This technique eliminates the difficulties involved in Fourier filtering in the imaging optics, thereby extending the reflection moiré' interferomelry to general plate structures and loading. The slope contours are obtained still by double exposure.     

Application of moiré technique in the stress analysis of cutting tools

Knowledge of the stress field in cutting tools is necessary for their strength evaluation. However, the experimental analysis of stresses in cutting tools during machining poses a number of problems in view of the steep stress gradient present. Photo-elasticity and the 'built-up tool' methods are the common techniques used for this purpose. In the present paper results on the stresses in cutting tools obtained during actual machining through the successful application of the transmitted moiré technique, are presented. In these experiments transparent epoxy tools carrying moiré gratings were used to machine commercial lead alloy at low speeds. The tool rake angle was varied. The effectiveness of moiré method, basically a displacement analysis technique, in providing useful data on tool stresses comparable with the other direct stress analysis techniques such as photo-elasticity is also discussed in the paper.     

Moiré from high speed deformations

A method is described for the photographic recording of moire "test gratings when deformations are changing with high speed. An ultra–short flash pulse is used to illuminate the grating. This is an important feature of the equipment and the necessary requirements for it are discussed in detail. The circuit diagram and constructional details are given.     

Strain Measurement Techniques in Explosives

Abstract:  Experiments to visualise the detailed microstructure of highly heterogeneous plastic-bonded explosives have utilised high-resolution moiré interferometry to measure localised deformations under quasi-static loading. The information obtained has proved valuable in the development of finite element material models, and offers the potential to isolate and examine the influence of particular microstructural features (like binder layer thickness, or explosive crystal size) on deformation characteristics. New insight into failure processes and the implications of ageing effects have been obtained. The experimental methods and modelling have broad applicability to microstructural studies of a variety of composites.     

An image-shearing Moiré method to record partial slopes of flexed plates

A new Moiré method is proposed to map partial slope contours of laterally loaded reflective plate models. The beam of light reflected by the plate is collected by a field lens and is optically sheared by a thin glass plate at the focus to produce slope modulated gratings at the image plane. No real grating is used. Sensitivities an order higher than those of classical Moiré methods are obtained.     

TRIANGULAR AND v 2 GRATINGS FOR THE MOIRÉ TECHNIQUE

Methods for constructing two new types of grating for the moiré technique are described, their use is illustrated in application to a simple beam, and the range of measurable strains is established. In certain circumstances the use of these gratings presents a means of accurate analysis without recourse to elaborate equipment.     

Investigation of Analytical Solutions for Bonded Structures by Photomechanical Methods

Abstract:  In this paper, both photoelasticity and moiré interferometry were successively incorporated with finite element method to investigate the predicted thermal stresses and lateral displacement of bonded structures calculated from different theories. It was found that the distributions of moment and transverse force play significant roles in making different values of thermal stresses in the adherends by authors' and Suhir's 1986 theories. On the other hand, the values of lateral displacement obtained from different theories are almost identical.     

APPLICATION OF GRIDS TO PLASTICS FOR THE MOIRE FRINGE METHOD OF STRAIN MEASUREMENT

There has been considerable interest during recent years on the measurement of strain using the moiré fringe method. This report reviews the techniques of applying grids to specimens and gives details of methods used at NEL. Strain measurements on a simple tensile test specimen using the moiré mismatch technique are compared with those obtained with a Linley extenso-meter.     

A hybrid experimental/computational approach to the assessment of crack growth criteria in composite laminates

While a number of criteria have been proposed for predicting the onset of crack growth in composite laminae, there has hitherto been no accepted method of assessing the value of alternative proposals. This study integrates the use of computational modelling, along with moiré interferometry for the quantitative mapping of near crack tip deformation fields. The moiré method was used to validate the computational model. It was then possible to use the computational model on its own to predict crack tip behaviour in more detail than could be done with the optical system alone.
This study chose one crack growth criterion - the normal stress ratio - to demonstrate the assessment routine. The value of the normal stress ratio (NSR) was calculated as a function of the ply angle to the loading direction. In order to obtain sensitive predictions of crack growth direction, it was desirable to proceed by analysing the crack tip behaviour down to radial distances from the crack tip of a few tens of microns. By a suitable choice of elements around the crack tip, the finite element model was able to predict values of the NSR down to a radius of 80 times10^-3 mm around the crack tip.     

Three dimensional displacement measurement using Moiré interferometry

Moiré interferometry is used to simultaneously measure the in plane and out of plane displacement components. The high sensitivity of displacement measurement is maintained and high contrast fringes are obtained without recourse to additional filtering.