Gary Cloud

Optical methods of engineering analysis

Introduction 1. Light and interference 2. Classical interferometry 3. Photoelasticity theory 4. Basic applied photoelasticity 5. Photoelasticity methods and applications 6. Geometrical moire theory 7. In-plane motion and strain measurement 8. Moire mapping of slope, contour and displacement 9. Diffraction and Fourier optics 10. Moire with diffraction and Fourier optical processing 11. Procedures of moire analysis with optical processing 12. Principles of moire interferometry 13. A moire interferometer 14. Experimental methods in moire interferometry 15. Holographic interferometry theory 16. Holographic interferometry methods 17. Laser speckle and combinations of speckle fields 18. Speckle photography 19. Speckle correlation interferometry 20. Electronic speckle pattern interferometry 21. Phase shifting to improve interferometry.

The application of interferometric techniques to the nondestructive inspection of fiber-reinforced materials

A method for increasing the sensitivity of dynamic materials evaluation (DME) to localized damage in fiber-reinforced composites was examined. To obtain this improved sensitivity, different aspects of DME were examined. These included an increase in the frequency used to evaluate the dynamic properties, utilization of mode-shape information and different procedures for evaluating the experimental data.The extent of the internal damage was determined using measured changes in the dynamic properties of the system (loss factor, dynamic stiffness and mode shape). To obtain the response information at higher frequencies a modalanalysis system was built around the performance characteristics of a laser doppler vibrometer (LDV) and an electronic speckle pattern interferometer (ESPI). These two devices provided complementary information for the determination of the dynamic characteristics of each vibration mode. With this system, damage-induced changes in the dynamic characteristics of composite materials were measured at frequencies up to 10 kHz.The results of this study showed the following. (1) Torsion modes provide the most sensitivity to localized internal damage. (2) The evaluation of higher frequency NDI data requires the ability to correlate the measured loss factor and resonant frequencies with the actual mode shape. (3) The data obtained over the frequency range of the test could be reduced to a series of slopes that provide a sensitive indication of the material condition. (4) The sensitivity of the dynamic method to localized damage is limited by the measurement of the loss factor.

On dispersion of birefringence of photoelastic materials

Several investigators have observed that birefringence depends upon the color of light used in its measurement. That is to say, the order of isochromatic is not simply inversely proportional to the wavelength. This dependence of the absolute and relative retardation, and consequently of the photoelastic coefficients upon wavelength, is called dispersion of birefringence.In this paper, the authors discuss previous reports and formulations of optical dispersion and dispersion of birefringence. These past descriptions, which are partially arbitrary, are thought to be inadequate in view of progress in photoelasticity, spectroscopy, polymer physics and, in particular, the extension of photoelastic methods into the longwave spectral regions. New measures of dispersion are presented, and their relationships to each other and to other quantities, especially the stress- and strain-optical coefficients, are discussed. The authors present dispersion measurements derived from tests of CR-39 and P-6 in a spectral region consisting of the visible and near infrared. Some results indicate that the association between dispersion of birefringence and linear limit stress may constitute the basis for less ambiguous definitions of the linear range and for the establishment of general analytical expressions which are more closely related to real physical behavior.

Whole-field strain measurement in a pin-loaded plate by electronic speckle pattern interferometry and the finite element method

The strain field in an epoxy plate loaded in tension through a steel pin is determined using electronic speckle pattern interferometry (ESPI) and the finite element method (FEM). In a dual-beam illumination speckle interferometer, the in-plane component of the displacement at the plates surface is accurately measured using a four-step phase-shifting algorithm. Digital image processing algorithms have been developed for noise reduction and strain calculation directly in the computer from the phase map with a strain gage length of about 0.4 mm. A whole-field strain map is obtained, as well as distributions of strain concentration factor, in critical regions near the hole of the plate. FEM is used to perform a nonlinear contact analysis accounting for friction effects at the pin/hole interface. The agreement between experimental results and numerical predictions is good. In terms of speed, accuracy and ease of use, dual-beam ESPI appears to be a superior method of whole-field strain analysis.

Strain gauge measurement of rein tension during riding: a pilot study

A pilot study was performed using a strain gauge transducer intercalated between the bit and the left rein to measure rein tension dynamically during riding. The strain patterns consisted of a series of spikes with frequencies corresponding to two per stride in walk and trot and one per stride in canter. The highest tension recorded in each gait was 43 N at walk, 51 N at trot and 104 N in canter. Based on the results of this study, it is recommended that the methodology should be adapted so that both reins are instrumented simultaneously, data are transmitted telemetrically to eliminate the need for a tether connecting the horse to the computer, and kinematic data are synchronized with the rein tension recordings.

Practical Speckle Interferometry for Measuring In-Plane Deformation

Speckle interferometry involves making a high resolution double-exposure photograph of an object that is illuminated with coherent light. When observed in a Fourier optical data processor, fringes are observed that are indicative of the changes, such as displacements, occurring in the body between exposures. Sensitivity of the measurement is variable and may be chosen after the data are recorded. A simple calibration procedure is described and its use in measuring in-plane displacements in deformable solids is demonstrated. Also given are some limitations on the range of sensitivity that can be effectively utilized with holographic films, simple procedures, and a small laser.

A Study on the Effects of Clearance and Interference Fits in a Pin-Loaded Cross-Ply FGRP Laminate:

The elastic behavior of a cross-ply fiber glass-reinforced epoxy laminate loaded in tension through a pin fitted with clearance and interference is examined. A nonlinear contact analysis using the Finite Element Method and an experimental study by Speckle Interferometry are performed, showing fairly good correlation. An interesting comparison is performed between FEM results and Hertzian contact theory. Major conclusions are: (I) clearance causes high compressive stress in the bearing area, and does not influence the peak tensile stress; (2) interference has beneficial effects on the joint, as (a) it lowers the peak circumferential stress, and (b) it decreases the change of stress due to the application of the load. This last conclusion suggests that interference can improve the fatigue life of the joint.

Mechanical-optical properties of polycarbonate resin and some relations with material structure: Polycarbonate is an excellent model material, but heat treatment is required. Dependence of birefringence on time and wavelength is described by normalized creep and normalized retardation parameters. Dispersion of birefringence is related to the spectral transmittance

Polycarbonate resin possesses optical and mechanical properties which make it particularly suitable for certain experimental investigations, including two-and three-dimensional photoelastic analysis. The ductility and transparency of this material might be usefully employed in photomechanical investigations of plastic and viscoelastic response. The similarity of the stress-strain law of polycarbonate to that of mild steel could simplify the similitude problem. In addition, its spectral transmittance in visible and infrared makes polycarbonate useful for studies of material properties and structure.The optical creep of polycarbonate is respresented by a normalized creep coefficient. The relationship of this factor to the theory of viscoelasticity is discussed, and the conditions for a valid calibration of birefringent materials are reviewed. The wavelength dependence of relative retardation is represented by the normalized retardation, from which the dispersion of birefringence can be deduced.The stress-birefringence-time-wavelength characteristics of two brands of polycarbonate resin were determined. Because of residual birefringence, it was necessary to heat treat the resin at about 146°C, and properties of both annealed and unannealed resins are presented. Retardation was measured over the visible and near-infrared portions of the electromagnetic spectrum (407 nm to 1900 nm). There exists a definite relationship between dispersion of birefringence, which amounts to 14 percent in visible, and the infrared spectral transmittance, which is indicative of material structure.

Strain-Relief Inserts for Composite Fasteners -An Experimental Study

The stress and strain distributions in the vicinity of single-pin lap joints in a glass-epoxy laminate were studied in a combined experimental-theoretical program. One objective was to evaluate the possibility of obtaining significant stress concentration relief through the use of isotropic material inserts. This paper concentrates on some experimen tal results. High sensitivity Moire interferometry was employed to measure surface strains in three directions within the insert ring and in the adjacent composite laminate. Strain rosette equations were then used to obtain maximum shear strains. Measurements were performed for no insert, a hard insert (aluminum), and a soft insert (epoxy) in an isotropic material and in a composite. Stress reductions were observed at bearing and ligament areas for both types of inserts, with the stiffer insert giving the greatest advantage.

Techniques in infrared photoelasticity - Paper discusses the equipment and experimental procedures which have been employed in the extension of photoelastic measurements into the infrared region

The extension o photoelastic methods to spectral ranges other than the visible offers several intersting and potentially profitable possibilities. These include a clearer understanding of the phenomenon of birefringence and its relationship to material structure and behavior, an increase in the number and types of useful photoelastic materials, and improved measurement of material behavior, including dispersion of birefringence. Indeed, confinement to the narrow visible spectrum unduly restricts this powerful experimental tool.Although the principles of measurement and the interpretation of data are not dependent on wavelength, several problems arise when nonvisible radiation is used in photoelastic investigations. The authors have developed apparatus and experimental procedures for studies in which monochromatic radiation of up to 2-μ wavelength is employed. It is believed that the basic techniques may be applied throughout an even broader spectrum, and equipment is being developed for use with radiation at wavelengths of more than 2 μ.In order to facilitate adaption of photoelastic procedures to different spectral ranges, a systems approach is employed in the design, development, and description of apparatus and techniques. This consists of carefully defining the basic functions to be served in the measuring system, then choosing devices and methods which best serve these individual functions at the particular wave-lengths used. Of course, compatibility of the system elements and proper functioning of the entire system must be assured.The most serious problem in instrumentation for infrared photoelasticity is created by the fact that the radiation is not visible. Observation and recording of data is accomplished by means of infrared converters, infrared photography and point-sensing techniques using a modulated infrared beam. Additional possibilities are provided by closed-circuit television and video tape.