The basic principle of the white light speckle method for strain analysis is reviewed with a theoretical analysis of the recording and reconstruction processes of specklegrams including the effect of defocus. Ways to increase the sensitivity are proposed. The method's a-plicability to measuring inplane displacement, out-of-plane displacement, surface strain on a curved object, interior strain in a 3-D object, and large deformation is exemplified.

The elastodynamic equations for the caustics formed from light rays reflected from the vicinity of the instantaneous position of the tip of a running crack were established for the case when the crack propagation velocity is considered as constant. For the study of elastodynamic stress intensity factors K'FI and lq, it is necessary to know the expressions for the stress components existing at the running crack tip at any instant of the propagation of the crack and the corresponding dynamic mechanical and optical properties of the material of the specimen under identical loading conditions. The distortion of the form of the corresponding reflected caustic from the lateral faces of a dynamically loaded transparent and optically inert specimen, containing a transverse crack, running under constant velocity, was studied on the basis of complex-potential elasticity theory, and the influence of this form on the value of the dynamic stress intensity factors was given. The method was applied to the study of a propagating crack under mode I or mixed-mode deformation in a PMMA specimen under various propagation velocities, and the corresponding dynamic stress intensity factors Ig and Kil were evaluated. As an application, the method was used for studying the propagation of cracks under an initially mode I deformation in iron-epoxy particulate composite specimens under various filler-volume fractions and in different types of composite biphase specimens, as well as in specimens under prevailing plane-stress conditions containing different kinds of material discontinuities.

Deformations of engineering structures are measured by applying a fine grid pattern to the surface and photographing this in white light with a specially modified 35 mm camera. The processed negatives are analyzed in a spatial filtering system to produce moire fringe maps which represent in detail the three-dimensional displacements that have occurred. Measurements have been obtained from brick built and concrete structures, from steel surfaces heated to 600°C, and from plastic and composite materials.

This paper is meant to be an illustration of some specific applications of the moire technique that are used almost daily in the Laboratory for Strength of Materials and Welding Technology of the State University of Gent to visualize the elasto-plastic displacement field in metal specimens and structures. The emphasis is mainly laid on the feasibility aspects; interpretation of the results are beyond the scope of this article. More specific informaion is available on request.

This paper summarizes several years' effort aimed at applying moire methods to the evaluation of structural performance of composite materials. Results relating to edge effects in laminates, bolted joint behavior, design of composite tension and shear test specimens, adhesive joint performance, fracture mechanics studies, and use of shaped holes for reduction of stress concentrations are described. The moire approach is invaluable for examining the effects of nonlinear material response, localized damage, and material inhomogeneity which are inherent in the structural response of fiber reinforced materials.

Determining strain with grids is one of the oldest and simplest methods of experimental stress analysis. Here, the method is reviewed. Various techniques that are used to print, record, and analyze grids are discussed, and the types of problems to which the grid method has been applied are presented.

Optical methods of shadow and projection moire are particularly suitable for mapping the absolute shape of a given surface or the difference between that surface and a master one or its accidental shape change. Essentially, product moire fringes are obtained on an image plane where a "filter" grating analyzes the image of the test surface which has been "coded" by projecting a master grating on it. The article resumes the theory in a unifying approach and presents a comprehensive list of applications.

The method of shadow moire topography has been used to characterize smooth craters formed on the surface of ductile metals. The craters were made by the impingement of hard spheres (diameter 5 X 10-3m) traveling at speeds of approximately 130 m s-1. A comparison of shadow moire with other methods, in particular the techniques based on holography, shows that the former is simpler to use, less costly, and relatively insensitive to vibrations. A mathematical description of a crater has been developed which describes the surface to a good approximation. It has only two independent parameters: the crater depth and the half-length, and can be integrated to give the volume of the crater. The parameters, which can be found directly from moire topographs, give a measure of the crater volume (10 to 0.1 X10-9 m3) accurate to within 8 to 15%. A more elaborate method for determining the volume, which is also described, optimizes the values of the parameters and has a final accuracy better than 3%.

The projection moire technique affords an accurate method of measuring the geometry of the necked region in a tensile specimen. Corrections to the measured stress, necessary to account for the triaxiality introduced by the neck, require values for the radius of curvature of the specimen at the minimum section. The spacing of fringes resulting from the interference between a grating projected on the specimen surface and a master grating not only provide information which permits calculation of the radius but also demonstrates any changes in symmetry resulting from plastic deformation. Results obtained by testing specimens of copper and mild steel having initially straight and initially curved profiles indicate that Bridgman's approach provides a self-consistent correction for triaxiality. However, comparison of incremental tests with continuous tests indicates that the development of neck geometry may be sensitive to the strain rate.

The term "isodynes" has been proposed by Pindera and Mazurkiewicz to denote a new family of characteristic lines of plane stress fields. These lines carry information on total normal forces acting on related cross sections and yield the distribution and values of related normal and shear stress components. Two families of isodynes related to two characteristic directions yield the values of all three components of a plane stress field and additional redundant information. Isodyne photoelasticity methods can be applied to determine all three stress components in photoelastic models and in original machine or structural parts using isodyne coatings. The term "gradient photoelasticity" has been proposed by Pindera and Hecker to denote a new method of photoelasticity which utilizes relationships between the curvature of light paths in a photoelastic object and the gradients of symmetrical and distortional parts of stress/strain tensors. Utilizing a basic mathematical model of photoelastic effect presented by Ramachandran and Ramaseshan, gradient photoelasticity yields the momentary values of absolute and relative photoelastic coefficients. Both methods can be applied to determine the values of stress intensity factors for arbitrary cracks and all stress components in composite structures.

The term "isodynes" has been proposed by Pindera and Mazurkiewicz to denote a new family of characteristic lines of plane stress fields. These lines carry information on total normal forces acting on related cross sections and yield the distribution and values of related normal and shear stress components. Two families of isodynes related to two characteristic directions yield the values of all three components of a plane stress field and additional redundant information. Isodyne photoelasticity methods can be applied to determine all three stress components in photoelastic models and in original machine or structural parts using isodyne coatings. The term "gradient photoelasticity" has been proposed by Pindera and Hecker to denote a new method of photoelasticity which utilizes relationships between the curvature of light paths in a photoelastic object and the gradients of symmetrical and distortional parts of stress/strain tensors. Utilizing a basic mathematical model of photoelastic effect presented by Ramachandran and Ramaseshan, gradient photoelasticity yields the momentary values of absolute and relative photoelastic coefficients. Both methods can be applied to determine the values of stress intensity factors for arbitrary cracks and all stress components in composite structures.

Experimental methods of stress analysis are very useful for composite materials and structures because of the complexities due to anisotropy. This paper deals with the application of photoelastic methods to glass fiber reinforced plastics. The basic principles governing the photoelastic response in birefringent orthotropic composites are briefly reviewed. Then, three important aspects of photo-orthotropic elasticity, namely, photoelastic calibration, separation of principal stresses, and application to problems of engineering importance, are considered in detail. A new calibration specimen is proposed for determining the three independent photoelastic constants: an orthotropic half-plane subjected to a concentrated edge load. From the isochromatic response, the three stress-fringe values can be obtained by a least-squares procedure based on the theoretical stress distribution. Next, four methods of separating the principal stresses in birefringent com-posite models are proposed: the method of drilling small holes at the points of interest, the method of oblique incidence, the method of combining the transmitted photoelastic response with the reflected photoelastic response, and the method of birefringence dispersion. Finally, results from a photoelastic study of pinloaded composite models are presented to show the usefulness of the techniques of photo-orthotropic elasticity. These models simulate bolted joints in composites, which have been receiving increasing attention in recent years due to their importance in composite structures.

The paper reviews integrated photoelastic methods for stress determination in axisymmetric bodies. The specimens are put in an immersion bath and viewed in a common transmission polariscope. If birefringence of the specimen is high, integral fringe pattern as well as quasi-isoclinics may be photographed. By point-by-point technique the characteristic directions as well as the characteristic phase retardation on a number of light rays are measured. This experimental data, often combined with the application of the equilibrium equations and macrostatic equilibrium conditions, permit one to determine stress distribution in the specimen. Investigation of stresses in isotropic speci-mens as well as in cubic single crystals is considered. A new method of determining stress in cylinders, including measurement of the angular deflec-tion of light rays, is described.

A major unsolved problem in fracture mechanics concerns the subcritical growth of cracks under fatigue loading to a critical size at which catastrophic fracture occurs. Such growth is the precursor to most service fractures and often involves non-self-similar flaw growth, nonplanar flaws, and nonuniform stress intensity distributions along flaw borders. These geometric complexities coupled with notch geometries and stress gradients which precede and accompany the flaw growth remove the problem from the realm of mathematical tractability. Beginning a decade ago, the author and his associates have been involved in a continuing effort to develop optical methods for extracting stress intensity factor estimates and, more recently, crack shapes and displacement fields from photoelastic models of such bodies. Methods developed involve a marriage between the field equations of linear elastic fracture mechanics with the optical techniques of frozen stress photoelasticity and moire interferometry. After briefly reviewing the analytical foundations of the methods and describing the techniques, this paper presents results obtained from applying the technique to problems of current technological interest in the aerospace and nuclear fields.

A multiple ruby laser system is used to provide intense, extremely short (30 ns) highly monochromatic light pulses for high speed photography. The light output from the lasers is transmitted to the two optical systems with fiber optic light guides. The ruby lasers are employed with a conventional Cranz-Schardin system in dynamic photoelastic studies of stress waves and in studies of crack branching in tempered glass. The ruby lasers are employed with an unorthodox Cranz-Schardin system in ballistics studies of armor penetration. Of particular importance is the use of a microprocessor which provides precise synchronization by computing in real time the anticipated arrival time and controlling the Q-switching of the lasers. Variations in projectile velocity due to projectile weight, charge density, and drag are accommodated by programming the microprocessor.

High speed photography has the great advantage of giving direct visual information about transient processes. This article reviews the techniques of high speed photography and describes a range of applications with examples from the author's group at the Cavendish. There are various types of high speed cameras that can operate in different speed, or framing rate, ranges. These are discussed in terms of increasing framing rate ability since this illustrates the techniques involved and also outlines the historical development of the subject. The applications illustrated include studies of fracture, impact, erosion, high rate of strain deformation, explosive initiation, and laser-induced damage.

Development of accurate constitutive relations for the mechanical behavior of materials at high strain requires continuously interactive testing. Tensile testing at constant true strain rate illustrates this process. In such a test a servo-controlled testing machine is driven by a control signal produced by the difference between a feedback and a command signal. Response of the specimen to the testing environment produces the feedback signal. This paper describes an optical sensor which produces a signal suitable for controlling a servohydraulic test system. The device employs a projection system and a scanning galvanometer mirror to translate a silhouette of a test specimen past a fixed slit covering a photodiode. The photodiode produces an ac waveform, which subsequent processing converts to a dc signal proportional to the minimum dimension of the silhouette scanned. For many materials this provides sufficient information to measure the instantaneous true strain rate in the deforming region. The paper concludes with a discussion of limitations of the device.

A family of optical methods are developed for the measurement of slope and curvature of flexed plate surfaces. Slope and curvature contour fringes are generated by using gratings as filters in the light path emerging from the flexed surface. General field equations are derived for gratings placed anywhere in the field. Depending upon the pitch and the position and the number of gratings used, a variety of techniques are obtained for the contouring of slopes or curvatures. When a low density grating is used, the resulting shadowgram yields slope contour fringes if it is placed at the real focal plane of the field lens. Otherwise, the shadow gram fringes are not slope contours. However, slope contours can be obtained if a double-exposure technique or a double grating is used to generate the moire fringes. If a grating of sufficiently high frequency is used, the resulting pattern is a curvature pattern, for which a monochromatic light source is needed. A successive plotting method is also proposed for curvature contouring whereby white light and a grating of arbi-trary pitch can be used. The methods are verified by a series of experiments using cantilever beams and clamped circular plates. Applications to a variety of other problems, including flexure wave propagation, are also demonstrated.

Superpolished mirrors lose their low-scatter finish in industrial atmospheres because of a corrosion process. This degradation is investigated by exposing mirrors in controlled gaseous environments, monitoring changes with scatter measurements, and microscopically and elementally analyzing the surface films. Blue haze results when nickel from the mirror combines with airborne pollutants and water to form an amorphous salt which can coat the nickel and etch it. Preventive techniques to keep the metal clean and free of water films are also discussed.

In laser-induced inertial confinement fusion the understanding of the physical process taking place in the plasma atmosphere surrounding the target is extremely important because it is here where the energy is absorbed and transported to the core region. Optical probing of the target atmosphere can provide information about this important region by using laser diagnostics such as interferometry, Faraday rotation, and Thomson scattering. Interferometry is used to determine electron density distributions in the plasma and has permitted the direct observation of effects due to ponderomotive forces in the plasma. Thomson scattering is another diagnostic which can help understand some of the interaction process taking place in plasma corona. This technique, apart from providing electron and ion temperature inside the plasma, can provide direct information about wave-wave decay process, ion turbulence, and other collective effects. It has been successfully used to isolate some nonlinear wave phenomenon in 10.6 um laser-produced plasmas.

The present status and potential future direction of the NASA space infrared astronomy program is reviewed. Projects and project concepts dis-cussed include the Infrared Astronomy Satellite, Small Infrared Telescope on Spacelab 2, Cosmic Background Explorer, Shuttle Infrared Telescope Facility, Space Telescope, large deployable reflector, molecular line survey, and infrared interferometer in space. Needs for continued engineering development in critical technology areas such as detectors, cryogenics, optics, and space structures are indicated.

The Extreme Ultraviolet Explorer Mission is described. The purpose of this mission is to search the celestial sphere for astronomical sources of extreme ultraviolet (EUV) radiation (100 to 1000 A). The search will be accom-plished with the use of three EUV telescopes, each sensitive to different bands within the EUV band. A fourth telescope will perform a higher sensitivity search of a limited sample of the sky in a single EUV band. In six months, the entire sky will be scanned at a sensitivity level comparable to existing surveys in other more traditional astronomical bandpasses.

The Cosmic Background Explorer (COBE) satellite, under study by NASA since 1976, will map the spectrum and the angular distribution of diffuse radiation from the universe over the entire wavelength range from 1 micron to 1.3 cm. It carries three instruments: a set of differential microwave radiometers (DMR) at 23.5, 31.4, 53, and 90 GHz, a far infrared absolute spectrophotometer (FIRAS) covering 1 to 100 cm-1, and a diffuse infrared background experiment (DIRBE) covering 1 to 300 microns. They will use the ideal space environment, a one year lifetime, and standard instrument techniques to achieve orders of magnitude improvements in sensitivity and accuracy, providing a fundamental data base for cosmology. The instruments are united by common purpose as well as similar environmental and orbital requirements. The data from all three experiments will be analyzed together, to distinguish nearby sources of radiation from the cosmologically interesting diffuse background radiations. Con-struction is planned to begin in 1982 for a launch in 1988.

The task of characterizing optical wave propagation through the earth's atmosphere under low-visibility weather conditions is extremely difficult; it requires, in essence, solution of the general multiple-scattering problem. This paper describes results from an integrated approach to propagation through the turbid atmosphere in which experiments are used to establish the magnitude of the propagation effects and to supply empirical relationships which permit simplifying approximations in the theory. In particular, line-of-sight laser measurements of multipath spread, angular spread, beam spread, and incoherent source measurements of angular spread and power transmission versus range are reported. From the behavior of these data two approximate propagation theories, viz., multiple forward scatter and strong multiple scatter, are developed.

A conceptually simple high speed infrared (IR) zoom lens system has been designed to operate in the 8 to 13µm wavelength region for the detection of missile signatures. Even though the optical system is designed to provide high resolution throughout the 3:1 zoom range at a large relative aperture, it is extremely lightweight and compact. The zoom concept has been derived from first-order optical principles which will be presented as the starting point of the optical system design. The use of the symmetry principle for aberration correction will be discussed. Paraxial ray trace was used as an aid in establishing lens powers and groupings in the initial layout. Considerations which governed material selection will also be discussed. The zoom system consists of six lens elements including a field flattener near the image plane. The total glass weight is only 172.4 grams and the length is 161.79 mm. There are two moving lens components linked together with a cam to provide mechanical compensation. The detail design was accomplished through the use of a computer optimization program. The 1 mrad image resolution requirement is achieved throughout the zoom range. Performance data will be presented and tolerance considerations will be discussed.

A low cost, four watt CO2 laser intended for use as either an expendable beacon or expendable source for proximity fuses has been developed by the Hughes Aircraft Company. The laser, excited by a 140 MHz rf power supply, has a modulation rate capability extending from dc to more than 100 kHz. The laser, dc-dc converter, and rf power supply weigh less than 1 kg. Although most of the intended applications require the laser to operate for only a few seconds, it has been tested for over 600 hours with negligible power degradation.

A simple incoherent speckle technique for color image retrieval with white-light processing is presented. The technique does not rely on coherent speckles. The encoding is performed by using incoherent color illuminations of a diffused color object, with different slit orientations, during the specklegram constructions. Color image retrieval is then obtained by color spatial filtering in the Fourier domain of a white-light optical processor.

A simplified derivation is presented which illustrates the effects of unmatched fields of view for a lidar system. The resultant equations are easily evaluated on a programmable calculator, and comparison with previous calculations are numerically similar.

Overtly and covertly many of our authors have indicated their feelings about referees' comments. Many of our authors are grateful for the referees' help. An equal number are hostile regarding referees as bumbling barriers to their work. The most common reaction is one learned from years of schooling; "I'll do what the teacher says to make the grade, but I know it's dumb." What is the best reaction? Gratitude? Sometimes. Hostility? Seldom or (I hope) never.