D. Post

High-sensitivity moiré interferometry—A simplified approach

High-quality moire-interferometry patterns were obtained with the following conditions: specimen of graphite-polyimide composite material; reflective cross-line specimen grating of 600 ℓ/mm (15,000 ℓ/in.); virtual reference grating of 1200 ℓ/mm (30,000 ℓ/in); fringe-multiplication factor of 2; carrier pattern of 10 fringes/mm (250/in.); carrier eliminated by optical filtering. The phase-type specimen grating was cast onto the composite material with silicone rubber, using a ‘homemade’ mold. The virtual grating was formed with a plane mirror in a collimated field. Precision-quality optical elements are not required.

Moiré interferometry for engineering mechanics: Current practices and future developments

In the past decade, the optical method called moire interferometry has matured rapidly to emerge as an invaluable tool, proved by many industrial and scientific applications. It has been applied to numerous problems in engineering mechanics. It measures in-plane displacement fields with high sensitivity and high spatial resolution. This paper reviews current practices of moire interferometry and its extensions. Applications in diverse fields are addressed to demonstrate the wide applicability of the method, especially the recent applications for thermal deformation analyses of microelectronics devices. Speculation on future developments and practices is presented.

Determination of thermal strains by moiré interferometry

An existing method is extended to measure thermal strain distributions on an absolute basis. Free thermal expansion and stress-induced deformations are separated, allowing the determination of coefficients of expansion, stress-induced strains, normal stresses and shear stresses. The method is applicable to many steady-state and transient thermal-strain problems.A crossed-line grating is replicated on the specimen at elevated temperature. A zero-expansion mold is used for the replication, so that the grating frequency at the replication temperature can be retained to null the moiré interferometer at room temperature. When the specimen is viewed in the moiré interferometer, the fringe patterns reveal theU andV displacements induced by any change from the replication temperature. In addition, carrier fringe techniques are introduced for steady-state problems to subtract off the free thermal expansion and produce fringe patterns of the stress-induced deformations themselves. The method is demonstrated by analysis of a bimaterial plate subjected to a uniform change of temperature.

Immersion interferometer for microscopic moiré interferometry

The basic sensitivity of moiré interferometry has been increased beyond the previously conceived theoretical limit. This is accomplished by creating the virtual reference grating inside a refractive medium instead of air, thus shortening the wavelength of light. Various optical configurations of moiré interferometry for operation in a refractive medium are introduced and one of them has been put into current practice. A very compact four-beamimmersion interferometer has been developed for microscopic viewing, which produces a basic sensitivity of 4.8 fringes per μm displacement (contour interval of 0.208 μm per fringe order), corresponding to moiré with 4800 lines per mm. Its configuration makes it inherently stable and relatively insensitive to environmental disturbances. An optical microscope is employed to obtain high spatial resolution. The method is demonstrated for deformation of a thick graphite/epoxy composite at the 0/90-deg ply interface.

Iosipescu shear characterization of polymeric and metal matrix composites

The paper outlines the results of an experimental/analytical investigation of deformation and stress fields in the test section of graphite/epoxy and boron/aluminum Iosipescu specimens using high-resolution moiré interferometry and finite-element analysis. Very good correlation between experimental and analysis is demonstrated for the graphite/epoxy speciments. This, in turn, justifies the use of correction factors employed in the previous investigations to eliminate differences in the inital shear moduli obtained from 0-deg and 90-deg losipescu specimens. The significant discrepancies observed in the case of boron/aluminum specimens are attributed to localized yielding at the notch tips caused by the clamping action, the associated in-plane bending effects and changing boundary conditions due to slippage in the grips.

The magic of carrier fringes in moiré interferometry

Practical applications in which carrier fringes are used with moiré interferometry for strain measurements are presented. Examples illustrate how moiré carrier fringes are applied to obtain the desired data in complex laminated composite specimens. In many cases, carrier fringes permit extraction of much more detailed information, with procedures that are easier and more accurate than those using loadinduced fringes alone. The fringe vector for carrier fringes is introduced and its application to the interpretation of fringe patterns is explained. In moiré interferometry, the carrier fringes are produced easily by adjustments of optical elements that control the virtual reference grating.

Stable Crack Growth in Aluminum Tensile Specimens.

Posts white-light moiré interferometry was used to obtain sequential records of the transientUy-displacement fields associated with stable crack growth in 7075-T6 and 2024-0, single-edge-notched (SEN) specimens with fatigued cracks. TheUy-displacement fields are used to evaluate the crack-tip opening displacement (CTOD), far- and near-fieldJ-integral values, Dugdale-strip-yield model, Williams polynomial function and the HRR field.

A Localized Hybrid Method of Stress Analysis : A Combination of Moire Interferometry and FEM

A method of combining moiré interferometry and the finite-element method to effect localized stress analysis is presented. The displacement data from local regions of interest in the optical experiment are used as boundary conditions for the finite-element stress analysis.The stability of the method is examined with data from simple numerical models one of which corresponded to the stress analysis of a pin-loaded plate with friction. These studies show that the method requires the sensivity of moiré interferometry for successful implementation, i.e., displacement data accuracy within 0.1 μm or 4 μin.This localized hybrid method of stress analysis provides a powerful and efficient method for the reduction of experimental data.

Full-field displacement and strain rosettes by moiré interferometry

A cross-line phase-type specimen grating was interrogated by moiré interferometry techniques to produce full-field fringe patterns of three displacement components:Ux,Uy, andU45. ForU45, sensitivity was 1/1700 mm/fringe (1/43,000 in./fr). Closely packed fringes with much information content were obtained. Fringe patterns of strain components⎵x,⎵y and⎵45 were produced by mechanical shearing (or optical differentiation) of the displacement patterns; a shearing distance of only 0.6 mm (0.025 in.) was used. The rosette method yields complete strain information from these three components of normal strain—which were derived from three direct derivatives of displacement. Consequently, the need for cross-derivatives of displacement, which are highly sensitive to accidental rigid-body rotations, is circumvented.

Thick composites in compression: an experimental study of micromechanical behavior and smeared engineering properties

The micromechanical behavior and mechanical properties of thick graph ite/epoxy composite laminates were studied experimentally using high-sensitivity moire interferometry and strain gages. Quasi-isotropic and cross-ply laminates were in vestigated. Specimens were loaded in in-plane compression and interlaminar compres sion. Strong free-edge effects were documented, including high interlaminar shear strains and normal strains. Average or smeared values of Youngs moduli and Poissons ratio were measured.