Processing-induced residual strains in solid cylinders of Nickel 200 were investigated using phase shifting moiré interferometry. Two different experimental approaches were used to study the strains produced during Tungsten-inert gas spot welding. A comparison of results for a ‘hot/fast’ and a ‘cool/slow’ weld demonstrate the capabilities of the general approach. Both experimental methods revealed distinct differences in the residual displacement and strain fields between the two types of welds.

Composite materials offer a unique advantage over conventional engineering materials in that structural properties can be tailored to suit specific applications. However, the inherent anisotropy and the discrete layer-by-layer fabrication method of composite materials lead to mechanical behavior and failure characteristics that are quite different from those of homogeneous materials. Consequently, failure modes such as delamination in polymer matrix composites and matrix cracking and damage in ceramic matrix materials prohibit these materials from being used in conventional engineering structures, as well as making their characterization in the laboratory difficult. In this paper, an experimental photomechanics technique called phase-shifting moiré interferometry is described. This technique is capable of providing analysts and designers (both material and structural) with detailed displacement and strain fields near discontinuities in these materials. The technique allows high resolution measurements of in-plane surface displacements to be made without introducing global smoothing errors, thus preserving the integrity of data near cracks, discontinuities and material interfaces. In this paper, the advantages of phase-shifting moiré interferometry will be illustrated through several problems involving composite materials.

Conventional measurements of energy release rates,GI andGII, for delaminations in composite materials, generally utilize loads, crack lengths and simple standard specimen geometries. In this work, a more widely applicable measurement method, using phase shifting moiré and the J integral, is presented. The experimental technique described requires only fringe-pattern information and the elastic constants for the measurements—thus it can be used when the standard methods are inapplicable. Using conventional double-cantilever beam and end-notched flexure specimens, the energy release rate has been measured simultaneously by the moiré method and the standard methods, with good agreement found between the two. This development will for the first time permit the experimental validation of new finite-element routines as they are developed.

Strain development in several notched ceramic-matrix composites (CMCs) has been monitored over a broad range of load. This was achieved by using phase-shifting moire interferometry, which provides a map of the surface strains. A sequence of fringe patterns was used to chart the evolution of strain redistribution as a function of load. The ensuing strains were related to the micromechanical damage mechanisms. Stress concentrations were estimated from the strains by using stress/strain curves. Implications for the notch sensitivity of CMCs are discussed.

The phase shifting technique has recently seen application to many types of interferometers, including holographic, speckle and moiré systems for strain analysis. In these applications, close control of the phase shifts may be impossible due to the effect of mechanical vibrations, and this presents difficulties when extracting the phase information, since the reference phase shifts are generally assumed to be known. In this work, several approaches to the problem of determining the reference phase shifts in a perturbing environment are critically evaluated using computer simulations. The analyses provide a general relationship between precision and reference phase shift errors and demonstrate the performance of each method as certain idealised fringe pattern error parameters are varied. A practical example confirms the predictions of the numerical simulations and demonstrates the feasibility of using reference phase shift estimates to improve data reduction procedures.

Understanding the mechanical behavior of jointed-rock masses is of critical importance to designing and predicting the performance of a potential nuclear waste repositiry. To this end we have studied the frictional sliding between simulated rock joints using phase shifting moire interferometry. Preliminary calibration models were made from stacks of Lexan plates that were sand-blasted to provide a uniform frictional interface. Load was applied monotonically and phase shifted moire fringe patterns were recorded at three different load states. Plots of slip along the interfaces for the model are presented to demonstrate the ability of the photomechanics technique to provide precise measurements of in-plane displacement, and ultimately the slip between the plates.

There is a great deal of interest in characterizing the dynamic mechanical behavior of laminated carbon-fiber/epoxy composites for military and aerospace applications. Current research efforts have been directed at measuring the strength lost because of accumulated damage. Very little work has been done to determine how this damage is accumulated during dynamic mechanical loading. Of particular interest is the effect of short duration (< 1 {micro}s) stress pulses on mechanical behavior such as delamination. In this paper, a magnetic flyer plate apparatus is presented for generating a short duration stress pulse in a unidirectional carbon-fiber/epoxy laminated composite. The stress pulse is characterized using a dynamic moire interferometer.

Experimental displacement and strain fields are presented for a generalized plane stress tensile specimen consisting of commercially pure titanium diffusion bonded to 6Al-4V titanium. Plastic flow initiates at the intersection of the interface with the specimen free edge. Further deformation results in concentrated shear bands emanating from the interface at both free edges. Interactions of the interface-free edge shear bands force a state of plane strain in the center of the specimen. These interface constraint effects have practical relevance on the testing of joined metals with property mismatch.

Phase Shifting Interferometry is a highly accurate data acquisition technique that efficiently utilizes several frames of information for each measurement. In this work, the advantages of phase shifting have been applied to a conventional moiré interferometer, yielding a system capable of recording phase shifted fringe patterns for both in-plane displacement components. Using this method, the phase of a wavefront of interest can be determined at each detector location, so that the resolution of the phase measurements is limited primarily by the detector discrimination and geometry. Unlike traditional Fourier fringe analysis, the noise rejection of phase shift processing algorithms does not degrade image fidelity in the presence of edges and discontinuities. A general discussion of both the phase shifting technique and the Fourier fringe analysis method is included to provide insight into the problems of processing discontinuous fringe patterns.

Originally developed as a method to assist in the assessment of optical components, the phase shifting technique has recently seen application to many other types of interferometers, including holographic, speckle, and moire systems for strain analysis. In these applications, close control of the phase shifts may be impossible due to the effect of mechanical vibrations, and this presents difficulties when extracting the phase, since the reference phase shifts are generally assumed to be known. Several approaches to the problem of determining the reference phase shifts in a perturbing environment are critically evaluated using computer simulations. The precision and accuracy of these methods are demonstrated through a practical solid mechanics example. It was found that the reference phase shifts can be determined with sufficient accuracy using all of the methods considered, and that the ultimate precision of the system was limited by additional factors -- particularly grating noise.

Moiré interferometry is employed to study toughening in partially stabilized zirconia (PSZ). Energy to fracture as a function of crack growth curves (R-curves) is derived from mode I compliance calculations and from near tip fitting of the moiré fringes. The effect of the tetragonal to monoclinic phase transformation in the zirconia is found by comparing the bulk compliance R-curves to the locally derived moiré R-curve. Localized strain field plots are produced from the moiré data for the PSZ zirconia. The observed transformation zone height compares favorably with that predicted by Okada et al. [Acta metall. mater.40, 1421 (1992)] in a companion paper, as does the qualitative nature of the R-curve with predictions by Stump and Budiansky [Acta metall.37, 3297 (1989)].

At certain grain sizes, alumina R curves have been shown to increase with crack growth. This phenomenon is in contrast to R curves for metals, which exhibit a steady-state plateau effect with increasing crack growth. Recent results show R curves for calcium silicate geomaterials also increase. The specimen geometry employed for both alumina and geomaterials is a generalized plane stress double cantilever beam (DCB). The specimens are under direct pin loading with fixed displacement. This note will discuss the common mechanisms of increasing R curve behavior of these quasi brittle materials. Finally, a hypothesis on the discrepancy between the fracture toughness of single crack rock samples and in large seismic events is presented.