The determination of the components of the strain tensor in holographic interferometry

The general problem of obtaining the derivatives of the displacements in holographic interferometry is analyzed. Expressions for the most general case are derived and a particular solution is suggested. A data-reduction method is proposed. Application examples are given.     

Measurement of apparent young's modulus in the bending of cantilever beam by heterodyne holographic interferometry

Detection of micromechanical phenomena in material requires a sufficiently high-accuracy measurement. This paper shows the possibility of applying heterodyne holographic interferometry to such experimental verification. The effects of the ratios of thickness to grain size on the apparent Young's modulus are evaluated based on the deflection patterns of the cantilever beam. The characteristic length ℓ of couple-stress theory is calculated by applying the analytical results of Koiter to our experimental results. This value is about one fifth of the grain size.     

A new holographic interferometer for stress analysis

In holographic interferometry with a photoelastic model, two families of fringes are generated simultaneously when the model is stressed. One family represents the isochromatic-fringe pattern normally associated with photoelasticity which yields the difference between the principal stresses. The other family represents the isopachic-fringe pattern associated with interferometry which yields the sum of the principal stresses. From these complementary patterns, the magnitudes of the principal stresses can readily be determined throughout the field of observation. Unfortunately, these fringe patterns are not completely independent but interact in such a way as to make interpretation difficult in critical regions of the model. A new system has been developed which readily permits simultaneous acquisition of these fringe patterns without their undesirable mutual interaction, as well as providing increased sensitivity. This new interferometer uses a double-pass object beam and an optical rotator to eliminate the isochromatic-fringe pattern and its effect from the isopachic interferogram. Such a system has considerable value in experimental mechanics for applications to both static and dynamic model studies and to materials investigation.     

The determination of stress and strain concentrations at an ellipsoidal inclusion in an anisotropic material

The aim of this paper is to produce elementary yet explicit formulae for the evaluation of stress and strain concentration factors at an ellipsoidal inclusion, for arbitrary anisotropy, under uniform loading at infinity. The results are such that the required formulae do not involve the solution of any boundary value prolems or the knowledge of any Green's functions. An important feature of the analysis is that the solution of the interface problem is intimately related to the solution of the inclusion problem.     

Residual-stress determination through combined use of holographic interferometry and blind-hole drilling

Holographic interferometry is used to determine in-plane radial displacements due to release of residual stresses by hole drilling. A method is derived for relating radial displacements measured in three directions of illumination to the state of residual stress, analogous to relations used in the conventional strain-rosette technique. Residual stress is produced by an interference fit of two circular tubes. Agreement between stress determined holographically with a computed value and with that determined by the conventional technique is good. Advantages of the holographic technique in overcoming various shortcomings of the conventional technique are discussed. A modification of the holographic technique involving data collection in only two directions of illumination is described.     

The determination of mode I stress-intensity factors by holographic interferometry

The use of linear elastic fracture mechanics generally depends upon the availability of suitable analytical or numerical solutions for the relevant crack-tip stress-intensity factor,K. Convenient experimental verification of such solutions is a valuable aid to their correct application and can provide a practical substitute in real design situations of great complexity. A convenient, new experimental technique for estimating the Mode I stress-intensity factor using holographic interferometry and test pieces cut from thin sheets of commercially available polymethylmethacrylate is described and demonstrated. The test pieces can readily be prepared to model any desired Mode I geometry and boundary conditions. In addition, a prior self-calibration procedure can be employed to enhance both convenience and accuracy. Real-time interference-fringe data from the crack-tip region are easily reduced and plotted to yield a straight line whose slope provides a one-parameter evaluation of the effect of geometry on the stress-intensity factor. This information, together with the crack length and applied stress, completely definesK.     

Strictly conjugate stress and strain tensors

a subclass of strictly conjugate tensors, namely, the tensors that satisfy the requirement for transformation by the same law upon rigid motion of the neighborhood of a material particle, is separated into the class of work-conjugate stress and strain tensors. The advantage of the use of strictly conjugate stress and strain tensors in formulating the variational principles for bodies from a hyperelastic material is shown. Lavrent'ev Institute of Hydrodynamics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 41, No. 3, pp. 149–154, May–June, 2000.     

Hybrid stress analysis of vibrating plates using holographic interferometry and finite elements

The work reported herein is concerned with an investigation of the potential of mating time-average holography and static-finite-element analysis (a ‘hybrid’ stress-analysis technique) in solving dynamic problems. The model studied is a vibrating thin rectangular cantilever plate. Realtime holography was used to locate natural vibrating modes of the plate. Experimental data were smoothed to remove scatter using a spline-like procedure in one or two dimensions, as applicable. Two methods of smoothing the experimental displacement data used in the hybrid procedure were considered. The first involved preprocessing (smoothing) the data before they were incorporated into the numerical model, while the second involved smoothing the data as part of the solution process. In both instances the amount of smoothing was specified by the user. The first and fourth vibrational modes were investigated. The results were compared to a NASTRAN dynamic solution and to a Ritz method series solution with very good results.     

A method of three-dimensional strain measurement on non-ideal objects using holographic interferometry

Holographic interferometry has been applied to determine the components of tensile, compressive and shear strain in the surface layer of a non-ideal object subjected to high values of stress. The method consists of determining the three components of the vector displacement at a sufficient number of discrete points on the surface of the object. Functions in the form of truncated power series are fitted to the fringe order-distance data using the method of least squares. Interpolation of these functions is then carried out to obtain the fringe-order numbers at closely spaced equal intervals. The displacement in each interval is calculated and the displacement-distance relationship is then numerically differentiated to obtain strain. Components of stress at a particular point are then estimated by multiplying the strain value by the appropriate modulus. These stresses result from the application of considerable forces that are likely to cause large rigid-body displacements unless the object can be satisfactorily restrained. The difficulty of providing the required restraining forces has been avoided by mounting the object on a kinematic mount from which it can be removed before stressing and replaced afterwards. The accuracy of relocation of the object is sufficient to ensure the satisfactory formation of holographic interference fringes. As the use of the two-exposure holographic method results in a direction ambiguity of the displacement vector at any point of the surface, this method is complemented by electrical strain measurement. The strain is sampled in two convenient orthogonal directions at some part of the surface. The application of the uniqueness theorem of elasticity then permits the direction of displacement to be uniquely specified at any point of the surface. The electrical strain measurement also provides a check of the strain values determined by interferometry at various positions on the surface. This check is necessary since precise knowledge of the surface geometry of a non-ideal object may be lacking. Incorrect assumptions made about the geometry are shown to result in serious errors in the evaluation of strain. The method described has been used to estimate the stresses in the vicinity of a threaded hole in a sample of pipe for representative values of torque applied in screwing a connecting member into the pipe. Possible reasons for failure of the pipe in service are then deduced.     

A fringe-compensation technique for stress analysis by reflection holographic interferometry

The use of holographic interferometry for stress analysis of nontransparent objects is limited by rigid-body displacements of the object. These displacements can alter the fringe patterns and often cause the fringes to disappear completely. A technique of compensation for this deterioration of the fringe pattern forreal-time holographic interferometry is described in this paper. It is especially designed to permit the accurate measurement of the out-of-plane component of strain near regions of stress concentration in plates that are subjected to in-plane loading. It is first shown that the fringes caused by a pure rigid-body displacement can be eliminated almost completely by translations of the hologram and rotation of the illumination wave. This procedure is first described when the displacement is known; then when it is unknown. A method to estimate the error made in the correction is presented. In actual stress-analysis problems, the object is both rigidly displaced and strained. Assuming the rigid displacement is known and corrected as previously, the analysis is developed to relate the fringe pattern to the strain-related displacement. This analysis takes into account the optical modifications of the system that are necessary to achieve the rigid-body-displacement correction. When the rigid-body displacement is unknown, the method is shown still to be workable through the use of various symmetries and boundary conditions. Two sample interferograms are presented as illustrations. Quantitative treatment of data from one of these are presented in a companion paper.     

A new notation for stress and strain

This note is concerned with a new notation for the stress and deformation gradients. It is intended to be both clear and elegant.     

The OPCM strain gauges for strain and stress measurement of orthotropic material structures

The orthotropic mechanical sensor of piezoelectric composite material made from piezoelectric ceramic and resin materials and their sensing mechanism are presented. The sensing equations of the adhered-and embedded-type sensing units are deduced, which are used to detect the stresses in orthotropic material structures. The surface strain of the orthogonal plate is measured under the action of the planar stress field, and the error is analyzed. Supported by the National Natural Science Foundation (No. 59635640), the Science Foundation of Jiangsu Education Committe (99KJD130001) and the Science Foundation of Jiangsu Province (BK99116).     

Application of the holographic interferometry method to determine the stress intensity factor

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 1, pp. 155–158, January–February, 1990.     

Measurement of the strain field near a crack tip in polymethylmethacrylate by holographic interferometry

An experimental method has been developed for the detailed, real-time study of the strain field near a crack tip in any transparent material. This method combines holography and interferometry to measure the field of averaged transverse strain in a sheet specimen under inplane loading. It is shown that the solution for an edge crack in a semi-infinite sheet based on two-dimensional linear elastic theory can be applied to the finite-width polymethylmethacrylate specimen. This solution is observed to become less valid as the crack tip is approached due to the breakdown of the plane-stress assumption. This effect is evaluated for a range of thicknesses from much less to much more than the crack length.     

On the determination of deformation from strain

The problem of finding a deformation corresponding to a given Cauchy–Green strain is approached with a procedure that employs the Gram decomposition of the deformation gradient. It is shown that the rotation occurring in that decomposition can be obtained by solving a system of partial differential equations in the group of rotations or in its Lie algebra. The equivalence between the integrability conditions of these two systems and those of the systems of equations arising in the usual procedures for determining a deformation from the strain is proved. Examples of application of the proposed procedure are given.     

The stress conjugate to logarithmic strain

A stress S is said to be conjugate to a strain measure E if the inner product S · E̊ is the power per unit volume. The logarithmic strain In U, with U the right stretch tensor, has been considered an interesting strain measure because of the relationship of its material time derivative (In U) with the stretching tensor D. In a previous article (Int. J. Solids Structures 22, 1019–1032 (1986)) a formula for (In U) was obtained in direct notation for the cases where the principal stretches are repeated, as well as for the case where they are all distinct. Here the formula for (In U) and the definition of conjugate stress are used to derive an explicit, properly invariant expression for the stress conjugate to the logarithmic strain.