Wrinkles widely existing in sheets and membranes have attracted a lot of attention in the fields of material science and engineering applications. In this paper, we present a new method to generate ordered (striplike) and steady wrinkles of a constrained dielectric elastomer (DE) sheet coated with soft electrodes on both sides subjected to high voltage. When the voltage reaches a certain value, wrinkles will nucleate and grow. We conduct both experimental and theoretical studies to investigate the wavelength and amplitude of the wrinkle. The results show a good agreement between theory and experiment. Moreover, the amplitude and wavelength of ordered wrinkles can be tuned by varying the prestretch and geometry of the DE sheet, as well as the applying voltage. This study can help future design of DE transducers such as diffraction grating and optical sensor.

The present paper introduces a variable sensitivity displacement interferometer (VSDI) used to monitor both normal and in-plane particle displacements in wave propagation experiments. The general system consists of two interferometers working in tandem. Normally reflected light is interfered with each of two symmetrically diffracted light beams generated by the specimen rear surface. In cases where the surface motion simultaneously exhibits both in-plane and normal displacements, the fringes represent a linear combination of the longitudinal and transverse components of motion. Decoupling of the normal and in-plane displacement histories may be achieved through a linear combination of the two VSDI records. Alternatively, it is always possible to decouple the components of motion by combining a VSDI record with an independent measurement of either component. Moreover, it is shown that in the case of pure normal motion, the VSDI system functions as a desensitized normal displacement interferometer (DNDI). Similarly, in situations involving purely in-plane motion, the VSDI is shown to function as a desensitized transverse displacement interferometer (DTDI). The DNDI and DTDI fringe sensitivities are in general shown to depend on the angle θ or equivalently, the frequency σ of a grating manufactured at the observation point and the order n of the diffracted beams. The variable sensitivity feature of the VSDI greatly desensitizes normal displacement measurements and is particularly well suited for wave propagation studies in which normal particle velocities in excess of 100 m/s are generated. Experimental results are presented which demonstrate the application of this technique to monitoring particle motion histories in plate impact recovery experiments.

Using the method of electron beam moire´, a small region at an interface of a [O 2 / 90] s glass fiber reinforced plastic composite has been examined during tensile testing. The tensile test was conducted inside a scanning electron microscope, with a high spatial frequency line grating (10,000 lines/mm) at the interface between a longitudinal ply and a transverse ply. During the test, this region was observed at a magnification of 1900 × . Local strain measurements were made by interpreting the moire´ fringe patterns over gage lengths that varied from 10 to 30 μm. The magnitude and distribution of the local strains depended on the damage that occurred with monotonically increasing load. Load shedding by the transverse ply was evident from the fringe patterns. Extremely high local strains were observed: longitudinal fiber strains up to three percent, normal strains up to three percent, and shear strains up to 40 percent in the epoxy matrix.

This paper investigates the possibilities for improving the accuracy and the sensitivity of the projected fringe moire technique in combination with digital image processing. Several techniques for improving the accuracy of various moire methods are reviewed. The method of translation of the grating between exposures is investigated, both theoretically and experimentally. It is found that the sensitivity and accuracy can be increased by at least one order of magnitude using this method. Also the “hills and valleys” problem is easily solved by this method.

Experimental results are presented for impact of two parallel plates of 6061-T6 Aluminum, skewed at an angle of 26.6° from the axis of the projectile. A transverse displacement interferometer (TDI) [1] with a 200 lines/mm grating is used to monitor the transverse motion of the rear surface of the aluminum target plate. Two first-order diffracted laser beams are used for this TDI with a resulting sensitivity of 2.5 μm per fringe. In addition the normal motion of the rear surface is monitored simultaneously by means of a velocity interferometer [2] in which the zeroth-order diffracted beam is used as the beam reflected from a moving mirror. Comparison of the velocity-time profiles of the target rear surface with those predicted by the analysis given by Abou-Sayed and Clifton [17] indicates that the computed transverse velocity-time profiles have regions of steeper slope than observed in the experiments. This discrepancy appears to be mainly due to the inadequacy of the assumption of isotropic hardening and the Huber-Mises yield function in the analysis [17]. The sensitivity of the transverse velocity profiles to the plastic flow characteristics of the material suggests that the pressure-shear impact experiment, when used with a TDI, is a good technique for the study of material properties at very high strain rates (10 4 ∼ 10 5 sec − 1 ) and under postshock conditions.

A method is proposed whereby the diffraction pattern produced by the distorted grating obtained from the Ligtenberg method can be suitably filtered to give lines of constant curvatures in problems of flexure.

This paper deals with the experimental determination of velocities of displacement in a sphere subjected to a diametral load. The maximum applied load reduced the diameter of this sphere by 27 percent. The velocities are given for each step of loading and were determined from a model made of polyurethane rubber that behaves linearly. The velocities were obtained by superposition of photographs of 500 lpi gratings embedded in the meridian plane of the sphere. The evaluation was made both point-by-point and by photographic recording of the whole field of isotachics. The errors produced as a consequence of the motion of points are evaluated and criteria are given on the best method to use and on whether corrections are necessary or not.

A transparent material with variable modulus of elasticity has been manufactured. It exhibits good photoelastic properties, and can also be strain analyzed by printing gratings on its surface and producing the moire´ effect. The new material is used to stress and strain analyze a disk of variable E, subjected to diametral compression. The discussion of the results obtained suggests that the stress distribution in the disk of variable E is practically the same as the stress distribution in the homogeneous disk. It also indicates that the strain fields in both cases are very different, but that it is possible, approximately, to obtain the strain field from the stress field for an homogeneous material using the value of E at every point, and Hooke’s law.

Using a spatial filtering technique, moire´ patterns can be observed that correspond to different forms of the indicial equation describing the parametric properties of gratings and moire´. In particular both the additive and subtractive moire´s are obtained individually over the field. This method, combined with a previously developed method for obtaining spatial partial derivatives from deformed gratings, permits the experimental determination of the whole field of Cartesian shears and rigid rotations.

The moire´ method is combined with hologram interferometry to obtain the three displacements of an arbitrarily deformed plane in the three-dimensional space. Double beam interference patterns are utilized. The interfering beams are obtained from the diffraction patterns of a grating printed in the analyzed plane. The in-plane and the out-of-plane displacements are measured in separate steps and yield separate patterns. The patterns are generated by double exposure and observed by a wave front reconstruction process. The experimental results included in the paper show a good agreement with theoretical results, proving the feasibility of the proposed technique.

The knowledge of the partial derivatives of displacement components is essential in strain analysis. Two methods of determining these partial derivatives using moire´ effects are presented in this paper. One consists of superposing two shifted copies of the same deformed grating of lines. The other consists of superposing two shifted copies of moire´ patterns of displacement components. Explanations of the phenomena, based on the parametric properties of superposed families of lines, are given. Applications to the determination of the nonlinear deformation and strain tensor are included.