Time-averaged moire method for in-plane vibrational analysis

A new method is proposed for the determination of the in-plane displacement components of bodies subjected to cyclic loading. The recording on a photographic film is done statically, with exposure times longer than the period of vibration (time-average). A grating printed on the body surface is perturbed by the motion and the displacements are analyzed by photographing the grating lines. For relatively small amplitudes of vibration an alternative technique is proposed requiring the use of a master. Fringes are then photographed directly. In this case it is convenient to use a carrier which can be obtained by rotation of the master over the surface of the vibrating body. It is proposed to use Fourier filtering to convert frequency variation moiré fringes to the conventional intensity variation fringes. The application of the method is illustrated with the determination of u and ν components of displacement in a ring and in a disk subjected to cyclic diametral compression. As do other time-averaged methods, the proposed method has the advantage of permitting the static study of vibration phenomena.     

Determination of strains in photoelastic coatings

Photoelastic coatings can be cemented directly to actual structural components and tested under field conditions. This important advantage has made them relatively popular in industry. The information obtained, however, may be misinterpreted and lead to serious errors. A correct interpretation requires the separation of the principal strains and, so far, this operation has been found very difficult. Following a previous paper by one of the authors, it is proposed to drill small holes in the coating and record the birefringence at points removed from the edge of the holes. The theoretical background of the method is reviewed, the technique necessary to use it is explained and two applications are described. The precision of the method is evaluated and found satisfactory in contradiction to information previously published in the literature.