A closer observation of a thin, compressed powder layer shows that the photoelastic effect in glass powder mass is proportional to the difference of the principal stresses. Technical problems in experimental procedure, such as preparation of powder, photographing, temperature control etc. are discussed. Displacement of the powder particles or point-to-point deformation can also be observed in the process of this experiment. Some examples of experimental results are given to show how much can be acquired by applying this method to engineering. For example, the results of the experiments determine an optimum punch width to extrude powder through a nozzle.
A new photoelastic method of experimentation for stress analysis of orthotropic materials made of polyester resin and fiber-glass cloths, the “Fiber Reinforced Plastics (F. R. P.), is presented. Required orthotropy and good photoelastic properties can be obtained by proper choice of resin and fiber-glass cloth. From the results of tests on such F. R. P. plates, the following basic rules are derived: (1) the strain sensitivity of F. R. P. plates is independent of the direction of strain and has the same value over the whole plate: (2) photoelastic fringe patterns obtained on usual outfits of photoelastic experiment give loci of points at which the difference between principal strains is constant. On the basis of these rules, the fundamental of this method of experimentation has been established. To test the availability of this method and to study the problem of stress concentration in orthotropic plates, experiments have been made on specimens of rectangular shape with a circular hole in center subjected to uniform tension applied at both ends with the result that the practical usefulness of this method is promising and that the circumferencial stress distribution of the hole obtained experimentally agrees fairly well with that obtained theoretically on a plate of infinite width.
The birefringence of single crystals of NaCl type caused by compressive stress is measured for mercury light 5461 A by the use of Babinet's compensator. Samples, NaCl, NaBr, KCl, KBr and KI, were prepared from melts. The sign of photoelastic coefficient is positive in Na salts and negative in K salts. With the increase of atomic number of anions, the coefficients for Na salts become larger and those for K salts smaller. The coefficient for Na salts decreases as the stress is progressively increased, whereas the coefficient for K salts increases up to the stress of about 40kg/cm2. The theoretical values of birefringence at the stress value of 1kg/cm2 are calculated to be 0.009 and 0.040cm/kg for NaCl and KCl, respectively. The former is in fair agreement with the experimental value of 0.008 for NaCl but the latter is not with that of -0.003 for KCl.
A new method for measuring stress-live, residual or that due to dead load-on the surface of metallic structures is described. The method is based essentially on the elastic redistribution caused by removal of a small portion of the concerned part of structure, that is the stress relief method in combination with the photoelastic coating method. A thin sheet of appropriate photoelastic material is cemented on the metal surface, and a small circular hole (for instance, 2.0mm in diameter) is drilled through the film into the metal to a certain depth, then the photoelastic stress pattern is visualized by means of a reflection type polariscope. Both the magnitudes and the directions of two principal stresses can be easily obtained by counting the photoelastic coloured or monochromatic fringes. However small the hole is drilled, the sensitivity of the method is not essentially Towered. Therefore this method is considered most non-destructive among the known mechanical methods of measuring residual stresses. The principle and technique of the method together with the sensitivity and the accuracy are described.