Experiments for the Determination of Transient Stress and Strain Distributions in Two-Dimensional Problems

The object of this investigation was to develop a method utilizing photoelasticity for determining transient stress and strain distributions in two-dimensional problems. Numerous investigators have approached this problem using the common photoelastic materials which have a relatively high modulus of elasticity and correspondingly high velocity of wave propagation. However, many difficulties were encountered in photographically recording data, and only a few reliable stress patterns were obtained. In order to avoid these difficulties, a material with a low modulus of elasticity and a correspondingly low velocity of wave propagation was developed. As a result the 8-mm. Fastax camera is capable of recording precise fringe patterns. The material used was a member of the epoxy-resin family, modified to give the desired properties. Both its static and dynamic properties were determined as accurately as possible. It was found that the strain-fringe value of the material is approximately constant, but the modulus of elasticity and stress-fringe value are different for static and dynamic loadings. Preliminary studies were conducted to develop the method using a circular disk under a radially applied concentrated impact load as the model. A simply supported beam under central impact was then studied, and deflection curves obtained were compared with curves theoretically predicted by Saint Venant and Flamant. The comparisons showed good agreement. An analysis of the formation of the fringe pattern for various times after impact also was made.