The Library Shelf

The first number of the sixteenth volume in this series contains eighteen papers covering the usual wide range of topics. The longest paper describes a photoelastic investigation carried out in the U.S. Naval Research Laboratory, Washington, D.C., to determine the dynamic stress distribution surrounding a running crack. Iso‐chromatics were obtained using the Schardin multiple spark technique, with the optical system modified for the method of fringe multiplication. Principal stress distributions were found by means of an optical interferometer with single spark illumination, using a number of similar models. Principal stresses were corrected to allow for the variation of stress‐optical sensitivity with time. It was found that the dynamic distribution approximates to that in a statically loaded specimen extended at its ends by a fixed displacement. The zone of stress disturbance grows in all directions proportionally to crack length. The average stress on the remaining uncracked cross section increases with length of crack to a maximum of about 1½ times the original value. Measured terminal crack velocities agreed well with calculated values. The discussion compares the results with theoretical analysis.