A method of studying the formation of cracks in a material subjected to stress

A method is described to determine the onset of cracking in specimens of concrete subjected to tension or compression in mechanical testing machines. While the loads were being applied, measurements were made at intervals of the velocities of ultrasonic pulses passing through the test-piece in the axial and/or transverse directions. In compression, the velocity of the ultrasonic pulses in the direction of loading remained constant while the load was increased to failure, but in the transverse direction a fall in the velocity started at only a fraction of the ultimate load, and the velocity then decreased with increase of load. This indicated that cracking occurred internally parallel to the direction of loading. The load at which it started depended on the strength of the concrete and the uniformity of the stress distribution. In tension, fracture was preceded by only a very small, and often insignificant, amount of cracking which occurred at right angles to the direction of loading.