In elasto-plastic fracture mechanics a number of methods for determining stable crack growth resistance curves are applied. Ultrasound is well suited for nondestructive in-situ measurements of crack propagation and has some well-known advantages over other single-specimen methods (compliance, electric potential difference method). The phenomenon exploited for crack growth monitoring in this technique is the diffraction at a crack tip. Receiving and transmitting angle-beam probes are symmetrically placed on the notched side of the test specimen. The time of flight of the tip-diffracted echo is measured. It is the most important feature of this configuration that the very first echo arriving at the receiver yields the information about the actual crack length.

Recently, an experimental device for ultrasonic crack growth monitoring on the basis of the time of flight of the tip- diffracted transverse wave has been applied to three-point bending tests of steel samples. The Elastodynamic Finite Integration Technique (EFIT) has been used to simulate the wave propagation in the cracked and bended specimen. By comparison of the simulated and measured echo trains it has become possible to identify the individual echoes. Furthermore, there is considerable agreement between the overall crack extension measured by ultrasonic means and fractographically. However, it has been found that, even when a transverse wave angle beam probe is used, the tip-diffracted transverse wave echo may be overlapped by a small longitudinal wave echo reflected at the backwall.

The present paper deals with crack growth monitoring based on the time of flight of the tip-diffracted longitudinal wave. Results of both measurements and EFIT simulations are shown and compared with one another. Pros and cons of the application of transverse and longitudinal waves are discussed.