The material chosen was a C-Mn steel plate (classification ASTM A-283, gr. C) with 38 mm thickness. The ultrasonic waves were generated by commercial surface transducers of 4 MHz and a Krautkramer equipment model USIP11. Tensile and fracture mechanics were conducted with MTS equipments.

Fracture mechanics test pieces were machined with fatigue cracks with length in the range of 7 to 13 mm, and inclined to the surface at angles of 15&3176, 30&3176 and 45&3176.

Crack depth sizing were made using the Rayleigh wave that contour the crack path, as already mentioned above, but also using the wave generated at the crack tip following the phenomenon known as conversion mode. This wave travels forth along the uncracked ligament of the test piece, and come back to the crack tip.

In order to simulate an "in service" inspection, where the loading of the equipment is to be expected, these specimens were submitted to a four point bending fracture mechanics test, so that the region of the crack tip become under an increasing tensile stress state. Thus, the ultrasonic crack sizing could be correlated to the stress intensity factor (K) existing at the crack tip. The relevance of this result is the following: a crack opens when it is mechanically loaded and its depth sizing become easier by the time-of-flight technique.

The questions is to be sure that this loading does not bring any risk of failure of the equipment being inspected.

The results showed that: - a tensile loading is necessary to bring crack sizing accuracy to an acceptable level, without any risk of failure;

• the greater the crack angle and/or the crack length, the greater is the subsizing;
• upto an angle of 15°, subsizing is still acceptable, since it does not go beyond 5% error, and this for both waves, Rayleigh and conversion mode;
• for 30° and 45°, only sizing by the conversion mode wave offers acceptable errors.

Ultrasonic Testing - Saft, TOFD, SAM and Tomography