It is well known that eddy current testing is sensitive only to the surface defects of bars, but not good to the internal defects of bars, especially much more difficult to the defects in the central parts of the bars' cross-section. Besides, the probability of defect appearance in the central parts of bars is greater. It is obvious that eddy current testing method is far from meeting the demands of the flow detection of small bars.

In order to carry out an omnibearing ultrasonic testing, namely, a no dead zone ultrasonic testing, for small bars whose diameters are from 3 to 25 mm, in this paper, a newly developed ultrasonic testing technique has been put forward, namely, a new method of immersion shear wave testing which combines a newly developed special 5 MHz composite line focused shear wave probe for no upside dead zone with a conventional ultrasonic flaw detector and chooses the artificial defects of FBH (flat bottomed hole) and crack to detect small bars. At the same time, this paper gives a detailed description to the principles, structures, functions and properties of the composite line focused shear wave probe for no

upside dead zone and its behaviour when it is used to detect the internal defects of bars with different diameters. The results of plenty of experiments show that this probe and a new ultrasonic testing technique are very satisfactory for an omnibearing ultrasonic testing of the bars with 3 to 25 mm diameters.