Setting standards of performance and reliability

The DryScan 410D or Shadow Technique is a proven method in the Non-Destructive testing of high technology materials used in the Aerospace and Aviation Industry, especially Hybrids, Composites and Honeycomb structures, which cannot be examined successfully by conventional NDT methods. This technique can be used in the through transmission mode i.e. with the signal passing directly through the test material from the transmitter to the receiver, and also when working with the transmitter and receiver transducers on the same side of the specimen.

The dry coupling technique offers a number of considerable advantages. For example when a wide spectrum of transmitted signals is used, many of the geometric considerations conventionally applied in ultrasonic's can be ignored. Typically, the transmitter and receiver transducers need not be in alignment, they can be oriented at a right angle to each other. This characteristic, plus the fact that the transducer tip can be as small as 5mm diameter, enables testing to be achieved that cannot be dealt with by conventional means.

With the Dryscan technique, highly effective signal transmission can now be achieved by using a completely dry contact system. The ultrasonic energy is coupled between the transducers and the material under test by using special plastic pads on the transducers, eliminating the need for a coupling medium. Transducers can be held or they may be roller type for continuous pass scanning using various manual or automatic manipulation systems, which can be linked to C-Scan recording systems. The Dryscan 410D concept is simple to apply and requires only a minimum of operator training. This technique results in no possible hazards of radiation, contamination or degradation of the test pieces.

Advantages of the System

The main advantages of the Shadow Technique is summarized below:

	No coupling medium is required to contact the transducers acoustically to the test material.
	Many of the geometric considerations applied to conventional ultrasonic detection methods can be ignored.
	The system is highly reliable for fault detection.
	The method can be used for many materials as a GO or NO-GO system - semi - or fully automatic.
	In many cases, materials surface preparation is not necessary.

Interpretation of the Results

With the Dryscan type of transducer, the received signals produce a typical pattern.

The first group of signals in this pattern usually contains from 7 -10 cycles.

The condition of the material is decided from three factors:

	The intensity of the received signal
	The displacement or the starting point of the first half circle of this wave on the Time Base.
	The shape of the displayed interference pattern.

As a general rule, the structure is considered to be good provided:

	The first group reaches full scale with a specified dB value.
	The waveform patterns look normal i.e. the first group contains between 7 and 10 cycles.
	There is no sudden change in intensity between adjacent cycles.

Applications