Liquid couplant is required or transmission of ultrasound by removing the layer of air between the face of ultrasonic probe and the material to be inspected. A preliminary survey was conducted and a number of readily available solid materials were identified which, were thought, would potentially replace the liquid couplant.

The amount of ultrasonic transmission through all the above materials were measured by applying varying amounts of pressure on the transmitter and receiver to squeeze out the air trapped in the interface. It was noticed that the amount of transmission varies with pressure in the case of all these materials. When closely watched, it was understood that in a few materials, this variation is minimum and maximum transmission can be attained at very low pressures. Common properties of all these materials were particularly investigated in order to identify which are the required properties that play dominant part in transmitting ultrasonic energy without applying external pressure at the transducers. It was the sticky nature of the surface of these materials that enabled transmission without pressure and experiments were conducted to quantify the sticky property.

After studying the effect of each ingredients of these compounds, a number of new compositions were prepared and the required properties were improved. Finally, a suitable composition was arrived at taking into account that an easily replaceable face of the conventional contact probe could be fabricated from the above material. Experiments were also conducted after making the replaceable face of the probe in order to arrive at a suitable thickness of the couplant material.

After successful fabrication of the replaceable face for conventional contact probe, procedures were developed to implement the system for mapping and locating the debonds. Studies are also continuing to improve the strength of the material to improve the life of the replaceable face so as to enable us to use this system for conventional ultrasonic flaw detection where use of liquid couplant is highly inconvenient.