Development of Ultrasonic and Microwave Techniques for Detection of Decay in Wooden Cross-arms

Most of the cross-arms of the 275 KV and 123 KV transmission lines in Malaysia and certain countries are using hard wood especially from chengal species. Due to natural weathering this wooden cross-arms is degraded and decayed. For safety reason there is a need for a simple, light, and accurate non-destructive decay detection. In this project the microwave reflection technique and ultrasonic method which is based on the transit time have been developed for in-situ measurement. In this study, we categorize the decay on the wood into three stages of decays namely severely decay, incipient decay and sound wood. In microwave method these stages can be related to the amount of water that can be absorbed by weathered wood and secondly on the wood density at particular moisture content especially at environmental moisture content. This method managed to detect inner decay up to 2 cm from the surface of the wood. In ultrasonic method, an investigation has been done to find suitable method of measurement, especially detection of decay under the metal block. Appling this detection system the transit times along l-l direction for sound wood, incipient wood, and cracked wood fall under the ranges of 100μs to 200μs, 200μs to 500μs, and above 500μs respectively. To complete the study, several prototypes have been developed, which can be used for field-testing or maintenance work. Introduction: In Malaysia, 90% wood poles and wooden cross-arms are used as transmission line supports for power pylon towers of power rating 275KV and 132KV. The hardwoods from chengal species are used for this purpose. The hot and humid climate in tropical countries like Malaysia facilitates the growth of fungi and other destructive microorganisms in wooden structures. Thereby, deterioration of wooden structures occurs in the form of splits, cracks, and other defects that permit the entry of fungi into the cores of the poles, rendering them unsuitable for use as line supports.