This case study shows how an innovative nondestructive testing technique solved a difficult, and potentially very expensive, inspection problem.
A novel means of examining TV transmission masts for fatigue cracks has been developed by TWI. It works by pressurising the internal region between incomplete penetration welds using compressed air. If leaks occur it is self-evident that through cracks exist.
The joints in question are welded flanges on the main mast legs. A typical 230 high mast may have over 200 such flanges. The operators are concerned to know how much fatigue damage has already occurred in these welds under windloading. Inspection by conventional methods is extremely difficult because of poor access, the harsh environment, and heavy galvanising and paint on the welds.
The air pressure method was originally proposed by TWI as a way of overcoming the NDT problem. However, the success of the method was dependent on the joint having a sufficient reserve of strength following first leak for remedial measures to be taken before total failure.
To test this, real joints from a disused mast, fitted with the pressure system, were fatigue tested to destruction. The tests showed that, even with an invisible surface breaking crack less than 2mm long underneath the galvanising and paint, the pressure dropped to below half its original value in less than a minute. The measured life to total failure gave ample margin for action to be taken to strengthen the joints by mechanical clamping.
The next step is to install the pressure system on the most vulnerable mast, using a purpose made fixture and drill for making the 75mm long pressurisation holes in the flanges. A special drilling procedure has been developed to ensure that a clear air path is obtained even into the tightest gap. The first mast to be examined was on a mountain in Scotland.
Six flanges at each level were 'plumbed' in to a central pressure point convenient to the access ladder. Future inspections will entail pressurising each level using a small air bottle and noting any pressure loss - no more difficult than checking a car tyre!
The principle has potential application wherever partial penetration joints are used, provided that the residual life can be shown to be acceptable after first leak, either by calculation or by test. It has particular value when access is difficult as in power plant, bridges, cranes and offshore and is best designed into structures at the drawing board stage.