NEWSLETTER

Structures subjected to stress may accumulate damage through cyclical mechanical and thermal loading, corrosion or other chemical degradation, radiation damage or sudden impact. Structural integrity monitoring may be used to identify the accumulation of damage and schedule repair or replacement of worn out parts to prevent failure whilst reducing operating costs and maintaining safety.

There are several aspects to AEA Technology's approach to structural integrity monitoring. One aspect is simply looking for defects directly - this is usually classed as inspection and whilst AEA Technology has developed and applies many inspection techniques, this is not what is implied in the SIMoNET context of structural integrity monitoring. There are, however, at least three more aspects to structural integrity monitoring.

First, there is the logical extension of inspection, namely, looking at structures in some way over time to see if damage accumulation can de detected without the need to seek individual defects. This might be applied to corrosion, for example. AEA Technology make a system called FLEXIMAT which consists of a flexible multi-element array of ultrasonic transducers, bonded to a thin flexible printed circuit strip, approximately 450 mm long and 40 mm wide. It has the flexibility to be permanently attached to piping or vessel walls without maintenance throughout the life of the component. It is equipped with a remote data retrieval facility which makes it ideal for situations that would otherwise necessitate insulation removal, erection of scaffolding, or plant shut down in hazardous areas.

Secondly, there is an aspect in which plant operating conditions such as pressure, temperature or rotational speed are continuously monitored and used to identify operating cycles and their associated thermo-mechanical stress as they occur. Creep fatigue damage models are incorporated to identify mechanical damage according to actual plant usage. This approach is used to monitor the damage to a number of power station boilers around the world . It is also forms part of the TURBOlife software suite for monitoring real time damage to gas turbine, hot gas path components and is installed in the UK on gas turbines used in natural gas compression. The approach allows operators of a wide range of equipment to arrange maintenance 'as needed' rather than 'as scheduled'.

Thirdly there are situations in which sensors can be embedded in a structure, or retrofitted, to measdure damage accumulation directly. For example, on average every day in Britain two railway bridges are struck by lorries. Trains then have to be halted so that experts can check that the bridge is safe. Passengers are meanwhile left frustrated and angry as their journeys are delayed, although in many cases the damage is minimal and the bridge structure remains perfectly safe. AEA Technology's new Bridge Bash Monitor system involves placing sensors on the bridge linked to a nearby computer. When a lorry hits the bridge, a warning is automatically faxed to a control room. The controller can then immediately survey the scene and decide whether the damage is serious enough to make it necessary to stop trains.

In a project with Cranfield University, AEA Technology have developed and tested a fibre optic sensor which has been used to measure strain in composite panels bonded to an old bridge as stiffereners. The sensor has alo been used on pressure vessels. Between 10 and 20 local strain sensor points can be put on to a fibre so two fibres can give up to 40 measurement points. Fibres can be wound to follow almost any desired path around a component. Results from these tests suggest that this will be a valuable, easily transportable, monitoring technique.