Specialised equipment for measuring residual stresses using the centre hole strain gauge method has solved the problem of locations with restricted access. Conventional centre hole equipment requires headroom of at least 200mm and clearance of about 30mm from internal corners. TWI's Structural Integrity Department has developed Tubestress for measuring residual stresses inside 70mm diameter tubes, and Cornerstress for measurement locations at 11mm from an internal corner.
Both sets of equipment have a borescope for viewing the centre hole gauge and a tungsten carbide nozzle for machining the centre hole - typically 2mm deep, 2mm diameter - using a jet of abrasive powder.
The borescope and nozzle are mounted on micrometer controlled, three-axis slides. The slides are used in conjunction with the borescope for initial alignment of the equipment with the strain gauge, moving from the viewing position to the machining position and measuring the diameter and position of the hole. Accurate alignment of the hole with the centre of the gauge is critical: the tolerance on hole eccentricity is 20 microns.
TWI Tubestress and Cornerstress equipment has been used in major projects for clients in power generation, defence and nuclear waste disposal applications. The equipment can be customised for measurements in other locations with limited access.