A small robust weldable gauge was developed. The gauge resistance, gauge factor, thermal output and thermal hysteresis were satisfactory. The required thermal draft was not achieved.
The strain gauge comprises a thin metal film which acts as a strain sensing element. This is encapsulated between 2 very thin glass films, which serve to passivate and to electrically isolate the gauge from a metal foil on which it is formed, and which is welded to the structure being monitored.
The strain gauges were made by forming a thin glass film on a stainless steel foil, using physical vapour deposition (PVD). The stainless steel was etched to contain stress relieving slots. A thin film of nickel chromium alloy was then grown by PVD on the glass film, then etched to form 2 resistors in the form of a half bridge. The device design was such that one element of the bridge, and the other was not. A further layer of glass was formed in top of the resistive elements to prevent high temperature oxidation. Contact pads were formed through the passivating glass layer to enable electrical connection between the gauge elements and the high temperature cables leading to the measurement instrumentation.
The gauges were enclosed in steel packages and spot welded to the structures where strain was to be measured. Both the gauges and their packaging configuration were intended to withstand continuous exposure to temperatures of at least 565 C.
THE AIM OF THE PROJECT IS TO DEVELOP A MINIATURE RESISTIVE STRAIN GAUGE WHICH CAN BE WELDED TO STEEL STRUCTURES REQUIRED TO OPERATE UNDER STRESS A HIGH TEMPERATURES AND THUS PERMIT IN SITU MEASUREMENT OF STRAIN IN THE STEEL.THE MAIN APPLICATION ENVISAGED IS MEASUREMENT OF LONG TERM CREEP IN HIGHLY STRESSED COMPONENTS IN ELECTRICITY GENERATING STATIONS WHERE ACCURATE ASSESSMENT OF THE SAFE OPERATING LIFE-CYCLE OF PLANT IS OF GREAT ECONOMIC IMPORTANCE.
Funding SchemeCSC - Cost-sharing contracts
EC1A 7A4 London
KT22 7SA Leatherhead, Surrey