REVERSIBLE BONDED STRAIN GAGE :: essays research papers

ABSTRACT For accurately measuring thermal strains, particularly on large structures where welded strain gages cannot be used, a reversible bonded strain gage was developed. Basically it is a special polyimide strain gage which is same on both the base side and cover side so that it can be used both ways. It can be used to measure strains at temperatures under 250oC (482oF) of a structure made of aluminum alloys or composites (to which its difficult to weld a strain gage).These gages can be can be peeled after taking required apparent strain measurements in a furnace and can be attached reverse side up at a required point on a structure. To measure mechanical stresses on structures at high temperatures it is essential to measure apparent thermal stresses accurately in the first place. In practice, several strain gages in a pack are used to obtain calibration data. The apparent strain and gage factor change of all the gages in the pack are assumed to be same which is not so in practice, in spit e of great efforts to reduce scatter of apparent strain. Since reversible strain gages can be reattached to the test structure after taking apparent strain readings, the error caused due to apparent strain scatter (by using different strain gages) can be reduced to great extent. In this paper the thermal characteristics of the reversible strain gage – repeatability of apparent strain, gage-factor change, creep, drift and the output for a given mechanical strain – were investigated. INTRODUCTION There are several problems associated with elevated temperature measurements, static or dynamic, the basic one being that alloys useful as strain gages at these temperatures are also excellent temperature sensors. Firstly, installation of the strain gage is a problem and secondly the apparent strain and change in gage factor makes it very difficult to measure the actual strain. In aerospace industry we come across a lot of situations when very accurate strain measurements at high temperatures are required, but in spite of a lot of improvement in new high temperature strain gages most of them are welded types. Hence, they cannot be used on materials like aluminum alloys or composites. In this paper, a reversible bonded strain gage is described for use at temperatures under 250o (482oF) that can be applied to a structure made of materials commonly used in aerospace industry like aluminum alloys and various composites. Aircraft wings are often subjected to high temperature and high acou stic noise level and the application of reversible strain gages to accurately measure the stresses is the main motivation behind choosing this paper for review.