INTRODUCTION

A strain-sensitive material is one whose electrical resistance is proportional to the instantaneous spatial-average strain over its surface. Such materials are of two types: metallic (i.e., foil or wire) or semiconductor (described in Chap. 12 under Piezoresistive Accelerometers). When such a material is stretched, its length increases and its cross-section decreases; consequently, there is an increase in its electrical resistance. This change in resistance is a measure of its mechanical motion. Thus, a strain gage is a device which uses change in electrical resistance to measure strain.

The resistance strain gage may be employed in shock or vibration instrumentation in either of two ways. The strain gage may be the active element in a commercial or special-purpose transducer or pickup, or it may be bonded directly to a critical area on a vibrating member. Both of these applications are considered in this chapter, together with a discussion of strain-gage types and characteristics, cements and bonding techniques, circuitry for signal enhancement and temperature compensation, and related aspects of strain-gage technology.

The electrical resistance strain gage discussed in this chapter is basically a piece of very thin foil or fine wire which exhibits a change in resistance proportional to the mechanical strain imposed on it. In order to handle such a delicate filament, it is either mounted on, encapsulated in, or bonded to some type of carrier material and is known as the bonded strain gage. Bonded strain gages are available in a wide range...