Reversible Bonded Strain Gage

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 spite 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 acoustic noise level and the application of reversible strain gages to accurately measure the stresses is the main motivation behind choosing this paper for review. These gages have an additional advantage that the adhesives used to fix the gage cures at room temperature unlike most adhesives for high temperature strain measurements which needs curing heat cycles (like 1 hr at 180o). As a result of this heating of a structure during curing can be avoided.

BACKGROUND

Gage Factor Change

The electrical properties of the active strain elements that are most critical to strain gage performance are gage factor (G) and temperature coefficient of resistance (TCR) [1], defined as

(1)

(2)

where T is temperature, the strain and R is resistance. This is particularly true when large temperature gradients are superimposed over the surfaces of engine components.

Since static strain gages are not only subjected to applied mechanical strains, but are also subjected to large thermal strains as well, the strain measurement must be capable of distinguishing the relative contributions of the two sources of strain. Specifically, the total contribution to the measured strain (fractional resistance change) for a given strain application is the sum of the actual mechanical strain at a given temperature and the temperature induced strain. The latter contribution is due to the TCR of the semiconductor and the differences in thermal coefficient of expansion (TCE) between the gage and the substrate. Thus, the relative contributions to the total strain can be represented by (3), (4) and (5).

(3)

Where,

(4)

(5)

and where s and g are the coefficients of thermal expansion for the substrate and gage, R the electrical resistance, T the temperature and is the strain. From the equations above, it is evident that the TCR should be as small as possible to avoid the need for temperature compensation. By minimizing the thermal component of static strain (apparent strain) and maximizing the gage factor, it should be possible to maximize the sensitivity of the sensor and permit reliable strain measurements at high temperatures.

Thus, as temperature increases the gage factor also change and in order to measure the actual mechanical strain correctly, we need to know the gage factor change when compared to the gage factor under calibration condition which is generally the room temperature.

Apparent Strain

Apparent strain is any change in gage resistance that is not caused by the strain on the force element. Apparent strain is the result of the interaction of the thermal coefficient of the strain gage and the difference in expansion between the gage and the test specimen. The variation in the apparent strain of various strain-gage materials as a function of operating temperature is shown in Figure1.In addition to the temperature effects, apparent strain also can change because of aging and instability of the metal and the bonding agent.

Figure 1

Apparent Strain Variation with Temperature

Compensation for apparent strain is necessary if the temperature varies while the strain is being measured. In most applications, the amount of error depends on the material used, the accuracy required, and the amount of the temperature variation. If the operating temperature of the gage and the apparent strain characteristics are known, compensation is possible.

It is desirable that the strain-gage measurement system be stable and not drift with time. In calibrated instruments, the passage of time always causes some drift and loss of calibration.

For accurately measuring the