TABLE OF CONTENTS The two types of strain transducers discussed in this section are the resistive and the capacitive strain gauges.
Strain is defined as the change in length per unit length of an object when that object is subjected to a load. If the length of a 2-inch long steel bar decreases by 0.006 inches when loaded, then for a unit length of 1 inch the length of the bar decreases by 0.003 inches, and the strain is 0.003/1 = 0.003 inches per inch. This is often stated as 3000 micro inches per inch or 3000 micro-strains.
The applied load creates a stress in the object that is equal to the applied force divided by the unit area over which the force is applied. For elastic materials, the stress is linearly proportional to strain, and the material will return to its original length when the load is removed. Almost all materials have an elastic region where the stress is linearly proportional to strain. If the strain is too high, the material will be permanently deformed and will not return to its original length when the load is removed. At some higher level of strain the material will fracture. The purpose of applying strain gauges to test objects is to determine the level of stresses that the material is experiencing as a result of the forces acting on the object. These forces can be tensile or compressive, and they can be static or dynamic.
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