Experimental Strain Analysis

As was seen in previous modules, stress analysis even of simple-appearing geometries can lead to complicated mathematical maneuvering. Actual articles | engine crankshafts, medical prostheses, tennis rackets, etc. | have boundary shapes that cannot easily be described mathematically, and even if they were it would be extremely difficult to fit solutions of the governing equations to them. One approach to this impasse is the experimental one, in which we seek to construct a physical laboratory model that somehow reveals the stresses in a measurable way. It is the nature of forces and stresses that they cannot be measured directly. It is the effect of a force that is measurable: when we weigh an object on a spring scale, we are actually measuring the stretching of the spring, and then calculating the force from Hooke's law. Experimental stress analysis, then, is actually experimental strain analysis. The difficulty is that strains in the linear elastic regime are almost always small, on the order of 1% or less, and the art in this field is that of detecting and interpreting small displacements. We look for phenomena that exhibit large and measurable changes due to small and difficult-to-measure displacements. There a number of such techniques, and three of these will be outlined brief y in the sections to follow. A good deal of methodology has been developed around these and other experimental methods, and both further reading1 and laboratory practice would be required to put become competent in this area.