Measurement of Residual Stresses by the Hole-Drilling * Strain Gage Method

Introduction Residual Stresses and Their Measurement Residual (locked-in) stresses in a structural material or component are those stresses which exist in the object without (and usually prior to) the application of any service or other external loads. Manufacturing processes are the most common causes of residual stress. Virtually all manufacturing and fabricating processes -- casting, welding, machining, molding, heat treatment, etc. -- introduce residual stresses into the manufactured object. Another common cause of residual stress is in-service repair or modification. In some instances, stress may also be induced later in the life of the structure by installation or assembly procedures, by occasional overloads, by ground settlement effects on underground structures, or by dead loads which may ultimately become an integral part of the structure. The effects of residual stress may be either beneficial or detrimental, depending upon the magnitude, sign, and distribution of the stress with respect to the load-induced stresses. Very commonly, the residual stresses are detrimental, and there are many documented cases in which these stresses were the predominant factor contributing to fatigue and other structural failures when the service stresses were superimposed on the already present residual stresses. The particularly insidious aspect of residual stress is that its presence generally goes unrecognized until after malfunction or failure occurs. Measurement of residual stress in opaque objects cannot be accomplished by conventional procedures for experimental stress analysis, since the strain sensor (strain gage, photoelastic coating, etc.) is totally insensitive to the history of the part, and measures only changes in strain after installation of the sensor. In order to measure residual stress with these standard sensors, the locked-in stress must be relieved in some fashion (with the sensor present) so that the sensor can register the change in strain caused by removal of the stress. This was usually done destructively in the past -- by cutting and sectioning the part, by removal of successive surface layers, or by trepanning and coring. With strain sensors judiciously placed before dissecting the part, the sensors respond to the deformation produced by relaxation of the stress with material removal. The initial residual stress can then be inferred from the measured strains by elasticity considerations. Most of these techniques are limited to laboratory applications on flat or cylindrical specimens, and are not readily adaptable to real test objects of arbitrary size and shape. X-ray diffraction strain measurement, which does not require stress relaxation, offers a nondestructive alternative to the foregoing methods, but has its own severe limitations. Aside from the usual bulk and complexity of the equipment, which can preclude field application, the technique is limited to strain measurements in only very shallow surface layers. Although other nondestructive techniques (e.g., ultrasonic, electromagnetic) have been developed for the same purposes, these have yet to achieve wide acceptance as standardized methods of residual stress analysis. *  Drilling implies all methods of introducing the hole (i.e., drilling, milling, air abrasion, etc).

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