Measurement of residual stress using interferometric moiré: A new insight

Abstract

ABSTRACT--The use of intefferometric moira and hole drilling to determine residual stress has been well reported and accepted for stress fields whose principal directions can be predicted well enough to permit the moir~ grids to be aligned with the principal strain axes. When the principal strains do not align themselves with the grid axes, a third strain component can be obtained by working with the diagonal pitch of the moir6 grid, but this requires resetting the optical bench to the lower frequency. Diffraction efficiency is lost, with an additional loss in sensitivity. In this paper, the authors determine the shear strain component by observing the rotation of the moir~ fringes in close proximity to the hole. The results of experiments on a specimen containing a model residual stress distribution are presented and compared with the theoretical prediction. Finally, the isothetic contours, based on elastic theory, were computed and plotted for several cases to verify this proposition. These results and the expected residual stress distribution are also compared to the experimentally obtained moira fringes. KEY WORDSnResidual stress, moir~ interferometry, hole drillingFabrication and processing operations often introduce residual stresses to structural components. Operations involving localized heating (e.g., welding), differential thermal expansion (e.g., heat treatment of composites) or plastic deformation (e.g., forming) create internal stresses that may be retained in the finished component. These stresses can be desirable (e.g., cold-worked holes) or undesirable (e.g., weld cracking). Measurement of the magnitude and direction of residual stresses is often necessary because the stresses can influence the service life of a component. X-ray diffraction, ultrasonic birefringence, Barkhausen noise and mechanical techniques are all used to measure residual stresses. 1 Of the mechanical techniques, hole drilling is routinely used. 2In the hole-drilling method, a small, shallow hole (~, 1 mm diameter by 1.5 mm deep) is drilled in the sample, and the local displacement of the surface, in response to the material removal, is measured, usually with strain gages. Because drilling the hole releases local stresses, the adjacent surface undergoes displacements in response to the new stress equilibrium, and measurement of these displacements allows

Incremental Hole Drilling for Residual Stress Analysis of Strongly Textured Material States – A New Calibration Approach

“…Afterwards, the residual stress is calculated from the measured strain using generalized Hooke's law and calibration data. An alternative to the strain gage rosettes are optical strain measurement techniques like digital image correlation [15,16], moiré interferometry [17][18][19] or holography [20,21]. However, these optical techniques are in general not as accurate as strain gages and more sensitive to external influences like vibrations or temperature variations [22,23].…”

Advances in Hole-Drilling Residual Stress Measurements

“…Moiré interferometry [29][30][31][32][33][34][35] provides a sensitive technique for measuring the small surface displacements that occur during hole drilling. Figure 2 schematically shows a typical optical arrangement [33].…”

Hole Drilling and Ring Coring

“…When using Moiré interferometry [20][21][22][23], a diffraction grating consisting of finely ruled lines, typically 600-1200 lines/mm, is attached or made directly on the specimen surface. This area is illuminated by two symmetric light beams that are derived from a single coherent laser source.…”