Residual stress is that which remains in a body that is stationary and at equilibrium with its surroundings. It can be very detrimental to the performance of a material or the life of a component. Alternatively, beneficial residual stresses can be introduced deliberately. Residual stresses are more difficult to predict than the in-service stresses on which they superimpose. For this reason, it is important to have reliable methods for the measurement of these stresses and to understand the level of information they can provide. In this paper, which is the first part of a two part overview, the effect of residual stresses on fatigue lifetimes and structural integrity are first summarised, followed by the definition and measurement of residual stresses. Different types of stress are characterised according to the characteristic length scale over which they self-equilibrate. By comparing this length to the gauge volume of each technique, the capability of a range of techniques is assessed. In the sec...

Residual stress. Part 1 – Measurement techniques

Residual stresses can be defined as those stresses that remain in a body after manufacturing or processing in the absence of external forces or thermal gradients. Virtually all manufacturing and fabricating processes introduce residual stresses into the manufactured article and extreme service loading may also change the state of residual stress in the component. The effects of residual stress may be either beneficial or detrimental, depending upon the sign, magnitude and distribution of the stress. For improved process and product control, design, performance and modelling it is increasingly important to have rigorous experimental procedures to determine the residual stresses to the best possible accuracy.
A wide variety of residual stress measurement techniques exist, but hole drilling is one of the most widely used. It is relatively simple, inexpensive, quick and versatile, and can be both laboratory-based and portable. However, achieving high quality, accurate stress data is not trivial. This guide provides both the inexperienced user and the expert with a practical guide to achieving better measurements. It draws together some background to the technique, discusses the current standards and highlights a number of key issues crucial to obtaining a good measurement, based on input from UK experts and some of the findings from a recent UK hole drilling residual stress intercomparison exercise. The currently available residual stress data analysis techniques are discussed, and a comprehensive bibliography of key references is included together with some information and links to UK hole drilling contacts and to relevant web sites, providing the reader with a valuable resource for further reading.

The measurement of residual stresses by the incremental hole drilling technique.

A new method combining Moire interferometry and the incremental hole-drilling method is developed to determine both uniform and nonuniform residual stress distribution in depth. The study is reported in two parts. In this first part, the theoretical development of the moire interferometry hole-drilling method is presented. The relationship between the in-plane surface displacements produced by introducing a blind hole and the corresponding residual stresses is established by employing the existing theoretical solution containing a set of undetermined coefficients. The coefficients are calibrated by the three-dimensional finite element method and they are processed nondimensionally for general use. The whole field in-plane surface displacements data U x and U y produced by the relaxation of residual stresses are obtained from moire interferometry after each increment of hole drilling. The high signal-to-noise ratio provided by moire interferometry allows accurate determination of fringe orders near the hole boundary which is essential for enhancing fidelity of residual stress determination. The experimental procedure to determine the signs of residual stresses is described and the accuracy of the method is also discussed.

Study of Residual Stress Distribution by a Combined Method of Moiré Interferometry and Incremental Hole Drilling, Part I: Theory

Based on the dual-beam illumination principle of Leendertz, an electronic speckle pattern interferometer (ESPI) is developed to de- termine separately two orthogonal components of the in-plane displace- ment field. In the arrangement described, the lengths of the illumination beams were reduced to make the optical setup compact. The CCD cam- era for recording images is placed outside the area enclosed by the optical path, and adequate working space is provided between the cam- era and the test rig, so that adjustments can be made without interfering with the light beams. Using a customized telephoto lens, different areas in the range of 5.634.0 and 14310 mm 2 can be examined at a distance of 550 mm from the test piece. Combined with the blind-hole-drilling method, it is applied to residual stress determination, which is a chal- lenging task due to stress concentration and severe decorrelation in the area neighboring the drilled hole. © 1998 Society of Photo-Optical Instrumentation Engineers. (S0091-3286(98)01808-X)

Two-dimensional in-plane electronic speckle pattern interferometer and its application to residual stress determination

Effects of residual stress and heat treatment on fatigue strength of weldments were investigated with lateral link specimens and sway bar bracket specimens. Residual stresses in the thickness direction at a weld toe location were measured by the X-ray diffraction (XRD) method and the hole drilling method for as-welded and post-heat-treated specimens. Compressive residual stresses on the surface were obtained from the as-welded specimens, while zero or very low tensile residual stresses were obtained from the heat-treated specimens. Microstructural differences between as-welded and heat treated specimens were also investigated in this study. Heat-treated specimens showed fine grains at the weld zone compared to that of as-welded specimens. Fatigue strength of as-welded specimens was found higher than that of heat-treated specimens due to compressive residual stresses induced at the weld toe areas. This study also predicted the fatigue life of as-welded specimens with commercial fatigue software and the life predictions were compared with experimental results.

Effects of residual stress and heat treatment on fatigue strength of weldments

A new six-element strain gage rosette is presented that can greatly improve residual stress measurement accuracy when using the hole-drilling method. The proposed rosette consists of three pairs of sector-shaped radial and circumferential gages connected as half-bridges. This rosette design increases effective strain sensitivity by a factor of 2.3 compared with a standard ASTM rectangular rosette, and can identify stresses at one-third greater depths from the measurement surface. Experimental measurements confirm theoretical strain response calculations within 3–4 percent. Apart from a small increase in time to complete the electrical connections, the practical use of the proposed rosette is identical to that of a conventional three-element rosette.

A new rosette design for more reliable hole-drilling residual stress measurements

A novel method for enhancing the strain sensitivity of the hole-drilling method for measuring uniform residual stresses is examined. Such enhanced strain sensitivity is important because it improves the accuracy of the residual-stress evaluation. The new method involves enlarging the effective hole size by drilling a reverse taper hole. A simple practical technique for drilling reverse taper holes is described. The strain sensitivity for this new method is compared with that of the conventional hole-drilling method. Experimental results show excellent correspondence with theoretical results. The reasons for the sensitivity improvement are explained.

Enhanced Sensitivity Residual-stress Measurements Using Taper-hole Drilling

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A new rosette design for more reliable hole-drilling residual stress measurements

Enhanced Sensitivity Residual-stress Measurements Using Taper-hole Drilling