The measurement of residual stress using the incremental hole drilling is well established, but the main limitations with the conventional strain gauge approach are the requirements for surface preparation, the need for accurate alignment and drilling, the restricted range of hole geometries commensurate with the specific gauge designs, and the limited range of strain data averaged over the footprint of the strain gauge grid. Recent attempts to extend the method have seen the application of full field optical techniques such as electronic speckle pattern interferometry and holographic interferometry for measuring the strain fields around the hole, but these methods are sensitive to vibration and this limits their practical use to controlled laboratory environments.There are significant potential benefits therefore of using a more robust technique based on Digital Image Correlation (DIC), and work is presented in this study on the development of the method for measuring surface displacements and strain fields generated during incremental hole drilling. Some of the practical issues associated with the technique development, including the optimization of applied patterns, the development of the optical system and integration with current hole drilling equipment are discussed, and although measurements are only presented for a single load case -the equi-biaxial stress state introduced during shot peening -the novel aspect of this work is the integration of DIC measurements with incremental drilling and an application of the Integral Method analysis to measure the variation of residual stress with depth. Validation data comparing results from conventional strain gauge data and FE models is also presented.

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This paper presents results which advance and improve the usefulness, accuracy and efficiency of incremental centre hole drilling as a method of measuring near surface residual stress fields. Particular emphasis is placed on providing optimal values for the number of drilling step increments to be used and their corresponding size. Guidelines on the optimal values for the number and size of steps to use during measurements are presented for various ratios of hole radius to strain gauge rosette radius in the form of tabulated data. These guidelines are subsequently incorporated into a new data analysis program which permits very near surface residual stress fields to be accurately determined in real components. The benefits of the new approach are highlighted by reporting the results of measurements made on three industrial components, each of which has been subjected to a well-known engineering process. These components are a shotpeened spring-steel, a friction stir welded aluminium alloy, and a titanium alloy subjected to three different machining processes. The results reveal that the improvements to the incremental centre hole drilling technique can provide measured residual stresses from depths ranging from about 10 mm to 1 mm.

Hole drilling is one of the most widely used techniques for measuring residual stress, but the conventional approach is limited in the near surface detail that can be resolved. Because of concerns about the levels of induced residual stress that might develop during machining and surface treatment processes, there is significant interest in developing a technique that can obtain near-surface residual stress information by the application of fine-increment hole drilling. Critical information can be lost if conventional, large depth increments are used and the fine incremental hole drilling approach, using depth increments as small as 20µm, offers a cost effective and rapid solution, with the possibility of measuring near surface stresses. Results focus on three different machining studies and a shot peened specimen, all cases where the stress field changes rapidly through the depth, particularly close to the surface. A systematic assessment of machining parameters is not within the scope of this paper and is not presented, but work has focused on highlighting the application and potential of the fine increment hole drilling approach.