Papers by Author: Jerry D. Lord

Abstract: The paper describes a novel method based on Digital Image Correlation (DIC) for measuring the static modulus of active PGA graphite specimens during conventional four-point bend strength tests. DIC has been used in combination with finite element and numerical modelling to monitor the displacement fields developing in the specimen during testing, and calculate representative modulus values. Details of the model and results are presented for two specimen geometries and a range of materials with different levels of exposure and density, tested in the perpendicular and parallel orientations. The calculated static modulus values from the DIC measurements confirm the trend between modulus, flexural strength and density. Comments and observations on the uncertainty in the measurement are also presented.

Abstract: The paper describes recent use of Digital Image Correlation (DIC) for in-situ measurements of deformation and cracking of large civil engineering structures like bridges and power plant. Recent work at NPL has demonstrated the potential of DIC as a novel NDT tool for measuring deformation and cracking in reinforced concrete structures. This has particular application where the area of interest is in a region where inspection is difficult or costly and where direct access may have safety implications. In this case accurate measurements from pairs of images can be very cost effective.

Abstract: 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.

Abstract: Hole drilling along with X-Ray diffraction, 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 regarding 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. Through a cross comparison with X-ray diffraction and neutron diffraction the procedure of fine incremental drilling has been validated, and the advantages of this technique demonstrated.