Measurement of Internal Strains in Graphite Using the Deep Hole Drilling Technique

Currently, measurement of internal strains deep inside graphite is extremely difficult. As a consequence there is a need to assess the ability of a deep-hole drilling (DHD) method to measure internal strains in reactor core graphite and explore the potential for in-situ measurement. DHD technique is a semi-destructive method for measurement of the through thickness residual stresses. The technique previously has been applied successfully to metallic and composite materials. In this paper, the method for stress measurement in graphite is examined particularly when a significant volume fraction of porosity is present. Two types of graphite were used, PGA and PG25 filter graphite. In PGA graphite the Young’s modulus of elasticity was orientation dependent. As a consequence samples were cut from blocks in two directions. PG25 filter graphite is a surrogate for service exposed material. Known loads were applied to graphite beam samples and the DHD method was used to measure the stress/strain profile through the material. The results were compared with the strain data obtained from strain gauges bonded to the samples. Overall, there was an excellent agreement between the DHD measured stress/strain and applied stress. It is shown that deep-hole drilling technique can measure linear stress distributions in graphite.