Metrological inspection of multi-material drilled holes using X-ray Computed Tomography and image processing

This paper demonstrates a novel method that combines X-ray Computed Tomography (CT) and image processing for the investigation of bonded materials with significantly different densities. CT is increasingly used in aerospace applications for the inspection of materials and identification of defects. The limitations of the scanning process and data reconstruction methods are continuously being researched so as to improve the quality of the final results. One of the common issues experienced in the scanning of multi-material samples using CT is beam hardening. The materials examined to demonstrate the method developed are Carbon Fibre Reinforced Polymers (CFRP) and titanium alloy Ti6Al4V. These materials are often combined together to optimise the strength to weight ratio and joined together with fixings and rivets which require holes to be drilled through the two materials together. The machining of these materials is difficult due to their higher specific properties and as a result tool wear can become an issue. CFRPs properties depend on the nature, orientation and bond of the fibres and as a result drilling also affects service life. The results of the method ensure the quality of the drilled holes by quantifying variation in the process by measuring maximum diameter and radius of the hole and examining the entrance delamination and exit burrs though image processing. These results can then be compared to the tolerances defined within the design specification to help certify product acceptance.