Contrast-enhanced micro-computed tomography visualization of adhesively bonded CFRP/aluminum tubular joints.

Multi-material adhesively bonded joints between carbon fiber reinforced polymers (CFRPs) and metals are used in many high-end applications in automotive and aerospace industries, with adhesives having multiple benefits over mechanical fasteners. However, automated quality control of the adhesive layer using suitable nondestructive testing techniques is lacking and hampers their use to the fullest extent. Micro-computed tomography (micro-CT) is an established technique in visualizing these materials, but requires contrast between the CFRP and carbon-based adhesive to enable automatic component differentiation and high-quality imaging. In this work, we introduce hafnium oxide nanocrystals (HfO2 NCs) as a contrast agent in the adhesive to visualize multi-material tubular joints. We present an optimized and scalable dispersion workflow that, in combination with careful surface chemistry design, results in long term stable NC dispersions. Micro-CT imaging of the CFRP/Al adhesively bonded tubular joints shows the increased contrast compared to an unloaded adhesive, which can be quantified based on their local attenuation coefficients. The increase in contrast leads to distinguishable peaks in the gray value histogram, allowing for fully automated multi-material segmentation.