To a large extent, Carbon Fiber Reinforced Polymer (CFRP) composites have become the material of choice in modern aircraft design, due to their high strength-to-weight ratio, corrosion immunity and fatigue properties. Many primary structure components (e.g. bulkheads, stabilizers, wing boxes), where an aluminum skin was previously joined to a metallic airframe by fasteners, adhesives or welding, have been replaced by composite equivalents, in which the CFRP skin and a web of orthogonal CFRP stiffening panels are bonded into a T-shaped joint (Fig. 1). In the pi-Joint, unidirectional skin and web panels are adhesively bonded in a 3D woven preform that is shaped like the Greek letter â. The joint may be covered by a CFRP, 3D woven overwrap and its adhesive layers may be an Electrically Conductive Film (ECF). Pi joints offer performance equivalent to, or better than, metal structures, as well as the possibility of significant cost savings in the manufacture of large-scale, complex net-shape components. Compared to metal structures, where flight-induced stresses are concentrated at each fastener, in a pi-joint stresses are distributed along the entire pi-web and pi-skin interfaces. Consequently, Nondestructive Inspection (NDI) must be performed on the entire pi-joint to confirm that it is well-formed and properly adhered to the skin and web, and that the component as a whole meets engineering allowances for voids, ply wrinkles or Foreign Object Debris (FOD).
