Strength improvement of bolted joints in composite materials by use of patched metal inserts

Abstract

Metal inserts are occasionally used to improve bearing load capacity of bolted joints in laminated composite materials. This paper investigates a new reinforcement concept where inserts are built by locally replacing composite plies with metal patches of various diameters, surrounding the holes. The inserts are built during composite manufacturing by alternately placing the metal patches through the thickness of the laminate at locations where holes are to be drilled after consolidation. An extensive experimental study including pin-loaded, open–hole tensile, and single-shear testing of bolted specimens is presented. Considerable improvement of the bearing strength – 50-60% – is attained for pin-loaded specimens with inserts, demonstrating the potential of the reinforcement concept. The open–hole tensile tests show that the by-pass strength can be maintained or even improved with up to 20% if the inserts are properly designed. Finally, the results from the single-shear tests of bolted joints show more than 25% improvement in strength for reinforced single- and double-bolt specimens. It is possible that the inserts would maintain clamping pressure over time, which could then almost double the imrovement (47%) for bolted joints.