Experimental investigation of the effect of half gap/half overlap defects on the strength of composite structures fabricated using automated fibre placement (AFP)

Abstract

The Automated Fibre Placement (AFP) process has gained traction as an efficient manufacturing method for large and complex composite structures. An AFP machine uses a computer-controlled arm with a head at the end capable of constant placement of uncured pre-impregnated carbon fibre tows grouped together to form bands. While AFP offers benefits through automation, certain defects inherent to the manufacturing method are induced. Primary defects observed by the aerospace industry are misaligned tows, often the result of trying to steer the tows. This defect occurs when a tow deviates from the intended path causing a combination of both side-to-side gaps and overlaps. In this first of a two-part paper, we experimentally investigate the effects of steered tow defects on the mechanical performance of carbon-epoxy composite structures made with AFP. The orientation, size and number of steered tow defects embedded in the laminates are varied, and their mechanical effects are examined by static tension, static compression and fatigue tests. The major experimental result is that compression is more critical than tension with the largest strength reductions reaching 25% and 10%, respectively. The configuration of the defects has a significant effect on the results. For example, the greater the number of defects, the lower the compressive strength for 45° oriented defects but the opposite is observed for 90° oriented defects. An increased defect width leads to a higher compressive strength in 45° configurations while it is the opposite for tensile tests. The experiments give insight into the behaviour of AFP panels and help to understand their failure modes.