Laminated carbon fibre reinforced polymers (CFRP), are materials providing exceptional mass specific mechanical properties and are therefore intensively used in the aeronautical and aerospace industries (certain modern aircrafts are made of more than 50 % of CFRP). Recycling methods for these materials including thermal (pyrolysis), chemical (solvolysis) or mechanical (cutting, shredding) methods exist. Nevertheless, they all have the disadvantage to start with the part size reduction and lead to a loss of the length and the orientation of fibres which in turn induces a down cycling of the material. A successful attempt to recover complete fibre layers embedded in matrix was performed by initiating a crack by an impact loading and by propagating the crack using a dynamic peel-like loading at a few meters per second at Fraunhofer EMI [1]. A good preservation of the mechanical properties of the recovered single layers could be demonstrated. Nevertheless, alternative methods for both crack initiation and propagation might also produce promising results and be more suitable for small size specimens. In this context, a novel mechanical method is proposed to separate the laminated CFRP layer-by-layer by cutting the interface between the layers using power-ultrasonics.

Experimental tests were conducted to evaluate the feasibility of this novel proposed method. A special cutting sonotrode made of CPM-Steel, with a resonance frequency of 20 kHz was fixed to an Ultrasonic plastic welding setup, which had been adapted to realise cutting experiments. The investigated specimen was a multi-layer carbon-fibre PEKK (Polyetherketonketone) composite material in the shape of rectangular specimen featuring a length of 70 mm, a width of 30 mm and thickness of 4 mm. The specimen was clamped with an in-house constructed anvil and successive cuts, featuring a depth of 25 mm (maximum depth of cut which could be achieved from the ultrasonic cutting setup) were initiated starting from one edge of the sample. Interlaminar cracks were successfully induced and a propagation 10 mm further than the edge of the ultrasonic tool could even be observed. In order to propagate the crack through the whole specimen and thus to reach complete layer separation, a dedicated setup has been developed at INATECH in cooperation with Fraunhofer EMI and was adapted to the existing tensile testing machine Zwick-Z20 with a 20 kN load cell. The layer separation setup consists of a base plate, above which the specimen is placed. One end of the specimen is fixed with a wedge-shaped clamp penetrating into the crack and the other end is fixed with a toggle clamp. The base plate is fixed to one side of the tensile testing machine. A pyramid surface profiled clamp is used to firmly grip the single layer which was partially delaminated with the cutting sonotrode. This profiled clamp is then fixed to the other end of the tensile testing machine. The separation force was measured during the delamination of each single layer and the mechanical properties of the recovered single layers were determined in a second step to evaluate the damage induced by the layer separation process. This concept will be extended to other organic composite laminates for a layer-by-layer separation and it will be also adapted to process larger specimens in the future.