Comparison of two NIR lasers for composite repair preparation by laser ablation and the effect on interlaminar fracture toughness of CFRP
Proceedings of the 15th Conference on Laser Ablation (COLA)
Type: Zeitschriftenaufsatz (non-reviewed)
Especially the aviation industry is a prominent user of carbon fibre reinforced plastics (CFRP). While this material class has already been used for the last decades, it has only recently become the major material in structural parts of modern aeroplanes. However, with increasing application of CFRP also arises the necessity for effective rework and repair strategies. Current state of the art is manual routing, but a lot of research on automation of the process by conventional tooling, abrasive water jet, and also laser is conducted. It has already been shown that laser radiation of various wavelengths (UV to MIR) can be used for preparation of the damage zone by ablating stepped or scarfed structures from the material . But the material characteristics, especially the optical properties, pose a challenge to the laser ablation process. Studies have shown that nanosecond-pulsed UV and MIR lasers are able to ablate CFRP without causing an increased risk for delamination as opposed to NIR laser radiation . This risk of delamination is supposed to result from the ablation mechanism of the composite material. For UV and MIR lasers the majority of the laser radiation is directly absorbed in the matrix material. This is not the case for the NIR radiation that is mostly absorbed on the carbon fibres’ surface indirectly removing the matrix material and potentially causing delamination below the fibres. This risk can be evaluated by performing mode I interlaminar fracture toughness (GIc) tests , which is used to test two adhesively bonded CFRP surfaces, where one surface received surface treatment. During a recently conducted performance comparison for laser ablation of CFRP between two nanosecond-pulsed NIR lasers with different beam characteristics the GIc results in Figure 1 were achieved. The ablated samples were compared to mechanically milled samples. A minimum toughness of GIc = 450 J/m² is required by some OEM. This limit was surpassed by one of the two NIR laser sources. This study focusses on the analysis of the two lasers’ characteristics with regard to the resulting interlaminar fracture toughness and the identification of a sweet spot for delamination free laser ablation of CFRP using budget-prices NIR laser.