Automatic control for laser-based repair preparation of composites by fiber orientation detection and short coherent interferometry

In automobile and aviation industries, composite materials are the key to lightweight efficiency. Composites containing carbon fiber reinforcements are of particular interest due to their supreme stiffness-to-weight-ratio, but the machining of this material class is still a challenge in production, rework, and repair. Laser technology presents a powerful solution to this obstacle. Laser ablation is a technique that can be utilized for repair preparation of composites. However, for large-scale industrial acceptance, laser processes must incorporate a high degree of automation. In this paper, an approach to fully automatic control of the laser-based repair preparation is presented. It features two process monitoring techniques: fiber orientation detection and short coherent interferometry. Both techniques aim to support the material ablation process by enabling homogeneous material removal to a specific material layer or specified depth. Between laser scanning cycles, a camera-based system scans the processed surface and measures the fiber orientation in each pixel with regard to an adjustable tolerance. When scarfing composite parts made from noncrimped carbon fibers, this technique can be used to detect the transition between two consecutive layers. Areas in the scarfing zone that reveal fibers of the next layer will be excluded from further laser scanning until the current fiber layer is completely ablated. Composite parts reinforced by crimped fabrics are similarly processed, but since single material layers are not easily identified, the ablation process is not controlled by fiber orientation measurements. Instead, short coherent interferometry is used to detect the ablated depth and compare it to the part's ideal layer thickness. Areas in the scarf zone that have reached the desired depth are excluded from further laser scanning. This technique is also applicable to noncrimped fabrics. The results show that when the laser-based repair preparation is controlled by these techniques, precise scarfing can be achieved.