u169 Crack Formation and Crack Path in CFRP Machining

During milling of carbon fiber reinforced plastics (CFRP) material a very distinct material removal process has been identified. Instead of forming continuous chips of a certain size, the material is removed in a powder-like fashion shooting out of the tool / workpiece contact area. The brittle, powder-like chip formation in cutting CFRP suggests crack formation as main material removal process. Analyzing the material removal for different fiber orientations characteristic matrix (epoxy) and fiber failure behavior can be identified. Some orientations show a frequent crack formation reaching deeper into the structure, while for others the matrix and fiber removal takes place only at the very surface. The crack path in the epoxy matrix and in the fibers depends on the fiber orientation relative to the trajectory of the cutting edge, respectively tool. Matrix removal and fiber fracture is minimum and limited to the surface itself when the cutting takes place parallel to the fiber orientation or the tool is running on fibers which gradually ascending to the surface. The most complex crack formation is observed when the tool runs head-on on the fiber ends at the surface. Here the crack path starts with a matrix / fiber interface failure which causes deeper cracks running from the surface into CFRP followed by fiber cracking below the cut surface.