4. Conclusions
The punching of through-holes on pseudo-ductile CFRP laminates and CFRP/metal hybrid composites utilizing a servo press was studied. The effect of the UD prepreg size on the punch-shear load, punching resistance (Ks), and the action of shearing was discussed on the basis of experimental results. The main points of this study are as follows:
• The punching of the laminate made with standard prepreg sheets resulted in a larger punch load than that for the thin-ply laminate, regardless of the punch speed and the thickness of the laminate. Nonetheless, the experimental results indicate that the punch force has a linear relationship with the machine speed for the punching of laminated CFRP composites; the reference maximum punch load tended to decrease with increasing machine speed.
• The punching resistance (Ks) is considered to be a suitable parameter for evaluating multilayer CFRP laminates made with various prepreg thicknesses in terms of the ultimate tensile strength (σUTS) and yield stress (σy). The actual experimental results in this study were closely fitted by linear functions such as Ks = −0.16σy + 386 and Ks = 0.59σUTS-525 (37 SPM punching velocity). Hence, the maximum punch load is applied that allows a straightforward assessment for design purposes.
• The shear mechanism in the punching of laminated CFRP composites is very similar to that in both standard and thin-ply laminates. Standard and thin-ply laminates subjected to half-punching showed that shear cracks through the fiber material at similar characteristic angles by the punch for outer shear band and die for inner shear band, as well as the large propagation of the outer crack and the inner crack was connected by the delamination.
