Laser-assisted cyclic chipless splitting for hard-to-cut thick wall tubes and fatigue fracture mechanism analysis

High-quality precision cropping of hard-to-cut thick wall tubes is urgently needed as the first process for precision forming of tubular parts. Due to the deficiencies of large cutting load and waste of chips and coolants in the present cutting methods, a laser-assisted cyclic chipless splitting (LACCS) method is developed, which prefabricates circumferential crack with large stress concentration by laser, and exerts cyclic rotary load at a low amplitude simultaneously to split the tube. The theoretical laser crack depth and critical loads are derivated for fatigue crack propagation and instant rupture, respectively, and an XFEM model based on cohesive damage criterion is established to reveal the fracture mode based on stress intensity factors. Finally, section quality and microfracture mechanism are investigated by experimental tests with different initial crack depths and load control curves. The results show that critical fatigue splitting load and instant rupture load gradually drop with crack depth at a decreasing rate; for crack depth of 0.7 mm, the critical loads are 0.67% and 34.36%, respectively, of that in the traditional cutting method. The type I crack is the primary fracture mode in this LACCS process, and the stress intensity factor K-1 varies as a cosine curve with peak amplitude of 436.09 MPa(m)(1/2). Step-reduction frequency load can smoothly transite the crack propagation to final rupture, and obtains high quality sections in 16-32 s, with outer roundness error of 0.6%, flatness error of 0.2-0.3 mm and final rupture area of 0.25-1.19 mm(2). The fractographies typically show laser affected zone, cracks propagation zone and final rupture zone. Micro-cracks initiate from the tangency points on the laser notch curve with maximal stress change rate, and the ductile striations propagate with growth rate of 3.3-5.1 mu m/cycle. Final rupture zone forms under shear stress, featured by quasi-parabolic dimples involving microvoid coalescence.