304L stainless steel DMLS FEA History of engineering & technology LCF adaptive control additive manufacturing blade-disc-Franc3D crack propagation cyclic loading dental implant abutment fatigue fatigue life improvement fatigue testing finite element analysis finite element method (FEM) fractography fracture materials characterization milling process mini specimen mixed-mode cracking mode II stress intensity factor mode III stress intensity factor monolithic zirconia crown non-proportional mixed mode loading part functionality rail steel research progress simply supported bending