Fatigue damage assessments of M10 bolted joint, made of 42CrMo4 heat treatable steel and strength class 10.9, were carried out for different preload forces and variable amplitude eccentric forces for high reliability. Assessments were done with preload forces of 50%, 70%, and 90% of force at bolt yield point and without preload force. The nominal approaches from Eurocode standard and VDI 2230 guidelines are mostly used for fatigue assessment. These nominal approaches cannot consider and describe in detail the local stress state at the thread root, which on bolt M10 have a radius of 217 μm. Threaded joints have several peculiarities that complicate the fatigue damage assessments. Range of dispersion is used to describe material cyclic scatter band with Gaussian normal distribution in logarithmic scales. Range of dispersion for notched structure of 42CrMo4 steel is taken from measurements. In order to take multiaxial stress field in thread root with high notch effect, multiaxial fatigue stress criterion based on a critical plane theory is applied for fatigue damage assessment. Critical plane approach is used for estimation of the fatigue damage and fatigue fracture plane position. Decrease of fatigue strength beyond the S-N curve knee point at 2·106 cycles is considered for the damage calculation. The main difficulties encountered in threaded joint fatigue damage assessment are due to the uncertainties and therefore, statistics and probability are applied. Assessments are carried out for 50%, 97.5%, 99%, 99.9%, 99.99%, 99.999%, 99.9999%, 99.99999%, and 99.999999% survival probability.
Keywords: fatigue damage, fatigue of thread root, failure probability, survival probability