Todor Ergić

Fatigue Damage Assessment of Bolted Joint Under Different Preload Forces and Variable Amplitude Eccentric Forces for High Reliability

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 was used to describe material cyclic scatter band with Gaussian normal distribution in logarithmic scales. Range of dispersion for notched structure of 42CrMo4 steel was 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 was 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 was 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 were applied. Assessments were carried out for 50, 97.5, 99, 99.9, 99.99, 99.999, 99.9999, 99.99999 and 99.999999 % survival probability.

Influence of Semi-Circular Cracks on Threaded Connection Fatigue by Means of Kitagawa-Takahashi Diagram and El Haddad Equation

Evaluation of semi-circular surface crack influence on threaded connection fatigue behavior, made of 42CrMo4 heat treatable steel, was carried out. Crack diameters were defined as 0.02, 0.05, 0.1, and 0.15 mm. Influence of semi-circular surface cracks was investigated by means of Kitagawa–Takahashi diagram and El Haddad equation. Assessments were done for survival probability of 99% on detailed FE model with normal metric thread profile and preload force at 70% of force at bolt yield point. The most critical location on threaded connections usually are thread roots which contain a very high notch effect. In order to accurately consider multiaxial stress field in thread root, multiaxial fatigue stress criterion based on a critical plane theory for fatigue assessment, was used. Mean stress influence was taken into account by means of Haigh diagram. Variable amplitude loading history of low-high (Lo-Hi) sequence spectrum was analyzed with the numerical algorithm of Rainflow cycle counting in the time domain. Fatigue damage was calculated according to the modified Palmgren-Miner linear damage accumulation hypothesis.