Experimental and numerical analysis of fatigue crack growth in carbonized bolts of gasoline engine cylinder head

The study on the fatigue cracks growth of the carbonized bolts of the cylinder head in the gasoline engine was performed in two experimental and numerical sections. The experimental section of the study included chemical composition determination, tensile properties, hardness, microstructure, and fractography. Scanning electron microscope (SEM) observations of the fracture surface identified fatigue as the main cause of premature bolts failure with an approximate fatigue life of 1.44×10^8 cycles. The crack initiates from the carbonized surface layer of the bolts, where impurities of oxide compounds and cavities help to propagate the crack until final faracture. The numerical section of the study included stress analysis and fatigue crack growth. Stress analysis showed that the maximum stress created in the bolts is about 4% more than the standard value. High-stress concentration in the threaded zone is one of the factors in crack initiation that the cyclical force of combustion pressure has caused the propagation of fatigue cracks. Numerical analysis of fatigue also showed that the presence of cracks to a depth of 0.12 mm is the origin of premature failure. Comparison of the two experimental and numerical sections showed that the calculated crack depths are consistently consistent with the observations of the oxide layer of optical and scanning electron microscopes.