The Fracture Failure Analysis On Main Bearing Bolts Of The Diesel Engine And Investigation On Its Loading Behaviours

A failure investigation has been conducted on four failed main bearing blots of diesel engine. The fracture surface of the failed bolts was observed by scanning electron microscope (SEM). The macro and micro fractography were performed in detail. The composition of the fracture surface were analyzed by X-ray energy dispersive analysis (EDAX). The mechanical properties and metallurgical quality of the failed bolts were examined. The results indicate that the chemical composition of the materials, mechanical properties and metallurgical structure of the failed bolts are within the specified technical range. However, it has been found that the transitional fillet radius for heads of the bolts is smaller, which is 40-50% smaller than that specified by the technical demand. Macrographs on the fractures show that the cracks initiated from the transitional fillet between the head and rod of the bolt. The beach marks can be observed clearly in the fracture surface, which is typical feature of the fatigue failure. It can be concluded that the multi-source failure mechanism is mainly operating processes. The stress concentration appears in the origin of the crack.In order to determine the axial stress and pre-tightening axial force of the failed bolts, the pre- tightening imitating tests of the bolts were performed by exerting a series of the torque to the bolts. The axial strain of the bolt rods under a series torque was evaluated by static strain meter. The experience relation of the assembling torque of the bolt (M) and the axial stress (a) or axial force (F) of the bolt can be concluded according to the Hook Law. When dipping oil at the thread, the relations are shown as follows:σa (MPa) =8.69 + 1.84M(Nm); F(KN) = 1.47 + 0.308 M(Nm)According to the practical assembling torque (275Nm) , the axial stress (a) exerting on the rod of the bolt is much lower than that of the fracture strength. The ratio of them (σ a/σ b) is approximately equal to 0.4.In view of the appearance of the stress concentration characteristics on the fracture, ANSYS software (Finite element analysis) used to calculate the stress profile and the stress concentration coefficient at the transitional fillet of the bolt's head under series of axial force. The effects of the size of the transitional fillet radius on the coefficient of the stress concentration were analyzed. The following conclusions can be obtained:1) When axial stress (a) < 550MPa, the rod and transitional fillet regions of the main bearing bolt are under elastic stage. When axial stress (σ) ≈ 550MPa, the yieldphenomena would take place at the transitional fillet region of the main bearing bolt. When axial stress (a) > 550MPa. the stress of the yield zone don't nearly increase but the yield zone would propagate from the transitional fillet toward the thread along axial direction.2) Stress concentration coefficient increases with decreasing the transitional fillet radius between the head and the rod of the bolt. The same diameter of the bolt is provided, the stress concentration coefficient at the transitional fillet for the bolt with a step is smaller than that for the bolt without a step.3) The transitional fillet radius of the failed bolts is small and their stress concentration coefficient is about 2.0. Under practical assembling pre-tightening torque (275Nm), the pre-tightening stress of the bolt is greater and their axial stress attains approximately to the yield strength Due to the smaller transitional fillet between the head and rod, the stress concentration coefficient is greater. Additionally, the end planes of the main bearing cover are not exactly parallel each other so that a vertical inclined load was exerted to the main bearing bolt. Under the alternating axial stress, the fatigue cracks initiated from the transitional fillet to lead to the fatigue fracture.On the basis of the analysis on the failure causes and determination of the failure mechanism, the recommendations and measurements in design and machining and assembling processes are proposed as follows:1) Increase...