An investigation of the effect of bolt tightening stress on ultrasonic velocity in cylinder head and main bearing cap bolts of diesel engine

Accurate estimation of clamping force is a major issue in the high-strength bolted joints. This study proposes a methodology to predict this force based on the measurement of the ultrasonic velocity using the pulse-echo method. The clamping forces of the cylinder head and main bearing cap bolts were also measured in a high-speed marine diesel engine using the torque–angle strategy. The new and used bolts were considered in both coated and uncoated conditions to investigate their effects on the clamping forces. The variations in the ultrasonic length, tightening torque, and bolt preload were measured in all stages of the assembly process. Results showed that the partial plastic deformation of threads reduced the clamping force in the first stage of tightening of new bolts. In the used bolts, preload was affected by an increase in the coefficient of friction under the bolt head. Moreover, the lack of zinc phosphate coating also increased the coefficient of friction and reduced the preload. It was concluded that the mentioned reduction can be successfully controlled by the torque–angle method. The comparison between the results of the present paper and previous studies showed that axial stress had a significant influence on the changes in the ultrasonic velocity. However, the presence of shear stress can further decrease the wave velocity. Finally, a relationship between the ultrasonic velocity variations in terms of the applied stress (up to the plastic region) was obtained in a real assembly process, and the clamping force can be estimated with an accuracy of 10%.