Study On Stress Analysis And Ultrasonic Nondestructive Testing Of High-Strength Bolt

In recent years,the further advance of power grid construction has put forward higher requirements on the reliability of high-strength bolt connection,and the stress analysis and nondestructive testing of high strength bolt have been more important.In this paper,nondestructive testing of in-service high strength bolts is studied by combining finite element simulation,fracture mechanics test and ultrasonic flaw detection experiment.The distribution of stress and strain in high strength bolt was analyzed by AN SYS software,and the influence of two kinds of mic-rocracks on the stress distribution in root of thread was studied.The results show that the distribution of stress and strain in the bolt is very uneven,and the stress concentration mainly occurs at the-root of the front two threads in the engagement.The microcrack can greatly increase the stress concentration of threads and accelerate material failure.In order to study the fracture mechanical behavior of high-strength bolts,the static load tensile test and fracture toughness KIC test of quenched 40Cr steel were carried out.The mechanical indexes were analyzed and the fracture morphology was observed under scanning electron microscope.The results showed that the continuous yield strength of 40Cr steel was 1025.5 MPa and the tensile strength was 1134.5 MPa.The value of KIC is 108 MPa·m1/2.Besides,the fracture morphology showed dimples,cleavage steps and river patterns,which indicated cleavage fracture mechanism with partial microporous coalescence.By analyzing the working condition of high-strength bolt in the transmission tower,ultrasonic nondestructive testing technique was selected for in-service testing.The longitudinal-wave straight probe was used to detect artificial cracks at different positions and depths.Based on in-situ detection of screw end face and supplemented by edge detection of nut end face,the detection method and quantitative experience curves were obtained,which can detect cracks over 1 mm.Further,a auxiliary device of ultrasonic testing for bolt was developed and the corresponding evaluation method about crack hazard level was summarized.