Acoustic Structural Analysis of Steel Samples Loaded with Rotational Bending During Fatigue Tests

Abstract

In order to study the effect of accumulated metal damage on the speed of ultrasonic shear and Rayleigh waves, steel bar stock samples were subjected to cyclic loading according to the cantilever bending with rotation scheme. The results of measuring the velocity of ultrasonic waves (shear and Rayleigh) in steel rods with low-cycle fatigue under the influence of cyclic loads are presented. Samples of bars of steel grade 40X in the state of delivery and after annealing at 700 ° C for 30 minutes were investigated. The length of the bars is 270 mm, their diameters are 10 and 19 mm. Cyclic loads with cantilever bending were provided with a specially designed installation for fatigue testing, in which the bar was clamped with one side in the cartridge of the machine providing rotation, and the load was hung on the other side through the bearing. Shear wave velocities in the directions of the cross section of a bar and Rayleigh waves along the envelope of the cylindrical surface of the bar in the zone of maximum stresses were estimated. The excitation and reception of waves were carried out by an electromagnetic - acoustic method. To estimate the velocity of shear (transverse) and Rayleigh waves, we used the method of multiple reflections, respectively, over the cross section and the envelope of the rod. The results of the speed variation in bars with increasing cantilever load and the number of cycles are presented. It was found that the shear wave velocity in a bar is maximally reduced in the zone of maximum stresses. With loads of 60 % of the yield strength in the samples, a sharp decrease in speed is observed at 300,000 cycles. The change in the structure of the metal after annealing leads to a decrease in the number of cycles, after which a sharp change in the velocity of the waves occurs. The Poisson's ratio, calculated from the velocities of shear and Rayleigh waves, also changes with an increase in the number of cycles and makes it possible to take into account the change in the bar diameter during the tests. The performed experiments on studying the effect of cyclic bar loads give good informative data on fatigue depending on heat treatment and structural changes in the material. The possibility of implementing an electromagnetic-acoustic control method on multiple reflections allows us to significantly increase the sensitivity to microdefects when analyzing a signal on long-range reflections and to ensure high accuracy in determining the speed of waves.