Study of the damage to 12Cr18Ni10Ti steel samples under low cycle fatigue using methods of nondestructive control

Abstract

The possibility of determining the degree of damage to flat samples in the area of low-cycle fatigue at the stage of accumulation of scattered damage before the appearance of a macroscopic crack is demonstrated. Flat samples were tested for cantilever bending at room temperature with a constant loading amplitude until a fatigue crack appeared. Austenitic steel 12Cr18Ni10Ti was used as a test material. The velocity (propagation time) of elastic waves in the material and the coercive force were measured upon testing at intervals of 500 cycles. The zone of damage development was analyzed and value of hardening (based on microhardness measurements) was determined within 15 mm of the developed crack. The analysis of the images of the alloy microstructures in the zone of the most likely crack propagation was performed following different number of cycles at the same point on the sample surface. The structural changes (γ – α transition (formation of martensite deformation)) that occur during cyclic loading, as well as nucleation and development of damage, followed by the formation of fatigue cracks, inevitably lead to changes in the elastic and magnetic properties of the material. At the initial stages of loading, we observed changes in the initial (both acoustic and magnetic) characteristics of the material, which later became stable or changed slightly. When operating time exceeded 80 % of the number of cycles before failure, a significant change in the measured parameters occurred. The obtained dependences of the acoustic and magnetic characteristics of the material provide reliable information about the current damage and can be used in estimating the residual life of the structures made of 12Cr18Ni10Ti steel.