Elastic-Plastic deformation studies in 4140 steel using the critically refracted longitudinal (Lcr)wave technique

Abstract

The critically refracted longitudinal (LCR) wave ultrasonic technique has been used to investigate a typical turbine disk material in the elastic-plastic region. Both applied tensile stress and residual strain effects are discussed. A sample of 4140 steel, typically used for turbine components, was tested. The acoustoelastic phenomenon was evaluated using probe sets of three different nominal frequencies, IMHZ, 2.25MHz and 5MHz. Although slightly different acoustoelastic constants were obtained for both 2.25 MHz and 5 MHz frequencies in the elastic region (up to 80% of the yield strength), statistically both have no significant difference in predicting the acoustoelastic constant. The I MHz procedure was found not to be adequate for the plate type structure. Data obtained in post yield tests on the same specimen using a 5 MHz probe set provided some very interesting results. When an applied load was below the yield point, the "return to no-load" LCR travel time varies by only 2︢ ns, (0.026% of total travel time). When the load passed the yield point, the "return to no-load" LCR travel-time showed a distinct change, due to the occurrence of residual strain. After load release, a 12.8% residual strain resulted in a 15 ns (0.198% of total travel time) change in the "no-load" travel time. The test results indicate that the LCR ultrasonic technique can linearly track the material stress-strain behavior until the yield stress level is approached. It is applicable even in a passed yield region. The LCR travel time, or the relative change on LCR velocity, can not only predict the applied stress level when a load below the elastic limit is on, it is also capable of indicating prior yielding, in terms of residual strain, when the component has been worked in an elastic-plastic region. In the elastic-plastic region, a simple quadratic relationship between the LCR travel time and the residual strain may replace the linear correlation between the LCR velocity and applied stress in the elastic limit.