Non-destructive prediction of material properties of steel and Al-alloys - state and challenge
The challenge for future nondestructive techniques is characterized by the two major demands of the users of n.d. techniques. One is the detectability of defects getting smaller and smaller, is the earlier detection of damages and the reliable characterization of states or evaluation of quantities describing the quality of the component. The second demand is the application of the n.d. techniques as early as possible in the manufacturing process in order to predict the properties of the final product and to enable the optimization of the process. This second demand and the applicability of electromagnetic and ultrasonic techniques will be addressed by the contribution. The application of multiple parametric regression algorithms yields values for yield and ultimate strength which are within ± 3% in agreement with those, evaluated destructively. Since the established destructive techniques come up to a ± 1% accuracy, the demand towards improvements of the n.d. techniques is obvious. The quality of the results is governed by the quality of the calibration of the electromagnetic quantities using representative samples of the material to be tested. Promising correlations are also found between combinations of second and third order elastic constants and the yield strength of steel samples. Improvements of the measuring technique are still needed to make this approach applicable. In daily practice the sound velocities together with the thickness and the content of the major alloying elements are taken to predict the yield strength of steel products. Whereas these and other activities result in a sufficiently accurate prediction of the strength of steel and Al-alloys, there is no promising technique to be seen in order to predict the toughness directly. The only process integrated application of n.d. techniques to determine material properties are three different electromagnetic techniques to evaluate the yield strength of cold rolled steel sheet and the application of ultrasonic technique to characterize the drawability of the sheet. The contribution informs upon the worldwide state of the art and discusses the limitations and the benefits of the application of the n.d. techniques. The authors will try to articulate the next steps towards an improvement of the present situation.