Nondestructive characterisation of damage evolution in advanced high strength steels

Damage evolution in two different Advanced High-Strength Steels (AHSS), namely a Dual-Phase (DP) and a Transformation Induced Plasticity (TRIP) assisted steel, has been analyzed with two specialized non-destructive methods (Micromagnetic Multiparameter Microstructure and Stress Analysis, 3MA, and ultrasonic analysis using ElectroMagnetic Acoustic Transducers, EMATs). The 3MA method was applied in order to characterize the interaction between domain walls and the internal defects, whilst the EMAT method allowed evaluation of the efficiency of the dynamic magnetostriction and ultrasonic attenuation. The analysis was carried out for two groups of samples: group one, samples deformed to pre-determined strain levels and group two, samples measured during deformation in a tensile testing machine. The results of all measurements were compared with a parallel series of deformed samples in which the damage was investigated with both optical as well as electron-microscopy techniques. An automated evaluation of the specially prepared samples allowed a quantification of the damage in a statistically representative manner. The microscopic analysis revealed that specific changes in the 3MA signal were coupled with the onset of the first micro-cracks. This signal was also related to the further evolution of damage in these samples. Finally, the characteristics of damage development in a DP and a TRIP-assisted steel were compared and related to stages of deformation such as uniform elongation, necking, and imminent failure.