Plastic damage assessment of Q690 high strength structural steel derived from the eddy current array technique

Abstract

This paper reports a novel use of eddy current array technique to assess the plastic damage of a high strength Q690 steel. The eddy current array parameters can not only effectively judge the severe damage area but also can characterize the early plastic damage of specimens. Under a detection frequency of 375KHz, the mean axial voltage increased from begin-stretch (0.01 V) to fracture (1.83 V). Meanwhile, the digital image correlation tests showed that the strain value increased from 0 % to 14.785 % at this process. Coherently, it was found that the eddy current array parameters had a certain linear relationship with the strain degree. Besides, the microstructure observation revealed that the voids generation/growth as well as the dislocation structure/density change in Q690 steel aggravated with the strain degree. In the early plastic deformation stage, the void defect could be the influence factor to the change of eddy current array parameters, since the axial voltage exhibited a significant increase in relation to both void area fraction and number density, ranging from 0 % to 1.519 % and 11.636 × 10⁴/mm², respectively. In later stage, the change of voltage could be due to the joint effect of void factor and dislocation factor, since its growth from the void defect slowed down, while it increased steadily with the increase of GND density, ranging from 2.12 to 6.31 × 10⁴/m². In summary, the current array eddy technique could be an effective detecting method, which can characterize the plastic damage degree of Q690 steel even at the early stage.