Shearographic Detection of Internal Defects in Austenitic Stainless Steels

Despite the formation of a passive layer, corrosion also occurs in austenitic stainless steels. One of the most common and most dangerous types is pitting corrosion, in which the material beneath the surface is completely dissolved. Since this type of corrosion only produces small holes on the surface, it is difficult to detect without nondestructive testing. This paper presents induction-excited shearography as an optical, nondestructive testing method for quick inspection of inner defects and corrosion. The investigations were carried out on test specimens with blind holes made of austenitic stainless steel 1.4301 (AISI 304). The detection limits of different defect sizes were determined objectively based on the signal-to-noise ratio (SNR). The results show that inner defects with a size of Ø 2 mm can be detected via SNR in a depth of up to 1.5 mm. Larger defects can be detected in greater depth. The data obtained were validated on real test specimens. The measurement of a more realistic defect geometry showed higher SNR values compared to the idealized test specimens. From the measured SNR values of realistic defect geometry, it can be concluded that real irregularities have higher SNR values than defects with flat bottoms. The results show that induction-excited shearography is suitable for the nondestructive testing of austenitic stainless steels.