Case Study: Surface Defect Analysis of Small Diameter, Cold Drawn Steel

One sample of 11.118mm to 11.112mm in diameter by 28cm length low carbon cold drawn steel was evaluated for detection of small surface defects of significantly less than 1% of volume. To enable adequate detection of small features of approximately .07mm deep (1) resonance-mode electromagnetic acoustic transducer (RM-EMAT) was utilized for this test. The transducer width is 28.7mm with an axial inspection window of about .19.05mm. The transducer has a .14.22mm ID and is a N:9 wavelength transducer that produced an axial shear resonant frequency of approximately 992.4 kHz. This produced significant shifts in resonant frequency on the defect as presented (greater than 800Hz). This sample was short so three screen plots were generated with information taken 7 cm before and behind the surface defect and one screen plot taken on the defect. The sample was repeatedly tested with the high-resolution transducer indicated a high degree of frequency stability and repeatability. There are other frequency indications on the sample that are caused by other smaller surface marks as also seen under low power microscope inspection however the frequency deviations are less than the deviation generated by the larger defect. The following plots indicate both the resolution and stability of this transducer operating with an ARIS unit. If there is a requirement to detect that are smaller in volume than those presented this toroidal transducer design can be configured for greater sensitivity by increasing its wavenumber up to N:11 with a resonant frequency of approximately 1.2 MHz.

Good Section Before Defect (long end)

Good Section after Surface Defect (short side)

Surface Defect