1987

Sundell, Henrik

The objective of this work has been to investigate the applicability of a switching direct current potential difference (DCPD) method to crack length estimations in ferromagnetic materials. The test materials have been stainless steel (AIS1316) pipes (diameter 10 mm) and compact tension (CT) specimens of different steel compositions. In creep crack measurements and with slow loading rates the direct current method is suitable. The crack extension can be calculated from the potential readings and the crack extension is in good agreement (error less than 10 percent) with optical measurements and 2-dimensional metal film measurements. The DCPD results seem to be better than the results from the unloading compliance method, which was used simultaneously. The DCPD equipment developed is very stable and is suitable for long term creep crack tests. The DCPD methods not suitable for rapid loading rates. It appears that the presence of stress in high current density fields induces magnetoelastic effects which overwhelm the DC potential changes during high loading rates with magnetic structural materials.