Correlation Study between the Surface-Magnetic Response and Sinusoidal Axial-Tensile Strain on a Butt-Weld Connection Performed on ASTM A36 Steel

A series of nine butt-welded coupons were tested dynamically following a sinusoidal load that reached 0.50, 0.60, and $0.70{\mathrm{F}}_{\mathrm{y}}$ for an A36 low-carbon steel. This load was applied at 5 Hz with an initial $0.10{\mathrm{F}}_{\mathrm{y}}$ offset to maintain a full-tension test at all times to avoid compression. Two main variables were of interest for the present research: (1) applied strain; and (2) surface magnetic field. Strain gages and magnetometers tracked the said variables at two locations: at the base metal and at the proximity of the heat affected zone (HAZ). The testing was aimed for fatigue at high-load levels, and it stopped once it reached 100,000 cycles. At lower levels, it remained a nondestructive test. The results present a semielliptical correlation between the strain and surface magnetic field similar to the classic mechanical correlation between stress and strain, presenting some type of magnetic hysteresis. The said semielliptical correlation is not steady in time, and it shows a presumable progression of damage accumulation through a second-order parameter extracted from the said correlation, which shows a clear agreement (by means of a complete reversal in magnitude) with classical S-N fatigue failure behavior. Also, when analyzing the behavior in time, complete reversals in phase or out-of-phase behavior between the semisinusoidal magnetic signals at early stages of the test differ from those at late stages. This particular change in time also indicates a physical change on steel as testing continues, and it can be traced in a timely fashion by tracking the magnetic response in both base metal and HAZ areas. This nondestructive analysis of surface magnetic fields suggests that magnetic fields, although small in magnitude (measured in $\mathrm{\mu T}$), can be used as a nondestructive means for determining damage accumulation and to separate safe stages from nonsafe stages of service for a butt-welded tension member.