Estimating the Depths of Cracks in SS Pipes by Varying the Frequency of an ACPD Sensor

Alternating Current Potential Drop (ACPD) is one nondestructive evaluation method for detecting cracks within stainless steel pipes at industrial power plants. It is well known that the penetration depth of the excitation current from the ACPD sensor depends on the frequency of the applied alternating current. With respect to a “skin effect,” lower frequencies produce deeper penetration. This frequency versus depth relationship can be used to judge the depth of cracks that may exist within stainless steel pipes. By varying the frequency of the alternating current supplied to the excitation circuit of an ACPD sensor, crack depths can be estimated. If the penetration depth of the excitation current nearly reaches the depth of the crack tip, then the existence of the crack will cause an increase in the current and a corresponding decrease in the potential drop measured by the ACPD sensor. On the other hand, if the penetration depth of the excitation current reaches beyond the depth of the crack tip, then the crack impedes the electrical current and causes a corresponding increase in the potential drop measured by the ACPD sensor. The frequency at which the measured potential drop flip-flops from a decrease to an increase corresponds to the depth of the crack tip, which can be calculated by using the standard equation for skin depth according to excitation frequency.