טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentHagit Saguy
SubjectIdentification of Cracks in Metal Conductors by Electrical
Potential Technique
DepartmentDepartment of Mechanical Engineering
Supervisor Full Professor Rittel Daniel
Full Thesis textFull thesis text - English Version


Abstract

 Electrical potential techniques for crack depth measurements have been used for over 40 years to monitor critical cracks in metallic fatigue and fracture specimens. These methods are based on the fact that there will be a disturbance in the electrical potential field around any discontinuity in a current-carrying body (AC or DC).   The magnitude of the disturbance depends directly on the size, location and shape of the discontinuity. 


  When an alternating current (AC) is passed through a conductor the current is confined to a layer or skin at the surface.   The existence of the “skin effect” phenomenon means that the effective cross section carrying the current is smaller compared to DC systems.  This fact, improves the sensitivity of crack size measurement.


Based on the literature review we found two limitations of the Altenating Current Potential Drop (ACPD) technique.  First, there is a good estimation of the crack depth for thin skin conditions, but the thick skin approximation gives a lower limit of the crack depth and is not a tight estimation. 

The second limitation of the ACPD technique is that it was applicable only to surface breaking cracks.  There was no solution for estimating the depth of bottom breaking cracks and internal cracks.

In this work, we specifically address the behavior of AC current flow in metal conductors with different flaws.  We focus on three different flaws: surface breaking notches/cracks, bottom notches/cracks and internal notches/cracks. 


We initially model the surface crack situation and examine the current behavior as function of the frequency. After identifying the current behavior close to the crack (corner effect) we develop a global theoretical solution which bridges the thin and the thick skin solutions for surface breaking cracks (uniform and non uniform crack depth). 

Next, we model the inner and bottom crack situations and examine the current behavior as a function of the frequency.  Through detailed numerical simulations, we develop a tomographic-like methodology and a new associated criterion to reveal and estimate the size and location of internal and bottom cracks.  

Finally, the proposed equations and methodology (surface and inner crack) are verified experimentally. This includes uniform depth notches and actual sharp fatigue cracks, either hidden or apparent.  The experimental results show an excellent agreement with the new theoretical framework.

The new methodology is deemed to significantly extend the range of applications field of the ACPD as an advanced Non Destructive Testing (NDT) tool.