Question

cat lest and explain the importance ID (b) What is Standard Depth of Penetration (6) for Eddy current inspection? who factors affecting standard depth of penetration? Estimate the per depth of penetration? Estimate the percentage of eddy current density at a hight of 4 8 from the surface ") for Eddy current inspection? What are the important

Answer 1

Eddy current inspection is a Non destructive testing method which generally makes use of electromagnetic induction process to inspect and object.

The standard depth of penetration(d) is the depth at which the eddy currents show a very good result while carrying any test.simply beyond this depth the eddy current inspection is not much reliable as beyond this point the eddy current density becomes low and there by the strength of the eddy currents to locate the flaw is also less, and the generally the standard depth of penetration is the depth at which the eddy current density has been decreased to 1/e or reduced to 37% of the surface density.

(when electricity flow in a conducting material the current density is more at the surface and decreases with depth )

now normally for one standard depth of penetration(d) the current density has been reduced to 37% of surface density, at two standard depth of penetration(2d) the current density is reduced to 13% of surface density and at 3d it is 5% and reduced with depth.

the standard depth of penetration is generally calculated by

d = 1/γπμσf

where d in mm.

Factors affecting standard depth of penetration

-> The magnetic permeability($\mu$ ) increases and the standard depth decreases

-> The frequency () of the excitation current increases and the standard depth decreases

-> The conductivity of the material also affects the standard depth of penetration as conductivity decrease the standard depth increases

-> The condition of the surface that is the roughness of the surface,surrounding temperature etc also effect the standard depth.

for 4d depth the % of eddy current density

generally current density

i =i,*e -r/d

where $i_{x}$ = current density

$i_{o}$ = current density at surface

x = depth below surface

d = standard depth of penetration

now for 4d and from this we get finally at 4d

iz = i, *e -4

   

iz _ lotet -= 0.183 20 20

hence % eddy current density at 4d is 1.83% or 2%(standard)