Submitted byName-amit kumar banra

Reg no-130301EER037

Branch - EEE

1 Introduction

2 Depth of penetration

3 Eddy current v/s current density

4 Eddy current depth of penetration

5 Eddy current how it penetrates

6 Equation to calculate standard depth

penetration

7 Example copper how it penetrates

8 problems 1 / 2

9 Numerical 1 / 2

10 conclusion

CONTENTS

The intensity of eddy currents decreases exp

onentially with depth in a material. The inte

nsity at any given depth isaffected by the sa

me variables that influence the surface inten

sity of eddy currents, although not always in

the samemanner or by the same amount. To put

it another way, the depth of penetration of a

specific intensity of eddy current sis affected by

the variables, as indicated in Table 4-

4. Generally, any parameter that increases the

depth of penetrationwould provide an equivalent

eddy current response at a greater depth in a

test part.

STANDERD DEPTH OF PENETRTION

Three of these variables (conductivity, relative magnetic permea

bility, and frequency) are used to define the standarddepth of

penetration. Standard depth of penetration is the depth below the

surface of the inspection article at which

themagnetic field strength, or the intensity of the induced eddy

currents, is reduced to 36.8 percent of the value at thesurface.

(0.368 = 1/e, where e = 2.71828, the base of natural logarit

hms). The standard depth of penetration isexpressed by the

following formula

Penetration depth

.electromagnetic radiation is incident on the surface of a material, it may be (partly) reflected from that surface and there will be a field containing energy

transmitted into the material. This electromagnetic field interacts with the atoms and electrons inside the material. Depending on the nature of the material, the electromagnetic field might travel very far into the material, or may die out very quickly. For a given material, penetration depth will generally be a function of

wavelength

EDDY CURRENT vs CURRENT DENSITY

Eddy current density does not remain constant across the depth of a material. The

density is greatest at the surface and decreases exponentially with depth (the "skin

effect"). The standard depth of penetration equation (shown to the right) is used to

explain the penetration capability of eddy current testing, which decreases with

increasing frequency, conductivity, or permeability

EDDY CURRENT DEPTH OF PENETRATION

Eddy current how it penetrates

The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (d). The word 'standard' denotes plane wave electromagnetic field excitation within the test sample (conditions which are rarely achieved in practice). Although eddy currents penetrate deeper than one standard depth of penetration, they decrease rapidly with depth. At two standard depths of penetration (2d), eddy current density has decreased to 1/e squared or 13.5% of the surface density. At three depths (3d), the eddy current density is down to only 5% of the surface density.

Eddy current penetratration

Since the sensitivity of an eddy current inspection depends on the eddy current density at the defect location, it is important to know the strength of the eddy currents at this location. When attempting to locate flaws, a frequency is often selected which places the expected flaw depth within one standard depth of penetration. This helps to assure that the strength of the eddy currents will be sufficient to produce a flaw indication. Alternately, when using eddy currents to measure the electrical conductivity of a material, the frequency is often set so that it produces three standard depths of penetration within the material. This helps to assure that the eddy currents will be so weak at the back side of the material that changes in the material thickness will not affect the eddy current measurements.

Equation to calculate standard depth penetration

Where

δ = Standard Depth of Penetration (mm)

π = 3.14

f = Test Frequency (Hz)

µ = Magnetic Permeability (H/mm)0 = Electrical Conductivity (% IACS)

Example of the material copper

questions

Q.1.What is the standard depth of penetration when performing an

eddy current inspection on a piece of 304 Corrosion Resistant Steel?

The material has a relative permeability of 1.05 and a conductivity

of 2.5% IACS. The frequency used to drive the eddy current probe is

50 kHz

Q.2. Determine the frequency needed to achieve a depth of

penetration of 2mm in 7075-T76 aluminum plate

-

soloutionsQuestion 1.

soloutionsQuestion 2 .

NUMERICALS 1

Numericals 2

conclusion“I would like to thank all of my friends and “EMT sir, MR

ARINDAM BHATTACHARJEE..for giving me this opportunity for

presenting a ppt on such a wonderfull topic,as In the class I didn’t

understand too much about this topic but when I was given the topic

“depth of penetration” as my topic of presentation I came to know

many things about this chapter,by researching it on google,books

etc!

And at last not the least, I thank you for taking out time to read out

my ppt.