nt ndt 006_crack depth measurement_electric potential drop techniques_nordtest method

8/10/2019 NT Ndt 006_Crack Depth Measurement_Electric Potential Drop Techniques_Nordtest Method

1/31

CRACK DEPTH MEASUREMENT:

ELECTRIC POTENTIAL DROP TECHNIQUES

1. INTRODUCTION

1.1 Scope

This NORDTEST method specifies procedure for depth measurements of

surface breaking cracks in metallic materials by the use of electric

potential drop techniques.

1.2 Acceptance criteria

This NORDTEST method does not specify acceptance criteria. It is

referred to relevant codes or other documents specifying such cri-

teria.

1.3 Extent of examination

The NORDTEST method does not specify the extent of examination, in

terms of number of measurements along the detected crack.

published by

NORDTESTTekniikantie 12

FIN-02150 ESBO

Finland

key words classification

drop techniques UDC 620.179.18crack depth measurement

test method ISSN 0283-720X


2/31

- 2 -

1.4 References

Magnetic particle examination

- NORDTEST Doc. 286-81 "Magnetic Particle Examination of welded

joints in steel".

Penetrant examination

- ISO 3452 "Non-destructive testing. Penetrant inspection. General

principles".

- ISO 3879 "Welded joints. Recommended practice for liquid penetrant

testing".

1. 5 Symbols and definitions

Symbol

AC

ACPD

DC

DCPD

A1 and A2

B1 and B2

DPE

I

K

MPE

R

Description

Alternating current

Alternating current potential drop

Direct current

Direct current potential drop

Positions of current electrodes for

current supply to test specimen

Positions of potential electrodes for

measurement of electric potential

Dye penetrant examination

Crack depth

Cross-section area

Frequency

Applied current

Coefficient defined by crack depth

divided by potential drop

Magnetic particle examination

Electrical resistance


3/31

Symbol

s

V0

Vc

Definitions

- 3 -

Description

Distance between the two potential

electrodes B1 and B2

Voltage measured between the electrodes

B1 and B2 without crack in between

Voltage measured between the two potential

electrodes B1 and B2 with a crack present

between them

Skin depth, a measure for the penetration

of current into the test object

Permeability

Relative permeability

Vacuum permeability

Conductivity

Specific electrical resistance = l/

(i) Skin depth

The skin depth is the depth below the metal surface at which

the current density amplitude has dropped to l/e of its am-

plitude at the surface.

(ii) Permeability

The magnetic permeability of a substance may be defined as

the ratio of the magnetic induction in the substance to the

magnetizing field to which it is applied.

(iii) Conductivity

Electric conductivity is a measure for the quantity of elec-

tricity tranferred across unit area, per unit potential gra-

dient per unit time. The conductivity is the reciprocal of

the resistivity.


4/31

- 4 -

(iv) Ferromagnetic materials

Those materials in which the magnetic moment of atoms or

ions in a magnetic domain tend to be aligned parallel to

one another in zero applied field, below a characteristic

temperature called the Curie Point. The relative permeabi-

lity is much higher than one, r > 1.

(v) Non-ferromagnetic materials

Those paramagnetic and diamagnetic materials within which

the magnetic induction is proportional to the applied field

with r 1.

2. APPLICATION

2.1 General

Whenever a crack depth measurement is specified to be performed ac-

cording to this document, the applications are restricted to:

* surface breaking cracks.

* electrical conductive metals.

Further details on applications and restrictions for each individual

technique are described in Chapter 9.

The crack depth measurement can be performed as:

* a single spot measurement on a periodic basis.

* a monitoring technique to follow crack growth in specimens subject

to fatique or other crack propagating mechanisms.

* crack profiling by using several potential measuring electrodes

along the crack.

2.2 Required information

Reference to this document is insufficient to specify a proper crack

depth measurement. At least the following additional information has

to be given:

(i) Extent of examination, eg the number of measurement points,

and the number of readings at each point.


5/31

- 5 -

(ii)

(iii)

(iv)

(v)

(vi)

Precautions in case unacceptable cracks are found.

Operational Manual for the equipment.

Material quality information (magnetic/non-magnetic, possible

stress history).

Report on crack detection, eg magnetic particle examination,

dye penetrant examination, eddy current examination.

Any information available on crack type, characterization,

and origin.

The operator may ask for additional information which can be helpful

in the evaluation of measurements.

In order to evaluate the crack significance, the acceptance criteria

must also be specified.

3. PERSONNEL

The personnel responsible for applying an electric potential drop

technique to measure crack depths should be thoroughly trained and

qualified to carry out this method of examination. Moreover, the

personnel concerned should:

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

have adequate experience to carry out the inspection.

be familiar with the characteristic properties of the equip-

ment used, the principles on which it operates, and the

checking of its performance.

be familiar with the possibilities and the limitations of

the method used to detect the crack, and also other means

to detect surface breaking cracks.

be conversant with other methods to measure or estimate crack

depths.

be adequately familiar with the properties of the material

to be tested.

be familiar with the importance of crack type and crack con-

figuration on the reliability of measured depth.

be able to independently report the results of the tests.


6/31

- 6 -

All the personnel concerned should be of a high integrity and they

should carry out all instructions, procedures and stipulations

meticulously and not make any decision contradictory to them.

The personnel in question should keep themselves informed about de-

velopments in crack depth sizing.

Whenever necessary, the personnel in question should be submitted

to qualification tests to prove their proficiency.

4. SURFACE PREPATION

The material surface in vicinity of the crack to be sized must allow

for proper contact between the metal surface and the current and

potential electrodes. Differences in the transition resistance can

cause false results.

The surface roughness will also affect the reliability and accuracy

of the measured crack values, and this influence will depend on the

frequency used. Sizing of shallow cracks require high frequency cur-

rent which in turn require smooth surface due to the very small skin

depth (see also Section 6.2).

5. PERIODIC CHECK OF EQUIPMENT

A quality check of the crack depth measuring equipment shall be per-

formed at intervals not exceeding 6 months. The check shall assure

that all the operating modes function satisfactorily and the follow-

ing apparatus shall be included:

(i) the power supply, e.g. generator, battery pack, charger.

(iv) the measuring probe with all its potential electrodes and

the cabling to the control panel.

Whenever any of the equipment has been exposed to rough treatment

(iii) the control panel, e.g. registration unit, amplifier, crack

(ii) the current electrodes and their cabling to the power supply

depth presentation.


7/31


8/31

- 8 -

The skin depth can commonly be expressed as:

= (l/r0f)1/2 (1)

where

r = relative permability

0 = vacuum permability

= 4 x 10-7 H/m

= conductivity

f = frequency

As seen from Eq (1), the skin depth can be altered by varying the

frequency of the applied current. The choice of frequency will there-

fore influence the applications and limitations of the method.

In order to illustrate the frequency dependency, a typical mild steel

quality (r = 500, = 5.8 x 106 -1 m-1) will have its skin depth

changed from 1.3 mm to 0.13 mm when the frequency is altered from 50

Hz to 5 kHz. Obviously, the crack detection sensitivity and the sizing

accuracy will be affected by such differences in skin depths.

By referring to Fig. 6.1, the crack depth measurement principle is

based on the measured increase in voltage between two potential mea-

suring electrodes caused by the crack as the current will follow the

crack around the crack tip. Generally, the current electrodes (A1 and

A2) should be widely spaced so that a current flow, essentially per-

pendicular to the suspected crack, is maintained. Further, if the

potential electrodes (B1 and B2) then are placed along a field line,

the potential drop between them will be proportional to the current

path length between them. Therefore, with a fixed spacing (s) betweenthe two potential electrodes, the crack depth (d) can be determined

by the following equation, assuming an uniform electric field and a

crack much deeper than the skin depth:

V0/s = Vc/(s+2d) (2)

or, when expressed as crack depth

d = (Vc/V0-1) s/2 (3)


9/31

-9 -

where

V0 = voltage measured without a crack in between the potential

electrodes.

Vc = voltage measured with a crack in between the potential electro-

des.

Hence, by making two potential drop measurements, one in an uncracked

area and one across the crack, and knowing the potential electrode

separation, it is possible to determine the crack depth at any point.

Sizing of cracks with depth comparable to the skin depth must be per-

formed (if at all possible) with utmost care and only after a thorough

calibration. The Eq. (2) will not be applicable for such cracks.

Neither does Eq. (2) account for small corrections in the measured

crack depth due to skin depth effects in the corners between specimen

and crack face, and around the crack tip.

current lines

Fig. 6.1 Crack depth measurement by alternating current potential

drop technique (ACPD).

6.3 Direct current potential drop (DCPD) techniques

The DC method involves passing a constant current through the speci-

men volume to be inspected, and the DC potential drop is a result of

reduction in cross sectional area, and not of an increase in current

path around the crack tip as for the ACPD method described in Ch. 6.2.

By referring to Fig. 6.2, the current is applied between the electro-

des A1 and A2, and the potential increase can be measured between the

electrodes B1 and B2 as a result of the increased electrical resistance.


10/31

- 10 -

current lines

lines

Fig. 6.2. Crack depth measurement by direct current potential drop

technique (ACPD).

By applying Ohm's Law, the voltage reading at an undamaged surface

will be given by:

V0 = RI

= s I/F

where

(4)

V0 = voltage reading between the two potential electrodes

R = electrical resistance

I = applied current

= specific electrical resistance

s = distance between the potential electrodes

F = cross-section area transversed by current lines

In presence of a crack between the two potential measuring electrodes,

the voltage will increase to read Vc, due to the increased resistance

caused by reduction in cross-section area. The crack depth will now

be a function of the difference between the two voltage readings, i.e:

d = K(Vc - V0) (5)

where K is a function determined by calibration.

The equation (4) assumes uniform field distribution.


11/31

- 11 -

If the current electrodes are placed near the crack, the potential

increase will be significantly more influenced by the distance around

the crack than the reduced cross section area.

7. INSTRUMENTATION

The crack depth measuring instrumentation shall essentially consist

of three elements:

1) A current supply.

ACPD

The alternating current supply shall have an amplitude which is

matched to the chosen frequency in such a way that potential

drops will have a sufficiently high value to be measured reliably

by sensitive AC voltmeters.

In general, ACPD instruments available today fall into one of the

two following categories:

a) A supply being able to deliver low current (0.3 A to 3.0 A)

with a corresponding high frequency (1 kHz to 10 kHz).

b) A supply being able to deliver high current (at least 300 A)

with a corresponding low frequency of typical 50 Hz.

DCPD

The constant or direct current supply shall have an amplitude

control in order to:

* adjust the current density as required.

* be able to obtain a sufficiently high voltage value to be

measured reliably by sensitive DC voltmeters.

In general, DCPD instruments available today have a current supply

need of at least a couple of Ampres. Some instruments are even

based on direct currents exceeding 1 kA.

The current supply may well be an integrated part of the measuring

system, for ACPD as well as DCPD.


12/31

- 12 -

2) A voltage (potential drop) measuring device.

The measuring device shall contain a touch probe with two poten-

tial electrodes for voltage measurements, signal treatment elec-

tronics and a crack depth readout. The touch probe can preferably

be equipped with more than the two absolute necessary electrodes

in order to perform differential measurements. This type of mea-

surement is done to compensate for differences in material quali-

ties on each side of the crack, and/or to do reference and crack

measurements in one operation.

3) A calibration block.

A calibration block with notches shall be available in the same

material quality, as electric conductivity and magnetic permea-

bility are concerned. Further requirements to calibration are

described in Chapter 8.

8. EQUIPMENT CALIBRATION

8.1 General

The crack depth reading is strongly dependent on electric conducti-

vity (or resistivity), magnetic permeability and test object geo-

metry. It is therefore mandatory to perform a calibration. Generally,

if the test object material quality differs from that of an available

calibration block, slight differences can be balanced out by the in-

strument zero setting, if such exists, and for DC also by adjusting

the current amplitude from knowledge of the specimen cross section.

Greater differences in material qualities can be approximately com-

pensated for by working out a correction factor according to manufac-

turers procedure, if such is applicable.

However, when crack depth measurements are requested according to

this NORDTEST method, a calibration block as described in Section 8.2

must be available.

8.2 Calibration blocks

In order to perform a crack depth measurement in an accurate and reli-able way, a calibration block satisfying the following requirements

shall be available.


13/31

- 13 -

8.2.1

ACPD

Requirements for an ACPD calibration block will be:

* conductivity and permeability values similar to that of the mate-

rial to be examined.

* narrow notches with a constant depth over a distance of at least

five times the depth.

* calibration block length of at least 200 mm in order to allow

for a current pole separation necessary to obtain a uniform cur-

rent field distribution.

* calibration block width of at least 40 mm and always at least

twice the crack depth in order to minimize edge effects.

* calibration block thickness of at least 20 mm and always at least

50% more than the notch depth.

8.2.2 DCPD

Requirements for a DCPD calibration block will be:

* specific electric resistivity similar to that of the material to

be inspected.

* material thickness the same as for the test object.

* narrow notches with a constant depth across the whole width of

the block.

* the notch depths must not exceed 40% of thickness.

* calibration block length of at least 200 mm in order to allow for

a current pole separation necessary to obtain a uniform current

field distribution, if independent current electrodes are used.

* calibration block width of at least 100 mm and always at least

four times the crack depth in order to minimize edge effects.

the configuration of potential measuring electrodes and current

supply electrodes in the chosen DCPD instrumentation will signi-

ficantly influence the dependency between the object cross section

area and the depth reading. Therefore, if an unchanged current den-

sity is required between calibration and crack depth measurements,

the width of the calibration block shall be chosen accordingly.

The required block width will hence depend on current adjustment

capabilities.


14/31

- 14 -

8.2.3 NORDTEST Calibration Blocks

The NORDTEST Calibration Blocks for ACPD satisfy the stated require-

ments in Sections 8.2.1, and the range of blocks has the following

identification:

* NORDTEST Calibration Block ACPD No. 1 with notch depths between

1 mm and 10 mm in steps of 3 mm (see Fig. 8.1).


5 mm and 20 mm in steps of 5 mm.


10 mm and 40 mm in steps of 10 mm.

Calibration blocks for even deeper cracks can be specified accordingto the same principles.

The NORDTEST calibration blocks for DCPD satisfy the stated require-

ments in Section 8.2.2 and is schematically shown in Fig. 8.2. The

width should be 100 mm or, if applicable, as required to achieve a

current density comparable to that expected in the test object. The

block identification DCPD No. 1 means 1 mm slit, DCPD No. 2 a 2 mm

slit and so on.

Fig. 8.1. NORDTEST Calibration Block ACPD No.1.


15/31

-15 -

Fig. 8.2. NORDTEST Calibration Block DCPD No. 1.

The test block thickness shall be as for the test object

(T) and the width shall be 100 mm. If constant current den-

sity is required for reliable measurements, the width shall

by chosen accordingly.

8.3 Reference blocks

The calibration blocks shall be used to calibrate the crack depth

readout from an "ideal" testing situation, i.e, with minimized in-

fluence from limiting factors, with proper current field distribu-

tion and with clearly defined artificial cracks (notches), all in

order to have a standard way of reference for the measurements to

be performed.

If, however, the test object represents any limiting factor, as lis-

ted below and also described in Chapter 9, it is strongly recommended

to manufacture a reference block which contains actual limiting

factors. Such reference blocks will help to establish correction cur-

ves and to compensate for effects not included in the standard cali-

bration. Many of the limiting factors can be compensated for by in-

strument adjustments. Even so, it is recommended to use a reference

block to increase the measurement reliability. The reference block

should reflect the object to be examined, with respect to:

* physical size (thickness, width, length)


16/31

- 16 -

* surface geometry (e.g. curved surfaces, corners)

* surface condition (e.g. smooth, rough, electric contact conditions)

* electric conductivity (for AC and DC) and magnetic permeability

(for AC)

* position of artificial cracks (e.g. in corners, near edges)

* profile of artificial cracks (e.g. elliptic, semi-circular, more

randomly variable)

* crack depths

* crack orientation (e.g. normal/oblique to the surface)

Several of the DCPD techniques require constant current density

between the calibration and the crack measurements. In such cases,

the use of a reference block will be of significant value.

9. APPLICATIONS AND LIMITATIONS

9.1 General

In the application of potential drop techniques for sizing of crack

depths there are some restrictions and weaknesses which should be

recognized by testing institutes, quality departments, operators and

also by those establishing testing specifications and setting up code

requirements. These limitations as well as recommended applications

are described in the following sections.

9.2 Types of crack

9.2.1 ACPD

The potential drop techniques are all intended for depth measurement

of surface breaking cracks. The crack to be sized must be open, which

means without metallic contact between the two crack faces. Fatique

cracks usually fall into this category.

9.2.2 DCPD

The potential drop techniques are all intended for depth sizing of

surface breaking cracks. The DCPD method is not as sensitive as the


17/31

- 17 -

ACPD to metallic contact between the two crack faces, if the contact

areas only will be a small fraction of the total crack area. Anyway,

sizing of cracks without metallic bridging, like most fatique cracks,

will be among the most favourable.

9.3 Material quality

9.3.1 ACPD

It is generally recommended to use the AC technique only on ferromag-

netic (ferritic) materials with r > 1.

Crack depth sizing in non-ferromagnetic materials will imply drop in

sensitivity and should only be performed when recommended equipment

and procedure are available from the manufacturer, and only after a

thorough calibration on the same material quality.

9.3.2 DCPD

Crack depth sizing with DCPD can be performed in non-ferromagnetic

as well as ferritic materials, but only after a thorough calibration

on the same material quality.

9.4 Specimen size

9.4.1 ACPD

The AC potential drop techniques do not have any significant limita-

tion with regards to specimen size, because the current is flowing

in a thin surface layer. Hence, an increase in thickness will not re-duce the sensitivity.

9.4.2 DCPD

The size of the DC potential drop caused by a crack depends on the

crack area combined with the current density in the specimen across

the defect plane. Adequate current densities can generally only be

achieved in small samples, which means that very large currents are

required to produce sufficiently large current densities for crack

sizing in large sections. The direct current (DC) method is therefore

impractical for larger components.


18/31

- 18 -

9.5 Material surface geometry

The AC and DC potential drop techniques can be used on various sur-

face geometries, such as plates, pipes, other curved surfaces, tu-

bular joints, T-butt welds and corners of various angles, and on

weldment as well as on parent metal. It is usually a matter of calibra-

tion procedure which defines the limits of applications.

One important difference is that the surface geometry much more signi-

ficantly will affect the DCPD measured results than results obtained

with ACPD.

9.6 Edge effects

9.6.1 General

If a crack runs towards an edge of the specimen, e.g. the specimen

side, and a depth sizing is performed in this area, the depth read-

ing will be significantly affected due to disturbances (compression

of current lines) in the current flow.

If a reading has to be performed close to a specimen edge, where

false readings are expected, correction curves, as illustrated in

Fig. 9.1, must be established in order to maintain the measurement

reliability, by using the edge on the calibration block.

DISTANCE FROM EDGE (MM)

Fig. 9.1. Illustration of correction curves for crack depth measure-

ments close to specimen edges.


19/31

- 19 -

9.6.2 ACPD

As a general rule, correct depth will not be indicated unless the

measuring position is located away from the edge by at least the

depth of the crack. For small cracks depths, the distance from the

edge should be even two or three times the depth.

Since the depth of the crack is usually unknown, the distance to be

kept has to be determined by a reading close to the edge, and then

this reading must be used as the basis for probe positioning.

If a crack runs parallel to one of the edges of the test piece, no

significant error in the readings is to be expected, even if the

distance to the edge is no more than that required to apply the probe.

9.6.3 DCPD

As a general rule, correct depth will not be indicated unless the

measuring position is away from the edge by at least 30 mm or at least

the depth of the crack, for depths exceeding 30 mm.

Since the depth of the crack is usually unknown, the distance to be

kept has to be determined by a reading close to the edge and then

this reading must be used as the basis for probe positioning.

9.7 Cracks at oblique angles

Basically, the extension along the crack is measured, and not the

actual projected depth to which it penetrates from the surface, un-

less the crack is penetrating normally. Hence, if the depth of an

oblique crack is the vital parameter, the oblique angle must be found

by some other gauging procedure, and then the depth can be calculated.

9.7.2 DCPD

Depth sizing of cracks at oblique angles will result in meter read-

ings lying between the depth of the crack as measured along the crack

faces and the projected depth to which it penetrates from the surface.

9.7.1 ACPD


20/31


21/31

- 21 -

than approximately:

* a few skin depths from through-thickness penetration for ACPD.

* 40% of the thickness for DCPD.

The use of a reference block is then recommended to maintain reli-

able depth measurements.

9.10 Multiple cracks

Great care should be taken in presence of multiple cracks. Additional

cracks in the vicinity of the crack to be measured influence the cur-

rent distribution and can be the reason for erroneous measurements.

As the geometry of the several cracks at first is unknown, no pos-sibility of proper correction exists.

9.11

Current supply

The current amplitude must be kept at a constant level and with a

minimum of fluctuations through all stages of the measurement, the

calibration, the zero adjustment and the measurement across the

crack.

9.12

Crack tip stresses

Crack tip stresses can cause a crack closure, and hence, an unde-

restimation of the crack depth, normally up to approximately 1 mm.

Crack tip stresses will also cause changes in material quality and

therefore also in the skin depth, which accordingly will produce

an error in the depth reading. An applied strain to the test speci-

men causing the crack to be open is recommended.

9.13

Bridging

A firm metallic contact between areas of the two crack faces will

cause electrical bridging as the current takes the path of least

resistance. The result is often a considerable depth underestimation.

The ACPD techniques are more sensitive to bridging than the DCPD

techniques.


22/31

- 22 -

10. ACCURACY AND RELIABILITY

10.1 General

The accuracy and reliability in crack depth sizing can generally be

affected by several factors. Among such factors are the equipment

itself, the crack geometry, the surface geometry as well as varia-

tions in the material quality, e.g. variations in the conductivity

and permeability from base material to welds. The operator and the

measurement procedure can also significantly affect the results. In

order to obtain the best achievements on surfaces other than those

plane and smooth, a reference block should be used (see Chapter 8).

A reference block or corresponding correction curves must also be

used to minimize uncertainties associated with edge effects, deep

and short cracks, and other limiting factors.

The accuracy in depth sizing will usually also be improved by using

a measuring probe with more than two potential electrodes, for the

purpose of achieving differencial measurements in order to eliminate

material differences in the vicinity of the crack.

Typical achievements for crack depth meters are listed in the two

following sections.

10.2 ACPD

The accuracy (standard deviation) in sizing will be of order:

- 0.2 mm on artificial cracks, measured in laboratory conditions.

- 1.0 mm on fatigue cracks, measured in in-service environments.

Crack closure stresses can cause underestimations of crack depths.

The accuracy and reliability in sizing crack types with a signifi-

cant probability for electrical bridging between the crack faces,

are poor.

The low frequency/high current techniques are more accurate than

the high frequency techniques on rough surfaces.


23/31

10.3

11.

11.1

- 23 -

The current supply leads as well as external electromagnetic noise

can cause electromagnetic fields which can be picked up by the vol-

tage probe leads or the probe itself, and create unreliable measure-

ments. The current supply leads should therefore by widely separated

from the probe cables.

The accuracy in relative measurements will be far better than the

values for absolute measurements indicated above. Relative measure-

ments can be obtained by monitoring crack growth with the potential

electrodes permanently attached to the specimen.

DCPD

The measuring accuracy (standard deviation) is generelly within 10%

of the actual crack depth. The use of a thoroughly prepared proce-

dure can improve the accuracy significantly.

The accuracy in relative measurements will be far better than the

values for absolute measurements indicated above. Relative measure-

ments can be obtained by monitoring crack growth with the potential

electrodes permanently attached to the specimen.

Crack closure stresses can cause underestimations of crack depths.

The accuracy and reliability are significantly lowered when sizing

crack types with a probability to have major areas for electrical

bridging between the crack faces.

The surface curvature is of particular importance for the measuring

accuracy when using DCPD.

EXAMINATION PROCEDURE

Crack detection

The potential drop techniques described in this NORDTEST method are

primarily designed for crack sizing, not crack detection. Hence, the

detection must be performed with some other means of non-destructive

testing, and then marked properly on the surface in order to make a

reliable positioning of the potential drop measuring equipment.


24/31

- 24 -

11.2 Preparation

Before the actual crack depth sizing is performed, the following

preparations should be carried out:

(i)

(ii)

(iii)

(iv)

(v)

The surface of the test piece, where the measuring probe

and the current electrodes will make contact, must be pro-

perly cleaned to allow for good electric contact (Chapter

4).

During set-up of the instrument, a visual check should be

performed to reveal possible damages of the equipment. The

instrument set-up shall be performed according to relevant

operating instructions.

The operator shall make himself/herself familiar with, and

be sufficiently trained on, the actual equipment in order

to obtain reliable measurements.

The operator shall provide for all information required, as

listed in Section 2.2.

The operator shall ensure the existence of quality check

documents, as described in Chapter 5.

11.3 Calibration

The calibration shall be performed on a NORDTEST Calibration Block

or an equivalent block satisfying the requirements listed in Chapter

8. The depths of the notches shall cover the expected measurement

range.

The current electrodes shall be positioned towards the ends of the

calibration block and with an electrode separation of at least 150

mm. In systems were the current electrodes are an integrated part

of the measurement system, the current electrodes shall by placed

as required by the operation manual procedure. The electrodes must

be positioned with one electrode on each side of the notch to be

sized.

The measuring probe must be placed in between the current electrodes,

as indicated in Fig. 6.1. The alignment of the potential electrodesmust be as parallel as possible to the current lines.


25/31

- 25 -

The calibration can now be performed by:

* Zero adjustment at an area of the calibration block without notch.

* Establishing a correspondance between instrument readings and real

notch depths. The alignment of the potential electrodes must be as

perpendicular as possible to the notch length direction, with thenotch in between the two potential drop measuring electrodes.

The calibration as described is mandatory.

For AC techniques this calibration must not be altered during a pres-

cribed measuring period.

For DC techniques the calibration can be altered as required to fit

the test object size to obtain constant current density.

An introduction of reference blocks to simulate real test conditions

shall not influence the calibration settings, but only be used to

establish correction curves referred to the calibrated values.

11.4 Crack sizing

When the required calibration has been carried out, the examination

procedure will be as follows:

(i) The current amplitude and frequency shall not be altered dur-

ing calibration.

(ii) If applicable, necessary correction curves may be established

by measuring depths of artificial cracks manufactored in a

reference block. The placement of current electrodes should

be similar to that possible at the surface of the test object,

and in accordance with the operating instructions of the

equipment. The reference values can then be found by:

* zero adjustment at an area of the reference block without

notch, and with a surface geometry similar to the one that

will be used for zero adjustment on the test object.

* establishing a correspondance between instrument readings

and real notch depths. The alignment of the potential

electrodes must be as perpendicular as possible to the

notch length direction, with the notch in between the two

electrodes.


26/31

- 26 -

The measuring probe must be placed in between the current

electrodes, and the alignment of the potential electrodes

must be as parallel as possible to the current lines.

(iii) The sizing of crack depths in the test object can then be

carried out. The placement of current electrodes should beas for the reference block measurements, if applicable, and

in accordance with the operating instructions of the equip-

ment.

The crack depth values can be found be the following general

procedure (it is referred to the operating instructions for

the actual equipment for further details):

* zero adjustment at an untracked area of the test object,and at a surface similar to the one used for zero adjust-

ment on the reference block, if applicable.

* carry out crack depth measurements in one or more places

along the crack. The alignment of the potential electrodes

must be as perpendicular as possible to the crack length

direction, with the crack in between the two electrodes.

The measuring probe must be placed in between the current

electrodes, and the alignment of the potential electrodes

must be as parallel as possible to the current lines.

(iv) All crack depth readings should be corrected according to

the calibrated values, according to established correction

curves from a reference block, and according to applicable

procedures described in the operation manual.

(v) A new calibration, as described in Section 11.3, shall com-

plete the crack sizing procedure.

(vi) A demagnetization shall be performed, if required,

(vii) The reporting shall be performed according to requirements

listed in Chapter 13.


27/31

- 27 -

12. EVALUATION OF RESULTS

The evaluation of results in electric potential drop techniques is

not a matter of defining reporting/registration levels since all

readings of crack depths should be registrated. Normally, the crack

depth reading shall be evaluated according to calibration data, and

in particular according to readings obtained on reference objects,

if such have been carried out.

However, sometimes the readings will be of a character not expected,

and additional information would be required in order to gain confi-

dence in the sizing. Therefore, the "requirements to evaluation"

will more have a character of general advice than strict require-

ments. Such advice may be:

(i) Unreliable readings or successive readings with significant

deviations in depth reading at the same location, and with

unchanged test conditions, could likely be cause by:

* unstable electrical bridging between the two crack faces,

or

* crack closure effects caused by stresses.

The general advice, if applicable, would be to apply strain

to the specimen with the purpose to open the crack and si-

multaneously measure the crack depth again.

(ii) Readings indicating zero crack depth or a significantly

underestimated crack depth, even in presence of verified sur-

face indication, would probably be caused by firm and stable

contact between the crack faces, which again is characteristic

of multiple cracks on line (cracks usually produced during

fabrication, and not by fatigue in operation). An advice in

such cases will be to carry out or verify the sizing by a

different technique, e g, ultrasonics or eddy current tech-

niques.

(iii) Depth measurements showing strong variations along the crack

length are probably caused by irregular crack profile.

Reliable depth estimates can then only be obtained by using

correction curves based on artificial crack similar to the

expected crack profile.


28/31

- 28 -

13. REPORTING

The results of crack sizing by potential drop techniques shall be

given in a report which should include all the necessary informa-

tion required to:

(i) Take decisions on acceptance of the crack revealed by the

ACPD or the DCPD technique.

(ii) Facilitate repair of unacceptable cracks.

(iii) Permit the crack sizing to be repeated.

The report should therefore include the following information:

a) Job identificationb) Test object identification, drawings and dimensions

c) Time and place of the examination

d) Ambient conditions of the examination

e) Name and signature of the operator

f) Relevant material characteristics

g) Surface condition and geometry

h) Data related to equipment, e g, manufacturer, serial no, type

(including ACPD or DCPD), current amplitude, frequency, probe

description

i) Description of calibration block and report on calibration data

j) Description of reference blocks, if used, and report on reference

data

k) Data concerning the results from the examination

l) Additional data which may concern limitations of the examina-

tion because of surface geometry, crack geometry or others

m) Specific requirements agreed upon by the parties involved

n) Crack profile description, if required.


29/31

- 29 -

Annex A

REFERENCES

1. Non-Destructive Testing Handbook.

Editor Robert C. McMaster, n, 9,218 (1981).

6. Hayashi, M. et al.: "Surface crack detection by direct current

potential drop technique". Proc. WCNDT, Las Vegas 1985, p.239.

7. Hayashi, M. et al.: "DC potential crack detection system for

single edge crack". Proc. WCNDT, Las Vegas 1985, p.245.

8. Dover, W.D. and Collings, R.: Recent advances in the detection

and sizing of cracks using alternating current field measure-

ments (A.D.F.M.). Br. Journ. of NDT, NovV. 1980, p.298.

9. Duncumb, A.C. and MudgCNDT, Las Vegas 1985, p.239.

7. Hayashi, M. et al.: DC potential crack detection system for

single edge crack". Proc. WCNDT, Las Vegas 1985, p.245.

8. Dover, W.D. and Collings, R.: "Recent advances in the detec-

tion and sizing of cracks using alternating current field

measurements (A.C.F.M.). Br. Journ. of NDT, Nov. 1980, p.298.

9. Duncumb, A.C. and Mudge, P.J.: "The alternating potential drop

method for surface crack depth measurements". The Welding

Research Bulletin, Jan. 1982, p.13.

10. Webborn, T.J.C.: "Crack depth measurements using AC potential

drop": Mat. Eval., Febr. 1982, p.156.

11. Aboutorabi, A.A. and Cowling, M.J.: "Measurement of crack pro-

file of semielliptical surface cracks using AC potential

technique". NDT INTERNATIONAL, Vol 16, No 3, June 1983, p.139.


30/31

- 30 -

12. Uemura, T. et al.: "An improved AC electric potential method

and its application to detection of micro fatigue cracks'.

The NDT Journ., Japan, Vol 1, No 3, 1983, p.139.

13. Tomlinson, F.R.: "Monitoring crack growth by a potential drop

method". Proc. Europ. Conf. on NDT, Mainz 1978, 72/637.

14. Haugen, R. and Rangnes, E.: "A new instrumentation for fatigue

crack depth measurement with AC potential drop technique.

Proc. Europ. Conf. on NDT, Vienna, 1982.

15. Dalberg, P., Haugen, R. and Myrhaug, 0.: "Crack depth determi-

nation. The AC potential drop technique". VERITAS report No 81-

1079, 1981.

16. Hicks, M.A. and Pickard, A.C.: "A comparison of theoretical and

experimental methods of calibrating the electrical potential

drop technique for crack length determination". Int. Journ. of

Fracture, 20 (1982), 91-101.

17. Dover, W.D., Charlesworth, F.D.W. and Taylor, K.A.: "Alternate

current field measurements. A new method for detecting and mea-suring fatigue cracks". Advances in Fracture Research, Vol 4,

Cannes, France 1981.

18. Operating Manual. Crack depth measuring instrument RMG 4011.

Karl Deutsch.

19. Operating Instructions. ACPD crack depth gauge U8 Crack Micro

Gauge. Wells Krautkramer.

20. Technical data. Universal crack depth meter X-RT 804.

Krautkramer.

21. CDP3 crack detector. Testwell Ltd. Equipment news. Materials

Evaluation, 44, July 1986, p.922.


31/31

22. CGM5 ACPD crack growth monitor. Matalect Ltd. Equipment News.

NDT INTERNATIONAL. June 1986, p.224.

23. Dover, W.D. and Bond, L.J.: "Weld crack characterization on

offshore structures using AC potential difference and ultra-

sonics". NDT Int. 19, (4), p.243-247, Aug. 1986.

nt ndt 006_crack depth measurement_electric potential drop techniques_nordtest method

Documents