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A surface-coating technique for monitoring stressed structures to detect open cracks
A surface-coating technique for monitoring stressed structures to detect open cracks M. Sorel A passive and continuous monitoring system for detecting open cracks on structures under dynamic stress, utilising a coating applied to critical areas, is described. The physical parameter monitored in this technique is electrical insulation loss measured by means of conventional apparatus. The application of the coating is easy, rapid and suitable for areas with complex geometry. Detection is accurate and reliable. The cost of the system as well as its possible uses make it suitable for industrial application.
Introduction Many current processes are capable of continuous detection of open cracks appearing on parts or structures under dynamic stresses, but all of them have specific limits of use. We have applied ourselves to work out a process which can meet the following conditions: •
unsophisticated design involving conventional equipment; possible adaptation to areas with comparatively complex geometry;
• passive, easy, rapid and reliable detection; •
The solution This element constitutes the very originality of this new process. The major features of that solution are as follows.
•
• quick and easy installation; •
The flexible coating This compound can cure at ambient temperature; once cured, it can fit in a quite flexible way the parts to be tested. Moreover, this coating is transparent and fully waterproof.
Its high wetting power enables the solution to deeply enter any interstice. This power results from the action of a non-ionic surface-active agent which reduces the interfacial tension between fluid and metal down to about 30 dynes/cm.
utilization in industrial environments;
• non-corrosive process and easy removal;
Flexiblecoating
/
Wettingand conductivesolution / Epoxyresin
• cheap material and installation.
Description of the process
/////,////.'//////l/lf//lf//A O~)-4"~0 -I-
Principle of operation The process principle is shown schematically in Figure 1. When the structure has no crack (a), the ohmmeter indicates perfect insulation (R -~ 10 '° ~2). As soon as that structure displays a crack (b), the initial insulation provided by the epoxy resin is interrupted, and an electrical link is established as the electrolytic solution penetrates the crack. Such a loss of insulation can be easily observed and acted upon.
hmmeter a
\
Structure
J
Flexiblecooting
/
Wettingand c°~ Ep~::::'s~:i°n
Process components Epoxy resin This resin consists of mixed base and hardener; cures after 24 hours in ambient temperature; this time may be reduced down to 4 hours if heated at 50°C. Apart from its fundamental property of being perfectly isolating, another advantage of this resin is that it closely fits the distortions of coated materials. Its service life in operation under + 1200 microdistortions exceeds 106 cycles and it keeps its mechanical properties at temperatures ranging between -50°C and +80°C.
b
Structure
Fig. 1 Principleof process: (a) Whenno crack exists, the ohmmeter indicates perfect insulation. (b) A crack interruptsthe insulation provided by the epoxy resin
0308-9126/81/050281-04 $02.00 © IPC Business Press 1981 NDT INTERNATIONAL. OCTOBER 1981
281
--4
Cl
C2
II
II
Zi
detection system being studied and the other represents the acoustic emission signal. The acoustic emission detection system operates both in linear mode and in cumulative count with return to zero after l0 s recorded events.
{--1
I
Zz
Eorthingwire
Wireimmersed in thesolution
Ao
oB
Fig. 2 Equivalentcircuit
diagram of the solution and structure between the terminals of the measuring circuit when a crack interrupts the insulation (seetext for explanation of symbols)
•
Its high electric conductivity ranks it as an electrolyte (59 mS/cm, ie approximately 17 ~2 cm resistivity).
•
Its passivating power prevents any alteration of the stripped metal.
After a comparatively slow increase of the acoustic emission event number, one can see an intense emission (numerous returns to zero) which reveals a stage of incipient crack which is confirmed 3 min afterwards by the incipient crack signal, in the form of a drop in the detection coating insulation. Thus, the recording very accurately indicates the performance of the acoustic emission and shows how quickly detection is achieved by means of the coating. The fracture surface After detection has put an end to the dynamic stresses applied to the flange, we have marked the crack and, by simple tension, completed the failure of the flange.
q
I F (crackingcross-sectlon)
Method of detection
Once ready for use on a structure, the crack detection coating is connected to a pair of conductive elements. One end of an element is immersed in the solution, whereas an end of the second element is connected to the structure earth. The free ends of the conductive elements are then connected to the detecting apparatus.
I-1
op l,catlonpoint
l/
I_J-4JiF
IA
F-F cross-section ~
The equivalent circuit diagram of the solution and structure between the terminals A and B of the measuring circuit when a crack interrupts the insulation is shown in Figure 2, whe re:
A-A c
C1 is capacitance resulting from the phenomenon of dual electric layer on the structure area as exposed by the crack.
r
~
I / ~ / ~ / . ~ / ~ Cyclicbending
Zz is faradic impedance representing the kinetic properties of physicochemical phenomena at the structure surface.
b
/ Resin Coating
/
R is solution resistance. C2 is capacitance resulting from the dual electric layer at the surface of the conductive wire immersed in the solution. Z2 is faradic impedance representing the kinetic properties of physicochemical phenomena at the surface of the wire immersed in the solution. T e s t results
'
C
Fig. 3
Blade
(a) Compressorblade tested under cyclic bending; (b) blade at FF through blade and coating
cross-section; (c) cross-section
Test on a compressor blade
A compressor blade (material: Z 12 C 13) was subjected to cyclic bending as shown in Figure 3. Recording and detection is shown in Figure 4. In particular this figure, redrawn from the record sheet, displays the sudden drop of insulation related to the structure sustaining fatigue damage.
(51,000 c y c l e s ~ Poperunwinding rate 500mm/h Crack-detection (33,000cycles)--~
I0!~
Test on a stamped flange
A stamped flange (material: E 24 I) was subjected to cyclic tension as shown in Figure 5. Recording Two traces appear on the recording of the fatigue damage to the flange (Figure 6). One line is related to the
Fig. 4
Recordingof
compressor blade fatigue
failure
I
]I
Paper unw=nding r o t e Comm/rmn
Cyclic tension
o
~COUStlC emission
~" slgnal
I
a
Crack detection s~gnal
b
Structure
~ / / A " "
viii;
L Fig. 6
Recording of flange failure
Fig. 7
Fracture surface on flange
Fig. 5 Stamped flange tested under cyclic tension: (a) plan; (b) A A cross-section; (c) tested section
The fracture photograph (Figure 7) shows the detected fatigue crack (a series of offset elongated crescents with, multiple origins at the bottom of the throat) as well as the cut resulting from the sudden failure. The depth of the cracked area is approximately one third of the sheet thickness, ie less than 1.5 mm deep cracking. Tests on fatigue specimen
I00
The test specimen A number of tests on cylindrical specimens (Figure 8) of various cross sections and materials have been conducted as yet. As an instance of these first results, we shall merely describe one of the tests which best illustrate the detection sensitivity in a comparatively awkward situation, ie subject to both tensile and compressive stress (1250 daN + 1300), under small resistant cross section and with a high cracking rate material (35 NCD 16 treated to 130 kgf/mm 2).
r=40
r=20
Dimensions in mm Solution
Resin
Recording is shown in Figure 9. The fracture surface As soon as a defect is detected, the crack can be marked as the dynamic stress generating machine is stopped. Once the crack is marked, stresses are applied again to the test specimen. The fracture surface displays two dark areas which correspond to two incipient cracks the relative importance of which has caused the detection system to be actuated. Lighter areas, adjacent to the dark ones, correspond to the
Coating
Fig. 8
Geometry o f cylindrical test specimens
Crock- deteition
f
i
I
i~- IOOKQ(macNne stoppingvalue)
I Fig. 9
I
J
Recording of cylindrical test specimen fatigue failure
crack evolution after detection (corresponding to ] 1,300 additional cycles); beyond is the final fracture area. The initial cross section of the test specimen is 50 mm 2 , the cracked and detected area occupies 6 mm 2 , ie about 12% o f the total cross-section, whereas the total cracking area before fractming is 22 mm 2 , ie about 44% o f the total cross-section.
Partial conclusions Considering the first tests we have completed and which, to the largest possible extent, were conducted on actual structures, we already may draw a number of inferences from this newly developed method o f crack-detection. •
The simple design consists in displaying the loss of initial insulation through resistance measurement.
•
The application of coating requires little skill or special care. The downtime before use can be reduced to 48 hours in normal conditions.
•
The method is efficient even on parts with a complex shape (very acute angles) as well as on symmetrical components; it can also be applied on areas irrespective of their orientation.
Fig. 10
Fracture surface on cylindrical test specimen (x 7.5)
•
The controlled area is prepared on request and ahnost without geometrical restrictions.
•
The technique does not induce any possible corrosion, for the solution is passivating.
•
The cracking signal is sudden and highly meaningful, and will therefore be used unambiguously to stop the machine or to set any warning signal into operation.
Author Mr Sorel is with Centre Technique des Industries M~caniques, Etablissement de Saint-Etienne, 10 rue Barrouin, 42029 Saint Etienne Cedex, France.
Introduction Many current processes are capable of continuous detection of open cracks appearing on parts or structures under dynamic stresses, but all of them have specific limits of use. We have applied ourselves to work out a process which can meet the following conditions: •
unsophisticated design involving conventional equipment; possible adaptation to areas with comparatively complex geometry;
• passive, easy, rapid and reliable detection; •
The solution This element constitutes the very originality of this new process. The major features of that solution are as follows.
•
• quick and easy installation; •
The flexible coating This compound can cure at ambient temperature; once cured, it can fit in a quite flexible way the parts to be tested. Moreover, this coating is transparent and fully waterproof.
Its high wetting power enables the solution to deeply enter any interstice. This power results from the action of a non-ionic surface-active agent which reduces the interfacial tension between fluid and metal down to about 30 dynes/cm.
utilization in industrial environments;
• non-corrosive process and easy removal;
Flexiblecoating
/
Wettingand conductivesolution / Epoxyresin
• cheap material and installation.
Description of the process
/////,////.'//////l/lf//lf//A O~)-4"~0 -I-
Principle of operation The process principle is shown schematically in Figure 1. When the structure has no crack (a), the ohmmeter indicates perfect insulation (R -~ 10 '° ~2). As soon as that structure displays a crack (b), the initial insulation provided by the epoxy resin is interrupted, and an electrical link is established as the electrolytic solution penetrates the crack. Such a loss of insulation can be easily observed and acted upon.
hmmeter a
\
Structure
J
Flexiblecooting
/
Wettingand c°~ Ep~::::'s~:i°n
Process components Epoxy resin This resin consists of mixed base and hardener; cures after 24 hours in ambient temperature; this time may be reduced down to 4 hours if heated at 50°C. Apart from its fundamental property of being perfectly isolating, another advantage of this resin is that it closely fits the distortions of coated materials. Its service life in operation under + 1200 microdistortions exceeds 106 cycles and it keeps its mechanical properties at temperatures ranging between -50°C and +80°C.
b
Structure
Fig. 1 Principleof process: (a) Whenno crack exists, the ohmmeter indicates perfect insulation. (b) A crack interruptsthe insulation provided by the epoxy resin
0308-9126/81/050281-04 $02.00 © IPC Business Press 1981 NDT INTERNATIONAL. OCTOBER 1981
281
--4
Cl
C2
II
II
Zi
detection system being studied and the other represents the acoustic emission signal. The acoustic emission detection system operates both in linear mode and in cumulative count with return to zero after l0 s recorded events.
{--1
I
Zz
Eorthingwire
Wireimmersed in thesolution
Ao
oB
Fig. 2 Equivalentcircuit
diagram of the solution and structure between the terminals of the measuring circuit when a crack interrupts the insulation (seetext for explanation of symbols)
•
Its high electric conductivity ranks it as an electrolyte (59 mS/cm, ie approximately 17 ~2 cm resistivity).
•
Its passivating power prevents any alteration of the stripped metal.
After a comparatively slow increase of the acoustic emission event number, one can see an intense emission (numerous returns to zero) which reveals a stage of incipient crack which is confirmed 3 min afterwards by the incipient crack signal, in the form of a drop in the detection coating insulation. Thus, the recording very accurately indicates the performance of the acoustic emission and shows how quickly detection is achieved by means of the coating. The fracture surface After detection has put an end to the dynamic stresses applied to the flange, we have marked the crack and, by simple tension, completed the failure of the flange.
q
I F (crackingcross-sectlon)
Method of detection
Once ready for use on a structure, the crack detection coating is connected to a pair of conductive elements. One end of an element is immersed in the solution, whereas an end of the second element is connected to the structure earth. The free ends of the conductive elements are then connected to the detecting apparatus.
I-1
op l,catlonpoint
l/
I_J-4JiF
IA
F-F cross-section ~
The equivalent circuit diagram of the solution and structure between the terminals A and B of the measuring circuit when a crack interrupts the insulation is shown in Figure 2, whe re:
A-A c
C1 is capacitance resulting from the phenomenon of dual electric layer on the structure area as exposed by the crack.
r
~
I / ~ / ~ / . ~ / ~ Cyclicbending
Zz is faradic impedance representing the kinetic properties of physicochemical phenomena at the structure surface.
b
/ Resin Coating
/
R is solution resistance. C2 is capacitance resulting from the dual electric layer at the surface of the conductive wire immersed in the solution. Z2 is faradic impedance representing the kinetic properties of physicochemical phenomena at the surface of the wire immersed in the solution. T e s t results
'
C
Fig. 3
Blade
(a) Compressorblade tested under cyclic bending; (b) blade at FF through blade and coating
cross-section; (c) cross-section
Test on a compressor blade
A compressor blade (material: Z 12 C 13) was subjected to cyclic bending as shown in Figure 3. Recording and detection is shown in Figure 4. In particular this figure, redrawn from the record sheet, displays the sudden drop of insulation related to the structure sustaining fatigue damage.
(51,000 c y c l e s ~ Poperunwinding rate 500mm/h Crack-detection (33,000cycles)--~
I0!~
Test on a stamped flange
A stamped flange (material: E 24 I) was subjected to cyclic tension as shown in Figure 5. Recording Two traces appear on the recording of the fatigue damage to the flange (Figure 6). One line is related to the
Fig. 4
Recordingof
compressor blade fatigue
failure
I
]I
Paper unw=nding r o t e Comm/rmn
Cyclic tension
o
~COUStlC emission
~" slgnal
I
a
Crack detection s~gnal
b
Structure
~ / / A " "
viii;
L Fig. 6
Recording of flange failure
Fig. 7
Fracture surface on flange
Fig. 5 Stamped flange tested under cyclic tension: (a) plan; (b) A A cross-section; (c) tested section
The fracture photograph (Figure 7) shows the detected fatigue crack (a series of offset elongated crescents with, multiple origins at the bottom of the throat) as well as the cut resulting from the sudden failure. The depth of the cracked area is approximately one third of the sheet thickness, ie less than 1.5 mm deep cracking. Tests on fatigue specimen
I00
The test specimen A number of tests on cylindrical specimens (Figure 8) of various cross sections and materials have been conducted as yet. As an instance of these first results, we shall merely describe one of the tests which best illustrate the detection sensitivity in a comparatively awkward situation, ie subject to both tensile and compressive stress (1250 daN + 1300), under small resistant cross section and with a high cracking rate material (35 NCD 16 treated to 130 kgf/mm 2).
r=40
r=20
Dimensions in mm Solution
Resin
Recording is shown in Figure 9. The fracture surface As soon as a defect is detected, the crack can be marked as the dynamic stress generating machine is stopped. Once the crack is marked, stresses are applied again to the test specimen. The fracture surface displays two dark areas which correspond to two incipient cracks the relative importance of which has caused the detection system to be actuated. Lighter areas, adjacent to the dark ones, correspond to the
Coating
Fig. 8
Geometry o f cylindrical test specimens
Crock- deteition
f
i
I
i~- IOOKQ(macNne stoppingvalue)
I Fig. 9
I
J
Recording of cylindrical test specimen fatigue failure
crack evolution after detection (corresponding to ] 1,300 additional cycles); beyond is the final fracture area. The initial cross section of the test specimen is 50 mm 2 , the cracked and detected area occupies 6 mm 2 , ie about 12% o f the total cross-section, whereas the total cracking area before fractming is 22 mm 2 , ie about 44% o f the total cross-section.
Partial conclusions Considering the first tests we have completed and which, to the largest possible extent, were conducted on actual structures, we already may draw a number of inferences from this newly developed method o f crack-detection. •
The simple design consists in displaying the loss of initial insulation through resistance measurement.
•
The application of coating requires little skill or special care. The downtime before use can be reduced to 48 hours in normal conditions.
•
The method is efficient even on parts with a complex shape (very acute angles) as well as on symmetrical components; it can also be applied on areas irrespective of their orientation.
Fig. 10
Fracture surface on cylindrical test specimen (x 7.5)
•
The controlled area is prepared on request and ahnost without geometrical restrictions.
•
The technique does not induce any possible corrosion, for the solution is passivating.
•
The cracking signal is sudden and highly meaningful, and will therefore be used unambiguously to stop the machine or to set any warning signal into operation.
Author Mr Sorel is with Centre Technique des Industries M~caniques, Etablissement de Saint-Etienne, 10 rue Barrouin, 42029 Saint Etienne Cedex, France.