EXPERIMENTAL RELATIONSHIP OF BREAK ELONGATION AND INDENT DATA FOR AGEING DEGREDATION OF CSP AND CR CABLE JACKET J.-S. Kim Korea Electric Power Research Institute(KEPRI), Korea Email address of main author: HL5JAA@kepri.re.kr Abstract Attempts to extend the lifetime of NPP have become one of the major concerns in the world nuclear industry. Consequently, life evaluation and lifetime management of cable to survive over plant lifetime has become major topic of discussion. For the life evaluation of cable, break-elongation test in acordance with ASTM D412 has been widely used. Since dumbbell specimen has to be supplied from plant cable for break-elongation test, it is not easy to apply this method to operating plant. One of alternatives is indent test which nondestructively measures the aging of cable jacket. Because indent test has no approved standard, we have to compare the test result with that of break-elongation test. Since two kinds of tests for one aging evaluation are time consuming, we decided to develop relational equation between elongation and indent which can deliver relative elongation value from indent data. Some relational equations between elongation and indent were obtained but coefficients in the equation were different according to the material type. Good convergence was observed when indent depth was used as equation factor instead of indenting modulus. It was verified that elongation data can be delivered from indent data if we had enough amount of experiment result for various material type. 1. Introduction For the life evaluation of cable, break-elongation test of ASTM D412 has been widely used[1]. Since dumbbell specimen has to be supplied from plant cable for break-elongation test, it is not easy to apply this method to operating plant. One of alternatives is indenting test which nondestructively measures the aging of cable jacket. Because indenting test has no approved standard, we have to compare the test result with that of break-elongation test. Since two kinds of tests for one aging evaluation are time consuming, we decided to develop relational equation between elongation and indenting which can deliver relative elongation value from indenting data. Relational equation between elongation and indenting based on the test results of CSP and CR cable jacket will be described herein. 2. Test method 2.1. Accelerated aging Electric ovens with air circulating fans were used for accelerated aging of cables. Dumbbell specimens for elongation test and 1mm long cables for indenting test were prepared for accelerated aging during 48hours, 72hours, 96hours, 168hours and 164hours at the each temperature of 116, 126, 136 and 146. Specimens were air cooled for 24 hours after finishing the accelerated aging. 1
2.2. Break-elongation test Break-elongation test is destructive aging evaluation method which measures the elongation length after elongates aged dumbbell specimens until they are broken. Dumbbell specimens were prepared in accordance with ASTM D412 Die C dimension. We kept the speed of 5±5mm/min during break-elongation test. Specially designed machine which can elongate 5 cable specimens at one time is used for break-elongation test as shown on FIG. 1. Part number1 is body plate of upper specimen grips. Number2 is upper specimen grip. Number 3 is lower specimen grip. Number4 is load cell. 2.3. Indenting test FIG. 1. Cable elongation machine Indenting test is nondestructive aging evaluation method which measures the hardness of cable by pressing a cable surface with steel probe in the vertical direction[2]. Portable cable indenter as shown on FIG.2 was used for this test. Part number1 is cable clamp assembly. Number2 is probe. Number3 is load cell assembly. Number4 is PDA. Number5 is replaceable Li-Ion Battery. 2mm round bar and.56mm edge of truncated con type probe was used for indenting[3]. Pressing load was limited to.5kg f to protect damage of cable jacket. Indenting depth didn t go over.7mm for every cable specimens. FIG.3 shows deviationspeed curve for various type of probes. In the legend box of the graph,.56con mean cone type probe of.56mm flat edge, 2.ball mean ball type probe of 2.mm ball edge..56mm flat edge probe and moving speed of.31 mm/sec were chosen for best accuracy[4]. FIG. 4 shows detail of indent probe. FIG. 2. Portable cable indenter 2
FIG. 3. Measuring deviations of modulus according to the probe types and speeds FIG. 4. Truncated cone probe 3. Relations between elongation and indenting 3.1. Elongation rate and indent modulus Relationships between elongation rate and indent modulus of CSP and CR cable jacket aged at 116 during several aging times are shown on FIG. 5 and FIG. 6. Aging rate in elongation test is break-elongation rate which comes from length of aged specimen divided by that of un- 3
aged specimen. Indent modulus is a value which comes from the indent load divided by depth. It was found that elongation rate and indent modulus have reverse linear relationship. 1 96hr 8 168hr E lo ng atio n rate(% ) 6 4 2 264hr 312hr 336hr 2 4 6 8 1 12 14 M o d u lu s (kg f/m m ) FIG. 5.Relationship between elongation rate and indent modulus for CSP cable jacket 1 E longation rate(% ) 8 6 4 2 96hr 168 hr 264hr 312 hr 5 1 15 2 25 3 35 M odulus(kgf/m m ) FIG. 6. Relationship between elongation rate and indent modulus for CR cable jacket 3.2. Elongation length and indent depth Relations between elongation length and indent depth of CSP and CR cable jacket aged at 116 during several aging times are shown on FIG. 7 and FIG. 8. elongation length is elongation length at break point. Indent depth is the value at the designated indent load. It was found that elongation length and indent depth have proportional linear relationship. 4
14 12 Elongation(m m 1 8 6 4 2..1.2.3.4.5.6.7.8 Indent Depth(m m ) FIG. 7. Relationship between elongation length and indent depth for CSP cable jacket E longation(m m ). 12 1 8 6 4 2..1.1.2.2.3.3.4.4 Indent D epth(m m ) FIG. 8. Relationship between elongation length and indent depth for CR cable jacket 4. Relational between elongation and indenting 4.1. Relational Equations As result of experiment, it was found that elongation and indent data have proportional relation. FIG. 9 and FIG. 1 show relationship between elongation rate and indent modulus for CSP and CR cables aged at 116, 126, 136 and 146 during 48, 72, 77, 88, 93, 94, 96, 168, 264, 312 hours. FIG. 11 and FIG. 12 show relationship between elongation length and indent depth for CSP and CR cables. Relational equations between elongation and indent are described as equation (1) ~ (4) 5
6 2.5 e / e = 3.9E ( P / dt ) for CSP cable (1) e / e for CR Cable (2) 6 2.14 = 2.2E ( P / dt ) 2.57 e= 31 ( ht ) for CSP cable (3) 1.96 e= ( h ) for CR cable (4) 52 t FIG. 9. Relationship between elongation and modulus for CSP cable FIG. 1. Relationship between elongation and modulus for CR cable 6
FIG. 11. Relationship between elongation and indent depth for CSP cable FIG. 12. Relationship between elongation and indent depth for CR cable 4.2. Review of test result After making the general equation between elongation and indent, we analyze the deviation between actual data and equation data. Deviation of indent modulus and indent depth for CSP and CR cables are described on table 1. 14.6% average deviation for CSP cable and 1% average deviation for CR cable at the equation of elongation rate and indent modulus is observed. 9.2% average deviation for CSP cable and 7.9% average deviation for CR cable at 7
the equation of elongation length and indent depth are observed. Equation for indent depth and elongation length had better accuracy than that for indent modulus and elongation rate. Table 1. Deviation of elongation corresponding to modulus and indent depth Cable Material Elongation for un aged specimen Deviations of Indent modulus at elongation Deviations of indent depth at elongation CSP 129 mm 18.87 mm(14.6%) 11.84 mm(9.2%) CR 125 mm 12.97 mm(1%) 9.87 mm(7.9%) 5. Comparison of activation energy between break-elongation and indent We calculated the activation energy of CSP and CR cable specimens by using breakelongation and indenter test. Instead of selecting a data at 5% break-elongation rate for the calculation of activation energy, we compared activation energies of every 5% decrement for break-elongation rates and every 5% increment for indenter modulus. FIG. 13 and FIG. 14 show the activation energies by break-elongation and indenter modulus for CSP and CR cables. Table 2 shows activation energy value of beak-elongation and indenter modulus For the cable of CSP, we found that activation energy at 5% break-elongation decreasing rate showed.885ev and.962 ~.945ev were showed at indenter modulus increasing rate of 15% ~ 25%. The value of indenter modulus showed 1.8 times value of break-elongation. For the cable of CR, activation energy at 5% break-elongation rate showed.973ev and.8 ~.89ev were showed at indenter modulus increasing rate of 3% ~ 4%. The value of indenter modulus showed 1.1 times value of break-elongation. A ctivation E nergy(ev) 1.4 1.2 E longation Indenter 1.8.6.4.2 % 2% 4% 6% 8% 1% R ate ofe/e and M /M (% ) FIG. 13. Activation energy of elongation and indenter modulus for CSP cable 8
A ctivatio n E nerg y(ev) 1.4 1.2 E lo n g a tio n In d e n te r 1.8.6.4.2 % 2% 4% 6% 8% 1% R a te o f e /e a n d M /M (% ) FIG. 14. Activation energy of elongation and indenter modulus for CR cable Table 2. Activation energy of beak-elongation and indenter modulus(ev) Rate(%) CSP Cable CR Cable Elongation Indenter Elongation Indenter 5 1.257.871 1 1.68.871 15.962.848 2.959.822 25.945.81 3.676.84 35.496.8 4.89 45.681 5.885.793.728 55.897.777 6.896.761 65.912.824 7.82.876 75.812.98 8.88.936 85.793.946 6. Conclusion Some relational equations between elongation and indent were obtained but coefficients in the equation were different according to the material type. Good convergence was observed when indent depth was used as equation factor instead of indenting modulus. It was verified that elongation data can be delivered by indent data if we had enough amount of experiment result for various material type. I expect the indentation methodology developed in this paper will 9
be useful to evaluate the aging condition of plant cable without destructing the operating cables. IAEA-CN-155-46 REFERENCES [1] INTERNATIONAL ATOMIC ENERGY AGENCY, Assessment and management of ageing of major nuclear power plant components important to safety, IAEA- TEDOC-1188, Vienna(2). [2] ELECTRIC POWER RESEARCH INSTITUTE, Cable Indenter Aging Monitor, EPRI NP-7348, USA(1991) [3] JONG-SEOG KIM, Evaluation of cable life based on condition monitoring, 1 fall meeting of Korean nuclear society, Korea(21). [4] JONG-SEOG KIM, Evaluation of Nuclear Plant Cable Aging through Condition Monitoring, Journal of Korean Nuclear Society, Volume 36, Number 5, pp 475-484, Korea(24). 1