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Nondestructive Testing and Fracture Mechanics

5th International CANDU In‐Service Inspection Workshop and NDT in Canada 2014 Conference June 16‐18, 2014Er/BAM 3.0

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Nondestructive Testing and FractureMechanics

Dir. u. Prof. Dr.‐Ing. A. ErhardBundesanstalt für Materialforschung und –prüfung (BAM)[email protected]

Dir. u. Prof. Dr.‐Ing. F. OtrembaBundesanstalt für Materialforschung und –prüfung (BAM)[email protected]



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NDT

FractureMechanics

Introduction

Basis for fracture mechanics analyses

Steam generator tube inspection

Conclusions



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Introduction

When experts in the two fields of NDT and fracture mechanics meet, the discussion usually rapidly shows the mis‐understanding that each side has developed for the methodology of the other side in recognising and acknow‐ledging its strengths and weaknesses. This paper therefore lays emphasis on better understanding.

G. Dobmann, D. D. Cioclov and J. H. KurzNDT and fracture mechanics. How can we improve failure assessment by NDT? Where we are – where do we go?Insight Vol. 53 No 12 December 2011

Five non‐destructive testing (n.d.t.) methods are widely used for defect detection: these are magnetic particle,dye penetrant, electrical eddy currents, radiography and ultrasonics. The first three can detect only surface‐breaking or immediately sub‐surface defects, while radiography and ultrasonics can also find embedded, remotedefects. Ultrasonics is far more sensitive to cracks than is radiography; moreover, of all the n.d.t. methods, onlyultrasonics can in general measure a crack's through‐wall position and size. Consequently only ultrasonics is fullycompatible with fracture mechanics requirements. Used in conjunction with fracture mechanics, ultrasonics hasproved a powerful technique for demonstrating component integrity.

J. M. Coffey and M. J. WhittleNon‐Destructive Testing: Its Relation to Fracture Mechanics and Component Design23 January 1981Phil. Trans. R. Soc. Lond. A 299 (1981) pp. 93 ‐ 110; Printed in Great Britain



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What is the capability of NDT?

What is the capability of fracture mechanics?

What is the advantage together?

Detection of defects (POD)Determination of the defect positionDetermination of the defect size

Requirements on the NDT in relation to inspection interval, ‐place, ‐sensitivityAssessment of crack growth and critical crack sizes depending on the loadBreak preclusion, deterministic methods against• failure• exceed of a given leakage rate of a pressurized component in the

whole operation time

Avoid catastrophic failure of components together with preventive methods

Introduction (cont.)



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At a first look, it can be misleading that the curves for the shallower flaws lie left of the curves for deeper flaws , which in a conven‐tional POD diagram would mean a higher POD. The right way to inter‐pret the diagram will be explained by an example. A flaw with an area of about 12.5 mm2 that is 5 mm deep will have a POD of only about 20%. A flaw of the same area but only 3 mm deep will have a POD of around 90%. Since both flaws have the same area, that means that the shallower flaw is longer than the deeper one.

M. Pavlovic, K. Takahashi, C. MüllerProbability of detection as a function of multiple influencing parametersInsight Vol 54 No 11 November 2012, pp. 606 ‐ 611

Multiparameter POD

Fracture Mechanics Independent of the used NDT methods, the detectability of critical defects is essential for component integrity assessment.



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Fracture Mechanics (cont.)



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The ratio of approximately 0.4 between the half axes of the elliptical shaped crack depends on the maximum on the stress intensity factor. This ratio is valid for the semi‐elliptical and the embedded crack.




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The assumed time must correspond with the time for periodical in‐service inspection. NDT methods used for in‐service inspection must be adapted on the crack growth rate, i.e. the sensitivity of the methods must guarantee a detectability of the crack sizes of a0∙2c0.

For surface breaking cracks a semi‐elliptical geometry come up the fracture mechanics demands. The result of crack growth calculation (a, 2c) for an elliptical crack size (a, 2c) regarding a specified or measured load during operation is after an assumed time as follows:

a0 = a +a2c0 = 2c + 2c




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NDT Methods for steam generator tube inspection



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Steam Generator Tube inspection



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Scanning direction (axial) Circum

ferential dire

ction 8 to 18 coils

T1

T3

R1

T2R2

R3

R4R5

Demonstration of the flexibility of the probe and cable arrangement at a SG U‐bend tube

Coil arrangement at three rows at the circumference

The number of the coils depends on the inner diameter of the tube regarding the resolution .Until now EC array probes with 8 to 18 single coils are available on the marked.

Signal transmission using the transmitter coil T1 , signal receiving using receiver R1 and R2. In continue the transmitter coil has to be changed from T1 to T2 and the receiver coils to R2 and R3, follow the algorithm Tn+1 , Rn+1 and Rn+2 respectively. By means of this procedure, an examination of axial defects in the whole tube circumference is guaranteed. For the detection and sizing of circumferential oriented defects the transmitter coils are T1 and T2 und the receivers are R4 and R5. In continue the transmitters and receivers are switched, follow the algorithm above.

Steam Generator Tube inspection (cont.)



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The signal is within the aperture (aperture size approx. 14mm) of the transmitter (T) and receiver (R) coils direct proportional the defect length extension.

The signal decreases lightly for defect length larger the aperture and change over to a saturation.

For defects extension larger than the aperture the signal of the middle of the defect extension is plotted too




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0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 5 10 15 20 25 30 35 40

Relativ

e crack de

pth a/t /

‐

Crack length 2c / mm

D

BA

B (S = 3)A (S = 3)D (S = 1.5)

Level B (acceptable)2c = 8.0 mma/t = 0.62

failure

acceptable

Loading Level

Critical through-wall crack lengthAcceptable through-wall crack length

S ≥ 3 for loading level A and BS ≥ 1.5 for loading level C and D (whereby S = Safety margin)The smallest acceptable axial through‐wall crack length of 2c = 8 mm was calculated for loading level B with a safety margin of 3.

Loading levels after KTA 3201.2

Level A The loading of normal operational conditions.

Level BFor level B the reliability of the reference loading has to be verified as well as the oscillation of this loadings

Level CThis level requires loadings in the frame of the stress analyses for primary stresses. Plastic deformation is possible on geometrical discontinuities.

Level D: Only primary loadings must be considered with the result that plastic deformation in bigger areas of the component could be happen.

Assessment of Steam Generator Tube inspection

Level D (critical crack length)2c = 17.0 mma/t = 0.75



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• Eddy current testing (EC) is the favoured steam generator tube in‐service inspection technique. Tube plugging is mostly based on these measurements.

• The detection of IGSCC at SG tubes near the tube sheet is one of the challenges for EC inspection techniques. In the past, the EC inspection of this area was difficult due to the big carbon steel mass of the tube sheet.

• With the application of eddy current arrays, the detection of cracks in this area has become more reliable. The results presented in this paper showed the advantage of the technique in relation to defect detection, sizing and characterisation by image presentation.

• The failure mechanisms such as IGSCC at Inconel 800 SG tubes have been considered within the integrity assessment of these tubes by the calculation of cracks using the fracture mechanic analyses.

• For an axial crack the calculated critical crack length is 17 mm for a relative crack depth of 0.75 without a safety factor and the stress stage D.

• Acceptable crack length of 8 mm was calculated for the stress stage B and a safety factor of 3.

• The results received with the employed EC technique and with the calculation based on fracture mechanics shows that with the application of both disciplines an assessment of the SG tubes integrity is possible.

Conclusion

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