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Pipelines constitute the integral part of primary
heat transport system of nuclear reactors, oil-gas transmission lines etc.
Fracture and fatigue testing of pipes is a crucial step in the integrity
assessment of pipes to ensure Leak Before Break design criteria.  Determination of crack growth during such
experiments can be done using various methods namely unloading compliance
technique, potential difference technique, digital image correlation etc. The
unloading compliance technique is the most commonly used method to determine
in-situ crack growth in fracture tests. This method involves computation of
crack length from the unloading compliance data obtained during fracture tests
of specimens/components. However the unloading compliance method is well
established only for standard specimens as given by ASTM E 1820-11. This paper
focuses on employing the existing analytical expression that correlates
unloading compliance and crack length measured during fracture test of
throughwall cracked pipe. The calculated crack growth using the adopted model
is compared with that of the preliminary experimental results from the fracture
tests of four throughwall circumferentially cracked straight pipes. The adopted
model is found to be effective in ensuring the smoothness and accuracy of
calculated crack growth.

words:  compliance;
fracture test; crack growth; crack mouth opening displacement; plastic load

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1. Introduction


Structural integrity assessment of pipes
used in nuclear industry, oil-gas transmission industry etc. is done by
preforming fracture tests on small scale standard specimens and full scale
components. The crack growth data obtained from the fatigue and fracture
experiments are employed in determining various fracture parameters to characterize
the crack tip stress field. The unloading compliance technique is a widely
adopted technique to evaluate crack growth during fracture experiments.  The pre-requisite for this technique is the availability
of equations correlating compliance measured during experiments and crack
length. ASTM E647-13 provides guidelines for measurement of crack growth using compliance.
ASTM E1820-11 provides the necessary correlation between compliance and crack
growth for standard specimens namely Compact Test specimen, C(T), Disk shaped
Compact Specimen, DC(T) and Single Edge Bend specimen, SE(B). Van Minnebruggen (2011)
et al. (2011) validated the standardised test procedure described in ASTM E1820
to estimate the ductile crack length extension from the experiments conducted
on SENB specimens made of pipeline material (Grade X65). However these
correlations cannot be used for evaluating crack growth on full scale piping
components. This is due to the fact that the equation should not only have
crack length as a function but also the current deformation level/load level as
a parameter*. Jan Džugan (2003) proposed a modified approach using unloading
compliance to estimate the crack length in Charpy size specimens. The
modification was made by introducing a function of specimen size and specimen
deformation to the compliance equation. Shen and Tyson (2009) proposed
compliance equations to estimate crack growth in Single Edged notched Tension
specimens, SE(T) incorporating the effect of rotation during testing. Chattopadhyay
et al. developed compliance correlations to estimate crack growth in
throughwall circumferentially cracked pipes subjected to four point bending.  Compliance correlations were derived by
generating compliance versus crack length by performing small displacement
linear elastic finite element analysis. However, it did not account for the
large geometric deformation that might occur during the loading of the specimen*.
This paper studies the model proposed by Chattopadhyay et al. and validates it
by comparing the results with that of preliminary experimental investigations
on the fracture tests of four throughwall circumferentially cracked straight

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