TY - GEN
T1 - Small-Specimen Testing, with Image-Based Analysis, for Crack Propagation Resistance in Polygranular Nuclear Graphite
AU - Marrow, Thomas James
AU - Scotson, Dan
AU - Jin, Xiaochao
AU - Chen, Hongniao
AU - Chen, Yang
AU - Koko, Abdo
AU - Earp, Philip
AU - Wu, Houzheng
N1 - Publisher Copyright:
Copyright 2022 by ASTM International,
PY - 2022/12/1
Y1 - 2022/12/1
N2 - To enable monitoring of graphite that has been exposed to irradiated, high-temperature environments, there is a need for analysis methods that are suitable for fracture toughness measurements in small test specimens. Quasistatic fracture propagation has been studied for two candidate graphites for next-generation nuclear energy, SNG742 and T220, using small specimens in the double-cleavage drilled-compression (DCDC) geometry (20 x 7 x 7 mm). Compression of the DCDC specimen initiated stable crack propagation, and the surface full-field displacements were measured by digital image correlation. A phase congruency method was applied to the displacement field to quantify the crack lengths, crack opening displacements, and crack tip opening angles. The classical analytical solution for the stress intensity factor in the DCDC specimen gave unrealistic results due to its boundary condition assumptions. A new analysis method is proposed in which the measured crack displacement field is injected as boundary conditions into a finite element model, allowing the J-integral to be evaluated via the contour integral method, which then provides the mode 1 stress intensity factor during quasistatic crack propagation. With the assumption of linear elasticity, the critical stress intensity factor in T220 was constant for crack propagation up to 6 mm and lower than that in SNG742, which showed rising fracture resistance for longer cracks. The analysis was validated using Macor, a linear elastic fine-grained glass ceramic with known fracture toughness without significant R-curve behavior. The small-specimen graphite results are consistent with the reported fracture toughness from large-specimen tests, but the values are overestimations due to the nonlinear behavior of unirradiated graphite. Methods to extract nonlinear elastic properties by inverse analysis are discussed. The outlook for fracture testing of irradiated graphite at elevated temperatures is considered.
AB - To enable monitoring of graphite that has been exposed to irradiated, high-temperature environments, there is a need for analysis methods that are suitable for fracture toughness measurements in small test specimens. Quasistatic fracture propagation has been studied for two candidate graphites for next-generation nuclear energy, SNG742 and T220, using small specimens in the double-cleavage drilled-compression (DCDC) geometry (20 x 7 x 7 mm). Compression of the DCDC specimen initiated stable crack propagation, and the surface full-field displacements were measured by digital image correlation. A phase congruency method was applied to the displacement field to quantify the crack lengths, crack opening displacements, and crack tip opening angles. The classical analytical solution for the stress intensity factor in the DCDC specimen gave unrealistic results due to its boundary condition assumptions. A new analysis method is proposed in which the measured crack displacement field is injected as boundary conditions into a finite element model, allowing the J-integral to be evaluated via the contour integral method, which then provides the mode 1 stress intensity factor during quasistatic crack propagation. With the assumption of linear elasticity, the critical stress intensity factor in T220 was constant for crack propagation up to 6 mm and lower than that in SNG742, which showed rising fracture resistance for longer cracks. The analysis was validated using Macor, a linear elastic fine-grained glass ceramic with known fracture toughness without significant R-curve behavior. The small-specimen graphite results are consistent with the reported fracture toughness from large-specimen tests, but the values are overestimations due to the nonlinear behavior of unirradiated graphite. Methods to extract nonlinear elastic properties by inverse analysis are discussed. The outlook for fracture testing of irradiated graphite at elevated temperatures is considered.
KW - digital image correlation
KW - fracture toughness
KW - graphite
KW - J-integral
KW - small specimen
UR - http://www.scopus.com/inward/record.url?scp=85168908935&partnerID=8YFLogxK
U2 - 10.1520/STP163920210051
DO - 10.1520/STP163920210051
M3 - Conference contribution
AN - SCOPUS:85168908935
T3 - ASTM Special Technical Publication
SP - 1
EP - 17
BT - Graphite Testing for Nuclear Applications
A2 - Tzelepi, Athanasia
A2 - Metcalfe, Martin
PB - ASTM International
T2 - 2021 Symposium on Graphite Testing for Nuclear Applications: The Validity and Extension of Test Methods for Material Exposed to Operating Reactor Environments
Y2 - 23 September 2021 through 24 September 2021
ER -