THE NON-CONSTANT CTOD/CTOA IN CRACK PROPAGATION Page: 4 of 25
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). They also discovered that the use of a constant CTOA as the fracture criterion
would underestimate the fracture load. Using the experimental J-R curves in Lam et al.
 and following the derivations in Kanninen and Popelar  and in Gullerud et al.
, the CTOA can be expressed as a function of the slope of the J-R curve (dJ/da) or the
tearing modulus. The prediction can then be compared with the CTOA that are directly
calculated from the finite element method using nodal release technique. It is shown in
this paper that the initially higher values of CTOD/CTOA are a natural consequence of
the crack growth resistance in a material (J-R curve), and therefore, it can be concluded
that CTOD and CTOA are essentially functions of crack tip constraint.
A set of single edge-notched bend (SENB) specimens was designed based on
American Society for Testing and Materials (ASTM) Standard Test Method for
Measurement of Fracture Toughness (E 1820) specifications, except for the initial crack
lengths that were varied to achieve different levels of crack tip constraint. These
specimens were originally used to develop a constraint-modified J-R curve for A285
carbon steel storage tanks . The same set of test data are used in the current study of
CTOD/CTOA during the course of crack extension.
The specimens were machined from A285 carbon steel Grade C, heat E400 plate with
0.18 wt.% carbon, 0.43 wt.% manganese, and 0.026 wt.% sulfur. The tensile test shows
that the 0.2% offset yield stress ((y) is 251 MPa (36.4 ksi), the ultimate tensile stress 415
MPa (60.2 ksi), the flow stress (ay) 333 MPa (48.3 ksi, defined as the average of the 0.2%
yield stress and the ultimate stress), and the Young's modulus (E) 207 GPa (30,000 ksi).
The Poisson ratio of the material is 0.3. The true stress-true strain curve is shown in
page 3 of 3
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LAM, POH-SANG. THE NON-CONSTANT CTOD/CTOA IN CRACK PROPAGATION, article, July 19, 2004; South Carolina. (digital.library.unt.edu/ark:/67531/metadc787592/m1/4/: accessed September 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.