Experimental Evaluation of the J or C* Parameter for a Range of Cracked Geometries

Abstract

In the current ASTM Standard Test Method for Measurement of Creep Crack Growth Rates in Metals (E 1457) the experimental C* parameter is related to the load and creep load line displacement rate through the geometry related η factor. In this work η factors for a range of geometries are presented. The geometries examined are the compact tension specimen, C(T), single edge notch specimen in tension, SEN(T), and bending, SEN(B), double edge notch specimen in tension, DEN(T), middle crack specimen in tension, M(T) and the C-shaped specimen in tension CS(T). Calculations have been performed for a linear elastic-power law hardening material but the resulting η factors are applicable to either power law plastic or power law creeping materials. Values for ηLLD and ηCMOD, based on the load line displacement and crack mouth opening displacement, respectively, have been determined. A wide range of crack depths, 0.1⩽a/W⩽0.7, where a is crack length and W is specimen width, and hardening exponents, 3 ⩽N ⩽10, under plane stress and plane strain conditions have been examined using the finite element method. The influence of specimen length, crack length, material properties and out of plane stress state on the η factor has also been considered. It has been found that for shallow cracks the value of η depends quite strongly on the exponent, N in the material power law, regardless of whether η is defined based on the load line displacement or crack mouth opening displacement. The ηLLD factor has also been found to be strongly sensitive to plane stress/strain conditions imposed, a/W and specimen length, whereas ηCMOD depends more weakly on a/W and is almost independent of specimen length for the cases examined. There is, however, no clear trend in these variations over the range of specimen geometries and a/W examined. These results are found to be consistent with those in the literature. Recommendations are made regarding the most appropriate values for η, depending on the specimen type and geometry while taking into account the variability due to the material properties, out of plane stress state and variations between the numerical analyses.

Predicting creep crack initiation in austenitic and ferritic steels using the creep toughness parameter and time‐dependent failure assessment diagram

“…(24). Note that sensible trendlines have not been obtained using Eqs (21) and (22) in conjunction with the CCG and CCI regression data detailed in Tables 2 and 3, respectively, and thus are not shown in the following figures.…”

“…where U p is the area under the load displacement curve associated with plasticity, W is the specimen width, B n is the net specimen thickness between any side-grooves, a is the crack length and n the secondary creep power-law stress exponent (see Eq. 15) and η a geometry function (η = 2.2 for C(T) specimens 21 ). Note that the K c mat relation has been modified since the work in Ref.…”

“…The CCI times have been predicted for the tests on homogenous PM and weldment C(T) specimens. 16,21,26 These tests have been used to derive the relationship of creep toughness on time, thus appropriate predictions are expected. In addition, CCI data are available for feature tests on P22 PM and weldment pipe components, as detailed in Ref.…”

Limit load based evaluation of plastic η factor for C(T) specimen with a mismatched weld

“…Wang and co-authors [2] proposed CTOD equations expressed in terms of weld height, mismatch level and strain hardening rate for specimens made of non-homogeneous and strain hardening materials. Smith [3], Panontin and co-authors [4], Cassanelli and co-authors [5], Kim and co-authors [6], Davies and co-authors [7], have proposed analytical and numerical solution to η expression for even match (M=1) C(T) specimens but varying strain hardening exponent. Xuan and co-authors [8] and Marie & Nedelec [9] have performed FE based analysis for various mismatch factors from 0.25 to 2 and 2.3 respectively.…”

Effects of side‐groove depth on creep crack‐tip constraint and creep crack growth rate in C(T) specimens

“…It has been shown that side grooves in fracture specimens can significantly alter the distributions of displacement, strain, and stress along the crack‐front, thus affect crack‐tip constraint level and fracture behavior of materials. Under creep condition, many experimental, theoretical analyses, and numerical simulations have been conducted to investigate creep crack initiation and growth, with the results showing that crack‐tip constraint can significantly influence CCG rate . It has been shown that the CCG rate raises as crack depth and specimen thickness (geometry constraint) increased.…”