Crack-tip chemistry modeling of stage I stress corrosion cracking Page: 4 of 19
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Introduction
1 he crack veloc i ty-sfress intensity relationship induced by stress
corrosion cracking (SC( ) in a wide variety of materials exhibits a
threshold, and stage ! and stage II regimes. Ihe threshold, referred to as
K src, is I lie stress intensify at whicli measurable crack extension occurs.
Stage I is tin; regime in which the crack velocity exhibits a very strong
dependence on stress intensity and stage II is the regime in which the
crack velocity exhibits a very weak and sometimes neglible dependence on
stress intensity,
lhe stage 1 regime plays a ci itical role in SCC, and yet most of the
theoretical and experimental effort has been focused on K^scc and stage 11.
lhe slope of the stage I regime is important, for determining design
stresses to avoid SC.C and for life prediction of a material should SCC be
unavoidable, for components that must perform for extended periods, Lhe
stage I slope is a critical parameter because it defines Ihe "long-term"
K . A laboratory define'! k,src, determined in hours, will approach the
"long-term" K|r(r with ini leasing stage 1 slope. the slope of the stage 1
regime is a iso a factor in life prediction because the crack velocity can
increase by several ordeis ol magnitude in this regime. Ihe total impact
on component life will In1 a (unction of incubation time, crack initiation
lime, transition from shnit crack to long crack electrochemistry and
mechanics, and then crack growth as defined by conventional long crack
K |SCf, stage 1 and stage 11.
Stage I be havior of material' has received vei y little1 experimental or
theoretical emphasis. Stacie I behavior is difficult to measure experi
mentally because' of t tie '.Irong sties1, intensity dependence .where tin1
velocity may increase by an order of magnitude with only a 1 ? MI'a/m
increase in Ihe stress inton ily, thus mating stage I a very transient,
phenomenon., Also, because ot I lie rapid increase in crack velocity, the
velocity is determined f 1 mu v'M"y small c lack extensions. Ibis results in
greater scatter in t lie data because velocities are based on a relatively
small sampling of the mat * ■ i i ,i I and ai e not an average ovet a range of
microchemua 1 and micrest tinluraI variations. theoretical emphasis has
been placed in the electrochemical, mechanical, and microstructural/micro
chemical aspects of KJS(( and stage II cracking. Most theoretical studies
have not considered tne stress or stress intensity aspects of SCC but. have
emphasized the physical piomsses involved. It is quite conceivable that
these processes are involved to different degi ees in K|Sa., stage I and
stage II cracking, but quantitative descriptions of the crack velocity
stress intensity relationship, in stage I are absent, from the literature.
because of the distinct ivenn > and significance of this regime, an
analytical description of stago | is of considerable value. Ibis
descript ion will also enhance1 our understanding of K,s c and stage II S(.C
regimes. Ihe purpose of Ihi . paper is to report on a modeling effort to
describe Ihe stage I behaviot of Ni tested in IN II^SO^ based on
modifications to a crack tip chemistry model developed by Danielson, Osier,
and Jones (1).
Bibkyround
Experimental crack veloc i t.y-s tress intensity data for SCC is presented in
one of the two following formulas:
:»i
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Jones, R. H. & Simonen, E. P. Crack-tip chemistry modeling of stage I stress corrosion cracking, article, October 1, 1991; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc1103922/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.