a[mm]/l - Memorial University of Newfoundland DAI

And.
otN+l = B., b,'V-? - ofN.? ]
ofN+' + BoatN-l = Boo,N-~
From Equalon (5 251. ~f the final strength of a specmen tr measured m r fracture ten.
the lnlllal strength can be computed by knowing the slrcrring rate a Conversely. tf the
nn!t~nl strength of s rpcclmcn IS known. @he fracture rmngth m any constant loudlng nte
lest can be defined from Equallan 15.251 by numencvl methods.
As reported by Nadeau. Bennet. and Fuller 11982). urlng the pnncnples of Ihnear
elanc fracrure mechan!er. the dependence of strength on the rare of loadmg cnn be
expressed by [he lagmthmlc fm of Equation (5.24). The erpresrlon can be wntlen ar
follows.
Ino, = L l n ~ + o -L ln,!o,'-' -
Ntl N+I af'v-2 )
(5.26)
Analysis of Equrtaon (5.261 tmpllcr that a plot of Ino, Venus In0 would have a
slope of [II(N+I)] ar lower valuer of o . Rnaily, s would reach I eonnant value lrem
rlapcl as high values of 6. Thts tseonsnstent with the suknl#cal cnck growth model that
ar very hlgh loadtng ntcs. the strcngth would be largely andependent of loading me.
Sbnee. there 3s not enough time for rukntlcal crack gmwth to occur. the mltlal md final
strength ue eswnually equal.
In Xcmt yean, there are three nndependent mcthcds for evaluating the constant B
in Equatlon (5.23) by detcrmnnnng the conswt N baed on.