A fully plastic microcracking model for transgranular stress-corrosion cracking in planar-slip materials
- PDF / 1,247,132 Bytes
- 11 Pages / 598 x 778 pts Page_size
- 92 Downloads / 187 Views
I.
INTRODUCTION
TRANSGRANULAR stress-corrosion cracking (T-SCC) in materials exhibiting planar slip (i.e., in materials having low stacking-fault energy) has been shown to be discontinuous,tl-Tl The fracture surfaces match and consist of microcracks on the order of 1/~m in size, frequently resulting in a "cobblestone" appearance. The details of this cracking are sensitive to the imposed potential and to the crack-growth direction, 14,s,s,9,1~ and are likely to be influenced by strain rate. t~] It has been shown in experiments on single crystals that crack blunting can occur when the crack-growth direction is such that the crack tip is roughly coplanar with the active slip planes, and this results in "crack arrests" which are spaced on the order of 50 to 200 ~m apart, t4,~,81 In this case, continued fracture requires that the crack be renucleated, t3~ but the renucleation time is on the order of minutes and is highly sensitive to the corrosion potential. t2,4,8,9] Transgranular stress-corrosion cracking is also discontinuous on a microscale, where the combined effects of the ubiquitous ligaments (which constrain the crack opening displacement) and the corrosive environment (which affects the value of Klc) lead to discrete microcrack advances, t51 This results in an effective velocity between renucleation events which is too large for Faradaic dissolution to be rate controlling, as claimed by classical slip-dissolution models, t~21 and too small for a single unstable crack-growth event to have occurred, as claimed by proponents of various versions of the filminduced cleavage model, tr'Tj Moreover, the latter models are also excluded in accounting for microcrack advances, since they depend on diffusional processes (to W.F. FLANAGAN and B.D. LICHTER, Professors of Materials Science, Department of Mechanical Engineering, and M. ZHU, Research Instructor, Orthopedics and Rehabilitation Department, are with Vanderbilt University, Nashville, TN 37235. MIN WANG is with Peterbilt Motors Co., Nashville, TN 37116-0487. Manuscript submitted August 16, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
form films or dealloyed layers) which cannot operate in the time interval of the microcracking events, t4,t3] Thus, average crack-growth rates for T-SCC can be very small when macrocrack arrests occur--as low as 0.1 /~m/s, this being determined mainly by the time required for crack renucleation. On the other hand, the instantaneous crack-growth rates are found to be several orders of magnitude higher, and describe the overall crack-growth rate in the absence of crack arrests, i.e., that rate which occurs discontinuously between crack arrests. By analysis of simultaneous current and load transients which accompany such instantaneous crack growth in experiments on oriented single crystals, we have been able to determine the instantaneous velocity and the prevailing stress state accompanying such crack growth.
II.
DATA ANALYSIS
The data for the following calculations come from the slow-strain-rate tests performed on disordered Cu-
Data Loading...
