Modeling the texture dependence of environmentally assisted growth of long and short cracks
- PDF / 2,179,781 Bytes
- 12 Pages / 612 x 828 pts Page_size
- 100 Downloads / 181 Views
The deflection of long cracks subject to far field Mode I loading is modeled for simply kinked and zig-zag configurations representing local Mode 1/Mode II loading and for simply twisted and doubly twisted configurations representing local Mode I/Mode III loading. Twisted crack geometries are considered in both plane stress and plane strain conditions. Crystallographic texture is incorporated explicitly into the model through the texture parameter, f, and model predictions express a normalized CTOD as a function off. Data for the propagation threshold in fracture mechanics specimens of ZIRCALOY* exposed to an iodine environment at 573 K agree well with model predictions. Discrepancies are explained by plastic processes complementing the transgranular cleavage. An analogous model predicts the threshold stress for propagation of short cracks as a function of texture using the normalized CTOD from the long crack data. Model predictions are correlated with threshold stress data for ZIRCALOY tubing exposed to an iodine environment at 593 K. Model assumptions are discussed relative to the applicability of LEFM to short cracks. The model predicts a low threshold stress in materials with a texture well oriented for transgranular cleavage.
I.
INTRODUCTION
THE propagation of cracks in Mode I loading is normally characterized by a far field stress intensity factor where the path of crack growth is assumed to be linear and perpendicular to the direction of loading. On a microscopic level, cracks seldom propagate in a linear fashion. Deflection or branching of the cracks is observed in fatigue, 1-5 in environmentally assisted fracture, 6-H and in quasi static fracture. ~2'~3 Similar crack deflection effects are observed in ceramic materials. 14'15'~6Often, the consequences of crack deflection are beneficial where a higher fatigue threshold (AK, h),2 a higher threshold stress intensity for environmental failure (Ktscc), 6 a higher failure stress, ~4'~5or improved toughness 17 results. Crack deflection is caused by a variety of microstructural features such as second phase particles, grain boundaries, TM slip bands, 3'19 and intentionally added obstacles as in the case of composites. 17 Crystallographic texture can exert a strong influence on crack propagation, particularly where cleavage along preferred crystallographic planes is the mode of failure. For hexagonal close packed systems, texture is known to profoundly influence susceptibility to environmentally assisted failure in zirconium alloys6'7's and in titanium alloys.~°'H'2° In unalloyed titanium and its near a alloys, the cleavage plane is identified as 15 deg from the basal plane in aqueous environments 2~'22'23and 0 deg or 10 deg from the basal plane in liquid metal environments. 22'23 In zirconium alloys no direct measurements of cleavage facet plane have been made, but the cleavage plane is inferred to be at or near the basal plane based on analogy with titanium and its alloys and on other convincing but indirect evidence. 24'25'26 This justifies the use of basal p
Data Loading...
