Role of Slip Mode on Stress Corrosion Cracking Behavior
- PDF / 1,051,195 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 10 Downloads / 257 Views
TRODUCTION
SERVICE failures of engineering materials in aggressive environments occur by stress corrosion cracking (SCC). Environments that cause this failure are usually gaseous (such as moisture or hydrogen) or aqueous solutions of sodium chloride (NaCl), etc. Stresses required to cause the SCC failures are below the yield stress of the alloy and are tensile in nature. These stresses can be externally applied or can be residual stresses from fabrication. Thus, the condition for SCC is the combination of an aggressive environment and an applied tensile stress that leads to a time-dependent subcritical crack nucleation and its growth. The SCC does not occur if only environment is present without stress. On the other hand, stress alone can cause overload failure even in the absence of environment. The relative roles of these two contributing factors (chemistry and stress) have been a subject of investigation over the past 50 years.[1–11] Precipitation-hardened alloys exhibit a wide variety of microstructures in solid solution from underage (UA) to peak age (PA) to severely overage (OA) conditions that lead to the variations in yield stress, work hardening, slip mode, fracture toughness (KIc), threshold stress for stress corrosion crack nucleation (KIscc), and steadystate crack propagation rates (da/dt). The environment reduces the required mechanical forces for breaking the bonds by reducing cohesive forces or surface energies. A.K. VASUDEVAN, Scientific Officer, is with the Office of Naval Research, Arlington, VA 22203. Contact e-mail: [email protected]. mil K. SADANANDA, Consultant, is with Technical Data Analysis, Inc., Falls Church, VA 22042. Manuscript submitted: February 12, 2010 Article published online December 8, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
Traditionally, experimental data on SCC are plotted in terms of crack velocity, (da/dt), vs stress intensity factor, K, indicating three distinct regions, as shown in Figure 1. In region I, the crack velocity is highly sensitive to the applied stress intensity factor, K, and the concentration or aggressiveness of the environment. A threshold stress intensity KISCC can be defined below which crack velocity is negligible. In region II, the crack growth rate is dependent primarily on the material/ environment system. However, in contrast to region I behavior, the crack velocity in region II is independent (case A) or weakly dependent on the applied K (case B). Region III of crack growth behavior is characterized by strong dependence on the applied K, where K approaches KIC. The (da/dt)-KI diagrams are considered to be useful in evaluating the SCC characteristic of a material. It is noted that the three-stage curve shown in Figure 1 is observed for most materials showing SCC behavior in gaseous, aqueous environments as well as for liquid metal embrittlement. In addition, these SCC curves can vary in magnitude for a given alloy, depending on the type of deformation mode such as planar to wavy slip. In the planar slip mode, the deformation is confined to a specific
Data Loading...
