Processes that Control Crack Growth

Reactions of the gas or gases with newly produced crack surfaces to evolve hydrogen, or surface oxygen (namely, physical and chemical adsorption). 

Diffusion of hydrogen or oxygen to the fracture (or embrittlement) sites. 

Partition of hydrogen or oxygen among the various microstructural sites. 

Hydrogen-metal or oxygen-metal interactions leading to embrittlement (i.e., the embrittlement reaction) at the fracture site. 

Transport Processes 1. Gas Phase Transport 2. Physical Adsorption  

---

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

The small slope of the stage II section of the crack-growth rate versus K curve is attributed to corrosion-related, diffusion-controlled processes in the crack. Steady-state diffusion mechanisms are required to account for the fact that the crack growth rate is essentially constant... 

This behavior is generally characterized by a plateau region, which prevails over a definite threshold It i usually referred to as stress CF as SCC systems usually exhibit this behavior, and the most common theory assumes that the crack growth rate is because of the addition of SCC and pure fatigue crack advance. This type of synergistic effect is observed in systems not sensitive to SCC such as ferritic stainless in seawater under cathode polarization. It is often associated with HE. It is possible that the plateau behavior is because of the control of crack growth rate by nonmechanical processes such as transport processes (73). 

Crack growth models in monolithic solids have been well document-ed. 1-3,36-45 These have been derived from the crack tip fields by the application of suitable fracture criteria within a creep process zone in advance of the crack tip. Generally, it is assumed that secondary failure in the crack tip process zone is initiated by a creep plastic deformation mechanism and that advance of the primary crack is controlled by such secondary fracture initiation inside the creep plastic zone. An example of such a fracture mechanism is the well-known creep-induced grain boundary void initiation, growth and coalescence inside the creep zone observed both in metals1-3 and ceramics.4-10 Such creep plastic-zone-induced failure can be described by a criterion involving both a critical plastic strain as well as a critical microstructure-dependent distance. The criterion states that advance of the primary creep crack can occur when a critical strain, ec, is exceeded over a critical distance, lc in front of the crack tip. In other words... 

Studies of environment-enhanced crack growth in gaseous environments have shown that crack growth may be controlled by (i) the rate of transport of the environment (along the crack) to the crack tip, (ii) the rate of surface reactions with the newly created crack surfaces to evolve hydrogen, or (iii) the rate of diffusion of hydrogen into the process zone ahead of the crack tip. In this simplified, chemical-based model, the competition between transport and surface reaction is considered. 

---