Crack initiation protection

The surface finish of the specimen may also affect impact behaviour. Machined surfaces usually have tool marks which act as stress concentrations whereas moulded surfaces have a characteristic skin which can offer some protection against crack initiation. If the moulded surface is scratched, then this protection no longer exists. In addition, mouldings occasionally have an embossed surface for decorative effect and tests have shown that this can cause a considerable reduction in impact strength compared to a plain surface. 

There are two main approaches to avoidance of ozone damage. One is the addition of antiozonants. The more extensively used antiozonants are listed in Table 13-12. These antiozonants have limited solubility in elastomers and tend to bloom to the surface. The antiozonant action is not well understood chemically, but probably involves formation of a protective film. Crack initiation occurs at critical stress, which is strongly... 

Stress corrosion cracking is a form of localized corrosion, where the simultaneous presence of tensile stresses and a specific corrosive environment prodnces metal cracks [157, 168]. Stress corrosion cracking generally occnrs only in alloys (e.g., Cn-Zn, Cu-Al, Cu-Si, austenitic stainless steels, titaninm alloys, and zirconinm alloys) and only when the alloy is exposed to a specific environment (e.g., brass in ammonia or a titaninm alloy in chloride solutions). Removal of either the stress on the metal (which must have a surface tensile component) or the corrosive environment will prevent crack initiation or cause the arrest of cracks that have already propagated. Stress corrosion cracking often occurs where the protective passive film breaks down. The continual plastic deformation of the metal at the tip of the crack prevents repassivation of the metal surface and allows for continued localized metal corrosion. 

As an example, an injection molded piece is formed by several layers of different orientation. In such parts the outer layers serve as a protective skin that inhibits crack initiation. In an injection molded article, cracks are likely to initiate inside the component by defects such as weld lines and filler particles. The gate region is also a prime initiator of fatigue cracks. Corrosive environments accelerate crack initiation and failure via fatigue. 

The surface finish of the specimen may also affect its impact behavior. Machined surfaces usually have tool marks that act as stress concentrators, whereas molded surfaces have a characteristic skin that may offer some protection against crack initiation. 

Moderate cathodic protection can improve a corrosion fatigue performance of low strength steels. In the case of high strength alloy, cathodic protection may accelerate fatigue crack initiation. Steel specimens, AISI 4040 Rc 20), in 3% NaCl were stressed 10% above and below the fatigue limit. The specimens were maintained at... 

---