Crack initiation finish

Poor surface finish of weld bead. Helps initiate fatigue cracks. Improve finish by grinding. 

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. 

Crack initiation is promoted by corrosion and, particularly, by pitting or crevice attack, even of low depth, which can cause local conditions of acidity and thus the development of hydrogen. The duration of this stage depends both on the characteristics of the steel and the environment (e. g. the surface finishing of the steel bars, the pH and chemical composition of the environment, etc.) and the time required to initiate the preliminary pitting or crevice attack. However, it does not depend on the stress applied to the steel. 

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. 

Weibull modulus, some information on the relationship between the detailed finishing grade of glass surface and Weibull modulus will be needed. It is noteworthy that Tempax has the highest crack initiation load in water, but not in heptane. 

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