Sampling in CF Tests

The fracture mechanics approach in CF provides the basics for many fatigue crack growth studies. The relationship is 

The growth or extension of a fatigue crack under cyclic loading is mainly controlled by the maximum load and minimum/maximum stress ratio. However, as in crack initiation, there are other factors that have an effect, especially in the presence of an aggressive environment. 

The established standard test method for CF crack growth rate is ASTM E647 (30). In this constant load test method, the crack length is measured visually as a function of elapsed cycles followed by numerical analysis of the data to obtain the crack growth rate. Crack growth rates are then expressed as a function of crack tip intensity range Ak-, which is based on linear-elastic stress analysis. 

The crack growth rate daldN where a is the e crack length and AT the number of cycles as a function of Ak- gives results that are independent of sample geometry, and this permits the comparison of data obtained from a variety of sample configurations and loading conditions (4). 

CF tests may be carried out in an apparatus designed by the Continental Oil Company. The apparatus consists of a Monel tank with four samples subjected to cyclic bending. The first step consists of determining the displacement caused by the applied load. The exact stresses are determined by strain gauges. The electrolyte is deaerated with 3% sodium chloride. Polished or sand blasted samples are used, and the behavior of the alloy in CF may be studied at the free corrosion potential under different percentages of stress amplitude of the elastic limit. From potentiokinetic curves, I =J E) the protection or pitting potential applied during the stress test. Each test can have four samples and the difference between results (85) for similar tests does not exceed 15%. 

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