Cyclic Stress-Strain Curve Determination

After few blocks of these fully reversed cycles with increasing and then decreasing strain amplitude the material stabilizes. Normally, the strain is 

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To determine the local strain range, the cyclic stress strain curve is used, since under the influence of cyclic loads, the material will soon approach the stable cyclic condition. The cyclic stress-strain curve is computed as... 

The cyclic stress-strain curve can be used, for example, to perform finite element simulations of cyclic loadings. To simulate the complete experiment in the computer, it would be necessary to obtain information on the hardening of the material (isotropic and kinematic hardening) and to determine a material model that correctly describes it. This would be an extremely comphcated procedure. Furthermore, the entire number of cycles would have to be calculated, which would require an immense amount of computing time. Instead, the flow curve, taken by the finite element software to be monotonous, can be replaced by the cyclic stress-strain curve. A single, monotonous loading of the component is then simulated. Stresses and strains calculated in this way correspond well with those in the cyclically loaded component. 

One of the most frequently used tests for fatigue-resistance evaluation is the well-known plotting of stress versus the number of cycles, usually referred to as the S-N (curve) relation . Various wave forms of cyclic stresses may be applied to a specimen to test its suitability to withstand prolonged strain. Machine elements are assessed to determine their practical endurance of industrial applications to which they may be exposed. Such tests focus on the nominal stress required to cause fatigue failure at some number of cycles. A logarithmic scale is almost always used for N, the number of cycles to failure. A schematic S-N plot is shown in Fig. 7.1. Note the horizontal line in plot (a), known as a knee , which represents the endurance limit . As implied by its name, at this level of stress, the specimen is characterized by its ability to endure a large number of stress-cycles at the stress level of the horizontal line and below it without failure. In plot (b), no such horizontal line is observed and the curve continues to decrease, indicating that the stress must be reduced for the test specimen to be able to withstand a certain number of cycles. 

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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