API REPORT 82-34
Variable Deflection Fatigue Properties of Welded Steel as Applicable to Offshore Structures
|Publication Date:||8 March 1985|
This report describes accomplishments during a three year research undertaking wherein variable deflection fatigue properties of butt welded ABS DH32 steel plate specimens in flowing, natural sea water have been determined. The two principal variables were a) type of spectrum and b) level of cathodic polarization. In the former case the existing, narrow band C/12/20 and a wide band spectrum that is based upon results from a previous API project were employed. Potential for the experiments ranged from freely corroding to −1.35. (Cu-CuSO4).
The results of the fatigue experiments have been evaluated and discussed with regard to the above two variables (type of spectrum and level of cathodic polarization). In addition, the data are discussed in perspective to previous constant deflection fatigue results, existing design criteria, load interaction effects and damage accumulation. Specific conclusions which have been reached include the following:
1. Cathodic polarization at each of the potentials investigated enhanced fatigue life for both the narrow and wide band spectra experiments relative to the freely corroding value. This substantiates that the freely corroding condition is a more severe type of exposure than cathodically protected, at least for non pre-cracked specimens.
2. The data were generally consistent with the principles of linear damage summation (Miner's rule), although cumulative damage may differ from unity in individual tests by as much as a factor of two or more.
3. Variable and constant deflection test results were best correlated with one another in terms of an effective stress parameter based upon the Miles Equation.
4. The experimental results support appropriateness of the API-X curve for design of welded connections that undergo variable deflection loading in sea water.