Lifetime Statistical Analysis of Welded Aluminum Light Pole Structures under Cyclic Loading

The fatigue life analysis of welded aluminum light poles under random fluctuating wind-induced loads is a challenging task. The statistical analysis of full-scale test data provides a means to examine the fatigue behavior of specimens and, when performed in sufficient quantity, to develop design recommendations. Thirty-five welded aluminum light pole specimens were fatigue-tested in the Structures Laboratory of the University of Akron, Ohio. Of the 35 tested, 22 specimens failed adjacent to the cast shoe-base during testing, whereas the remainder did not fail and were considered run-outs. The fatigue data was used to develop a best-fit S-N curve, as well as a lower bound assuming a log-normal distribution. The lower-bound curve provides an estimate of the 97.7% probability of survival for the welded shoe-base details and may be used as the basis for fatigue design. In an attempt to better understand the results and develop an estimate of the constant amplitude fatigue limit (CAFL), several statistical methods were applied to the test data, including (1)the Dixon-Mood method for modified staircase test results, (2)the maximum likelihood method (MLE) for estimating the Weibull distribution parameters, and (3)the Kaplan-Meier methodology for survival times. The up-and-down modified staircase method provided an estimate of the CAFL at 16.1MPa (2.3ksi) at 20million cycles for the welded aluminum shoe-base details. The shape parameter for the two-parameter Weibull distribution was found to be 0.43, indicative of a gradually decreasing hazard rate. The Kaplan-Meier was used to provide estimates of the probability of failure as well as the CAFL. A value for the CAFL based on the Kaplan-Meier intervals was found to be 19.9MPa (2.9ksi). There were no failures of specimens after interval 162 (32.4million cycles). An endurance limit (the CAFL) from the application of a Weibull distribution function was estimated to be 20.7MPa (3.0ksi). The constant amplitude fatigue limit calculated with a modified S-N curve is recommended for the design of shoe-base aluminum light details and is in good agreement with the results of statistical methods and recent design specifications.