Fatigue life prediction for the development of ship board exposed equipment

A mechanically scanned array type radar antenna was developed for several classes of US NAVY vessels. A light weight requirement as well as the RF properties required that substantial parts of the equipment be composed of aluminum alloys. The combined stress corrosion behavior of aluminum alloys exposed to cyclic loading in the ocean environment suggests that fatigue aggravated by corrosion was a concern. The author developed a fatigue life prediction technique using Microsoft Excel that took into account the winds, ship speeds, ship motions, Concept Of Operations (CONOPS), and the rotational parameters to predict the fatigue loading on the antenna over a 20 year life. The load levels based on the sum of the wind and ships motions were surprisingly high. Among other things, this analysis showed that the customer´s specification of the environmental loading was woefully inadequate. This paper describes the step by step methodology and discusses the options available at each step of the procedure. The validity and sensitivity of the results are discussed. The technique is easily scalable to other shipboard equipment.