A Fatigue Comparison Of High Strength Steel, Stainless Steel, And Titanium In Asimulated Ocean Environment
Authors: Beach Jeffrey E. , Marchica Nicholas V. , Taylor David W. , Ichter Larry L. 
Published: October 2008

The fatigue performance of two high-strength steels (NY-80, HY-130), stainless steel 17-4 PH and a titanium alloy (Ti 6A1-4V) were compared in a simulated ocean environment. The laboratory test specimens (box beams) were representative of any high-strength marine appendage such as rudders, hydrofoil foils, etc. Single-sided welds, by necessity, are used to connect the cover plating to the internal stiffeners of the load-carrying structure. These welds are considered poor from a fatigue standpoint. Three methods of closure joints studied were slot welds, single-sided "T" welds and single-sided butt welds. The box beam fatigue testing was designed for 7.5 × 106 cycles. Fatigue failures, the occurrence of a visible through crack in the test section, were observed prior to this for air and salt-water tests for all materials. Slot welds and single-sided "T" welds lasted twice as long as butt welds. Information obtained, can be used to determine the suitability of these engineering materials and weld joints in ocean structures. Since the box beams tested were designed on an "equivalent load concept", the test results compare fatigue performance of equivalent structural systems.

INTRODUCTION

Marine structures are subject to fatigue loads and potentially an aggressive environment throughout their lives like any other long-lived structure. The magnitude of loads, effects of environment, and fatigue performance of the structure depend on many factors including the vehicle type, mission and details of the structure involved. One type of marine structure that is particularly susceptible to fatigue damage is the multicellular wing-type structure used for rudders, control fins and planes and hydrofoil struts and foils. This type of structure normally is comprised of internal cross directional stiffeners with closure plates on both sides (see Fig. 1). This sometimes is referred to as "egg-crate" construction. Because of the thinness of the structure, cover plates on one side of the structure normally are attached with some form of single-sided welds.

The single-sided welds themselves present potential fatigue damage sites. In addition, the whole structure potentially can present fatigue problems because of the fact that the internal surfaces are not accessible for inspection. For hydrofoil foils, the single-sided welds can be selectively located so that stresses they experience are nominally compressive. For struts, rudders, and other types of structures, this is not possible since both sides of the structure see both tensile and compressive stresses.

In order to evaluate properly the fatigue and crack growth performance of these types of structures a large-scale experimental program was developed at the David W. Taylor Naval Ship R&D Center. This program was developed around a hydrofoil foil test article; however, the comparative results would be applicable to any type of marine structure that encompasses egg-crate stiffening and single-sided welds. The basic test article is a tapered box beam approximately 4 ft in length that is mounted vertically (see Fig. 2). One end is fixed and the other end point loaded. The cantilevered box beam has a tapered design so that the resulting stress distribution is approximately constant over 70 percent of its length.