The S-N curves for two spoke materials are compared in the diagram below. The surface condition of each material used in the test was smooth and corrosion free. The carbon steel is subject to corrosion in extended use on a bike and this makes it less desirable for long term use.

The Carbon steel is expected to show a fatigue limit as shown in the right hand diagram. Other material types show a decreasing life as S increases.

From: McMahon and Graham, :"The Bicycle and the Walkman," Merion (1992)