ALL metals will fatigue from repetive cycle loading. They just have to be designed to keep from flexing too sharply either through beefier materials or additional structure to put the number of cycles required for failure well outside it's expected life span. Some metals will take cycle stresses better than others. I've seen some of those aluminum trailers have problems and I've seen some that have lasted for 10's of years. Regardless, it's always a good idea to closely inspect your trailer for stress cracks - though aluminum trailer owners should look more often.