The question of Stainless steel vs Aluminum often arises when considering components for a number of applications.
Fatigue Strength and Life –* Stainless steel* has 5-10 times the fatigue strength (resistance to bending & vibration) of aluminum pipe. The fatigue strength of stainless steel is 25-75,000 psi, compared to aluminum at 6-18,000 psi.
Components made of stainless steel have infinite fatigue life, which means that they will never fail unless the load exceeds a certain level (ie. single excessive load), which causes “plastic deformation” (i.e. bending). The critical shortcoming of aluminum is its finite fatigue life, which means that aluminum components will ALWAYS eventually break. ALWAYS. It’s just a question of how many fatigue cycles the aluminum will survive. The beefier the frame, the longer it will last. Design, assembly and weld quality will also affect a welded assemblies life, but there is no such thing as an aluminum part that will last forever – if it gets used. An aluminum component subjected to frequent use, heavy loads, and pounding may only last a few years.
Lower fatigue loads relative to the strength of the frame will increase the number of fatigue cycles to make an Aluminum part last long enough for it’s intended use, but given enough use, every aluminum part will eventually fail. This has been demonstrated over and over, and is taught in all the engineering textbooks. Aluminum’s eventual failure when subjected to cyclical loads is not a question of “if”, but a question of “when.”
Stainless steel samples shows a different pattern. The peak stress continues to drop until it reaches 300 MPa. After this it remains constant. This means that so long as the stainless steel component’s stress was kept below 300 MPa then it would never fail due to fatigue. This “fatigue limit” is a common feature of most steel and titanium alloys whilst most metals such as aluminum and copper have no fatigue limit and so will fail eventually, regardless of how small the peak cyclic stress is.
The significance of the fatigue limit is that if the material is loaded below this stress, then it will not fail, regardless of the number of times it is loaded. Material such as aluminum, copper and magnesium do not show a fatigue limit, therefore they will fail at any stress and at any number of cycles.
Strength – Stainless steel is 2-3 time stronger than aluminum. The tensile strength of 304L stainless is 79,000 psi(316 stainless steel is 80,000 psi), compared to 25-40,000psi for aluminum (depending on grade used).
TIG welding stainless steel produces welds that are as strong as or stronger than the tubing it joins. Used by NASA, nuclear power plants, food service, rocket manufacturers, chemical and food/beverage processing plants, TIG welding stainless steel is the only option considered world-wide for consistent high strength, high quality welds. By contrast, Aluminum does not weld very well, and the parts are usually anodized prior to welding. When welded, Aluminum’s hard anodized surface requires extreme heat and resists welding and “pops” off, making the weld joint brittle, and removing all anodized surface protection/shine in the weld “heat affected zone”. Longer term, these aluminum welds eventually succumb to hydrogen cracking. Aluminum welds are rarely anodized after welding, due to the cost of the anodizing equipment and the size of the immersion tank that would be required to fit the entire assemblies into it, so the welded aluminum areas are subject to corrosion and pitting.
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