Stretching infers deformation. Over-reved rods stretch. Aluminum flexes more than steel. Not only does heat treat effect strength, the material itself makes a BIG diference. 7075 T-6 aluminum is stronger than mild/low carbon steel. Some production steel rods are low carbon steel. Anything better than that is stronger than stronger than aluminum. That's part of the reason aluminum rods are so m***ive. Aftermarket steel rods are typically made of MUCH stronger(and more fatigue resistant) 4340. Airplanes are made of aluminum. Aside from some of the airplane mechanics I have known, aluminum fatigue is just one of the reasons I don't fly.
Al, I am a degreed metallurgical engineer. The official HAMB metallurgist from way back. Be careful getting into an argument here. I am only trying to make sure the correct terms and facts are stated. Elastic stretch is not the same as plastic deformation (permanent deformation, caused by exceeding the yield strength). The rod stretches elastically under normal conditions. No matter what the heat treat, the elastic modulus stays the same. Alum will always be one third as stiff compared to steel. A heat treated alum may be higher yield and ultimate strength than a mild steel, that is tensile strength. Not the same as elastic modulus. Alum rods are m***ive for two related reasons, one to help make the applied stress lower, and two because the lower stress make for less (elastic) stretch problem. Aftermarket rods are made from higher alloyed materials because even without heat treating, the alloying increases the strength. It also increases other mechanical properties such as percent elongation, ductility, toughness, etc. Fatigue limit for steel is typically about 50% of yield strength. Stay below this stress and you will not have a fatigue crack problem. Airplanes are mostly aluminum and they also have regular strict inspection intervals. Yes, they do also get fatigue cracks. I used to work commercial aircraft and have done many failure ****ysis on parts due to fatigue cracking. Airplanes also tend to have multiple load paths, so there is not one single failure point if it can be avoided.
I'm running aluminum rods in strip/limited street use engine. The engine builders only real concern was high oil temp on the street. He warned going over 270 oil temp would risk spinning a bearing due to the expansion rate of aluminum. Has anyone had such experience?
Not trying to argue, just clarifying. I don't see anything in your response that contradicts what I posted. In the automotive world, a stretched rod is one that's longer than it was when new. Intentional or not, you come off like you are pretty impressed with your own credentials. I am reminded of the old saying that's something like; people who are good at something don't need to go around pointing it out. I have worked with many designers and engineers on lots of different projects. Some are truly amazing, the rest are, to one extent or another, essentially along for the ride. But, they all have degrees. The most knowledgeable and capable people I know have made mistakes in their own area of expertise. I have seen cases of those people having misconceptions about things relating to their fields. Point being, even the best/smartest/most knowledgeable experts aren't perfect and don't know everything. So much for degrees and unquestioned righteousness. Regardless of how much you actually do or do not know, or the fact you see yourself as the "official HAMB metallurgist from way back", when it's all said and done you and I both are just a couple guys on an internet forum. Because of that, I feel no obligation to treat you or your posts different any of the other guys here. Not trying to start a dispute or hurt your feelings. That's just how it is.
read this from Bill Miller Engineering [ BME ] scroll down to the part about " only streetable aluminum rods " http://www.bmeltd.com/rods.htm They've been running them for 20 years on the street. I think the biggest problem is with "Redneck or Shadetree Engine Builders" The urban myth has saved a lot of lawsuits
]read this from Bill Miller Engineering [ BME ] scroll down to the part about " only streetable aluminum rods " http://www.bmeltd.com/rods.htm They've been running them for 20 years on the street. I think the biggest problem is with "Redneck or Shadetree Engine Builders" The urban myth has saved a lot of lawsuits[/QUOTE] Good info, thanks for sharing the link. Sounds like my builder was right on with the oil temp. I'll be putting an oil temp gauge in just to be safe.
Al, I don't have to defend my credentials. I have contributed numerous statements on metals and metallurgy info over many years here, to many persons benefit. Not bragging, just the way it is. I even used to have "HAMB Metallurgist" under my name ***igned by Ryan before the software changed and now we don't have special ***les anymore. I never said I was perfect, but I do come from a position of knowledge and try to keep incorrect information from being repeated. I did not say you were wrong, only that the terms stated could use some clarification. I will refrain from any further comment, it does not serve the orig question any benefit. Let's get back to cars.
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