EssForty
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Sunshine, Surf & Suzuki
Posts: 382
Melbourne, FL
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I didn't go into much detail of my calculations since it probably bores most people who do not wear pocket protectors or have electrical tape on their glasses. Here goes: Youngs modulus relates stress to strain for a given material. Since the assumption is that the steel is constrained by the aluminum case, the length the steel expands is zero. So I actually started as you suggest by determining the length it would expand by being heated from 70 to 250 degrees if unconstrained, and then figured out how much force it would take to squish a piece of steel back to the original length. That puts length on both sides of the equation and length drops out as a factor in calculating stress in this unique case. The resultant equation is simply stress =coeff of thermal expansion x temp diff x youngs modulus. It always amazes me how much stress can be developed in this scenario. Often the constrained material will bow significantly if the constraining walls do not deform or fail. So now we have a piece of steel applying 1,000 psi,,,then 10,000 psi...then 25,000 psi against the outer case wall as it heats up but remains constrained by the aluminum. Aluminum loses the arm wrestling match and shears as the steel finally builds up sufficient stress to exceed the shear strength of the cast aluminum. The aluminum will of course fail at the weakest point, usually where there is a defect such as porosity or sand inclusion.
This was just a proverbial "back of the envelope calculation" to see if there was any way to generate even a ballpark number that would explain it. Perhaps it is too simplistic to reduce the filter to a single element.
The failure mode looks like the way a frozen soda can blows up in a freezer. Of course, since water is one of those odd materials that expands, instead of contracts, on freezing, it gives the same failure mode by freezing that steel would provide on heating. It expands, deforms the can a bit then explodes catastrophically when the force exceeds the shear strength of the aluminum.
At any rate, I was just trying to make sense out of it. It would be hard to believe that high cycle fatigue of 10-20 psi oil pressure would fatigue the casting. Constrained thermal expansion is the best idea I could come up with. I'm open to other explanations and more accurate calculations that would eliminate this theory.
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