5 Replies Latest reply on Oct 9, 2008 5:29 PM by Mike Ramsey

    Stress from casting

    Mike Ramsey
      Here's my problem. I want to figure the stress in a casting as it cools. How I've set it up is I first run a thermal study I give it two loads the first is my initial temp as the melting temperature of 2800 F. The next load is I'm saying it cools to 80 F. I set this up as a transient study and I'm letting it cool for 4 hours and setting it to solve in 4 steps. I used the variation with time option for my 80 F temp and it varies from the 2800 to 80. My results for the temp seem reasonable.

      The next step is to find stress in the casting from it cooling. Under the thermal options of my static study I import temperatures from my thermal study at time step 4 and use my melting point for iron as the reference temp at zero strain. I get reasonable displacement in the .005" range but I'm seeing 300-400 ksi over a lot of the part when I have a yield of 35 ksi

      If it makes a difference in the case I'm using the faces the core of the casting would touch as a fixed restraint. Any ideas of what's happening?
        • Stress from casting
          I am guessing that the fixed faces are preventing them from shrinking resulting in the high stresses. Suppress the restrains and use either soft springs to stablize the model of turn on Use intetial relief.
            • Stress from casting
              Vince Adams
              Hi Mike, this is a great problem! I was contracted by a manufacturer of enamel coated cast iron sinks to investigate why the enamel cracked after cooling in some areas and not in others. It didn't appear predictable.

              The conclusion we came to is that... well this is a tough problem... If you cool a body uniformly throughout the volume & it is unrestrained, there is no stress. Any restraint that inhibits shrinkage will generate unreasonable stress as thermally induced expansion/contraction is a force of nature that is hard to stop. Only mathematically perfect restraints in FEA can do it.

              In this case, the sinks sat on a table, unrestrained. However, thinner sections cooled more quickly and would shrink at different rates than thicker sections did. This caused stress. Also, we identified that convection conditions varied throughout the product. Some areas were exposed to a measureable airflow in the plant while others were blocked from moving air due to the geometry of the product itself. This again caused unequal cooling.

              Finally, and this might not be as applicable to your problem, the enamal contracted at different rates than the cast iron but was subjected to the same cooling variations.

              You certainly need to watch your restraints as Wayne suggested but also consider all the thermal effects in your problem before signing off on the results.

              -- Vince
              • Stress from casting
                Mike Ramsey
                Wayne, you're my new hero. You have a poster or something I can hang on my wall? Mr. T is getting torn down.
              • Stress from casting
                Bill McEachern
                the tricky part about this problem is that at high temperatures the part has no strength and hence can't develop any stress till it cools and the modulus and the strength start to increase. When its a liquid is has no significant strength and when it is really quite hot but still frozen it has a very low stiffness and not much strength - hence it can stress relieve itself quite easily. Once it gets a bit cooler then the stiffness gets significant and the strength starts to climb then the stresses can materialize and get locked in to some degree. I doubt very much you can get a decent estimate with the approach proposed. That said though it might provide some insight if some approximation can be worked out that at least models the development of the frozed in stresses in a sensible way even if the absolute numbers are not reliable. In any case the way to handle it is by a non-linear analsyis and you woul dneed some decent material property data from melting temp to room temp to get a decent estimate.
                  • Stress from casting
                    Mike Ramsey
                    Really what we're looking for is an estimate. We have a part that is building stress as it cools and when it's being machined it will twist during the machining process causing the bores to be out of spec. What we want to know is if we add mass to the web of the part if that will help us avoid this problem without a secondary stress relieving process.

                    Changing the restraint swung my results in the other direction to where I'm at about 300psi now. In case anybody is wondering.