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Our group has been working to design a combustion chamber of a relatively small hybrid engine with a chamber pressure of 30 bars. We are currently faced with the problem that we cannot easily access materials from certified places that have the relevant information documented. That would be a problem for us because we cannot calculate the chamber thickness otherwise. I was wondering if there are easier alternatives, and if we were to find a place that can provide a detailed report of the material, what would be the properties we need to calculate the chamber thickness that we should request for?

Thank you to you all in advance.

Baldwin Chen
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  • Would the chamber be at elevated temperature ? If so you need to pick a temperature. ASME ( boiler code) has stress vs temperature listings , mostly for steels. At elevated temperatures you use stress rupture or creep strength , not yield. – blacksmith37 Jan 04 '23 at 01:40
  • Yes, the chamber would be at an elevated temperature of around 300-400 degree celcius. I will look into the two terms, thank you so much. – Baldwin Chen Jan 05 '23 at 06:04
  • Your temperature are just below creep and S R ranges. In relaxation range for steels . Steels will work with very minor distortion with time Traditional Cr : Mo pressure vessel steels would be good , Plate -ASTM 387 and A542 if you can find it. Pipe may cover sizes you want and be more available. – blacksmith37 Jan 05 '23 at 17:00

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For isotropic materials, a property which is simple enough to get and useful enough is the ultimate tensile strength, or (even better for design properties the proof stress - stress at 0.2% elongation).

Alternatively, one other option (not so accurate though) is to get a batch of the material and do a hardness test and the convert the value of the hardness test to material strength. Depending on the material the charts may vary but the following is an example from Bossard Hardness comparison table according to ISO 18265.

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If the material are anisotropic then things can get complex and the geometry will have a significant effect on which strength property is more crucial (tensile or shear).

NMech
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  • If we do decide that we want to use stainless steel, we could use the ultimate strength then for SS 316 and SS 304 given that they are generally isotropic, is that correct? (Thank you so much) – Baldwin Chen Jan 05 '23 at 06:06
  • generally the properties of stainless steel can vary a lot depending on composition and heat treatment. For a ball park you could but generally its best if you actually calculate the minimum specs and they order by those specs. – NMech Jan 05 '23 at 07:14