Of the four consumer products in the figure above, one does not belong. Which one is it?
The disposable propane cylinder is made of steel and holds liquid petroleum gas, which is a two-phase mixture of liquid and gaseous propane. At normal indoor temperatures the pressure inside the container, called its internal pressure, is about 130 psi (pounds per square inch).
The soda can is made of aluminum alloy 3004 for the body and 5182 for the top (both are highly formable alloys of mostly aluminum with some magnesium). Like the propane cylinder, the soda can has an internal pressure, in this case from the carbon dioxide dissolved in the contents. The internal pressure of a soda can is approximately 60 psi.
The CO2 cartridge is made of ferritic stainless steel and holds carbon dioxide at an internal pressure of around 1000 psi. These cartridges are used for air pistols, paintball guns, tire inflators and even slip-test tribometers.
The water filter housing is made of a common plastic called polypropylene. Such filter housings are part of water filtration systems designed for domestic use with house plumbing where the water pressure, and hence the internal pressure of this water filter housing, is around 75 psi.
All four products have in common that they must withstand internal pressure in order to function as intended, and all four employ thin-walled cylindrical bodies. But what are the differences?
The most obvious difference among the four is that three are made of metal and one is made of plastic. Pressurized plastic is not a problem though, as long as the designer appreciates that polypropylene has a different set of strength and stiffness properties compared to steel, aluminum and stainless steel.
The most important difference in the four products is that three have domed ends – the propane and CO2 cartridges are convex and the soda can is concave – and one is flat. The significance is that according to solid mechanics theory the tensile stress in a flat end is about ten times more than in a domed end. This was significant for the water filter because although the stress in its cylindrical body was amply resisted by the plastic from which it was made, the much higher stress in its flat bottom caused the polymer to crack so that the whole bottom broke off. Water flowed under pressure out the bottom for hours creating a costly amount of water damage.
Spotting the significance of the water filter’s flat bottom is an example of how failure analysts, applying fundamental engineering principles, find the root cause of failures without necessarily being, in this example, plumbing experts. A mastery of fundamental engineering principles serves any failure analyst well when confronting a vast assortment of product, machinery, device and system failures.