Pressure vessels, from the simplest aerosol can to the biggest boiler, are
designed, for safety, to yield or leak before they break. The details of this design
method vary. Small pressure vessels are usually designed to allow general
yield at a pressure still too low to cause any crack the vessel may contain to
propagate (“yield before break”); the distortion caused by yielding is easy to
detect and the pressure can be released safely. With large pressure vessels this
may not be possible. Instead, safe design is achieved by ensuring that the smallest crack that will propagate unstably has a length
greater than the thickness of the vessel wall (“leak before
break”). The leak is easily detected, and it releases pressure
gradually and thus safely (Table 6.19). The two criteria
lead to different material indices. What are they?