CREEP

摘要

In the later 1960s, I could see that some Triumph air-cooled 500 twins started AMA National races strongly and finished them sounding a bit flat and emitting smoke. Then in 1968, they were swept into obsolescence by, of all things, Harley's resurgent (and supposedly obsolete) side-valve KR. In the fullness of time, I suspect that what I was seeing was very likely the result of the slow heat distortion of solids far below their melting point, known as "creep." The classic example of creep usually given is the gradual elongation of jet engine turbine blades, constantly loaded in tension by their rapid rotation and kept at high temperature by the passage of hot air-diluted combustion gas among them. At installation, the blades clear the inside of their casing by enough to allow for heat expansion and some elongation. As blades grew longer in service from total-time-at-temperature, they would eventually begin to scrape against the casing if not replaced. Another such example is the movement of glaciers, which although solid are able to deform and flow under the urging of gravity.