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242 J. M. CALHOUN Vol 43, tfo. 4
If a small constant tension is applied to a strip of film and the elongation measured against time, a complicated series of phenomena occur as illustrated in Fig. 10. Following the first instantaneous elastic extension, elongation does not cease, but continues at a gradually decreasing rate. This is due to a combination of delayed elasticity and plastic deformation, and the material appears to flow or creep. If the load is removed at the end of a given time, there is first an instantaneous elastic contraction corresponding to the original instantaneous elastic extension. The film then continues to contract, a behavior called creep recovery. However, the film may not regain its original size or form, even in a very long time. The portion of the creep which is recoverable has been termed delayed elasticity or primary creep, and the portion which is not recoverable has been termed plastic flow, secondary creep, or more commonly, cold flow* A material which has undergone cold flow is said to have taken a permanent set.
Plastic behavior of the type described above is important in motion picture film, because of the permanent deformations or distortions which may result from the application of stress during storage, handling, processing, or projection. The magnitude of the cold flow which may occur in a given film increases with increase in the load applied, the time during which it acts, the temperature, and the moisture content of the film. If the temperature of a piece of film under tension is reduced before the load is removed, creep recovery is retarded or prevented. This phenomenon has already been referred to as the "freezing-in of strain." If the film is reheated at some later time after removal of the load, recovery of primary creep will occur and produce a shrinkage. For a more detailed description of the elastic and plastic behavior of materials of this type, the reader is referred elsewhere.2' 5 15 16
* There is some lack of uniformity in the literature concerning the terminology employed for the various types of deformation which plastic materials exhibit under load. Both the recoverable and nonrecoverable portions of creep are sometimes included in the term, plastic flow. This may be due to the fact that an analogy is sometimes drawn between recoverable creep and the behavior of a model consisting of both elastic and viscous elements connected in parallel.2 15 However, recoverable creep is more logically considered as an elastic deformation in the sense that it is eventually recovered, although it is a delayed, or imperfect elasticity. We prefer to reserve the terms plastic flow and cold flow for the nonrecoverable creep. Leaderman5 overcomes this difficulty by the use of the terms primary creep (recoverable) and secondary creep (nonrecoverable) .