Engineering Physics of High-Temperature Materials. Nirmal K. Sinha. Читать онлайн. Newlib. NEWLIB.NET

Автор: Nirmal K. Sinha
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119420460
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Bressers et al. (1981).

Schematic illustration of short-term (200 s) and longer-term (2341 s) tensile SRRTs on a single specimen of polycrystalline nickel-base superalloy Waspaloy at 1005 K (732 degrees Celsius) for 650 MPa using linear timescale.

      Source: N. K. Sinha.

      There are innumerable sets of creep curves for a wide variety of manufactured and natural materials illustrating transient and tertiary creep stages, but the recovery on full unloading is rarely reported. There are, however, examples of stress‐dip tests in which creep continues after a short recovery on partial unloading during the steady state or actually at mcr that occurs at evolved microstructure corresponding to this state. Unfortunately, stress‐dip tests do not provide useful information on transient creep at the beginning of a creep test and the characteristics of neither the delayed elastic deformation nor the viscous flow corresponding to the original, undeformed and undamaged microstructure.

      Figure 1.4 exemplifies a unique set of results for a complex nickel‐based aerospace alloy. It brings out the fact that the delayed elastic strain, ε d, recovered on full unloading well within the tertiary stage of creep, after mcr, was not negligible and not “absorbed” within the viscous component. The long‐term test (2341 s) is noticeably larger than that of the 200 s test. Hence, ε d increases with time. This raises the question as to the mechanism(s) responsible for generating delayed elasticity in polycrystalline materials that may have far‐reaching consequences, presented in Chapter 5, in developing physically based creep models.

Image described by caption.

      Source: N. K. Sinha.

      Figure 1.5 also shows the “average viscous strain rate”, ModifyingAbove epsilon Subscript v Baseline With ampersand c period dotab semicolon, during the creep time or the “strain relaxation” time, t SR, of 200 s and 2432 s, respectively, given by

      where εv is the “measured permanent or viscous strain” after full recovery, as shown in Figure 1.5.

Schematic illustration of strain–time curves.