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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temperature rises, complex issues related to time–temperature effects complicate matters.

      On the experimental front, constant‐load or constant‐stress creep tests are customarily performed at elevated temperatures. Room temperature creep tests are also performed on certain materials exhibiting low‐temperature ductility. The uniaxial tension test or compression creep test is the simplest and fundamentally most important test for the evaluation of material properties. The tradition is to load a specimen and monitor the evolution of strain. No specific efforts are made to determine the elastic modulus, such as Young's modulus (E), corresponding to the initial microstructure of the test specimen. At some stage, either the load is removed intentionally or by rupture. The post‐test analysis concentrates typically on stress–time–temperature dependence of strain and strain rate, and sometimes on microstructural examinations at room temperature. Almost invariably, the characteristics of the minimum creep rate (mcr), often considered as the steady‐state flow rate, are discussed. It is trendy to report only the mcr, time to rupture (t f), and elongation (engineering strain) at failure, ε f. Efforts are also sometimes made, but not necessarily as a normal practice, in fitting the creep curves for the transient creep, especially, for example, in the case of rocks.

      High‐temperature deformation processes are continuous, and each regime depends on earlier deformation and microstructural history. Materials remember their thermomechanical history! What happens if the load is applied (rise time) in fractions of second and if the creep (strain relaxation) test is terminated by unloading in fractions of second after a short creep or strain relaxation time of t SR and the strain ε (recovery) is monitored continuously for a long time? It should provide a historical record of strain that recovers immediately (elastic, ε e), strain that recovers with time (delayed elastic, ε d), the permanent or viscous strain, ε v, accumulated during t SR, and an average viscous strain rate ModifyingAbove epsilon With ampersand c period dotab semicolon v = ε v/t SR for the period t = 0+ to t SR. How does ε d vary with time? How does ModifyingAbove epsilon With ampersand c period dotab semicolon v vary with time, stress, temperature, and initial (constant) microstructure?

      Why not stop the test, unload the specimens completely (unlike partial unloading used in “strain‐ or stress‐transient dip tests”) during the creep test, as well as during other tests, such as constant‐strain‐rate strength tests and constant‐strain SRTs, and monitor the strain–time response for extended periods and evolution of strain trinity? This is like looking backward (hindsight) at the growth history of elastic, delayed elastic, and viscous characteristics.

      This book revolves around the concept of opening up the door for hindsight and using the opportunity it offers for developing both experimental and theoretical approaches. This is a recurring theme of various chapters. Experimental procedures were developed to examine not only total deformation, but also the three strain components: elastic, recoverable delayed elastic, and permanent viscous strain. Most importantly, theoretical developments can also be judged not only by how well they predict the total deformation under specific external conditions, but also how well they predict the strain components.

      It is well known that viscous flow (dislocation creep creep) exhibits stress‐wise highly nonlinear response, with stress exponent, n v, varying from a value of 4 for pure metals to significantly higher values for complex alloys. It is shown in Chapters 5 and 6 that delayed elastic response could exhibit nearly linear to highly nonlinear response, with stress exponent, s, varying from 1 to 4 for complex nickel‐base superalloys, so far examined experimentally. However, the ratio, n v/s, may not vary significantly for different materials examined so far. The n v/s (n v = 11.8 and s = 4.0) ratio of ≈3 for the nickel‐base superalloy IN‐738LC is similar to that of 4.3 for another nickel‐base superalloy – Waspaloy is also very close to that of 3.3 for titanium‐base alloy Ti‐6246 (n v = 4 and s = 1.2) and is exactly like that of polycrystalline ice with n v/s = 3 (n v = 3 and s = 1); however, ice is not a metal!