Technical Brief

A Basis for Selecting the Most Appropriate Small Specimen Creep Test Type

[+] Author and Article Information
T. H. Hyde, C. J. Hyde, W. Sun

Department of Mechanical,
Materials and Manufacturing Engineering,
The University of Nottingham,
University Park,
Nottingham NG7 2RD, UK

Contributed by the Pressure Vessel and Piping Division of ASME for publication in the JOURNAL OF PRESSURE VESSEL TECHNOLOGY. Manuscript received August 23, 2012; final manuscript received October 24, 2013; published online January 7, 2014. Assoc. Editor: Osamu Watanabe.

J. Pressure Vessel Technol 136(2), 024502 (Jan 07, 2014) (6 pages) Paper No: PVT-12-1139; doi: 10.1115/1.4025864 History: Received August 23, 2012; Revised October 24, 2013

Many components in conventional and nuclear power plant, aero-engines, chemical plant etc., operate at temperatures which are high enough for creep to occur. These include plain pipes, pipe bends, branched pipes etc., the manufacture of such components may also require welds to be inserted in them. In most cases, only nominal operating conditions (i.e., pressure, temperatures, system load, etc.) are known and hence precise life predictions are not possible. Also, the proportion of life consumed will vary from position to position within a component and the plant. Hence, nondestructive techniques are adopted to assist in making decisions on whether to repair, continue operating or scrap certain components. One such approach is to use scoop samples removed from the components to make small creep test specimens, i.e., sub-size uniaxial creep test specimens, impression creep test specimens, small punch creep test specimens, and small ring (circular or elliptical) creep test specimens. Each specimen type has its own unique advantages and disadvantages and it may not be obvious which one is the most appropriate test method to use. This paper gives a brief description of each specimen and associated test type and describes their practical limitations. The suitability of each of the methods for determining “bulk” material properties is described and it is shown that an appropriate test type can be chosen.

Copyright © 2014 by ASME
Topics: Creep
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Fig. 1

Small material samples

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Fig. 2

“Standard” uniaxial creep test specimen (GL ≈ 30–50 mm; dGL ≈ 6–10 mm; L = 100–130 mm)

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Fig. 3

Shapes and dimensions of small creep test specimens: (a) conventional sub-size uniaxial specimen (GL ≈ 5–12 mm; dGL ≈ 1–3 mm); (b) SPT specimen (D ≈ 8 mm; to ≈ 0.5 mm); (c) ICT specimen (w = bi ≈ 10 mm; di ≈ 1 mm; h ≈ 2.5 mm); (d) circular SRT (R ≈ 5 mm, d ≈ 1 mm, and depth bo ≈ 2 mm); and (e) elliptical SRT (a ≈ 15 mm, b ≈ 7.5 mm, d ≈ 1 mm, and depth bo ≈ 2 mm)

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Fig. 4

Schematics diagrams showing the small specimen loading arrangements: (a) uniaxial; (b) SPT; (c) ICT; (d) circular SRT; and (e) elliptical SRT

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Fig. 5

(a) Uniaxial minimum creep strain rate data obtained with standard specimen (Fig. 2) on a P91 steel at 600  °C [3] and (b) uniaxial creep rupture data obtained with standard specimen (Fig. 2) on a P91 steel at 600  °C [3]

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Fig. 6

Minimum creep stain rate data for 316 stainless steel at 600 °C and 2-1/4r1Mo weld metal at 640 °C, obtained from uniaxial and impression creep tests (e.g., Ref. [17])

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Fig. 7

(a) Minimum creep strain rate data for a P91 steel at 650 °C obtained from uniaxial and SRC tests [8] and (b) minimum creep strain rate data for a Nickel base superalloy at 800  °C obtained from uniaxial and SRC tests

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Fig. 8

(a) Converted creep rupture data (using Eq. (3), with Ks = 1) obtained from a SPT on a P91 steel at 650  °C, compared with corresponding uniaxial data [16] and (b) converted creep rupture data (using Eq. (3), with Ks = 1.275) obtained from a SPT on a P91 steel at 650  °C, compared with corresponding uniaxial data [16]




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