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Research Papers

# Analysis and Design of a Small, Two-Bar Creep Test Specimen

[+] Author and Article Information
Balhassn S. M. Ali

e-mail: eaxba@nottingham.ac.uk

Wei Sun

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

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received December 21, 2012; final manuscript received August 1, 2013; published online August 30, 2013. Assoc. Editor: Marwan K. Khraisheh.

J. Eng. Mater. Technol 135(4), 041006 (Aug 30, 2013) (9 pages) Paper No: MATS-12-1294; doi: 10.1115/1.4025192 History: Received December 21, 2012; Revised August 01, 2013

## Abstract

In this paper, a new small-sized (two-bar) specimen type, which is suitable for use in obtaining both uniaxial creep strain and creep rupture life data, is described. The specimen has a simple geometry and can be conveniently machined and loaded (through pin-connections) for testing. Conversion relationships between the applied load and the corresponding uniaxial stress, and between the measured load-line deformations and the corresponding uniaxial minimum creep strain rate, have been obtained, based on the reference stress method (RSM), in conjunction with finite element analyses. Using finite element analyses the effects of the specimen dimensions on reference stress parameters have been investigated. On this basis, specimen dimension ratio ranges are recommended. The effects of friction, between the loading pins and the specimen surfaces, on the specimen failure time, are also investigated. Test results obtained from two-bar specimen tests and from corresponding uniaxial specimen tests, for a P91 steel at 600 °C, are used to validate the test method. These results demonstrated that the specimen type is capable of producing full uniaxial creep strain curves. The advantages of this new, small, creep test specimen, for determining uniaxial creep data, are discussed and recommendations for future research are given.

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## Figures

Fig. 1

(a) Photograph of a scoop sample and (b) dimensions of a typical scoop sample [14]

Fig. 2

Conventional uniaxial creep test specimen dimensions are in mm [15]

Fig. 3

Variation of log(Δ·ssc(n)/B(ασnom)n) with n [8]

Fig. 4

Two bar specimen geometry and dimensions

Fig. 5

(a) TBS experiment setup and loading application; (b) the TBS specimen, and (c) tested specimen

Fig. 6

Finite element mesh and the boundary conditions

Fig. 7

Creep deformation versus time for P91 at 600 °C, obtained from TBS-FE analyses for different stress levels

Fig. 8

Contour plot of damage parameter, ω, in the TBS for P91 at 600 °C, (σ = 180 MPa, tf = 90.81 h)

Fig. 9

Determination of β and η for the TBS

Fig. 10

Variations of β and η parameters with various Lo/Di ratios; the other ratio k/Di and b/Di for the specimen are 0.62 and 0.12, respectively

Fig. 11

Variations of β and η parameters with various k/Di ratios; the other ratios Lo/Di and b/Di for the specimen are 2.16 and 0.12, respectively

Fig. 12

Variations of β and η parameters with various b/Di ratios; the other ratios Lo/Di and k/Di for the specimen are 3.12 and 1.25, respectively

Fig. 13

Variations of β and η parameters with Di, for specimens with Lo = 18 mm and b = (0.5, 1, 1.5, 2, and 2.5 mm)

Fig. 14

The steady state deformation rates obtained from the TBS theoretically and numerically (FE) using β =1.4557

Fig. 15

The steady state deformation rates obtained from the TBS theoretically and numerically (FE) using β = 1.179

Fig. 16

Deformation versus time curves obtained from the two bar specimens for a P91 steel at 600 °C

Fig. 17

Minimum creep strain rate data for P91 steel at 600 °C

Fig. 18

Converted TBS creep strain together with the corresponding uniaxial creep strain for P91 steel at 600 °C, stresses in (MPa)

Fig. 19

Creep rupture data obtained from TBSs and uniaxial specimens for P91 steel at 600 °C

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