Article

Damage Morphology of Mod.9Cr-1Mo Steel under Biaxial Stress

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Abstract

Evaluation of creep rupture lifetime of high temperature components which undergo multiaxial loading is important. This paper investigates biaxial creep properties, especially creep deformation, from comparison of biaxial tensile creep tests with principal stress ratios (λ) of 0.0 and 1.0 at 923 K using a cruciform specimen and an originally designed biaxial creep machine. Here, λ is the principal stress ratio defined as λ=σy/σx. σx and σy are the principal stresses in x- and y-directions, respectively. The λ=0.0 test corresponds to a uniaxial tensile creep test, while the λ=1.0 test corresponds to an equi-biaxial tensile creep test. The creep rupture lifetime in the λ=1.0 test was comparable to that in the λ=0.0 test at same Mises equivalent stress of σeq=100 MPa. In contrast, creep curve in the λ=1.0 test was different from that in the λ=0.0 test. Creep deformation in the λ=0.0 test proceeded at late stage. On the other hand, creep deformation in the λ=1.0 test proceeded from early stage. In order to clarify the creep deformation mechanism, observation of fractured surface was performed using a scanning electron microscope (SEM). Although many dimples were observed in both tests, the number of dimples in the λ=1.0 test is twice than that in the λ=0.0 test. In the λ=1.0 test, all directions on the xy plane are principal stress directions, which leads to promotion of the dimple generations. It was confirmed that accelerated creep deformation was caused by the dimples generated from early stage.

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... There are several types of loading in multiaxial creep testing methods [1][2][3][4][5][6][7][8][9][10][11][12][13]. One is conventional uniaxial tensile loading using a notched bar specimen. ...
... Sometimes, a testing strategy applies biaxial tensile loads to a cruciform specimen. The authors have completed this study using the biaxial tensile loading test on the cruciform specimen, because it can carry out a wide range of multiaxial stress tests under different conditions [5,8,10,13]. Although the cruciform specimen has some advantages when used in multiaxial testing, its current size is too large at 190 mm × 190 mm. ...
... Because the biaxial tensile load test can simulate a wide range of multiaxial stress conditions, a miniature cruciform specimen was designed so that a multiaxial creep test could be carried out even with a small specimen size. Fig. 3(a) shows the last shape and dimensions of a miniature cruciform specimen [5,8,10,13,[22][23][24][25] for a multiaxial creep test designed for this study. The specimen has a total area of 50 mm × 50 mm, a 5 mm × 5 mm square shaped gauge section at the intersections of its arms, and its central gauge is 1 mm thick. ...
Article
High temperature components such as boiler tubes and jet engine turbine blades undergo multiaxial creep damage due to extreme temperature conditions and their complex shapes. A multiaxial creep investigation is required for the safety of high temperature components in the design phase of the manufacture of such a component. However, there are only a few commercial testing machines that can conduct multiaxial loading at high temperatures. A miniature cruciform specimen having a plane stress condition gauge section that is 5 mm square was designed using finite element (FE) analysis. Further, a biaxial testing machine capable of tensile loading was designed to conduct multiaxial creep tests. The testing machine had a 2 kN loading capacity and a 1 kW furnace. We also developed a non-contact displacement measuring method for the miniature specimen, which had an excessively small gauge section to permit the attachment of a mechanical extensometer. The proposed method uses a conventional optical camera to capture some images of the surface. We obtained the displacement value of the specimen, by tracking the trace of the target marks painted on the surface of the specimen. The measured strain value that was obtained from the non-contact displacement measuring method corresponds to the strain obtained by the mechanical method at room and high temperatures. By using the developed multiaxial creep testing machine and the non-contact observation system, we can investigate not only the deformation of the testing specimen, but also the surface conditions of materials during the creep test.
Article
High temperature components such as boiler tubes and jet engine turbine blades undergo multiaxial creep damage due to their complex shapes and combination of mechanical stress and inner pressure for a steam tube. Although multiaxial creep investigation is required for the safety of high temperature components designing, there are only a few commercial testing machines that can conduct multiaxial loading at high temperatures. The authors have developed a miniature cruciform specimen having a 5 mm square size plane stress condition gauge section and a biaxial tensile loading testing machine. This study discusses multiaxial creep rupture time and creep properties of type 304 stainless steels at 923 K using the miniature cruciform specimen. A specimen deformation during the creep tests could be observed clearly and the displacement of the specimen also could be measured using a developed non-contact measuring method. Creep rupture time under multiaxial stress condition was shorter than that of uniaxial stress loading.
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