Kee Bong Yoon

Criteria for Small Scale Creep Testing Condition and Estimation of Ct

As new creep resistant materials are developed for the temperature range of 800–1100°C, the improved creep properties sometimes require the modification of the conventional approaches in high temperature life assessment procedures which have been applied for low alloy steels and chromium steels usually used in the temperature range of 500–650°C. In this study, finite element analyses were carried out by using the creep properties of a gas turbine material to establish the small scale creep testing requirements and to propose a corrected Ct estimation scheme. The maximum allowable creep zone size was estimated through the analogy between the small scale yielding conditions and the small scale creep conditions. The maximum time during which the small scale creep condition holds was also defined. The creep zone boundary with the definition of ec=ee was more suitable for Ct estimation than that with the definition of ec=0.002, since it was easy to find a proper correction function by applying the former definition. The newly estimated value of β r c =1/7=1/12 was derived for determining Ct instead of the conventional value of β r c =1/7.5. Excellent correlations were shown between the estimated Ct values obtained with the new β r c and the finite element calculated Ct values in all creep condition ranges. This result strongly supports the validity of the suggested small scale creep criterion and the new correction value β r c =1/12 proposed in this study.

Effects of Defect Size on Crush Test Load of Butt Fusion Welded MDPE Pipes

It is expected that the size of welding defect affects the mechanical performance of welded medium density polyethylene (MDPE) pipe joints. In this study, butt fusion welded MDPE pipe joints with a single spherical or planar defect of various sizes were studied using experimental crush testing and also by finite element method. The crush test showed that the mechanical performance of crush was not affected by the size and geometry of a single welding defect when the defect size was increased to 45% of the pipe’s wall thickness. The simulation results indicated that the effect of the single welding defect on the Von Mises stress distribution near the defect explained the reason of the test results.

Crack Tip Creep Deformation Behavior in Transversely Isotropic Materials

Theoretical mechanics analysis and finite element simulation were performed to investigate creep deformation behavior at the crack tip of transversely isotropic materials under small scale creep (SCC) conditions. Mechanical behavior of material was assumed as an elastic-2 nd creep, which elastic modulus ( E ), Poisson’s ratio (ν ) and creep stress exponent ( n ) were isotropic and creep coefficient was only transversely isotropic. Based on the mechanics analysis for material behavior, a constitutive equation for transversely isotropic creep behavior was formulated and an equivalent creep coefficient was proposed under plain strain conditions. Creep deformation behavior at the crack tip was investigated through the finite element analysis. The results of the finite element analysis showed that creep deformation in transversely isotropic materials is dominant at the rear of the crack-tip. This result was more obvious when a load was applied to principal axis of anisotropy. Based on the results of the mechanics analysis and the finite element simulation, a corrected estimation scheme of the creep zone size was proposed in order to evaluate the creep deformation behavior at the crack tip of transversely isotropic creeping materials.

Assessment of Strength Characteristics of Al 2024 ECAP Metal Using Small Punch Testing

When subjected to severe shear deformation by ECAP, microstructure of Al2024 becomes extremely refined. To measure the strength of that, small punch(SP) testing method was adopted as a substitute for the conventional uniaxial tensile testing because the size of material processed by ECAP were limited to in transverse direction. SP tests were performed with specimens in longitudinal and transverse directions of Al2024 ECAP metal. For comparing the strength values with those assessed by SP tests, uniaxial tensile tests were also conducted with specimens in longitudinal direction. Failure surfaces of the tested SP specimens showed that failure mode was shear deformation and Al 2024 ECAP metal has an anisotropy in strength. Thus, conventional equations proposed for assessing the strength characteristics were improper to assess those of Al2024 ECAP metal. In this paper a way of assessing the strength of Al 2024 ECAP metal was proposed and was proven to be effective.

Strength and Fracture of Ultra-Fine Grained Aluminum 2024 ECAP Metal

When subjected to severe shear deformation by ECAP (Equal Channel Angular Pressing), microstructure of Al2024 becomes extremely refined. To measure the strength of this ultra fine grained metal, the small punch (SP) testing method was employed as a substitute for the conventional uniaxial tensile testing since the size of metal bar processed by ECAP were limited to 12 mm in transverse direction as shown in Fig. 1. The small punch tests were performed with specimens in longitudinal and transverse directions of Al 2024 ECAP metal. For comparing the strength values with those assessed by SP tests, the uniaxial tensile tests were also conducted with specimens in longitudinal direction, in which specimen sampling is possible. Failure characteristics were investigated using scanning electron microscopy (Fig. 2). The surfaces of the tested SP specimens showed that failure mode was shear deformation (Fig. 3). Based on this observation it was argued that the conventional equations proposed for assessing the strength of the material by the SP test were improper to assess those of Al2024 ECAP metal.