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Low-Cycle Fatigue Behavior of Reduced Activation Ferritic-Martensitic Steel at Elevated Temperatures
Godwin Kwame Ahiale,Won‑Doo Choi,Seungyon Cho,Yi‑Hyun Park,Young‑Bum Chun,Yong‑Jun Oh 대한금속·재료학회 2023 METALS AND MATERIALS International Vol.29 No.1
Changes in the low-cycle fatigue (LCF) behavior of reduced activation ferritic-martensitic (RAFM) steel containing trace amount of Zr—namely, Advanced Reduced Activation Alloy (ARAA) were presented in the temperature range of 573 to 873 K. The fatigue lifetime decreased significantly as the temperature increased from 573 to 823 K, but it was rather similar between 823 and 873 K, where the tensile properties deteriorated most rapidly. The decrease in lifetime at elevated temperatures is mainly due to the disappearance of the initial tempered martensitic structure during cycling resulting in rapid cyclic softening and local oxidation at surface slip bands which led to earlier crack initiation. The fatigue lifetime and cyclic softening behavior of ARAA were compared with those of the other conventional RAFM steels.
BEST RANDOM PROXIMITY PAIR THEOREMS FOR RELATIVELY U-CONTINUOUS RANDOM OPERATORS WITH APPLICATIONS
Godwin Amechi Okeke 영남수학회 2017 East Asian mathematical journal Vol.33 No.3
It is our purpose in this paper to introduce the concept of best random proximity pair for subsets A and B of a separable Banach space E. We prove some best random approximation and best random proximity pair theorems of certain classes of random operators, which is the stochastic verse of the deterministic results of Eldred et al. [22], Eldred et al. [18] and Eldred and Veeramani [19]. Furthermore, our results generalize and extend recent results of Okeke and Abbas [42] and Okeke and Kim [43]. Moreover, we shall apply our results to study nonlinear stochastic integral equations of the Hammerstein type.
Godwin Kwame Ahiale,Doo-Hyun Kim,Won-Jon Yang,Jong-Hoon Lee,Yong-Jun Oh 대한금속·재료학회 2018 METALS AND MATERIALS International Vol.24 No.4
We investigated the change of precipitation behavior and impact resistance as a function of depth from the surface of thickblock of SAF 2507 super duplex stainless steel with the thickness (T) of 200 mm after water quenching from 1050 °C. Theamount of detrimental sigma phase increased smoothly until the depth of 0.25T, followed by a rapid increase from 0.25T tothe center. However, the impact strength decreased signifi cantly with only 1.3% of area fraction of sigma phase as the depthincreased past 0.1T. Based on fractography analysis for the samples at such small depth ranges, the distance between thesigma phase particles aff ected the relative amount of initiating brittle cracks in front of the notch and was one of the crucialfactors that dramatically reduced impact resistance with depth.
Correlation between the Microstructure and Mechanical Properties of W-Bearing Ti-6Al-4V Alloys
Godwin Kwame Ahiale,계인석,권영삼,오용준 대한금속·재료학회 2021 대한금속·재료학회지 Vol.59 No.6
W-containing Ti-6Al-4V alloys (W=0, 1, and 5 wt%) were fabricated by the powder injection molding process, and the corresponding effects of tungsten content on the mechanical properties and microstructure of the alloys were investigated. The alloy powders were sintered at 1200 °C and then hot-isostatically-pressed at 900 °C. The fabricated alloys were subjected to microstructural and chemical analyses, and tensile and nano-indentation tests. The yield strength and tensile strength proportionally increased as the W content was increased from 0 wt% to 5 wt%. Ductility was not affected by the addition of up to 5 wt% W due to its complete dissolution in the matrix. Higher W addition induced finer α/β lamellar microstructures and increased the β to α phase ratio. Moreover, the added W dissolved preferentially in the β phase by solid solution hardening, increasing the hardness of the β phase, which originally was significantly softer than the α phase. For the alloys containing up to 5 wt% W, the strengthening without ductility loss was attributed to the finer α/β lamellae and the volume increase in the β phase hardened by W. These results suggest that adding W to Ti-6Al-4V alloy is a promising method for developing Ti alloys with both high strength and toughness.
Image Re-branding in a Fragile State: The Case of Nigeria
Godwin Uyi Ojo,Iro Aghedo 서울대학교행정대학원 2013 Journal of Policy Studies Vol.28 No.2
In today’s globalized world, nations are increasingly concerned about how they are perceived by their publics, often carefully planning and executing policies to maintain a positive identity or shed a negative image. While some countries are able to execute nation branding, others are not. Using the Nigerian experience as a case study, this article shows how the character of a state can undermine its capacity for successful nation branding. We argue that countries are judged not just by what they say but also by what they do, and that inherent contradictions such as sectarian violence, ethnic tension, and corruption underline Nigeria’s underdevelopment and its reputation at home and abroad. We assess this trajectory in the context of state fragility and “prebendalization” politics in a country with a powerful colonial legacy, and suggest a duality of order and disorder involving formal and informal structures of governance that impinge significantly on re-branding efforts.
FIXED POINT THEOREMS FOR CONVEX MINIMIZATION PROBLEMS IN COMPLEX VALUED CAT(0) SPACES
Godwin Amechi Okeke,Mujahid Abbas,Manuel de la Sen 경남대학교 기초과학연구소 2020 Nonlinear Functional Analysis and Applications Vol.25 No.4
In this paper, we introduce the concept of a complex valued CAT(0) space andpropose a new proximal point algorithm for certain nonlinear operators satisfying rationalexpressions in the framework of complex valued CAT(0) spaces. We prove existence of fixed point of these nonlinear mappings in such spaces. And we prove strong and -convergence of the iterative sequence generated through our proposed algorithm to the minimizer of aconvex function and the xed point of these mappings. We show that the new iterative algorithm converges faster than the modified Mann-type and Ishikawa-type proximal point algorithms.