Designing tough isotropic structural composite using computation, 3D printing and testing

Kim, Yongtae,Kim, Youngsoo,Libonati, Flavia,Ryu, Seunghwa Elsevier 2019 Composites. Part B, Engineering Vol.167 No.-

<P><B>Abstract</B></P> <P>Staggered platelet composites found in nature, such as nacre, bone, and conch-shell, exhibit a remarkable combination of high toughness, strength, and/or stiffness, and have inspired the development of bio-inspired composites mimicking their characteristic features. However, those excellent mechanical properties are primarily observed under specific loading conditions due to their mechanical anisotropy, which originates from the aligned microstructures consisting of high aspect ratio inclusions. In this study, we combine numerical simulations and 3D-printing to define a design strategy of isotropic two-dimensional structural composites consisting of stiff and soft constituents that are arranged in square, triangular, and quasicrystal lattices. For relatively isotropic structures, the soft tile/stiff boundary configuration significantly outperforms the stiff tile/soft boundary configuration in terms of normalized toughness, strength, and stiffness with respect to the simple rule of mixture estimates for each, because the former provides more extrinsic toughening mechanisms and effectively lowers the stress concentration near the crack tip. The quasicrystal lattice offers the best isotropy in elastic response, while its absolute values of stiffness, strength, and toughness turn out to be similar or lower than those of triangular lattice composites due to more irregular stress distribution. In contrast, for the highly anisotropic staggered platelet structure, the stiff tile/soft boundary configuration significantly outperforms the inverted one, owing to its unique load-transfer mechanism, which relies primarily on the shear-lag effect.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

On the structures of class seminroups of quadratic non-maximal orders

( YongTae Kim ) 호남수학회 2004 호남수학학술지 Vol.26 No.3

Samuel Johnson as a Moralist in "The Vanity of Human Wishes," The Rambler, and Rasselas.

Kim, Yongtae 한국현대영미어문학회 2001 The Journal of KACELLS Vol.6 No.2

Dealing with a series of his writings written from his late thirties to late forties, such as "The Vanity of Human Wishes," The Rambler, and Rasselas, this article sets out its argument with a premise that through these writings Samuel Johnson carries out experiments on moral problems. A crucial source of moral problems will be drawn from the phrase of "hunger of imagination" mentioned by Imlac in his visitation to the Pyramids in Rasselas and considered to be the root from which any other moral evils branched forth. Johnson's understanding of imagination has two sides: positive and negative. In its positive sense he praises man's capacity to create something special through his industrious working of imagination. Yet, it is the very negative side of imagination that I am going to examine in this article. He offers his negative view on imagination emphasizing the preying capacity of human imagination. The nature of preying imagination is that whatever we have we are always to imagine more; to imagine more leads to wanting more; and finally wanting more brings about fatal loss. Unchecked imagination is bound to give rise to other negative faculties of human mind: pride and wishing for fame delude us into thinking that we are standing over other people in terms of wealth, learning or political power, and, consequently, pride arouses envy and hostility among the opponents. Seeing that those moral problems are mainly caused by man's detachment from outside realities, therefore, Johnson suggests as an effective therapy the genuine contact with the variety of realities.

Development of a generalized scaling law for underwater explosions using a numerical and experimental parametric study

Yongtae Kim,Seunggyu Lee,Jongchul Kim,Seunghwa Ryu 국제구조공학회 2021 Structural Engineering and Mechanics, An Int'l Jou Vol.77 No.3

In order to reduce enormous cost of real-scale underwater explosion experiments on ships, the mechanical response of the ships have been analyzed by combining scaled-down experiments and Hopkinson’s scaling law. However, the Hopkinson’s scaling law is applicable only if all variables vary in an identical ratio; for example, thickness of ship, size of explosive, and distance between the explosive and the ship should vary with same ratio. Unfortunately, it is infeasible to meet such uniform scaling requirement because of environmental conditions and limitations in manufacturing scaled model systems. For the facile application of the scaling analysis, we propose a generalized scaling law that is applicable for non-uniform scaling cases in which different parts of the experiments are scaled in different ratios compared to the real-scale experiments. In order to establish such a generalized scaling law, we conducted a parametric study based on numerical simulations, and validated it with experiments and simulations. This study confirms that the initial peak value of response variables in a real-scale experiment can be predicted even when we perform a scaled experiment composed of different scaling ratios for each experimental variable.

Detection of BER Enzymes Based on Fluorescence Resonance Energy Transfer

Yongtae Kim,홍인석 대한화학회 2009 Bulletin of the Korean Chemical Society Vol.30 No.9

THE MULTILEVEL SECURITY PROBLEM OVER CLASS SEMIGROUPS OF IMAGINARY QUADRATIC NON-MAXIMAL ORDERS

KIM, YONGTAE 호남수학회 2006 호남수학학술지 Vol.28 No.2

A scheme based on the cryptography for enforcing multilevel security in a system where hierarchy is represented by a partially ordered set was first introduced by Akl et al. But the key generation algorithm of Akl et al. is infeasible when there is a large number of users. In 1985, MacKinnon et al. proposed a paper containing a condition which prevents cooperative attacks and optimizes the assignment in order to overcome this shortage. In 2005, Kim et al. proposed key management systems for multilevel security using one-way hash function, RSA algorithm, Poset dimension and Clifford semigroup in the context of modern cryptography. In particular, the key management system using Clifford semigroup of imaginary quadratic non-maximal orders is based on the fact that the computation of a key ideal $K_0$ from an ideal $EK_0$ seems to be difficult unless E is equivalent to O. We, in this paper, show that computing preimages under the bonding homomorphism is not difficult, and that the multilevel cryptosystem based on the Clifford semigroup is insecure and improper to the key management system.

An extended analytic model for the elastic properties of platelet-staggered composites and its application to 3D printed structures

Kim, Youngsoo,Kim, Yongtae,Lee, Tae-Ik,Kim, Taek-Soo,Ryu, Seunghwa Elsevier 2018 COMPOSITE STRUCTURES -BARKING THEN OXFORD- Vol.189 No.-

<P><B>Abstract</B></P> <P>The load transfer mechanism in the unique staggered platelet structure of nacre has been intensively analyzed, and its structural features have been mimicked by various manufacturing methods such as 3D printing. However, due to the limited applicability of most analytical models and the limitations of 3D printers, it remains difficult to quantitatively predict and design the properties of nacre-inspired synthetic structures. In this study, we improve the analytic models to predict the effective elastic properties of various staggered platelet structures for a wide range of constituent materials’ elastic moduli and geometric parameters by carefully accounting for the volume average of elastic properties. We also systematically investigate the effect of the printing orientation and width of the composite structure fabricated by a 3D printer. We show that our improved analytic model accurately predicts the properties of a 3D printed composite when the manufacturing-condition-dependent material properties are used as input. Our study reveals the origin of discrepancy between theory and 3D printed structures and enables a rational design of nacre-inspired structures.</P>

An Accuracy Enhanced Error Tolerant Adder with Carry Prediction for Approximate Computing

Yongtae Kim 대한전자공학회 2019 IEIE Transactions on Smart Processing & Computing Vol.8 No.4

This paper presents a new approximate adder design to improve the computation accuracy of the conventional error tolerant adder by leveraging a carry prediction technique with a sum generator. The proposed carry speculation scheme exploits inputs from a single bit position and effectively increase the bit width of the accurate addition. Implemented in a 65-nm CMOS technology, the proposed approximate adder is up to two times faster than, and twice as power efficient as, the traditional adders. Compared to the other approximate adders considered in this paper, the proposed adder achieves up to 3.7%, 15.5%, 79.9% and 79.9% reductions in the error rate (ER), mean relative error distance (MRED), mean error distance (MED) and normalized MED (NMED) respectively, at an extra cost of merely 4% to 6% in area, delay, and power. In addition, the proposed adder offers a good tradeoff between power/energy and accuracy and improves on power/energy-NMED products by up to 46%, outperforming other approximate adders.

A Novel Approximate Adder with Enhanced Low-cost Carry Prediction for Error Tolerant Computing

Yongtae Kim 대한전자공학회 2019 IEIE Transactions on Smart Processing & Computing Vol.8 No.6

This paper proposes an approximate adder that employs a novel carry speculation scheme to enhance the computation precision of the existing error tolerant adder (ETA) designs with extremely little hardware overhead. The proposed carry prediction technique leverages two input bits to increase the prediction accuracy while the conventional ones do only one bit. This leads to a reduction of the carry prediction error rate from 25% to 18.75%. Compared to the existing ETA design, the proposed adder reduces normalized mean error distance (NMED) and mean relative error distance (MRED) by up to 10% and 28%, respectively, at the cost of only a two-input OR gate. Moreover, the proposed design outperforms the conventional ETAs when jointly evaluating hardware cost and computation accuracy. Specifically, the new design allows 11% and 17% reductions of area-power-NMED and power-NMED products, respectively, compared to the traditional ETA.

Preconception care for men

( Yongtae Kim ) 한국모자보건학회 2009 한국모자보건학회 학술대회 연제집 Vol.2009 No.2