• International Journal of Automotive Technology
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• v.7 no.5
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• pp.579-584
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• 2006

Aluminum Resistance Spot Weld(Al RSW) is an enabling technology for body assembly of low mass fraction vehicles. Due to the unreliable durability of spot-welded joints, applications of Al RSW are limited. This study presents experimental investigation on the use of a post-weld cold working process to improve the fatigue strength of Al RSW. The post-weld cold working process includes special shaped indenters that are pressed or driven into the structure to induce compressive residual stresses. The mechanical properties of the post-weld cold worked Al RSW were investigated, including the experimental results of fatigue and micro-hardness tests. Comparisons of the mechanical properties and qualitative results between the as-welded RSW specimens and the post-weld cold worked RSW specimens are discussed. The post-weld cold worked Al RSW samples had an increase in both microhardness and fatigue life.

• Journal of Ocean Engineering and Technology
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• v.25 no.6
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• pp.66-71
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• 2011

Recently, the production of shipbuilding and offshore plant industries, with a trend toward large structures, has led to an increased use of high strength ultra-thick plates. The use of ultra-thick plates increases the welding tasks, and the welding process generates distortion and residual stress in the weldment because of the rapid heating and cooling. Welding distortion and residual stress in the welded structure resulte in many troubles such as deformation and life deterioration. In particular, the welding residual stress has an important effect on welding deformation, fatigue, buckling strength, brittleness, etc. The purpose of this study was to evaluate the residual stress at a multi-pass weldment using an experimental method for EH36 high-tension steel. In this experimental method, AIS3000 was used to measure the residual stress of a welded part, HAZ, and base metal; EPMA and XRD were used to study the material properties.

• JSTS:Journal of Semiconductor Technology and Science
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• v.5 no.1
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• pp.38-44
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• 2005

Pyrolyzed polyimide is explored in terms of MEMS material. This paper describes chemical, electrical, mechanical properties of pyrolyzed polyimide (PIX-1400) thin film as MEMS material. When polyimide thin film was pyrolyzed at $800^{\circ}C$ for 60 minutes in $N_{2}$ ambient, the residual ratio of pyrolyzed film thickness measured with a surface profiler is about 49 %, and the resistivity is about $2.17{\times}10^{-2}\;{Omega}cm$. From the result of the load-deflection test, the estimated Young's modulus and initial average stress of pyrolyzed polyimide are 67 GPa and 30 MPa, respectively. As one demonstration of MEMS structures of pyrolyzed polyimide, the fabrication method of the microbridge structure is proposed for a micro heater and a resonator.

• Proceedings of the KIEE Conference
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• 2000.07c
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• pp.1877-1879
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• 2000

This paper deals with the change of the micro-structure of ZnO associated with lightning surge current and ageing test. In this work, a surge current generator which can produce 8/20 [${\mu}s$], 6 [kA] impulse current is designed and fabricated to simulate the lightning impulse current. The residual voltage and leakage current flowing to ZnO blocks are observed. Also a compensation circuit was used in resistive current measurement. The micro-structures of ZnO arrester block were significantly changed by lightning surge current and accelerated temperature ageing test.

• Structural Engineering and Mechanics
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• v.53 no.1
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• pp.105-130
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• 2015

In this paper, the classical continuum mechanics is adopted and modified to be consistent with the unique behavior of micro/nano solids. At first, some kinematical principles are discussed to illustrate the effect of the discrete nature of the microstructure of micro/nano solids. The fundamental equations and relations of the modified couple stress theory are derived to illustrate the microstructural effects on nanostructures. Moreover, the effect of the material surface energy is incorporated into the modified continuum theory. Due to the reduced coordination of the surface atoms a residual stress field, namely surface pretension, is generated in the bulk structure of the continuum. The essential kinematical and kinetically relations of nano-continuums are derived and discussed. These essential relations are used to derive a size-dependent model for Mindlin functionally graded (FG) nano-plates. An analytical solution is derived to show the feasibility of the proposed size-dependent model. A parametric study is provided to express the effect of surface parameters and the effect of the microstructure couple stress on the bending behavior of a simply supported FG nano plate.

• Journal of Welding and Joining
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• v.2 no.2
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• pp.54-61
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• 1984

Construction of welding structure is greatly dependent upon welding heat cycle. Fracture toughness is decreased remarkablely due to coarse grained HAZ and inequal residual stress of three dimensions to originate in welding. Post weld heat treatment(PWHT) is carried out to increase the fracture toughness of HAZ and to remove the residual stress. There occur some problem such as toughness decrement and stress relief cracking(SRC) in the coarse grained HAZ subject to the effect of tempering treatment. Therefore, in this paper, the effect of heat inputs affecting cooling rate and PWHT under the no stress on fracture toughness were evaluated by crack opening displacement (COD), SEM and micro-hardness test. Experimental results are as follows; 1. Fracture toughness of weld HAZ is dependent upon weld heat cycle and it is decreased with increment of heat input, but the degree of improvement of fracture toughness after PWHT was linearly increased with heat input. 2. Hardness of the parent metal is not changed, but the softening of coarse grained HAZ is remarkable due to PWHT. 3. Fracture surface of as-weld show the perfect brittle fracture with the cleavage fracture, but after PWHT they appear the ductile fracture surface with dimple.

• Transactions of the Korean Society of Mechanical Engineers
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• v.18 no.2
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• pp.519-528
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• 1994

Degradation problem due to long term service in machine or structure is now one of important problems in whole industrial field. In this study, pressure vessel steel, Cr-Mo steel, which was used more than 60,000 hours, was surface-modified by laser beam radiation for the improvement of fatigue strength. To find out optimum radiation condition, hardness, residual stress measurement and fatigue tests were carried out with the specimen of different radiation conditions. Experimental results show that micro-hardness values on the surface of the radiated specimens were approximately 2.2 times higher than those of un-radiated ones. In the depth direction of the specimen, hardness on the surface showed maximum value and was decreased at the inside the specimen. Different hardness values are due to the energy density Q which was absorbed by the specimen. Fatigue tests show that fatigue life was improved by the compressive residual stress after laser beam radiation. However, some specimens with differednt conditions show the shorter fatigue life. It means that laser beam radiation with optimum parameter can improve thae fatigue strength.

• Journal of IKEEE
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• v.1 no.1 s.1
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• pp.1-10
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• 1997

It is well-known that rectangular bulk-Si sensors prepared by etch or epi etch-stop micromachining technology are already in practical use today, but the conventional bulk-Si sensor shows some drawbacks such as large chip size and limited applications as silicon sensor device is to be miniaturized. We consider a circular-shape SOI(Silicon-On-Insulator) micro-cavity technology to facilitate multiple sensors on very small chip, to make device easier to package than conventional sensor like pressure sensor and to provide very high over-pressure capability. This paper demonstrates the cross-functional results for stress analyses(targeting $5{\mu}m$ deflection and 100MPa stress as maximum at various applicable pressure ranges), for finding permissible diaphragm dimension by output sensitivity, and piezoresistive sensor theory from two-type SOI structures where the double SOI structure shows the most feasible deflection and small stress at various ambient pressures. Those results can be compared with the ones of circular-shape bulk-Si based sensor$^{[17]}. The SOI micro-cavity formed the sensors is promising to integrate with calibration, gain stage and controller unit plus high current/high voltage CMOS drivers onto monolithic chip.

• Proceedings of the Korean Vacuum Society Conference
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• 2012.08a
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• pp.380-381
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• 2012

Bioinspired sea urchin-like structures were fabricated on silicon by inductively coupled plasma (ICP) etching using lens-like shape hexagonally patterned photoresist (PR) patterns and subsequent metal-assisted chemical etching (MaCE) [1]. The lens-like shape PR patterns with a diameter of 2 ${\mu}m$ were formed by conventional lithography method followed by thermal reflow process of PR patterns on a hotplate at $170^{\circ}C$ for 40 s. ICP etching process was carried out in an SF6 plasma ambient using an optimum etching conditions such as radio-frequency power of 50 W, ICP power of 25 W, SF6 flow rate of 30 sccm, process pressure of 10 mTorr, and etching time of 150 s in order to produce micron structure with tapered etch profile. 15 nm thick Ag film was evaporated on the samples using e-beam evaporator with a deposition rate of 0.05 nm/s. To form Ag nanoparticles (NPs), the samples were thermally treated (thermally dewetted) in a rapid thermal annealing system at $500^{\circ}C$ for 1 min in a nitrogen environment. The Ag thickness and thermal dewetting conditions were carefully chosen to obtain isolated Ag NPs. To fabricate needle-like nanostructures on both the micron structure (i.e., sea urchin-like structures) and flat surface of silicon, MaCE process, which is based on the strong catalytic activity of metal, was performed in a chemical etchant (HNO3: HF: H2O = 4: 1: 20) using Ag NPs at room temperature for 1 min. Finally, the residual Ag NPs were removed by immersion in a HNO3 solution. The fabricated structures after each process steps are shown in figure 1. It is well-known that the hierarchical micro- and nanostructures have efficient light harvesting properties [2-3]. Therefore, this fabrication technique for production of sea urchin-like structures is applicable to improve the performance of light harvesting devices.

• Journal of Welding and Joining
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• v.29 no.3
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• pp.76-81
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• 2011

Recently, the structural failures due to fatigue occur frequently with the increase of size of ships and offshore structures. In this respect, the assessment of fatigue life and the residual strength are very important. Currently, the smart materials technology has demonstrated a variety of possibilities for a diagnosis of structural strength and structural health condition for large structures. The benefits and feature of the MFC sensor are more flexible, durable and reliable than conventional smart material. In this study, Micro Fiber Composite (MFC) sensor for the measurement of stress intensity factor (SIF) of two dimensional cracks induced in a structure is developed. Two MFC sensors are placed in the vicinity of the crack tip close to each other with the crack tip in between them. The SIFs of Mode I($K_I$) as well as of Mode II($K_{II}$) based on the piezoelectric constitutive law and fracture mechanics are calculated. In this study, the SIF values measured by MFC sensors are compared with the theoretical results and measured value.