• Journal of the Microelectronics and Packaging Society
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• v.26 no.4
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• pp.133-140
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• 2019

The effects of thermal cycle conditions on the bonding strength and crack propagation behaviors in electroplated Cr/electroplated Ni-P coatings were systematically evaluated. 1^st heat treatment was performed at 500℃ for 3 hours after electroplating Ni-P, and then, 2^nd heat treatment was performed at 750℃ for 6 hours after electroplating Cr. The measured bonding strength by ASTM C633 were around 25.6 MPa before thermal cycling, while it increased to 47.6 MPa, after 1,000 cycles. Increasing thermal cycles led to dominant fail mode with cohesive failure inside adhesive, which seemed to be closely related to the increasing bonding strength possibly not only due to higher Cr surface roughness, but also to penetrated channeling crack density. Also, increasing density of penetrated channeling cracks in electroplating Cr layer led to slightly stronger bonding strength due to mechanical interlocking effects of adhesive inside channeling cracks.

• Tribology and Lubricants
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• v.15 no.1
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• pp.39-45
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• 1999

The wear behavior of $Al_2$O$_3$-40%TiO$_2$deposited on casted aluminum alloy (ASTM A356) by APS (Air Plasma Spray) against SiC ball has been investigated in this work. Wear tests were carried out at room temperature. The friction coefficient of $Al_2$O$_3$-40%TiO$_2$coating is lower than that of pure $Al_2$O$_3$coating(APS). $Al_2$O$_3$-40%TiO$_2$coating indicated the existence of the optimal coating thickness. It is found that voids and pores of coating surface resulted in the generation of cracks, and the cohesive of splats and the porosity of surface play a role in wear characteristics. It is suggested that the mismatch of thermal expansion of substrate and coating play an important role in wear performance. Tension and compression under thermo-mechanical stress may be occurred by the mismatch between thermal expansion of substrate and coating. The crack propagation above interface is observed in SEM.

• Structural Engineering and Mechanics
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• v.73 no.5
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• pp.585-598
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• 2020

We develop design model within which nucleation and propagation of crack in a fibrous composite is described. It is assumed that under loading, crack initiation and fracture of material happens in the composite. The problem of equilibrium of a composite with embryonic crack is reduced to the solution of the system of nonlinear singular integral equations with the Cauchy type kernel. Normal and tangential forces in the crack nucleation zone are determined from the solution of this system of equations. The crack appearance conditions in the composite are formed with regard to criterion of ultimate stretching of the material's bonds. We study the case when near the fiber, the binder has several arbitrary arranged rectilinear prefracture zones and a crack with interfacial bonds. The proposed computational model allows one to obtain the size and location of the zones of damages (prefracture zones) depending on geometric and mechanical characteristics of the fibrous composite and applied external load. Based on the suggested design model that takes into account the existence of damages (the zones of weakened interparticle bonds of the material) and cracks with end zones in the composite, we worked out a method for calculating the parameters of the composite, at which crack nucleation and crack growth occurs.

• Computers and Concrete
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• v.34 no.2
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• pp.137-149
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• 2024

In order to study the triaxial mechanical behavior of steel fiber reinforced high performance concrete (SFRHPC), the standard triaxial compression tests with four different confining pressures are performed on the cylindrical specimens. Three different steel fiber volumes (0, 1% and 2%) are added in the specimens with diameter of 50 mm and height of 100 mm. Test results show that the triaxial compressive strength and peak strain increase with the increasing of fiber content at the same confining pressure. At the same steel fiber content, the triaxial compressive strength and peak strain increases with the confining pressure. The compressive strength growth rate declines as the confining pressure and steel fiber content increases. Longitudinal cracks are dominant in specimens with or without steel fiber under uniaxial compression loading. While with the confining pressure increases, diagonal crack due to shear is obvious. The Mohr-Coulomb criterion is illustrated can be used to describe the failure behavior, and the cohesive force increases as steel fiber content increases. Finally, the numerical model is built by using the PFC3D software. In the numerical model a index is introduced to reflect the effect of steel fiber content on the triaxial compressive behavior. The simulating stress-strain curve and failure mode of SFRHPC are agree well with the experimental results.

• Journal of the korean academy of Pediatric Dentistry
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• v.25 no.4
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• pp.761-771
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• 1998

In this study, attempt has been made to evaluate the effect of $CO_2$ laser irradiation on enamel surface structure and the bond strength of sealant material. Conventional acid etching was used as a control technic for comparison. The results obtained from this experiment were as follows; 1. The highest mean shear bond strength value was observed in samples of Group I (acid-etching) with the statistical significance(p<.05) between all the other groups. 2. The shear bond strength in Group IV was the lowest among laser etching groups. but there were no significant difference between them(p>.05). 3. Scanning electron microscopic observation showed that the rough and irregular surface was created by $CO_2$ laser treatment with the formation of numerous pores, micro-cracks, and small bubble-like inclusion. Increasing the energy density induced localized surface melt with a thin smooth glaze-like appearance. 4. In acid-etched control specimen cohesive failure predominated, whereas adhesive failure was the main mode in laser-treated group. Based upon the above-mentioned results, it can be assumed that the $CO_2$ laser is not an adequate substitute for the acid-etch technique in enamel preconditioning. More studies are required to explore the effective condition of laser irradiation which could attain the better bond strength of restorative materials.

• Journal of the Korean Ceramic Society
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• v.55 no.5
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• pp.492-497
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• 2018

This paper reports the efficacy of tape casting using an Ag-10 wt% CuO filler for the successful joining of a sintered $Ce_{0.9}Gd_{0.1}O_{2-{\delta}}-La_{0.7}Sr_{0.3}MnO_{3{\pm}{\delta}}$ (GDC-LSM) ceramic with a SUS 460 FC metal alloy by reactive air brazing. The as-prepared green tape was highly flexible without drying cracks, and the handling was easy when used as a filler material for reactive air brazing. Heat treatment for the GDC-LSM/SUS 460 FC joint was performed at $1050^{\circ}C$ for 30 min in air. Microstructural observations indicated a reliable and compact joining. The room temperature mechanical shear strength of the as-brazed joints was $60{\pm}8MPa$ with a cohesive failure. The flexural strength of joints was measured from room temperature up to $850^{\circ}C$, where the strength retention revealed to be almost 100% at $500^{\circ}C$. However, the joints showed a degradation in strengths at 800 and $850^{\circ}C$, exhibiting strength retentions of 57% and 37%, respectively.

• Restorative Dentistry and Endodontics
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• v.22 no.1
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• pp.144-155
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• 1997

This study evaluated shear bond strength between porcelain and resin cement according to various surface treatments of porcelain, and surface condition of debonded porcelain. 50 porcelain specimens(Celay block A2M7) and composite resin specimens(Clearfil Photo-Bright) were prepared, and divided into 5 experimental groups according to the treatment method of porcelain surface. 5 experimental groups by surface treatments were as follows; CONTROL Group : No surface treatment was done on the surface of porcelains. SAND Group : The surface of porcelains were sandblasted with $50{\mu}m$ aluminum oxide for 5 seconds. HF Group: The surface of porcelains were etched with 8% Hydrofluoric acid for 4 minutes. SIL Group: The surface of porcelains were coated with silane coupling agent and heated at $100^{\circ}C$ for 5 minutes. SAND+HF+SIL Group : The surface of porcelains were sandblasted with $50{\mu}m$ aluminum oxide for 5 seconds and etched with 8% Hydrofluoric acid for 4 minutes, and coated with silane coupling agent and heated at $100^{\circ}C$ for 5 minutes. After surface treatments on the prepared porcelain surface two pastes of Panavia 21$^{(R)}$ were mixed, they were applied between composite resin block and porcelain surface, and then excessive resin cements were removed, and its margin was surrounded with Oxyguard II. All specimens were stored for 24 hours in water at $37^{\circ}C$ and tested with Instron testing machine between porcelains and resin cements, and debonded porcelain surfaces were observed under Scanning Electon Microscope(Hitachi S-2300) at 20kvp. The values from each group were compared statistically by Student's t-test. The obtained results were as follows; 1. The shear bond strength without surface treatment of porcelain was the lowest among all experimental groups(p<0.05). 2. The detached porcelain surface with sandblasting alone had more remarkable cracks than with only Hydrofluoric Acid or Silane coupling 2gent, but showed the lowest value of shear bond strength among surface treated groups(p<0.05), 3. When porcelain surface was treated by hydrofluoric acid, it affected shear bond strength more than silane coupling agent, but there were no significant statistical differences(p>0.05). 4. When three methods were combined to increase shear bond strength between porcelains and resin cements, its value was the highest than the others(p<0.05). 5. In Scannig Electron Micrograph of detached porcelain surface with no treatment, the sample revealed adhesive failure between the porcelain and resin cement whereas detached porcelain surface with combination of three method cohesive failure on the porcelain.