• The Journal of Korean Academy of Prosthodontics
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• v.44 no.6
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• pp.683-689
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• 2006

Statement of problem. Interfacial toughness is important in the mechanical property of layered dental ceramics such as core-veneered all-ceramic dental materials. The interfaces between adjacent layers must be strongly bonded to prevent delamination, however the weak interface makes delamination by the growth of lateral cracks along the interface. Purpose. The purpose of this study was to determine the effect of the reaction layer on the interfacial fracture toughness of the core/veneer structure according to the five different divesting. Materials and methods. Thirty five heat-pressed Lithia-based ceramic core bars (IPS Empress 2), $20mm{\times}3mm{\times}2mm$ were made following the five different surface divesting conditions. G1 was no dissolution or sandblasting of the interaction layer. G2 and G3 were dissolved layer with 0.2% HF in an ultrasonic unit for 15min and 30 min. G4 and G5 were dissolved layer for 15min and 30min and then same sandblasting for 60s each. We veneered bilayered ceramic bars, $20mm{\times}2.8mm{\times}3.8mm$(2mm core and 1.8mm veneer), according to the manufacturer's instruction. After polishing the specimens through $1{\mu}m$ alumina, we induced five cracks for each of five groups within the veneer close to interface under an applied indenter load of 19.6N with a Vickers microhardness indenter. Results. The results from Vickers hardness were the percentage of delamination G1:55%, G2:50%, G3:35%, G4:0% and G5:0%. SEM examination showed that the mean thickness of the reaction layer were G1 $93.5{\pm}20.6{\mu}m$, G2 $69.9{\pm}14.3{\mu}m$, G3 $59.2{\pm}20.2{\mu}m$, G4 $0.61{\pm}1.44{\mu}m$ G5 $0{\pm}0{\mu}m$. The mean interfacial delamination crack lengths were G1 $131{\pm}54.5{\mu}m$, G2 $85.2{\pm}51.3{\mu}m$, and G3 $94.9{\pm}81.8{\mu}m$. One-way ANOVA showed that there was no statistically significant difference in interfacial crack length among G1, G2 and G3(p> 0.05). Conclusion. The investment reaction layer played important role at the interfacial toughness of body ceramic bonded to Lithia-based ceramic.

• 전자공학회논문지 IE
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• v.49 no.2
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• pp.1-8
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• 2012

The effects of the addition of either monoclinic $ZrO_2(pure)$ or tetragonal $ZrO_2$ containing 5.35wt% $Y_2O_3(Y-TZP)$ on the mechanical properties and thermal shock resistance of $Al_2O_3$ ceramics were investigated. The addition of $ZrO_2$(m) and Y-TZP increased sintered density of $Al_2O_3$. The Vickers hardness also increased as the volume fraction of Y-TZP increased going through a maximum at 20wt%. The hardness of the specimens was found to be dependent on the sintered density. The higher volume fraction of either $ZrO_2(m)$ or Y-TZP resulted in the higher fracture toughness of the composite was. This result may be taken as evidence that toughening of $Al_2O_3$ can be achieved by not only the transformation toughening but microcrack toughening of $ZrO_2$. The thermal shock property for $Al_2O_3-ZrO_2$ composites was improved by increasing the volume fraction of monoclinic $ZrO_2(pure)$. The grain size increased as the volume fraction of $ZrO_2$ did.

• Geophysics and Geophysical Exploration
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• v.4 no.4
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• pp.124-132
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• 2001

Electrical resistivity survey has been conducted for delineating geological fault structure in Kaekok-ri near Kyungju. In general, electrical resistivity survey has an advantage of searching buried faults and its traces compared with other geophysical survey methods. Distribution of electrical conductivity in the ground is influenced by the ratio of pores, groundwater and clay minerals. These properties are evidenced indirectly to explain for weathering condition, faults and fracture Bones. Thus the electrical resistivity survey can be an effective method to find buried faults. We have carried out two dimensional (2-D) interpretation by means of smoothness-constrained least-squares and finite element method. Field data used in this paper was acquired at Kaekok-ri, Wuedong-eup, Kyungju-si, where is Ulsan Fault and is close to the region in which debatable quaternary fault traces were found recently. The dipole-dipole array resistivity survey which could show the 2-D subsurface electrical resistivity structure, was carried out in the area with three lines. The results showed good property of fault, fracture zone and fault traces which we estimated were congruous with the results. Through this study, 2-D electrical resistivity survey interpretation for fault is useful to apply.

• Journal of the Korean Society for Precision Engineering
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• v.3 no.1
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• pp.40-49
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• 1986

Crack, craze and void are common defects which may be found in the bulk of polymeric materials such as either themoplastics or thermosets. The healing phenomena, autohesion, of these defects are known to be a intrinsic material property of various polymeric materials. However, only a few experimental and theoretical investigations on crack, void and craze healing phenomena for various polymeric materials have been reported up to date [1, 2, 3]. This may be partly due to the complications of healing processes and lacking of appropriate theoretical developments. Recently, some investigators have been urged to study the healing phenomena of various polymenic materials since the significance of the use of polymer based alloys or composites has been raised in terms of specific strength and energy saving. In the earlier published reports [1, 2, 3, 4], the crack and void healing velocity, healing toughness and some other healing mechanical and physical properties were measured experimentally and compared with predicted values by utilizing a simple model such as the reptation model under some resonable assumptions. It seems, however, that the general acceptance of the proposed modeling analyses is yet open question. The crack healing processes seem to be complicate and highly dependent on the state of virgin material in terms of mechanical and physical properties. Furthermore, it is also strongly dependent on the histories of crack, craze and void development including fracture suface morphology, the shape of void and the degree of disentanglement of fibril in the craze. The rate of crack healing may be a function of environmental factors such as healing temperature, time and pressure which gives different contact configurations between two separated surfaces. It seems to be reasonable to assume that the crack healing processes may be divided in several distinguished steps like stress relaxation with molecular chain arrangement, surface contact (wetting), inter- diffusion process and com;oete healing (to obtain the original strength). In this context, it is likely that we no longer have to accept the limitation of cumulative damage theories and fatigue life if it is probable to remove the defects such as crack, craze and void and to restore the original strength of polymers or polymer based compowites by suitable choice of healing histories and methods. In this paper, we wish to present a very simple and intuitive theoretical model for the prediction of healed fracture toughness of cracked or defective polymeric components. The central idea of this investigation, thus, may be the modeling of behavior of chain molecules under healing conditions including the effects of chain scission on the healing processes. The validity of this proposed model will be studied by making comparisons between theoretically predicted values and experimentally determined results in near future and will be reported elsewhere.

• International Journal of Highway Engineering
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• v.20 no.1
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• pp.87-95
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• 2018

PURPOSES : A geo-grid pavement, e.g., a stress-absorbing membrane interlayer (SAMI), can be applied to an asphalt-overlay method on the existing surface-pavement layer for pavement maintenance related to reflection cracking. Reflection cracking can occur when a crack in the existing surface layer influences the overlay pavement. It can reduce the pavement life cycle and adversely affect traffic safety. Moreover, a failed overlay can reduce the economic value. In this regard, the objective of this study is to evaluate the bonding properties between the rigid pavement and a SAMI by using the direct shear test and the pull-off test. The predicted fractural energy functions with the shear stress were determined from a numerical analysis of the moving average method and the polynomial regression method. METHODS : In this research, the shear and pull-off tests were performed to evaluate the properties of mixtures constructed using no interlayer, a tack-coat, and SAMI with fabric and without fabric. The lower mixture parts (describing the existing pavement) were mixed using the 25-40-8 joint cement-concrete standard. The overlay layer was constructed especially using polymer-modified stone mastic asphalt (SMA) pavement. It was composed of an SMA aggregate gradation and applied as the modified agent. The sixth polynomial regression equation and the general moving average method were utilized to estimate the interlayer shear strength. These numerical analysis methods were also used to determine the predictive models for estimating the fracture energy. RESULTS : From the direct shear test and the pull-off test results, the mixture bonded using the tack-coat (applied as the interlayer between the overlay layer and the jointed cement concrete) had the strongest shear resistance and bonding strength. In contrast, the SAMI pavement without fiber has a strong need for fractural energy at failure. CONCLUSIONS : The effects of site-reflection cracking can be determined using the same tests on cored specimens. Further, an empirical-mechanical finite-element method (FEM) must be done to understand the appropriate SAMI application. In this regard, the FEM application analy pavement-design analysis using thesis and bonding property tests using cored specimens from public roads will be conducted in further research.

• Journal of Adhesion and Interface
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• v.16 no.2
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• pp.69-75
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• 2015

Stone board for the rock bed was needed to reduce weight using thin thickness and reinforced materials. In this work, stone/wood board for rock bed was studied. Stone and wood were attached to reduce total weight of stone for rock bed. For reinforcing wood heat treatment method was used to change surface and mechanical properties. Mechanical strength of heat treated wood increased more than neat condition. The optimum heat treatment condition was set on $100^{\circ}C$ under tensile, flexural loads whereas surface energy was also obtained by contact angle measurement. Optimum adhesive condition was to get the maximum adhesion between stone and wood. Lap shear test was performed for stone/wood board with different adhesives such as amine type epoxy, polyurethane, chloro-rubber and vinyl chloride acetate type. Fracture surface of lap shear test was shown at wood fracture part on stone using amine type epoxy adhesive. It was found that for high adhesion between stone and wood the optimum adhesive was epoxy type for the rock bed.

• Resources Recycling
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• v.24 no.2
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• pp.55-61
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• 2015

In this study, mechanical properties of waste ground rubber tire powder were investigated to evaluate the influence of pre-treatment process for recycling. The tensile test, fracture test and morphology observation were carried out using various kinds of waste ground tire powders, which were produced by grinding and devulcanization process, respectively. As a results, it was found that the produced rubber powder through grinding process increased its tensile strength and elongation with decreasing particle size because of decreasing surface area. Devulcanized rubber powder also increased its tensile strength and elongation by de-crosslink with sulfur. It could be also suggested that devulcanization treatment after grinding process was more efficient recycling process for both increasing tensile property and fracture elongation of waste ground rubber tire powders.

• Polymer(Korea)
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• v.36 no.1
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• pp.59-64
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• 2012

The impact strength of material is considered the most important design factor for small and light products. Impact strength is a unique material property, thus the impact strength should not depend upon the geometry of specimen. However it varies according to specimen thickness, notching method, and notch shape. In this study, the variations of impact strength have been investigated according to thickness, notch shape, and notching method of specimen. Engineering plastics such as PC, ABS and POM have been used in this study. Experimental results showed impact strength increased as thickness decreasesd. PC showed the highest increment of impact strength when the thickness was thin. Fractured section of PC showed brittle fracture behavior when the specimen was thick. However it showed ductile fracture behavior when it was thin. The impact strength of in-mold notched specimen showed higher than that of milling notched specimen. PC showed the highest notch sensitivity among the materials used in this experiment.

• Composites Research
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• v.36 no.4
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• pp.281-287
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• 2023

Environmental pollution from waste and the climate crisis, due to rising global average temperatures, are reaching critical levels threatening human survival. Research is ongoing across various fields to solve this problem, with a key focus on developing eco-friendly, carbon-neutral materials. Our study aimed to integrate natural fibers, known for their environmentally friendly properties and lower carbon emissions, with carbon fibers. In general, combining high-strength and low-strength materials results in intermediate properties. However, we found that certain properties in our study exceeded those of typical carbon fiber composite materials. To validate this, we produced both carbon fiber composite materials and carbon fiber/natural fiber hybrid composite materials. We then compared their mechanical properties using a range of specific tests. Our results revealed that the hybrid composite material exhibited superior bending strength and fracture toughness compared to the carbon fiber composite material. We also identified the underlying mechanisms contributing to this strength enhancement. This breakthrough suggests that the use of hybrid composite materials may allow the production of stronger structures. Moreover, this can play a significant role in mitigating environmental pollution and the climate crisis by reducing carbon emissions, a major contributing factor to these global challenges.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.36 no.5
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• pp.339-346
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• 2023

The uncertainties of microstructural features affect the properties of materials. Numerous pores that are randomly distributed in materials make it difficult to predict the properties of the materials. The distribution of pores in cementitious materials has a great influence on their mechanical properties. Existing studies focus on analyzing the statistical relationship between pore distribution and material responses, and the correlation between them is not yet fully determined. In this study, the mechanical response of cementitious materials is predicted through an image-based data approach using a convolutional neural network (CNN), and the correlation between pore distribution and material response is analyzed. The dataset for machine learning consists of high-resolution micro-CT images and the properties (tensile strength) of cementitious materials. The microstructures are characterized, and the mechanical properties are evaluated through 2D direct tension simulations using the phase-field fracture model. The attributes of input images are analyzed to identify the spot with the greatest influence on the prediction of material response through CNN. The correlation between pore distribution characteristics and material response is analyzed by comparing the active regions during the CNN process and the pore distribution.