• The Journal of Advanced Prosthodontics
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• v.5 no.1
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• pp.36-43
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• 2013

PURPOSE. The objective of this study was to compare the fracture toughness ($K_{Ic}$) obtained from the single edge V-notched beam (SEVNB) and the fractographic analysis (FTA) of a glass-infiltrated and a zirconia ceramic. MATERIALS AND METHODS. For each material, ten bar-shaped specimens were prepared for the SEVNB method ($3mm{\times}4mm{\times}25mm$) and the FTA method ($2mm{\times}4mm{\times}25mm$). The starter V-notch was prepared as the fracture initiating flaw for the SEVNB method. A Vickers indentation load of 49 N was used to create a controlled surface flaw on each FTA specimen. All specimens were loaded to fracture using a universal testing machine at a crosshead speed of 0.5-1 mm/min. The independent-samples t-test was used for the statistical analysis of the $K_{Ic}$ values at ${\alpha}$=0.05. RESULTS. The mean $K_{Ic}$ of zirconia ceramic obtained from SEVNB method ($5.4{\pm}1.6\;MPa{\cdot}m^{1/2}$) was comparable to that obtained from FTA method ($6.3{\pm}1.6\;MPa{\cdot}m^{1/2}$). The mean $K_{Ic}$ of glass-infiltrated ceramic obtained from SEVNB method ($4.1{\pm}0.6\;MPa{\cdot}m^{1/2}$) was significantly lower than that obtained from FTA method ($5.1{\pm}0.7\;MPa{\cdot}m^{1/2}$). CONCLUSION. The mean $K_{Ic}$ of the glass-infiltrated and zirconia ceramics obtained from the SEVNB method were lower than those obtained from FTA method even they were not significantly different for the zirconia material. The differences in the $K_{Ic}$ values could be a result of the differences in the characteristics of fracture initiating flaws of these two methods.

• Korean Journal of Materials Research
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• v.28 no.12
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• pp.738-747
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• 2018

For the aerospace structural application of high-strength 2xxx series aluminum alloys, stress corrosion cracking(SCC) behavior in aggressive environments needs to be well understood. In this study, the SCC sensitivities of 2024-T62, 2124-T851 and 2050-T84 alloys in a 3.5 % NaCl solution are measured using a constant load testing method without polarization and a slow strain rate test(SSRT) method at a strain rate of 10-6 /sec under a cathodic applied potential. When the specimens are exposed to a 3.5 % NaCl solution under a constant load for 10 days, the decrease in tensile ductility is negligible for 2124-T851 and 2050-T84 specimens, proving that T8 heat treatment is beneficial in improving the SCC resistance of 2xxx series aluminum alloys. The specimens are also susceptible to SCC in a hydrogen-generating environment at a slow strain rate of $10^{-6}/sec$ in a 3.5 % NaCl solution under a cathodic applied potential. Regardless of the test method, low impurity 2124-T851 and high Cu/Mg ratio 2050-T84 alloys are found to have relatively lower SCC sensitivity than 2024-T62. The SCC behavior of 2xxx series aluminum alloys in the 3.5 % NaCl solution is discussed based on fractographic and micrographic observations.

• Korean Journal of Metals and Materials
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• v.47 no.12
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• pp.797-810
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• 2009

A study was made of the effects of a Sn addition on the microstructure and microfracture mechanism of squeeze cast AZ51-xSn magnesium alloys. Microstructural observation, in situ fracture testing, and fractographic observations were conducted on these alloys to clarify the microfracture process. The microstructural analyses indicated that $Mg_2Sn$ particles as well as $Mg_{17}Al_{12}$ particles precipitated mainly along the solidification cell boundaries; the volume fraction of these hard particles increased as the amount of added Sn increased, with increased the strength. From in situ fracture observations of the AZ51-7Sn alloy, coarse precipitates located on the cell boundaries worked as easy crack propagation sites and caused abrupt intercellular fracturing. On the other hand, the overall fracture properties of the AZ51-3Sn alloy improved because crack propagation proceeded into the Mg matrix rather than into the cell boundaries as twins developed actively, as confirmed by an R-curve analysis. These findings suggest that the addition of 3~5 wt.% Sn is effective in improving both the tensile and fracture properties on the basis of well-developed twins, the blocking of crack propagation, and crack blunting.

• Korean Journal of Metals and Materials
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• v.56 no.12
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• pp.845-853
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• 2018

To understand the effect of microstructural factors (i.e., the size of ${\alpha}$ phase, equiaxed vs bimodal structure) on high cycle fatigue (HCF) and fatigue crack propagation (FCP) behaviors of mill-annealed Ti-6Al-4V (Ti64) alloy, three specimens of EQ (equiaxed)-8 (8 indicates the size of ${\alpha}$ grain), BM (bimodal)-8, and BM-16 were studied. The uniaxial HCF and FCP tests were conducted at an R ratio of 0.1 under sinusoidal fatigue loading. The microstructural influence (i.e., EQ vs BM) was not significant on the tensile properties of mill-annealed Ti64 alloy, and showed an increase in tensile strength and elongation with decreasing gauge thickness from 50 mm to 1.3 mm. The microstructure, on the other hand, affected the resistance to HCF substantially. It was found that the EQ structure in mill-annealed Ti64 has better resistance to HCF than the BM structure, as a result of different crack initiation mechanism. Unlike HCF behavior, the effect of microstructural features on the FCP behavior of mill-annealed Ti64 was not significant. Among the three specimens, BM-16 specimen showed the highest near-threshold ΔK value, probably because it had the greatest slip reversibility with large ${\alpha}$ grains. The effect of microstructural factors on the HCF and FCP behaviors of mill-annealed Ti64 alloy are discussed based on fractographic and micrographic observations.

• Korean Journal of Metals and Materials
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• v.48 no.1
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• pp.28-38
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• 2010

High temperature high cycle and low cycle fatigue deformation behavior of automotive heat resistant aluminum alloys (A356 and A319 based) were investigated in this study. The microstructures of both alloys were composed of primary Al-Si dendrite and eutectic Si phase. However, the size and distribution for eutectic Si phase varied: a coarse and inhomogeneous distributed was observed in alloy B (A319 based). A brittle intermethallic phase of ${\alpha}-Fe\;Al_{12}(Fe,Mn)_3Si_2$ was detected only in B alloy. Alloy B exhibited high fatigue life only under a high stress amplitued condition in the high cycle fatigue results, whereas alloy A showed high fatigue life when stress was lowered. With regard to the low-cycle fatigue result ($250^{\circ}C$) showing higher fatigue life as ductility increased, alloy A demonstrated higher fatigue life under all of the strain amplitude conditions. Fractographic observations showed that large porosities and pores near the outside surface could be the main factor in the formation of fatigue cracks. In alloy B. micro-cracks were formed in both the brittle intermetallic and coarse Si phasese. These micro-cracks then coalesced together and provided a path for fatigue crack propagation. From the observation of the differences in microstructure and fractography of these two automotive alloys, the authors attempt to explain the high-temperature fatigue deformation behavior of heat resistant aluminum alloys.

• Journal of the Microelectronics and Packaging Society
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• v.10 no.3
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• pp.29-35
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• 2003

The Sn-based lead-free solders with varying microstructure were prepared by changing the cooling rate from the melt. Bulky as-cast SnAg, SnAgCu, and SnCu, alloys were cold rolled and thermally stabilized before the creep tests so that there would be very small amount of microstructural change during creep (TS), and thin specimens were water quenched from the melt (WQ) to simulate microstructures of the as-reflowed solders in flip chips. Cooling rates of the WQ specimens were 140∼150 K/sec, and the resultant $\beta-Sn$ globule size was 5∼10 times smaller than that of the TS specimens. Subsequent creep tests showed that the minimum strain rate of TS specimens was about $10_2$ times higher than that of the WQ specimens. Fractographic analyses showed that creep rupture of the TS-SnAgCu specimens occurred by the nucleation of voids on the $Ag_3Sn$ Sn or $Cu_6Sn_5$ particles in the matrix, their subsequent growth by the power-law creep, and inter-linkage of microcracks to form macrocracks which led to the fast failure. On the other hand, no creep voids were found in the WQ specimens due to the mode III shear rupture coming from the thin specimens geometry.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.36 no.4
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• pp.421-428
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• 2012

In this study, the effects of surface oxide film formation on the fatigue behavior of 7075-T6 aluminum alloy were analyzed in terms of the corrosion time of the alloy. The aluminum material used is known to have high corrosion resistance due to the passivation phenomenon that prevents corrosion. Aluminum alloys have been widely used in various industrial applications such as aircraft component manufacturing because of their lighter weight and higher strength than other materials. Therefore, studies on the fatigue behavior of materials and passivation properties that prevent corrosion are required. The fatigue behavior in terms of the corrosion time was analyzed by using a four-pointing bending machine, and the surface corrosion level of the aluminum material in terms of the corrosion time was estimated by measuring the surface roughness. In addition, fractographic analysis was performed and the oxide films formed on the material surface were studied by scanning electron microscopy (SEM). The results indicated that corrosion actively progressed for four weeks during the initial corrosion phase, the fatigue life significantly decreased, and the surface roughness increased. However, after four weeks, the corrosion reaction tended to slow down due to the passivation phenomenon of the material. Therefore, on the basis of SEM analysis results, it was concluded that the growth of the surface oxide film was reduced after four weeks and then the oxide film on the material surface served as a protection layer and prevented further corrosion.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.37 no.12
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• pp.1497-1502
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• 2013

When using hydrogen gas as an ecofriendly energy sources, it is necessary to conduct a safety assessment and ensure thereliability of the hydrogen pressure vessel against hydrogen embrittlement expected in the steel materials. In this study, by applying the in-situ SP test method, the gas hydrogen embrittlement behaviors in SA372 steel, which is commonly used as a pressurized hydrogen gas storage container, were evaluated. To investigate the hydrogen embrittlement behavior, SP tests at different punch velocities were conducted for specimens with differently fabricated surfaces at atmospheric pressure and under high-pressure hydrogen gas conditions. As a result, the SA372 steel showed significant hydrogen embrittlement under pressurized hydrogen gas conditions. The effect of punch velocity on the hydrogen embrittlement appeared clearly; the lower punch velocity case indicated significant hydrogen embrittlement resulting in lower SP energy. The fractographic morphologies observed after SP test also revealed the hydrogen embrittlement behavior corresponding to the punch velocity adopted. Under this pressurized gas hydrogen test condition, the influence of specimen surface condition on the extent of hydrogen embrittlement could not be determined clearly.

• Restorative Dentistry and Endodontics
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• v.35 no.2
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• pp.69-79
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• 2010

The purpose of this study was to evaluate the effect of film thickness of various resin cements on bonding efficiency in indirect composite restoration by measurement of microtensile bond strength, polymerization shrinkage, flexural strength and modulus, fractographic FE-SEM analysis. Experimental groups were divided according to film thickness (< $50\;{\mu}m$-control, $50\;{\mu}m$-T50, $100\;{\mu}m$-T100, $150\;{\mu}m$-T150) using composite- based resin cements (Variolink II, Duo-Link) and adhesive-based resin cements (Panavia F, Rely X Unicem). The data was analyzed using ANOVA and Duncan's multiple comparison test (p < 0.05). The results were as follows ; 1. Variolink II showed higher microtensile bond strength than that of adhesive-based resin cements in all film thickness (p < 0.05) but Duo-Link did not show significant difference except control group (p > 0.05). 2. Microtensile bond strength of composite-based resin cements were decreased significantly according to increasing film thickness (p < 0.05) but adhesive-based resin cements did not show significant difference among film thickness (p > 0.05). 3. Panavia F showed significantly lower polymerization shrinkage than other resin cements (p < 0.05). 4. Composite-based resin cements showed significantly higher flexural strength and modulus than adhesive-based resin cements (p < 0.05). 5. FE-SEM examination showed uniform adhesive layer and well developed resin tags in composite-based resin cements but unclear adhesive layer and poorly developed resin tags in adhesive-based resin cements. In debonded surface examination, composite-based resin cements showed mixed failures but adhesive-based resin cements showed adhesive failures.

• Restorative Dentistry and Endodontics
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• v.32 no.4
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• pp.343-355
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• 2007

The objectives of this study was to evaluate the durability of 4 resin cements by means of microtensile bond strength test combined with thermocycling method and fractographic FE-SEM analysis. Experimental groups were prepared according to thermocycling (0, 1,000, 5,000) and the kind of resin cements, those were Variolink II, Multilink, Panavia F 2.0, Rely X Unicem. Flat dentin surfaces were created on mid-coronal dentin of extracted third molars. Then fresh dentin surface was grounded with 320-grit silicon carbide abrasive papers to create uniform smear layers. Indirect composite block (Tescera, Bisco Inc., Schaumburg, IL, USA) was fabricated ($12\;{\times}\;12\;{\times}\;6\;mm^3$). It's surface for bonding to tooth was grounded with silicon carbide abrasive papers from 180- to 600-grit serially, then sandblasted witk $20\;-\;50\;{\mu}m$ alumina oxide. According to each manufacturer's instruction, dentin surface was treated and indirect composite block was luted on it using each resin cement. For Rely X Unicem, dentin surface was not treated. The bonded tooth-resin block were stored in distilled water at $37^{\circ}C$ for 24 hours. After thermocycling, the bonded tooth-resin block was sectioned occluso-gingivally to 1.0 mm thick serial slabs using all Isomet slow-speed saw (Isomet, Buehler Ltd, Lake Bluff, IL, USA). These sectioned slabs were further sectioned to $1.0\;{\times}\;1.0\;mm^2$ composite-dentin beams. The specimens were tested with universal testing machine (EZ-Test, Shimadzu, Japan) at a crosshead speed of 1.0 mm/min with maximum load of 500 N. The data was analyzed using one-way ANOVA and Duncan's multiple comparison test at $p\;{\leq}\;0.05$ level. Within the limited results, we conclude as follows; 1. The bond strength of Variolink II was evaluated the highest among experimental groups and was significantly decreased after 1,000 thermocycling (p < 0.05). 2. The bond strength of Multilink was more affected by thermocycling than the other experimental groups and significantly decreased after 1,000 thermocycling (p < 0.05). 3. Panavia F 2.0 and Rely X Unicem showed the gradually decreased tendency of microtensile bond strength according to thermocycling but there was no significant difference (p > 0.05). 4. Adhesive based-resin cements showed lower bond strength with or without thermocycling than composite based-resin cements. 5. Variolink II & Multilink showed high bond strength and mixed failure, which was occurred with a thin layer of luting resin cement before thermocycling and gradually increased adhesive failure along the dentin surface after thermocycling. The bonding performance of resin cement can be affected by application procedure and chemical composition. Composite based-resin cement showed higher bond strength and durability than adhesive based-resin cement.