• 수산해양기술연구
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• 제20권1호
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• pp.23-29
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• 1984

고역알루미늄합금의 파괴특성에 미치는 시효열처리의 영향에 대하여 검토한 결과는 다음과 같다. 1. 현미경 조직의 관찰결과 $190^{\circ}C,$ 12hr로 시효열처리한 것이 시효강화된 양호한 조직과 미세한 석출분포를 나타내고 있음을 알 수 있었다. 2. 계단식시효열처리에 의하여 정상시효열처리보다 시효시간을 반정도로 단축시킬 수 있었고, 이들 조직도 유사하였다. 3. 인장파단면의 SEM 전자현미경 사진관찰결과는 $190^{\circ}C,$ 12hr으로 시효열처리한 것은 딤플형 입계 및 입내연성파괴를 나타내나, 이를 제외한 대부분은 시효열처리 시간과 온도에는 관계없이 입계연성파괴인 것이 관찰되었다.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.59-64
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• 2002

Effects of the aging treatments on the microstructure and strength of heat affected zone(HAZ) in the welds of a age-hardened Al-Mg-Si alloy, 5N01-T5, were investigated. The base metal aging treatments before MIG welding were conducted at 423K to 473K for 28.8ks Post weld heat treatment(PWHT) to recover the HAZ strength was performed at 448K for 28.8ks. Microstructure observations, hardness measurements and tensile tests were conducted to study properties of the MIG weld joints. The position of the softest region in HAZ where the hardness insufficiently recovered after natural aging and PWHT was at a distance of approximately 15mm from the center of the fusion zone. Hardness of the softest regions after natural aging and PWHT decreased with increase in the base metal aging temperature. TEM observation clarified that strengthening ${\beta}$"(Mg$_2$Si) precipitates and coarse ${\beta}$′ precipitates affected the hardnes of HAZ. Incomplete recover of hardness in HAZ after PWHT was caused by the precipitating of non-hardening ${\beta}$′ phase during the weld thermal cycle. In order to examine the effects of weldheat input and welding speed, the laser weld joints were also investigated and compared with the MIG weld ones. Laser welding had the narrower width of the softened regions in HAZ compared with MIG welding. The hardness of the softest regions of the laser welds after PWHT was higher than that of the MIG welds. Quantitative relations between hardness of the softest region and base metal aging temperature were obtained for both welding processes. Accordingly, the equations to estimate the strength of the weld joints after PWHT with varying base metal temperatures were proposed for MIG welding and laser welding.

• 한국재난관리표준학회지
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• 제2권4호
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• pp.59-65
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• 2009

현재 대부분 사용되고 있는 반발경도법을 이용한 강도 추정식을 서울의 대표적인 지하구조물에 적용할 경우 준공년도에 따른 재령계수의 적용이 부적합한 것으로 판단됨에 따라 본 연구에서는 최근 8년간 수행된 서울지하철 정밀안전진단결과를 이용하여 코어의 일축압축강도와 동일위치에서 실시된 반발경도 시험결과와의 상관관계에 준공년도를 고려하여 공용중인 박스 및 터널구조물에 대한 합리적인 강도평가 추정식을 제시하였다.

• International Journal of Aeronautical and Space Sciences
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• 제18권1호
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• pp.56-62
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• 2017

Because adhesively bonded joints are used in many structural systems, it is important to predict accurate adhesive strengths. Composite aircraft with many joints are easily exposed to low temperatures and high relative humidity. This paper presents a humidity aging effect on the adhesive strength of a composite single-lap joint (SLJ). The adhesive strength of the SLJ is predicted using a finite element analysis with a cohesive zone model (CZM) technique. The humidity aging effect is evaluated based on the adhesive strength and CZM parameters. A lap joint test is carried out on the composite SLJ specimens, which are exposed for four months of 100% R.H. at $25^{\circ}C$. The predicted strengths are in good agreement with experimental data, and the actual crack propagation is satisfactorily simulated using the local CZM technique.

• The Journal of Advanced Prosthodontics
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• 제9권5호
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• pp.364-370
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• 2017

PURPOSE. To explore the influence of different surface conditionings on surface changes and the influence of surface treatments and aging on the bond strengths of composites to non-aged and aged resin nanoceramics. MATERIALS AND METHODS. Rectangular-shaped non-aged and aged (5000 thermocycles) resin nanoceramic specimens (Lava Ultimate) (n=63, each) were divided into 3 groups according to surface treatments (untreated, air abrasion, or silica coating) (n=21). The surface roughness was measured and scanning electron microscopy was used to examine one specimen from each group. Afterwards, the specimens were repaired with a composite resin (Filtek Z550) and half were sent for aging (5000 thermocycles, n=10, each). Shear bond strengths and failure types were evaluated. Roughness and bond strength were investigated by two- and three-way analysis of variance, respectively. The correlation between the roughness and bond strength was investigated by Pearson's correlation test. RESULTS. Surface-treated samples had higher roughness compared with the untreated specimens (P=.000). For the non-aged resin nanoceramic groups, aging was a significant factor for bond strength; for the aged resin nanoceramic groups, surface treatment and aging were significant factors. The failures were mostly adhesive after thermal cycling, except in the non-aged untreated group and the aged air-abraded group, which had mostly mixed failures. Roughness and bond strength were positively correlated (P=.003). CONCLUSION. Surface treatment is not required for the repair of non-aged resin nanoceramic; for the repair of aged resin nanoceramic restorations, air abrasion is recommended.

• 한국지반공학회:학술대회논문집
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• 한국지반공학회 2005년도 춘계 학술발표회 논문집
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• pp.916-923
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• 2005

In this study, to observe aging effect of undrained shear behavior for Nak-Dong River sand, undrained static and cyclic triaxial tests were performed with changing relative density ($D_r$), consolidation stress ratio($K_c$) and consolidation time. As a result of the test, the modulus of elasticity to all samples estimated within elastic zone by the micro strain of about 0.05% in case of static shear behavior increased with the lapse of consolidation time significantly, so aging effect was shown largely. Also strength of phase transformation point(S_{PT}$) and strength of critical stress ratio point($S_{CSR}$) increased with the lapse of consolidation time. Undrained cyclic shear strength($R_f$) obtained from the failure strain 5% increased in proportion to relative density($D_r$) and initial static shear stress($q_{st}$), $R_f$ of consolidated sample for 1,000 minutes increased about 10.6% compared to that for 10 minutes at the loose sand, and $R_f$ increased about 7.0% at the medium sand. In situ application range of $R_f$ to the magnitude of earthquake for Nak-Dong River sand was proposed by using a increasing rate of $R_f$ as being aging effect shown from this test result.

• 한국재료학회지
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• 제24권8호
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• pp.401-406
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• 2014

In this study, we investigated the precipitation behavior of the R-phase precipitated at the initial stage of aging and its effects on the mechanical properties of 25%Cr-7%Ni-2%Mo-4%W super duplex stainless steel. The R-phase was mainly precipitated at the interface of ferrite/austenite phases and inside of the ferrite phase during the initial stage of aging. It was transformed into the ${\sigma}$-phase with an increase of the aging time. The ferrite phase was decomposed into a new austenite(${\gamma}_2$)phase and the ${\sigma}$-phase by an aging treatment. The R phase was an intermetallic compound showing higher molybdenum and tungsten concentrations than the matrix and also showed higher molybdenum and tungsten concentrations than the ${\sigma}$ phase. In the initial stage of aging, precipitation of the R-phase did not change the hardness, the strength and the elongation. The hardness and the strength increased upon a longer aging time, but the elongation rapidly decreased. These results show that the R-phase did not significantly affect the hardness and the strength, though it did influence the elongation.

• The Journal of Advanced Prosthodontics
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• 제12권5호
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• pp.265-272
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• 2020

PURPOSE. The aims of this study were to investigate mechanical properties and hydrothermal degradation behaviour of the cubic-containing translucent yttrium oxide stabilized tetragonal zirconia polycrystal (Y-TZP). MATERIALS AND METHODS. Four groups of Y-TZP (T, ST, XT, and P), containing different amount of cubic crystal, were examined. Specimens were aged by autoclaving at 122℃ under 2 bar pressure for 8 h. Phase transformation was analyzed using X-ray diffraction (XRD) to measure phase transformation (t→m). Kruskal-Wallis test was used to determine the difference. Surface hardness, biaxial flexural strength, and fracture toughness in values among the experimental groups and verified with Wilcoxon matched pairs test for hardness values and Mann Whitney U for flexural strength and fracture toughness. RESULTS. XRD analysis showed no monoclinic phase in XT and P after aging. Only Group T showed statistically significant decreases in hardness after aging. Hydrothermal aging showed a significant decrease in flexural strength and fracture toughness in group T and ST, while group XT and P showed no effect of aging on fractural strength and fracture toughness with P<.05. CONCLUSION. Hydrothermal aging caused reduction in mechanical properties such as surface hardness, biaxial flexural strength, and fracture toughness of Y-TZP zirconia. However, cubic-containing zirconia (more than 30% by volume of cubic crystal) was assumed to have high resistance to hydrothermal degradation. Clinical significance: Cubic-containing zirconia could withstand the intraoral aging condition. It could be suggested to use as a material for fabrication of esthetic dental restoration.

• 한국공작기계학회:학술대회논문집
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• 한국공작기계학회 2004년도 추계학술대회 논문집
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• pp.60-66
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• 2004

Hardness value decreased about 3% for annealed specimens and increased about 60% for one hour aged specimens. But the values of the other specimens aged two hours or more showed almost the same. The yield strength was the highest about 1,800㎫ in 0.06%Nb specimen having twice as much as the base meta1 specimen. Also, the elongation was the highest in 0.03%Nb specimen showing the same as base metal specimen. The higher aging temperature and the longer aging time, the higher fatigue life. On the other hand, the 0.03%Nb specimen showed the highest fatigue life which increased about 12% more than base metal specime. 0.06%Nb specimen aged at 482$^{\circ}C$ for 8 hours simultaneously satisfied the 250 grade strength and 200 grade elongation having the most superior mechanical properties.

• 한국소성가공학회:학술대회논문집
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• 한국소성가공학회 2008년도 추계학술대회 논문집
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• pp.203-206
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• 2008

Transformation of austenite to martensite during cold rolling has been widely used to strengthen metastable austenitic stainless steel grades. Aging treatment of cold worked metastable austenitic stainless steels, including ${\alpha}'$-martensite phase, results in the further increase of strength, when aging is performed in $200^{\circ}C$ to $450^{\circ}C$ temperature range. The purpose of the present study was to evaluate the effect of time and temperature on the stress-strain behavior of cold worked austenitic stainless steels. The amount of ${\alpha}'$-martensite during cold working and aging was examined by ferrite scope and X-ray diffraction (XRD). During aging at $450^{\circ}C$ for 1hr, tensile strength dramatically increased by 150MPa. Deformed metastable austenitic steels containing the "body-centered" ${\alpha}'$-martensite are strengthened by the diffusion of interstitial solute atoms during aging at low temperature.