연속철근 콘크리트 포장 시공 후 가로균열 순환거동 계측

유태석,조철민 한국도로학회 2018 한국도로학회논문집 Vol.20 No.5

PURPOSES : The purpose of this study was to suggest a quantitative trend of the daily and seasonal cyclic movement of transverse crackwidth based on measurements in CRCP(Continuously Reinforced Concrete Pavement) within the first year of construction. METHODS : Crack gauges were installed in eight normal cracks, two induced cracks, and two construction joints of newly constructed CRCP. Crack width movements were continuously collected for about a year to investigate the cyclic behavior after construction. The daily and seasonal crack width movements were quantitatively analyzed and compared. RESULTS: Crack width movement in hot weather was relatively less than in cold weather. As a result of frequency analysis of the daily cyclic behavior, it was revealed by measurement that the minimum crack width from 2 p.m. to 4 p.m. was caused by expansion of the concrete; and that the maximum width from 6 a.m. to 8 a.m. was caused by contraction. Average crack width movements were calculated for every month and showed seasonal cyclic behavior. Maximum crack width was measured from December to January. Average crack width was investigated from March to April. Daily crack width movement in relation to concrete temperature was calculated from -0.00017 to -0.03844 mm/℃ and showed gradual decrease in absolute value with time caused by change in the crack spacing. It was found that the relationships between the monthly average crack width and concrete temperature are from -0.004 to -0.012 mm/℃. CONCLUSIONS : Crack-width movement shows a daily and seasonal cyclic behavior. Crack-width measurement in any time or season will have variance caused by daily and seasonal cyclic movement. Variances and trends were obtained in this study based on measurements for various cracks. The long-term behavior of cracks should be surveyed and compared with these measurements to investigate trends of convergence with time, caused by convergence of crack spacing.

Investigation of Temperature Dependent Cracking Behaviors of Asphalt Mixtures using the Modified Overlay Tester

Christopher Jabonero,곽평진,조윤호 한국도로학회 2017 한국도로학회 학술대회 발표논문 초록집 Vol.2017 No.10

The discontinuity movements of the Portland cement concrete (PCC) layer due to temperature fluctuations and traffic loading are primary causes of the reflection cracking in asphalt overlays. The thermal expansion and contraction of the discontinuities at the PCC layer induces tension at the bottom of the asphalt overlay layer creating excessive strains which causes cracking. The additional cyclic discontinuity movements from the thermal fluctuations and traffic loads propagates the cracks initiated until failure of the overlay layer. However, the crack behaviors of asphalt mixtures varies with temperature due to its viscoelastic property. As such, there is a need to investigate the cracking behavior of asphalt mixtures with varying temperatures and loading conditions. A modified overlay tester developed to evaluate the cracking resistance of asphalt mixtures in various loading directions and different confining temperatures was used to investigate the behavior of asphalt materials with various temperatures and loading conditions. The laboratory test was conducted in 2 segments. The first segment investigates the asphalt cracking behavior subjected to horizontal loading in 3 varying temperatures (10, 25 and 40C) which simulates the cyclic thermal contraction and expansion at the discontinuity. The second segment examines the cracking propagation of the asphalt mixture subjected to vertical loading in 3 varying temperatures. A load dissipation curve per loading cycle is generated in each test along with the images taken on the face of the specimen to monitor the crack propagation. Results have shown that asphalt mixtures undergo a 3-phase cracking behavior: initiation, propagation and failure. This is evident in the load dissipation curve when the initiation phase shows a rapid reduction of peak loads in first series of loading cycles which is followed by a slow and constant load reduction over a certain number of cycles. Failure occurs when there is a sudden decline in peak load and the percent reduction of the load is achieved. Figure 1 shows a fine dense grade asphalt mixture subjected to horizontal movement at 10C. Meanwhile, the load dissipation curve is further investigated by analyzing the images captured during testing. It can be seen that the first visible crack can be identified after 40 cycles which steadily propagates up to 600 cycles. However, between 600 and 700 loading cycles, there is a sudden dip in peak load which shows that at that the stage the crack has already propagated to the top of the test specimen as shown in Figure 2. Other tests have shown that the cracking patterns and load dissipation curves vary with different testing temperatures signifying that low temperature is more susceptible to early failure with constant differential movement. Further tests signify that using a general formula, parameters are calculated which refer to fracture properties of the material.

A Study on the Determination of Closing Level for Finite Element Analysis of Fatigue Crack Closure

Choi, Hyeon-Chang The Korean Society of Mechanical Engineers 2000 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.14 No.4

An elastic-plastic finite element analysis is performed to investigate detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The finite element analysis performed under plane stress using 4-node isoparametric elements can predict fatigue crack closure behavior. The mesh of constant element size along crack surface can not predict the opening level of fatigue crack. The crack opening level for the constant mesh size increases linearly from initial crack growth. The crack opening level for variable mesh size, is almost flat after crack tip has passed the monotonic plastic zone. The prediction of crack opening level using the variable mesh size proportioning the reversed plastic zone size with the opening stress intensity factors presents a good agreement with the experimental data regardless of stress ratios.

高溫下 304 스테인리스鋼의 複數表面龜裂의 成長學動에 관한 實驗的 硏究

서창민(C. M. Suh),신형섭(H. S. Shin),황남성(N. S. Hwang),정대윤(D. Y. Jung) 한국해양공학회 1995 韓國海洋工學會誌 Vol.9 No.1

The crack which is discovered in various structures and machine elements is multi-cracks. Multi-cracks may cause serious problems because they grow individually, and coalesce into one and it leads to fracture.<br/> Fatigue tests have been carried out to study the growth and coalescence behavior of multi-surface-cracks initiated at the semicircular surface notch in type 304 stainless steel at elevated temperature.<br/> The results are as follows;<br/> When multi-surface-cracks are lying on the surface of material, the major surface crack has greater influence on the the fatigue life than the subcracks.<br/> The aspect ratio of multi-surface-cracks is lower than that of single crack because of the interaction and coalescence of surface cracks.<br/> Crack growth shape turns to semiellipse from the semicircle notch. After coalescence, the surface crack length increases rapidly, and it leads to fracture.<br/> Further, the slope transition of Paris law was found in the da/dN-ΔK₁ plots.

Numerical Analysis for Prediction of Fatigue Crack Opening Level

Choi, Hyeon Chang The Korean Society of Mechanical Engineers 2004 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.18 No.11

Finite element analysis(FEA) is the most popular numerical method to simulate plasticity-induced fatigue crack closure and can predict fatigue crack closure behavior. Finite element analysis under plane stress state using 4-node isoparametric elements is performed to investigate the detailed closure behavior of fatigue cracks and the numerical results are compared with experimental results. The mesh of constant size elements on the crack surface can not correctly predict the opening level for fatigue crack as shown in the previous works. The crack opening behavior for the size mesh with a linear change shows almost flat stress level after a crack tip has passed by the monotonic plastic zone. The prediction of crack opening level presents a good agreement with published experimental data regardless of stress ratios, which are using the mesh of the elements that are in proportion to the reversed plastic zone size considering the opening stress intensity factors. Numerical interpolation results of finite element analysis can precisely predict the crack opening level. This method shows a good agreement with the experimental data regardless of the stress ratios and kinds of materials.

Investigation on mechanical behavior and crack coalescence of sandstone specimens containing fissure-hole combined flaws under uniaxial compression

Qian Yin,Hongwen Jing,Haijian Su 한국지질과학협의회 2018 Geosciences Journal Vol.22 No.5

This study focuses on the effect of pre-existing flaw geometry on mechanical behavior and crack coalescence modes of sandstone specimens containing combined flaws with different fissure angle, ligament length and fissure length under uniaxial compression. The flaw geometry is a combination of a single hole and an inclined fissure underneath, which is generated by a high pressure water-jet cutting machine and is different from that reported in previous studies. The effect of flaw geometry on mechanical behavior of sandstone specimens is analyzed. Basically, mechanical parameters including the peak strength, peak axial strain, elastic modulus and secant Young’s modulus for the flawed specimens are lower than those for the intact specimens, with the reduction extent related to the fissure angle, ligament length and fissure length. Variation trends of the crack initiation stress for all tested cases are studied. Initiated crack types and cracking modes also depend on the combined flaws geometry. For the flawed specimens with a small fissure angle, ligament length or fissure length, cracking modes are generally characterized by cracks initiated from the hole-wall and evolved to the specimen boundary. However, when the fissure angle, ligament length or fissure length is increased, cracks initiated from both the hole-wall and fissure tips produce the main failure planes, accompanied by a free-standing “triangular prism structure” within the specimens. Numerical simulations using RFPA2D (Rock Failure Process Analysis in two dimensions) are carried out on the flawed sandstone specimens and agree well with the experimental results in the peak strength and overall cracking behavior.

Crack-healing behavior and bending strength of Al2O3/SiC composite ceramics according to the amount of added Y2O3

남기우 한양대학교 세라믹연구소 2010 Journal of Ceramic Processing Research Vol.11 No.4

Three Al2O3/SiC composite ceramics were prepared, which included 1, 3 or 5 wt.% Y2O3, and their high-temperature bending strengths and in-situ crack-healing behaviors examined. A surface elliptical-crack of about 100 ߦ in diameter was introduced in the specimens using a Vickers hardness indenter. From in-situ observations, the Al2O3/SiC composite ceramic with 3 wt.%Y2O3 showed superior crack-healing ability than the 1 and 5 wt.% Y2O3 ceramics. The as-cracked specimen with 3 wt.% Y2O3showed strength recovery on heating for one hour at 1473 K in air, which may have been due to the lower crack-healing temperature on the addition of 3 wt.% Y2O3. The heat-resistance limit temperatures of the crack-healed Al2O3/SiC composite ceramics were 1073, 1373 and 873 K for 1, 3 and 5 wt.% Y2O3, respectively.

잔골재의 배합비에 따른 고성능 섬유 보강 시멘트 복합체의 휨 거동 및 균열 특성

신경준(Shin Kyung-Joon),장규현(Jang Kyu-Hyoun) 대한토목학회 2007 대한토목학회논문집 A Vol.27 No.5A

인장에 취약한 단점을 가지고 있는 시멘트계 혼합재료인 모르타르와 콘크리트의 인장성능을 보강하기 위한 다양한 방법이 사용되고 있으며, 주요한 한가지 방법으로 인장에 강한 섬유를 콘크리트에 혼입하여 주는 섬유보강 콘크리트가 꾸준히 연구 사용되어 왔다. 최근에는 부족한 인장강도를 보완하여 준다는 개념에서 한 단계 발전하여 여러 개의 균열을 동시에 발생시켜 각각의 균열폭을 최소화시키면서 극한변형율을 증가시키는 고성능 섬유보강시멘트복합체(HPFRCC)에 대한 연구가 활발히 진행되고 있다. 이러한 연구들은 주로 섬유의 배합특성과 물-시멘트비와 결합제에 대한 연구에 초점을 맞추어 수행되었으며, 잔골재의 배합 특성에 대한 연구는 부족한 실정이다. 따라서, 본 연구에서는 고인성 HPFRCC 배합의 기본이 되는 잔골재에 따른 역학적 특성 연구를 수행하였고, 이에 대한 결과를 바탕으로 잔골재 비에 따른 HPFRCC의 거동 및 균열 특성에 대한 연구를 수행하였다. 실험결과, 잔골재로써 규사를 사용한 경우 압축강도와 탄성계수를 유지하며 파괴인성이 낮아지는 경향을 보였으며, 휨인성의 증가와 효과적인 균열분산을 위한 최적 배합은 잔골재(규사)-시멘트비가 0.8~1.0인 것으로 나타났다. Various methods have been used to reinforce the cementitious material such as mortar and concrete that have weak tensile strength. Major reinforcing method is to mix matrix with fibers which have strong tensile strength. Recently, High Performance Fiber Reinforced Cementitious Composites (HPFRCC) has been developed which shows multiple cracking behavior, that is different from conventional FRC. Related studies mainly have focusing on the mechanical behaviors according to fiber types and mixture proportions such as W/C and binder contents. However, study related to fine aggregate type and contents are insufficient. Therefore, this paper examines the fracture characteristics related to fine aggregate and contents, and reports the cracking behavior as well as mechanical behavior for various mixtures which have different fiber type and mixture proportions. From the results, it was useful to use silica sand as a fine aggregate for reducing fracture toughness as maintaining compressive strength and elastic modulus. It is showed that optimal silica-cement ratio was 0.8~ 1.0 to maximize flexural toughness and number of crack.

Fatigue Crack Propagation Behavior in STS304 Under Mixed-Mode Loading

Lee, Jeong-Moo,Song, Sam-Hong The Korean Society of Mechanical Engineers 2003 JOURNAL OF MECHANICAL SCIENCE AND TECHNOLOGY Vol.17 No.6

The use of fracture mechanics has traditionally concentrated on crack growth under an opening mechanism. However, many service failures occur from cracks subjected to mixed-mode loading. Hence, it is necessary to evaluate the fatigue behavior under mixed-mode loading. Under mixed-mode loading, not only the fatigue crack propagation rate is of importance, but also the crack propagation direction. In modified range 0.3$\leq$a/W$\leq$0.5, the stress intensity factors (SIFs) of mode I and mode II for the compact tension shear (CTS) specimen were calculated by using elastic finite element analysis. The propagation behavior of the fatigue cracks of cold rolled stainless steels (STS304) under mixed-mode conditions was evaluated by using K$\_$I/ and $_{4}$ (SIFs of mode I and mode II). The maximum tangential stress (MTS) criterion and stress intensity factor were applied to predict the crack propagation direction and the propagation behavior of fatigue cracks.

Post-uniform elongation and tensile fracture mechanisms of Fe-18Mn-0.6C-xAl twinning-induced plasticity steels

Yu, Ha-Young,Lee, Sang-Min,Nam, Jae-Hoon,Lee, Seung-Joon,Fabrè,gue, Damien,Park, Myeong-heom,Tsuji, Nobuhiro,Lee, Young-Kook Elsevier 2017 Acta materialia Vol.131 No.-

<P>The objective of the present study was to elucidate the complicated interrelationship between necking, post-uniform elongation (e(pu)), strain rate sensitivity (SRS), fracture mechanism and Al concentration in Fe-18Mn-0.6C-xAl twinning-induced plasticity steels. Many tensile tests were conducted for in- and ex situ observations of necking, fracture surfaces, crack propagation and the density and size of micro-voids with the assistance of a high-speed camera and X-ray tomographic equipment. The addition of Al increased epu, SRS and reduction ratios in dimension of the neck part of tensile specimens, and also changed fracture mode from quasi-cleavage to ductile fracture at the edge part. The quasi-cleavage surface of Al-free specimen was induced by edge and side cracks occurring along grain boundary junctions and twin boundaries within the edges and side surfaces where local deformation bands meet. The ductile-fracture surface of 1.5 %Al-added specimen was formed by the coalescence of micro-voids. While the side-to-middle crack propagation occurred in Al-free and 1 %Al-added specimens due to side cracks, the middle-to-side crack propagation was observed in 1.5 %Al-added specimen. The Al-free specimen had the larger size of the 20 largest voids compared to the 1.5 %Al-added specimen despite its lower void density and local strain due to the accelerated growth of voids near the tips of side cracks. Evaluating the negligible epu of Al-free specimen by SRS is not deemed to be reasonable due to its inappreciable necking and side cracks. The improvement of epu in 1.5 %Al-added specimen is primarily due to disappearance of edge and side cracks. (C) 2017 Acta Materiali a Inc. Published by Elsevier Ltd. All rights reserved.</P>