• Computers and Concrete
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• v.34 no.1
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• pp.15-31
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• 2024

This study explores the failure mechanisms of 'I' shaped non-persistent cracks under uniaxial loads through a combination of experimental tests and numerical simulations. Concrete specimens measuring 200 mm×200 mm×50 mm were manufactured, featuring 'I' shaped non-persistent joints. The number of these joints varied from one to three, with angles set at 0, 30, 60, and 90 degrees. Twelve configurations, differing in the placement of pre-existing joints, were considered, where larger joints measured 80 mm in length and smaller cracks persisted for 20 mm with a 1 mm crack opening. Numerical models were developed for the 12 specimens, and loading in Y-axis direction was 0.05 mm/min, considering a concrete tensile strength of 5 MPa. Results reveal that crack starting was primarily influenced by the slope of joint that lacks persistence in relation to the loading direction and the number of joints. The compressive strength of the samples exhibited variations based on joint layout and failure mode. The study reveals a correlation between the failure behavior of joints and the number of induced tensile fracture, which increased with higher joint angles. Specimen strength increased with decreasing joint angles and numbers. The strength and failure processes exhibited similarities in both laboratory testing and numerical modeling methods.

• Geomechanics and Engineering
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• v.39 no.1
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• pp.43-54
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• 2024

Cross-flaws are frequently encountered in practical rock engineering projects near horseshoe-shaped cavities, and their presence can significantly impact the failure mode of these cavities. This study utilizes a combination of laboratory experiments and numerical simulations to investigate the influence of cross-flaws on the failure mode of a horseshoe-shaped cavity. During the experimental tests, we varied the length of secondary flaw and the angle of the cross-flaws in the specimens, followed by subjecting them to biaxial compression. Our experimental results show that when the angle α between the primary and the secondary flaws is small (0° and 45°), only one crack is initiated at the vault of the cavity, resulting in a shear failure mode. Conversely, when the angle α is large (90° and 135°), two cracks are more likely to initiate at the vault of the cavity, leading to the failure mode of falling blocks in the surrounding rock. Furthermore, the circumferential stress at the cavity vault from numerical simulations results is consistent with this observed phenomenon. When the angle α is small, only one circumferential tensile stress concentration is observed at the cavity vault, resulting in the initiation of a single crack. In contrast, when the angle α is large, two stress concentrations appear at the vault of the cavity, leading to the initiation of two cracks from these locations.

• Journal of the Korea institute for structural maintenance and inspection
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• v.21 no.6
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• pp.98-105
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• 2017

In this study, the fiber blending ratio and strain rate effect on the tensile behavior of hybrid fiber reinforced cement composite was evaluated. Hooked steel fiber and polyvinyl alcohol fiber were used for reinforcing fiber. The fiber blending ratio of HSF+PVA were 1.5+0.5, 1.0+1.0 and 0.5+1.5vol.%. As a results, the tensile strength, strain capacity and fracture toughness of the hooked steel fiber reinforced cement composites were improved by the increase of the bond strength of the fiber and the matrix according to increase of strain rate. However, the tensile stress sharply decreased after the peak stress because of the decrease in the number of straightened pull-out fibers by micro cracks in the matrix around hooked steel fiber. On the other hand, PVA fiber showed cut-off fracture at strain rate $10^{-6}/s$ with multiple cracks. However, at the strain rate $10^1/s$, the multiple cracks and strain capacity were decreased because of the pull-out fracture of PVA fiber. The HSF1.5PVA0.5 shown the highest tensile strength because the PVA fiber suppressed the micro cracks in the matrix around the hooked steel fiber and improved the pull-out resistance of hooked steel fiber. Thus, DIF of strain capacity and fracture toughness of HSF1.5PVA were greatly improved. In addition, the synergistic response of fracture toughness was positive because the tensile stress was slowly decreased after the peak stress by improvement of the pull-out resistance of hooked steel fiber at strain rate $10^1/s$.

• Proceedings of the Korea Concrete Institute Conference
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• 2001.11a
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• pp.777-780
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• 2001

Corrosion products of reinforcement in the concrete exerts pressure to the adjacent concrete that the concrete is subject to tensile stress. If the tensile strength exceeds the tensile strength, cracks are initiated around steel and propagates through concrete cover. Cracking of the cover means that the lifetime of the structure is ended. So the amount of corrosion which introduces crack in the concrete cover is a crucial factor in the reinforcement corrosion problem. In this study, relation between internal pressure and amount of corrosion are pursued by way of corrosion experiment and finite element analysis.

• Proceedings of the Korean Geotechical Society Conference
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• 2010.03a
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• pp.907-910
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• 2010

This study has developed a numerical analysis technique newly which can evaluate the damage propagation characteristics of civil infrastructures. To do this, numerical techniques are incorporated for the concrete members up to the compressive damage due to the bending compressive forces after the tensile crack based on the deformation mechanism. Especially, for the compressive damage stage after the tensile crack, the crack propagation process will be analyzed numerically using the concept of an equivalent plastic hinged length. Using this concept, we investigate the reasonability of the developed module by comparing commercial program for the tunnel structure. It can be established from this study that section forces, such as axial forces and the moment cracks takes place, can be related to the width of the crack making it possible to analyze the crack extension.

• Applied Microscopy
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• v.43 no.2
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• pp.81-87
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• 2013

The influence of carbide formation on mechanical properties has been investigated in carburized steels. Through controlled diffusion and precipitation processes, the morphologies of carbides could be changed and then fine, networked, and spherical shapes at carburized layers were obtained. These morphological changes affected tensile and bending fatigue properties of the steel. The fine and the spherical carbides acted as resistance sites against crack propagation, which improved the mechanical properties. However, the networked carbides deteriorated the properties because the cracks propagated along the boundaries of them. These results indicate that the morphological control of carbides is one of important keys to improve the mechanical properties.

• Structural Engineering and Mechanics
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• v.36 no.4
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• pp.447-461
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• 2010

Fiber reinforced polymer (FRP) bars have been widely used as reinforcement for concrete structures. However, under elevated temperatures, the difference between the transverse coefficients of thermal expansion of FRP rebars and concrete may cause the splitting cracks of the concrete cover. As a result, the bonding of FRP-reinforced concrete may not sustain its function to transfer load between the FRP rebar and the surrounding concrete. The current study investigates the cracking resistance of FRP reinforced concrete against the thermal expansion based on a mechanical model that accounts for the tensile softening behavior of concrete. To evaluate the efficacy of the proposed model, the critical temperature increments at which the splitting failure of the concrete cover occurs and the internal crack radii estimated are compared with the results obtained from the previous studies. Simplified equations for estimating the critical temperature increments and the minimum concrete cover required to prevent concrete splitting failure for a designated temperature increment are also derived for design purpose.

• Proceedings of the Korea Concrete Institute Conference
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• 2004.11a
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• pp.441-444
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• 2004

This paper presnets the tensile behavior of 8 high performance fiber-reinforced cementitious composites (HPFRCCs) members, each reinforced with one deformed bar 16mm in diameter. The variables included HPFRCC(Ductal, steel cord and polyethylene hybrid fiber, PE fiber) versus normal concrete. Fibers used in HPFRCC significantly increased tensile strength, ductility, and tension stiffening of cementitious materials. For HPFRCC, after first cracking, tensile load continue to rise without fracture localization. Sequentially developed parallel cracks contributed to the inelastic strain at increasing stress level. After yielding of the reinforcing bars, HPFRCC showed increases in loads with increasing strains.

• Transactions of the Korean Society of Pressure Vessels and Piping
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• v.14 no.1
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• pp.15-23
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• 2018

A compact heat exchanger is one of critical components in a very high temperature gas-cooled reactor (VHTR). Alloy 617 (Ni-Cr-Co-Mo) is considered as one of leading candidates for this application due to its excellent thermal stability and strengths in anticipated operating conditions. On the basis of current ASME code requirements, sixty sheets of this alloy are prepared for diffusion welding, which is the key technology to have a reliable compact heat exchanger. Optical microscopic analysis show that there are no cracks, incomplete bond, and porosity at/near the interface of diffusion weldment, but Cr-rich carbides and Al-rich oxides are identified through high resolution electron microscopic analysis. In high-temperature tensile testing, superior yield strengths of the diffusion weldment compared to the code requirement are obtained up to 1223 K ($950^{\circ}C$). However, both tensile strength and ductility drop rapidly at higher temperature due to the insufficient grain boundary migration across the interface of diffusion weldment. Best fit curves for minimum yield strength and average tensile strength are drawn from the experimental tensile results of this study.

• Geomechanics and Engineering
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• v.14 no.2
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• pp.203-209
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• 2018

Reliable estimation of compressive as well as tensile in-situ stresses is critical in the design and analysis of underground structures and openings in rocks. Kaiser effect technique, which uses acoustic emission from rock specimens under cyclic load, is well established for the estimation of in-situ compressive stresses. This paper investigates the Kaiser effect on marble specimens under cyclic uniaxial compressive as well as cyclic uniaxial tensile conditions. The tensile behavior was studied by means of Brazilian tests. Each specimen was tested by applying the load in four loading cycles having magnitudes of 40%, 60%, 80% and 100% of the peak stress. The experimental results confirm the presence of Kaiser effect in marble specimens under both compressive and tensile loading conditions. Kaiser effect was found to be more dominant in the first two loading cycles and started disappearing as the applied stress approached the peak stress, where felicity effect became dominant instead. This behavior was observed to be consistent under both compressive and tensile loading conditions and can be applied for the estimation of in-situ rock stresses as a function of peak rock stress. At a micromechanical level, Kaiser effect is evident when the pre-existing stress is smaller than the crack damage stress and ambiguous when pre-existing stress exceeds the crack damage stress. Upon reaching the crack damage stress, the cracks begin to propagate and coalesce in an unstable manner. Hence acoustic emission observations through Kaiser effect analysis can help to estimate the crack damage stresses reliably thereby improving the efficiency of design parameters.