• Restorative Dentistry and Endodontics
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• v.31 no.2
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• pp.96-102
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• 2006

There are various factors affecting the fracture of NiTi rotary files. This study was performed to evaluate the effect of cross sectional area, pecking motion and pecking distance on the cyclic fatigue fracture of different NiTi files. Five different NiTi $files-Profile^{(R)}$ (Maillefer, Ballaigue, Switzerland), $ProTaper^{TM}$(Maillefer, Ballaigue, Switzerland), $K3^{(R)}$ (SybronEndo. Orange, CA) , Hero $642^{(R)}$ (Micro-mega, Besancon, France), Hero $Shaper^{(R)}$ (Micro-mega, Besancon, France)-were used. Each file was embedded in temporary resin, sectioned horizontally and observed with scanning electron microscope. The ratio of cross-sectional area to the circumscribed circle was calculated. Special device was fabricated to simulate the cyclic fatigue fracture of NiTi file in the curved canal,. On this device, NiTi files were rotated (300rpm) with different pecking distances (3 mm or 6 mm) and with different motions (static motion or dynamic pecking motion) . Time until fracture occurs was measured. The results demonstrated that cross-sectional area didn't have any effect on the time of file fracture. Among the files, $Profile^{(R)}$ took the longest time to be fractured. Between the pecking motions, dynamic motion took the longer time to be fractured than static motion. There was no significant difference between the pecking distances with dynamic motion, however with static motion, the longer time was taken at 3mm distance. In this study, we could suggest that dynamic pecking motion would lengthen the time for NiTi file to be fractured from cyclic fatigue.

• Geomechanics and Engineering
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• v.32 no.4
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• pp.375-385
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• 2023

There are many joint fissures distributed in the engineering rock mass. In the process of geological history, the underground rock mass undergoes strong geological processes, and undergoes complex geological processes such as fracture breeding, expansion, recementation, and re-expansion. In this paper, the damage-stick-slip process (DSSP), an analysis model used for rock mass failure slip, was established to examine the master control and time-dependent mechanical properties of the new and primary fractures of a multi-fractured rock mass under the action of stress loading. The experimental system for the recemented multi-fractured rock mass was developed to validate the above theory. First, a rock mass failure test was conducted. Then, the failure stress state was kept constant, and the fractured rock mass was grouted and cemented. A secondary loading was applied until the grouted mass reached the intended strength to investigate the bearing capacity of the recemented multi-fractured rock mass, and an acoustic emission (AE) system was used to monitor AE events and the update of damage energy. The results show that the initial fracture angle and direction had a significant effect on the re-failure process of the cement rock mass; Compared with the monitoring results of the acoustic emission (AE) measurements, the master control surface, key blocks and other control factors in the multi-fractured rock mass were obtained; The triangular shaped block in rock mass plays an important role in the stress and displacement change of multi-fracture rock mass and the long fissure and the fractures with close fracture tip are easier to activate, and the position where the longer fractures intersect with the smaller fractures is easier to generate new fractures. The results are of great significance to a multi-block structure, which affects the safety of underground coal mining.

• Geomechanics and Engineering
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• v.11 no.5
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• pp.657-670
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• 2016

The energy dissipation of coal under dynamic loads is a major issue in geomechanics and arising extensive concerns recently. In this study, dynamic loading tests of coal were conducted using a split Hopkinson pressure bar (SHPB) system, the characteristics of dynamic behavior and energy dissipation of coal were analyzed, and the mechanism of energy dissipation was discussed based on the fracture processes of coal under dynamic loads. Experimental results indicate that the energy dissipation of coal under dynamic loads has a positive linear correlation with both incident energy and dynamic compressive strength, and the correlation coefficients between incident energy, dynamic compressive strength and the energy dissipation rate are 0.74 and 0.98, respectively. Theoretical analysis demonstrates that higher level of stress leads to greater energy released during unstable crack propagation, thus resulting in larger energy dissipation rate of coal under dynamic loads. At last, a semi-empirical energy dissipation model is proposed for describing the positive relationship between dissipated energy and stress.

• Proceedings of the Korean Society For Composite Materials Conference
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• 2002.10a
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• pp.273-275
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• 2002

In this studies, curing behavior and mechanical properties of tetrafunctional epoxy resin (4EP)/ fluorine-containing epoxy resin (FEP) blend systems was investigated with 4, 4'-diaminodiphenol methane (DDM) as a curing agent. The cure activation energies $(E_a)$) were studied by Flynn-Wall-Ozawa's equation with dynamic DSC method. For the fracture toughness of the casting specimens, the critical stress intensity factor ($K_{IC}$) and the specific fracture energy ($G_{IC}$) were determined by fracture toughness test.

• Journal of Advanced Marine Engineering and Technology
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• v.12 no.1
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• pp.47-54
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• 1988

TMCP steel manufactured by controlled rolling followed by accelerated cooling process is known to have extra-ordinary mechanical properties such as tensile strength and toughness. However, there is much uncertainty about the fatigue fracture characteristics of this steel. In this paper, the fatigue fracture behaviour of the TMCP steel in base metal and weldment were inspected through the Dynamic Implant test method. Those results were quantitavely compared with those of the ordinary normalized steel of same strength level. Moreover, the effect of the diffusible hydrogen included in the welded part on the fatigue fracture behaviour were made clear. As the experimental results, the fatigue fracture characteristics of the TMCP steel in case of base metal proved out to be superior to that of the normalized steel. However, the TMCP steel weldment including the diffusible hydrogen appeared to have inferior fatigue characteristics compared with the same conditioned normalized steel weldment.

• Maxillofacial Plastic and Reconstructive Surgery
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• v.18 no.3
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• pp.470-487
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• 1996

The purpose of this study was to investigate the dynamic response of the mandible to impact and provide insight into the fracture mechanism of the mandible, by 3-dimensional finite element method. The finite element model of the mandible was developed and calculated using NASTRAN/XL (MSC co. U.S.A.) and the linear dynamic transient analysis was performed according to the impulsive force direction, force type and impulse time to the mandible. At first, the load was applied on the mandibular symphysis, body, angle and subcondylar area in the horizontal mandibular plane and the computed stress-time histories at 14 locations of the mandible were obtained. Secondly, the impulsive force was directed to the symphyseal area with changing the force magnitude and impulse time, and calculated the node displacement at 8 locations of mandible. The conclusions from from this study were as follows. 1. The appearance of impulsive energy transmission was different to the direction of impulse to the mandible. 2. The impulsive stress and deformation were larger in lingual or medial side than buccal or lateral in the mandible. 3. The velocity, appearance of energy transmission and the fracture pattern in mandible were affected rather impulse time than force. 4. The horizontal impact to the one side of mandible did not have effect on the stress and displacement of contralateral mandible. From the above results, fracture pattern in symphysis can be showed as simple or comminuted, multiple or associated in body and angle and solitary in subcondyle area.

• Proceedings of the KWS Conference
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• 2003.11a
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• pp.179-181
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• 2003

The present study focuses on the effect of geometrical discontinuity, strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on the ductile crack initiation using two-parameter criterion. Fracture initiation testing has been conducted under static and dynamic loading using circumferentially notched round-bar specimens. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal elastic-plastic dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out.

• Tunnel and Underground Space
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• v.22 no.5
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• pp.339-345
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• 2012

Information on the deformation behavior and fracture strength of rocks subjected to dynamic loadings is important to stability analyses of underground openings underground vibration due to rock blasts, earthquakes and rock bursts. In this study, Split Hopkinson Pressure Bar (SHPB) system was applied to estimate dynamic compressive and tensile fracture strengths of limestone and also examine deformation behavior of limestones under dynamic loadings. A micro-focus X-ray CT scanner was used to observe non-destructively inside the impacted limestone specimens. From the dynamic tests, it was revealed that the limestone have over 140MPa dynamic compressive strength and the strain-rate dependency of the strength. Dynamic Brazilian tensile strength of the limestone exceeds 21MPa and shows over 3 times static Brazilian tensile strength.

• Computational Structural Engineering
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• v.26 no.4
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• pp.25-30
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• 2013

• Computational Structural Engineering
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• v.26 no.4
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• pp.39-44
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• 2013