• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.10a
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• pp.227-230
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• 2009

In order to predict failure behavior of advanced high-strength steel sheets (AHSS) in hole expansion tests, damage model was developed considering surface condition sensitivity (with specimens prepared by milling and punching: 340R, TRIP590, TWIP940). To account for the micro-damage initiation and evolution as well as macro-crack formation, the stress triaxiality dependent fracture criterion and rate-dependent hardening and ultimate softening behavior were characterized by performing numerical simulations and experiments for the simple tension and V-notch tests. The developed damage model and the characterized mechanical property were incorporated into the FE program ABAQUS/Explicit to perform hole expansion simulations, which showed good agreement with experiments.

• Journal of Welding and Joining
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• v.26 no.3
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• pp.51-60
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• 2008

This paper is to evaluate fracture characteristics of API 2W Gr.50 TMCP steel weldment typically applied for offshore structures, with the focus on the influence of heat input arising from flux cored arc welding. Based on the results and insights developed from this study, it is found that the toughness for both CTOD and impact exhibits a tendency to decrease as the weld heat input increases. The reheated zone of weldmetal exhibit lower hardness than solidified zone and microstructure that are liable to affect the toughness are acicular ferrite and martensite-austenite constituents (M-A). In particular, M-A is a more effective micro-phase for CTOD toughness than impact toughness.

• International Journal of Concrete Structures and Materials
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• v.4 no.1
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• pp.37-43
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• 2010

Theoretical models based on modern interpretations of the morphology and interactions of cement hydration products are developed for prediction of the mechanical properties of hydrated cement paste (hcp). The models are based on the emerging nanostructural vision of calcium silicate hydrate (C-S-H) morphology, and account for the intermolecular interactions between nano-scale calcium C-S-H particles. The models also incorporate the effects of capillary porosity and microcracking within hydrated cement paste. The intrinsic modulus of elasticity and tensile strength of hydrated cement paste are determined based on intermolecular interactions between C-S-H nano-particles. Modeling of fracture toughness indicates that frictional pull-out of the micro-scale calcium hydroxide (CH) platelets makes major contributions to the fracture energy of hcp. A tensile strength model was developed for hcp based on the linear elastic fracture mechanics theories. The predicted theoretical models are in reasonable agreements with empirical models developed based on the experimental performance of hcp.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1995.10a
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• pp.14-18
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• 1995

Although structural ceramics have excellent mectanical properties, it is very difficult to grind with high efficiency and high quality because of their high strength, hardness, and brittleness. Unfortunately machined ceramics often contain surface damages such as micro fracture and crack on account of brittle fracture. Therefore, is is important to minimize the brittle fracture. The present paper examines grinding characteristics of representative structural ceramics,such as Al /sab 2/O /sab 3/, SiC, Si /sab 3/ N /sab 4/. Effects of grinding variables including table speed and depth of cut on the grinding performance were investigated. Experimental results show that the surface quality is related to the specific grindings energy. The higher specific energy results in the better surface quality.

• Proceedings of the Korean Society of Machine Tool Engineers Conference
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• 1998.03a
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• pp.287-292
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• 1998

In this study behavior of fatigue crack and fatigue fracture is observed under rotary bending fatigue testing in friction welded dissimilar materials. Fatigue fracture most occurred in SM15C heat affected zone around Interface. In case of fatigue test, stress is reduced the position of fracture gradually moves to the welded Interface. Micro crack of heat affected zone surface on SM15C is observed at any different stress.

• Transactions of the Korean Society of Mechanical Engineers
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• v.6 no.3
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• pp.239-246
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• 1982

The effect of the second phase morphology on the fracture ductility of dual phase steel was studied by means of tensile tests carried out room temperature. In this case the second phase morphology is characterized by two kinds; one is the MEF microstructure in which martensite encapsulated islands of ferrite, the other is the FEM microstructure in which ferrite encapsulated islands of martensite. The fracture ductility is improved by variation of the second phase morphology, but is essentially uneffected in the range of high strength ratio (4.7). Also the variation of ductility is well understood according to the difficulty of cleavage crack formation of the ferrite grain and to the brittleness of the martensitic structure.

• The Korean Journal of Ceramics
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• v.1 no.4
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• pp.197-203
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• 1995

Nearly fully dense PZT samples both with tetragonal and with morphotropic phase boundary compositions were prepared by the conventional powder processing and sintering. A micro-indentation technique was used to evaluate the dependence of fracture toughness on remanent polarization, crack length and the direction of crack propagation. The result shows that the toughness increases with the remanent polarization along the poling direction and decreases in the transverse direction. The dependence of toughness on the remanent polarization is neither symmetric nor linear but rather shown to be saturated quickly with the increase in remanent polariztion. R-curve behaviors are observed in both poled and unpoled samples. Sequential SEM and XRD studies on annealed, poled, ground, fractured and etched samples show that domain switching is evident as a viable toughening mechanism but might depend upon the rate of crack propagation. Grain bridging is also observed as one of the active toughening mechanisms.

• Advances in materials Research
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• v.3 no.3
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• pp.151-161
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• 2014

Geopolymer composites reinforced with different layers of woven flax fabric are fabricated using lay- up technique. Mechanical properties, such as flexural strength, flexural modulus and fracture toughness of geopolymer composites reinforced with 2.4, 3 and 4.1 wt% flax fibres are studied. The fracture surfaces of the composites are also examined using scanning electron microscopy. The results show that all the mechanical properties of the composites are improved by increasing the flax fibre contents. It is also found that the mechanical properties of flax fabric reinforced geopolymer composites are superior to pure geopolymer matrix. Micro-structural analysis of fracture surface of the composites indicated evidence of various toughening mechanisms by flax fabrics in the composites.

• Smart Structures and Systems
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• v.2 no.4
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• pp.357-363
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• 2006

Equations describing deformation defects, damage accumulation, and fracture condition have been suggested. Analytical and numerical solutions have been obtained for defects produced by a shear in a fixed direction. Under cyclic loading the number of cycles to failure well fits the empirical Koffin-Manson law. The developed model is expanded to the case of the micro-plastic deformation, which accompanies martensite accommodation in shape memory alloys. Damage of a shape memory specimen has been calculated for two regimes of loading: a constant stress and cyclic variation of temperature across the interval of martensitic transformations, and at a constant temperature corresponding to the pseudoelastic state and cyclic variation of stress. The obtained results are in a good qualitative agreement with available experimental data.

• Geomechanics and Engineering
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• v.38 no.1
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• pp.57-68
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• 2024

Tunnels offer myriad benefits for modern countries, and understanding their behavior under loads is critical. This paper analyzes and evaluates the damage to buried horseshoe tunnels under soil pressure and traffic loading. To achieve this, a numerical model of this type of tunnel is first created using ABAQUS software. Then, fracture mechanics theory is applied to investigate the fracture and damage of the horseshoe tunnel. The numerical analysis is based on the damage plasticity model of concrete, which describes the inelastic behavior of concrete in tension and compression. In addition, the reinforcing steel is modeled using the bilinear plasticity model. Damage contours, stress contours, and maximum displacements illustrate how and where traffic loading alters the response of the horseshoe tunnel. Based on the results, the fracture mechanism proceeded as follows: initially, damage started at the center of the tunnel bottom, followed by the formation of damage and micro-cracks at the corners of the tunnel. Eventually, the damage reached the top of the concrete arch with increasing loading. Therefore, in the design of this tunnel, these critical areas should be reinforced more to prevent cracking.