• Steel and Composite Structures
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• v.53 no.2
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• pp.169-194
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• 2024

Although the excellent characteristics of functionally graded materials (FGMs) cracks could be found due to manufacturing defects or extreme working conditions. The existence of these cracks may threaten the material or structural strength, reliability, and lifetime. Due to high cost and restrictions offered by practical operational features these cracked components couldn't be replaced immediately. Such circumstances lead to the requirement of assessing the dynamic performance of cracked functionally graded structural components especially under moving objects. The present study aims to comprehensively investigate the dynamic behavior of functionally graded cracked Timoshenko beams (FGCTBs) resting on Winkler foundation and subjected to moving load through shear locking free finite elements methodology. The through thickness material distribution is simulated by the exponential gradation law. The geometric discontinuity due to cracks is represented using the massless rotational spring approach. The shear locking phenomena is avoided by using the different interpolation functions orders for both deflections and rotations. Based on Timoshenko beam element, a shear locking free finite elements methodology is developed. The unconditionally stable Newmark procedure is employed to solve the forced vibration problem. Accuracy of the developed procedure is verified by comparing the obtained results with the available results and an excellent agreement is found. Parametric studies are conducted to explore effects of the geometrical, material characteristics, crack geometrical characteristics, the elastic foundation parameter, and the moving load speed on the dynamic behavior for different boundary conditions. Obtained results revealed the significant effect these parameters on the dynamic performance of FGCTBs.

• Explosives and Blasting
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• v.23 no.4
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• pp.31-44
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• 2005

In highly stressed conditions, the excavation damage zone induced by stress redistribution and disturbance must be evaluated after tunnel excavation. Therefore, the investigation of stress-induced deformation and fracture in rock is indispensable. In this study, fracture and damage mechanisms of rock induced by the accumulation of microcracks were investigated by the moving point regression technique as well as acoustic emission measured during uniaxial compression tests. Especially, the modified procedures to determine damage thresholds more systematically were newly proposed, and successfully applied to rock. From experiments, crack initiation and track damage stress levels were estimated to be $33{\~}36\%$ and $84{\~}89\%$ of uniaxial compressive strength respectively, for both of Hwangdeung granite and Yeosan marble. However, the normalized crack closure stress level for Yeosan marble was much higher than for Hwangdeung granite. In addition, the largest proportion of total axial strain in Hwangdeung granite was attributable to elastic deformation and initial microcracking. However, the greatest part of axial deformation in Yeosan marble arose from initial crack closure and unstable cracking. Finally, it was seen that unstable cracking after the crack damage stress level played a key part in the lateral deformation in rocks under uniaxial compression.

• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.3
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• pp.259-267
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• 2004

A finite element method is used to simulate interaction of laser-based ultrasounds with surface breaking tracks in elastic media. The laser line source focused on the surface of semi-infinite medium is modeled as a shear dipole in 2-D plane strain finite elements. The shear dipole-finite clement model is found to give correct directivity patterns for generated longitudinal and shear waves. The interaction of surface waves with surface breaking cracks (2-D machined slot) is considered in two ways. Both the source and receiver are fixed with respect to the cracks in the first case, while the source is moving in another case. It is shown that the crack depth tested in the range of 0.3-5.0mm $({\lambda}_R/d=0.21{\sim}3.45)$ can be measured using the corner reflected waves produced by the fixed laser source. The moving laser source is found to cause a large amplitude change of reflected waves near crack, and the crack whose depth is one order lower than the wavelength ran be detected from this change.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.29 no.6
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• pp.583-590
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• 2016

This paper presents a dynamic crack propagation algorithm with Rayleigh damping effect based on the MLS(Moving Least Squares) Difference Method. Dynamic equilibrium equation and constitutive equation are derived by considering Rayliegh damping and governing equations are discretized by the MLS derivative approximation; the proportional damping, which has not been properly treated in the conventional strong formulations, was implemented in both the equilibrium equation and constitutive equation. Dynamic equilibrium equation including time relevant terms is integrated by the Central Difference Method and the discrete equations are simplified by lagging the velocity one step behind. A geometrical feature of crack is modeled by imposing the traction-free condition onto the nodes placed at crack surfaces and the effect of movement and addition of the nodes at every time step due to crack growth is appropriately reflected on the construction of total system. The robustness of the proposed numerical algorithm was proved by simulating single and multiple crack growth problems and the effect of proportional damping on the dynamic crack propagation analysis was effectively demonstrated.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.22 no.4
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• pp.335-341
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• 2009

In this paper, review of developed MLS-based finite elements and a proposal for their applications are described. The shape functions and their derivatives of MLS-based finite elements are constructed using Moving-Least Square approximation. In MLS-based finite element, using the adequate influence domain of weight function used in MLS approximation, kronecker delta condition could be satisfied at the element boundary. Moreover, because of the characteristics of MLS approximation, we could easily add extra nodes at an arbitrary position in MLS-based finite elements. For these reasons, until now, several variable-node elements(2D variable element for linear case and quadratic case and 3D variable-node elements) and finite crack elements are developed using MLS-based finite elements concept. MLS-based finite elements could be extended to 2D variable-node triangle element, 2D finite crack triangle element, variable-node shell element, finite crack shell element, and 3D polyhedron element. In this paper, we showed the feasibility of 3D polyhedron element at the case of femur meshing.

• Journal of Institute of Control, Robotics and Systems
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• v.15 no.9
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• pp.877-880
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• 2009

A curing process in post-printing section of R2R process is required for an electrical property of the printed pattern when devices such as RFID, Solar cell are printed. PEN as well as heat-stabilized PET which is used as a plastic substrate would be deformed at high temperature due to change of its elastic modulus. And crack in the printed pattern, which is on the plastic substrate is occurred due to the deformation of the substrate. The occurrence of crack causes electrical resistance to increase and the quality of the device to deteriorate. In case of RFID antenna, the range of reading distance is shortened as the electrical resistance of the antenna is increased. Therefore, the deformation of the plastic substrate, which causes the occurrence of crack, should be minimized by setting up low operating tension in R2R process. In low tension, slippage between a moving substrate and a roller would be generated when the operating speed is increased. And scratch would be occurred when slippage is generated due to an air entrainment, which is related to the thickness of the air film. The thickness of the air film is increased when operating speed is increased as shown by simulation based on mathematical model. The occurrence of scratch in conductive pattern printed by roll to roll process is a critical damage because it causes degradation or failure of electrical property of it.

• Journal of the Computational Structural Engineering Institute of Korea
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• v.24 no.5
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• pp.577-588
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• 2011

This paper provides a novel extended Moving Least Squares(MLS) difference method for the potential problem with weak and strong discontinuities. The conventional MLS difference method is enhanced with jump functions such as step function, wedge function and scissors function to model discontinuities in the solution and the derivative fields. When discretizing the governing equations, additional unknowns are not yielded because the jump functions are decided from the known interface condition. The Poisson type PDE's are discretized by the difference equations constructed on nodes. The system of equations built up by assembling the difference equations are directly solved, which is very efficient. Numerical examples show the excellence of the proposed numerical method. The method is expected to be applied to various discontinuity related problems such as crack problem, moving boundary problem and interaction problems.

• Structural Engineering and Mechanics
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• v.75 no.1
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• pp.19-32
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• 2020

Due to repetitive traffic loadings and environmental attacks, reinforced concrete (RC) bridge deck slabs are suffering from severe degradation, which makes structural repairing an urgency. In this study, the fatigue performance of an RC bridge deck repairing technique using ultra-high performance fiber reinforcement concrete (UHPFRC) overlay is assessed experimentally with a wheel-type loading set-up as well as analytically based on finite element method (FEM) using a crack bridging degradation concept. In both approaches, an original RC slab is firstly preloaded to achieve a partly damaged RC slab which is then repaired with UHPFRC overlay and reloaded. The results indicate that the developed analytical method can predict the experimental fatigue behaviors including displacement evolutions and crack patterns reasonably well. In addition, as the shear stress in the concrete/UHPFRC interface stays relatively low over the calculations, this interface can be simply simulated as perfect. Moreover, superior to the experiments, the numerical method provides fatigue behaviors of not only the repaired but also the unrepaired RC slabs. Due to the high strengths and cracking resistance of UHPFRC, the repaired slab exhibited a decelerated deterioration rate and an extended fatigue life compared with the unrepaired slab. Therefore, the proposed repairing scheme can afford significant strengthen effects and act as a reference for future practices and engineering applications.

• Steel and Composite Structures
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• v.25 no.1
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• pp.57-65
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• 2017

The fatigue fracture of studs is the main reason for failure of composite beams based on massive engineering practices. Hence, studying the laws of cracks initiation and propagation are of great directive significance. eXtended Finite Element Method (XFEM) is an effective method in solving moving discontinuous problems in recent years. This paper extends our recent work on the fatigue damage analysis of stud shear connectors in the steel and crumble rubber concrete (RRFC) composite beams based on XFEM. The process of crack initiation to failure of the stud is simulated and an effective calculation criteria for the fatigue life of the composite beams is put forward. After the reliability of the numerical analysis is verified based on tests results, the extensive parametric study is conducted concerning effects of different rubber contents, shear connection degrees and the stress amplitudes. Results show that with the increasing rubber contents and shear connection degrees, the fatigue lives of composite beams increase obviously. Furthermore, the relationship between the fatigue life of the stud at the edge of the shear span and the whole composite beams is studied. Finally, the S-N curves of the single stud and the whole composite beams are put forward based on XFEM.

• Structural Engineering and Mechanics
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• v.58 no.2
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• pp.199-216
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• 2016

A large number of bridges were built several decades ago, and most of which have gradually suffered serious deteriorations or damage due to the increasing traffic loads, environmental effects, and inadequate maintenance. However, very few studies were conducted to investigate the vibration behaviors of a damaged bridge under moving vehicles. In this paper, the vibration behaviors of such vehicle-bridge system are investigated in details, in which the effects of the concrete cracks and bridge surface roughness are particularly considered. Specifically, two vehicle models are introduced, i.e., a simplified four degree-of-freedoms (DOFs) vehicle model and a more complex seven DOFs vehicle model, respectively. The bridges are modeled in two types, including a single-span uniform beam and a full scale reinforced concrete high-pier bridge, respectively. The crack zone in the reinforced concrete bridge is considered by a damage function. The bridge and vehicle coupled equations are established by combining the equations of motion of both the bridge and vehicles using the displacement relationship and interaction force relationship at the contact points between the tires and bridge. The numerical simulations and verifications show that the proposed modeling method can rationally simulate the vibration behaviors of the damaged bridge under moving vehicles; the effect of cracks on the impact factors is very small and can be neglected for the bridge with none roughness, however, the effect of cracks on the impact factors is very significant and cannot be neglected for the bridge with roughness.