• Transactions of the Korean Society of Mechanical Engineers A
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• v.25 no.5
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• pp.823-833
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• 2001

In recent years, the subject of remaining life assessment has drawn considerable attention in chemical plants, where various structural components typically operate at high temperature an pressure. Thus a life prediction methodology accounting for high temperature creep fracture is increasingly needed for the components. Critical defects in such structures are generally found in the form of semi-elliptical surface crack, and the analysis of which is consequently an important problem in engineering fracture mechanics. On this background, we first develop an auto mesh generation program for detailed 3-D finite element analyses of axial and circumferential semi-elliptical surface cracks in a piping system. A high temperature creep fracture parameter C-integral is obtained from the finite element analyses of generated 3-D models. Post crack growth module is further appended here to calculate the amount of crack growth. Finally the remaining lives of surface cracked pipes for various analytical parameters are assessed using the developed life assessment program.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.27 no.8
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• pp.1388-1397
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• 2003

Since the reliability of adhesively bonded joints for composite structures is dependent on many parameters such as the shape and dimensions of joints, type of applied load, and environment, so an accurate estimation of the fatigue life of adhesively bonded joints is seldom possible, which necessitates an in-situ reliability monitoring of the joints during the operation of structures. In this study, a self-sensor method for adhesively bonded joints was devised, in which the adhesive used works as a piezoelectric material to send changing signals depending on the integrity of the joint. From the investigation, it was found that the electric charge increased gradually as cracks initiated and propagated in the adhesive layer, and had its maximum value when the adhesively bonded joint failed. So it is feasible to monitor the integrity of the joint during its lifetime. Finally, a relationship between the piezoelectric property of the adhesive and crack propagation was obtained from the experimental results.

• Journal of the korean Society of Automotive Engineers
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• v.15 no.2
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• pp.112-120
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• 1993

Variable thickness plates are commonly encountered in the majority of mechanical/structural components of industrial applications. And, as a result of the unsymmetry of the structure or the load and the anisoptropy of the materials, the cracks in engineering structures are generally subjected to combined stresses. In spite of considerable practical interest, however, a few fracture mechanics study on combined mode crack in a variable thickness plate have carried out. In this respect, combined mode 1/3 stress intensity factors $K_{1}$ and $K_{3}$ at the crack tip for a variable thickness plate were obtained by 3-dimensional finite element analysis. Variable thickness plates containing a central slant crack were chosen. the parameters used in this study were dimensionless crack length .lambda. crack slant angle .alpha, thickness ratio .betha. and width ratio .omega. Stress intensity factors were calculated by crack opening displacement(COD) and crack tearing displacement(CTD) method proposed by Ingraffea and Manu. The effect of thickness ratio .betha. on $K_{1}$ is relatively great in comparison to $K_{3}$.

• Journal of the Korean Society for Precision Engineering
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• v.18 no.10
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• pp.154-160
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• 2001

For major structural components periodic inspections and integrity assessments are needed for the safety. However, many flaws are undetectable because sampling inspection is carried out during in-service inspection. Probabilistic integrity assessment is applied to take into consideration of uncertainty and variance of input parameters arise due to material properties and undetectable cracks. This paper describes a Probabilistic Fracture Mechanics(PFM) analysis based on the Monte Carlo(MC) algorithms. Taking a number of sampling data of probabilistic variables such as fracture toughness value, crack depth and aspect ratio of an initial surface crack, a MC simulation of failure judgement of samples is performed. for the verification of this analysis, a comparison study of the PFM analysis using a commercial code, mathematical method is carried out and a good agreement was observed between those results.

• Journal of KSNVE
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• v.9 no.5
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• pp.1019-1028
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• 1999

Free vibration characteristics of cantilevered laminated composite beams with a transverse non0propagating open carck are investigated. In the present analysis a special ply-angle distribution referred to as asymmetric stiffness configuration inducing the elastic coupling between chord-wise bending and extension is considered. The open crack is modelled as an equivalent rotational spring whose spring constant is calculated on the basis of fracture mechanics of composite material structures. Governing equations of a composite beam with a open crack are derived via Hamilton's Principle and Timoshenko beam theory encompassing transverse shear and rotary inertia effect. the effects of various parameters such as the ply angle, fiber volume fraction, crack depth, crack position and transverse shear on the free vibration characteristics of the beam with a crack is highlighted. The numerical results show that the natural frequencies obtained from Timoshenko beam theory are always lower than those from Euler beam theory. The presence of intrinsic cracks in anisotropic composite beams modifies the flexibility and in turn free vibration characteristics of the structures. It is revealed that non-destructive crack detection is possible by analyzing the free vibration responses of a cracked beam.

• Structural Engineering and Mechanics
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• v.75 no.5
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• pp.595-605
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• 2020

In this study, the flexural behaviors of full-scale prestressed concrete box girders are experimentally investigated. Four girders were fabricated using two types of concrete (compressive strengths: 50 MPa and 70 MPa) and tested under four-point bending until failure. The measured parameters included the deflection, the stress and strain in concrete and steel bars, and cracks in concrete. The measurement results were used to analyze the failure mode, load-bearing capacity, and deformability of each girder. A finite element model is established to simulate the flexural behaviors of the girders. The results show that the use of high-performance concrete and reasonable combination of prestressed tendons could improve the mechanical performance of the box girders, in terms of the crack resistance, load-carrying capacity, stress distribution, and ductility.

• Smart Structures and Systems
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• v.15 no.5
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• pp.1233-1252
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• 2015

This study investigates the problem of crack detection in post-buckled beam-type structures. The beam under the axial compressive force has a crack, assumed to be open and through the width. The crack, which is modeled by a massless rotational spring, divides the beam into two segments. The crack detection is considered as an optimization problem, and the weighted sum of the squared errors between the measured and computed natural frequencies is minimized by the bees algorithm. To find the natural frequencies, the governing nonlinear equations of motion for the post-buckled state are first derived. The solution of the nonlinear differential equations of the two segments consists of static and dynamic parts. The differential quadrature method along with an arc length strategy is used to solve the static part, while the same method is utilized for the solution of the linearized dynamic part and the extraction of the natural frequencies of the cracked beam. The investigation includes several numerical as well as experimental case studies on the post-buckled simply supported and clamped-clamped beams having open cracks. The results show that several parameters such as the amount of applied compressive force and boundary conditions influences the outcome of the crack detection scheme. The identification results also show that the crack position and depth can be predicted well by the presented method.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2009.05a
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• pp.401-404
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• 2009

The purpose of this study is investigating the joint characteristics of advanced high strength steel DP780 and Al5052 alloy sheet in the clinching process. It is difficult to join the advanced high strength steel with light-weight materials like aluminum alloy, because of low formability of DP780. The defects of clinching joint such as necking of the upper sheet, cracks of the lower sheet and no interlocking were occurred by different ductility between advanced high strength steel and aluminum alloy. The clinching conditions should be optimized to interlock without any defects. In this study, the effect of process parameters of clinching process on joinability of advanced high strength steel with Al5052 alloy was investigated by using FE-analysis. From the result of FE-analysis, the clearance between clinching punch and die, die depth and the shape of die cavity mainly affected the joinability of advanced high strength steel with Al5052 alloy.

• Korean Journal of Computational Design and Engineering
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• v.19 no.1
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• pp.12-18
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• 2014

Shot-peening is frequently used on various mechanical parts because it can improve the fatigue life of components by generating compressive residual stresses on the surface. This can be done by repeatedly hitting the work-piece surface with small balls and making indentations on it. In fact, finding optimal peening time among various peening parameters is the most important. Under-peening can not improve the fatigue life sufficiently while over-peening causes cracks and reduces fatigue life in contrast. In general, optimal peening time is experimentally determined by measuring arc-height using Almen-strip in accordance with SAE J442 standard. To save the time and efforts spent in carrying out experiments to find optimal peening time, this paper presents a computer simulation algorithm for the estimation of surface coverage made by impeller type shot-peening machines (PMI-0608). Surface coverage is defined as the proportion of the work-piece surface that has been indented in a given time of shot-peening. An example (standard tensile test specimen) is presented to validate the proposed method.

• Journal of the Korean Ceramic Society
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• v.53 no.6
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• pp.628-634
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• 2016

The 8 wt% yttria($Y_2O_3$) stabilized zirconia ($ZrO_2$), 8YSZ, a typical thermal barrier coating (TBC) for turbine systems, was fabricated under different starting powder conditions and coating parameters by atmospheric plasma spray (APS) coating process. Four different starting powders were prepared by conventional spray dry method with different additive and process parameter conditions. As a result, large- and small-size spherical-type particles and Donut-type particles were obtained. Dense structure of 8YSZ coating was produced when small size spherical-type or Donut-type particles were used. On the other hand, 8YSZ coating with a porous structure was formed from large-size spherical-type particles. Furthermore, a segmented coating structure with vertical cracks was observed after post heat treatment on the surface of dense structured coating by argon plasma flame at an appropriate gun distance and power condition.