• Transactions of the Korean Society of Mechanical Engineers A
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• v.41 no.2
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• pp.129-136
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• 2017

This study aims to assess the damage tolerance of the wing structure of aged aircraft with long-term service through the fatigue crack growth analysis and tests. For the fatigue-critical locations (FCL) W2 and W4 in the wing structure, the fatigue stress spectrum was derived based on a previous study. Thereafter, a crack propagation analysis for the FCLs was conducted using the commercial software $NASGRO^{TM}$. The algorithm for the fatigue stress spectrum was verified. Fatigue crack growth tests were then performed for two types of specimens: Type #1 was extracted from the wing structure of aged aircraft, and Type #2 was made of the same material as the wing structure. By comparing the experimental results of these specimens, we assessed the damage tolerance of the wing structure of aged aircraft with service time.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.3 s.234
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• pp.462-469
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• 2005

Single phase bainite structure which is obtained by the conventional austempering treatment reduces the ductility of ductile cast iron. Because of the reduction of ductility it is possible to worsen the fatigue properties. Therefore, semi austempered ductile iron which is treated from ${\alpha}+{\gamma}$ is prepared to investigate the static strength and fatigue properties in comparison with fully austempered ductile iron (is treated from ${\gamma}$). In spite of semi austempered ductile iron shows the $86{\%}$ increase of ductility. Also, semi austempered ductile iron shows the higher fatigue limit and lower fatigue crack growth rate as compared with fully austempered ductile iron. By the fractographical analysis, it is revealed that the ferrite obtained by semi austempering process brings about the plastic deformation(ductile striation) of crack tip and gives the prior path of crack propagation. The relatively low crack growth rate in semi austempered specimen is caused by above fractographical reasons.

• Smart Structures and Systems
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• v.6 no.4
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• pp.349-362
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• 2010

Ultrasonic Guided Waves (UGWs) are a useful tool in structural health monitoring (SHM) applications that can benefit from built-in transduction, moderately large inspection ranges and high sensitivity to small flaws. This paper describes a SHM method based on UGWs, discrete wavelet transform (DWT), and principal component analysis (PCA) able to detect and quantify the onset and propagation of fatigue cracks in structural waveguides. The method combines the advantages of guided wave signals processed through the DWT with the outcomes of selecting defect-sensitive features to perform a multivariate diagnosis of damage. This diagnosis is based on the PCA. The framework presented in this paper is applied to the detection of fatigue cracks in a steel beam. The probing hardware consists of a PXI platform that controls the generation and measurement of the ultrasonic signals by means of piezoelectric transducers made of Lead Zirconate Titanate. Although the approach is demonstrated in a beam test, it is argued that the proposed method is general and applicable to any structure that can sustain the propagation of UGWs.

• Smart Structures and Systems
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• v.18 no.3
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• pp.569-584
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• 2016

This paper presents a method for stochastic modelling of fatigue crack growth and optimising inspection and maintenance strategy for the structural members of steel bridges. The fatigue crack evolution is considered as a stochastic process with uncertainties, and the Gamma process is adopted to simulate the propagation of fatigue crack in steel bridge members. From the stochastic modelling for fatigue crack growth, the probability of failure caused by fatigue is predicted over the service life of steel bridge members. The remaining fatigue life of steel bridge members is determined by comparing the fatigue crack length with its predetermined threshold. Furthermore, the probability of detection is adopted to consider the uncertainties in detecting fatigue crack by using existing damage detection techniques. A multi-objective optimisation problem is proposed and solved by a genetic algorithm to determine the optimised inspection and maintenance strategy for the fatigue affected steel bridge members. The optimised strategy is achieved by minimizing the life-cycle cost, including the inspection, maintenance and failure costs, and maximizing the service life after necessary intervention. The number of intervention during the service life is also taken into account to investigate the relationship between the service life and the cost for maintenance. The results from numerical examples show that the proposed method can provide a useful approach for cost-effective inspection and maintenance strategy for fatigue affected steel bridges.

• Structural Engineering and Mechanics
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• v.6 no.4
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• pp.467-471
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• 1998

A mathematical model is developed to predict the fatigue notch factor of butt welds subject to number of parameters such as weld geometry, residual stresses under dynamic combined loading conditions (tensile and bending). Linear elastic fracture mechanics, finite element analysis, dimensional analysis and superposition approaches are used for the modelling. The predicted results are in good agreement with the available experimental data. As a result, scatters of the fatigue data can be significantly reduced by plotting S-N curve as ($S{\cdot}K_f$) vs. N.

• Journal of Welding and Joining
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• v.21 no.6
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• pp.71-76
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• 2003

For the purpose of clarifying the influence of welding residual stress to the fatigue crack propagations behaviour, an analytical investigation based on finite element method is performed to examine the opening behaviour of tip-closed crack in the compressive residual stress. A finite element model comprised of contact elements for the crack plane and plane stress elements for the base material is used to evaluate crack opening stress of the crack existing in the residual stress field. Also an analytical method based on the superposition principle to estimate the length of opened part of tip closed crack and the stress distribution adjacent to the crack during uploading is applied to the finite element model. The software for the analysis is ABAQUS, which is a general purpose finite element package. The results show that stresses distributed on the crack surfaces are reduced and approached to zero as the applied stresses are increased up to crack tip opening stress and no mechanical discontinuity is found at the boundary of contact elements and plane stress elements. It is verified that the opening behavior of the fatigue crack in the residual stress can be predicted by finite element method with the proposed analytical method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.3
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• pp.528-533
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• 1997

In the analysis of fatigue crack propagation behavior, the crack length is one of the most important factors. In the test of crack propagation, compliance method is widely used to detect crack length. The measurement of surface crack length is not so easy with compliance method. In this study, the image processing technique was applied to measure the surface crack length. CCD(Charge-coupled device) camera was used to observe the crack image and the computer program to detect crack length from stored crack image was developed. CCT(Center Cracked Tension) specimen was used to compare the compliance method with the image processing technique. The crack length which detected by the image processing techniques was found to be well consistent with that from the optical measurement.

• Journal of the Korean Society for Heat Treatment
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• v.14 no.4
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• pp.212-219
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• 2001

The fatigue test was carried out to evaluate the fatigue characteristics of fire resistance steel for frame structure and heat affected zone (HAZ) by the one side Gas Metal Arc Welding (GMAW). In this paper, the fatigue crack growth behavior was investigated with the compact tension specimen of base metal and the HAZ according to chemical composition and rolling end temperature, respectively. And the acoustic emission signals obtained from the fatigue test were analyzed by the time-frequency analysis method as a nondestructive evaluation. Main results obtained are summarized as follows; The hardness was appeared softening phenomenon that weld metal and HAZ are lower than that of base metal. Fatigue life of welded specimen was longer than that of base metal. m was 3~4.5 in base metal and 3.8~5.8 in HAZ. The main frequency range of acoustic emission signal analyzed from time-frequency method is different with the range by noise and crack. Also, it could be classified that it was also generated by fracture mechanics of dimple, inclusion etc.

• Proceedings of the Korea Concrete Institute Conference
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• 1992.04a
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• pp.126-130
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• 1992

Recently, fatigue problem become a critical issue in the design of prestressed concrete bridges due to the increase of traffic volumes and use of high-strength materials. Most existing studies are mainly concerned with the fatigue behavior of component materials only such as concrete, reinforcing bars, and prestressing steels and few studies exist that deals with the fatigue behavior of bridge members. An improved analytic formulation for both uncracked and cracked prestressed concrete composite section with cyclic creep effect is developed to take into account the change of neutral axis with crack propagation. The procedure also enables to investigate serviceability limit states, deflection and crack width. The present study allows more realistic analysis and design of prestressed concrete composite girder bridges under fatigue loadings.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 1992.04a
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• pp.30-35
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• 1992

Since many design problems in the railroad, aerospace and machine structures involve considerations of the effect of cyclic loading, manufacturing and quality control processes must fully account for fatigue of critical components. Due to the sensitivity of the Paris law, it is very important to calculate the ΔK numerically to minimize the error of predicted fatigue life in cycles. It is shown that the p-version of FEM based on LEFM analysis is far better suited for computing the stress intensity factors than the conventional h-version. To demonstrate the proficiency of the proposed scheme, the welded T-joint with crack problem of box car body bolster assembly and a crack problem emanating Iron a circular hole in finite strip have been solved.