• Metals and materials international
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• v.24 no.6
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• pp.1412-1421
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• 2018

Cold-expanded austenitic high nitrogen steel (HNS) was subjected to investigate the effects of grain size on the stress-controlled high cycle fatigue (HCF) as well as the strain-controlled low cycle fatigue (LCF) properties. The austenitic HNSs with two different grain sizes (160 and $292{\mu}m$) were fabricated by the different hot forging strain. The fine-grained (FG) specimen exhibited longer LCF life and higher HCF limit than those of the coarse-grained (CG) specimen. Fatigue crack growth testing showed that crack propagation rate in the FG specimen was the same as that in the CG specimen, implying that crack propagation rate did not affect the discrepancy of LCF life and HCF limit between two cold-expanded HNSs. Therefore, it was estimated that superior LCF and HCF properties in the FG specimen resulted from the retardation of the fatigue crack initiation as compared with the CG specimen. Transmission electron microscopy showed that the effective grain size including twin boundaries are much finer in the FG specimen than that in the CG specimen, which can give favorable contributions to strengthening.

• International Journal of Highway Engineering
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• v.1 no.1
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• pp.85-96
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• 1999

This study was performed to evaluate performance of polymer-modified asphalt mixtures and specially designed reinforcement techniques against reflection cracking of the asphalt pavement overlay. Selected polymers were used for asphalt modification and polyester fiber, a polypropylene film (vinyl) and a grid were used for mixture reinforcement. Using the asphalt mixture with optimum asphalt content, a slab was made and cut into two pieces of specimen. A layer of grid or vinyl was placed at the bottom of specimen to strengthen the pavement layer against crack. A repeated loading was applied to the asphalt mixture specimen which is Placed on a cement concrete with a pseudo-crack. Crack propagation under repeated loading was monitored and effectiveness of the devised crack retarding techniques was evaluated. From the test results. a significant retardation of mode I crack progress was monitored from some of the modified and reinforced asphalt mixtures.

• Journal of Welding and Joining
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• v.14 no.2
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• pp.79-89
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• 1996

일정진폭하중과 과대하중비 2.5의 단일 인장과대하중에 의한 4140강 용접부 의 피로균열성장거동을 실온과 -45.deg.C의 저온에서 피로시험과 파면관찰을 통하여 고찰하였다. 이때, 용접부 미시조직의 영향을 평가하기 위해 모재(parent metal), 열영향부(as-welded HAZ), 열처리된 열영향부(PWHT HAZ)로 나누어 응력비 0과 0.5로 CT시험편을 이용하여 피로시험을 실시하였다. 피로균열성장거동은 재료의 미시조직과 온도변화보다는 응력비에 크게 영향을 받았으며, 단일 과대하중에 의한 피로균열성장 지연효과가 모든 재료에서 상당히 크게 나타났다. 전자현미경에 의한 피로파면 관찰 결과, 실온에서는 연성의 스트라이에이숀과 -45.deg.C에서는 의벽개파면과 같은 피로 균열성장거동을 나타내고 있다.

• Journal of Ocean Engineering and Technology
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• v.11 no.4
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• pp.49-60
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• 1997

This paper deals with the effect of coating layer on the fatigue strength of TiN coated 1Cr-1Mo-0.25V steel prepared by using the arc ion plating (AIP) process, in which it was characterized by the presence of extractor grid (ion filter). The rotary bending fatigue tests were carried out under room air conditions, and the fatigue crack initiation and growth bwhavior were observed by using plastic replica method. As experimental results, it was found that the obvious improvement of fatigue life at lower stress region was confirmed in TiN coated specimen processed with ion filter. It was also exlained that the increase of fatigue life in the case of an improved AIP process with ion filter was attributed to the retardation of crack initiation of the substrate surface due to hard coating layer, more densly formed with the reduced size and density of droplets.

• Proceedings of the KSME Conference
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• 2001.06a
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• pp.603-608
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• 2001

Recently, cold expansion of fastener holes is commonly used in the aerospace industry to increase the fatigue endurance of airframes. Cold expansion process is used as the retardation of crack initiation in the hole. This treatment leads to an improvement of fatigue behavior due to the compressive residual stresses developed on the hole surface. The residual stress profile depends on the cold expansion ratio. In the present paper, it is shown that residual stress is redistributed due to the application of cold expansion process for CT specimen. It is further shown that residual stress increases in proportion to cold expansion ratio. It is thought that crack growth rate increases as cold expansion ratio.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.4D
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• pp.387-393
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• 2010

This paper studies static crack growth of asphalt pavement using the extended finite element method (XFEM). To consider nonlinear characteristics of asphalt concrete, a viscoelastic constitutive equation using the Maxwell chain is used. And a linear cohesive crack model is used to regularize the crack. Instead of constructing the viscoelastic constitutive law from the Prony approximation of compliance and retardation time measured experimentally, we use a smooth log-power function which optimally fits experimental data and is infinitely differentiable. The partial moduli of the Maxwell chain from the log-power function make analysis easy because they change more smoothly in a more stable way than the ordinary method such as the least square method. Using the developed method, we can simulates the static crack growth test results satisfactorily.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.10
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• pp.1299-1306
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• 2011

In this study, crack-growth model parameters subjected to variable amplitude loading are estimated in the form of a probability distribution using the method of Bayesian parameter estimation. Huang's model is employed to describe the retardation and acceleration of the crack growth during the loadings. The Markov Chain Monte Carlo (MCMC) method is used to obtain samples of the parameters following the probability distribution. As the conventional MCMC method often fails to converge to the equilibrium distribution because of the increased complexity of the model under variable amplitude loading, an improved MCMC method is introduced to overcome this shortcoming, in which a marginal (PDF) is employed as a proposal density function. The model parameters are estimated on the basis of the data from several test specimens subjected to constant amplitude loading. The prediction is then made under variable amplitude loading for the same specimen, and validated by the ground-truth data using the estimated parameters.

• International Journal of Highway Engineering
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• v.7 no.2 s.24
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• pp.1-12
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• 2005

This study was conducted to evaluate effect of geogrid and fabric, which are used underneath the overlaid asphalt pavement for retarding reflection cracking by simulated laboratory test. In this study, an interlayer at the interface between old concrete pavement surface and overlaid asphalt mixture, and polymer-modifier were used as an effort of retarding reflection crack initiation and for strengthening mixture. Five products were used in preparation of asphalt concrete beam specimen which was tack coated on top of jointed concrete block. Simulated Mode I and II fracture test were conducted under wheel loading and results were compared among those products. From the test results, several material and reinforcement combinations were observed to have a significant retardation effect against reflection cracking. The most effectively strengthened pavement against reflection cracking was found to be the LDPE-modifier asphalt mixture with a grid reinforcement at the bottom.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.11
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• pp.2383-2389
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• 2002

This paper describes an experimental study on fatigue life extension by using cold working process in fastener hole of aircraft structure. Cold working process was applied for A12024-7351 specimens by considering the effect of edge margin on fatigue life. It is generally recognized that cold working process offers a protective zone around fastener hole of aluminum aircraft structure due to the residual compressive stresses which lead to retardation of crack growth. Thus this process provides the beneficial effect of increasing the fatigue life of the component. there by decreasing maintenance costs. It has also been successfully incorporated into damage tolerance and structural integrity programs. Cold working specimens were tested at constant amplitude peak cyclic stresses. Fatigue life of cold working specimen compared with that of specimen fabricated with base material. The increase of fatigue life for cold working specimen is discussed by both considering the effect of residual compressive stresses measured by X-ray diffraction technique and quantitative effect of edge margin.

• Journal of the Korean Society for Heat Treatment
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• v.26 no.6
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• pp.306-316
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• 2013

Microstructure and fracture behavior of a multi-phase low-density steel were investigated. After hot-rolling and heat treatment, the microstructure of low-density steel was composed of coarse ferrite grains and elongated bands which include second phases such as austenite, martensite and ${\kappa}$-carbide depending on holding time during isothermal heat treatment. After tensile test, microcracks were observed at martensite or ${\kappa}$-carbide interface in the elongated bands. Coarse ferrite grains showed cleavage fracture behavior regardless of second phase. The cleavage fracture of ferrite could be attributed to their coarse grain size and solute atoms that increase ductile-to-brittle transition temperature of ferrite. Despite of the tendency of cleavage fracture in coarse ferrite grains, a specimen having coarse spheroidized ${\kappa}$-carbide particles in the elongated bands showed high total elongation of 30%. Thus, the easiness of plastic deformation in the elongated band seems to play an important role in retardation of cleavage crack formation in coarse ferrite grains.