• Journal of the Korean Society of Fisheries and Ocean Technology
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• v.28 no.1
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• pp.61-70
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• 1992

In this study, corrosion fatigue test of SAPH45 steel was performed by the use of plane bending fatigue tester in marine environment and investigated fracture surface growth behavior of base metal and heat affected zone corrosion fatigue. The main results obtained are as follows: 1) Fracture surface growth of heat affected zone (HAZ) is delayed more than that of base matel (BM), and they tend to faster in seawater than in air. 2) Corrosion sensitivity to corrosion fatigue life of HAZ is more susceptible than that of BM. 3)In the case of the corner crack by corrosion fatigue, the correlation between the propagation rate of fracture surface area(dA/dN) and stress intensity factor range(ΔK) for SAPH45 are applied to Paris rule as follows: dA/dN=C(ΔK) super(m) where m is the slope of the correlation, and is about 6.60-6.95 in air and about 6.33-6.41 in seawater respectively.

• Journal of the Korean Society for Precision Engineering
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• v.9 no.1
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• pp.106-117
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• 1992

In this study, the propagation behaviour and the closure phenomena of physically small surface cracks were investigated by the techinque of the Kikukawa-unloading elastic compliance method using a back face strain gage. The surface cracks initiated and propagated from notched specimens under constant amplitude bending load. The crack shape (aspect ratio) with approximately semi-circular at the early stage was changed to semi-elliptical as the cracks grew larger. The crack depth (a) could be expressed uniquenly by the crack length (c). The dependence of the crack propagation rate on the stress ratio R was strongly related in the lower ${\Delta}K$ range. The deceleration of the surface crack propagation rate was prominent in lower R during the crack length was small. When the propagation rate was rearranged with the effective stress intensity factor range ${\Delta}$K_{eff} the dependence of the crack propagation rate on the stress ratio R was found to be diminshed. These were caused by the crack closure phenomena that was most prominent at the lower propagation rate. The mechanism of crack closure phenomena was dominated by the plasticity-induced mechanism.

• Structural Engineering and Mechanics
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• v.19 no.6
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• pp.639-652
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• 2005

In this paper, a full-scale K-joint specimen was tested to failure under cyclic combined axial and in-plane bending loads. In the fatigue test, the crack developments were monitored step by step using the alternating current potential drop (ACPD) technique. Using Paris' law, stress intensity factor, which is a fracture parameter to be frequently used by many designers to predict the integrity and residual life of tubular joints, can be obtained from experimental test results of the crack growth rate. Furthermore, a scheme of automatic mesh generation for a cracked K-joint is introduced, and numerical analysis of stress intensity factor for the K-joint specimen has then been carried out. In the finite element analysis, J-integral method is used to estimate the stress intensity factors along the crack front. The numerical stress intensity factor results have been validated through comparing them with the experimental results. The comparison shows that the proposed numerical model can produce reasonably accurate stress intensity factor values. The effects of different crack shapes on the stress intensity factors have also been investigated, and it has been found that semi-ellipse is suitable and accurate to be adopted in numerical analysis for the stress intensity factor. Therefore, the proposed model in this paper is reliable to be used for estimating the stress intensity factor values of cracked tubular K-joints for design purposes.

• Journal of Ocean Engineering and Technology
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• v.6 no.2
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• pp.76-84
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• 1992

In this study, corrosion fatigue test of SAPH45 steel was performed by the use of plane behavior of base metal (BM) and heat affected zone (HAZ) of SAPH45. The main results obtained are as follows: 1) The more aspect ratio (b/t) of corner crack decreases, the more aspect ratio (b/a) takes greatly effect by corrosion. 2) The correlation between the stress intensity factor range ($\Delta$k) and crack growth rate (da/dN) for weldment in seawater is given by Paris rule as follow: da/dN=C($\Delta$K) super(m). Where m is constant, and the value is 3.82-3.84. 3) The accelerative factor ($\alpha$) of BM and HAZ under seawater is about 1.1-1.9, and ($\alpha$) of HAZ increases more and more under the low $\Delta$K region. 4) HAZ is more susceptible to corrowion than BM because of potential of electrode (E sub(c)) of HAZ becomes more less noble potential than that of BM.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2003.06a
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• pp.1402-1405
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• 2003

The main object of this study was evaluated by the delamination damage for fiber stacking angle. Therefore, this work need to compare the shape of delamination for a different fiber stacking angie. So this study uses a method of fatigue test which was created [0]$_2$,[+45]$_2$[90]$_2$. The extension of the delamination zone formed between aluminium alloy and glass fiber-adhesive layer were measured by an ultrasonic C-scan image. As a result, the shapes of delamination zone don't depend upon the crack propagation. We could know that the delamination zone grew interaction between stress flow of fiber layer and crack driving force. Hence, the existing study were applied to the stress transfer, fiber bridging effect, delaminantion growth rate should need to the develop useful factor because of change of fiber stacking angle.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2002.10a
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• pp.816-819
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• 2002

Accurate stress intensity factor analyses and crack growth rate of surface-cracked components in inhomogeneous materials are needed for reliable prediction of their fatigue lift and fracture strengths. This paper describes an automated system for analyzing the stress intensity factors of three-dimensional (3D) cracks in inhomogeneous materials. 3D finite element method (FEM) was used to obtain the stress intensity factor for subsurface cracks and surface cracks existing in inhomogeneous materials. To examine accuracy and efficiency of the present system, the stress intensity factor for a semi-elliptical surface crack in a plate subjected to uniform tension is calculated, and compared with Raju-Newman's solutions. Then the system is applied to analyze cladding effect of subsurface cracks in inhomogeneous materials. The results were compared with those surface cracks in homogeneous materials. It is clearly demonstrated from these analyses that the stress intensity factors for subsurface cracks are less than those of surface cracks.

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.61-61
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• 2010

The purpose of this study is to prevent the unstable fracture at the FCA butt weldment of hatch coaming deck in the ultra large containership during service life. In order to do it, the behavior of the embedded crack at the weldment under design loading conditions was evaluated in accordance with BS7910. Here, the level of primary stress induced by ship motion was evaluated by the design code of classification society and FEA. The level of residual stress as secondary stress was calculated in consideration of the restraint degree of weldment and welding heat input by using the predictive equation proposed by authors in the previous study. The fatigue crack growth rate at the weldment was evaluated using CT specimen in accordance with ASTM E647. According to the results, although the allowable defect for embedded crack specified in the classification society exists at the weldment, the occurrence possibility of unstable fracture at the weldment could not be negligible, regardless of CTOD value given in this study. So, in this study, the effect of initial defect size, welding heat input, restraint degree and CTOD on the fracture mechanical characteristics of embedded crack at the weldment was evaluated by the comprehensive fracture assessment. Based on the results, the design criteria including allowable defect, residual stress level and CTOD value was established to prevent the unstable fracture at the FCA butt weldment of hatch coaming deck in an ultra large containership during service life of 20years.

• 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.

• Journal of the Korean Society for Precision Engineering
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• v.9 no.3
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• pp.140-148
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• 1992

The effect of mechanical, properties in the plastic zone near the crack-tip was investigated, under various controlled loading conditions, i.e., ${\Delta}K$ increasing, ${\Delta}K$ decreasing, and single overload test. For both ${\Delta}K$ decreasing test and ${\Delta}K$ increasing test with constant stress ratio, it is found that the ratio of material constant m'( ${\Delta}K$ decreasing test) to material constant m( ${\Delta}K$ increasing test) is larger than 1 for n<0.1, and it is equal to 1 for 0.10.2. A modified crack growth rate equation based on Forman's equation which applied stable region of fatigue crack propagation in ${\Delta}K$ decreasing test is proposed. Within the limit of this single overload test, an empirical relation between among the retardation ratio (Nd/ $N^{*}$), the strain hardening exponent (n) and the percent peak load (%PL) has been established.established.

• Journal of Advanced Marine Engineering and Technology
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• v.15 no.1
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• pp.51-58
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• 1991

Corrosion fatigue test was performed by the use of plane bending fatigue tester in marine environment having various specific resistance from 25(natural sea water) to 5000.ohm.cm. It is in order to investigate the effects of marine environmental factor on the corrosion fatigue fracture of SS41 steel. The main results obtained are as follows; 1. The aspect ratio(b/a) of corner crack growing in natural sea water is lower than that in air. 2. The surface crack growth rate(da/dN) in marine environment is faster than that in air and da/dN delaies with the specific resistance increased. 3. The experimental constant m of paris rule [da/dN=C(${\delta}$K)$^m$] decrease with the specific resistance decreased and the effect of corrosion in proportion to the specific resistance is more sensitive than that of stress intensity factor range(${\delta}$K) under region II. 4. The accelerative factor(${\alpha}$) in marine environment is about 1.1-2.7 and .alpha. is increase under the low region of stress intensity factor range(${\delta}$K). 5. The electrode potential($E_0$) gets less noble potential with the specific resistance decreased.