• Structural Engineering and Mechanics
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• v.53 no.5
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• pp.867-880
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• 2015

Mechanical fastening is still one of the main methods used for joining components. Different techniques have been applied to reduce the effect of stress concentration of notches like fastener holes. In this work we evaluate the feasibility of combining laser shock peening (LSP) and cold expansion to improve fatigue crack initiation and propagation of open hole specimens made of 6061-T6 aluminum alloy. LSP is a new and competitive technique for strengthening metals, and like cold expansion, induces a compressive residual stress field that improves fatigue, wear and corrosion resistance. For LSP treatment, a Q-switched Nd:YAG laser with infrared radiation was used. Residual stress distribution as a function of depth was determined by the contour method. Compact tension specimens with a hole at the notch tip were subjected to LSP process and cold expansion and then tested under cyclic loading with R=0.1 generating fatigue cracks on the hole surface. Fatigue crack initiation and growth is analyzed and associated with the residual stress distribution generated by both treatments. It is observed that both methods are complementary; cold expansion increases fatigue crack initiation life, while LSP reduces fatigue crack growth rate.

• 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.

• Structural Engineering and Mechanics
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• v.81 no.5
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• pp.565-574
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• 2022

The infrastructures such as offshore, bridges, power plant, oil and gas piping and aircraft operate in a harsh environment during their service life. Structural integrity of engineering components used in these industries is paramount for the reliability and economics of operation. Two regression models based on the concept of Gaussian process regression (GPR) and Minimax probability machine regression (MPMR) were developed to predict stress intensity factor range (𝚫K). Both GPR and MPMR are in the frame work of probability distribution. Models were developed by using the fatigue crack growth data in MATLAB by appropriately modifying the tools. Fatigue crack growth experiments were carried out on Eccentrically-loaded Single Edge notch Tension (ESE(T)) specimens made of API 5L X65 Grade steel in inert and corrosive environments (2.0% and 3.5% NaCl). The experiments were carried out under constant amplitude cyclic loading with a stress ratio of 0.1 and 5.0 Hz frequency (inert environment), 0.5 Hz frequency (corrosive environment). Crack growth rate (da/dN) and stress intensity factor range (𝚫K) values were evaluated at incremental values of loading cycle and crack length. About 70 to 75% of the data has been used for training and the remaining for validation of the models. It is observed that the predicted SIF range is in good agreement with the corresponding experimental observations. Further, the performance of the models was assessed with several statistical parameters, namely, Root Mean Square Error (RMSE), Mean Absolute Error (MAE), Coefficient of Efficiency (E), Root Mean Square Error to Observation's Standard Deviation Ratio (RSR), Normalized Mean Bias Error (NMBE), Performance Index (ρ) and Variance Account Factor (VAF).

• Proceedings of the KSME Conference
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• 2008.11a
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• pp.725-726
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• 2008

This paper proposes elastic stress intensity factors and crack opening displacements (CODs) for a slanted axial through-wall cracked cylinder under an internal pressure based on detailed 3-dimensional (3-D) elastic finite element (FE) analyses. Based on the elastic FE results, the stress intensity factors along the crack front and CODs through the thickness at the center of the crack were provided. These values were also tabulated for three selected points, i.e., the inner and outer surfaces and at the mid-thickness. The present results can be used to evaluate the crack growth rate and leak rate of a slanted axial through-wall crack due to stress corrosion cracking and fatigue. Moreover, the present results can be used to perform a detailed Leak-Before-Break analysis considering more realistic crack shape development.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.21 no.10
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• pp.1626-1635
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• 1997

It is important to ensure the reliability of the first localized reactor vessel steel. To satisfy with this purpose, a study on the impact/hardness, low cycle fatigue(LCF), crack growth rate(da/dN) and fracture toughness( ) of base material(BM) and weld metal(WM) were performed under room temperature air and corrosion conditions. A summary of the results is as folows : (1) Charpy impact absorbed energy of BM was the highest value, heat affected zoon(HAZ) and the lowest, WM. The hardness of BM was similar to HAZ. (2) Coefficients of Manson equation using the monotonic tensile test data were obtained for the present material. (3) The effects of stress ratio and ambient (120.deg. C and NaCl) condition on da/dN were investigated, da/dN with NaCl condition expressed the highest value. (4) The results of Charpy V-notch impact test had good correlation with $K_{IC}$ characteristics and the lowest curve of $K_{IC}$ for BM was derived, more researches about WM and HAZ are required hereafter.

• Bulletin of the Society of Naval Architects of Korea
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• v.27 no.2
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• pp.47-54
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• 1990

In this study, the effects of gas ratio($N_2:H_2$) and treatment time on the fatigue fracture behavior, fatigue crack growth behavior and corrosion fatigue fracture behavior for the ion-nitrided SM45C steel were investigated. The results show that the fatigue limit and corrosion fatigue strength increased in porportion to $N_2$ gas and treatment time for all kinds of specimen, used in the experiment. Compare to the non-nitrided specimen, the fatigue limit and the corrosion fatigue strength increased about $24{\sim}29%$ at $10^7$ cycles in air and $32{\sim}48%$ at $10^6$ cycles in 3% NaCl aqueous solution, respectively. Similar results were derived with SM45C steel under Compression-Tension $24{\sim}29%$ at $10^6$ cycles in air and $32{\sim}48%$ either in 3% NaCl aqueous solution or in tap water, respectively. Ion-nitrided SM45C steel showed a slow fatigue crack growth rate at relatively low range of ${\Delta}K$ compared to the non-nitrided specimen. To the contrary, its rate increase at higher range of ${\Delta}K$.

• Nuclear Engineering and Technology
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• v.45 no.2
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• pp.265-276
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• 2013

Pressure tubes made of Zr-2.5 wt% Nb alloy are important components consisting reactor coolant pressure boundary of a pressurized heavy water reactor, in which unanticipated through-wall cracks and rupture may occur due to a delayed hydride cracking (DHC). The Canadian Standards Association has provided deterministic and probabilistic structural integrity evaluation procedures to protect pressure tubes against DHC. However, intuitive understanding and subsequent assessment of flaw behaviors are still insufficient due to complex degradation mechanisms and diverse influential parameters of DHC compared with those of stress corrosion cracking and fatigue crack growth phenomena. In the present study, a deterministic flaw assessment program was developed and applied for systematic integrity assessment of the pressure tubes. Based on the examination results dealing with effects of flaw shapes, pressure tube dimensional changes, hydrogen concentrations of pressure tubes and plant operation scenarios, a simple and rough method for effective cooldown operation was proposed to minimize DHC risks. The developed deterministic assessment program for pressure tubes can be used to derive further technical bases for probabilistic damage frequency assessment.