Yooil Kim

Low Cycle Fatigue Analysis of Marine Structures

There are situations where a marine structure is subjected to stress cycles of such large magnitude that small, but significant, parts of the structural component in question experiences cyclic plasticity. Welded joints are particularly vulnerable because of high local stress concentrations. Fatigue caused by oscillating strain in the plastic range is called “low cycle fatigue”. Cycles to failure are typically below 104 . Traditional welded joint S-N curves do not describe the fatigue strength in the low cycle region (< 104 number of cycles). Typical Class Society Rules do not directly address the low cycle fatigue problem. It is therefore the objective of this paper to present a credible fatigue damage prediction method of welded joints in the low cycle fatigue regime.Copyright

Extraction of the mode shapes of a segmented ship model with a hydroelastic response

Abstract The mode shapes of a segmented hull model towed in a model basin were predicted using both the Proper Orthogonal Decomposition (POD) and cross random decrement technique. The proper orthogonal decomposition, which is also known as Karhunen-Loeve decomposition, is an emerging technology as a useful signal processing technique in structural dynamics. The technique is based on the fact that the eigenvectors of a spatial coherence matrix become the mode shapes of the system under free and randomly excited forced vibration conditions. Taking advantage of the simplicity of POD, efforts have been made to reveal the mode shapes of vibrating flexible hull under random wave excitation. First, the segmented hull model of a 400 K ore carrier with 3 flexible connections was towed in a model basin under different sea states and the time histories of the vertical bending moment at three different locations were measured. The measured response time histories were processed using the proper orthogonal decomposition, eventually to obtain both the first and second vertical vibration modes of the flexible hull. A comparison of the obtained mode shapes with those obtained using the cross random decrement technique showed excellent correspondence between the two results.

Flaw Assessment on an Offshore Structure using Engineering Criticality Analysis

조선해양산업의 차세대 성장동력으로서 해양플랜트가 주목 받 고 있다. 심해 자원을 채굴하는 해양플랜트산업은 이전보다 고도 의 기술력이 요구되며 심해 자원의 경제성이 완전히 소모될 때까 지 해양플랜트 설비가 안정적으로 운영되어야 하는 특성이 있다. 해양구조물의 극후판으로 구성되는 구조부재의 경우 용접으로 연결되어 있기 때문에 미세한 초기결함이 존재할 가능성이 크다. 비파괴 검사 과정 중에 구조부재에 초기결함이 발견 된다면 지속 적인 파랑하중으로 유기되는 반복 하중을 받아 초기결함이 진전 하면서 해양구조물은 피로파괴에 상당히 취약할 수 있다. 따라서 주어진 피로 하중 수명 동안 해양 구조물이 구조적으로 안전한 지에 대한 평가 해석이 필요하며 이를 테면 용접 부나 구조적 취 약부에 존재하는 초기결함의 정보를 이용한 파괴역학 기반의 결 함 평가 해석이 수행되어야 한다. 파괴역학 기법을 이용한 피로 수명 예측 및 구조 안전성 평가 를 위한 연구가 국내에서 활발히 이루어졌다. Noh, et al. (1993) 는 독립형 LNG탱크의 총체적인 구조안전성 평가과정을 다뤘다. 파랑 응력의 장기분포 해석을 수행하여 S-N선도를 이용한 피로 강도 평가를 실시하였고 LBF(Leak Before Failure)이론을 바탕으 로 관통균열의 진전해석을 수행하여 급속 취성파괴에 대한 안전 성을 검증하였다. Yim and Byun (1993)은 BSI PD 6493의 3단 계 평가방법에 균열 성장 효과를 고려한 건전성 평가 방법과 균 열의 안정성장을 엄밀히 고려할 수 있는 안정성 평가방법 (Stability Assessment Method: SAM)과 더불어 소성붕괴를 평가 하는 극한 하중해석을 고려한 건전성 평가방법을 정식화 하였다. Jeon, et al. (2001)은 선체 구조에서 피로손상을 받기 쉬운 용접 부위인 T형 이음부 및 호퍼 너클 이음부 모델의 피로실험 및 선 형탄성 파괴역학을 이용한 피로균열 진전해석을 수행하여 피로 균열 진전 수명을 정확히 예측하였다. Kim, et al. (2002)은 구조 pISSN:1225-1143, Vol. 52, No. 6, pp. 435-443, December 2015

Development of Fatigue Damage Model of Wide-Band Process by Artificial Neural Network

For the frequency-domain spectral fatigue analysis, the probability mass function of stress range is essential for the assessment of the fatigue damage. The probability distribution of the stress range in the narrow-band process is known to follow the Rayleigh distribution, however the one in the wide-band process is difficult to define with clarity. In this paper, in order to assess the fatigue damage of a structure under wide band excitation, the probability mass function of the wide band spectrum was derived based on the artificial neural network, which is one of the most powerful universal function approximation schemes. To achieve the goal, the multi-layer perceptron model with a single hidden layer was introduced and the network parameters are determined using the least square method where the error propagates backward up to the weight parameters between input and hidden layer. To train the network under supervision, the varieties of different wide-band spectrums are assumed and the probability mass function of the stress range was derived using the rainflow counting method, and these artificially generated data sets are used as the training data. It turned out that the network trained using the given data set could reproduce the probability mass function of arbitrary wide-band spectrum with success.Copyright

Wet Damping Estimation of the Segmented Hull Model using the Random Decrement Technique

This paper presents the wet damping estimation of the segmented hull model using the random decrement technique together with the continuous wavelet transform. The tested 16 sea states are grouped together based on the speed of the ship in order to figure out the possible influence of the ship speed on the damping ratio. The measured time histories of vertical bending moment for each tested sea state were processed with random decrement technique to derive the free decay signal, from which the damping ratios are estimated. Also, the autocorrelation functions of the filtered signal were calculated and comparison was made with the free decay signal obtained from the random decrement technique. Then the wet damping ratios for each sea state group, as well as precise wet natural frequencies, are estimated by using continuous wavelet transform. It turned out that the wet natural frequencies derived from the measured signal did not show any significant discrepancy compared with those obtained by wet hammering test, whereas the damping ratio did. It was considered that the discrepancy of the damping ratio between in calm and moving water may be attributed to the viscous effects caused by dramatically different flow pattern and relative velocity between the vibrating structure and surrounding fluid particles.

First Ocean Going Ships With Springing and Whipping Included in the Ship Design

It is well known that ships vibrate due to waves. The wave induced vibrations of the hull girder are referred to as springing (resonance) and whipping (transient vibration from impacts). These vibrations contribute to the fatigue damage of fatigue sensitive details. An Ore Carrier of 400 000 dwt is currently being built by DSME, and at time of delivery, it will be the world’s largest bulk (ore) carrier. The scantlings of large ships must be carefully designed with respect to global loading, and when extending the design beyond experience, it is also wise to consider all aspects that may affect operation and the life time costs. The vessel will also enter a long term contract and is therefore to be evaluated for 30 year Brazil-China operation. In order to minimize the risk of fatigue damage, the vessel is designed according to DNV’s class notation CSA-2 requiring direct calculations of the loading and strength. Further it has been requested to include the effect of springing and whipping in the design. Reliable numerical tools for assessing the additional fatigue effect of vibrations are non-existing. DNV has, however, developed an empirical guidance on how the additional effect may be taken into account based on previous development projects related to the effect of vibrations on large ore carriers Due to the size and route of operation of the new design, it has, however, been required by the owner to carry out model tests in both ballast and cargo condition in order to quantify the contribution from vibration. The results from this project have been used for verification and further calibration of DNV’s existing empirical guidance. A test program has been designed for the purpose of evaluating the consequence in head seas for the Brazil to China trade. Full scale measurements from previous development projects of ore carriers and model tests have been utilized to convert the current model tests results into estimated full scale results for the 400 000 dwt vessels. It is further important to carefully consider how the vibrations are to be included in the design verification, and to develop a procedure for taking into account the vibrations which results in reasonable scantlings based on in-service experience with similar designs and trades. This procedure has been developed, and a structural verification has been carried out for the design. The final outcome of the model test was in line with previous experience and in overall agreement with DNV’s empirical guidance, showing a significant contribution from vibrations to the fatigue damage. The springing/whipping vibrations more than doubled the fatigue damage compared to fatigue evaluation of the isolated wave induced loading. The cargo condition vibrated relatively more than experienced on smaller vessels. Various sources to establish the wave conditions for the Brazil to China ore trade were used, and the different sources resulted in significant differences in the predicted fatigue life of the design.Copyright

Fluid-structure coupled acoustic analysis of vibrating Basilar membrane within the cochlea of inner ears

This paper presents the preliminary study on the dynamic characteristics of the basilar membrane (BM) within the cochlea of inner ear. The BM is a vibrating element that varies in width and stiffness like a string on an instrument. While low frequency sounds vibrate near the apex (at the maximum length), high frequency sounds vibrate near the base of the cochlea (near the round and oval windows). Over the last decades, this frequency selectivity has been utilized for acoustic transducers by mimicking the cochlea tonotopy: passive frequency selectivity and transform from acoustic sound into frequency signal of hair cells in the organ of Corti. In previously reported studies, the frequency selectivity was simply achieved by physical parameters, such as length and thickness of beam array although the motion of the BM is generally described as a traveling wave. In this study, fluid-structure coupled acoustic analysis of vibrating BM within the cochlea of inner ear is performed to describe the actual motion of BM. The new approach different from the cantilever beam array –based approach will be then investigated for improved frequency selectivity.

Residual Stress Prediction in Multi-layer Butt Weld Using Crack Compliance Method

Abstract It depends on the joint configuration, dimensions and constraints of the joint whether the residual stress at the root of single-sided butt weld is tensile or not. Therefore, recommendation is generally made that highR ratio should be used in the fatigue test of welded joint in order to prevent excessively long life caused by compressive residual stress. In this research, the residual stress profile in butt weld joint was obtained through compliance method, using successive extension of a slot and measurement of the variation of strainduring the slot extension. The residual stress profile was firstly assumed to be the linear summation of Legendrepolynomials up to 9th order excluding 0th and 1st order. Strain variation on the surface was measured while the slot was being extended by cutting to find out the 8 unknown coefficients of each polynomial term. The cut was made by the electric discharge machine. It was concluded that the residual stress near the surface is positive valued, however, it turned into the negative value as soon as it passed through 2 or 3 mm of the depth. Key Words : Crack compliance method, Residual stress, Electric discharge machine, Finite element analysis,Butt weld

Hydroelasticity Vibration of a Rectangular Tank Wall

A theoretical study is carried out on the hydroelasticity vibration of a rectangular tank wall. It is assumed that the tank wall is clamped along the plate edges. The fluid velocity potential is used for the simulation of fluid domain and to obtain the added mass due to wall vibration. In addition, the vibration characteristics of stiffened wall of the rectangular tank are investigated. Assumed mode method is utilized to the stiffened plate model and hydrodynamic force is obtained by the proposed approach. The coupled natural frequencies are obtained from the relationship between kinetic energies of a wall including fluid and the potential energy of the wall. The proposed analytical approach was found to be in good agreement with the results of a well-known commercial three-dimensional finite element program.Copyright

A Novel Frequency Selectivity Approach Based on Travelling Wave Propagation in Mechanoluminescence Basilar Membrane for Artificial Cochlea

This study presents the initial assessment for a new approach to frequency selectivity aimed at mimicking the function of the basilar membrane within the human cochlea. The term cochlea tonotopy refers to the passive frequency selectivity and a transformation from the acoustic wave into a frequency signal assisted by the hair cells in the organ of Corti. While high-frequency sound waves vibrate near the base of the cochlea (near the oval windows), low-frequency waves vibrate near the apex (at the maximum distance from the base), which suggests the existence of continuous frequency selectivity. Over the past few decades, frequency selectivity using artificial membranes has been utilized in acoustic transducers by mimicking cochlea tonotopy using cantilever-beam arrays with defined physical parameters such as length and thickness. Unlike the conventional cantilever-beam array type, the travelling wave propagation based-mechanoluminescence (ML) membrane made of ZnS:Cu- polydimethylsiloxane (ZnS:Cu-PDMS) composite that we describe here provides new frequency selectivity more similar to that demonstrated by the human membrane. Here, we explored the potential of the ML membrane to deliver new frequency selectivity by using a non-contact image sensor to measure visualized frequencies. We report that the ML basilar membrane can provide effective visualization of the distribution of strain rate associated with the position of maximal amplitude of the travelling wave.