Tae Hyun Baek

Hybrid Full-Field Stress Analysis Around Holes in Tensile-Loaded Plates by Phase-Shifting Photoelasticity

A hybrid stress determination around circular and elliptical holes utilizing photoelastic phase-shifting and nonlinear least-squares methods is presented. The method was demonstrated by calculating fringe orders of distant points along straight lines using 8-step phase-shifting method. The data was used to evaluate the coefficients in the complex stress functions for hybrid analysis. Tangential stresses around the boundary of the holes were obtained using conformal mapping technique. Different number of terms in a power-series representation of the complex type stress function was tested to qualitatively observe the effects of varying stress field. Actual fringes were related with the reconstructed and sharpened fringes along with the change in the number of terms, m. Good agreement was obtained when m in stress functions was equal to nine. At high stress concentration, the result obtained from the hybrid method agrees with FEM by two and five percent for circular and elliptical hole, respectively. The results show that the established numericalexperimental method for stress analysis is considerably reliable.

An Improved Hybrid Full-Field Stress Analysis of Circularly Perforated Plate by Photoelasticity and Finite Element Analysis

hybrid experimental-numerical method is presented for determining the stresses around a circular hole in a finite-width, tensile loaded plate. Calculated fringes obtained by FEA provided the information about the external boundary of the hybrid element, and those fringes on straight lines were used for hybrid analysis. In order to see the effects of varying stress field, different numbers of terms in a power-series representation of the complex type stress function were tested. Actual and reconstructed fringes were compared. The hybrid results were highly comparable with those predicted by FEA. The result showed that this approach is effective and promising because isochromatic data along the straight lines in photoelasticity can be conveniently measured by use of phase shifting photoelasticity.

Analysis of Photoelastic Stress Field Around Inclined Crack Tip by Using Hybrid Technique

In this paper, a hybrid technique is presented. First, the isochromatic fringe data of a given set of points are calculated by the finite element method and are used as input data in complex variable formulations. Then the numerical model of the specimen with a central inclined crack is transformed from the physical plane to the complex plane by conformal mapping. The stress field is analyzed and the mixed-mode stress intensity factors are calculated for this complex plane. The stress intensity factors are calculated by the finite element method as well as by a theoretical method and compared with each other. In order to conveniently compare these values with each other, both actual and regenerated photoelastic fringe patterns are multiplied by a factor of two and sharpened by digital image processing.

Measurement for In-Plane Displacement of Tensile Plates with Through-Thickness Circular Hole and Partly Through-Thickness Circular Hole by Use of Speckle Interferometry

Speckle interferometry with phase shifting method is used to measure in-plane displacements of a steel plate with a partly through-thickness circular hole and a steel plate with a through-thickness circular hole. The circular hole of steel plate with a partly through-thickness circular hole is cut on the rear side of the plate, so that it is not visible during experiment. The speckle noises of fringe patterns acquired by optical experiment are processed with image processing algorithm of Gaussian blur and the in-plane displacements of the two specimens are obtained by use of the processed fringe patterns. Also the in-plane displacements of the two specimens are calculated by use of ANSYS. The results of optical experiments are quite comparable to those of calculation with ANSYS.

Test and Evaluation of a Newly Built Multi-Purpose Transmission Type Polariscope

A multi-purpose polariscope was developed by applying an electro-mechanical control system to a diffused transmission-type circular polariscope. A conventional polariscope is only good for manual control of optical elements. The new polariscope system was devised to be controlled through two stepping motors and two magnetic clutches. The new polariscope can be used not only for the point-wise measurement using Tardy compensation technique but also for the full-field fringe analysis using conventional and/or phase measuring techniques, if applicable. The distributions of digitally determined unwrapped isoclinics and isochromatics using the digitized images obtained from the developed polariscope were in close agreement to manual measurements.

Material Stress Fringe Constant Measurement of Specimen under Pure Bending Load by Use of Photoelastic Phase Shifting Method

In a photoelastic experiment, it is necessary to know the material stress fringe constant of the photoelastic specimen to determine the stresses from the measured isochromatic fringe orders. The material stress fringe constant can be obtained using a simple tension specimen and/or a circular disk under diametric compression. In these methods, there is generally a need to apply numerous loads to the specimen in response to the relationship of the fringe order. Then, the least squares method is used to obtain the material constant. In this paper, the fringe orders that appear on a four-point bending specimen are used to determine the fringe constant. This method requires four photoelastic fringes obtained from a circular polariscope by rotating the analyzer to 0, /4, /2, and 3 /4 radians π π π . Using the four-point bending specimen to determine the material stress fringe constant has an advantage because measurements can be made at different locations by applying a constant load. The stress fringe constant measured with this method is within the range suggested by the manufacturer of the photoelastic material. 이 논문은 년도 대한기계학회 호남지회 춘계 § 2014 학술대회 군산대 발표논문임 (2014. 5. 9., ) . Corresponding Author, thbaek@kunsan.ac.kr 2014 The Korean Society of Mechanical Engineers C 류관용 김명수 백태현 · · 1388 는 실험에 의한 광역학 분야 중의 (photomechanics) 한 가지 방법이다 광탄성법은 광신호의 이차원 . 신호처리 특성을 활용하여 시편에서 전제 응력장 을 가시적으로 관찰할 수 있 (whole field stress) 다. 광탄성법으로 시편에 발생된 응력을 측정 하기 위해서는 광탄성 실험 장치인 편광기로부터 얻어지는 등색선 프린지와 등경선 (isochromatic) 프린지를 이용한다 등색선은 시편에서 (isoclinic) . 발생된 주응력 차이 에 (principal stress difference) 의해 나타나며 등경선은 주응력 방향 , (principal 에 의해 나타난다 측정된 등색 프 stress direction) . 린지 차수로부터 응력으로 환산하기 위해서는 광 탄성 재료 응력 프린지 상수(material stress fringe 를 알아야 한다 일반적으로 재료 응력 constant) . 프린지 상수는 단순하중을 받는 인장시편이나 압 축하중을 받는 원형디스크에 여러 하중을 가하여 나타나는 프린지 측정한 후 하중과 프린지 차수 , 선도로부터 결정한다. 이 경우 프린지 차수는 , 육안이나 수작업으로 타디 보정법(Tardy compensation 등을 이용하여 측정해야 하는 불편한 점 method) 이 있다. 본 논문에서는 점 굽힘 시편 4 (4-point bending 에 하중을 가하여 나타나는 프린지로부 specimen) 터 재료 응력 프린지 상수를 결정하고자 한다. 4 점 굽힘 시편의 순수 굽힘 구간에서는 주응력 방 향이 일정하므로 광탄성 단계 위상이동법 4 의 적용 (photoelastic 4-step phase shifting method) 이 가능하다 이 방법에서는 원형편광기에서 검 . 광판을 그리고 라디안 회전시 0, /4, /2, 3 /4 π π π 켜 얻은 개의 광탄성 프린지를 필요로 한다 4 . 4 점 굽힘 시편을 이용하여 재료 응력 프린지 상수 를 결정하기 위하여 일정 하중을 가한 후 서로 , 다른 위치에서 측정하여 평균값을 구하고 유한 , 요소법에 의한 응력분포와 서로 비교하고자 한 다.

Fringe Analysis around an Inclined Crack Tip of Finite-Width Plate under Tensile Load by Photoelastic Phase-Shifting Method

Photoelasticity is a technique of experimental methods and has been widely used in various domains of engineering to determine the stress distribution of structures. Without complicated mathematical formulation, this technique can conveniently provide a fairly accurate whole-field stress analysis for a mechanical structure. Here, stress distribution around an inclined crack tip of finite-width plate is studied by 8-step phase-shifting method. This method is a kind of photoelastic phase-shifting techniques and can be used for the determination of the phase values of isochromatics and isoclinics. According to stress-optic law, the stress distribution could be obtained from fringe patterns. The results obtained by polariscope arrangement combined with 8-step method and ABAQUS FEM simulations are compared with each other. Good agreement between them shows that 8-step phase-shifting method is reliable and can be used for determination of stress by experiment.

Elasto-Plastic Stress Distribution in Triplate Joint Assembly under Bending Moment

Pre-produced triplate transition joint assemblies are widely used in shipbuilding industry to make welds between aluminum and steel for a number of years now. The straight-shaped transition joint assemblies are bent during shipbuilding. So it is necessary to study the residual stresses created by punch forming, which would have heavy effects on the quality of structural parts. ABAQUS is a suite of powerful engineering simulation programs, based on the finite element method. In this paper, ABAQUS was used as the main tool to simulate the residual stresses in a triplate transition joint after unloading. Punch-pressing was carried to simulate bending moment in ABAQUS. The triplate is consisted of baselayer (steel: Lloyd’s Shipplate Gr. A), interlayer (pure aluminum: Al99.5) and superlayer (Al-Mg alloy: AlMg4.5Mn). Results from the ABAQUS analysis showed that increasing the radius of punch significantly reduced the von Mises residual stresses in steel. Changes of von Mises residual stresses in interlayer (Al99.5) and superlayer (AlMg4.5Mn) were negligible.

Analysis of Residual Stress Components Developed in Cold Drawing by Finite Element Method

Predicting the behavior of steel during deformation process under service conditions is one of the main challenges in cold drawing. In this paper, finite element method was used to analyze the distribution of residual stress components in a rod with respect to different area reductions in cold drawing process. Cold drawing brought sunken-in deformation at the bottom land of the rod that was in accord with the result obtained from the analysis using ABAQUS. The results obtained from finite element analysis were in close agreement with the determined high-accuracy measurements.

Fatigue Life Simulation of a High Pressure Breech System

In order to evaluate the structural integrity of the breech system used for a thick-walled cylinder subjected to pulsating high internal pressure, fatigue life simulation of a breech system was performed. A stress analysis of the breech was performed to locate the critical region vulnerable to crack initiation. Low-cycle fatigue behavior of the breech material was investigated to obtain the fatigue crack initiation properties. Elastic-plastic finite element stress analysis resulted in a stress concentration at the breech ring groove root. Strains at the breech ring and block were experimentally measured using strain gages and resulted in similar values compared to the calculated strains. Local strain approach was employed to estimate the fatigue life of the breech system for crack initiation at the groove root of the breech ring. Fatigue tests using simulation specimens were performed and an averaged fatigue life was obtained, showing a very good agreement with the calculated fatigue life within a factor of two.