Si-Young Kwak

Structural analysis considering shrinkage defect of cast part

Abstract Flow and solidification of fusion metal at high temperature may introduce defects in cast components. As a results, many components have unsound internal areas. However, most engineers do not consider the effect of shrinkage defects when designing components; it is generally assumed that the material is completely sound. The material property reduction method is one approach to taking into account the effect of unsound areas, but it cannot consider stress concentration effects around the shrinkage. To compensate for this limitation, a shape simplification method is proposed. The method reconstructs shrinkage defects as hollow spheroid primitives based on shrinkage shape data obtained from industrial computerised tomography. The shape simplification method offers a smaller number of elements than other methods for modelling of shrinkages, and is also able to calculate the stress concentration. The present study examines the effect of shrinkage on a component structure subject to practical loads. It is possible to improve the productivity and reliability of cast products by such considerations.

Contribution Analysis Using Shape Simplification Method for Casting Structure Shrinkage

Most structure engineers give the casting components over-estimated factor of safety without any reasonable foundation due to the worries about the unavoidable defects such as shrinkages and porosity in castings; the engineers have little knowledge on the relation between the defect and structural behavior. And the workers in casting field also do not know how to control the defects by manufacturing; they do not know to where the defects move or until how size they reduce the defects. In this study, shrinkage defect was scanned by industrial computerized tomography instrument (CT), and subsequently was modeled to a spheroid primitive for structural analysis. Using these simplified models of shrinkage, we observed the effects of the defect on the results of the structural analysis. A commercial structural analysis code was used to do the analysis works. Considering the conclusions, it is possible to manage the shrinkages effectively in casting process and to design the products with more reliable

ESPI combined with hole drilling method to evaluate heat treatment induced residual stresses

Heat from manufacturing processes like casting and heat treatment may cause localized expansion in parts. When these parts are cooled subsequently, some areas cool and contract more than others, leaving residual stresses, which may exert a considerably negative influence on both the structures static strength and fatigue lifetime. So it is necessary to measure or predict the stress distribution in parts after heat treatment. In this study, residual stresses in the specimen are measured by ESPI (Electronic Speckle-Pattern Interferometry) combined with the hole drilling method. The material of specimen is SUS 304 austenitic stainless steel. It is quenched and water cooled subsequently. We also simulated the thermal stresses field induced by casting process by numerical analysis. As a result, comparisons of results from presented method as well as numerical solution are presented finally to give a conclusion on this residual stress measurement method in this paper.

Analysis of Impact Behavior of Al-Alloy Castings Considering Internal Defects

In general, internal defects, such as shrinkage in casting, cause stress concentration and can be a starting point for cracks. Therefore, it is important to understand the effects of internal defects on the mechanical properties including the impact behavior. This study aim is to evaluate the effects of internal casting defects on the impact performance of Al-alloy castings. Both an experimental method and computational analysis were used to achieve the research objective. The internal defects in the casting were scanned using an industrial CT scanner, and their shape was simplified using ellipsoidal primitives for impact analysis. The good agreement between the experimental and computer simulation results verified the reliability of the proposed computational method for the FEA of casting components with internal defects.

Shape-Simplification Analysis Model for Fatigue Life Prediction of Casting Products Considering Internal Defects

Internal defects are a major concern in the casting process because they have a significant influence on the strength and fatigue life of casting products. In general, they cause stress concentration and can be a starting point of cracks. Therefore, it is important to understand the effects of internal defects on mechanical properties such as fatigue life. In this study, fatigue experiments on tensile specimens with internal defects were performed. The internal defects in the casting product were scanned by an industrial CT scanner, and its shape was simplified by ellipsoidal primitives for the structural and fatigue analysis. The analysis results were compared with experimental results for casting products with internal defects. It was demonstrated that it is possible to consider internal defects of casting products in stress and fatigue analysis. The proposed method provides a tool for the prediction of the fatigue life of casting products and the investigation of the effects of internal defects on mechanical performance.

Combination of Different Numerical Methods for Efficient Thermal Stress Analysis of Casting Process

This paper proposes a method that involves a combination of FDM and FEM for analyzing casting process. At present, many numerical analysis methods such as FDM, FEM, and BEM are used for solving engineering problems. For a given problem, a specific method that is suited to the problem is adopted; in general, FDM or FVM is favored for problems related to fluid flow or heat transfer, and FEM is adopted in stress analysis. However, there is an increasing need for using a combined method for complex and coupled phenomena analysis. Hence, we proposed a method in which FDM and FEM are coupled in three-dimensional space, and we applied this method to analyze casting process. In the proposed method, solidification and heat transfer was analyzed by using FDM. The field data such as temperature distribution were converted into a format suitable for FEM analysis that was used for calculating thermal stress distribution. Using the proposed method, we efficiently analyzed the analysis process from the viewpoints of work and time. § 이 논문은 2010 년도 대한기계학회 CAE 및 응용역학부문 춘계학술대회(2010. 3. 4.-5., 서울대) 발표논문임. † Corresponding Author, vlvwlw@kitech.re.kr

Numerical and experimental study on thermal stress in grey iron castings

Abstract Research on thermal stress analysis using the hybrid finite difference/finite element (FDM/FEM) computational method is still in its early stages, as the brevity of the bibliography attests. Inhomogeneous temperature distributions during the casting process lead to the generation of various thermal stresses during cooling from the pouring temperature to room temperature and from surface to core. It is impossible to measure residual stress experimentally during casting because the casting is covered by the moulds and its temperature is too high. Thus, the final thermal stress state and deformation must be inferred from measurements made at room temperature. Four moderately complex specimens, designed to be sensitive to residual thermal stress, were cast from grey cast iron. Residual stress levels were measured at various positions by the centre hole method using transducer strain gauges. These values, and cooling curves derived from thermocouple measurements, show reasonable agreement with the results of FDM/FEM simulations using a hybrid numerical model developed by the authors. Whole stage simulations suggest that section width has a stronger influence on residual stress than section length. The discrepancies between measured and calculated temperatures are attributed to the fact that the model does not incorporate the air gap or mould porosity effects.

Fatigue Life Prediction of Non-Load-Carrying Cruciform Welded Joint using Master S-N Curve based on Structural Stress Approach

Welding process is of importance to assemble products or structures, but also the process is structural weakness due to stress concentration in welding joint. The fatigue design of welded joint requires time & labor consuming fatigue test because the fatigue life is various according to the depth of joint, joint type and load type etc. In fatigue design codes, they guide to classify welding joints with their shape( BS7608, IIW Documents) and provide fatigue assessment information. In terms of numerical method for fatigue analysis, it is also difficult to decide the stress peak in joint because of mesh sensitivity which means that stress value is varies with element type or size on stress concentration zone. Hot-spot method is used generally, but Battelle of United States proposed Master S-N Curve based on structural stresses converted by mechanical equilibrium theory. In this research, we extracted master S-N curve from Battelle`s fatigue test DB including test data of various welding joints to apply on Non-Load-Carrying cruciform Joint. Comparing fatigue results between the case of using normal stress and case of structural stress cor the cruciform Joint, The suggested Battelle method showed successive results.

Development of casting shrinkage cavity modelling toolkit for finite element analysis using ellipsoidal approximation technique

Abstract This paper presents a technique for automatically approximating casting shrinkage cavity models and its application in finite element analysis. First, reverse engineering was utilised to obtain the stereolithography format models of internal shrinkage cavities. After that, an in-house python code was used to analyse the shape of shrinkage cavity and then decompose it to ellipsoids, whose volume sum covers all points of the given shrinkage cavity obtained with reverse engineering. Finally, the ellipsoids were merged into a single ellipsoidal blob shaped three-dimensional computer aided design model, which can be used to model the casting with internal shrinkage for finite element analysis. The code was developed and implemented into ABAQUS as a customised plug-in toolkit, which makes it easy for engineers to model the casting with internal shrinkage cavities.

Approach to impact analysis of automobile Al alloy wheel in presence of casting shrinkage defect

Abstract The objective of the research was to evaluate the effect of casting shrinkage defects on the impact properties of cast aluminium alloy wheel for automobile. Most structure design engineers give the casting parts an overestimated factor of safety, specify extensive quality tests and perform extensive performance testing due to worries about unavoidable defects, such as shrinkages; however, all of these methods do not necessarily guarantee performance, and they are all somewhat costly. In this paper, an approach using computational impact analysis for an Al alloy, where shrinkage defect exists, was proposed to solve the motioned problem. The shrinkage defects were modelled by the shape simplification method. The method reconstructs the shrinkage defects to hollow spheroid primitive based on the shrinkage shape data obtained from industrial computerised tomography. The results of the simulation of wheel with shrinkage defect were compared with the results of ignoring shrinkage. Comparative results showed that when the impact analysis was applied to an Al alloy wheel, a traditional analysis without considering the shrinkage defect estimated the wheel safety; however, the results considering shrinkage defect show that the wheel would fracture under the same test condition.