Papers by Author: Dong Il Kwon

Abstract: Predicting the residual life of a structure is an essential issue in structure management. Many researchers have used different methods to predict structure lifetimes, such as the creep rupture test. However, this test is costly and time-consuming, and since it is also destructive, an unused specimen must be tested rather than an actual specimen in use. The instrumented indentation test (IIT), on the other hand, is easier and faster than conventional test methods, most important of all, it is a non-destructive method to obtain mechanical properties that can be performed on the actual structure in use. In this study, we obtained mechanical properties of a SA213-T23 tube material for a thermal power plant degraded for 1~2,000 hours at high temperature. and observed the degradation by analyzing the microstructure. We found a relation between the Larson-Miller Parameter (LMP) and degradation tensile properties considering the temperature and degradation time and suggested a method to predict the residual life by applying a failure criterion. Also, we confirmed that our interaction formula and the residual life are reasonable by comparison with statistical rupture time data from materials that have in fact degraded.

Abstract: The instrumented indentation test (IIT) is a mechanical testing method to determine the hardness and elastic modulus of materials by putting an indenter into a material surface. This technique has now gone beyond normal hardness tests by evaluating additional properties of materials and by allowing testing at much lower forces and indentation depths (micro/nano ranges). This study presents analytic models and procedures for evaluating tensile flow properties and residual stress state using IIT; the tensile flow properties are treated by defining a representative stress/strain beneath a spherical indenter and the residual stress by using a stress-insensitive contact hardness model. The IIT results are compared with those from conventional methods such as uniaxial tensile test and X-ray diffraction. In addition, IIT can be used as a multiscale mapping tool for the mechanical properties of composite materials and constituent phases by using macro/micro/nano indentation system: we made a hardness map of multiphase steel and measured the strength/residual stress distributions of welded pipeline.

Abstract: This study focused on the determination of fracture toughness by instrumented indentation technique. A theoretical model to estimate the fracture toughness of ductile materials is proposed and used to verify those results. Modeling of IIT to evaluate fracture toughness is based on two main ideas; the energy input up to characteristic fracture initiation point during indentation was correlated with material’s resistance to crack initiation and growth, and this characteristic fracture initiation point was determined by concepts of continuum damage mechanics. The estimated fracture toughness values obtained from the indentation technique showed good agreement with those from conventional fracture toughness tests based on CTOD. In addition, we confirmed that the proposed model can be also applied in the brittle material through modification of void volume fraction.

Abstract: The instrumented indentation technique (IIT) is a powerful method for evaluating mechanical properties of materials such as elastic modulus, tensile strength, fracture toughness and residual stress. Especially, IIT is a promising alternative to conventional methods of residual stress measurement such as hole drilling, saw cutting, X-ray/neutron diffraction, and ultrasonic methods because of its various advantages of nondestructive specimen preparation, easy process, characterization of material properties on local scales and measurement of in-service structures. Evaluation of residual stress using IIT is based on the key concepts that the deviatoric-stress part of the residual stress affects the indentation load-depth curve and that the quantitative residual stress in a target region can be evaluated by analyzing the difference between the residual stress-induced indentation curve and residual stress-free curve. To verify the applicability of the suggested technique, indentation tests were performed on the welded zone.

Abstract: The development of the instrumented indentation test (IIT), which gives accurate measurements of the continuous variation in indentation load as a function of depth, has paved the way to assessing tensile properties and residual stress in addition to hardness by analyzing the indentation load-depth curve. In this study, analytic models and procedures are presented for evaluating tensile flow properties and residual stress states using IIT. Tensile properties were obtained by defining representative stress and strain beneath the spherical indenter. The evaluation of residual stress is based on the concepts that the deviatoric stress part of the residual stress affects the indentation load-depth curve, and that analyzing the difference between the residual stressinduced indentation curve and the residual stress-free curve permits evaluation of the quantitative residual stress in a target region.

Abstract: The Johnson-Kendall-Roberts (JKR) theory was combined with the instrumented indentation technique to evaluate the work of adhesion and modulus of an elastomeric polymer. An indentation test was used to obtain the load-displacement data for contacts between a diamond indenter and poly(dimethylsiloxane), PDMS. The JKR theory, modified to avoid the effect of ambiguous contact radius and depth for nanocontact, was applied to take into account surface adhesion and viscoelastic effects of the compliant polymer. Future work will include experimental verification that polymer stiffness in JKR contact is a time-dependent function.

Abstract: The instrumented indentation technique (IIT) has recently attracted significant research interest because it is nondestructive and easy to perform, and can characterize materials on local scales. Residual stress can be determined by analyzing the indentation load-depth curve from IIT. However, this technique using a symmetric indenter is limited to an equibiaxial residual stress state. In this study, we determine the directionality of the non-equibiaxial residual stress by using the Knoop indentation technique. Different indentation load-depth curves are obtained at nonequibiaxial residual stresses depending on the Knoop indentation direction. A model for Knoop indentation was developed through experiments and theoretical analysis.

Abstract: Surface roughness is main source of error in instrumented microindentation when it is not negligible relative to the indentation depth. The effect of a rough surface on the results of instrumented microindentation testing using spherical indenter was analyzed by applying the contact depth model, which takes surface roughness into account. Improved variations in hardness and Young’s modulus were shown for W and Ni when the results were analyzed by this rough-surface model, while these values were underestimated with increasing surface roughness when analyzed by the flat-surface model. The deformation state of asperities underneath spherical indenter was also discussed.

Abstract: This study combined microscratch test and fracture-mechanical analysis to assess the interfacial reliability of Nafion and Pt/Ru catalyst layers in micro fuel cells. Scratch test was used to determine the critical load for interfacial failure, while fracture-mechanical analysis was used to quantify the adhesion between Nafion (the electrolyte polymer substrate) and Pt/Ru alloy (catalyst coating). We also proposed a key of solving ambiguous problems in indentation cracking test by determining geometric information from crack propagation and critical points, as for a hard porous coating on a soft substrate. A comparative analysis of three coating methods, spray, decalcomania and their mixed process, was done to assess the validity of our new method.

Abstract: The material characterization on the weak points of the structural systems is essential to evaluate safety accurately. However, general material characterization methods such as uniaxial tensile test and CTOD (crack tip opening displacement) test are destructive, therefore, it cannot be applied to the system in use. To overcome this problem, the material characterization using instrumented indentation technique was developed. However, current researches on instrumented indentation technique focus on the hardness measurement. The evaluation of flow property, residual stress and fracture toughness using instrumented indentation technique is not sufficiently performed. In this paper, we introduce the evaluation method of the flow property, the residual stress near the weldment and the fracture toughness developed from damage mechanics. The algorithm of flow property evaluation, the residual stress evaluation model and the fracture toughness model by using indentation were verified comparing with the experimental results.