• Transactions of the Korean Society of Mechanical Engineers A
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• v.29 no.9 s.240
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• pp.1253-1261
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• 2005

Cast stainless steel (CSS) is thermally aged by a long term exposure in the range of nuclear power plant operating temperature. The thermal aging is a cause of concern for the continued safe and reliable operation of CSS nuclear components. Therefore, an assessment of degradation in material properties of these components has been importantly considered. In this study the ball indentation tests were performed on four cast stainless steels aged at $400^{\circ}C$ for 3600 hours, to investigate the applicability of ball indentation test to the assessment of aging degradation of cast stainless steels. Thus, the reliability of ball indentation test for aged CSS was analyzed by evaluating the scattering of data tested from each material and by comparing tensile properties obtained from ball indentation test and standard tensile test. Also, the tensile properties of aged CSS obtained from ball indentation test were compared with those predicted by the evaluation procedure developed on the basis of material database for aged CSS.

• Transactions of Materials Processing
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• v.13 no.1
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• pp.21-26
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• 2004

Instrumented indentation tests have been used for estimating material properties. In order to analyze deformation characteristics with various factors, the unloading stiffness should be properly determined from the elastic behavior. The unloading stiffness is generally obtained from the shifted power functions fitting with the experimental unloading data. However, the functions often give rise to a poor representation of actual data, and also the unloading stiffness is governed by unloading condition. In this study, both numerical and experimental conditions to obtain proper unloading stiffness were investigated. The result showed that the amount of unloading ratio and hold time played an important role in fitting the unloading curves. The current efforts can successfully provide the unloading stiffness for indentation material properties.

• Proceedings of the KSR Conference
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• 2001.05a
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• pp.230-237
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• 2001

For the structural integrity of large and complex structures such as railway vehicle, the in-field diagnosis of mechanical properties of the structures is needed, and especially, the mechanical characteristics of the weldment must be carefully evaluated. But, conventional standard testing methods having destructive procedures are not applicable to in-field assessment of mechanical property variations within weldment because they needs the limitations of specimen size and geometry. In this paper, to overcome this problems, the advanced indentation technique (AIS) is introduced for simple and non-destructive/in-field testing of weldment of industrial structures. This test measures indentation load-depth curve during indentation and analyzes the mechanical properties related to deformation and fracture. First of all, flow properties such as yield strength, tensile strength and work hardening index can be evaluated through the analysis of the deformation behavior beneath the spherical indenter. Additionally, case studies of advanced indentation techniques are introduced.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2003.05a
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• pp.62-62
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• 2003

A statistical evaluation method was proposed to evaluate mechanical properties by using small specimens and nano-indentation for irradiation study. The method is empirically based on nano-indentation which values are statistically treated. The nano-indentation in function of indentation depth (h) is expressed using the variation factor V(h). Statistical parameters of the indentation are given by histograms. Analytical and experimental relation between histograms of phase dimension distribution and parameters V(h) and G(h) is considered using the condition of additivity of phases' microhardness. The method is applied to estimate mechanical properties of irradiated materials.

• Transactions of Materials Processing
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• v.13 no.8
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• pp.731-737
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• 2004

Mechanical properties such as Young's modulus and hardness of thin film in coated steel are difficult to determine by nano-indentation from the conventional analysis using the load-displacement curve. Therefore, an analysis of the nano-indentation loading-unloading curve was used to determine the Young's modulus, hardness. A new method is recently being developed for elastic-plastic properties of materials from nano-indentation. Elastic modulus of the thin films shows relatively small influence whereas yield strength is found to have significant effect on measured data. The load-displacement curves of material tested with a Berkovich indenter and nano-indentation continuous stiffness method is used to measure the modulus and hardness through thin films, and then these are computed using the analysis procedure. The developed neural networks apply also to obtain reliable mechanical properties.

• Proceedings of the KSME Conference
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• 2004.04a
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• pp.132-137
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• 2004

In this work, effects of hyper-elastic rubber material properties on the indentation load-deflection curve and subindenter deformation are first examined via [mite element (FE) analyses. An optimal data acquisition spot is selected, which features maximum strain energy density and negligible frictional effect. We then contrive two normalized functions. which map an indentation load vs. deflection curve into a strain energy density vs. first invariant curve. From the strain energy density vs. first invariant curve, we can extract the rubber material properties. This new spherical indentation approach produces the rubber material properties in a manner more effective than the common uniaxial tensile/compression tests. The indentation approach successfully measures the rubber material properties and the corresponding nominal stress.strain curve with an average error less than 3%.

• Proceedings of the KSME Conference
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• 2001.11a
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• pp.266-271
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• 2001

Spherical indentation technique was developed to evaluate the flow properties of metallic materials in carbon steel, stainless steel and alloys, etc. Through the spherical indentation test, differently degraded 1Cr-1Mo-0.25V steel's mechanical properties were observed and compared with conventional standard test data. The flow properties of 1Cr-1Mo-0.25V steels were estimated by analyzing the indentation load-depth curve. To characterize the flow property, we used material yield slope and constraint factor index rather than strain-hardening exponent because the variation of strain-hardening exponent was very little and the data showed irregularly. And the constraint factor's effect was small when the material yield slope was taken into account.

• Journal of the Korean Society for Railway
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• v.7 no.2
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• pp.142-148
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• 2004

When material properties depend much on positions in a material or it is difficult to make test specimens from a material or component, an instrumented indentation test described in ISO 14577-1, 14577-2 or KS B 0950 can be used to measure material properties and damage. In this study, first of all, the principals of the instrumented indentation test, KS B 0950 are introduced and yield strengths, tensile strengths and work hardening exponents of base materials, heat affected zones and weld materials are measured. In addition, the influence of post-weld heat treatment on the material properties is investigated. Finally the fatigue lift of butt welded specimens are evaluated by the local strain approach. To calculate local strains and stresses, elasto-plastic finite element analysis is conducted using the measured properties.

• Journal of the Korean Society for Precision Engineering
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• v.22 no.10 s.175
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• pp.186-194
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• 2005

This study was designed to investigate the mechanical properties of the coating layer on electronic galvanized sheet steel as a part of the ongoing research on the coated steel. Those properties were determined using nano-indentation, the finite element method, and artificial neural networks. First and foremost, the load-displacement curve (the loading-unloading curve) of coatings was derived from a nano-indentation test by CSM (continuous stiffness measurement) and was used to measure the elastic modulus and hardness of the coating layer. The properties derived were applied in FE simulations of a nano-indentation test, and the analytical results were compared with the experimental result. A numerical model for FE simulations was established for the coating layer and the substrate separately. Finally, to determine the mechanical properties of the coating, such as the stress-strain curve, functional equations of loading and unloading curves were introduced and computed using the neural networks method. The results show errors within $5\%$ in comparison with the load-displacement measured by a nano-indentation test.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.148-151
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• 2004

Mechanical properties such as Young's modulus and hardness of thin film in coated steel are difficult to determine by nano-indentation from the conventional analysis using the load-displacement curve. Therefore, an analysis of the nano-indentation loading curve was used to determine the Young's modulus, hardness and strain hardening exponent. A new method is recently being developed for plasticity properties of materials from nano-indentation. Elastic modulus of the thin films shows relatively small influence whereas yield strength and strain hardening are found to have significant effect on measured data. The load-displacement behavior of material tested with a Berkovich indenter and nano-indentation continuous stiffness method is used to measure the modulus and hardness through thin films.