• Proceedings of the KSME Conference
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• 2000.11a
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• pp.256-261
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• 2000

In this study, we derived work-hardening exponent using continuous indentation test technique. Continuous indentation test technique is a powerful method to evaluate mechanical properties, such as hardness, modulus, ${\sigma}-{\varepsilon}$ curves and etc. It has many merits conventional indentation test has. The relationship between true stress and mean contact pressure and between strain and indentation depth were derived. While the indenter pushes the materials, the region around the indenter is deflected elastically. It is called elastic deflection. And pile-up phenomenon related to plastic deformation around the indenter increased the contact depth, and sink-in phenomenon decreases. So we calibrated contact depth change by considering elastic deflection and pile-up/sink-in. Using calibrated contact depth we redefined the relationship between true stress and mean contact pressure and between strain and contact depth. Through these relationship we could derive work-hardening exponent by analyzing load-depth curves. And it showed good agreement with tensile test results.

• Transactions of Materials Processing
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• v.14 no.6 s.78
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• pp.541-546
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• 2005

Apparent mechanical properties in structural components can be different from the initially designed values due to the formation of the residual stress in metal forming and welding. Therefore, the evaluation of residual stress has great importance in the reliability diagnosis of structural components. A nondestructive continuous indentation technique has been proposed to evaluate various strength concerning mechanical properties from the analysis of load-depth curve. In this study, quantitative residual stress estimation on API X65 welded joints for natural gas pipeline was performed by analyzing the variation of indentation loading curve by residual stress through a new proposed theoretical model. The residual stress from the indentation method was compared with that from the saw-cutting method.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2005.05a
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• pp.126-129
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• 2005

Apparent mechanical properties in structural components can be different from the initially designed values due to the formation of the residual stress in metal forming and welding. Therefore, the evaluation of residual stress has great importance in the reliability diagnosis of structural components. A nondestructive continuous indentation technique has been proposed to evaluate various strength concerning mechanical properties from the analysis of load-depth curve. In this study, quantitative residual stress estimation on API X65 welded joints for natural gas pipeline was performed by analyzing the variation of indentation loading curve by residual stress through a new proposed theoretical model. The residual stress from the indentation method was compared with that from the saw-cutting method.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.5
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• pp.881-889
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• 2002

Newly developed continuous indentation technique has made nondestructive assessment of tensile properties possible. The present study was undertaken to determine new parameters for indicating time-dependent material degradation of petro-chemical plant. Continuous indentation tests were performed for Cr-Ni steel and Cr-Mo steel which are widely used as facilities of petro-chemical plant. From the results, it was found that yield strength and tensile strength cannot be used as general degradation parameters because the changes in the strengths with aging time didn't show any tendency. On the other hand, work hardening exponent and yield ratio showed consistent tendency with increasing aging time. Therefore, nam attention of this work was paid on them as new degradation parameters, and the in-field applicability of the parameters was evaluated and discussed.

• Journal of the Korean Society for Nondestructive Testing
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• v.24 no.2
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• pp.158-162
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• 2004

Reliability of welded structures in power plant facilities is very important, and their reliability evaluation requires exact materials properties. But, the conventional PQR (Procedure Qualification Record) can hardly reflect the real material properties in the field because the test is only done on specimens with simulated welding. Therefore, a continuous indentation technique is proposed in this study for simple and non-destructive testing of in-field structures. This test measures the indentation load-depth curve during indentation and analyzes the mechanical properties such as the yield strength, tensile strength and work hardening index. This technique has been applied to evaluate the tensile properties of the weldment in the main steam pipe and hot reheater pipe in power plants under construction and in operation.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2002.05a
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• pp.191-194
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• 2002

The tensile properties of materials can be obtained just in accordance with conventional tensile testing methods which are described in several standards. However, the standard testing methods cannot be applicable due to the destructive testing procedure and specimen size requirement for some cases including on-service facility materials. Therefore, simple, non-destructive and advanced indentation technique was proposed. This test measures indentation load-depth curve during indentation and analyzes the mechanical properties related to deformation and fracture. In this paper, the research trend of non-destructive evaluation of tensile properties using advanced indentation system and its application fields are reviewed and discussed.

• 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.

• 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 KWS Conference
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• 2005.06a
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• pp.288-290
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• 2005

Apparent mechanical properties in structural components can be different from the initially designed values due to the formation of the residual stress in metal forming and welding. Therefore, the evaluation of residual stress has great importance in the reliability diagnosis of structural components. A nondestructive continuous indentation technique has been proposed to evaluate various strength concerning mechanical properties from the analysis of load-depth curve. In this study, quantitative residual stress estimation on API X65 welded joints for natural gas pipeline was performed by analyzing the variation of indentation loading curve by residual stress through a new proposed theoretical model.

• Journal of the Korean Society for Heat Treatment
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• v.15 no.4
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• pp.188-193
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• 2002