Kyung-Woo Lee

Mechanical characterization of nano-structured materials using nanoindentation

The principal strengths of the nanoindentation technique, which is used extensively to measure the mechanical properties of nano/micro materials, are easy sample preparation and simple experimental method. Hardness and Youngs modulus are essential properties measured by nanoindentation; hardness corresponds to resistance to plastic deformation whereas Youngs modulus is related to elastic deformation. Two key difficulties arise in association with nanoindentation on small volumes: measurement accuracy and material response. Here we discuss the indentation size effect (ISE) considering tip bluntness and variation in hardness of nano-multilayers with a bilayer period, representative research on measurement improvement, and material response at nanoscales.

Nondestructive Estimation of Fracture Toughness Using Instrumented Indentation Technique

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.

Feasible Evaluation of Flow Properties and Stress State of Structural Materials Using Instrumented Indentation Tests

Flow properties and stress state are indispensable factors for safety assessment of structural materials in operation, which were evaluated using instrumented indentation tests (IITs). Flow properties were obtained by defining representative stress and strain, and IIT results for 10 steel materials were discussed by comparing with those from uniaxial tensile tests. The indentation load-depth curve is significantly affected by the presence of residual stress, and the stress-induced load change was converted to a quantitative stress value. The stress state of a friction stir-welded joint of API X80 steel was evaluated and compared with that measured by energy-dispersive X-ray diffraction.

Application of Nondestructive Instrumented Indentation Technique in Small-Scale Testing of Pressure Vessel and Piping Systems

Most small-scale testing techniques are essentially scaled-down versions of conventional testing techniques: they use specimens of similar geometry applied in a similar manner to estimate properties equivalent to those obtained for larger specimens. However, using these techniques for safety assessment of structures and piping systems requires general agreement about the techniques and validation of their results. In addition, these techniques all require destructive testing. In this study we adopt a new nondestructive method to measure the mechanical properties using the instrumented indentation technique. This technique can be applied directly in small-scale and localized sections because of its high spatial resolution. It also has the significant advantage of simplicity of specimen preparation and experimental procedure. During instrumented indentation testing, the load and penetration depth of an indenter tip driven into the sample are monitored, and material properties such as strength, fracture toughness and residual stress are evaluated from this information: the tensile properties by defining a representative stress and strain underneath a spherical indenter; the residual stress values near weldments by using the stress-insensitive contact hardness model.Copyright

A New Method for Nondestructive Evaluation of Mechanical Properties Using Instrumented Indentation Technique

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.

Assessment of Residual Stress and Determination of Stress Directionality by Instrumented Indentation Technique

The instrumented indentation technique has taken the limelight as a promising alternative to conventional residual stress measurement methods for welds with rapid microstructural gradients because of its easy and nondestructive testing procedure. The technique is based on the key concept that the deviatoric-stress part of residual stress affects the indentation load-depth curve. By analyzing the difference between the residual stress-induced curve and residual stress-free curve, the quantitative residual stress of the target region can be evaluated. To determine the stress-free curve of the target region, we take into consideration microstructural changes that accommodate strength differences. In addition, we determine the ratio of the non-equibiaxial residual stress by using an asymmetric Knoop indenter, which has an elongated four-sided pyramidal geometry. We find that the load-depth curve is changed on penetration direction of the long diagonal for Knoop indenter, and derive a quantitative relation between the stress ratio and the load difference through both theoretical analysis and experiments. Finally, indentation tests and conventional tests were performed on the welded zone to verify the applicability of the technique. The estimated residual stress values obtained from instrumented indentation technique agreed well with those from conventional tests.Copyright

Application of Instrumented Indentation Technique for Small Scale Testing

Instrumented indentation technique (IIT) is a novel tool to estimate mechanical properties such as tensile properties, residual stress and fracture toughness by analyzing indentation load-depth curve measured during loading-unloading of indentation. It can be applied directly in small-scale and localized sections of pressure vessel and pipeline since the preparation of specimen is very easy and the experimental procedure is feasible and nondestructive. We present the principles developed for measuring mechanical properties using IIT; the tensile properties by defining the representative stress and strain underneath a spherical indenter, the residual stress near the weldments using the stress-insensitive contact hardness model, and the fracture toughness of ductile metal based on critical indentation energy model. The experimental results from IIT were verified by comparing the results from the conventional methods such as uniaxial tensile test for tensile properties, mechanical saw-cutting and hole-drilling methods for residual stress, and CTOD test for fracture toughness. In particular, the applications of IIT in small scale materials and localized sections of the pressure vessel and pipeline in-use and in-fields are presented.© 2007 ASME

Non-destructive evaluation of strength and residual stress for weldments using continuous indentation technique

Residual stress, generated by the processing and welding of metallic materials, changes the structural configuration, intensifies external effects, such as chemical and dynamic environmental factors, reduces plastic deformation, fatigue and fracture resistance and reduces the structural life. In particular, welding residual stress generated by heat treatment during welding, is a crucial factor which is capable of causing fracture in the structures of facilities, so control by quantitative evaluation during construction and in service is essential. Furthermore, evaluation of strength non-uniformity and variation in the microstructures of respective regions of the weld metal, HAZ and base metal is also critical for weld zone integrity monitoring. A range of destructive testing methods, such as creep, tensile and residual stress evaluation testing should be implemented in order to evaluate appropriateness of welding and material suitability for such an in-service facili ty. However, with these destructive testing methods it is difficult to collect standardized test specimens and to measure the properties accurately due to varied strength properties depending upon non-uniform microstructures and specimen extraction locations. Existing non-destructive evaluation methods to measure the residual stress such as the X-ray diffraction method, neutron diffraction method and ultrasonic method have disadvantages, such that it is susceptible to the microstructure and measurement temperature, so specific information is required in advance. Furthermore, destructive methods, such as hole-drilling and saw cutting methods, have problems since not only do they have the drawbacks of being destructive but also there is the possibility of causing additional stress during removal of a coupon. On the other hand, the uniaxial tensile testing is standardized and employed for the evaluation of tensile strength properties but, due to the problems of specimen extraction required to achieve standard test specimens and processing, it is impossible to employ this method either during operation or for installed structures. Consequently, the development of a new method of testing is required where there is little restriction in extracting test specimens and the strength and residual stress characteristics of local regions can be accurately evaluated. In order to respond to the requirement described above, the continuous indentation technique was developed as a non-destructive testing method which can evaluate the residual stress and tensile strength characteristics of in-service facili ty structures. The continuous indentation technique can evaluate various mechanical properties, such as flow stress characteristics, fracture toughness and residual stress by means of the indentation load-displacement curve analysis, as indicated in figure 1, by continuous measurement of the indentation depth which is accompanied by an increase in the indentation load. Furthermore, with this technique, it is feasible to evaluate within a narrow region of maximum dimensions of several hundreds micrometers so, for example, the technique is capable of evaluating the characteristics of weld zone with materials with locally varied properties. This technique can be regarded as a non-destructive testing method since post-tested materials are left only with extremely small indentations. Non-destructive evaluation of strength and residual stress for weldments using continuous indentation technique

Determination of Precise Contact Area During Spherical Indentation for Metallic Materials

The continuous indentation technique, because it is fast, precise, and nondestructive, has been widely used to determine such mechanical properties as flow properties, residual stress, fracture properties, viscoelastic properties and hardness of materials and structural units. In particular, continuous indentation by a spherical indenter can provide hardness and flow properties such as yield strength, tensile strength, and work-hardening exponent, using the characteristic that strain from the loaded indenter changes with indentation depth. Since the stress and strain values on the flow curve are defined based on the contact area between the indenter and material in the loaded state, accurate determination of the contact area is essential. Determination of the contact area is closely connected with elastic deflection and plastic pile-up/sink-in behavior. In this study, the pile-up phenomenon is considered as two independent behaviors, elastic deflection and plastic pile-up/sink-in, which can each be described by a formula. The formulas can be obtained from FE simulation with conditions reflecting real indentation tests for materials used for various purposes and with a wide range of material properties. By analyzing indentation morphology from the FE simulation, the two phenomena were quantified as formulas. In particular, plastic pile-up/sink-in behavior was formulated in terms of work-hardening exponent and indentation ratio.Copyright