Dong Il Kwon

Residual Stress Estimation with Identification of Stress Directionality Using Instrumented Indentation Technique

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.

Fracture Behavior of Single- and Polycrystalline Silicon Films for MEMS Applications

Fracture behaviors of silicon films were evaluated by microtensile methods. We fabricated two types of specimens using surface micromachining, one for a test device for microtensile testing only and the other for a microtensile-compatible resonating device driven by alternating electrostatic force. The piezoelectric-driven uniaxial stress-strain measurement system was designed to evaluate the mechanical properties of thin films. We used UV adhesive to grip the device to the microtensile system, and the grip was made of UV-transparent glass in order to cure the underlying UV adhesive layer. To assess fracture toughness, we used newly proposed methods combining resonance frequency and microtensile methods. The fracture strength of single- and polycrystalline silicon showed dependence on geometry and doping condition. By varying the geometry, we analyzed the effect of the CMP side and the dry-etched side on changes in the mean fracture strength. Atomic force microscopy observation showed that the larger flaws of the dry-etched side were significant in decreasing the mean fracture strength. Fracture toughness based on fracture mechanics with a precrack was evaluated by newly proposed methods combining resonance frequency and microtensile techniques. The measured toughness was independent of specimen geometry but dependent on doping condition. The measured fracture toughness of notched specimens was 33% higher than that of pre-cracked specimens, even though the notch radius was as small as 1.4µm. The effects of notch-tip radius and doping on fracture toughness of silicon film were also analyzed.

Nondestructive Evaluation of Welding Residual Stress in Power Plant Facilities Using Instrumented Indentation Technique

The weld joints in power-plant pipelines have long been considered important sites for safety and reliability assessment. In particular, the residual stress in pipeline weldments induced by the welding process must be evaluated accurately before and during service. This study reports an indentation technique for evaluating welding residual stress nondestructively. Indentation load-depth curves were found to shift with the magnitude and direction of the residual stress. Nevertheless, contact depths in the stress-free and stressed states were constant at a specific indentation load. This means that residual stress induces additional load to keep contact depth constant at the same load. By taking these phenomena into account, welding residual stress was obtained directly from the indentation load-depth curve. In addition, the results were compared with values from the conventional hole-drilling and saw-cutting methods.

Influence of Biaxial Surface Stress on Mechanical Indenting Deformations

A theoretical model has been proposed to assess quantitative residual stress from a stress-induced shift in an indentation curve, but the assumption in this model of equibiaxial surface stress has obstructed its application to real structures in complex stress states. Thus we investigated the influence of non-equibiaxial surface stress on contact deformation through instrumented indentations of a biaxially strained steel plate in order to extend the model to a general surface stress by considering a ratio of two principal stresses.

Applications of Advanced Indentation Technique to Pre-Qualification and Periodic Monitoring of Strength Performance of Industrial Structures

The demand for new in-field technology which can non-destructively evaluate the key material properties has been increased for safe and economic operations of industrial structures/facilities whose material properties can be significantly degraded during operation in hostile environments. As a promising method to meet the needs, an advanced indentation technique is suggested here. This novel technique can evaluate the true-stress-true-strain relationship and quantitative tensile properties (such as yield strength, tensile strength and work-hardening exponent) non-destructively by analyzing indentation load-depth curve. In this paper, two recent applications of the indentation technique to preand in-service-inspection (PSI and ISI) of industrial structures are introduced and discussed. First, pre-qualification of strength performance of welded joint in power plant pipes is successfully performed using this indentation technique while conventional pre-qualification of welded joints through other non-destructive techniques are just focusing on exact crack detection. Second, it was proved that the advanced indentation technique has the strong potential to be used for in-field periodic monitoring of strength change of oil/gas transmission pipeline, which is required by new regulations for safe maintenance and economical repair.

Residual Stress Assessment in API X65 Pipeline Welds by Non-Destructive Instrumented Indentation

Conventional residual stress-measurement methods are difficult to use in industrial structures largely because of their complexity and destructive nature. In particular, the rapid microstructural gradients across welds impede a quantitative assessment of residual stress. An instrumented indentation technique has been developed to overcome the current problems in measuring the residual stress. Here this instrumented indentation technique was used to characterize the residual stress in API X65 steel welds in a natural gas transmission pipeline. The difference in indentation load between the stressed and unstressed regions at a fixed penetration depth was converted to a quantitative residual stress value through an analytical procedure. The stress distribution assessed across the welds agreed roughly with that from destructive saw-cutting tests. The discrepancy in absolute stress values from the two methods was considered to arise from the effects of microstructural difference and stress directionality in the welds. Introduction The quantitative measurement of welding residual stress is very important for the safe operation and economical maintenance of industrial structures and facilities such as gas/oil transmission pipelines, power plants, petrochemical plants, and so on. Over the last few decades, various lab-scale stress-measurement methods have been developed [1]. Destructive hole-drilling and saw-cutting methods are sensitive to macroscopic stress and evaluate the residual stress quantitatively without any reference sample, but they have limitations in industrial applications due to their destructive characteristics and the possibility of creating a new stress state by material removal. Thus, such nondestructive methods as X-ray and neutron diffraction, ultrasonic method, and Barkhausen noise have been investigated to establish relationships between the physical or crystallographic stress-detection parameter and the residual stress. However, these techniques are difficult to apply to welds with rapid microstructural gradients because the stress-detection parameter is also highly sensitive to metallurgical factors. Initial indentation studies for measuring residual stress focused on deriving the stress dependency of the contact hardness [2]. However, the stress-induced alteration in the hardness was less than 10% of its value in the unstressed specimen [3], so that the use of the contact hardness as a residual stress parameter was dubious. Recently, an instrumented indentation technique that measures elastic/plastic deformation behavior beneath a rigid indenter as a curve of indentation load versus indenter penetration depth has been developed and applied to evaluate various mechanical properties such as contact hardness, elastic modulus [4], yield and tensile strengths, work-hardening index [5] and fracture toughness [6]. The stress sensitivity and application potential of this technique had been recognized earlier; however, the relevant studies were tentative and somewhat empirical for a good while. Tsui et al. [3], studying the influence of in-plane stress on indentation plasticity by investigating both the shape of the indentation curve and the contact impressions, reported that hardness is invariant regardless of the applied stress within the elastic limit. An indentation model Key Engineering Materials Vols. 270-273 (2004) pp. 35-40 online at http://www.scientific.net

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.

Effects of Rough Surface on Contact Depth for Instrumented Microindentation Using Spherical Indenter

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.

Characterization of Polymer Adhesion through Modified JKR Theory and Instrumented Indentation Technique

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.

Yield Property Characterization for Au and TiN Thin Films by Applying Nanoindentation Technique

We tried to apply the nanoindentation technique to yield strength characterization by modifying a previous research. Although the yield strength determining technique developed by Kramer et al. has been successfully demonstrated for large scale indentations on bulky metals, its applicability is still doubtful to nanoscale indentations on thin films with severe roughness, anisotropy, and interfacial constraint. In order to overcome these problems, we combined the nanoindentation technique with a three-dimensional indent visualization technique in this study. Nanoindentation tests were performed for Au and TiN thin films and their corresponding indents were scanned by using an atomic force microscope. From the three-dimensional pile-up morphology, a circular pile-up boundary was measured and input into the yield strength formulation as an effective yielded zone radius. The yield strengths calculated were directly compared with those from the microtensile test.