Yeol Choi

Analysis of sharp-tip-indentation load-depth curve for contact area determination taking into account pile-up and sink-in effects

Hardness and elastic modulus of micromaterials can be evaluated by analyzing instrumented sharp-tip-indentation load–depth curves. The present study quantified the effects of tip-blunting and pile-up or sink-in on the contact area by analyzing indentation curves. Finite-element simulation and theoretical modeling were used to describe the detailed contact morphologies. The ratio f of contact depth, i.e., the depth including elastic deflection and pile-up and sink-in, to maximum indentation depth , i.e., the depth measured only by depth sensing, ignoring elastic deflection and pile-up and sink-in, was proposed as a key indentation parameter in evaluating real contact depth during indentation. This ratio can be determined strictly in terms of indentation-curve parameters, such as loading and unloading slopes at maximum depth and the ratio of elastic indentation energy to total indentation energy. In addition, the value of f was found to be independent of indentation depth, and furthermore the real contact area can be determined and hardness and elastic modulus can be evaluated from f. This curve-analysis method was verified in finite-element simulations and nanoindentation experiments.

Assessing welding residual stress in A335 P12 steel welds before and after stress-relaxation annealing through instrumented indentation technique

Abstract Conventional nondestructive techniques for welding residual stress measurement have many disadvantages in the field because of poor repeatability, large scatter in data, complex procedures, inaccurate results, etc. To overcome these difficulties, an instrumented indentation technique was applied to evaluate the welding residual stress in A335 P12 steel welds in electric power-plant facilities before and after stress-relaxation annealing. Comparison of our results with stress values obtained from a destructive saw-cutting test showed that the instrumented indentation technique is very useful for quantitative/nondestructive evaluation of welding residual stresses in industrial facilities such as power-plants.

Analysis of mechanical property distribution in multiphase ultra-fine-grained steels by nanoindentation

Abstract The strength characteristics of microphases in ultra-fine-grained steels were analyzed using nanoindentation and AFM. It was found that there were fine ferrite grains ( 1–2 μ m) formed by a strain-induced dynamic transformation in ultra-fine-grained steels. They had equiaxed and polygonal grain shape, and higher hardness and elastic modulus than coarse ferrite transformed statically. Strengthening factors of strain-induced dynamic transformation ferrite were analyzed in terms of cementite particles and dislocation density.

Derivation of tensile flow properties of thin films using nanoindentation technique

Abstract By regarding the tip blunting as a ball indentation at very low depth range (within about 80 nm in our experiments), the flow properties of Au thin films were derived from the indentation load–depth curve obtained by nanoindentation technique. The effects of pile-up or sink-in were considered in determining the real contact between the indenter and the specimen. The representative strain in indentation was defined in various ways and examined by comparing the flow properties derived from indentation load–depth curve with those measured by tensile test. The best definition was found to be the shear strain at contact edge multiplied by 0.1. When we considered the effects of pile-up or sink-in, the representative stress in indentation could also be determined, and was found to be one third of the mean contact pressure for fully plastic regime. As a more intrinsic property than hardness, the yield strengths of Au films with thickness of 0.56 and 0.99 μm were extrapolated from the derived true stress–true strain curve as 261±30 and 154±18 MPa, respectively.

Nondestructive observation on tensile property change of hydrogen-exposed Cr-Mo-V steel HAZ using an instrumented indentation technique

Long-term service of materials in environments of high hydrogen pressure can cause various sorts of damage such as temper embrittlement, hydrogen-assisted degradation and hydrogen attack [1‐3]. The demand in the petrochemical refinery industry for superior resistance to this damage has promoted the use of V-modified Cr-Mo steels for petrochemical reactor vessels. Like other structural steels, the Cr-Mo-V steels undergo welding during vessel construction. The superior properties of this steel must be carefully reconsidered because, as is well accepted, welding can seriously alter metallurgical and mechanical properties, generally for the worse. In particular, the coarse-grained heataffected zone (CGHAZ) adjacent to the fusion line is known as one of the weakest regions in the welded joints [4]. In addition, it has been reported that the CGHAZs of Cr-Mo steels are much more sensitive to hydrogeninduced damage than the base material [2]. Therefore, proper evaluation of the time-dependent change in mechanical properties of Cr-Mo-V steel joint CGHAZs exposed to hydrogen is very important in assessing the safety performance of the petrochemical reactor vessels under high hydrogen pressure. The present work, the first of a series of studies of the time-dependent degradation of CGHAZs in Cr-Mo-V steels according to increasing hydrogen exposure time, uses a nondestructive instrumented indentation technique to clarify the change in tensile properties.

Substitutional boron-doping of carbon nanotubes ☆

The substitutional placement of boron within the lattice of carbon nanotubes yields quite different transport properties for single walled nanotubes (SWNTs) as compared to multi-walled nanotubes (MWNTs). Boron ‘‘doping’’ of the MWNTs results in an acceptor state in the local density of states (LDOS) that lies near the Fermi level and can be directly correlated with features in the thermoelectric power (TEP) of B-doped MWNT mats. Transport measurements of individual B-doped MWNTs exhibit features associated with variable range hopping. In contrast, B-doping of SWNTs results in features in the density of states further from the Fermi level, and transport of the SWNTs shows an unusual variability in rectification not observed in the MWNT case. This suggests that boron has been introduced into the lattice of these two morphologies of nanotubes in very different ways. Interest in the electrical transport properties of both

Determining Stress-strain Curves for Thin Films by Experimental/Computational Nanoindentation

The nanoindentation technique has great promise in evaluating mechanical properties such as nanohardness and elastic modulus at micrometer or nanometer scales, since sample preparation and testing procedures are very easy. However, the nanohardness and elastic modulus cannot be directly related to basic material flow properties. Here a novel and simple experimental/computational method is proposed to extract stress-strain curves based on finite-element modeling (FEM) of nanoindentation. This method was verified for bulk Al by comparing the stress-strain curves extracted with those obtained from tensile testing, and was applied to Al thin films (0.5 μm and 1 μm) deposited on a Si substrate.

Determination of Brinell Hardness through Instrumented Indentation Test without Observation of Residual Indent

Hardness test is performed for determination of the other properties, such as strength, wear resistance and deformation resistance, as well as hardness itself. And it is performed for prediction of residual lifetime by analysis of hardness reduction or hardness ratio. However, hardness test has limitation that observation of residual indent is needed for determination of hardness value, and that is the reason for not to be widely used in industrial field. Therefore, in this study, we performed researches to obtain Brinell hardness value from quantitative numerical formula by analysing relationship between indentation depths from indentation load-depth curve and mechanical properties such as work hardening exponent, yield strength and elastic modulus.

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.

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.