Dongil Kwon

Derivation of plastic stress–strain relationship from ball indentations: Examination of strain definition and pileup effect

The ball indentation technique has the potential to be an excellent substitute for a standard tensile test, especially in the case of small specimens or property-gradient materials such as welds. In our study, the true stress–true strain relationships of steels with different work-hardening exponents (0.1–0.3) were derived from ball indentations. Four kinds of strain definitions in indentation were attempted: 0.2sinγ, 0.4hc/a, ln[2/(1 + cosγ)], and 0.1tanγ. Here, γ is the contact angle between the indenter and the specimen, hc is the contact depth, and a is the contact radius. Through comparison with the standard data measured by uniaxial tensile testing, the best strain definition was determined to be 0.1tanγ. This new definition of strain, in which tanγ means the shear strain at contact edge, reflected effectively the work-hardening characteristics. In addition, the effects of pileup or sink-in were considered in determining the real contact between the indenter and the specimen from the indentation load–depth curve. The work-hardening exponent was found to be a main factor affecting the pileup/sink-in phenomena of various steels. These phenomena influenced markedly the absolute values of strain and stress in indentation by making the simple traditional relationship Pm/σR ≈ ≈ 3 valid for the fully plastic regime.

Film-thickness considerations in microcantilever-beam test in measuring mechanical properties of metal thin film

Since metal thin films have broad applications in micromechanical devices, their mechanical properties are very important in designing and controlling such devices. In this study, the Young’s modulus and yield strength were estimated by using a microcantilever beam-bending technique. Al and Au cantilever beams of various thicknesses and lengths were fabricated by silicon bulk micromachining and were loaded in bending by a nanoindenter; displacement was simultaneously obtained at high resolution. The load–displacement data showed that the yield strengths of the two films increase with decreasing film-thickness. However, the Al and Au films showed different mechanical behaviors: the yield strength of the Au film followed the grain-size dependency of Hall–Petch type, but that of the Al film was not fully described by the grain-size effect alone. This difference arises mainly from the different surface conditions of each film. The effect of the surface on a film’s mechanical properties is discussed and a microstructure effect was suggested that a modified misfit dislocation theory used to predict the yield strength. 2003 Elsevier Science B.V. All rights reserved.

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.

Residual stresses in DLC/Si and Au/Si systems: Application of a stress-relaxation model to the nanoindentation technique

Residual stress in a thin film was analyzed by the nanoindentation technique. Two dominant effects of residual stress to indentation were summarized as the slope change in loading curve and the invariant value of intrinsic hardness. A stress-sensitive reversibly deformed zone around contact was modeled to explain the indentation behaviors under a residually stressed state. Finally, the residual stress was evaluated from the changes in contact shape and applied load during stress relaxation under the condition of constant indentation depth. The residual stresses in diamond-like carbon and Au films analyzed from this model agreed well with the average values measured by the curvature method.

Transformation strengthening by thermomechanical treatments in C-Mn-Ni-Nb steels

The purpose of this study is to clarify the correlation between microstructural factors and mechanical properties of ultrafine steels processed by thermomechanical controlled treatments. Three steels deformed at high strain rates in a pilot plant rolling mill showed very fine ferritic microstructure, whose grains became more equiaxed and finer with increasing fraction of alloying elements, and had good tensile and fracture properties, although they contained only about 0.01 pct carbon. Especially in the Ni-added steel, tensile properties were greatly improved because of the high dislocation density and the fineness of the ferritic substructure, readily satisfying the requirements for commercial-grade high-strength, high-toughness steels. The formation of ultrafine equiaxed grains in the steels might be explained by a possible strain-induced dynamic transformation mechanism associated with the austenite → ferrite transformation caused by heavy deformation in the austenite range.

Micromechanical estimation of composite hardness using nanoindentation technique for thin-film coated system

Abstract The hardness characteristics of thin-film coated composite were analyzed using the plastic-zone volume-law of mixtures theory. In calculating the plastic-zone volumes, the elastic/plastic indentation stress field was analyzed; the substrate was assumed to undergo radial deformation, while the cylindrical deformation mode was applied to the film. In addition, the effect of interface was incorporated. The deformation of the softer material was assumed to be constrained by the harder one under the condition of strain matching at the interface. In experiments, the hardness (H) and elastic modulus (E) of Al2O3 film, glass, and 304 stainless steel were evaluated by analyzing the load–depth curve measured by nanoindentation technique. And, the yield strength (Y) was calculated using the relationships between H, E, and Y. Next, we measured the composite hardness values of Al2O3/glass and Al2O3/304 stainless steel systems with increasing load up to 100 mN, and compared with those predicted. For Al2O3/glass system, the measured trend of composite hardness correlated well with the predicted one. On the other hand, there were some deviations for Al2O3/304 stainless steel system, which was expected to result from the film cracking and/or the change of load distribution for hard coating on very soft substrate.

Correlation of microstructure and fracture properties in weld heat-affected zones of thermomechanically controlled processed steels

This article presents a correlation study between the microstructural parameters and fracture properties in the weld heat-affected zones (HAZs) of high-strength low alloy (HSLA) steels,i.e., a normalized steel and four thermomechanically controlled processed (TMCP) steels. The influence of the local brittle zone (LBZ) on toughness was investigated by means of simulated HAZ tests as well as welded joint tests. The intercritically reheated coarse-grained HAZ ex-hibited the lowest impact energy over the testing temperature range, indicating that this region was the LBZ. By comparing the volume fraction of martensite islands with impact energy val-ues, this LBZ was attributed mainly to the significant increase in the amount of martensite. Niobium was also found to have a deleterious effect on the HAZ fracture toughness because of martensite hardening. This suggests that the formation of martensite islands must be controlled by proper design of chemical compositions to reduce the carbon equivalent and by using the proper welding conditions to limit cooling rates in order to optimize the fracture toughness of welded joints of TMCP steels.

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.

Effects of microstructural change on fracture characteristics in coarse-grained heat-affected zones of QLT-processed 9% Ni steel

Abstract This study investigates the correlation between the microstructural change and fracture characteristics in the coarse-grained heat-affected zones (CGHAZs) of the newly developed quenching, lamellarizing and tempering (QLT)-processed 9% Ni steel. The microscopic fracture behaviors of the various sub-zones within the HAZs including local brittle zone (LBZ) were estimated using simulated HAZ specimens. Both results of Charpy impact tests and in situ scanning electron microscopy (SEM) observations on simulated CGHAZ specimens show that the inter-critically reheated coarse-grained HAZ (IC CGHAZ) is a primary LBZ of this steel at cryogenic temperature, but not at room temperature. Microstructural analysis suggests that, unlike in other studies, the cryogenic LBZ phenomenon of the IC CGHAZs cannot be explained simply by the amount of martensite–austenite (M–A) constituents, but is mainly associated with the carbon contents in them. From all results obtained, a mechanism for microscopic toughness change among the CGHAZs is proposed and discussed.

KIC Modelling for a critical strain criterion involving the stress triaxiality effect

Abstract An analytical model for predicting fracture toughness KIC is proposed based on a stress-modified critical strain criterion, that reflects the effect of stress triaxiality on ductile fracture. For KIC modelling, the notch-tip strain and stress state are given by introducing asingular field for the case of power-law-hardening materials. Notch fracture toughness is interpreted in terms of the notch-root radius (ϱ): KIC is predicted to increase with increasing ϱ, but has a minimum at a small ϱ. The microstructuralls characteristics distance and the reference critical strain can be estimated by fitting the KIC vs ϱ data on the model equation. Finally, previous notch fracture-toughness data are re-analyzed with the proposed model: the current analysis explains well the interaction effect between the notch-tip strain field and the local-fracture-controlling microstructure even in the small ϱ range.