Norimasa Chiba

Representative strain of indentation analysis

Indentation analysis based on the concept of representative strain offers an effective way of obtaining mechanical properties, especially work-hardening behavior of metals, from reverse analysis of indentation load–displacement data, and does not require measuring of the projected contact area. The definition of representative strain adopted in previous studies [e.g., Dao et al., Acta Mater. 49, 3899 (2001)] has a weak physical basis, and it works only for a limited range in some sense of engineering materials. A new indentation stress-state based formulation of representation is proposed in this study, which is defined as the plastic strain during equi-biaxial loading. Extensive numerical analysis based on the finite element method has shown that with the new formulation of representative strain and representative stress, the critical normalized relationship between load and material parameters is essentially independent of the work-hardening exponent for all engineering materials, and the results also hold for three distinct indenter angles. The new technique is used for four materials with mechanical properties outside the applicable regime of previous studies, and the reverse analysis has validated the present analysis. The new formulation based on indentation stress-state based definition of representative strain has the potential to quickly and effectively measure the mechanical properties of essentially all engineering materials as long as their constitutive behavior can be approximated into a power-law form.

On radial crack and half-penny crack induced by Vickers indentation

Based on the stress fields formed upon Vickers indentation, the radial crack and half-penny crack systems are analysed separately using finite-element analyses. The crack length is correlated with the stress intensity factor (SIF) at the crack front and material elastoplastic properties via explicit relationships, from which a reverse analysis can be carried out such that the critical SIF (fracture toughness) can be readily derived once the crack length is measured. The proposed technique is validated by comparing with experimental data in the literature as well as parallel experiments of Vickers indentation-induced cracking.

A flexible automatic hexahedral mesh generation by boundary-fit method

A general-purpose automatic hexahedral mesh generation system for FEA (Finite Element Analysis) was developed, based on a shape recognition technique and the boundary-fit method. In this system, a solid model is analyzed and decomposed into single-connected sub-models. Then, other sub-models topologically identical to the original ones are constructed using only orthogonal angles. Cubes are used to construct intermediate models, which reassemble these, and finally, hexahedral meshes are generated by mapping the cubes back onto the original solid model. In order to make the automatic system more flexible and friendly to the user, this system also has the capability to interact with the user to modify the intermediate models used in the system to improve the quality of generated mesh, or to help the system to circumvent various topological difficulties.

Molecular dynamics study of shear and tensile deformation of bicrystalline aluminum

This paper describes shear and tensile deformation of bicrystalline aluminum by computer molecular dynamics. A bicrystal model with a [001] (310) ∑=5 tilt grain boundary is used for simulations. The simulations show that the effect of temperature on both the shear and tensile deformation is represented by a Boltzmann factor exp (-Q/kBT) and that the deformation is thermally activated in a typical manner. We found that the activation energy Q for the high temperature range, where T is higher than approximately 500 K to 600 K, is significantly larger than that for the low temperature range. This result shows that there are different deformation mechanisms between high and low temperatures. The activation energy difference is considered to be caused by a structural transition at the grain boundary.

Contact surface damage evaluation by infrared thermography

The detectability of contact surface damage is discussed using damaged surface models employing infrared thermography and finite element analysis. Temperature distribution measured by infrared thermography on a contact surface model with air layer is compared with that of calculated results obtained from finite element analysis. From the comparison it is concluded that thermographic measuring is applicable to fretting fatigue damage evaluation.

Residual Stress of Thin-Wall Pipe Subjected to Axisymmetric Plastic Expansion

A straight thin-wall pipe was plastically expanded at one end in the radial direction by inserting a rigid disk. The residual stress measured after withdrawal of the disk at the inner surface in the hoop and in the longitudinal direction shows a strong tensile peak beyond the region where the pipe was directly expanded by the insertion of the disk. The reason why the residual stress reaches its peak at the location far inward of the pipe, not in the region where the pipe was directly expanded, is discussed. From the FE analysis, it is concluded that the residual stress reaches its tensile peak on the inner surface at the plastic region front that was developed during the pipe expansion, and a simple formula for the tensile peak location is proposed. The similarities and differences between the residual stress distribution of the thin-wall pipe and the thick-wall pipe are discussed.Copyright

Critical Penetration Depth for Nano/Micro Indentation Test to Determine Elastic-Plastic Film Properties Deposited on Hard Substrates

The critical indentation depth to obtain proper elastic-plastic properties of thin film when the indentation tests are done on film/substrate system with sharp indenters is investigated. We focus on the characterization problem of soft film material, whose material properties are unknown, deposited on hard substrates. The critical depth is analyzed based on the finite element analysis (FEA) results. In order to extract the mechanical properties of the film from those of the film/substrate compound, we have to restrict the maximum penetration depth within a certain value. In this paper the relation between the load, P, and the depth, h, is analyzed in a power law relation, P = Chm , where the exponent m is a function of h. From extensive FEA results, we found that this exponent m starts to depart from 2 faster with increasing indenter apex angle and increasing hardening exponent of the film material. This means that the critical indentation depth decreases with increasing indenter apex angle and increasing hardening exponent. Based on this analysis, we propose a simple formula to evaluate the critical penetration depth h0 , as a function of apex angle, θ, of the indenter: h0 /d = 0.243cot θ, where d is the film thickness.Copyright

Molecular Dynamics Study of Impurity Effects on Grain Boundary Grooving.

Grain boundary grooving in crystalline aluminum is simulated by computer molecular dynamics, and impurity effects are investigated. We use a Morse potential that includes equilibrium spacing, γA1, and potential well depth, |uA1| to characterize aluminum-aluminum interaction. We also use a two-body interatomic potential that includes equilibrium spacing, γm, and potential well depth, |umin| to characterize aluminum-impurity interaction. The simulations show that when γm is smaller than γA1 and when |umin| is close to |uA1| (with the relative difference smaller than 20%), grain boundary grooving is prevented. This effect is explained by a decrease in the ratio of grain boundary diffusion to surface diffusion. Diffusion coefficients obtained by these simulations show that impurities at the grain boundaries which satisfy the above conditions (e. g., copper) strengthen surface diffusion without strengthening grain boundary diffusion.

Molecular Dynamics Simulation of Shear Deformation of Bicrystalline Aluminum

We investigate the shear deformation of bicrystalline aluminum using a molecular dynamics simulation. The computational cell contains a [001](310)Σ=5 tilt grain boundary. In simulations, we use a Morse potential. The simulations show that when the strain rate is small, and when the temperature is high, the strain rate is proportional to the shear force and the Boltzmann factor. The activation energy for the deformation agrees well with the activation energy for grain-boundary sliding and migration induced by thermal activation. This means that diffusions of the same type occur in both cases. However, this type of diffusion does not seem to occur when the temperature is low. When the strain rate is sufficiently large, yielding is observed in the grain. The critical shear force becomes smaller as the temperature is raised.

Molecular dynamics simulation of grain-boundary sliding and migration induced by thermal activation in aluminum

This paper describes the investigation of the grain-boundary behavior of coupled sliding and migration induced by thermal activation, using a molecular dynamics simulation. We simulate the two [001] (310) σ = 5 tilt grain boundaries in aluminum, where Morse potential and periodic boundary conditions are used. The simulations show that when the temperature is low, or when the distance between the two boundaries is long, the boundary structure is likely to be preserved.