Elasto-plastic Mechanical Properties Research Articles

Fracture failure analysis of plastic fiber based on thermodynamic strength theory and experiment

In order to analyze the mechanical behavior of plastic fiber bundle, a theoretical model considering elastic and plastic behavior of single fiber is established. Based on uniaxial tensile tests of polythene single fiber, elastoplastic mechanical properties, such as elastic modulus and plastic modulus, are obtained. Moreover, distributions of yield strength and breaking strength of single fiber are determined. The stress–strain curve of polythene fiber bundle is predicted by theoretical model. The correctness of this theoretical model is verified by uniaxial tensile test of polythene fiber bundle. All experimental data points are within the range predicted by theoretical model. Some key parameters which regulate mechanical behavior of single fiber are investigated theoretically. Results show that Weibull parameters of yield strength and breaking strength of single fiber have great effect on mechanical performance of fiber bundle. The concentration of distribution area of breaking strength can improve the overall fracture strength of fiber bundle. Results and conclusions in this investigation can extend and perfect fiber bundle model in terms of plastic behavior.

Elastoplastic Mechanical Properties and Kinematic Hardening Model of 35CrNi3MoVR.

The existing tensile-compression elastoplastic models are not suitable for varies of materials. An accurate constitutive model of the elastoplastic mechanical properties more suitable for 35CrNi3MoVR was produced by optimizing the existing fitting equations based on uniaxial tensile-compression tests, which are able to describe the elastoplastic stress-strain relation and Bauschinger effect varying with the maximum tensile plastic strain. A UMAT subroutine of the constitutive model in ABAQUS was proposed and conducted for FEM calculation. Hydraulic autofrettage tests were carried out under different pressures on thick-walled 35CrNi3MoVR tubes, and the results were compared with those of FEM calculations to further validate the accuracy of the fitting model. The results show that the constructed power function kinematic hardening model can effectively describe the elastoplastic mechanical properties of 35CrNi3MoVR and can be applied to the autofrettage calculation of this material. The comparison among the calculation results of different models proved that the model proposed in this research has better performance compared to other existing models. Taking the Mises stress at the inner surface of the thick-walled tubes as the evaluation criterion, the error of the power function kinematic hardening model reaches less than 3%, decreasing the error by at least 50%.

Cross-scale modeling and elasto-plastic mechanical properties of TBC with 3D hill-like interface

Failure of thermal barrier coating (TBC) systems is closely related to the temperature field and stress state of their components. The lack of accurate cross-scale analysis methods has made it difficult to clarify the mechanical properties and damage mechanisms of TBC interfaces. This study proposes a cross-scale modeling method for TBC called the thermal barrier coating master–slave model (TMSM). First, the TMSM is established based on Saint-Venant's principle, thermal elastoplastic mechanics theory, and the von Mises yield criterion, and its applicability is discussed considering mesh conditions, geometric conditions, and cyclic conditions. The advantages of the TMSM are then demonstrated by comparison with the slave model. Finally, the effects of material plasticity and oxide thickness on the damage of the TBC were analysed using the TMSM. The results showed that the boundary effect can be eliminated using the slave model with an interface side length of 160 μm. The interfacial axial stress obtained by the TMSM was greater than that obtained by the slave model. The thermal mismatch stress caused by thermal material expansion at high temperatures and the residual stress caused by material plasticity at the moment of cooling are the two main causes of TBC failure. When the thickness range of the oxide layer is 2–3 μm, the oxide layer will inhibit the maximum thermal stress during thermal cycles, which is supported by the results of fatigue experiments conducted under the same condition. This study provides a new approach for the cross-scale analysis of TBC and demonstrates the positive effect of thermally grown oxide thickness on the thermal shock resistance of TBC using numerical analysis.

Molecular dynamics study on the dislocation evolution mechanism of temperature effect in nano indentation of FeCoCrCuNi high-entropy alloy

ABSTRACT In this paper, the atomic model of high-entropy alloy is established by molecular dynamics simulation. By applying nano-indentation, the influence of Cu atomic ratio and temperature on the deformation of FeCoCrCuNi high-entropy alloy was studied, and the micro-scale dislocation movement evolution mechanism was obtained. The results show that, with the increase of temperature, the load of FeCoCrCuNi high-entropy alloy under nano indentation decreases. The total length of dislocations in each stage of nanoindentation decreases monotonically, and the types of dislocation reactions decrease. Under the action of high temperature, the number of perfect dislocations decomposed into Shockley partial dislocations decreases, resulting in more Frank dislocations and Stair-rod dislocations, which makes the extended dislocations recombine into perfect dislocations, and the length of dislocations decreases sharply. The dislocation defect is replaced by a cavity at high temperature, which makes the high-entropy alloy lose elastoplastic mechanical properties.

The Effect of Clamping Force and Friction Coefficient on The Clamping Clamp

In order to study the influence of clamping force and friction coefficient on the stress distribution of drill pipe joint during the buckling process, the nonlinear contact problem and elastoplastic mechanical properties of Φ139.7mm × 9.17mm drill pipe were simulated by finite element analysis method in this paper, and the stress distribution and size of drill pipe joint during the buckling process were obtained. And the effect of clamping force and friction coefficient on the joint of drill pipe. It is found that the stress of drill pipe joint is mainly concentrated in the contact position of drill pipe and the stress value of drill pipe joint shows a trend of increasing. When the friction coefficient is 0.15 and the clamping force is 600KN, although there is stress concentration in the contact position between the clamp teeth and the drill pipe joint, it does not exceed the yield limit of the drill pipe joint, and no plastic deformation occurs. Through the simulation calculation, it is found that the maximum clamping force and the maximum torque are 1200KN and 81.9KN.m respectively. When the torque is fixed, the change of friction coefficient has a great influence on the maximum clamping force of drill pipe joint. With the increase of friction coefficient, the maximum clamping force of drill pipe joint decreases continuously, and the friction coefficient is inversely proportional to the maximum clamping force. The research in this paper enriches the understanding of the buckling process of punching pliers and provides a reference for the rational buckling of pipe string.

Analysis of Elastoplastic Mechanical Properties of Non-Uniform Frozen Wall Considering Frost Heave

The aim of this study was to analyze the force and deformation law of an artificial frozen wall. Thus, the frost heave coefficient was introduced to describe the frost heave characteristics, and the frozen wall was regarded as a heterogeneous material whose material properties changed in a parabolic pattern with the radius. The elastoplastic stress and displacement formulas of a non-uniform frozen wall considering frost heave characteristics were derived according to different strength criteria. Consequently, the derived formulas were used to calculate and analyze the mechanical characteristics of the artificial frozen wall. The results showed that the radial stress of the frozen wall changed linearly, whereas the circumferential stress change followed a parabolic pattern after considering the non-uniform characteristics. Moreover, the displacement of the outer edge of the frozen wall was always greater than that of the inner edge, and the displacement of the inner edge changed with the increasing temperature, significantly greater than that of the outer edge. When the frozen wall was in the elastic state, its displacement caused by frost heave was constant. When the frozen wall entered the elastic–plastic state, the displacement of its inner edge caused by frost heave increased with the increase in the radius of the plastic zone, whereas the displacement of the outer edge caused by frost heave decreased with the increase in the radius of the plastic zone.

Dimensionless Analysis to Determine Elastoplastic Properties of Thin Films by Indentation

By assuming the elastoplastic properties of thin-film materials, a reverse analysis method is proposed by deriving a dimensionless function for the indentation process. The substrate effect is taken into account by assuming a perfect interface between thin-film and substrate materials. In order to obtain the applied load–penetration depth (P-h) curves, the indentation process is numerically modeled as an axisymmetric problem with a rigid-body Berkovich indenter on the semi-infinite substrate when performing finite element (FE) simulations. As a typical soft film/hard substrate problem, the elastic substrate is assumed and the power–law model is used to describe the constitutive properties of thin-film materials. Varying elastic modulus (10–50 GPa), yield strength (60–300 MPa), and hardening exponent (0.1–0.5) characterize different elastoplastic mechanical properties of thin-film materials with film thickness of 10–30 μm. Owing to the good trending P-h curves with the maximum indentation depth up to the 2/3 film thickness for different elastoplastic thin-film materials, a dimensionless function is derived and validated based on the predictions by reliable FE simulations. The proposed dimensionless function elegantly elucidates the essential relationship between the elastoplastic mechanical properties of the thin-film material and indentation responses (e.g., loading and unloading variables). The elastoplastic constitutive curves predicted by the proposed reverse method are confirmed to be in good agreement with the stress-strain curves of materials by FE simulations with the randomly selected elastoplastic mechanical properties and film thicknesses. This study provides a theoretical guidance to understand the explicit relationship between elastoplastic mechanical properties of the thin-film material and indentation responses.

Mechanical properties of shear-type hysteretic dampers supported by studs

The shear-type hysteretic damper of an energy-absorbing member reduces the damage to the main member, such as column or beam, by absorbing seismic energy as the shear deformation of steel. As a support element for connecting the damper to the structure, brace-type and stud-type systems are mainly used. In the stud support, the stress transmission mechanism to the damper is simple, and the degrees of freedom of the plane are higher than that of the other support types. For such a shear-type damper, an H-shaped cross-section with a panel made of a highly ductile material, such as a low-yield-point steel, is generally adopted. This study used a low-yield-point steel circular hollow-section damper, which was installed in the frame as a stud support form, and a case was used to receive the in-plane force. For the damper to function effectively in the frame, it was necessary to determine the elastoplastic mechanical properties of the damper and the dampers supported by the stud. The mechanical properties of the damper unit and damper system with the stud were examined by cyclic loading test. An H-shaped shear panel damper was used as the control group with almost the same full plastic shear strength, initial stiffness, and weight as the circular hollow-section damper and the cyclic loading test was conducted under the same conditions. The effectiveness of the low-yield-point steel circular hollow-section damper was examined as a shear-type hysteretic damper by comparing both. In addition, a nonlinear finite element analysis was performed to determine the influence of the initial stiffness on the damper supported by the stud.

Determination of the true stress-strain relations of high-grade pipeline steels based on small punch test correlation method

The small punch test (SPT) has been widely used to evaluate the mechanical properties of small regions in pipeline steels. However, its application has thus far been limited to the determination of elastoplastic mechanical properties such as the ultimate tensile strength, and few works on estimating he true stress-strain curves. This paper aims to propose a new method for determining the true stress-strain relations of high-grade pipeline steels based on the SPT results, and apply it to the welded joints of X80 grade pipeline steel where the traditional mechanical characterizations cannot be carried out. Mechanical properties of several high-grade pipeline steels were obtained by empirical formulas that were determined by the correlation relationships between the conventional tensile tests and SPT results. Then the Ramberg-Osgood(R–O) power law which can describe the stress-strain relations of high-grade pipeline steels was simplified and expressed by four determined mechanical property parameters so that the true stress-strain curves could be generated using these parameters. Meanwhile, the SPT correlation method was used to estimate the true stress-strain curves of the heat-affected zones and weld zones of the welded joints of X80 steel and the accuracy was verified by numerical simulations. The results demonstrate that the true stress-strain curves determined by the SPT correlation method are in good agreement with the uniaxial tensile results for base materials of high-grade pipeline steels and the predicted results of different areas of the welded joints of X80 steel are reliable.

A simple analytical model of wellbore stability considering methane hydrate saturation-dependent elastoplastic mechanical properties

Methane hydrates (MHs) have received widespread attention as a new clean energy resource. However, because of the sharp reduction/change in the mechanical/physical properties of methane hydrate bearing sediments (MHBS) after hydrate dissociation, as well as the strong multi-field coupling, wellbore instability is more likely to occur during drilling in MHBS under the seafloor. To fast predict the wellbore stability in MHBS, an elastoplastic analytical model is proposed in this study that fully considers the saturation-dependent mechanical properties and water/heat production/absorption due to hydrate dissociation. Axisymmetric problems at the horizontal cross-sections of the vertical wellbore are concerned. Based on an analytical model of pore pressure, temperature and salt concentration for drilling in MHBS that considers both conduction/diffusion and convection effects, the produced/absorbed water/heat at the dissociation front related to the MH dissociation rate are additional considered in the compatibility conditions in the modified models. Compared with the results of previous study, the improved solutions show a much lower temperature and slightly higher pressure around the dissociation front. Furthermore, the saturation-dependent mechanical properties of MHBS are simulated by an elastic-brittle-plastic constitutive model with unified strength criterion, where Young's modulus and cohesion are functions of hydrate saturation. The analytical solutions for the stresses, displacement and plastic radius are finally addressed under various locations of the plastic zone and principal stress orders. The analytical models are verified by their good agreement with the results from the FEM model under the same assumptions, and they are consistent with the results from complex numerical models. Through numerous calculations, the influences of the drilling fluid properties and initial conditions on the plastic radius and displacement at the wellbore wall are investigated in detail, and the analytical formula for the safest drilling fluid pressure is presented as a function of the initial pore pressure and in situ stress.

Effects of duty ratio on properties of Ni–P–(sol)Al2O3 coating prepared by pulse-assisted chemical deposition

With the wide application of nanocomposite protective coatings in various fields, there are higher requirements for their properties. At the same time, the traditional chemical deposition and electrodeposition processes have defects with different degrees. Nanocomposite coating was prepared by pulse current assisted chemical deposition process, which has the advantages of effectively improving the chemical deposition rate and ensuring the uniformity and compactness of the deposition layer surface morphology. In this paper, the effect of different duty ratios on Ni–P-(sol)Al2O3 nanocomposite deposition was studied. The surface morphology, composition, hardness, wear resistance and elastoplastic mechanical properties of the nanocomposite coating were tested through experiments. The experimental results showed that, when the duty ratio of pulse current was 20%, the prepared Ni–P-(sol)Al2O3 nano-composite deposition layer has the best performance, and its deposition rate reached 45.99 μm/h, hardness value was 629 HV, friction coefficient was 0.395, wear-width was 126.33 μm, Young modulus was 135.216 GPa, elastic recovery ratio he/hmax reached the maximum value of 0.3976, H3/E2 reached the maximum value of 0.0505 GPa. Compared with the coatings prepared by the traditional chemical deposition process, it is found that both coatings show good compactness and uniformity. However, the pulse-assisted chemical deposition method is significantly superior to the latter in hardness and wear resistance. The deposition layer prepared by pulse-assisted chemical deposition not only has a uniform and compact surface, but also has high hardness, good toughness and optimal wear resistance.

Predicting the macroscopic response of electrospun membranes based on microstructure and single fibre properties

In the present paper, the three-dimensional structure and macroscopic mechanical response of electrospun poly(L-lactide) membranes is predicted based only on the geometry and elasto-plastic mechanical properties of single fibres supplemented by measurements of membrane weight and volume, and the resulting computational models are used to study the non-affine micro-kinematics of electrospun networks. To this end, statistical parameters describing the in-plane fibre morphology are extracted from scanning electron micrographs of the membranes, and computational network models are generated by matching the porosity of the real mats. The virtual networks are compared against computed tomography scans in terms of structure, and against uniaxial tension tests with respect to their macroscopic mechanical response. The obtained virtual network structure agrees well with the fibre disposition in real networks, and the rigorous prediction of the mechanical response of two membranes with mean diameters of 1.10μm and 0.70μm captures the experimental behaviour qualitatively. Favourable quantitative agreement, however, is obtained only after lowering the Young's moduli, yield stresses and hardening slopes determined in single fibre tests, and after reducing the density of inter-fibre bonds in the model of the membrane with thinner fibres. The simulations thus demonstrate the validity and merits of the approach to study the multi-scale mechanics of electrospun networks, but also point to potential discrepancies between the properties of electrospun fibres within a network and those produced for single fibre characterisation, and highlight the existing uncertainty on the density and quality of bonds between fibres in electrospun networks.

A patient-specific numerical modeling of the spontaneous coronary artery dissection in relation to atherosclerosis

The spontaneous coronary artery dissection (SCAD) is a clinical complication of angioplasty leading to an initiation of a tear/crack in the intima layer of the artery. The crack can propagate to the interface of the intima-media layer following by intramural hematoma. The relation between the SCAD and atherosclerosis is a controversial issue, as some studies stated no correlation between them while others showed that a crack can initiate in the intima but cannot propagate into the atrophied media layer. To investigate the relation between the intraluminal crack propagation in the atherosclerotic artery and SCAD, this study numerically investigated the initiation and propagation of a crack in the intraluminal and radial locations of the healthy and atherosclerotic human coronary arterial walls. The energy release rate, namely J-integral, is computed as a numerical derivative of the strain energy with respect to a crack extension using a user-defined virtual crack method (VCE) of extended finite element method (XFEM). Experimental measurements were carried out to calculate the elasto-plastic mechanical properties of the healthy and atherosclerotic human coronary arteries. The experimental data were then assigned to our own established patient-specific FE model of the coronary artery. Cracks were sketched in the intraluminal and radial locations of the arterial wall and allowed to propagate to the virtual interface of the intima-media to form a false lumen. The results revealed a higher stress at the crack tip of the healthy arterial wall compared to the atherosclerotic one. Lower crack tip opening displacement (CTOD) and crack tip opening angle (CTOA) were observed in the intraluminal crack of the atherosclerotic artery. J-integral of the atherosclerotic arterial wall was also found to be higher than the healthy one at the intraluminal crack. The results revealed that although a crack can initiate in the intraluminal of an atherosclerotic artery, it cannot propagate into the media layer due to a relatively higher rate of the strain energy release in the atherosclerotic arterial wall compared to the healthy one.

Elasto-Plastic Mechanical Properties and Failure Mechanism of Innovative Ti-(SiCf/Al₃Ti) Laminated Composites for Sphere-Plane Contact at the Early Stage of Penetration Process.

A novel silicon carbide (SiC) continuous ceramic fiber-reinforced (CCFR) Ti/Al3Ti Metal-Intermetallic-Laminate (MIL) composite was fabricated. A high-efficiency semi-analytical model was proposed based on the numerical equivalent inclusion method (NEIM) for analyzing the small-strain elasto-plastic contact in the early stage of the penetration process. The microstructure and interface features were characterized by the scanning electron microscopy (SEM). Quasi-static compression tests were performed to determine the contact response and validate the proposed model. A group of in-depth parametric studies were carried out to quantify the influence of the microstructure. The comparison between results under the sphere-plane and plane-plane contact load indicates that, under the first sphere-plane, the compressive strength and failure strain are both lower and the SiC reinforcement effect on strength is very clear while the effect on ductility is not clear. The maximum plastic strain concentration (MPSC) in the Al3Ti layer is closest to the upper boundary of the central SiC fiber and then extends along the depth direction as the load increases, which are also the locations where cracks may initiate and extend. Moreover, the CCFR-MIL composite shows better mechanical properties when the center distance between adjacent SiC fibers is four times the fiber diameter and the volume fraction of Ti is 40%.

Composite structural modeling and tensile mechanical behavior of graphene reinforced metal matrix composites

Owing to its distinguished mechanical stiffness and strength, graphene has become an ideal reinforcing material in kinds of composite materials. In this work, the graphene (reduced graphene oxide) reinforced aluminum (Al) matrix composites were fabricated by flaky powder metallurgy. Tensile tests of pure Al matrix and graphene/Al composites with bioinspired layered structures are conducted. By means of an independently developed Python-based structural modeling program, three-dimensional microscopic structural models of graphene/Al composites can be established, in which the size, shape, orientation, location and content of graphene can be reconstructed in line with the actual graphene/Al composite structures. Elastoplastic mechanical properties, damaged materials behaviors, graphene-Al interfacial behaviors and reasonable boundary conditions are introduced and applied to perform the simulations. Based on the experimental and numerical tensile behaviors of graphene/Al composites, the effects of graphene morphology, graphene-Al interface, composite configuration and failure behavior within the tensile mechanical deformations of graphene/Al composites can be revealed and indicated, respectively. From the analysis above, a good understanding can be brought to light for the deformation mechanism of graphene/Al composites.

Characterization and methodology for calculating the mechanical properties of a TRIP-steel submitted to hot stamping and quenching and partitioning (Q&P)

Thermomechanical simulation of quenching, hot stamping, and quenching and partitioning processes of a high-strength TRIP-assisted steel were carried out in a Gleeble®3S50 thermo-mechanical simulator, coupled to the synchrotron X-ray diffraction line. The microstructures and mechanical properties were analyzed using Field Emission Gun Scanning Electron Microscopy (FEG-SEM), X-ray diffraction, and nanoindentation. The microstructures of thermomechanical treated specimens were modeled using the Object Oriented Finite Element (OOF) technique. The modeled microstructures were then fed into a finite element model to predict the mechanical behavior. By using a reverse algorithm method, the elasto-plastic mechanical properties of different microconstituents were determined. This was done through the analysis of instrumented nanoindentation loading-penetration curves. Tensile properties of the thermomechanical processed steels were measured by tensile testing of subsized specimens cut from samples processed on the Gleeble®3S50. The comparison between the experimental results and those of the reverse algorithm and the OOF modeled microstructure showed quite good agreement.

Structural modeling and mechanical behavior of Metal-Porous-Polymer-Composites (MPPCs) with different polymer volume fractions

Since the past five decades, the study of new materials has become attractive in both scientific research and industrial applications. For purpose of enhancing mechanical properties of metallic foam, a new composite entitled “Metal-Porous-Polymer-Composite (MPPC)” consisting of open-cell metallic foam and filling polymers was developed by infiltrating the filling polymers throughout the connected pores in open-cell metallic foam. Based on a three-dimensional (3D) structural modeling, microscopic structural models of Al metallic foam and MPPCs with spherical pores and Kelvin’s pores are established in the finite element code. Introducing the elastoplastic mechanical properties, the polymer–metal interfacial behaviors and the proper loads and boundary conditions on these structural models of Al metallic foam and MPPCs, the numerical compressive behaviors of Al metallic foam and PA6/Al & LDPE/Al MPPCs are successfully performed. The results show that the mechanical properties of PA6/Al & LDPE/Al MPPCs are mainly influenced by the filling polymers, the kind of pores, the pore size, the polymer–metal interface, the polymer volume fraction and the distribution of pores, respectively. It means that a good understanding can be provided concerning the deformation mechanism and engineering applications of both Al metallic foam and MPPCs.

Theoretical prediction and experimental investigation on formability of tailor-welded blanks

Based on the elastoplastic mechanical properties of the weld and heat affected zone metals obtained by a nanoindentation test, a theoretical calculation model was established for the forming limit diagram (FLD) of tailor-welded blanks (TWBs) on the basis of plastic constitutive relations and Hosford yield criteria. Hemispherical punch bulging tests were performed to verify the FLD theoretical calculation results. The results demonstrated that not only the FLD theoretical calculation of base materials but also that of TWBs had a good agreement with their experiments. Besides, poorer formability of TWBs caused its FLD significantly lower than that of base materials. The theoretical calculation model offers a reliable approach to obtain the specific FLD of TWBs.

Determination of elastoplastic mechanical properties of the weld and heat affected zone metals in tailor-welded blanks by nanoindentation test

The elastoplastic mechanical properties of the weld and heat affected zone metals have comparatively major impact on the forming process of tailor-welded blanks. A few scholars investigated the elastoplastic mechanical properties of the weld and heat affected zone, but they only simply assumed that it was a uniform distribution elastoplastic material different from the base materials. Four types of tailor-welded blanks which consist of ST12 and 304 stainless steel plates are selected as the research objects, the elastoplastic mechanical properties of the tailor-welded blanks weld and heat affected zone metals are obtained based on the nanoindentation tests, and the Erichsen cupping tests are conducted by combining numerical simulation with physical experiment. The nanoindentation tests results demonstrate that the elastoplastic mechanical properties of the weld and heat affected zone metals are not only different from the base materials, but also varying between the weld metals and the heat affected zone metals. Comparing the Erichsen cupping test resulted from numerical with that from experimental method, it is found that the numerical value of Erichsen cupping test which consider the elastoplastic mechanical properties of the weld and heat affected zone metals have a good agreement with the experimental result, and the relative error is only 4.8%. The proposed research provides good solutions for the inhomogeneous elastoplastic mechanical properties of the tailor-welded blanks weld and heat affected zone metals, and improves the control performance of tailor-welded blanks forming accuracy.

Elasto-plastic characteristics and mechanical properties of as-sprayed 8mol% yttria-stabilized zirconia coating under nano-scales measured by nanoindentation

Abstract The elasto-plastic characteristics and mechanical properties of as-received 8 mol% Y 2 O 3 –ZrO 2 (8YSZ) coatings were studied by nanoindentation at ultra-low loads with a Berkovich indenter at room temperature. All experimental data including hardness H and elastic modulus E were analyzed by the Weibull statistical method due to the porous and heterogeneous nature of the tested samples. It was found that the hardness firstly exhibits interesting reverse indentation size effect, and then shows normal indentation size effect within different indentation scales. The average elastic modulus of 8YSZ was estimated as 214.8 ± 13.2 GPa. In order to reveal the elasto-plastic characteristics of 8YSZ at nano-scales, the distribution of resolved shear stresses underneath the indenter tip region was evaluated by Hertzian contact theory when the deformation behavior of 8YSZ changed from fully elastic to elasto-plastic stages. The results shed light on understanding possible foreign object damage mechanisms of thermal barrier coating systems.