Papers by Keyword: Spherical Indentation

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Authors: Wen Yi Yan, Qing Ping Sun, Peter D. Hodgson
Abstract: The spherical indentation obeys Hertz contact theory when the applied load is within the elastic limit. Once the applied load is over the elastic limit, the indentation curve starts to deviate from the original purely elastic indentation curve. This deviation point, which indicates the start of the nonlinear deformation, is an important characteristic of a spherical indentation curve. The indentation force corresponding to the deviation point is related to a basic material constant, which is the yield stress for an elastic-plastic material or the transformation stress for a shape memory alloy. This relationship can be applied to measure the yield stress or the transformation stress from a simple spherical indentation curve. Detailed discussion on the relationship and the method is presented in this short paper.
Authors: Amir Hossein Mahmoudi, Mitra Ghanbari-Matloob, Soroush Heydarian
Abstract: In the present study an Artificial Neural Network (ANN) approach is proposed for residual stresses estimation in engineering components using indentation technique. First of all, load-penetration curves of indentation tests for tensile and compressive residual stresses are studied using Finite Element Method (FEM) for materials with different yield stresses and work-hardening exponents. Then, experimental tests are carried out on samples made of 316L steel without residual stresses. In the next step, multi-layer feed forward ANNs are created and trained based on 80% of obtained numerical data using Back-Error Propagation (BEP) algorithm. Then the trained ANNs are tested against the remaining data. The obtained results show that the predicted residual stresses are in good agreement with the actual data.
Authors: Zhuang Jin, Jian Ping Zhao
Abstract: Cao and Lu had built a method to acquire the properties of materials. But they neglected the influence of strain hardening exponent n by introducing the representative strain which didan’t have any physical meaning. A new method from a continuous spherical indentation test was built, the influence of strain hardening exponent n were considered and the formulas of dimensionless functions defined in their work were improved in this present paper. Then the computational results from the new method and the actual results were compared and the error is about 8%.
Authors: Chun Ping Guan, Hong Ping Jin
Abstract: Through dimensional analysis of indentation parameters in this study, we propose an artificial neural network (ANN) model to extract the residual stress and strain-hardening exponent based on spherical indentation. The relationships between indentation parameters and the residual stress and material properties are numerically calibrated through training and validation of the ANN model. They enable the direct mapping of the characteristics of the indentation parameters to the residual stress and the elastic-plastic material properties. The proposed ANN model can be used to quickly and effectively determine the residual stress and strain-hardening exponent.
Authors: Mohamad Idriss, Olivier Bartier, Gérard Mauvoisin, Charbel Moussa, Eddie Gazo Hanna, Xavier Hernot
Abstract: This work consists of determining the plastic strain value undergone by a material during a forming process using the instrumented indentation technique (IIT). A deep drawing steel DC01 is characterized using tensile, shear and indentation tests. The plastic strain value undergone by this steel during uniaxial tensile tests is determined by indentation. The results show that, the identification from IIT doesn’t lead to an accurate value of the plastic strain if the assumption that the hardening law follows Hollomon law is used. By using a F.E. method, it is shown that using a Voce hardening law improves significantly the identification of the hardening law of a pre-deformed material. Using this type of hardening law coupled to a methodology based on the IIT leads to an accurate determination of the hardening law of a pre-deformed material. Consequently, this will allow determining the plastic strain value and the springback elastic strain value of a material after a mechanical forming operation.
Authors: Moon Kyu Lee, Kui Won Choi, Tae Soo Lee, H.N. Lim
Abstract: The indentation test has been in the spotlight due to easy and non-destructive testing characteristics. However, there are little studies for the indentation test of porous materials in the evaluation aspect of methodology. The goal of this study was to evaluate a spherical indentation test in the aspect of indenter-size and indentation depth by measuring elastic modulus of porous materials such as a cancellous bone using a FEM. We developed a microstructure-based FE model of cancellous bone with apparent density 0.2~0.8 g/cm3 in order to simulate uniaxial compression test and indentation test in the light of anatomical observation with a scanning electron microscope (SEM). We obtained a load-displacement curve through the indentation simulation and calculated the Young’s modulus of cancellous structure based on Pharr's hypothesis. The result indicated that indenter diameter has to be more than five times of pore size and indentation depth should be about 8% of indenter diameter at least to obtain the appropriate result of the indentation test. It is expected that this result may guide to the design and the simulation of indentation test for porous materials
Authors: Xiu Min Gao, Yi Wang Bao, Guang Lin Nie
Abstract: The spherical indentation combined with acoustic emission was used to evaluate the local strength of glass, which is a nondestructive testing approach. However, stress time effect on the local strength of glass during spherical indentation has not been studied before. In the present work, stress time effect was investigated by examining the local strength of unstrengthened and strengthened glass at different loading rates. It is discovered that the local strength of glass increased greatly with the loading rate, which confirmed the time dependence of the fracture on glass. As a typical brittle material, the discreteness of strength date of glass measured by spherical indentation was also analyzed to evaluate the strength of glass correctly.
Authors: Peter Ogar, Denis Gorokhov
Abstract: A method for determining contact characteristics occurring in spherical indentation depending on the properties of an elastic-plastic material governed by the hardening Hollomon power law is proposed. In this case the empirical Meyer law relating a spherical indentation load with an indentation diameter d is used. Basically, the Meyer law is not related to the mechanical characteristics of the test material. The study used the relations between the strain hardening exponent n and the Meyer law constant obtained by S.I. Bulychev. The effects of «sink-in / pile-up» are considered. It is shown that there is no need to define Meyer law constants. The scope of application of the proposed equations is defined. A comparison of the results obtained with the published results based on the finite element (FE) analysis is given.
Authors: I. Nyoman Budiarsa, Mikdam Jamal
Abstract: In this work, finite element (FE) model of spherical indentation has been developed and validated. The relationships between constitutive materials parameters (σy and n) of elastic-plastic materials, indentation P-h curves and hardness on spherical indenters has been systematically investigated by combining representative stress analysis and FE modelling using steel as a typical model material group. Parametric FE models of spherical indentation have been developed. Two new approaches to characterise the P-h curves of spherical indentation have been developed and evaluated. Both approaches were proven to be adequate and effective in predicting indentation P-h curves. The concept and methodology developed is to be used to predict Rockwell hardness value of materials through direct analysis and validated with experimental data on selected sample of steels. The Hardness predicted are compared with the experimental data and showed a good agreement. The approaches established was successfully used to produce hardness values of a wide range of material properties, which is then used to establish the relationship between the hardness values with representative stress.
Authors: Wen Yi Yan, Qing Ping Sun
Abstract: Spherical indentation of superelastic shape memory alloys (SMAs) has been theoretically analyzed. Two characteristic points on the superelastic indentation curve have been discovered. The bifurcation force corresponding to the bifurcation point relies on the forward transformation stress and the return force corresponding to the return point relies on the reverse transformation stress. Based on these theoretical relationships, an approach to determine the transformation stresses of superelastic SMAs has been proposed. To improve the accuracy of the measurement, a slope method to locate the two characteristic points from the slope curves is further suggested. Additionally, the spherical indentation hardness was also analyzed.
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