Evaluation of the tensile properties of a material through spherical indentation: definition of an average representative strain and a confidence domain

In the present article, a new method for the determination of the hardening law using the load displacement curve, F–h, of a spherical indentation test is developed. This method is based on the study of the error between an experimental indentation curve and a number of finite elements simulation curves. For the smaller values of these errors, the error distribution shape is a valley, which is defined with an analytic equation. Except for the fact that the identified hardening law is a Hollomon type, no assumption was made for the proposed identification method. A new representative strain of the spherical indentation, called “average representative strain”, εaR was defined in the proposed article. In the bottom of the valley, all the stress–strain curves that intersect at a point of abscissa εaR lead to very similar indentation curves. Thus, the average representative strain indicates the part of the hardening law that is the better identified from spherical indentation test. The results show that a unique material parameter set (yield stress σy, strain hardening exponent n) is identified when using a single spherical indentation curve. However, for the experimental cases, the experimental imprecision and the material heterogeneity lead to different indentation curves, which makes the uniqueness of solution impossible. Therefore, the identified solution is not a single curve but a domain that is called “solution domain” in the yield stress–work hardening exponent diagram, and “confidence domain” in the stress–strain diagram. The confidence domain gives clear answers to the question of uniqueness of the solution and on the sensitivity of the indentation test to the identified hardening laws parameters. 1-Introduction The indentation test is widely used for the determination of the stress–strain curve of materials [1-22]. One of the advantages of this technique is that it is nondestructive and can be applied to materials in some conditions for which the classic tensile test cannot be done. For example, the indentation test can be applied both to small material samples and to plastically graded materials. One of the disadvantages of the indentation test is that the field of strain in the deformed sample is not homogenous which makes it difficult to identify the hardening law of the material. The application of the concept of the representative strain can significantly simplify the analysis of the indentation response and has often been used in the stress–strain curve determination from the indentation test [1-19]. In 1908, Meyer [23] established a relationship between the mean pressure of the material under spherical indentation and the contact radius over indenter radius (a/R) ratio. For a given a/R ratio, the mean pressure should