Cross-Scale Indentation Scaling Relationships Of Advanced Materials

Hardness is a comprehensive index describing the mechanical properties related to the strength and ductility of materials.The material hardness and its indentation test methods have received widespread attention from many scholars.In recent years,a substantial amount of material tests has shown that advanced materials,usually with microscopic designs,have a significant size effect at micro and nano scales.Since the indentation test has its advantages over other methods for cross-scale hardness testing,it has become the most used method for micro and nano hardness measurement,despite its strong localization feature and the need for more caution in testing small-scale inhomogeneous materials.In this thesis,based on the principle of the indentation test,the influence of the indentation size effect on the hardness testing of advanced materials is investigated,a series of quantitative cross-scale indentation scaling relationships are obtained,numerical models are developed to investigate the mechanism of inclusions near the indenter tip of inhomogeneous materials,and a study of the related work hardening mechanism of heterogeneous materials is also carried out.The significance of the thesis is listed below.In this thesis,the influence of the indentation size effect on micro and nano scale homogeneous materials is investigated by numerical simulations in combination with a conventional theory of mechanical-based strain gradient plastic theory and the dimensional analysis to compensate for the inability of the conventional indentation scaling relationships at small size scales.The results show that there are coupled effects of yield stress,workhardening effect,and indentation size effect on the indentation response of the material,which dominates the overall indentation response in different circumstances.This part of the analysis is validated by experimental results of micro and nano indentation hardness.A reference stress that takes into account the indentation size effect is then defined,and indentation hardness experimental results are also used to validate the work to obtain the corresponding quantitative cross-scale indentation scaling relationships.These new derived scaling relationships extend the conventional quantitative indentation scaling relationships to a broader testing range,facilitating the development of indentation testing at different size scales.And this quantitative method based on a large amount of simulation data is still further expandable.In order to improve the study of the indentation scaling relationships affected by material non-uniformity at different size scales,an equivalent numerical model is developed to investigate the effect of inclusions near the indenter tip on the overall indentation response.The results show that heterogeneous materials with hard inclusions have a higher enhancement effect at smaller size scales,and this enhancement at microscopic scales is mainly generated from the non-uniform deformation of the material due to the heterogeneous configuration and the corresponding geometrically necessary dislocations.In addition,this thesis proposes a new intrinsic constitutive model for metallic materials following the dislocation mechanism based on the Taylor dislocation model and related assumptions of the dislocation evolution,which is less dependent on the phenomenological relationship but maintains the simplicity of the conventional theory of mechanism-based strain gradient plasticity,and is also verified by experimental results.It is shown that an interface affected zone with an intrinsic width appears near the interface of heterogeneous materials,and this width is comparable to the size of the intrinsic length of the material.The Taylor hardening effect and back stress hardening effect in the interface affected zone are believed to be the main contributors to the additional hardening of heterogeneous materials.The research in this thesis deepens the understanding of the cross-scale indentation of advanced materials,provides quantitative results of the cross-scale indentation scaling relationships,obtains information on the influence of the material heterogeneity on the indentation response at different size scales,and explains the mechanism of the significant work-hardening effect of heterogeneous materials at micro and nano scales,which facilitates the research and application of advanced materials at small size scales.