Abstract
Concrete is still the most widely used material in construction worldwide.With the development of concrete science,scholars are increasingly exploring the influence of concerete structures on macroscopic performance from the interior of concrete.In modern concrete science,concrete is considered a cement-based material with multiple scale microstructures from nanoscopic to macroscopic[1].
Nanoscale characterization of cement based materials has shown that nanoindentation is an adequate technique for extracting nanomechanical properties[2].During a nanoindentation test,a diamond indenter tip is pressed into a material up to a specified force or depth and then withdrawn.At the same time,the indentation force and depth are recorded which can be extracted the mechanical properties such as hardness and the Young's modulus.
For a heterogeneous material such as concrete,how to obtain the random distribution characteristics of the properties of each phase medium is a fundamental and urgent task for researchers.In this paper,systematic concrete nanoindentation tests were performed at Shanghai Institute of Materials Research to obtain the random distribution characteristics of the mechanical properties of the nanoscale of concrete.On this basis,the indentation test data of different levels is modeled analysis by using the random field theory and mathematical statistics method as shown in Figure 1.
Figure 1 Results of nanoindentation test for cement paste
The mechanical analysis of the contact process between the indenter and the material to be tested in the indentation test is very complicated,which need to consider the elastoplasticity of the material,in the meanwhile,the boundary conditions between the indenter and the material specimen are constantly changing.These factors make it difficult to analyze the problem by analytical means.So numerical simulation shows an advantage on this issue.
In this paper,the nanoindentation experiments performed on the cement paste were simulated using the finite element method(FEM).A three dimensional(3D)finite element(FE)model was created using ABAQUS finite element software as shown in Figure 2.The commonly used Berkovich geometric model was selected for the finite element model of the indenter,which allows a fine description of the indenter tip,namely an imperfection such as the one which occurs in the real geometry[3].
Figure 2 3D model of the cement paste and indenter developed in ABAQUS
An elastoplastic damage constitutive model based on irreversible thermodynamics was employed for to simulate the loaddisplacement data recorded during the nanoindentation test.It is shown that the load-ondentation curves can be well reproduced by the finite element model correctly in Figure 3.
Figure 3 Comparison of load-displacement curves between nanoindentation test and numerical simulation