To characterize the elastic-plastic properties of thin film materials on elastic-plastic substrates,a simple theory model was proposed,which included three steps:dimensionless analysis,finite element modeling and data fitting.The dimensionless analysis was applied to deriving two preliminary nondimensional relationships of the material properties,and finite element modeling and data fitting were carried out to establish their explicit forms.Numerical indentation tests were carried out to examine the effectiveness of the proposed model and the good agreement shows that the proposed theory model can be applied in practice.
In this paper, a modified shear-lag model is developed to calculate the surface crack density in thermal barrier coatings(TBCs). The mechanical properties of TBCs are also measured to quantitatively assess their surface crack density. Acoustic emission(AE) and digital image correlation methods are applied to monitor the surface cracking in TBCs under tensile loading. The results show that the calculated surface crack density from the modified model is in agreement with that obtained from experiments. The surface cracking process of TBCs can be discriminated by their AE characteristics and strain evolution. Based on the correlation of energy released from cracking and its corresponding AE signals, a linear relationship is built up between the surface crack density and AE parameters, with the slope being dependent on the mechanical properties of TBCs.
An inverse method for extracting the elastic-plastic properties of metallic thin films from instrumented sharp indentation has been proposed in terms of dimensional analysis and finite element modeling.A wide range of materials with different elastic modulus,yield strength,and strain-hardening exponent were examined.Similar to the Nix-Gao model for the depth dependence of hardness H,(H/H0)2=1+h*Hh,the relationship between elastic modulus E and indentation depth h can be expressed as(E/E0)4=1+h*Eh.By combining these two formulas,we find that there is a relationship between yield stress σ y and indentation depth h:σy = σy0·(1+h*Hh)f(n)·(1+h*Eh)[0.25-0.54f(n)],where σ y0 is the yield strength associated with the strainhardening exponent n,the true hardness H0 and the true elastic modulus E0.f(n)= 1/2(1-n) is constant,which is only related to n,and h*H and h*E are characteristic lengths for hardness and elastic modulus.The results obtained from inverse analysis show that the elastic-plastic properties of thin films can be uniquely extracted from the solution of this relationship when the indentation size effect has to be taken into account.
The reverse analysis provides a convenient method to determine four elastic-plastic parameters through an indentation curve such as Young s modulus E, hardness H, yield strength σy and strain hardening exponent n. In this paper, mathematical analysis on a reverse algorithm from Dao model (Dao et al., Acta Mater., 2001, 49, 3899) was carried out, which thought that only when 20 ≤E*/σ0.033≤ 26 and 0.3
Yongli HuangXiaofang LiuYichun ZhouZengsheng MaChunsheng Lu
