Theories And Methods On The Determination Of The Mechanical Properties Of Sheet Metals Based On Instrumented Indentation Techniques

Fully understand the basic mechanical properties of materials is the basis and precondition to perform the material plasticity modelling and processing technology investigations.Sheet metals have many different types.They can be made of various materials,and their manufacturing process are very complex.The sheet metals have been widely used in the morden aviation and aerospace manufacturing industries.Currently,experimental testing of the mechanical properties of sheet metals highly relies on many traditional methods,such as uniaxial tension,bi-axial tension,shearing and so on.However,these previous testing methods have many disadvantages and inconvenient apsects.For example,the specimen preparation stage is very tedious and time consuming,and the experiment testing process can usually be limited by the finite volume/geometry of the material.Besides,these traditional mechanical testing methods can not realize the in-situ testing of the mechanical properties of materials.The disadvantages of these previous testing methods will easily lead to the difficulties in accurately and fully obtaining the mechanical properties of materials,thus impose the tremendous restrictions on the accurate plasticity modelling of sheet metals in the avation and aerospace manufacturing industries,and further limite the material plastic forming analyses and processing technology optimization studies.Indentation testing technique is a new method for the mechanical testing of material properties.It is rapidly developing in the recent years.In its experiment process,the rigid indenter was pressed against the surface of specimen,and it can induce the local multi-axial stress states,and thus provides a non-destructive testing way.Its experiment preparation process is very simple,and the experiment data is highly credible.The advantage of this testing method is that,it can be used in many situations where the trational testing methods are not applicable.Therefore,the indentation testing method does provide a more effective way for the mechanical testing of the material properties of sheet metals.In the thesis,the study aims at the theories and methods on the mechanical testing of material properites of sheet metals in aviation and aerospace industries based on instrumented indentation.Through the analysis of indentation experiment data,finite element simulation and numerical computation,the methods are build to identify the material properties based on indentation responses,and the attention is mainly focused on the uniqueness,sensitivity and confidence interval of the inverse identified material properties.The ultimate goal of the present study is to realize the mechanical testing of material properties of the sheet metals based on indentation techniques.The above research works are very singnificant for the development of the techniques in mechanical testing of material proeprties.The main research contents in the thesis are summarized as the following:(1)Spherical indentation experiment was performed on 2099-T83 Al-Li alloys,the residual imprint left on the surface of specimen was obtained by using the 3D Measuring laser microscope,and the experiment data was analyzed.Based on the proper orthogonal decomposition algorithm and the residual imprint snapshots from numerous spherical indentation FE(finite element)simulations,the sub-space of indentation imprint snapshots was constructed,and the corresponding cost function based on the error from imprint snapshots between experiment and simulations was built in the well-established sub-space in order to identify the unknown elastic-plastic properties of materials.The “Interior-point” constrained optimization algorithm was used to solve the cost function,and the elastic-plastic properties of2099-T83 Al-Li alloys were obtained.The uniqueness and sensitivity problems were analyzed,and the effectiveness of the established parameters identification method was verified.(2)Based on finite element simulation,the parametric analyses were perfomed on the residual imprint snapshots of the plastic anisotropic materials.The average and difference amounts S and(35)S of the residual imprint snapshots between the longitudinal and transverse directions of anisotropic materials were regarded as the effective material responses.The proper orthogonal decomposition algorithm was used to separately construct the sub-spaces of the average and difference amounts of the residual imprint snapshots.Based on the proper weighting of the sub-space coordinates errors of the difference and average amounts between experiment and simulation,the cost function was build in order to identify the anisotropic plastic properties of materials.The “Interior-Point” constraint optimization algorithm was used to solve the parameters identification process,and the anisotropic properties of Si Cw/A6061 were obtained.The effectiveness of the established parameters identification method was verified.The influence of weighting factor on the numerical results and the sensitivity of the inverse identification material parameters to the experiment error were analized.(3)The potential problems in the identification of the plastic properties of materials and their confidence interval using solely the indentation loading P-h curve were studied.Based on the statistical Bayesian inference theory,the formula was established to calculale the posterior distribution of material plastic properties,of which the potentional model and experiment uncertainties were considered.By combining the proper orthogonal decomposition algorithm,the Bayesian model updating and the corresponding parameters identification processes were performed in the well established sub-space of indentation P-h snapshots.By using the experiment loading P-h curve,the Hollomon and Ludwigson plasticity parameters of 20 Mn B5steel were separately calculated,and the effectiveness of the numerical method was verified.Based on the posterior distribution of the material plastic properties,the 95% confidence interval of the inverse identified materials parameters was defined,and corresponding confidence region on the 2D kernel density distribution and stress strain curves were provided.The influence of the pre-defined hardening model and indentation experiment depth on the numerical reuslts were analyzed.(4)The cross-sectional indentation experiment was performed on magnesium alloy AZ31 B,and the cross-sectional displacement distribution contours were obtained by using the white light interface method.Based on the cross-section indentation FE simulation of anisotropic materials and the parametric analysis,a method was proposed to describe the deformation characteristics of the out-of-plane deformation contours.The influence of anisotropic constitutive parameters and relative position between normal direction of crosssectional surface and material coordinate on the deformation characteristics was analyzed.The relationship between the deviation characteristics and the yield stress ration between two orthogonal directions was build.Based on this,the method is proposed to identify the anisotropic parameter based on cross-sectional indentation experiment,and it effectiveness was verified.The research contents in the thesis can be effectively used to obtain the isotropic elasticplastic and anisotropic plastity properties of sheet metals based on indentation experiments.They are very promotive for the further development of the indentation experiment techniques,as well as the accurate material plasticity modelling and relevant processing simulation and optimization investigations.