| The continuous ball indentation technique is suitable for non-destructive in-service tests upon mechanical properties due to ease of non-destructiveness,operability,and applicability to local weak areas.The accuracy of calculation results has been widely recognized.Nevertheless,particular test conditions may be encountered in in-situ indentation tests,such as thin sample thickness and limited surface area that can be pressed,etc.What is the impact of such conditions on indentation results and how to eliminate such effects are of concern to researchers.For widely used power hardening and linear hardening metallic materials,how to distinguish these two types of materials during calculation of indentation parameters and how to choose appropriate constitutive relation to fit the calculation results are also worthy of attention.Consequently,indentation tests and finite element simulation researches have been carried out to study the influence of test parameters and factors on indentation results.Through researches on tensile properties characterization process of power hardening and linear hardening metallic materials,the judgment and selection method of material constitutive relations has also been obtained.In this paper,characterization methods of partial indentation parameters have been studied.The data selection range and calculation method when fitting the unloading stiffness has been determined: firstly,select 100%~85% of the maximum indentation load on each unloading section.Secondly,determine the initial value of relevant parameters according to Doerner-Nix method.Finally,obtain the final results by Oliver-Pharr method;Compare the calculation results of different representative strain formulas,and obtain the best expression defined according to the ball indentation contact edge shear strain as Kwon;As indentation depth increases,the calculated value of the elastic modulus gradually deviates from the true value,therefore,the average result obtained in first three cycles is more accurate.Continuous ball indentation tests have been carried out.The applicability of parameters such as times of loading and unloading cycle,unloading ratio and maximum indentation ratio have been verified by comparison with the results of uniaxial tensile tests.Based on this setting,the influence of test parameters on indentation results has been studied.The indentation results of different indenter loading rates,distance between indenter center and sample boundary,and surface roughness of the sample have been compared and analyzed.The results show that: maximum loading speed of the ball indenter should be less than 1mm/min;Distance between indenter center and the sample boundary should be greater than 1.1mm;The sample surface should be pretreated with at least 800 mesh sandpaper,and this paper recommends grinding up to 1200 mesh sandpaper.Three-dimensional finite element modeling of continuous ball indentation process has been carried out by using ABAQUS.The Influence of element mesh division and friction coefficient on simulation results have been studied.Compared with the results of two-dimensional finite element modeling,indentation tests and uniaxial tensile tests,the accuracy of certain method has been verified.Furthermore,the influencing factors of indentation results have been studied by simulation.Taking 6061 aluminum alloy material as research objects,the results show that critical sample thickness affecting indentation calculation result is four times the indenter radius.Correction factor method and additional substrate method are proposed to improve the accuracy of indentation results when the thickness of the test sample is too thin.The critical indentation interval affecting indentation results is three times the indenter radius.It is proposed that when the indentation interval is too small,selected the latter data points can improve the accuracy of indentation results.Comprehensive analysis on the influencing zone of a single indentation point has also been carried out,three areas have been divided according to the degree of influence which may provide a reference for testing area selection and measuring points arrangement of on-site continuous ball indentation measurement.At the end of this paper,the validity and accuracy of the method for characterizing the tensile properties of power hardening and linear hardening metallic materials have been verified according to the indentation results of aluminum alloy and stainless steel materials.Based on the stress-strain characteristics of these two types of materials,the method of judging hardening characteristics of typical power hardening and linear hardening materials is proposed based on the trend of the adjacent representative stress-strain points slope.The feasibility of certain method has already been verified by “blind test” on 921 A ship steel without specifying the material constitutive relationship type. |
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