| Aluminum alloy of 2024 exhibits characteristics such as high specific strength,good plasticity,and excellent welding performance,making it widely used in the aerospace,aviation,and automotive industries.In practical service,the micro-mechanical properties such as elastic modulus,hardness,yield stress,ultimate tensile strength and crack propagation of the material play an important role in its performance.The micromechanical properties of 2024 aluminum alloy was studied by nanoindentation,in situ scanning electron microscopy loading technique and geometric phase analysis.The main contents of this paper are as follows:(1)The mechanical responses of aluminum alloys at different indentation depths and different radius of indenter were analyzed using nanoindentation continuous stiffness method(CSM).The indentation stress-strain(σ-ε)relationship is transformed from the load-indentation depth relationship through different analysis methods,and compared with macroscopic properties.The results show that the average elastic modulus measured by indenter with radius of 250μm at the maximum indentation depth of 550 nm is66.4GPa,which is 2.3% different from the macroscopic tensile elastic modulus.The accurate plastic properties can be obtained by combining geometric contact radius with Tabor strain formulas when the constraint factor 1.6 was used as the stress conversion coefficient.The nominal yield stress is 290 MPa,which is about 1.4% deviation from the macroscopic tensile yield stress.(2)The mechanical properties of the grating fabricated by ZEP-520 photoresist on the surface of aluminum alloy material,and the distribution of micro-nano strain field around prefabricated crack tip and surface defects of 2024 aluminum alloy specimens under load was investigated by in-situ SEM loading and geometric phase analysis(GPA).The results show that the critical stress corresponding to cracking at the crack tip of the grating is 40 MPa.The strain fields near the crack tip is dominated by the normal strain component perpendicular to the crack direction.When the stress is 12 MPa,there is obvious strain concentration at the crack tip along the vertical direction of the crack.The maximum strain is 5.2%,the radius of plastic yield zone is about 880 nm,and the theoretical strain predicted value is lower than the experimental value within 500 nm from the crack tip.The experimental strain distribution is consistent with the theoretical predictions of linear elastic fracture mechanics when the distance from the crack tip is greater than 500 nm.At the same time,the strain fields of different shape defects under load are analyzed.Stress concentration around hexagon defects is more likely to occur and the influence range is wider.The maximum strain value of hexagon defect is 0.072,which is 1.5 times of that of circular defect and has a maximum influence radius of0.8μm.(3)The in-situ tensile test of aluminum alloy with pre-fabricated crack was simulated and analyzed by ABAQUS extended finite element method.The distribution of stress and strain fields around the crack tip was investigated and compared with the experimental measurements and theoretical calculation results.The results show that the yield size of plastic zone obtained by numerical simulation is the same as that of crack tip plastic zone calculated by theory of elastic-plastic fracture mechanics.The finite element calculation results of the strain field distribution at the crack tip agree well with the experimental results.The maximum strain at 30 N is 5%,which is 3.8% deviation from the experimental results,and is consistent with the change trend of theoretical calculation results of linear elastic fracture mechanics.The accuracy and feasibility of the geometric phase analysis method to analyze material strain field at micro and nanoscale are verified. |
PDF Full Text Request