Study On Mechanical Behavior Of Alloy Materials Under Biaxial Quasi-static And Fatigue Loads

The mechanical properties of alloy materials under biaxial loading play an important role in guiding material design,studying material yield behavior and crack propagation properties.In this paper,the shape and size of the biaxial tensile cruciform specimen were optimized for the 6061 aluminum alloy material.The biaxial quasi-static load and fatigue load test were carried out.The yield behavior of the 6061 aluminum alloy under biaxial tensile stress was studied.And the expansion performance of crack under biaxial fatigue loading,the main research contents of this paper are as follows:Firstly,according to the design requirements of the cruciform specimen,the mathematical criterion based on the standard deviation of the test area of the specimen proposed by Demmerle was applied to the finite element method to construct the optimization criterion.The stress distribution of the cruciform specimen was calculated by the finite element analysis,and the optimization algorithm was used to achieve the optimal geometric parameters for the cruciform specimen.A scaled biaxial tensile test was conducted on the optimized specimen to verify the application feasibility of the optimized specimen.Secondly,the optimized specimen size was used and the biaxial tensile test with different loading ratio was carried out.The yield behavior of specimen under biaxial load was analyzed.The true stress-true strain curve in the biaxial direction under different loading ratios was calculated based on the data collected from the test.Based on the theory of Drucker,both the the biaxial stress-equivalent stress relationship and the biaxial strain-equivalent strain relationship were derived.Combining the experimental data,the Mises yield criterion and the Hill48 yield criterion was respectively adopted to calculate the true stress in the biaxial direction at different loading ratios.The accuracy and error of different yielding criteria were analyzed via the comparison between true strain curve and the test curve,to determine the yield criterion which could reasonably describe the biaxial mechanical properties of 6061 aluminum alloy under different loading paths.Finally,a fixed length of crack in the test area was prefabricated on the optimized cruciform specimen and the fatigue crack growth test for different pre-cracked angle specimens under the condition of two-way synchronous normal sinusoidal alternating cyclic load was conducted.The test studied the effect of different pre-crack angles on the crack direction and expansion rate of fatigue loads under biaxial fatigue loading.