Study On The Fracture Mode Transition Phenomenon Of 7075Al Ring Under Dynamic Tensile Loading

Metal materials are widely used in people's daily life and national defense science and technology industry.However,due to the lack of ability to predict the fracture behavior of metal materials,a large number of safety accidents are caused,which seriously threaten the safety of people's life and property.In this paper,a series of experiments and numerical simulation are focused on the fracture mechanics of metal materials under dynamic tensile loading,which focuses on the analysis of the effect of loading strain rate on the fracture mode of metal materials and the sensitivity of the metal material to the tensile and shear fracture mode and the crack expansion.The main controlling factors of the path are studied to study the material selection for 7075Al in aerospace.It is hoped that this study can improve the understanding of fracture problems,especially the prediction ability of fracture modes of metallic materials.The main content and innovative success of the paper are as follows:1)a reasonable 7075Al standard tensile specimen was designed,and a quasi static tensile test was carried out at 4 loading rates(0.00004/s,0.0004/s,0.004/s,0.04/s),and 3 experiments were carried out at each strain rate.The stress strain curve of 7075Al under different loading strain rates was very good before the specimen was broken.The results show that the results have high confidence.The stress-strain curve analysis shows that the 7075Al material used in the experiment has certain strain hardening characteristics.By observing and analyzing the fracture of the sample,it is found that with the increase of strain rate of tensile loading,the fracture mode of 7075 aluminum standard tensile specimen is transition from single tensile fracture to tensile shear mixed fracture,the tensile crack in the tensile shear mixed fracture mode is much more,and the ratio of shear crack is less.The fracture strain of the sample is obtained by using the stress strain curve and the method of measuring the length of the sample.The two ones have higher coincidence degree.It is found that the fracture position and the fracture strain of the standard tensile specimen under the same loading strain rate have certain randomness,but the statistical average fracture strain is along with the loading speed.The increase is slightly increased.2)the expansion and fracture experiment of the 7075Al ring under the electromagnetic solenoid loading is designed and carried out.By adjusting the charging voltage of the capacitor,the loading of different speeds of the 7075Al ring is successfully realized,the maximum speed is from 80m/s to 180m/s,and the corresponding maximum strain rate is from 3478s-1 to 7652s-1.In the experiment,Doppler laser interferometry was used to obtain the history of the expansion speed of the 7075Al ring.The fractographic SEM and metallographic analysis of the recovered samples were carried out.The experimental results show that with the increase of the dynamic loading strain rate,the fracture mode of the 7075 aluminum ring changed,from the tensile fracture under the low strain rate to the tensile shear mixing under the medium strain rate,and then to the shear fracture mode under the high strain rate;the fracture strain of the 7075Al ring gradually increased from 0.09 to 0.18;the number of fragments of the 7075Al ring was the number of fragments.It first increases and then decreases,then increases again,and its inflection point is the transformation point of fracture mode.3)the fracture process of 7075Al standard tensile test piece and 7075Al expansion ring under different loading strain rates is carried out by using the finite element software ABAQUS with powerful function in nonlinear field.The fracture model adopts the damage accumulation model,and the concrete function form combines the JC damage model and the continuous plastic damage model proposed by Liang Xue et al.The effects of strain rate,stress three axis and Lode angle on the fracture are considered.The model parameters are fitted by the standard tensile test and the experimental data of the expansion ring.The simulation of standard tensile specimens showed that with the increase of loading speed,the fracture gradually changed from tensile fracture to shear fracture.The simulation of electromagnetic expansion ring found that the fracture also changed from tensile fracture to shear fracture as loading speed increased.The stress three axis and Lode parameters in the fracture process of standard tensile specimen and expansion ring specimen under different loading strain rates are compared.It is found that with the increase of loading strain rate,the stress three axis and Lode parameters in the sample increase,which makes the fracture mode change from tensile to shear.The fracture strain of the standard tensile specimen and the expansion ring specimen obtained by simulation is in good agreement with the experimental results.The number of fragments of the electromagnetic expansion ring simulated is higher than that of the experimental results.