Dynamic Mechanical Properties And Fracture Behaviors Of Damage Tolerance Titanium Alloys Under High Strain Rate

Damage tolerance titanium alloys have an increasing application in aircraft main load-bearing structure due to their high fracture toughness and low fatigue crack propagation rate. It has great significance for aircraft maintenance, life prediction, even ensuring flight safety to study the dynamic mechanical behaviors and fracture behaviors of damage tolerance titanium alloys.In this paper, the dynamic mechanical properties and fracture behaviors of TC4-DT and TC21 alloys under high strain rate were studied. Dynamic compression experiments of TC4-DT and TC21 alloys were performed at a range of strain rate from 1000s-1 to 8000s-1 by high split Hopkinson Pressure Bar at room temperature,the curves of true stress-strain under high strain rates were obtained. The result shows that these two Ti alloys both have obvious strain work-hardening and effects of strain rate at strengthening, plasticity-increasing. With the increase of strain rate, they have stronger effects of strain rate at strengthening and plasticity-increasing, and TC4-DT alloy has more obvious effects of strain rate at plasticity-increasing than TC21 alloy. Microscope observation and analysis of the samples were carried out by Optical Microscope and Scanning Electronic Microscopy techniques,and micro-hardness was measured by Micro-hardness Tester. Compared with TC21 alloy, obvious adiabatic shear bands were observed in TC4-DT alloy and their width increased from 10μm to 20μm with strain rate ranging from 1000s-1 to 8000s-1. The micro-hardness of TC4-DT decreases with the increase of the distance along the vertical direction of adiabatic shear bands. But the distribution of micro-hardness of TC21 is uniform and the micro-hardness increases from 478.5HV to 535.2HV with the increase of strain rate. The crack initiation, propagation and polymerization take place on the specimens along the direction of adiabatic shear bands with the increase of strain rate. The experimental result shows adiabatic shear failure occurs in the tested specimens of those two Ti alloys, two typical areas of dimple and smooth with different features are alternatively distributed on the whole fracture surface of these two alloys. There are more dimples in TC4-DT alloy and the dimples along the direction of maximum shear are longer. Based on the Johnson-Cook constitutive model, the dynamic constitutive model of TC21 was obtained by fitting the experimental results and the dynamic constitutive model of TC4-DT was also obtained by fitting the experimental results with new temperature item.