Research on the Expansion Failure Problem of TA2 Titanium Coil Tubes

The expansion and fracture behavior of titanium tube shell walls is not only influenced by the characteristics of the burst load, but also related to material properties, structural features and dimensions of the cylindrical shell. Here, the analysis mainly focuses on the TA2 titanium alloy tube with specific structural dimensions under different burst pressures.

Researchers employed a combined approach of finite element analysis and experimental analysis to study the failure mechanism and influencing factors of titanium circular tubes under burst expansion. By analyzing the propagation of shock waves, the evolution of strain and stress states within ideal titanium circular tubes under different explosive burst loads, the fragmentation phenomenon of titanium circular tubes was explored. The results show that:

The failure of TA2 titanium coil under different burst pressures was all shear fracture along the 45° or 135° direction from the radial direction, but the crack initiation and failure process were different. Under higher burst pressure, microstructure of the fracture surface of the fragments showed that there were holes and microcrack bands distributed in the middle of the wall thickness, and the crack formed in the middle of the sample cross-section and extended to the inner and outer surfaces to cause fracture; while under lower burst pressure, the crack initiated from the inner wall of the sample and extended outward to cause fracture. The finite element results were in good agreement with the experimental phenomena.

Finite element analysis shows that at higher burst pressures, due to the repeated reflection of shock waves between the inner and outer walls of the sample, a secondary plastic zone is formed, resulting in a larger equivalent plastic strain in the middle region of the sample's wall thickness compared to the inner and outer surfaces. Fracture occurs during the shock wave loading stage, with damage cracks initiating from the middle of the sample's wall thickness and propagating towards the inner and outer walls, leading to failure. However, at lower burst pressures, failure occurs during the free expansion stage, when the sample as a whole enters a tensile stress state. The larger the equivalent plastic accumulation at the inner wall, the more the fracture initiates from the inner surface and propagates towards the outer wall in a shear manner.

Some outer wall tensile fracture phenomena observed in the relevant experiments may be related to factors such as the geometry of the specimens and material defects. The influence of defects on the burst failure characteristics of titanium coils deserves further attention.