Experimental and numerical investigation of damage state of Ti-6Al-4V alloy

In this thesis, the damage behavior of the Ti-6Al-4V alloy sheet in the tensile test and hydroforming (HF) process was investigated experimentally and numerically. In this context, tensile tests of pure shear, standard unnotched, and notched samples with different stress triaxiality values at tensile speeds of 1, 3, and 25 mm/min were carried out. After the tensile tests, the fracture surfaces of the samples were examined with SEM images in terms of the deformation rate and stress triaxiality values. It has been observed that the fracture surfaces generally have morphologies showing ductile fracture, and fracture surfaces are affected by stress triaxiality and deformation rate. It has been observed that the fracture surfaces of the tensile test specimens generally have morphologies showing ductile fracture and are affected by stress triaxiality and deformation rate. For this reason, it has been determined that it is appropriate to use the Johnson-Cook (JC) hardening and damage criterion, which takes into account the stress state and deformation rate, which is one of the ductile fracture material models, in Finite Element Analysis (FEA). The tensile test and FEA of the HF process were performed by determining the optimum values of JC hardening and damage criterion parameters. The tensile curves of Ti-6Al-4V alloy obtained from FEA showed excellent agreement with the experimental curves. In the hydraulic bulging process, the burst pressure, bulge height, and maximum thickness reduction were predicted correctly with a rate of 76.2%, 97% and 96.8%, respectively. In the HF process performed at 10 mm die height, the undulation distance and base radius after springback without any damage were determined with an average deviation of 0.73% and 0.13%, respectively, from the experimental results. In the HF process performed at a die height of 12 mm, the tearing pressure was estimated correctly with a rate of 92.5%. Die entry radius and base radius were determined with a success rate of 95.5% and 97.8%, respectively. The thickness distribution largely matched the experimental results. The fracture zones in both processes showed good agreement with the experimental results. As a result, it has been determined that the JC hardening and damage criterion can be successfully used in hydroforming of Ti-6Al-4V alloyed sheet in terms of formability and damage prediction.