Session: 06-03-02: Advances in Aerospace Structures and Materials-2

Paper Number: 172992

Quasi-Static and Dynamic Fracture Behavior of Additively Manufactured Inconel 718 

Inconel 718 (IN-718) is a superalloy with many aerospace and automotive applications. It is used in extreme environments such as rocket engine manifolds and automotive exhaust systems due to its ability to maintain high strength under intense heat. In this work, edge notched three-point bend specimens were additively manufactured using laser beam powder bed fusion (LBPF) for evaluating the fracture behavior under quasi static and dynamic loading conditions. Specimens were manufactured with identical print architecture but with three different laser process parameters, two heat treatment protocols, and two shielding gas usage during printing. Single edge notches were cut in specimens using electrical discharge machining (EDM) normal to the raster direction. A pair of EDM grooves was added to the front and back faces along the uncracked ligament of each specimen to constrain crack growth along a mode-I path, suppress shear lip formation and mitigate crack front tunneling. Random speckle patterns were sprayed on to specimens for performing 2D digital image correlation (DIC) and measure surface deformations. During quasi static tests, specimens were loaded slowly in a Instron universal testing (crosshead speed of 0.5 mm/min) machine until the crack initiated and grew. To evaluate the material’s fracture behavior under dynamic loading, similarly notched three-point bend specimens were tested using a split-Hopkinson compression bar apparatus and 2D DIC for mapping deformations. The fracture events were recorded using an ultrahigh speed digital camera at 400,000 frames per second (impactor velocity ~ 12 m/sec). Using measured surface displacement fields, the J-Integral was extracted at each load-step (in quasi-static experiments) or time instant (in dynamic experiments) using a hybrid DIC-Finite Element (DIC-FE) scheme that accounts for elastic-plastic stress vs. strain response. Quasi static results were validated with a companion finite element analysis based on far-field boundary conditions. The results show that the fracture energy release rate (ERR) continues to increase in the test window after crack initiation offering significant resistance to crack growth. The critical value of ERR has significant variation depending on the type of shielding gas employed while printing and heat treatment protocol used during LBPF.  However, only marginal variations occurred due to changes in the laser power and scan speed during printing. Fractography and microscopy of fractured specimens further support measurements. The dynamic fracture tests did not produce the expected fast fracture events (crack speeds were < 50 m/sec) due to material ductility. Accordingly, the measured fracture parameters closely followed quasi static counterparts. 

Presenting Author: Hareesh Tippur Auburn University

Presenting Author Biography: Hareesh Tippur is a McWane Endowed Chair Professor of Mechanical Engineering and Associate Chair for Graduate Studies at Auburn University, AL. He is the Director of the Laboratory for Failure Characterization and Optical Techniques.

Authors:

Hareesh Tippur Auburn University