Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 100045

100045 - Statistical Microvoid Characterization at the Instance of Ductile Fracture in Steels 

Ductile fracture is a common fracture initiating mechanism in engineering metals such as aluminum, stainless steel, copper, titanium, etc. The ductile fracture mechanism is started by the nucleation of microvoids, followed by their growth, and subsequent coalescence under the influence of stress triaxiality in a plastically deforming metallic matrix. Although several existing models predict the strains corresponding to the ductile fracture initiation by tracking material scale damage as a function of stress and strain states, the model parameters are usually not directly calibrated from experiments. With this, the present study aims to bridge the gap between the ductile fracture experiments and the prediction models by employing microscopy studies to calibrate the fracture model parameters.    

The objectives of the present study are: (a) to experimentally characterize the microvoid statistical attributes at the instance of fracture by employing microscopy on fracture surfaces; and (b) to understand the relationship between the experimentally extracted microvoid features at the instance of fracture, and state of stress and strain. To this end, uniaxial tension tests were performed on circumferentially notched cylindrical specimens made of 17-4 PH stainless steel. The fractured steel specimens were observed using a 4K digital microscope to identify the fracture mechanism and quantify the fracture surface. To understand the underlying microscopic fracture mechanism, the fracture surfaces were divided into three distinct fracture zones and were further studied at higher magnification by extracting high-resolution fractographs corresponding to each of the locations. Two rectangular areas of dimension 75µm × 75µm were randomly drawn on the high-resolution micrograph of each of the distinct fracture zones. Subsequently, 25 micro void areas were extracted using an image analysis software from each of the rectangular areas drawn on the micrographs. The present study assumes that the coalesced microvoids are equiaxed with approximately one-half of the microvoid lying on the fracture surface of the test specimens. Based on this assumption, microvoid radii were computed and statistical analyses of the microvoid size distributions were conducted. Furthermore, non-linear finite element analysis was performed on the notched steel specimens to extract the stress triaxiality and plastic strain variation in the test specimens. Equivalent plastic strain-averaged stress triaxiality was computed for the test specimens to capture the stress triaxiality variation throughout the loading history. Finally, an uncoupled fracture criterion was calibrated based on the experimentally extracted microvoid sizes. The experimentally calibrated fracture criterion was then used to predict the fracture initiation location and elongation corresponding to ductile fracture initiation in the steel test specimens. The microvoid statistical attributes along with the experimentally calibrated fracture criterion predictions will be presented in the poster.        

Presenting Author: Surajit Dey North Dakota State University

Presenting Author Biography: Surajit Dey is a Graduate Research Assistant in the Civil, Construction and Environmental Engineering department at North Dakota State University.

Authors:

Surajit Dey North Dakota State University
Ravi Kiran North Dakota State University