Issue 19

P. K. Pradhan et alii, Frattura ed Integrità Strutturale, 19 (2012) 51-60; DOI: 10.3221/IGF-ESIS.19.05 51 Micro void coalescence of ductile fracture in mild steel during tensile straining P. K. Pradhan, P. R. Dash Mechanical Engineering, VSS University of Technology, Burla, Odisha, India prasant2_pradhan@yahoo.com P. S. Robi Mechanical Engineering, IIT, Guwahati, India. Sankar K. Roy Mechanical Engineering, IIT Kharagpur A BSTRACT . The ductile fracture occurs mainly in three stages i.e. void nucleation, void growth and the void coalescence. The present work focuses on the study the coalescence of existing micro void in a ductile material, mild steel. The specimen with holes in square array at various angle to load axis have been tested. The holes were machined in the specimen and assuming those hole as the voids. The growth and coalescence behaviours during tensile straining were observed both in macro and micro levels. Since the existing facility is not adequate to make hole size in micron, this work has been carried out by making hole upto 500 micron. The results are compared with other specimen with bigger size hole and without any hole. Also the effects of micro voids (present in the material) on the progress of crack have been studied. It is found that with same amount of voids, present in different positions, the mechanical properties of the material are altered. K EYWORDS . Ductile fracture; Micro void; Coalescence; Crack propagation; Microstructure. I NTRODUCTION racture of structural components has been a common phenomenon faced by men for as long as structures have been used. Griffith [1] identified that surface energy, i.e. the amount of energy is required for a crack to grow. This theory in the original form is applicable only to a perfectly brittle material such as glass. Later Irwin and Orwan [2] modified this for the ductile material considering the energy required for plastic deformation at the crack tip. Fracture may be classified into two groups: One is brittle fracture and the other one is ductile fracture. Brittle fracture occurs suddenly and with no or very less deformation. Ductile fracture is always associated with large amount of plastic deformation and it consists mainly of three stages, (i) void nucleation, (ii) void growth and (iii) void coalescence. Nucleation of voids in materials occurs generally at second phase particles or at inclusions. Goods and Brown [3] developed a dislocation-based model of void nucleation. According to them, when the total stress (summation of local stress, flow stress and hydrostatic stress) exceeds the interfacial bond strength, the voids nucleate. Further increase in the external remote stress result in the continuous volumetric growth of voids. Rice and Tracey [4] found that the micro void undergoes a volumetric growth and shape change due to the continued plastic flow of matrix. Bandstra [5] has done some experiments with simulation and found that growth rate accelerates with the strain rate with the transverse orientations F