Author: Li, Wenting
Title: Investigation on ductile fracture behaviour and the modified GTN-Thomason fracture criterion in micro-scaled plastic deformation of materials
Advisors: Fu, Ming Wang (ME)
Degree: Ph.D.
Year: 2019
Subject: Hong Kong Polytechnic University -- Dissertations
Deformations (Mechanics)
Fracture mechanics
Department: Department of Mechanical Engineering
Pages: xxi, 147 pages : color illustrations
Language: English
Abstract: A constitutive model considering the compositions of surface grain, grain interior and grain boundary and their contributions to the flow strength or stress of materials in micro-scaled plastic deformation was developed and termed as a combined surface layer and grain boundary strengthening model in this dissertation. To determine the composition of the three interior microstructural parts of materials, optical microscope and digital image processing technologies were employed. A series of micro-tensile experiments using the round bar specimens with three different geometrical shapes and microstructural grain sizes were conducted for study of the deformation and ductile fracture behaviours of materials. The constitutive equation was implemented in finite element analysis and validated via physical experiments. The relationship among fracture strain, grain size and stress triaxiality of the round bar specimen was thus established. It was found both fracture strain and stress triaxiality are increased with the decrease of grain size, while the high stress triaxiality leads to a smaller fracture strain for a given grain size. Through observation of the material fractographs, it was revealed that the domination of shear fracture in the 'cup-cone' fracture is increased with grain size. The proposed constitutive model can thus be employed to investigate the deformation and ductile fracture more accurately in micro-scaled deformation of metallic materials. To explore the interactive influence of the deformation stress state and material microstructural grain size on the fracture behaviour in micro-scaled deformation, a series of micro-scale copper specimens with various geometrical dimensions and microstructural grain sizes were prepared and deformed to achieve different stress states represented by stress-related variables, such as the normalised third deviatoric stress invariant and the stress triaxiality. The speckle pattern method of continuous tracking was used to investigate the mechanical responses of materials in various deformation stress states and material microstructures, and a finite-element simulation of each deformation was performed with the combined surface layer and grain boundary strengthening constitutive model, which considers the contributions of the surface grain, grain interior and grain boundary in representing the grain and geometry sizes. The interactive effects of the normalised third invariant, stress triaxiality and microstructural grain size on the fracture strain were identified and established by accounting for the correlation between the results of simulation and those of physical experimentation. Their influences on the fracture mechanism, mode and behaviour were further explored. The results revealed that greater stress fosters the growth and coalescence of micro-defects and thus decreases the fracture strain in the dimple-dominant fracture mode of the round bar tensile deformation and in the shear-dominant fracture mode of the cylindrical compression. The fracture strain is increased in sheet shear and tensile deformations mainly as a result of the transformation from the shear-dominant fracture with an inter-void shearing mechanism in a low stress state to the dimple-dominant fracture with an inter-void necking mechanism in a high stress state. The stress state and the material microstructure both affect ductile fracture. The larger grain size generates fewer micro-voids, more uneven grain distribution and severer localisation deformation, which accelerates fracture failure. Furthermore, stress triaxiality and the normalised third invariant at a low stress triaxiality are decreased with a larger grain size, which in turn affects the occurrence of fracture. These effects coexist and compete to increase the fracture strain in some deformation scenarios, whilst an opposite effect is seen in other cases. In view of these influences, it was concluded that a larger grain size and a higher stress state inhibit the occurrence of fracture for sheet specimens with fracture modes from shear-dominant to dimple-dominant; in contrast, a smaller grain size and a lower stress state impede the occurrence of fracture and thus result in a larger fracture strain for other cases.
The investigation of the interactive influence of stress state and grain size on micro-scale ductile fracture aids the development of ductile fracture criterion suitable for micro-scaled deformation by considering the micro-mechanism of ductile fracture. To explore the ductile fracture criterion under different stress states and grain sizes, five more miniaturised copper sheets were designed and annealed. The mechanical property of sheet was described by the combined constitutive model. It was found that fewer voids, lower flow stress, smaller void shear effect and faster void coalescence are caused by a larger grain size. Shear factor and size effect were thus introduced into the newly modified GTN-Thomason criterion to predict the micro-scale ductile fracture of sheets under different stress states and grain sizes. The subroutine VUMAT was employed to code the developed criterion into ABAQUS software. The modelled load-stroke responses and deformation profiles of five sheets with different grain sizes agree well with the experiments. The extended criterion was thus validated. According to the accurate simulation results, the influences of stress triaxiality, the normalised third invariant and grain size on void evolution were further analysed. The results indicatedthat the increase of the normalised invariant prevents void shear and leads to a better ductility, while the increase of stress triaxiality facilitates the growth of voids to result in a worse ductility. When the deformation with a relatively high normalised invariant has an ignored void shear, the increase of grain size accelerates the onset of void coalescence and the rate of void growth, leading to a rapid failure. However, when the deformation with a relatively low normalised invariant has an unneglected void shear, finer grains increases the rates of void nucleation, growth and shear, causing a smaller fracture strain. These findings and the modified GTN ductile fracture criterion enhance the understanding and prediction of ductile fracture in micro-scaled plastic deformation, respectively, and further facilitate the development of microparts using by deformation-based micro-manufacturing process.
Rights: All rights reserved
Access: open access

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