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Experimental and numerical investigation of formability and ductile fracture in incremental sheet forming
Get PDFIncremental sheet forming (ISF), also referred to as single point incremental forming (SPIF), is a relatively new, flexible forming process. The ISF process has excellent adaptability to CNC milling machines due to the fact that it does not require the use of high capacity presses or specific dies and is easy to automate for both symmetric and non-symmetric shapes. In SPIF, a blank sheet is fixed by a holder onto a CNC milling machine. A workpiece is formed by the action of a small hemispheric tool in contact with the blank moving along a user-specified tool path and incrementally deforming the blank into the desired shape. According to the technical characteristics of ISF, it can be used to manufacture small batch and customised products. SPIF is a promising technique, which could be further developed as an alternative manufacturing approach for customised medical parts, e.g. cranial plates, with considerable benefits in lead time and cost reduction.
This thesis reports the findings of finite element (FE) analyses and experimental investigations into the formability and fracture in the ISF process, and their application with regard to manufacturing cranial plates, as well as the applicability of using the ISF process to form aluminium matrix composite (AMC).
A modified Gurson–Tvergaard-Needleman (GTN) damage constitutive model was developed with the consideration of shear. The developed GTN model was implemented in Abaqus/Explicit via a VUMAT user subroutine to predict ductile fracture in the ISF process due to void nucleation, growth and coalescence. In comparison with experimental testing, there is a good agreement in the occurrence of fracture. The results also show that shear plays a role under meridional tensile stress in accelerating fracture propagation in ISF processes.
The Nakajima test was used to construct the forming limit curve at necking (FLCN) and fracture (FLCF) for pure titanium sheet. The results of the FLCF are compared with those of the ISF to evaluate the ability of the Nakajima test to describe the fracture in ISF. Due to the fact that the GTN model can be used to capture fracture occurrence at high stress triaxiality, and the shear modified GTN model was developed to predict the fracture at zero stress or even negative stress triaxiality, the original GTN model and shear modified GTN model may not be suitable to predict the fracture in all samples of the Nakajima test, as some samples are deformed under moderate stress triaxiality. Therefore, another GTN based model with the consideration of stress triaxiality was developed to predict fracture in the Nakajima specimens under different stress triaxiality. The experimental and FE results showed that the shear modified GTN model predicted the fracture accurately with samples under uniaxial tension conditions due to low stress triaxiality and the original GTN model was suitable for an equi-biaxial strain state (high stress triaxiality). On the other hand, the stress triaxiality modified GTN model should be used for samples which have moderate stress triaxiality. The Nakajima test can be applied to establish the FLCF, which in turn can be utilized to describe the formability of the SPIF test for conical and pyramidal truncated specimens.
Experiments and FE analyses were conducted to evaluate the influence of major forming parameters, including the step size, feed rate and tool size, on the formability and fracture of pure titanium. Two types of pure Ti (grades 1 and 2) were used to evaluate the sensitivity of the material type to forming parameters. It was found that the ISF parameters have different degrees of effect on the formability and fracture occurrence of pure titanium, and that grade 2 pure Ti is more sensitive to forming parameters. The optimal parameters from the parametric study were used to investigate the feasibility of using ISF to form a cranial plate; the results show that ISF can satisfactorily form a cranial plate.
By evaluating the deformation and fracture mechanisms of a 6092Al alloy metal matrix composite reinforced with 17.5p vol. % SiC particles (6092Al/SiCp), the applicability of the ISF process to form the 6092Al/SiCp aluminium matrix composite (AMC) was investigated. Tensile tests were carried out at different strain rates to study the microstructure and topography of the 6092Al/SiCp sheet by scanning electron microscopy (SEM), coupled with energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results show that this AMC material at T6 condition is brittle due to the formation of precipitations as a result of either the interaction between the Al and SiCp or from hardening precipitation treatment e.g. Al2Cu, Al4Cu2Mg8Si7 and MgAl2. O-condition annealing was therefore used to reduce the detrimental effect of the intermetallic compounds in the interface region and can improve the fracture toughness and ductility of the material. However, the Al/SiC sheet treated with O-condition annealing is more sensitive to the strain rate than that treated with T6.
By investigating the feasibility of using SPIF to form AMC sheets for the first time, the results show that the 6092Al/SiCp sheets can be satisfactorily formed after O-condition annealing
Experimental and numerical investigation of formability and ductile fracture in incremental sheet forming
Get PDFIncremental sheet forming (ISF), also referred to as single point incremental forming (SPIF), is a relatively new, flexible forming process. The ISF process has excellent adaptability to CNC milling machines due to the fact that it does not require the use of high capacity presses or specific dies and is easy to automate for both symmetric and non-symmetric shapes. In SPIF, a blank sheet is fixed by a holder onto a CNC milling machine. A workpiece is formed by the action of a small hemispheric tool in contact with the blank moving along a user-specified tool path and incrementally deforming the blank into the desired shape. According to the technical characteristics of ISF, it can be used to manufacture small batch and customised products. SPIF is a promising technique, which could be further developed as an alternative manufacturing approach for customised medical parts, e.g. cranial plates, with considerable benefits in lead time and cost reduction. \ud
This thesis reports the findings of finite element (FE) analyses and experimental investigations into the formability and fracture in the ISF process, and their application with regard to manufacturing cranial plates, as well as the applicability of using the ISF process to form aluminium matrix composite (AMC).\ud
A modified Gurson–Tvergaard-Needleman (GTN) damage constitutive model was developed with the consideration of shear. The developed GTN model was implemented in Abaqus/Explicit via a VUMAT user subroutine to predict ductile fracture in the ISF process due to void nucleation, growth and coalescence. In comparison with experimental testing, there is a good agreement in the occurrence of fracture. The results also show that shear plays a role under meridional tensile stress in accelerating fracture propagation in ISF processes.\ud
The Nakajima test was used to construct the forming limit curve at necking (FLCN) and fracture (FLCF) for pure titanium sheet. The results of the FLCF are compared with those of the ISF to evaluate the ability of the Nakajima test to describe the fracture in ISF. Due to the fact that the GTN model can be used to capture fracture occurrence at high stress triaxiality, and the shear modified GTN model was developed to predict the fracture at zero stress or even negative stress triaxiality, the original GTN model and shear modified GTN model may not be suitable to predict the fracture in all samples of the Nakajima test, as some samples are deformed under moderate stress triaxiality. Therefore, another GTN based model with the consideration of stress triaxiality was developed to predict fracture in the Nakajima specimens under different stress triaxiality. The experimental and FE results showed that the shear modified GTN model predicted the fracture accurately with samples under uniaxial tension conditions due to low stress triaxiality and the original GTN model was suitable for an equi-biaxial strain state (high stress triaxiality). On the other hand, the stress triaxiality modified GTN model should be used for samples which have moderate stress triaxiality. The Nakajima test can be applied to establish the FLCF, which in turn can be utilized to describe the formability of the SPIF test for conical and pyramidal truncated specimens.\ud
Experiments and FE analyses were conducted to evaluate the influence of major forming parameters, including the step size, feed rate and tool size, on the formability and fracture of pure titanium. Two types of pure Ti (grades 1 and 2) were used to evaluate the sensitivity of the material type to forming parameters. It was found that the ISF parameters have different degrees of effect on the formability and fracture occurrence of pure titanium, and that grade 2 pure Ti is more sensitive to forming parameters. The optimal parameters from the parametric study were used to investigate the feasibility of using ISF to form a cranial plate; the results show that ISF can satisfactorily form a cranial plate. \ud
By evaluating the deformation and fracture mechanisms of a 6092Al alloy metal matrix composite reinforced with 17.5p vol. % SiC particles (6092Al/SiCp), the applicability of the ISF process to form the 6092Al/SiCp aluminium matrix composite (AMC) was investigated. Tensile tests were carried out at different strain rates to study the microstructure and topography of the 6092Al/SiCp sheet by scanning electron microscopy (SEM), coupled with energy dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM) and X-ray diffraction (XRD). The results show that this AMC material at T6 condition is brittle due to the formation of precipitations as a result of either the interaction between the Al and SiCp or from hardening precipitation treatment e.g. Al2Cu, Al4Cu2Mg8Si7 and MgAl2. O-condition annealing was therefore used to reduce the detrimental effect of the intermetallic compounds in the interface region and can improve the fracture toughness and ductility of the material. However, the Al/SiC sheet treated with O-condition annealing is more sensitive to the strain rate than that treated with T6.\ud
By investigating the feasibility of using SPIF to form AMC sheets for the first time, the results show that the 6092Al/SiCp sheets can be satisfactorily formed after O-condition annealing
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Surface roughness analysis of medical grade titanium sheets formed by single point incremental forming
Get PDFSingle point incremental forming (SPIF) process has proven benefits in terms of formability, flexibility, and low cost as compared to conventional sheet forming, although it has some issues such as dimensional accuracy and low quality of surface finish for some materials. The SPIF is considered as a potential method of producing customised medical parts such as craniofacial implants using a titanium sheet. This investigation aims to analyse the overall surface roughness of grade 1 pure titanium along the wall of the hyperbolic truncated cone formed by the SPIF process with different forming parameters (i.e., forming tool diameter, step size, and feed rate). Focus variation microscopy was used to measure the surface roughness experimentally along the wall of the truncated cone. Abaqus/Explicit was used to predict the equivalent stress and equivalent plastic strain along the wall of the truncated cone part from the top to the fractured region to evaluate the relationship between the stress, strain, and roughness distribution. It was found that the surface roughness changes with the deformed part height and rough surface could be produced in the region of high equivalent stress and low equivalent plastic strain. Such a surface roughness and equivalent stress and plastic strain correlation has a clear implication to the design and the surface quality of sheet parts made by SPIF
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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