@article{ngaile_yang_kilonzo_2011, title={Real-Time Friction Error Compensation in Tube Hydroforming Process Control}, volume={133}, ISSN={["1087-1357"]}, DOI={10.1115/1.4005430}, abstractNote={Tube hydroforming (THF) is a metal-forming process that uses a pressurized fluid in place of a hard tool to plastically deform a given tube into a desired shape. In addition to the internal pressure, the tube material is fed axially toward the die cavity. One of the challenges in THF is the nonlinear and varying friction conditions at the tube-tool interface, which make it difficult to establish accurate loading paths (pressure versus feed) for THF. A THF process control model that can compensate for the loading path deviation due to frictional errors in tube hydroforming is proposed. In the proposed model, an algorithm and a software platform have been developed such that the sensed forming load from a THF machine is mapped to a database containing a set of loading paths that correspond to different friction conditions for a specific part. A real-time friction error compensation is then carried out by readjusting the loading path as the THF process progresses. This scheme reduces part failures that would normally occur due to variability in friction conditions. The implementation and experimental verification of the proposed model is discussed.}, number={6}, journal={JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING-TRANSACTIONS OF THE ASME}, author={Ngaile, Gracious and Yang, Chen and Kilonzo, Obadiah}, year={2011}, month={Dec} }
 @article{yang_ngaile_2010, title={Preform design for forging and extrusion processes based on geometrical resemblance}, volume={224}, ISSN={["2041-2975"]}, DOI={10.1243/09544054jem1799}, abstractNote={ Preform design in multi-stage forging processes is critical to ensure the production of defect-free parts. Moreover, owing to the geometry and material-flow complexities in forging processes, finding the optimal preform shapes could be difficult and time consuming. This paper proposes an efficient preform design methodology based on geometrical resemblance, which requires several finite element analysis simulation iterations to obtain a good preform shape. The initial and subsequent simulations are carried out by constructing a slightly larger part that geometrically resembles the desired part. Initial finite element analysis simulation of the larger part is performed with a reasonably guessed preform shape, whose forming defects or flash formation would be corrected in subsequent steps. Then a series of intermediate parts of similar shape and between the largest part and the desired part in size are constructed. The undeformed shape corresponding to an intermediate part can be obtained by backwards tracing of material flow from the simulation results of the larger part. This undeformed shape is then taken as the preform shape of the intermediate part. The procedure is repeated until the intermediate part is geometrically close to the desired part, which leads to the preform shape. In order to verify this preform-design methodology, several case studies on forging and extrusion processes have been carried out. The methodology has been shown to be computationally efficient, requiring as few as three finite element iterations to obtain a good preform shape. }, number={B9}, journal={PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART B-JOURNAL OF ENGINEERING MANUFACTURE}, author={Yang, C. and Ngaile, G.}, year={2010}, pages={1409–1423} }
 @article{ngaile_yang_2009, title={Analytical Model for the Characterization of the Guiding Zone Tribotest for Tube Hydroforming}, volume={131}, ISSN={["1528-8935"]}, DOI={10.1115/1.3090888}, abstractNote={Common part failures in tube hydroforming include wrinkling, premature fracture, and unacceptable part surface quality. Some of these failures are attributed to the inability to optimize tribological conditions. There has been an increasing demand for the development of effective lubricants for tube hydroforming due to widespread application of this process. This paper presents an analytical model of the guiding zone tribotest commonly used to evaluate lubricant performance for tube hydroforming. Through a mechanistic approach, a closed-form solution for the field variables contact pressure, effective stress/strain, longitudinal stress/strain, and hoop stress can be computed. The analytical model was validated by the finite element method. In addition to determining friction coefficient, the expression for local state of stress and strain on the tube provides an opportunity for in-depth study of the behavior of lubricant and associated lubrication mechanisms. The model can aid as a quick tool for iterating geometric variables in the design of a guiding zone, which is an integral part of tube hydroforming tooling.}, number={2}, journal={JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING-TRANSACTIONS OF THE ASME}, author={Ngaile, Gracious and Yang, Chen}, year={2009}, month={Apr} }
 @article{ngaile_yang_2008, title={Analytical model for characterizing the pear-shaped tribotest for tube hydroforming. Part 1}, volume={222}, ISSN={["0954-4054"]}, DOI={10.1243/09544054JEM1057}, abstractNote={ An analytical model to characterize the pear-shaped tribotest is presented. In this test, a tubular specimen is pressurized, forcing the material to flow towards the apex of a pear-shaped die. The height of the pear-shaped tube is a function of the magnitude of friction stress at the tube—die interface. Through a mechanistic approach, a closed-form solution for field variables die—tube contact pressure, effective stress/strain, longitudinal stress/strain, and hoop stress/strain can be computed as a function of input pressure loading. The model has been validated by finite element analysis. The closed-form solution can be used rapidly to establish the calibration curves for determination of friction coefficient in the pear-shaped tribotest. Of equal importance, the analytical model can be used to optimize both process and die geometric variables to suit specific needs such as die wear studies through monitoring local interface pressure loading, types of material to be tested, tube sizes, and so on. Details on the applications of the developed analytical model can be found in Part 2 of this paper. }, number={7}, journal={PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART B-JOURNAL OF ENGINEERING MANUFACTURE}, author={Ngaile, G. and Yang, C.}, year={2008}, month={Jul}, pages={849–863} }
 @article{yang_ngaile_2008, title={Analytical model for planar tube hydroforming: Prediction of formed shape, corner fill, wall thinning, and forming pressure}, volume={50}, ISSN={["0020-7403"]}, DOI={10.1016/j.ijmecsci.2008.05.006}, abstractNote={An analytical model for planar tube hydroforming based on deformation theory has been developed. This analytical model can be used to predict hydroformed shape, corner fill, wall thinning, and forming pressure. As the model is based on a mechanistic approach with bending effects included, local strain and stress distribution across the wall thickness can be determined. This includes strain and stress distributions for the outer layer, inside layer, and middle layer. The model is validated using finite element analysis and tube hydroforming experiments on irregular triangular, irregular quadrilateral, and pentagonal hydroformed shapes.}, number={8}, journal={INTERNATIONAL JOURNAL OF MECHANICAL SCIENCES}, author={Yang, Chen and Ngaile, Gracious}, year={2008}, month={Aug}, pages={1263–1279} }
 @article{ngaille_yang_2008, title={Applications of analytical model for characterizing the pear-shaped tribotest for tube hydroforming. Part 2}, volume={222}, ISSN={["2041-2975"]}, DOI={10.1243/09544054JEM1058}, abstractNote={ Applications of the analytical model for characterizing the pear-shaped tribotest are presented. Details on the derivations of the analytical model can be found in Part 1 of this paper (published in Proceedings of the Institution of Mechanical Engineers, Journal of Engineering Manufacture, 2008, Vol. 222). In this test, a tubular specimen is pressurized, forcing the material to flow towards the apex of a pear-shaped die. The height of the pear-shaped tube is a function of the magnitude of friction stress at the tube—die interface. The analytical model is used rapidly to establish the calibration curves for determination of friction coefficient in the pear-shaped tribotest. The model is also used to optimize both process and die geometric variables to suit specific tribological needs. The paper presents examples of how optimal pear-shape tribotest conditions pertaining to die geometry, tubular material properties, tube sizes, and pressure loading can be achieved via this model. }, number={7}, journal={PROCEEDINGS OF THE INSTITUTION OF MECHANICAL ENGINEERS PART B-JOURNAL OF ENGINEERING MANUFACTURE}, author={Ngaille, G. and Yang, C.}, year={2008}, month={Jul}, pages={865–873} }