Hui Yun Hwang

Design and manufacture of a carbon fiber epoxy rotating boring bar

Abstract High speed boring operations of deep holes with steel or tungsten boring bars are often hindered by the chatter vibration of boring bars because of their low dynamic stiffness and natural frequencies. The chatter is a self-excited vibration that occurs in metal cutting if, either the chip width is too large with respect to the dynamic stiffness of the system, which is proportional to the damping and static stiffness of the boring bar, or the rotating speed of the boring bar approaches one of its natural frequencies, which is proportional to the specific stiffness (E/ρg) of the material used. In this study, a rotating boring bar was designed and manufactured with high stiffness pitch-based carbon fiber epoxy composite to meet the requirements of boring at high rotating speed because carbon fiber epoxy composite materials have a much higher specific stiffness and higher damping than conventional boring bar materials. The optimum design parameters for the composite boring bar were experimentally determined with respect to material types and dimensions of the boring bar through vibration tests. The dynamic characteristics of the composite boring bar developed were measured by the boring operation of aluminum engine blocks. From experiments, it was found that the dynamic stiffness of the composite boring bar was about 30% higher than that of the tungsten carbide boring bar. Also, chatter did not occur up to the ratio of length to diameter (l/d) of 10.7, which is about a 30% improvement compared to the tungsten boring bar of l/d of 8.0.

Prediction of crack length and crack growth rate of adhesive joints by a piezoelectric method

As adhesive joints have been widely used for fastening thin adherends, the damage tolerance design of adhesive joints has become important, and the estimation of initiation and propagation of a fatigue crack in the adhesive has become necessary. However, the measurement of crack length of tubular joints has been difficult because the observation of crack initiation and growth in the adhesive layer by conventional methods is not easy. In this work, a prediction method for the fatigue crack length in the adhesive layer of tubular single-lap adhesive joints was developed by the piezoelectric method. In order to obtain the relationship between the fatigue crack length and the piezoelectric signal, finite element analysis was conducted and verified by experiments. The damage of the adhesive joints was monitored by the piezoelectric method during torsional fatigue tests on tubular single-lap adhesive joints. Using the damage monitoring signals and the relationship between the fatigue crack length and the piezoelectric signal, a method for predicting fatigue crack growth in the adhesive layer of tubular single-lap adhesive joints was developed.

Torsional fatigue characteristics of aluminum-composite co-cured shafts with axial compressive preload

Power transmission shafts such as driving shafts or automotive propeller shafts should transmit static and dynamic torques with vibrational stability. Hybrid shafts made of unidirectional fiber-reinforced composite and metal have high fundamental bending natural frequency as well as high torque transmission capability: composite increases the fundamental bending natural frequency due to its high specific stiffness and metal such as aluminum or steel transmits the required torque. However, fabrication-induced thermal residual stresses due to the coefficient difference of thermal expansion of the composite and the metal are developed during manufacturing hybrid shafts so that the high residual stresses decrease fatigue resistance of the hybrid shafts, especially at low operating temperatures. In this paper, the torsional fatigue characteristics of aluminum–composite co-cure joined shafts with axial compressive preload were investigated. To change the thermal residual stresses, an axial compressive preload was given to the aluminum tube by a compressive jig during the co-cure bonding operation. In order to determine the thermal residual stresses with respect to the preload and temperature difference, stress analyses were performed by simple equations from mechanics of materials and finite element method. Then the static torque capacities and fatigue strengths of the hybrid shafts from static and fatigue torsional tests were correlated with the calculated thermal residual stresses. The fatigue strength of the hybrid shaft was much improved by the axial compressive preload, exceeding that of a pure aluminum shaft. Also, the degradation of the fatigue resistance of the hybrid shaft at subzero operating temperature was overcome by the axial compressive preload.

Temperature effects on the torsional fatigue characteristics of adhesively bonded tubular single-lap joints

Rubber-toughened structural epoxy adhesives are widely used for bonding structures because of their high toughness characteristic. However, the mechanical properties of structural epoxy adhesives degrade much at high environmental temperature, which affects the performance of the adhesive joints severely. Therefore, in this study, the temperature effects on the static and fatigue characteristics of adhesively-bonded tubular single-lap joints were measured. Then the relations between the parameters of S-N curves and the static strength of adhesively-bonded tubular singlelap joints were obtained, from which a fatigue life prediction model for the adhesively-bonded tubular single-lap joints at elevated temperatures was developed.