Book Description

Bolt mechanics has been developed over thousands of years. Today, bolted joints researchers and designers are still facing challenges in many areas especially when a joint works at elevated temperatures. Temperature changes of a bolted joint in such systems induce catastrophic cyclic axial and bending strain to the fasteners and gasket degradation. Now, with many studies, the axial strain issue can be addressed by adjusting the clamping load to maintain the stiffness of the joint or by changing the jointed materials to have appropriate coefficient of thermal expansions (CTE) to have a suitable clamping load. Dealing with the bending strain caused by cyclic thermal changes, however, is more complicated. It is still a big challenge for engine designers and fastener researchers. During heat up and cool down, most bolted joints have dissimilar thermal expansion rate of jointed components or different temperature profiles which will cause bolted joint expansion or contraction motions in axial and transverse directions. The amplitude of the bolt transverse motion is called bending motion in this dissertation which is associated with the relative motions between jointed components and bolt head. Friction force acting on the bolt head bearing area causes the bolt bending motion resulting in bolt bending fatigue. In this research, mathematical modeling and FEA methods have been employed to specify the design space based on the high temperature bolted joint operating conditions. The design target can be defined based on design control factors including demission, materials and operating temperature of the bolted joint. With this developed method, a specific high temperature bolted joint has been designed with a frictional target of the bolted joint. To meet this target, a series of tribological bench tests were conducted to evaluate washer with different coatings which have potential good high temperature characteristics under different temperatures. A washer with CrN46 multi-layer coating was found to close to the design target which was effective under high pressure and high temperature with a relatively low coefficient of friction. The tested coating has been analyzed after test and found that, with the aid of the formation of W2C, tungsten could allow the DLC coating to perform well under high temperature environment. This verified J. Luthin and C. Linsmeier finding that the W2C formation starts at 500°C. Delamination is the main wear mechanism for the CrN46 coating, but the wear depth met the design requirement which was 1500 reciprocating cycles. A sensitivity study using Taguchi method was conducted and showed that the dry coefficient of friction depends on many factors, including temperature, materials, surface finishing and sliding speed. The material is the most significant design control factor, which affects the friction coefficient. The CrN46 coasting sliding against CrN46 coasting was found to provide even lower coefficient of friction which was used in the design. Then the design of the joint with the coated washer was validated by engine dyno data which was collected by a novel mutli-camera 3D-DIC system developed in this research. This might be the first time 3D-DIC system has been successfully applied in an engine dyno to measure the displacement of the exhaust manifold and fasteners exposed to high temperatures, mechanical and dynamic loads. This application illustrated that a 3D-DIC system could be applied in a severe thermal and vibration environment with a proper system design. The data collected by the 3D-DIC system was compared to traditional LVDT data. It showed that 3D-DIC was reliable and could provide whole field contour data. The measured data has been used to validate the CAE analysis and improve the computational methodology. Measuring the strain field was still a challenge for this 3D-DIC system because of the high thermal strain generated at high temperature. A high-resolution speckle pattern was needed as well. To systematically address the high temperature bolt joint with thermal deformation, a robust CAE method for the MLS gasket design has been developed to overcome the current design method which was not an effective method to evaluate MLS gasket sealing performance with consideration of gasket material thermal degradation. A new method was developed in this research. In the new method, a thermal degradation factor was used to correct the gasket sealing pressure calculated by the current CAE method. With the new method, it was found that a gasket with high thermal degradation resistance showed better sealing performance than a gasket with low thermal degradation resistance, while the current CAE method could not have predicted any difference. The results from the new method matched with the dyno real engine test observation very well and can be used for high temperature bolted joint designs. Overall, this research found a metrology to predefine a friction requirement and a robust "engineed refiles"/"engineered coated washer" to provide a robust hig temperature bolted joint design. Four main sections were focused on in this research. The first was developing a systematic method which combined virtual analyses with bench tests to develop a low friction coated washer to solve the bolt bending fatigue issue due to the engine cyclic thermal changes. The second was to design tests to validate the performance of the low friction coated washer to meet the predefined frictional requirements. The third was to improve a process for virtual analysis for the high temperature bolted joint with more accurate temperature simulation and frictional interface model. The fourth was to develop a method to predict the gasket sealing behavior at the elevated temperature when considering the creep and relaxation of the bolted joint. The objective of this research has been met by the complete of the below tasks. 1. Developed a systematic method for designing a high temperature bolted joint for bolt bending fatigue. 2. A coating has been developed and evaluated by a series of A bench tests to solve the bolt bending fatigue issue due to the engine cyclic thermal changes. 3. A unique multi-camera 3D-DIC system has been successfully designed and applied in an engine dyno to measure the displacement of the exhaust manifold and fasteners exposed to high temperatures, mechanical and dynamic loads. 4. Virtual analysis method for the high temperature bolted joint has been improved with more accurate temperature simulation and frictional interface model. 5. Developed a method to predict the gasket sealing behavior at the elevated temperature with consideration of the creep and relaxation of the bolted joint. 6. Design and method have been validated by engine dyno tests.