Finite Element Analysis to Investigate Friction Behavior and Effects of Process Variables on the Residual Stress Distribution and Pulling Force Progression for the Split Sleeve Cold Expansion Process

Md. Saddam Hossen

doi:10.7273/000005010

The primary objective of this study was to investigate the effects of some of the fundamental properties, i.e., frictional behavior and different process variables, on the Split-sleeve Cold Expansion (SSCE) process. This study mainly focused on numerical analysis with subsequent experimental validation. SSCE process is an effective and widely used technique in the aerospace industry by improving the fatigue life of alloyed (especially Al alloy) aerospace structures with fastener holes. This SSCE process induces beneficial compressive residual stress surrounding the hole, reducing the stress concentration factor and allowing retardation against crack initiation and propagation at the critical zones near the hole edges. Consequently, this delay in crack initiation and propagation will improve the structures' fatigue life. This study can provide a comprehensive understanding of the effects of fundamental factors, such as friction properties, on the SSCE process. A friction model was developed to formulate contact pressure-dependent variable friction coefficients through simplified mandrel pulling operations results. The findings of this study were implemented in the numerical analysis, and the outcomes from both experimental and numerical studies matched well. Previous literature regarding the standard SSCE process undermined the importance of the frictional behavior of the SSCE process, conducting numerical simulations considering the process as either frictionless or maintaining a constant friction coefficient as a contact interaction property. From that perspective, the outcomes of this friction modeling study exhibited that incorporating the fundamental frictional properties will make the SSCE process analysis more robust and comprehensive. An in-house capability was developed to conduct both experimental and numerical analysis to enhance the understanding of the Split-sleeve Cold Expansion (SSCE) process. The contact pressure-dependent variable friction coefficients were utilized in the SSCE simulations. Pulling force evolution was considered the measuring parameter for comparing the experimental and numerical findings. The pulling force trend matched well for both experimental and FE results, rendering the effectiveness and accuracy of the FE modeling. In addition, a comprehensive numerical analysis was conducted to understand the effects of different manufacturing and process variables, i.e., geometric variation of the holes, angular pulling of the mandrel, and the friction coefficient in SSCE analysis. In the final stage of this study, a predictive analysis was performed on the bush insert cold expansion process. This process is a promising technique for the effective repair of corroded holes. The primary focus of this investigation was to examine the compressive residual stress distribution around the hole, including the bush insert. The impact of the bush insert wall thickness was also critical for this investigation. The analysis also included calculating the final size of the hole after expansion for different bush wall thicknesses.

Finite Element Analysis to Investigate Friction Behavior and Effects of Process Variables on the Residual Stress Distribution and Pulling Force Progression for the Split Sleeve Cold Expansion Process

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