A fundamental investigation of retention phenomena in snap-fit features

This study explores issues related to modeling and analysis of insertion and retention phenomena in snap-fit features. An analytical methodology for developing detailed models of snap-fits is proposed and formulated. The strategy involves idealizing the feature as a rigid body supported on a flexible structure. A set of equations that comprehensively describes the system in its deformed configuration is formulated. The equation system is iteratively solved for several such configurations to obtain a model of insertion and retention processes for snap-fits. It has been implemented for the cantilever hook in this research. The model shows good agreement with experimental results for most snap-fit geometries. However, it still needs to be refined to provide better predictions for high-angle retention. Suggestions for possible improvements and future research directions are provided.; An experimental study of high-angle retention has been undertaken with a view to establishing a fundamental understanding of governing phenomena. Force-displacement curves are correlated to still frames from high speed video of the experiment, to identify the location of peaks in the insertion and retention force curves. Samples were fabricated from aluminum and steel to eliminate complexities due to material behavior. The maximum retention force and the shape of the insertion and retention force curves was found to be very sensitive to catch surface conditions. Large drops in insertion and retention force values were observed between successive tests on the same sample. Retention force curves exhibit evidence of stick-slip contact. Experiments were also conducted on injection molded ABS samples. Once again, large differences in maximum insertion and retention force values were caused by change in contact edge conditions.; Initial steps for the development of predictive capability of time dependent effects in plastic parts were taken. A test material (polycarbonate) was characterized using a constitutive model capable of modeling long-term non-linear viscoelastic behavior. The model parameters were characterized and a master stress relaxation curve for the material was developed using time-temperature and time-strain superposition principles. The constitutive relationship was converted into an incremental form suitable for implementation in a commercial finite element analysis package.