Optimizing the structural finite element method based process to evaluate the sustainable human accidental sliding load of automobile airvent knob design made with thermoplastic PC - ABS

The research work elaborated the structural integrity of airvent by skipping the assembly level snap fit analysis of knob to reduce computational complexity of air vent knob sliding test post stopper. During assembly, the strain based mechanical breakage prediction of airvent sliding knob snaps is investigated in prestressed condition. The function of airvent knob is to rotate the vertical fins to divert the air flow as per comfort in intended direction identified by passenger. The airvent knob slides on central horizontal fin on a grove feature provided. Post sliding the knob to extreme end it gets stopped at end of grove feature of H fin. The sliding knob is assembled by compressing it against a silicon rubber elastomer inlay. The compression of inlay produces a constant tension on the knob snaps. The knob assembly has to sustain a passenger accidental and abrupt sliding loads while in stopped condition. The knob snaps in this press fit condition are the weak link in the accidental load transfer path and are the first to mechanically break. A load greater than 100 N need to be sustained by snaps to take care of these accidental loads exerted by passengers. The research work proposes a FEM based analysis approach to capture the mechanical breakage by skipping the assembly level insertion and silicon rubber inlay compression, which produces tension in snaps, as the prerequisite simulation is computationally expensive and complex due to polymer plasticity and elastomer hyper-elasticity generated by material nonlinear stress strain graphs and moving frictional contacts between parts. If the accidental load case without considering pre-tension on snaps is simulated, the load causing mechanical failure in the FEM based analysis is not correlating with physically field tested force values on samples. The strain at break for polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) material, used for knobs, is 100%. By considering the lower strain criteria of 5%, the analysis with physical testing in terms of load causing mechanical breakage of snaps is co-related. To validate the established breakage strain criteria of 5%, another air vent design with same material of knob is considered for study. A good correlation is established between FEM approach and physical testing based on load casing mechanical breakage of knob snaps. It is concluded from the outcomes of the FEM based analysis that the 5% strain as breakage criteria set for capturing knob strength for post stopper accidental load is efficient. The same process set can be utilized for developing more efficient FEA models.