Bridging Design and Standardization: Structural Analysis and Optimization of Direct Screw Fastening in Automotive Thermoplastic Panels

Sammanfattning

The automotive industry aggressively pushes for lightweighting. This accelerates the transition toward directscrew fastening into thermoplastic components. The primary goal aims to eliminate heavy, costly metal compressionlimiters. Applying legacy metallic fastening standards to viscoelastic polymers introduces a criticalengineering conflict. High assembly torques risk immediate localized yielding under the screw head. Overly conservativetorques jeopardize long-term joint stability. This thesis investigates and optimizes the structural limitsof insert-free thermoplastic joints within automotive A-pillar and IC-ramp assemblies developed in collaborationwith Volvo Cars.A fully integrated engineering workflow challenges existing conservative torque practices. The methodologysynthesizes physical friction characterization, an evolutionary optimization framework, high-fidelity nonlinearfinite element analysis (FEM), and destructive physical validation. The FEM incorporates 1000-hour viscoelasticcreep. Feeding experimental friction data directly into the computational loop significantly enhances thepredictive accuracy of the long-term structural models.A unifying structural principle emerges across the computational optimization, virtual simulations, and physicaltesting. Contact geometry dictates joint survivability entirely more than bulk material stiffness. Captive washersfundamentally transform the mechanical load path. They reduce localized contact pressure and drasticallyincrease ultimate torque capacity. This geometric optimization allows all evaluated thermoplastic joints tosafely withstand the strict 10 Nm Volvo Cars internal standard without requiring metal inserts. Transitioningfrom elongated oval clearance holes to minimized, circular geometries proves critical. This maximizes continuousbearing area and actively prevents macroscopic deformation.Significant variations in long-term durability exist across the tested material matrix. Rigid amorphous blends(PC-ABS) demonstrate excellent structural stability at ambient temperatures. They exhibit severe clampload decay under elevated thermal conditions (60◦C). Glass-fibre reinforced matrices (PP-GF) provide superiorlong-term preload retention. The internal glass fibers mechanically arrest viscoelastic flow. Unreinforcedpolypropylene (PP) exhibits critical sensitivity to massive creep and structural collapse across all configurations.The algorithmic DOE confirms that standard M5 fasteners lack sufficient bearing area for structural interiortrim. This establishes M6 hardware as the absolute necessary baseline.This research directly challenges internal company standards, exposing a broader disciplinary gap betweencomplex polymer mechanics and rigid mechanical standardization. The absence of a shared technical languageacross engineering, standards, and production functions represents a systemic barrier, not unique to this case,that prevents evidence-based design rules from reaching the factory floor. By translating viscoelastic behaviourand algorithmic optimization into actionable design guidance, this work demonstrates how silo-breaking, crossfunctionalframeworks can bridge that gap, offering a replicable model for insert-free thermoplastic fasteningbeyond the automotive context studied here.