Design of Mechanically Fastened (Bolted) Joints in Automotive Composites 940623

This paper discusses the design issues related to bolted joints in automotive composites. Pin bearing strength and fatigue test results of bolted joints in both random fiber as well as continuous fiber sheet molding compound composites are reviewed. Fatigue failure modes in bolted joints of these composites are presented. Based on these tests, design guidelines for bolted joints in sheet molding compound composites are proposed.

Sheet molding compound (SMC) composites and resin transfer molded (RTM) composites are being considered for applications in automotive chassis components, such as cross members, wheels, and front end structures. Mechanical fasteners will be used to attach these composite components to their supports or other structures. While adhesive joints are preferred in body applications, mechanical fasteners, such as bolts and screws, have some distinct advantages in chassis applications. They can be quickly and repeatedly disassembled for inspections, repairs or replacements. Adhesively bonded joints are more permanent and cannot be disassembled without either damaging or destroying the substrate.

Mechanical fasteners, such as rivets and bolts, are used extensively with aerospace composites. Materials in these fasteners are titanium, aluminum and stainless steels. They are also specially designed and developed to account for the unique nature of the carbon fiber reinforced epoxies, namely their brittleness and relatively low damage tolerance. Automotive fasteners follow the SAE specifications and are typically made of low to medium carbon steels. Due to their low cost, wide availability and standardization, they will continue to be used with both metallic and composite materials. Even if new fasteners are developed to suit the composite joints, it will be difficult to implement their use in repair shops and garages. Unlike the aerospace industry, there is very little control on the servicing of automobiles.

This paper reviews the design parameters that are important in the design of bolted joints in two automotive composites, namely random fiber sheet molding compound (SMC-R) and continuous/random fiber sheet molding compound (SMC-CR). The fatigue data, reported in details in References 1 and 2, were obtained in load-controlled cyclic tests of single lap joints between one of these composites and steel (Fig. 1). SAE fasteners were used to join composite specimens at both ends to 12 mm thick steel plates. The important joint parameters are the edge distance (e), specimen width (w), bolt tightening torque (T) and washer size. The influence of these parameters on fatigue life and failure modes at the bolted joints are summarized.