It's no secret that well-designed composite components can shave weight from cars without giving up an ounce of structural performance. And at the right production volumes, composites can cost less too. Yet composite components have a downside when it comes time to join them to the rest of the vehicle—in part because holes required by mechanical fasteners can cause stress-concentration and fatigue-life problems down the road.
So what's the best way to attach composite components? Four presentations from the Society of Plastics Engineers Automotive Composites Conference, held recently in Troy, MI, examined some of the joining difficulties and solutions associated with composites.
Vanquishing mechanical fasteners
For an object lesson in the potential for adhesive bonding of automotive composites, look no further than Ford Motor Company's 2002 Aston Martin Vanquish V12. "From its extruded aluminum space-frame to its carbon-fiber transmission tunnel and energy-absorbing crash structures, the entire vehicle is adhesively bonded together," reports John Hill of Ford's Research and Advanced Engineering group (Dearborn, MI). For the aluminum components, engineers picked a toughened single-component epoxy. "However, some of the most challenging parts on the vehicle are those made of composites," he says.
In all, the Vanquish V12 has 25 composite components, mostly made through resin transfer molding processes. These include not just the transmission tunnel but also the front-end crash assembly, the strut brace between the front shock towers, the rear assembly, and the body sides. "Several of these composite components were highly loaded structural members that contributed significantly to the vehicle's performance," Hill recalls.
And like many composite parts they weren't easy to join. Hill's presentation outlines the difficulties faced in one key joint, the one between the 60 kg carbon- and glass-fiber-reinforced front crash structure and the vehicle's cast aluminum shock towers. This joint has to withstand both frontal and offset crash loads as well as accommodate build tolerances that later allow the hood and fender to fit.
Joint design helped out on both scores. Hill describes the joint as having a large bond area that helps minimize stresses. The joint also features a tapered groove that has been carefully designed to take up some of the build tolerances and also oriented so that the joint remains mainly in compression under the crash loads.
Testing for strength
Ford Research engineers also went through an exhaustive testing process to identify the best adhesive for the job. After an initial screening, they put three two-component adhesives—two polyurethanes and one methymethacrylate—through a battery of lab tests before settling on one of the polyurethanes. This evaluation process went well beyond the usual shear-strength tests to include tests for stressed corrosion durability and creep as well as a dynamic mechanical thermal analysis (DMTA) and a determination of the shear modulus.
Why so many tests? One reason has to do with shortcomings of the lap shear test, perhaps the most common strength test in the adhesives business. "Other than confirming that an adhesive bonds to a substrate, simple lap shear specimens reveal little about the suitability of an adhesive to a given application," Hill says. And he argues that shear strength is rarely the limiting factor in an application problem because it's usually possible to increase a joint's bond area and lower its stress concentrations.
The other tests, by contrast, provided much more useful information about the real-world performance of the adhesive—particularly under corrosive, high-temperature conditions. The MMA, for example, exhibited the best corrosion performance of the three candidates. But the creep testing and measurement of tensile modulus via the DMTA highlighted this adhesive's sensitivity to elevated temperatures, which sealed the deal for the polyurethane.