Strength in unity - European ... Strength in unity Bonding increases vehicle bodies' stiffness without

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    European Coatings Journal

    12/2006

    46

    Strength in unity

    Bonding increases vehicle bodies' stiffness without weight penalties. High torsional and dynamic stiffness is highly desirable in automotive design, but so too is weight reduction. Crash-durable adhesives are vital for lightweight automotive body construction, and also significantly increase body stiffening. It is shown that high modulus glass bonding adhesives can further increase stiffness, while adhesive bonding of body panels also introduces new benefits. Detlef Symietz* * Contact: Dr. Detlef Symietz, Dow Automotive, 8807 Freienbach, Switzerland. dsymietz@dow.com Design requirements in automobile construction are many and often contradictory: costs and weight should both be reduced, while at the same time improved crash strength, better durability, lower environmental impact, greater structural stiffness and improved acoustic damping are all sought. Over time, the mass of vehicle bodies has crept up - a representative automobile from the middle class weighs 60% more today than three decades ago. In the more luxurious market segment the percentage change is less, but the upward trend remains. This increase leads to reduced fuel economy, even though the European motor industry has already agreed to meet the stipulations of the Kyoto Protocol (fleet emissions to lie below an average of 140 g/km carbon dioxide). This environmental goal is mainly being sought through improved engine design and management, although reduction in vehicle mass forms a second important goal for the industry. To achieve this, designers are considering the increased usage of high-strength steels, lightweight metals, metal foams and plastics. These increased demands on performance must not come at the expense of vehicle safety, durability and comfort. Newer materials, often used in combination with one another, will require alternative solutions to minimise galvanic corrosion. Crash-durable adhesives have much to offer in this new design realm. But structural bonding of the body helps to achieve even more than greater rigidity. The direct glazing technique also contributes significantly to the body stiffness, with potential for further stiffening.

    Directly bonded glazing increases stiffness Direct glazing with polyurethane adhesives has been used for more than three decades. It is a true multifunctional approach and helps to achieve: - Better tightness; - Higher safety (meeting the US norm FMVSS 212); - Lower drag value Cw due to closer gaps; - High manufacturing quality through robotic application; - Increased body stiffness. Today the main aims of development work are to give ease of application and faster safe-drive-off in the case of windshield replacement etc. This paper focuses on the body stiffening effect of direct glazing and the potential for further increasing it. Typically, direct glazing adhesives are moisture-cured polyurethanes. Their tensile shear strength is about 5 MPa, while the tensile strength of a free film is around 5-10 MPa with high elongation at break. The shear modulus (measured with a torsion pendulum) over the first two decades of these standard adhesives has been around 3 MPa. Today the figures are usually measured in a manner similar

    to DIN 54 451, but using a higher bond line thickness and evaluating the stress-strain curve at 10% strain as the so called G10% 'modulus'. These figures are no longer a physically real modulus. The figures for the 'normal' adhesive modulus achieved by this test method are close to 1 MPa. By integrating the rigidity of the glass into the body, the stiffness of the body has been significantly increased. The stiffening gain is around 20-30 % of the body stiffness measured without the bonded glasses (in the case of the windshield and rear window). The degree of stiffening is dependent on the car design.

    High modulus adhesives further enhance stiffness In the 1990s, so called 'high modulus' direct glazing adhesives entered the market. The G10% values were about three times higher, the torsional rigidity of the body increased again and this was achieved without any additional weight. This effect can be on the order of a further 10-20%. Figure 1 shows a typical relationship between stiffening and G10% 'modulus'. With increasing modulus, the stiffening effect tends asymptotically towards a limit. The absolute figure for such a limit is specific to each car design, that is, a more rigidly designed car reaches the limit at higher modulus values. Today, adhesives having a higher modulus have G10% values of about 3-4 MPa. Higher levels are not chosen for two reasons. First, the further stiffening effect is rather small, and a risk of glass fracture is presumed. Now the situation has been changed a little by designing more rigid bodies (without the bonded glass) and new results on the strength of glass. The higher initial body stiffness comes from new materials, sophisticated design and, for example, by using structural adhesives in the body itself. This means the modulus limits for stiffening are shifted to higher values.

    Risk of glass fracture has been re-examined The glass fracture risk has been analysed by measuring the strength of glasses (designed as typical for a windshield) and the real stresses on the windshield installed in a car. The car was subjected to simulated road inputs to determine these stresses. Different profiles were used to induce high body twist and displacements under different frequencies. Stress and strain at various locations on the windshield were measured. Table 1 shows windshield results from testing a car of the upper middle class category; the table also includes mean edge strength and mean biaxial strength results, reported by M. Khaleel et. al. [1]. Both mean and minimum strengths approach their limiting values after about 20,000 km of driving. The measurements were made at six locations on the windshield while the car was subjected to various road simulations. For an adhesive having a G10% close to 2 MPa and for an experimental ultra high modulus adhesive with G 10% around 6 MPa, the measured stress figures have been found to be between 5 MPa (lowest stress with lower modulus adhesive) and 25 MPa (maximum stress with experimental high modulus adhesive under worst road simulation conditions). Figure 2 shows the results obtained with the experimental ultra-high modulus adhesive, for the worst road simulation profile and the glass strength figures. The results seem to indicate a clear potential for further weight-neutral body stiffening without causing a significantly higher risk of glass fracture. Some more testing and development work for this approach is justified.

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    European Coatings Journal

    12/2006

    46

    Welding can usefully be combined with adhesives Epoxy based adhesives dominate the crash-durable application field. The adhesives are of the one-part, heat curing type. Three classes of these adhesives can be distinguished: - Basic structural adhesives - Semi-crash-durable adhesives - Crash-durable adhesives Table 2 shows some typical figures for performance, especially the impact peel strength, at various temperatures. But the Young's modulus is also important, because the main function is stiffening and crash resistance. Other outstanding results such as service durability etc. will be discussed later. Since heat-curing epoxy systems do not produce immediate bond stability, hybrid methods of joining are often used. During this procedure, the adherends are bonded both adhesively and by using traditional discrete techniques, often spot-welding. Such structures are stiffer, offer much better energy-management options in the event of a crash and are able to retain their structural stiffness better during their lifetime [2]. These newer adhesives add improved strength, improved stiffness and are able to resist any cracking in the material in a stable manner, far above the yield points of the adherends involved. All of this is possible at high strain rates and lower temperatures (with only a minor reduction in performance). Figure 3 shows a comparison of the torsional stiffness change for a body with the introduction of adhesive in addition to the usual spot-welding. Even where the number of spot-welds is reduced to 50% of its former value, the change in torsional stiffness is minimal. It should be clear from this illustration that a significant cost saving potential exists through the reduction of spot-welds in the body.

    Adhesives also improve crash performance In Figure 4 results are shown which were obtained with side impact protection profiles. The photo shows the original profile before being impacted, the currently used, spot-welded only, profile after impact testing and the corresponding less deformed sample, bonded only with a crash durable adhesive without any spot-welds. The very good impact performance of the adhesive is achieved through a patented toughening process known as Synergistic Rubber Toughening Technology. Application of the adhesive is achieved by the extrusion of a bead directly on the adherend substrate - in certain cases it is even possible to spray the adhesive onto the surface at a maximum speed of 500mm/s. This process allows well-controlled application, which helps to ensure good quality control. Experience gained from this procedure in series production of real bodies bodes very well for the future of this technology.

    High strength steels demand adhesive bonding Modern high strength and ultra-high strength steels are being increasingly considered as potential BIW (body in white) materials, allowing