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European Coatings Journal
12/2006
46
Strength in unity
Bonding increases vehicle bodies' stiffness withoutweight penalties.High torsional and dynamic stiffness is highly desirable inautomotive design, but so too is weight reduction.Crash-durable adhesives are vital for lightweight automotivebody construction, and also significantly increase bodystiffening. It is shown that high modulus glass bondingadhesives can further increase stiffness, while adhesivebonding of body panels also introduces new benefits.Detlef Symietz** Contact: Dr. Detlef Symietz, Dow Automotive, 8807Freienbach, Switzerland. [email protected] requirements in automobile construction are manyand often contradictory: costs and weight should both bereduced, while at the same time improved crash strength,better durability, lower environmental impact, greaterstructural stiffness and improved acoustic damping are allsought.Over time, the mass of vehicle bodies has crept up - arepresentative automobile from the middle class weighs60% more today than three decades ago. In the moreluxurious market segment the percentage change is less,but the upward trend remains. This increase leads toreduced fuel economy, even though the European motorindustry has already agreed to meet the stipulations of theKyoto Protocol (fleet emissions to lie below an average of140 g/km carbon dioxide).This environmental goal is mainly being sought throughimproved engine design and management, althoughreduction in vehicle mass forms a second important goal forthe industry. To achieve this, designers are considering theincreased usage of high-strength steels, lightweight metals,metal foams and plastics. These increased demands onperformance must not come at the expense of vehiclesafety, durability and comfort. Newer materials, often usedin combination with one another, will require alternativesolutions to minimise galvanic corrosion.Crash-durable adhesives have much to offer in this newdesign realm. But structural bonding of the body helps toachieve even more than greater rigidity. The direct glazingtechnique also contributes significantly to the body stiffness,with potential for further stiffening.
Directly bonded glazing increases stiffnessDirect glazing with polyurethane adhesives has been usedfor more than three decades. It is a true multifunctionalapproach 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 easeof application and faster safe-drive-off in the case ofwindshield replacement etc. This paper focuses on the bodystiffening effect of direct glazing and the potential for furtherincreasing it.Typically, direct glazing adhesives are moisture-curedpolyurethanes. Their tensile shear strength is about 5 MPa,while the tensile strength of a free film is around 5-10 MPawith high elongation at break. The shear modulus(measured with a torsion pendulum) over the first twodecades of these standard adhesives has been around 3MPa.Today the figures are usually measured in a manner similar
to DIN 54 451, but using a higher bond line thickness andevaluating the stress-strain curve at 10% strain as the socalled G10% 'modulus'. These figures are no longer aphysically real modulus. The figures for the 'normal'adhesive modulus achieved by this test method are close to1 MPa.By integrating the rigidity of the glass into the body, thestiffness of the body has been significantly increased. Thestiffening gain is around 20-30 % of the body stiffnessmeasured without the bonded glasses (in the case of thewindshield and rear window). The degree of stiffening isdependent on the car design.
High modulus adhesives further enhance stiffnessIn the 1990s, so called 'high modulus' direct glazingadhesives entered the market. The G10% values were aboutthree times higher, the torsional rigidity of the bodyincreased again and this was achieved without anyadditional weight. This effect can be on the order of a further10-20%. Figure 1 shows a typical relationship betweenstiffening and G10% 'modulus'.With increasing modulus, the stiffening effect tendsasymptotically towards a limit. The absolute figure for such alimit is specific to each car design, that is, a more rigidlydesigned 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 fortwo 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 designingmore rigid bodies (without the bonded glass) and newresults on the strength of glass. The higher initial bodystiffness comes from new materials, sophisticated designand, for example, by using structural adhesives in the bodyitself. This means the modulus limits for stiffening are shiftedto higher values.
Risk of glass fracture has been re-examinedThe glass fracture risk has been analysed by measuring thestrength 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 determinethese stresses. Different profiles were used to induce highbody twist and displacements under different frequencies.Stress and strain at various locations on the windshield weremeasured. Table 1 shows windshield results from testing acar of the upper middle class category; the table alsoincludes mean edge strength and mean biaxial strengthresults, reported by M. Khaleel et. al. [1]. Both mean andminimum strengths approach their limiting values after about20,000 km of driving.The measurements were made at six locations on thewindshield while the car was subjected to various roadsimulations. For an adhesive having a G10% close to 2 MPaand for an experimental ultra high modulus adhesive with G10% around 6 MPa, the measured stress figures have beenfound to be between 5 MPa (lowest stress with lowermodulus adhesive) and 25 MPa (maximum stress withexperimental high modulus adhesive under worst roadsimulation conditions). Figure 2 shows the results obtainedwith the experimental ultra-high modulus adhesive, for theworst road simulation profile and the glass strength figures.The results seem to indicate a clear potential for furtherweight-neutral body stiffening without causing a significantlyhigher risk of glass fracture. Some more testing anddevelopment work for this approach is justified.
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Welding can usefully be combined with adhesivesEpoxy based adhesives dominate the crash-durableapplication field. The adhesives are of the one-part, heatcuring type. Three classes of these adhesives can bedistinguished:- Basic structural adhesives- Semi-crash-durable adhesives- Crash-durable adhesivesTable 2 shows some typical figures for performance,especially the impact peel strength, at various temperatures.But the Young's modulus is also important, because themain function is stiffening and crash resistance. Otheroutstanding results such as service durability etc. will bediscussed later.Since heat-curing epoxy systems do not produce immediatebond stability, hybrid methods of joining are often used.During this procedure, the adherends are bonded bothadhesively and by using traditional discrete techniques,often spot-welding. Such structures are stiffer, offer muchbetter energy-management options in the event of a crashand are able to retain their structural stiffness better duringtheir lifetime [2].These newer adhesives add improved strength, improvedstiffness and are able to resist any cracking in the material ina stable manner, far above the yield points of the adherendsinvolved. All of this is possible at high strain rates and lowertemperatures (with only a minor reduction in performance).Figure 3 shows a comparison of the torsional stiffnesschange for a body with the introduction of adhesive inaddition to the usual spot-welding. Even where the numberof spot-welds is reduced to 50% of its former value, thechange in torsional stiffness is minimal. It should be clearfrom this illustration that a significant cost saving potentialexists through the reduction of spot-welds in the body.
Adhesives also improve crash performanceIn Figure 4 results are shown which were obtained with sideimpact protection profiles. The photo shows the originalprofile before being impacted, the currently used,spot-welded only, profile after impact testing and thecorresponding less deformed sample, bonded only with acrash durable adhesive without any spot-welds. The verygood impact performance of the adhesive is achievedthrough a patented toughening process known asSynergistic Rubber Toughening Technology.Application of the adhesive is achieved by the extrusion of abead directly on the adherend substrate - in certain cases itis even possible to spray the adhesive onto the surface at amaximum speed of 500mm/s. This process allowswell-controlled application, which helps to ensure goodquality control. Experience gained from this procedure inseries production of real bodies bodes very well for thefuture of this technology.
High strength steels demand adhesive bondingModern high strength and ultra-high strength steels arebeing increasingly considered as potential BIW (body inwhite) materials, allowing a significant reduction in the sheetthickness ans weight. However, the drop-off in bendingstiffness, which is proportional to the cube of the thickness,often leads to an unacceptable loss of stiffness. This can becompensated for by the continuous line joining afforded bybonding. These grades of steel also show significantheat-affected zones after welding, a problem which iscompletely sidestepped by the bonding process. In Table 3,different joining methods are compared to bondingtechnology.
Durability of bond strength is increasedFigure 5 illustrates some considerations for a high strengthsteel in relation to durability. In all three cases (spot-welding,adhesive bonding and spot-weld bonding) the same varyingload has been applied. The number of cycles to failure forthe different joining concepts is self-explanatory.Practical experience from the field shows, for example, thatthe body stiffness after some years' driving is much morestable for spot-weld bonded cars than for simplespot-welded cars. Figure 6 shows these results.Joining through adhesive bonding, which has been anaccepted technique for direct glazing of windscreens andindeed has solved critical safety issues, is finally beingaccepted in the body structure. Bonded joints offerimprovements over current techniques with respect tostiffness, energy management and improvement of thenatural frequencies of the structure.The initial joint strength, as required during production, hasbeen resolved through hybrid methods. Newer techniquesfor simulation also make the choice of reducing the numberof spot-welds much easier, which then promises asignificant cost saving, while at the same time improvingvehicle driveability.
Structural bonding will offer further gains in futureAdhesive bonding for vehicle manufacture delivers excellentperformance and durability of the cars. Direct glazingprovides more benefits than simply holding the windshield inthe frame in crash situations; it assists significantly inproducing a stiffer car body without any more weight.It appears that this stiffening effect may have a furtherpotential by using ultra-high-modulus adhesives. The glassfracture risk has been analysed and seems to be undercontrol, though more development work is justified.Crash-durable structural bonding is a rather newer fasteningapproach, but also with outstanding results. This technology,preferably used as hybrid bonding, is a multifunctionalapproach, making the body stiffer, quieter, safer and moredurable.
REFERENCES[1] M. A. Khaleel et al, Measurement of biaxial strength ofnew vs. used windshields, SAE 2000-01-2721[2] H. Flegel, The future of bonding as a joining technique,AutoTechnology, 2, 2002, p.64-67
Results at a glance- There are conflicting demands in automotive design for areduction in weight combined with increased body stiffness,acoustic damping and durability.- The adhesive bonding of automotive glass to the bodyworkis already known to significantly increase stiffness, with noincrease in weight and smoother airflow over the body.- It is shown that this stiffness can be further improved byusing high modulus glass bonding adhesives, and by using'hybrid' adhesive and welding techniques to bond bodyparts.- Adhesive bonding of body panels has the potential to allowfurther weight reductions while improving crash impactresistance.
The author:> Dr. Detlef Symietz is an R&D scientist at Dow Automotivein Freienbach, Switzerland. He is involved in the field ofadhesive bonding and is the author of a number of scientificpapers.
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Figure 1: Body stiffening versus G10% 'modulus' of adhesives.
Figure 2: Windshield stress results (bonded with ultra-high modulus adhesive) andglass strength figures.
Figure 3: Torsional stiffness comparison between spot-welding, hybrid bondingapproach and a hybrid approach in which welds have been reduced by 50%.
Figure 4: Side impact profiles before (rear) and after impact testing, fastened byspot-welding (front) and adhesive bonding (middle).
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Figure 5: Durability of different joining methods when using high strength steel.
Figure 6: Torsional body stiffness over operational lifetime for car bodies assembledby different methods .
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