Considering total vehicle cost, adhesives could let automotive OEMs economically join panels made from different materials while simultaneously acting as seals. Adhesives also lower the inevitable stress raisers common with conventional, discrete-point joining techniques. However, to successfully compete with other primary structural joining methods, adhesives must let automotive assemblies pass the crash scenario. Automakers need the reassurance that during a crash, the various energy-absorbing parts of the vehicle will remain essentially intact and perform their function. Recent advances in adhesive chemistries are opening new possibilities for automotive structural joints. One structural adhesive making headway is a crash-stable version called Betamate. Betamate adhesives are based on epoxy. Toughener polymers are added to the epoxy and finely disperse throughout its matrix. The toughener-'islets' range from a few hundred to a few thousand microns in diameter. They boost the dynamic fracturing behavior of the adhesive such that any cracking in the material takes place in a stable, controlled manner. Instead of concentrating all the energy at any potential crack, the toughener distributes the energy by a process known as cavitation - a continuous adhesive deactivation at each individual toughener site. Each deactivating step absorbs energy, generating localized dilationary (triaxial) stress within the adhesive layer. This complex stress state leads to many localized fractures. The little cracks spread throughout the material and dissipate energy in a controlled manner. It is possible to dramatically influence how an adhesive responds in the presence of a crack by fine-tuning adhesive formulations. Betamate structural adhesives have toughening technology in the form of a complex internal arrangement of polymers. The polymer arrangement is specifically put in place to make the adhesive resist unstable crack growth. The crack resistance is especially important under dynamic conditions, typical of a crash scenario. To help automotive designers understand the advantages that adhesives offer, Dow Automotive computer simulates what combination of stresses or loads acting on the joint will lead to its failure. Detail response of the complex and the nonlinear toughening behavior has been modeled using the explicit finite-element program from Abaqus Inc., Providence, R.I. The computational model predicts localized failure as well as the ever-changing state of the adhesive as the toughener polymer is extended and exhausted. Simulations agree well with experiments and give significant engineering insight into the details of the toughening mechanism and will help chemists improve adhesive formulations. Betamate adhesives bond well to the slightly oily surfaces typical of stamped steel and aluminum parts. Their chemistry can be tailored giving vehicle manufacturers a number of processing options. They can be jet-streamed or extruded onto substrates and do not wash off during subsequent gross dipping operations of the whole automobile body. Once cured, the adhesives stand up to salt-spray attack and other harsh environments. At low temperatures Betamate adhesives retain a significant proportion of their toughness.


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    Title :

    The glued-together car body


    Additional title:

    Neuer Klebstoff für Fahrzeugbaukörper


    Contributors:

    Published in:

    Machine Design ; 77 , 7 ; 90-95


    Publication date :

    2005


    Size :

    3 Seiten, 4 Bilder



    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




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