Finite Element Analyses of structural glazing elements in civil engineering and adhesively bonded windshields in automotive engineering - a comparison

21. Internationales Klebtechnik-Symposium: Verbindungstechnik im Bauwesen

14. Januar 2003

Dipl.-Ing. Thorsten Böger

Dr.-Ing. Gerhard Schmöller

Abstract

Due to numerous technologic and economic advantages and constant advancement adhesive and sealing technologies are used in increasing extent both in mechanical engineering and civil engineering. Especially structural adhesive joints with multi functionality and high potential for the future are targeted.

In this presentation the possibilities of Finite Element Analyses are dicussed from the mechanical engineer's point of view, taking as an example structural glazing facades in civil engineering and adhesively bonded windshields (direct glazing) in automotive industries.

In civil engineering structural glazing has been a success in Europe for a couple of years. Due to their high durability and a wide range of adhesive properties only silicones are approved, which usually are used as classical sealant materials. Dimensioning is done by analytical formulae and safety factors based on experience of many years. Standardized test methods and test items similar to real components help to characterize mechanical properties of the adhesive systems and their durability under climatic influences. The facade elements are subject to construction inspections.

In automotive industries, direct glazing is a typical application of adhesive technology. Windshields and rear windows are adhesively bonded to the car body. Apart from an increased torsional stiffness of the structure combined sealing functions and acoustic damping effects are further advantages. For experimental characterization of adhesive and compound properties standardized methods and part modules are used likewise. Today, FEA is used as standard tool for technical design layout (digital automobile). Simulating various load cases leads to constructive upgrades and helps to develop direct glazing concepts with features tailored to technical demands.

In general, structural adhesive joints in automotive applications are made of epoxy systems with a high shear modulus. The adhesive layer thickness is about some tenths of a millimetre. Due to low flexibility of the bond, these adhesives cannot be used for direct glazing because of critical stress concentrations within the glass panels and the risk of fracture in case of higher distortion (parking one tire up the curbstone). This problem can be resolved by a higher flexibility of the joint (lower shear modulus of the adhesive) and thicker adhesive layers (allows for relative motion through the joint). To achieve this, PUR adhesives (shear modulus approx. 3 MPa) with layer thickness of about 5 mm are used. Thus, by customising the flexibility of the joint the quasi rigid glass panel contributes to global stiffness of the car body (semi-structural adhesive joint). These aspects also play an important role with structural sealant glazing (official denotation) with regard to an even lower shear modulus of silicones and a layer thickness of approx. 7 mm.

The following paragraphs give an overview about relevant aspects for both mentioned applications and methods for FE simulations are described.