Comparison of the Mechanical Behaviour Between Stiff and Flexible Adhesive Joints for the Automotive Industry

Abstract

Adhesive bonding is increasingly being used in structural applications such as in automotive joints. The theoretical analyses and experimental data are generally for rigid and strong epoxy adhesives. Elastomeric adhesives such as polyurethanes are used in structural applications such as windshield bonding because they present important advantages in terms of damping, impact, fatigue, and safety which are critical factors in the automotive industry. However, there are other structural applications in the main body where polyurethanes may also be used. The main objective of the present project is to compare the behaviour of structural joints used in the automotive industry, such as single lap joints and T-joints made of rigid adhesives, and those made of elastic adhesives in terms of stiffness, strength, impact, damping, and fatigue. The elastomeric adhesive selected was a polyurethane from Sika (Sikaflex® 256) and the structural adhesive selected was an epoxy from Huntsman (Araldite® AV138/HV998). The shear strength of the polyurethane is approximately four times lower than that of the epoxy. However, the polyurethane shear failure strain is 330%, whereas that of the epoxy is only 6%. The benefits of using elastomeric adhesives in structural adhesive joints used in the automotive industry are described, especially in terms of ductility, impact, and fatigue.

Keywords:

• Damping
• Epoxy adhesive
• Fatigue
• Impact
• Polyurethane adhesive
• Single lap joint
• Steel
• T-joint

ACKNOWLEDGMENTS

The help of Mr. Joaquim Fonseca for the design of the T jig and of Prof. José Dias Rodrigues for the damping tests is gratefully acknowledged. Sika Portugal is gratefully acknowledged for supplying free polyurethane samples.

Notes

One of a Collection of papers honoring David A. Dillard, the recepient in February 2010 of The Adhesion Society Award for Excellence in Adhesion Science, Sponsored by 3M.