EFFECTS OF STRAINING RATE ON THE MECHANICAL BEHAVIOR OF ADHESIVE BONDS UNDER SHEAR DEFORMATION Herzl Chai Department of Solid Mechanics, Materials and Systems Faculty of Engineering, Tel-Aviv University, Israel Martin Y.M. Chiang and Carl R. Schultheisz Polymers Division, National Institute of Standards and Technology Gaithersburg, MD 20889 INTRODUCTION The mechanical behavior of adhesive bonds is of interest in a variety of industrial and technological applications, including traditional adhesive bonding, laminated structures and composite materials. The main approach for characterizing the mechanical performance of the bond is based on the bulk form of the material. However, such tests may lead to an early specimen failure due to the presence of high stress concentration. Also, monolithic material testing fails to produce the unique geometrical constraints imposed on the adhesive by relatively rigid adherends; such interaction may greatly alter the performance of the bond as compared to the bulk form, and lead to a considerable deformation in the adhesive prior to failure, particularly for very thin bonds [1]. Gsell et al. [2], in their testing of bulk polymers in shear, were able to eliminate some of these difficulties by localizing the failure with the introduction of a thickness gradient along the direction of maximum shear stress. This work extends a previous effort for characterizing the mechanical behavior of adhesive bonds into the effect of strain rate [3-5], which plays an important role on the long-term performance of the joint, particularly for polymeric adhesives. In this work, the Napkin Ring Test specimen [6] is used to obtain the stress-strain response of adhesive bonds undergoing simple shear. This specimen has a number of advantages over other forms of adhesive bond tests, including that the state of stress in the joint is nearly pure shear and the variation of shear stress is minimal, irrespective of the deformation level. Also, the deformation in the bond is localized, allowing for very large straining prior to catastrophic fracture. APPARATUS AND TESTING The test specimens were based on the napkin ring configuration; details of fabrication and testing are similar to those described in Ref. 1. As shown in Fig. 1, the specimens are made of two cylindrical rods with edges machined to a ring shape and joined end to end by a thin layer of adhesive. The ring is sufficiently narrow so shear stress variation in the radial direction is small. The adherends were made of 5086 aluminum alloys while the adhesive was a toughened epoxy adhesive (BP-907) 1 having a curing temperature of 180 o C. Prior to bonding, the adherend surfaces were cleaned and etched in accordance with the FPL etching procedure [6]. The adhesive thickness was controlled by placing shims over part of the bondline. However, despite great care and in the machining and processing of the test samples, some thickness variations along the bondline occurred. Pending a resolution of this technical aspect, the present work is limited to relatively thick bonds. Torsion tests were 1 Certain commercial materials and equipment are identified in this paper in order to specify adequately the experimental procedure. In no case does such identification imply recommendation or endorsement by the National Institute of Standards and Technology (NlST) nor does it imply that they are necessarily the best available for the purpose.