INVESTIGATIONS ON DAMAGE IN A FILLED EPOXY RESIN Nicolas Depoorter 1,2 , Daniel Coutellier 2 , Markus Mužic 1 , Antje Berg-Pollack 3 , Ye Cai 4 and Andre Zimmermann 1 1: Robert Bosch GmbH, FV/PLK5, Postfach 1131, D-71301, Germany; e-mail: Nicolas.Depoorter@de.bosch.com 2: LAMIH, UMR CNRS 8350, Université de Valenciennes et du Hainaut Cambrésis, F-59313 Valenciennes Cedex 9, France; 3: Fraunhofer-Institut für Betriebsfestigkeit, Bartningstraße 47, D-64289 Darmstadt, Germany; 4: School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, USA; ABSTRACT The demand for high-end products is rising. Thus, the ability to predict the deterioration of materials under service conditions, including the evolution of the constitutive equations, becomes a critical task. The aim of this work is to propose a method to study the damage evolution in a filled epoxy resin submitted to low cycle fatigue loading. The TEM analysis performed indicates damage mechanism which corresponds well to the decreasing slope of the stress-strain hysteresis loop observed for the experiments. Besides, the suggested damage model appears to be suitable for the simulation of strain-controlled cyclic tests and fits the damage evolution of the filled epoxy resin well. INTRODUCTION New technologies require unusual combinations of properties that cannot be met with conventional materials. Composites offer a way to achieve a unique combination of properties within a single material. Examples of this strategy are filled epoxy resins reinforced with particles [1]. One the one hand, fillers allow tailoring of the properties, on the other hand, fillers are often cheaper than the matrix material. Thus, it is possible to obtain a resin with extraordinary mechanical, electrical and thermal properties for a relatively low price. Filled epoxy resins are therefore widely used in industrial products, for example in the field of electronic packaging. More than ever, products tend to be used at their limits, and the prediction of the evolution of material properties becomes a critical task. The aim of this work is to study the damage evolution of a filled epoxy resin submitted to strain-controlled low cycle fatigue at several temperatures. Even if such tests are common for metals, there are relatively few data on polymers [2-4]. While the mechanisms of fatigue for metals are well-known, this phenomenon still must be better understood for polymers. The literature on polymers shows that two distinct mechanisms govern the fatigue process: crazing and shearbands [2, 5, 6]. Under a tensile loading, the onset of crazing is observed. Crazes are micro-cavities perpendicular to the maximum principal stress with load bearing fibrils connecting the craze walls. Because exceeding a certain stress level will break the fibrils and form a microcrack, crazes are generally assumed to be the precursors of cracks. When the polymer is submitted to torsion another mechanism takes place, namely shear yielding. The material yields via the formation of shearbands under 45° to the direction of the torque. Only for sufficiently high stresses, the crack propagates in a direction normal to the main principal stress. Shear yielding behavior is favored for rising temperatures [7].