Thermal and mechanical fatigue of a PA66/glass fibers composite material

Polymeric matrices are viscoclastic materials and dissipate a part of the mechanical energy in the form of heat when they are submitted to a cyclic strain. During a fatigue test, the temperature of the material can increase and stabilize to some extent. Sometimes it increases until the sample fracture occurs which is called a thermal fracture. When the sample fracture is governed by crack initiation and propagation, it is called a mechanical fracture. The glass transition temperature of the PA66 matrix is not far from the ambient temperature and a temperature increase possibly induces a change of the matrix physical state. So the fatigue behaviour of our material strongly depends on temperature variations. The fatigue tests were carried out with an alternative bending device with a ratio R = -1, at 23 degrees C, 45% RH and two different frequencies, 2 Hz or 10 Hz. The applied strain varies between 0.0116 and 0.0223. During the test, the surface temperature was measured in the most constrained part of the specimen by an infrared camcorder. For a 10 Hz frequency and although the test temperature is 23 degrees C, the sample temperature at the surface is much higher and whatever the applied strain, the matrix is in a rubbery state at the material fracture. For a 2 Hz frequency, the PA66 matrix remains in the glassy state. The fatigue failure surface observed by scanning electron microscopy for both kinds of samples will be discussed. (c) 2006 Elsevier Ltd. All rights reserved.