تعیین سریع حد فرسودگی
Abstract
1-Introduction
2- Experimental data
3-Behaviour modelling
4-Material characterisation in shear direction
5-Results
6-Conclusions
References
Abstract
This paper proposes a method for on the fast fatigue limit estimation for composite materials using a simulation of a self-heating test. The experimental method based on the material self-heating was successfully applied for composite materials and allows to determine a material’s endurance limit in a few hours. In order to increase the potential of this experience, the thermomechanical modelling is proposed with the precise description of the intrinsic energy dissipation. The rise of material’s temperature is linked with development of plastic strains, material damage, viscoelastic proprieties of polymer matrix and intro-crack friction. Therefore, the collaborative behaviour model including the hysteresis loops is used to represent the visco-elastoplastic damage composite behaviour. The collaborative model consists of two sub-models. The first one describes an envelope of the loading curves and insures the computation of elastic and the in-elastic strains as well as the in-ply damage. The second part deals with modelling of hysteresis loops during unloading path using a fractional derivative approach. Just a few parameters are required to represent the hysteresis loops. Using a proposed modelling, the dissipation due to the in-ply damage propagation, the material hardening and the viscoelastic effects can be precisely calculated. The fatigue limit is determined by using thermodynamic simulation the quasi-static cyclic test with the dissipation measurements as an equivalent to self-heating test. The method is validated for composite materials with thermosetting and thermoplastic matrixes.
Introduction
Nowadays, carbon fiber reinforced plastics (CFRP) are widely used in the different industrial fields. Their lightweight, high strength and long durability make them superior to classical metallic materials in the complex structures design. In order to increase the structure’s safety and their economic potential, the validation phase has to be made including the mechanical test and numerical simulations of the material behaviour. In the present work, the fatigue loading is concerned as the essential point in the certification of the industrial structures. The experimental measurement of fatigue limit for composite materials required a significant number of tests in the different orthotropic directions. The classical method consists in the applying of the cyclic loading for the given laminate during a substantial period of time (usually several weeks) in order to obtain Wöhler curve. The alternative method allows to determine the fatigue limit in several hours using the material self-heating effect. This method has been initially proposed for isotropic materials such as metal alloys [1], elastomers [2] and more recently for the short fiber composite materials [3]. In the case of carbon fiber composites, the self-heating method consists of applying a sequence of constant amplitude cyclic loading blocks. The stabilized temperature of the composite specimen is measured during each block. When the value of the stabilized temperature increases significantly, it is considered that the fatigue limit is attainted. Tomographic studies and the traditional fatigue tests allow to justify the developed method.