Study of the crack growth in composite rotor blade skin
Résumé
In rotorcrafts, blades are among the most critical components. They undergo a high cycle fatigue of a complex multi-axial loading and a close attention has to be paid to the safety of their design. We can roughly consider the structure of a blade as a stiff spar and a foam filling covered with a thin composite material skin. In case of in flight damage or unexpected stress concentration, a through-the-thickness crack, (also called “translaminar crack”) can initiate on the skin. For a fail-safe design of blades, this damage phenomenon has to be understood and its potential propagation along the skin has to be quantified. For that purpose, crack propagation experiments on rotor blades have been carried out in Eurocopter. Besides, a study is currently conducted with Institut Clément Ader, (Toulouse) for several years in order to understand the phenomenon and to develop a numerical model to simulate crack propagation. The study focuses on glass fiber based composite materials with [0/90] and [+-45] stacking sequence. First, experiments were carried out on small samples to study the effect of static and fatigue loadings on the material. Samples were cut out and polished to observe the damaged material with the aid of optical microscopy and scanning electron microscopy (SEM). It revealed that the type of damage was different for tension and shear loadings. Then, propagation tests were carried out on structural shaped samples under cyclic tension-tension and shear loading, to simulate opening and in-plane shear failure mode on blade skin It was shown that in fatigue loading, when a through-the-thickness crack propagates, a fibre bundle can only collapse entirely. Thus, the crack propagation speed is strongly dependant on the width of the fibre bundle and an original propagation model has been developed according to this observation. Between the micro-scale approach aiming at representing separately the two components of the material, and macro-scale models where properties are homogenized, and continuum damage mechanics is used, we developed a modelling where the material is meshed according to the bundle width, in order to understand the behaviour of a notched woven composite and predict the propagation of the crack under cyclic loading. More accurately, in this approach, two oriented meshes representing the warp and weft directions of the woven fabric are superimposed. Each tow is represented by one row of quadrangles, and spring elements link warp and weft tows to each other (see Figure 2). These interface elements contain all the potential damages of the material through their evolving stiffness. The stiffness of spring elements can evolve in function of static and fatigue loading. Finally, we consider with respect to experimental observations that a bundle can only break entirely and collapse collapsing? elements are introduced so that each bundle can break. We use a S-N curve, and a cumulative Miner-Palmgreen law to compute the damage into each tow and determine which one is critical and how many cycles it can sustain before its failure. It is then possible to deduce the direction of the propagation, its speed, and the extent of damage area. This modelling was implemented on commercial finite element software SAMCEF. It has been integrated into classical rotor blade FEM under structural loading, by kinematic sticking between macroscopic structural mesh of the blade and the tow-by-tow mesh close to the crack. It allows us from the solution of a macroscopic FE problem to obtain boundary condition for the propagation model. Crack propagation has then been simulated on blade structures under multi-axial loading and results were compared with experimental data of tests.