Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Validated composite mechanics and FEM guidelines and recommendations

The objectives of Task Group 2 in investigating blade material behaviour under complex stress states were to generate test results of basic plies and multidirectional laminates for the reference material to implement advanced FEM formulations, to define and validate experimentally multi-axial failure theories in static and fatigue loading and to quantify complex stress state effect on blade design by contributing with design recommendations.

Concerning in-plane static and fatigue characterization, a comprehensive experimental program was performed for the UD material in the frame of TG2. To this end a number of at least 25 tests per category were performed, to have a statistical description as well, for measuring 19 engineering constants including elastic constants, strength properties and thermal expansion coefficients. All experiments were performed at the same lab (UP), same test rig, same procedures so as to minimize variations in material property. Test coupons were delivered directly by LM. Test methods and detailed results were presented in a number of OPTIMAT reports.

For fatigue characterization of the UD material, definition of S-N curves at three R-ratios, 0.1, -1 and 10, both in the fibre and the transverse direction as well as in-plane shear fatigue strength under R=0.1 was performed.

In all static and fatigue tests, except for in-plane shear characterization tests which were based on ISO 14129 geometry and test method, the standard OB coupon geometry was used, introducing the concept for just one geometry coupon valid for all types of tests, e.g. static or fatigue in tension and compression, residual strength etc. Only thickness was varying from coupons tested in the fibre direction to those loaded transversely to the fibre. Comparison of test results with ISO based test methods and coupon geometries revealed that strength and elastic moduli are in very good agreement except the compressive strength in the fibre direction where the OPTIMAT specimen performs not so well due to bending deformation.

Concerning in-plane shear strength and modulus, a detailed comparison of several different methods and standards was presented showing large discrepancies in test results. It seems that the method selected in TG2, i.e. ISO 14129, tensile test of +/-45°laminated coupon yields fair results concerning both the shear modulus and strength.

It is believed that the database created for the in-plane characterization of the UD material can be used for similar epoxy laminates if a single batch of confidence testing for a material combination gives similar results.

With respect to the complex stress states in a rotor blade, it is believed that a rotor blade even if loaded in a complex mode, it is after all a thin-wall beam structure. As such, the tangential stress resultant in the shell thickness is negligible compared to the axial one. However, the shear stress resultant is of comparable magnitude to the axial one in areas as the shear webs or trailing and leading edges respectively.

In conclusion, 1D stress analysis, i.e. beam theory formulations, could be acceptable but failure prediction should be done in a layer-by-layer basis, i.e. complex stress states at the ply level should be taken into account.

As for failure prediction under complex stress states, typical plane stress states as those developed in the layers of the shells of a rotor blade can be simulated, besides sophisticated biaxial tests, by uniaxial testing in off-axis UD coupons. Failure prediction under static loading is performed satisfactorily using criteria such as Tsai-Wu, Puck and Tsai-Hill. Under cyclic loading, life prediction is also satisfactorily performed when using similar quadratic in stress functions compared to simplistic approaches, such as to consider separately damage from each stress component and add at the end.

Experimental results from cyclic biaxial tests, using specimens either of cruciform or tubular geometry, were not conclusive. Static tests results from bi-axial tension of cruciform specimens made of MD lay-up suggest that failure prediction according to limit functions as those cited in the above is fair, in general, although many other parameters such as stiffness degradation scenarios or material non-linearity for example are of paramount importance.

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Reported by

University of Patras
Department of Mechanical Engineering & Aeronautics, P.O.Box 1401 Panepistimioupolis Rio
265 04 Patras
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