Features of fatigue and damage-tolerance tests of full-scale metal-composite aircraft structures

The main features of the mechanisms of damage during cyclic loading of metal and composite materials have been studied. The results obtained are used to develop the approaches to fatigue and damage tolerance tests of metal-composite (hybrid) structures to confirm their safe operation. It is shown that composites exhibit higher scattering characteristics compared to metals due to their heterogeneity attributed to the anisotropy of the material properties, the presence of technological defects and various natural inhomogeneities in their structure, both in terms of static and fatigue properties. This leads to the necessity of testing two identical full-scale structures: one to confirm the service life of the metal part, and the other of the composite part. The fatigue tests of structures should be carried out for different ranges of variable loads, which differ in the size of the block of variable loads, the levels of «high» and «small» loads and the load enhancement coefficient. To carry out fatigue tests of the composite part of the structure during an acceptable time, it is necessary either to increase the loads of the variable loading spectrum, or to introduce additional «overload» cycles. Proceeding from the condition of the equality of the probabilities of non-destruction for the metal and composite parts of the structure, the relations for determination of the load enhancement coefficient k_σ and the number of «overload» cycles n are obtained. The load enhancement coefficient is determined by the ratio: , where m_c and m_m are the exponents of fatigue curves of the composite and metal, η_c and η_m are the reliability coefficients when testing composite and metal parts. The required number of «overload» cycles is determined by the ratio: where σ_{eq o}, σ_eq are the equivalent stresses of the overload cycle and the spectrum of variable loads.

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