Main research group: Prof. W. Ostachowicz, Prof. M. Krawczuk, Dr A. Żak

The past twenty years have brought a considerable increase in the use of composite materials. Modern composite materials are characterised by very high strength-to-weight ratios and very high durability in extreme conditions (under high stresses, temperatures, in moist atmospheres etc.). For these reasons composite materials are nowadays exploited in many fields of civil and mechanical engineering. They are widely used in the aircraft industry, motorization and also as structural elements in civil engineering. Because of their properties composite materials have found many applications in sport, space technology and even in medicine.

Elements of machines made of composite materials similarly to those made of isotropic materials can be exposed to many forms of fatigue damage that may grow during their exploitation. In case of unidirectional composite materials it is mostly fibre and matrix cracking, fibre and matrix debonding, and for laminated composite materials it is delamination of layers. The wide range of the applications for composite materials and the high requirements for their durability force us to carry on research programs in order to determine the influence of fatigue damage on their safe exploitation.

Various diagnostic methods for estimation of the location and size of damage in elements of structures are still being developed. The influence of these faults on the static and dynamic behaviour of structural elements has been investigated not only experimentally but also theoretically. It is found that fatigue damages results in changes in deflection and shape of structural elements, changes in natural frequencies and in modes of vibration. Changes in the amplitudes of forced vibration, resonant frequencies, and coupling of vibration modes are also observed in these cases. These reasons encourage many researchers to undertake extensive work on developing new models and new methods to study the influence of fatigue damage on changes in static and dynamic characteristics of structural elements. The results of this work obtained for elements of structures made of isotropic materials are widely presented in the literature. Because of the more complicated nature of composite materials there is a lack of suitable models as well as corresponding results of numerical investigations.

The main objective of the research in the area of the vibration and stability of structures is to develop new finite element models of composite constructional elements with damage in the forms of cracks or delamination. The influence of the damage (crack or delamination) location and size, as well as the influence of composite material anisotropy defined by the orientation and volume fraction of reinforcing fibres, have all been studied. Changes in natural frequencies and modes of vibration, changes in amplitudes of forced vibration, and resonant frequencies due to the damage, as well as changes in critical loads have been investigated.

The results obtained from numerical simulations have been also validated experimentally. For the purpose of the experimental measurements special composite specimens (beams and plates) have been prepared. The developed finite element models have been successfully verified for their compliance with the results of experimental measurements.

The influence of the delamination length
on natural frequencies of a laminated cantilever beam

Glass/epoxy and graphite/epoxy composite beams
and plates with cracks or delaminations for vibration tests

Vibration test of a delaminated glass/epoxy composite plate