always does not start with the growth and development of macro-crack at an only point. Also there are good possibilities that cracks formation and propagation are already present within the materials (cracks in the matrix, broken fibres). Under fatigue load, these defects give rise to the formation of cracks and to their propagation. Defects growth is microstructural in these materials and includes the presence of cracks in the matrix, delamination and fibres breakage. Fatigue life prediction of composite materials is very difficult.
In unidirectional composites with not-axial loading (angle between 0° and 90°), the apex of the crack is subjected to two displacement components: an opening normal mode and a sliding parallel mode to the fibres. The effects of two damage types depend on the loading angle. For example, crack propagation is only as opening mode with a load of 90°, while the crack propagation is only as sliding mode with an angle of 0°. Then there is mixed mode crack propagation for angles between 0° and 90°. In multidirectional composite fibres failure is related to transverse fibres; the crack propagates through the interface of the layers, it causes a stress concentration. Multidirectional laminates also create induced stresses in the edges as a result of different elastic properties of layers, often increasing the delamination between planes.
2.2 Failure mode
In some cases the failure is coincident with rupture (brittle behaviour), in other cases it coincides with the deviation from linear elastic behaviour (ductile behaviour). Failure can occur in various modes:
1. broken fibres;
2. fibre-matrix separation (debonding);
3. crack of the matrix (typical of fatigue);
4. separation of the sheets (delamination).
Sometimes these failure modes occur separately, sometimes they coexist.
4.2 Fatigue in composite materials
The growing use of composite materials is mainly due to their high strength and stiffness, coupled with a low density compared with steel. The fatigue load involves the formation of cracks in the matrix and then in the fibres. Composite materials can show different combinations (fibre orientation) and forms, so the fatigue study is particularly complex and demanding.
The fatigue failure occurs first in brittle materials characterized by low values of strain, so the matrix is damaged before the formation of cracks. The fatigue failure can take various forms in composite materials: broken fibres, fibre-matrix interface failure, delamination, presence of cracks in the matrix. Cracks decrease the stiffness and strength of the composite materials. The formation and numbers of cracks depends on fibres orientation. In composite materials with different fibre orientation cracks arise first in the weaker plans and then in other plans until the stronger.
Fibreglass composite materials have a greater versatility of geometry, but laminates offer a lower stiffness and static strength due to the distortion of fibreglass. Another possible reason is the percentage of fibres that is less than 60-65%.
