Issue34

R. Brighenti et alii, Frattura ed Integrità Strutturale, 34 (2015) 59-68; DOI: 10.3221/IGF-ESIS.34.05 60 The reliability and durability of composite structural components must be assessed through the evaluation of the damage phenomena taking place in such a class of materials due to in-service loading, especially when the external actions act cyclically on the structure. The degrading effects caused by repeated loading are responsible for a significant loss of the mechanical performances, and can be related to the fibre-matrix delamination (debonding), fibre breaking, fibre buckling, matrix plastic deformation or cracking, matrix damage [3, 4]. The present research aims at developing a micromechanical approach for the assessment of the fatigue behaviour of short-fibre-reinforced composites under cyclic loading causing a multiaxial stress state. The assessment of the degrading effects in such non-homogeneous materials under fatigue loading is complex, and requires a reliable mechanics-based model for their quantitative evaluation. Damages in the matrix and in the fibres are quantified by a damage mechanics approach, whereas the loss of fibre-matrix bonding is examined through fracture and fatigue mechanics. All these degrading phenomena are taken into account and quantified by analysis at micro-scale level. Finally, the behaviour of a fibre-reinforced material under multiaxial fatigue conditions is examined and compared with experimental data. M ULTIAXIAL FATIGUE OF MATERIALS he safety evaluation of engineering structures under variable loading is a key aspect of the material reliability [5-7]. Fatigue damage is a mechanical phenomenon affected by several factors such as stress values, stress gradients, size of structural components, surface roughness. Moreover, the load variability with time (cyclic or random) and the type of stress field in the material (uniaxial or multiaxial) play a crucial role in fatigue assessment. Many approaches have been formulated to analyse fatigue problems: empirical models based on the experimental Wöhler curves [8, 9], power laws for propagating cracks such as the Paris law [10], the critical plane approach [11-13], the average stress criterion and the stress invariant approach [14-16], the energy approach [17], the damage mechanics approach [18, 19], the fatigue fractal approach in the framework of the Wöhler approach [20]. The above problem becomes even more awkward in presence of relevant stress gradients, such as in structural components with notches (holes, fillets, welding, etc.), or geometrical irregularities. As is well-known, the cyclic loading reduces the mechanical properties of materials also if the stresses are below the yielding stress value, due to the irreversible rearrangement of the lattice structure at the microscopic level. In composite materials, the cyclic loadings are responsible for the decreasing mechanical properties of the matrix and for the reduction of the fibre-matrix bond effectiveness. Further, a fibre-matrix detachment can also take place, leading to a reduction of the useful fibre length for the load bearing purpose. Multiaxial fatigue assessment of the reinforced matrix material In fibre-reinforced composite materials, the fatigue assessment must take into account the matrix damage as well as the fibre-matrix interface. Since the local mechanical behaviour of a material containing a straight cylindrical fibre can be supposed to be transversally isotropic (Fig. 1a), the presence of a multiaxial stress state can be represented only by the stresses acting along the transversally isotropic axes, i.e. the radial axis and the axial one. According to such simplification, the multiaxial fatigue damage can be assessed by considering only the variability of such stresses. Figure 1 : (a) Cylindrical fibre surrounded by a cylindrical portion of matrix material; (b) partially detached fibre under radial and axial stresses. T (b) (a)