A successful exploitation of nanocomposite materials founds on the development of models capable of predicting the macroscopic mechanical behaviour as a function of the nanostructure. To this end the ...most critical issue to overcome is the identification of the inherent mechanisms at the very nanoscale which might depend on the type, the morphology and the functionalisation of the nanofilleras well as on the loading conditions. Within this scenario, this work aims to review the main damage mechanisms reported in the literature for nano-reinforced thermosetting polymers, to include new insights and to discuss predictive models incorporating these mechanisms.
•The matrix-dominated fatigue response of UD composites under tension–torsion is investigated.•Attention is focused on the initiation and propagation of transverse cracks.•S–N curves and Paris-like ...curves are presented for several biaxiality ratios.•Shear stress severely affects damage onset and evolution and the transverse fatigue strength.•Microscopic damage modes are influenced by the local multiaxial stress state.
The matrix-dominated behaviour of unidirectional composite laminae subjected to multiaxial fatigue loading was extensively investigated. Tubular specimens with three different lay-up (90n, 0F/90U,3 and 0F/90U,3/0F) were tested under combined tension/torsion loadings. Attention was focused on the initiation and propagation of cracks in the 90° layers, and fatigue data are presented in terms of S–N curves (for crack initiation) and Paris-like curves (for crack propagation) under several biaxiality ratios. Damage onset and evolution have been found to be strongly affected by the multiaxial loading condition, showing a particularly detrimental effect of the shear stress component. SEM investigations on the fracture surfaces confirmed a strong dependence also of the microscopic damage modes on the local multiaxial stress state.
In the present work a criterion for the non-fibre (i.e. matrix or fibre–matrix interface) – controlled fatigue behaviour of composite unidirectional laminae subjected to multiaxial loading is ...proposed. The criterion is based on experimental observations of the damage modes occurring at the microscopic scale leading to the fatigue failure of a lamina. In particular two parameters have been identified as representative of the driving forces for the damage initiation, and they have to be used for design purposes depending on the local multiaxial condition. These parameters, i.e. the local maximum principal stress and local hydrostatic stress, are calculated from the local stresses in the matrix, obtained from Finite Element analyses of a fibre/matrix unit cell subjected to periodic boundary conditions. The application of the proposed criterion to fatigue data taken from the literature shows that only two scatter bands can be used to completely describe the fatigue response of unidirectional laminae under generic multiaxial stress states.
In this work, the transverse crack initiation, propagation, and the crack density evolution in cross-ply laminates are analysed under two-stage block loadings. A theoretical formulation for the ...cracking probability under variable amplitude blocks is obtained based on the linear damage accumulation rule, often adopted in crack density prediction models from the literature. The comparison between the experimental results and the predictions for the cracking probability and the crack density evolution validate, from the phenomenological point of view, the adequacy of the linear damage accumulation rule for crack initiation under block loadings. Investigations of the damage evolution at the micro-scale are also presented, providing a mechanistic support to this approach.
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This paper presents an overview on the experimental investigation of the response of composite materials under multiaxial cyclic loading, starting from the pioneering works in the early 1970s to ...arrive to the research activities run in these days by the scientific community and concluding with the areas still open. The experimental techniques and sample geometries are compared, illustrating advantages and drawbacks of each solution, also with reference to the outcomes of the research. The influence of the main design parameters are illustrated, taking advantage of experimental results from the extensive investigation carried out in the last years at the University of Padova. Needs of future research are eventually addressed.
The very early stages of damage evolution under fatigue loadings were investigated by testing 45/-45/0s glass/epoxy specimens. Microscope observations revealed that the first damage event was the ...initiation of micro-cracks in the 45° plies within the inter-fibre region. Off-axis cracks then formed by the coalescence of these micro-cracks and propagated along the fibres direction with the same mechanism. The orientation of the micro-cracks was proven to be normal to the direction of the local maximum principal stress in the matrix. The results presented here are the first published evidences of fatigue damage initiation at the inter-fiber scale and represent a further validation of the fatigue crack initiation criterion recently presented by the authors.
In the present work, a multi-scale modelling strategy to assess the fracture toughness of nanoparticle filled thermosetting polymers is presented. The model accounts for the main damaging mechanisms ...arising in this kind of materials, i.e. nanoparticle debonding, plastic yielding of nanovoids and plastic shear banding of the polymer. Further, the proposed analytical framework considers the influence of an interphase around nanoparticles, a particular feature of nanocomposites.
Comparison of the theory to a bulk of experimental data from the literature shows a very good agreement.
An innovative procedure is proposed for the prediction of the crack density evolution in multidirectional laminates subjected to cyclic loadings. The crack initiation and propagation phases are ...treated separately and described by means of a master S-N curve and a Paris-like curve, respectively. A damage-based multiscale strategy is adopted for the prediction of multiple crack initiation, accounting for the statistical distribution of fatigue strength and crack propagation resistance within a ply. The procedure has been implemented in a Matlab® code for the simulation of the fatigue damage evolution in multi-directional symmetric laminates. Comparisons with experimental results taken from previous works show a very good agreement.
In the present work, a simple procedure to build periodic representative volume elements (RVE) is developed for UD composite laminae. The procedure is based on the alteration of an initially regular ...fibre distribution, and includes the possibility to start from different fibre configurations, to use of non-uniform fibre diameter, and for fibres to cross the RVE edges.
The obtained RVEs were found to differ from a theoretical hard-core process in terms of Voronoi equivalent radius distribution and nearest neighbour distribution, for which a novel analytical method is proposed. The fibre distributions were instead in good agreement with real microstructures in terms of both short- and long-range morphological indexes, especially when the actual fibre diameter distribution is used. A novel short-range index, called neighbour distance diagram, is also proposed to represent the distances of all the fibre neighbours.
The RVEs were then compared to real microstructures in terms of stress distributions. The Cumulative Distribution Function (CDF) of Local Maximum Principal Stress (LMPS) and the Local Hydrostatic Stress (LHS) in the matrix are calculated under transverse, shear and longitudinal stresses and under a temperature gradient, validating the RVE also under a mechanical point of view. Using uniform fibre diameter was found not to have a significant influence on the stress distributions.
Eventually, a parametric analysis was carried out to analyse the influence of the fibre volume fraction on LMPS and LHS CDF, under both mechanical and thermal loading conditions.
•0/902S and 0/452/0/−452S G/E laminates were produced by vacuum resin infusion.•Using different process parameters, different void contents were obtained.•Fatigue damage initiation, evolution and ...stiffness drop were deeply affected by voids.•A larger influence of voids was found for the cross-ply laminates.•Both void content and size contributed to decreasing the fatigue performances.
In the present work, glass/epoxy panels with stacking sequence 0/902S and 0/452/0/−452S were produced by vacuum resin infusion. Using different process parameters, which may lead to cost saving, also the presence of porosity was obtained in the laminates. Specimens were tested under uniaxial tensile fatigue loadings, monitoring the initiation and propagation of cracks in the off-axis plies. The performance of the material and the effect of porosity were evaluated in terms of life to crack initiation, crack growth rate, crack density evolution and stiffness degradation. A detrimental effect of the porosity was found even for a very low void content, highlighting the importance of properly accounting for their presence in the design of composite parts.
