Marcelo F. S. F. de Moura

Developing reliable numerical tools and analytical methodologies is essential to optimize the design phase of composite structures. The quasi-static fracture behaviour of glass-fibre reinforced polymer bonded joints was evaluated for bidirectional and quasi-isotropic sequences. The specimens were bonded with a two-component epoxy-based adhesive system. The complete mixed-mode I+II fracture envelope was obtained using double cantilever beam, three-point bending End-Notched Flexure, and mixed-mode bending configurations. A data reduction scheme based on the equivalent crack method was adapted to calculate fracture energies as a function of the compliance evolution recorded during the tests. The power law energy fracture criterion was used to describe the complete mixed-mode I+II fracture envelope. The criterion proved to be very accurate in predicting the mixed-mode fracture behaviour for both series. A cohesive zone model with a trapezoidal softening law was used to validate the experimental procedure of the three test configurations. The results obtained for the fracture envelope using the power law criterion and the local strengths were used as input for the numerical simulations. The model efficiently described the full fracture envelope of both stacking sequences, with a deviations below 3% for the B series and below 7.5% for the Q series.

Nowadays, the application of different materials is quite common to optimise the performance of structural components. In this context, bonding between metals and composites becomes a frequent strategy. However, the appropriate design requires special care owing to the complex stress profile in the adhesive layer, caused by the stiffness mismatch inherent to these solutions. A crucial aspect relies on fracture characterisation under mode I loading of bi-material bonded joints. Therefore, in this work, two different approaches are analysed regarding the attainment of an almost pure mode I of Carbon–Epoxy/Aluminium bonded joints using the asymmetric double cantilever beam test. One method relies on a global approach that matches the bending stiffness of the specimen arms, while the second, the longitudinal strain-based criterion, matches the longitudinal strain distributions of the two adherends at the bondline, making it a local method. The global process was validated against experimental results and subsequently compared to the local methodology. Additionally, several combinations of metallic and fibre-reinforced polymers were analysed numerically using the finite element method with a cohesive zone model to compare results from the two procedures. The consistently low presence of mode II loading across all cases demonstrates that both the local and global approaches accurately capture a predominant mode I loading condition under the investigated scenarios.

Composites of poly(vinyl alcohol) (PVA) in the shape of braids, in combination with crystals of hydroxyapatite (HAp), were analyzed to perceive the influence of this bioceramic on both the quasi-static and viscoelastic behavior under tensile loading. Analyses involving energy-dispersive X-ray spectroscopy (EDS) and scanning electron microscopy (SEM) allowed us to conclude that the production of a homogeneous layer of HAp on the braiding surface and the calcium/phosphate atomic ratio were comparable to those of natural bone. The maximum degradation temperature established by thermogravimetric analysis (TGA) showed a modest decrease with the addition of HAp. By adding HAp to PVA braids, an increase in the glass transition temperature (Tg) is noticed, as demonstrated by dynamic mechanical analysis (DMA) and differential thermal analysis (DTA). The PVA/HAp composite braids’ peaks were validated by Fourier transform infrared (FTIR) spectroscopy to be in good agreement with common PVA and HAp patterns. PVA/HAp braids, a solution often used in the textile industry, showed superior overall mechanical characteristics in monotonic tensile tests. Creep and relaxation testing showed that adding HAp to the eight and six-braided yarn architectures was beneficial. By exhibiting good mechanical performance and most likely increased biological qualities that accompany conventional care for bone applications in the fracture healing field, particularly multifragmentary ones, these arrangements can be applied as a fibrous fixation system.

In the present work, single- and double-dowel joints following different geometric configurations are experimentally and numerically investigated to derive the splitting behaviour of beech wood (Fagus sylvatica L.), one of the most widespread hardwood species in Europe for structural purposes. The influence of the spacing between dowels, their distance to the supports, and the slenderness of the beams is analysed. The correlation of the experimental failure loads with those predicted numerically by cohesive zone finite element-based models using the fracture properties of the species is discussed. The experimental results are also compared with those obtained from the normative expression included in Eurocode 5 and two other design models reported in the literature. The splitting failure loads predicted by both the analytical and numerical models were found to be conservative, the latter being closer to the experimental values.

In this work, the fracture behaviour of repaired honeycomb/carbon–epoxy sandwich panels under edgewise compression and three-point bending loading was analysed. Assuming the occurrence of damage resulting from a complete perforation leading to an open hole, the followed repair strategy consists of plug filling the core hole and considering two scarf patches with an angle of 10° in order to repair the damaged skins. Experimental tests were performed on undamaged and repaired situations in order to address the alteration in the failure modes and assess the repair efficiency. It was observed that repair recovers a large part of the mechanical properties of the corresponding undamaged case. Additionally, a three-dimensional finite element analysis incorporating a mixed-mode I + II + III cohesive zone model was performed for the repaired cases. Cohesive elements were considered in the several critical regions prone to damage development. The failure modes and the resultant load–displacement curves obtained numerically were compared with the experimental ones. It was concluded that the numerical model is suitable for estimating the fracture behaviour of sandwich panel repairs.