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Date palm fibers have been studied as potential reinforcement in composite materials. Their density is equal to 1.22 g.cm−3; making them suitable for use in lightweight applications. In order to efficiently use these fibers and understand their mechanical properties, quasi-static tensile tests and cyclic fiber fatigue tests were performed to determine the mechanical properties. Their internal structure, elementary fibers and porosity were analyzed by scanning electron microscopy. The results show that the mechanical properties are clearly improved by taking into account the porosity by calculating the real section of the fibers by using the imageJ software a cross-sectional pore fraction of the fibers has been determined and the ultimate properties of the pore-free bulky fibers has been calculated with stress and Young's modulus increased by approx. 54% as compared to the highly porous natural fibers. [ABSTRACT FROM AUTHOR]

Highlights: [•] Stiffness loss model for cyclic fatigue of sandwich panels in 3-point bending. [•] Model is identified through reduced number of measured parameters. [•] Model allows assessing the fatigue life of sandwich panels with limited number of available data. [•] Comprehensive experimental investigation with fracture modes of composite sandwich panels. [ABSTRACT FROM AUTHOR]

Abstract: The work describes the quasistatic tensile and fatigue behaviour of as-received sisal fibres. Natural-based reinforced composites are gaining significant interest within the structural community, due to their interesting mechanical properties, recyclability and environmentally-sustainable production and use. Natural fibres such as sisal constitute an excellent reinforcement material, due to the low extraction costs from plants, and high level of recycling involved in their manufacturing process. In this work the diameter, Young’s modulus, strength and strain to failure over 15 different samples are measured and compared against data from open literature. Tensile cyclic fatigue loading at eight loading levels (from 0.6 to 0.95) has been carried out. The maximum forces involved (between 9N and 22N) are considerably higher than the ones used previously in open literature, and lead to significant dependence of the hysteresis loops, energy dissipation and S–N behaviour of the sisal fibres versus the cycle and loading ratio levels. The results obtained from this work can be used to predict from a fatigue and structural integrity point of view the behaviour of sisal-based reinforced composites with high load bearing capability, and extend the design envelope of this class of natural-reinforced materials. [Copyright &y& Elsevier]

Abstract: We present the experimental results related to the mechanical behaviour under tensile static, fatigue and compressive relaxation loading of closed cell PVDF electret foams under different loading ratio conditions. The specimens are statically loaded until 60% of their ultimate displacement, and subsequently subjected to cyclic loading under displacement control. The static tests show a stress–strain behaviour and failure mode similar to the one of other polymeric closed cell materials, such as polymetacrylimide-based foams. The fatigue tests show an evolution of the stiffness degradation over the levels of cycles characterised by three distinct phases. The behaviour of the energy dissipated versus the loading ratios and numbers of cycles applied is discussed in this paper, as well as the comparison between the compressive fatigue behavior recorded on similar classes of foams. The compressive relaxation behaviour shows a two-phase dependence over the loading time, with increasing modulus for higher loading ratios, and absence of an asymptotic modulus for long time exposure. [Copyright &y& Elsevier]

Abstract: In this paper, we present a comparative analysis between the cyclic loading tensile behaviour of conventional and auxetic thermoplastic PU foams. While the two types of foam share the same base material (open cell PU–PE), one batch is transformed into an auxetic one (i.e., negative Poisson’s ratio) using a special manufacturing process involving moulding and exposure to particular temperature profiles to stabilise the transformation of the microstructure. The effect of the stiffness degradation and accumulation of energy dissipation versus the number of cycles are discussed for different loading levels r. The results show that the fatigue behaviour until failure, subjected to cyclic loading depends on the loading levels and occur in three stages. The results obtained shows also that the auxetic foam have enhanced characteristics under static loading and tensile fatigue compared to the conventional parent phase form. [Copyright &y& Elsevier]

Abstract: This work presents a comparative analysis between the cyclic loading compressive behaviour of conventional, iso-density non-auxetic and auxetic (negative Poisson’s ratio) thermoplastic polyurethane foams. While the three types of foam share the same base material (open cell rigid PU), one batch is transformed into auxetic (i.e., negative Poisson’s ratio) using a special manufacturing process involving moulding and exposure to particular temperature profiles to stabilize the microstructure transformation. The specimens have been loaded in cyclic compression with a sinusoidal waveform in displacement control. The static tests show the specific stress–strain compressive mechanical behaviour of these auxetic thermoplastic foams, opposite to conventional ones and other similar data on auxetics available in open literature. The effect of the load loss, stiffness degradation, the evolution of dynamic rigidity and accumulation of energy dissipation versus the number of cycles are discussed for different loading levels. The analysis of the results shows that the fatigue behaviour until failure occurs in two stages, subjected to cyclic loading, depends on the loading level. The hysteresis loop tends to close itself as function as the number of cycles N, while the slope of the dynamic stiffness decreases with increasing N, therefore with decrease of dissipated energy. The energy dissipated by the auxetic foams is significantly higher than the one from conventional parent phase and the iso-density foams at every number of cycles and loading level. [Copyright &y& Elsevier]

Abstract: The authors discuss the use of an artificial neural network (ANN) to estimate fatigue lifetime of a sandwich composite material structure subjected to cyclic three-point bending loads. A total of 27 samples (three different loading levels for nine samples each) were investigated to provide training, validation and testing data for a series of multi-layer perceptron ANNs. The networks were implemented using both conventional maximum likelihood and Bayesian evidence based training algorithms. It was found that the Bayesian evidence based approach provided a superior and smoother fit to the experimental data. Completely independent fatigue tests were conducted at intermediate levels of loading to evaluate the capacity of the fitted ANN model to interpolate to previously unseen regions of data. Excellent agreement was obtained between the model predicted outputs and the new experimental data. The capability and estimation of predication error when using the Bayesian technique is discussed along with the application of the model to generate classical S–N lifetime curves. [Copyright &y& Elsevier]

• Quasi-static and cyclic fatigue traction of long fibers of date palm fruit branches. • Characterization by SEM of the fiber of date palm fruit branches. • Analysis of load-displacement hysteresis curves. • Interpretation and discussion of S-N curves. This work describes the quasi-static tensile behavior and cyclic fatigue of technical fibers from date palm tested for the first time according to the authors' knowledge, which can be used in the reinforcement of green composites.The values of the stress and strain at failure and Young's modulus for18 samples tested under quasi-static tensile loading are compared with results reported in the literature for other fibers. Cyclic fatigue tests were performed at a frequency of 2 Hz under sinusoidal solicitation for different loading levels ranging from 0.6 to 0.95 corresponding to maximum recorded forces of 15.36 and 39.82 N, respectively. The evolution of the force as a function of the number of cycles was recorded. This allows us to follow the stiffness degradation. The load-displacement hysteresis curves show the influence of the number of cycles and the loading level on the size and shape of the hysteresis loops, allowing the calculation of the dissipated energy per cycle for the technical fibers. [ABSTRACT FROM AUTHOR]

Abstract: The application of interval set techniques to the quantification of uncertainty in a neural network regression model of fatigue lifetime is considered. Bayesian evidence training was implemented to train a series of multi-layer perceptron networks on experimental fatigue life measurements in glass fibre composite sandwich materials. A set of independent measurements conducted 2 months after the training session, and at intermediate fatigue loading levels, was used to provide a rigorous test of the generalisation capacity of the networks. The robustness of the networks to uncertainty in the input data was investigated using an interval-based technique. It is demonstrated that the interval approach allowed for an alternative to probabilistic-based confidence bounds of prediction accuracy. In addition, the technique provided an alternative network selection tool, and also allowed for an alternative to estimating the lifetime prediction error that was found to be a significant improvement over the Bayesian-derived estimate of confidence bound. [Copyright &y& Elsevier]