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Fatigue life prediction of materials can be modeled by deterministic relations, via mean or median S-N curve approximation. However, in engineering design, it is essential to consider the influence of fatigue life scatter using deterministic-stochastic methods to construct reliable S-N curves and determine safe operation regions. However, there are differences between metals and composites that must be considered when proposing reliable S-N curves, such as distinct fracture mechanisms, distinct ultimate strengths under tension and compression loading, and different cumulative fatigue damage mechanisms including low-cycle fatigue. This study aims at conducting a review of the models used to construct probabilistic S-N fields ( P-S-N fields) and demonstrate the methodologies applied to fit the P-S-N fields that are best suited to estimate fatigue life of the selected materials. Results indicate that the probabilistic Stüssi and Sendeckyj models were the most suitable for composite materials, while, for metals, only the probabilistic Stüssi model presented a good fitting of the experimental data, for all fatigue regimes.7

Knowledge of the stochastic nature of fatigue life of composite materials can be modeled by the failure time with the Weibull distribution. This task becomes complex when the samples are small and scattered. In this way, it is necessary to know and to improve robust models of estimation of the parameters of the distribution of Weibull. The aim of this work is to compare the performance of least squares, least squares weighted, maximum likelihood estimator and momentum method and to suggest a method that obtains better performance in life behavior to fatigue with small samples. Monte Carlo simulations were performed to estimate the distribution parameters with different sample sizes and an application with real fatigue data that compares performance using goodness-of-fit. The results of the simulations showed that the weighted least squares estimation was able to generate more reliable estimators for fatigue behavior during its useful life. In this way, it is possible to conclude that small samples make the real representation of life difficult to the material fatigue, but using the weighted least squares estimation method, it is possible to obtain more estimates.

A new formulation of a Logistic deterministic S-N curve is applied to fatigue data of metallic materials from ancient Portuguese riveted steel bridges. This formulation is based on a modified logistic relation that uses three parameters to fit the low-cycle-(LCF), finite-life-and high-cyclefatigue (HCF) regions. This model is compared to the Kohout-Věchet fatigue model, which has a refined adjustment from very low-cycle fatigue (VLCF) to very high-cycle fatigue (VHCF). These models are also compared with other models, such as, power law and fatigue-life curve from the ASTM E739 standard. The modelling performance of the S-N curves was made using the fatigue data considering the stress fatigue damage parameter for the materials

The polypropylene with load of reinforcement of curauá fiber (PP-curauá) is a compound developed to be applied in the process of injection of automotive plastic parts. Polypropylene composites reinforced with mineral fillers (talc) or glass fibers have been widely applied in this segment. However, natural fibers are an important alternative considering the aspects of sustainability, recyclability, abundance and low cost, when compared with glass fibers, and industrial talc. This study was prompted by the growing need for materials that meet the constant cycles of use, disposal and reuse, and avoid the harmful effects on the environment. The mechanical properties of a curaua fiber and polypropylene-based ecocomposite reprocessed one, three and five times were assessed, since reprocessing is known to change the strength of these materials. Pure polypropylene (0%) and ecocomposites with 10 and 30 wt% of fiber were submitted to tensile and three-point bending tests. The results showed that polypropylene ecocomposites with 30% curaua fibers exhibited a higher modulus of elasticity. Moreover, reprocessing did not significantly affect the ecocomposite properties, demonstrating their viability for reuse.

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