Problemas atuais da juticultura amazônica.

No presente trabalho, os autores procuraram expor os problemas atuais que entravam o desenvolvimento da juticultura amazônica, ressaltando a importância marcante da produção das fibras desta tiliácea como fonte da manutenção do mercado interno brasileiro de matéria prima necessária ao abastecimento do parque nacional de aniagem. Os problemas considerados, no trabalho em questão, foram: 1) problemas de ordem agrícola, 2) problemas de ordem sócio-econômica. Procurou-se, também, de forma sintética, expor a situação do mercado nacional e as possibilidades de exportação. Como conclusão, os autores admitem a necessidade urgente e prioritária de se estimular econômicamente o juticultor a fim de que se possa posteriormente mostrar que a quantidade e qualidade do produto depende em grande parte dos problemas culturais e tecnológicos.Separata da Pesquisa Agropecuária Brasileira, v. 1, p. 1-6, 1966

Design and characterization of a biomimetic composite inspired to human bone

Many biological materials are generally considered composites, made of relatively weak constituents and with a hierarchical arrangement, resulting in outstanding mechanical properties, difficult to be reached in man-made materials. An example is human bone, whose hierarchical structure strongly affects its mechanical performance, toughness in particular, by activating different toughening mechanisms occurring at different length scales. At microscale, the principal toughening mechanism occurring in bone is crack deflection. Here, we study the structure of bone and we focus on the role of the microstructure on its fracture behaviour, with the goal of mimicking it in a new composite. We select the main structural features, the osteons, which play a crucial role in leading to crack deflection, and we reproduce them in a synthetic composite. The paper describes the design, manufacturing and characterization of a newly designed composite, whose structure is inspired to the Haversian structure of cortical bone, and that of a classic laminate developed for comparative reasons. We conclude with a critical discussion on the results of the mechanical tests carried out on the new composite and on the comparative laminate, highlighting strengths and shortcomings of the new biomimetic material

Mechanics of collagen-hydroxyapatite model nanocomposites

Bone is a hierarchical biological composite made of a mineral component (hydroxyapatite crystals) and anorganic part (collagen molecules). Small-scale deformation phenomena that occur in bone are thought tohave a significant influence on the large scale behavior of this material. However, the nanoscale behaviorof collagen–hydroxyapatite composites is still relatively poorly understood. Here we present a molec-ular dynamics study of a bone model nanocomposite that consist of a simple sandwich structure ofcollagen and hydroxyapatite, exposed to shear-dominated loading. We assess how the geometry of thecomposite enhances the strength, stiffness and capacity to dissipate mechanical energy. We find that H-bonds between collagen and hydroxyapatite play an important role in increasing the resistance againstcatastrophic failure by increasing the fracture energy through a stick-slip mechanism

Effect of delamination on the fatigue life of GFRP: A thermographic and numerical study

Delamination is the major failure mechanism in composite laminates and eventually leads to material failure. An early-detection and a better understanding of this phenomenon, through non-destructive assessment, can provide a proper in situ repair and allow a better evaluation of its effects on residual strength of lightweight structural components. Here we adopt a joint numerical-experimental approach to study the effect of delamination on the fatigue life of glass/epoxy composites. To identify and monitor the evolution of the delamination during loading, we carried out stepwise cyclic tests coupled with IR-thermography on both undamaged and artificially-damaged samples. The outcome of the tests shows that IR-thermography is able to identify a threshold stress, named damage stress ?D, which is correlated to the damage initiation and the fatigue performance of the composite. Additionally, we performed FE-simulations, implementing the delamination by cohesive elements. Such models, calibrated on the basis of the experimental fatigue results, can provide a tool to assess the effect of parameters, such as the delamination size and location and composite stacking sequence, on the residual strength and fatigue life of the composite material

A juta na Amazônia.

O cultivo da juta na Amazônia. Conclusões dos trabalhos de pesquisa realizados pelo Instituto Agronômico do Norte

Droughts Over Amazonia in 2005, 2010, and 2015: A Cloud Cover Perspective

Amazon forests experienced recent severe droughts in an anomalous short period induced by different mechanisms and had different length periods and spatial patterns. Droughts of 2005 and 2010 were attributed to anomalous Sea Surface Temperature (SST) over the Tropical North Atlantic (TNA) during the dry season, but the 2010 drought was more severe and remained for a longer period because it was also induced in late 2009 by a moderate to strong El Niño (EN). Drought in 2015 led to unprecedented warming and extreme soil moisture deficits over some regions, and it was attributed to a very strong EN. Several studies analyzed these drought events regarding different climatic factors such as anomalies in SST, vegetation, temperature, precipitation, soil moisture deficits, solar radiation, etc. However, we have not identified a complete analysis of total cloud cover (TCC) over Amazonia during these drought events in the context of long-term trends and past strong EN events. This brief report aims to present a preliminary analysis of anomalies in TCC over Amazon using reanalysis data with a focus on the last recent drought events into a long-term context. Results show a significant decreasing trend (p < 0.05) for TCC over southern Amazonia during the dry season (around −2% per decade), in contrast to the significant increasing trend found over northern Amazonia during this season and the significant widespread increasing trend during the wet season (between +2 and +4% per decade). Correlation analysis between SST and TCC anomalies is also indicative of the different West-East and North-South patterns linked to EN events or drought episodes driven by TNA warm anomalies

Fatigue behaviour of a GFRP laminate by thermographic measurements

Composite materials are widely used to build structural components, thanks to their mechanical properties. Those are generally considered ‘engineering materials’, since they are tailored to meet specific requirements. Due to their use for structural components, it is important to know their mechanical behaviour, especially under cyclic loads. At present, there is a common interest, among researchers, to study the mechanical behaviour of composites, by means of both traditional and innovative techniques, with the final purpose of making previsions regarding their service life. In fact, due to their composite nature, they behave in a different mode compared to homogeneous materials. This study is focused on a glass fibre-reinforced plastic (GFRP); the aim of this work is to study its fatigue behaviour, from both the mechanical and the thermal points of view. The main reason is that there is a lack of knowledge, in the literature, about the fatigue of composites. In this study, a GFR laminate was characterized under static and dynamic loading conditions; during the experimental tests, thermal measurements were carried out by means of an IR-thermal camera. Temperature measurements were done during the static tests, whereas in the dynamic tests the dissipated energy was measured, by using the dissipation method (D-mode). Then, various criteria for fatigue life estimation were applied fitting the experimental data. Since different thermographic techniques have been used to estimate the fatigue behaviour, a final comparison between the experimental data and the predicted fatigue behaviour is proposed and discussed, showing a good agreement

Mechanical and Interface Properties of Biominerals: Atomistic to Coarse Grained Modeling

none5mixedA. K. Nair; LIBONATI, FLAVIA; Z. Qin; L. S. Dimas; M. J. BuehlerA. K., Nair; Libonati, Flavia; Z., Qin; L. S., Dimas; M. J., Buehle