The application of a mesoscopic scale approach in fretting fatigue

This work concerns the application of a mesoscopic scale fatigue criterion to predict the initiation of cracks in components under fretting conditions. A verification of the analysis is carried out by considering available experimental data for fretting fatigue in a high strength Aluminium alloy commonly used in the aerospace industry. These experiments revealed there is a contact size effect in fretting fatigue life. The results show that the mesoscopic scale fatigue criterion can correctly predict the initiation of fretting cracks for larger contact configurations. It is concluded that the reason for the poor performance of the criterion in predicting failure at smaller contacts may well be related to the effect of the stress gradient, a variable not accounted for in the mesoscopic criterion here assessed

Effect of a deep cryogenic treatment on wear and microstructure of a 6101 aluminum alloy

The aim of this work is to evaluate the effect of a deep cryogenic treatment (DCT) on the wear behavior and on the microstructure of an aluminum alloy. In order to compare the level of improvement on the wear resistance provided by the DCT with a more traditional technique, a test matrix which included DCT, CrN coated specimens, and combinations of both modification methods was conducted.The wear behavior was investigated using microabrasive wear tests.Thecryogenic treated specimens proved to have similar lowwear rates as the specimens coated with CrN. The most distinct improvementwas reachedwith a combination of both techniques. In the case of the DCT, the performed microstructural analysis identified the generation of additional GP-zones as the reason for the improved wear resistance

A notch methodology to estimate fretting fatigue strength

The aim of this work is to propose a methodology to estimate the fatigue limit of cylindrical contacts under a partial slip regime. Taylors point stress method, usually applied to estimate fatigue limit for notched structures, was associated with the Modified Wöhler Curves to define the fretting crack initiation thres hold methodology. Twenty-nine tests on cylindrical contacts were selected from the literature and considered to evaluate the quality of the estimates. The results agree well for twenty-three experimental data. As the fatigue limit under fully reversed bending is the fatigue parameter usually available for most metallic alloys, it was also showed how the second fatigue limit needed to calibrate the proposed procedure could be estimated by taking full advantage from other standard predictive methodologies previously devised to estimate the mean stress effect under uniaxial fatigue loading

The recycling of sugarcane fiber/polypropylene composites

Mechanical recycling is utilized to reuse waste and obtain other plastic products via the reprocessing of a material in industrial equipment. Natural fiber composites have become more popular in recent years; however, these composites' mechanical behavior remains less well-understood than single polymers’ behavior after recycling. Therefore, the objective of this work was to study the degradation of different sugarcane fibers/polypropylene composites using new grinding and injection processes and to evaluate the mechanical properties of these materials using analysis of variance (ANOVA). This work reveals the mechanical behaviors of recycled natural fiber composites that contain thermal stabilizer additives. Polypropylene composites reinforced with differently treated bagasse and straw fibers (10 and 20 wt%/PP) were obtained through melt mixing using a high-intensity thermokinetic mixer and were subsequently injected. The recycled composites exhibited decreased tensile strength relative to the original composites. However, when thermal stabilizers were added, the mechanical properties were maintained or increased, depending on the fiber and additive types

Análise comparativa da resistência à fadiga por fretting entre os cabos condutores feitos de liga de alumínio 900 MCM e 823 MCM

Com a necessidade de aperfeiçoamento do processo de transmissão e distribuição de energia elétrica, as indústrias e concessionárias desse setor estão em busca de componentes mais otimizados para suas linhas de transmissão. Em consequência, há a necessidade de aprimorar a formação do cabo condutor, que é o elemento mais oneroso das linhas elétricas. Nestas circunstâncias, condutores formados apenas por fios de alumínio aparentam ser mais vantajosos em relação aos cabos com alma de aço, obtendo melhores índices de ampacidade/peso, diminuindo assim, o custo de transmissão. Neste cenário, o objetivo deste artigo, foi verificar a influência do fenômeno de fadiga por fretting sob condições controladas, nos fios de liga de alumínio AA 6201 T81 e AA 1120 H19, que formam os condutores CAL 900 MCM e CAL 823 MCM, respectivamente. Para alcançar tal fim, criou-se um programa experimental capaz de adquirir os dados sobre a resistência à fadiga dos cabos condutores e dos fios de forma separada, a fim de analisar a influência do fretting sob o parâmetro de carga normal de contato de 750 N. Ao examinar os resultados, o condutor CAL 900 MCM obteve a menor resistência à fadiga, apesar de ser constituído por uma liga com melhores propriedades mecânicas. O mesmo resultado foi obtido nos ensaios de fadiga por fretting nos fios, deste modo, pôde-se inferir que a liga AA 6201 T81, embora possua melhor resistência à fadiga, ela é mais suscetível a falha na presença de fretting quando comparada a liga AA 1120 H19.Palavras chaves: fadiga por fretting em fios, contato entre fios de alumínio liga AA 1120 H19, liga AA 6201 T81.

Conductividad y evaluación microestructural de Electrolitos sólidos SGDC sintetizados por Pechini y precipitación controlada

In this work, controlled precipitation synthesis methods and polymeric precursors (Pechini) were used to obtain gadolinium-doped cerium oxide solid electrolytes (Ce0.8Gd0.2O1.9) and double-doped cerium oxide with gadolinium and samarium (Ce0.8Gd0.2−xSmxO1.9 x = 0.01; 0.03 and 0.05). The main results indicate that the synthesized powders by the Pechini method present better microstructural characteristics such as larger specific surface area and smaller particle size. Test specimens were compacted (187 MPa) and sintered (1500 °C/6 h) and subsequently characterized. Through the impedance spectroscopy analysis, an increase in the percentage of the samaria caused an increase in the grain boundary and grain conductivity in all the samples under study. In the grain interior, samples without samaria showed heterogeneous regions, where the cation-vacancy pairs are tightly bound, precluding these vacancies from participating in ion conduction. In the samples with samaria, these defect groups are smaller, indicating structural homogeneity, resulting in greater ionic conductivity. Grain boundary specific conductivities show slight increase for samples codoped with samarium oxide. This effect suggests that Sm3+ additions change the charge equilibrium in space-charge layer favoring better dopant distribution.En este trabajo se utilizaron los métodos de síntesis precipitación controlada y precursores poliméricos (Pechini) para obtener electrolitos sólidos de ceria dopada con gadolinio (Ce0.8Gd0.2O1.9) y óxido de ceria doblemente dopada con gadolinio y samario (Ce0.8Gd0.2−xSmxO1.9 x = 0.01; 0.03 and 0.05). Los principales resultados indicaron que los polvos sintetizados por el método Pechini presentaron mejores características microestructurales: mayor área de superficie específica y menor tamaño de partícula. Los polvos fueron compactados (187 MPa), sinterizados (1500 °C/6 h) y, por fin, caracterizados. Con análisis de los espectros de impedancia se observó que el aumento del porcentaje de samario provocó aumento de la conductividad del grano y del contorno de grano en todas las muestras en estudio. En el interior del grano las muestras sin samaria mostraron regiones heterogéneas (grupos de defectos) donde pares catión-vacancia están estrechamente unidos, lo que impide su participación en la conducción iónica. En las muestras con samaria, estos grupos de defectos son más pequeños, indicando homogeneidad estructural, obteniendo una conductividad iónica mas alta. La conductividad específica del límite de grano muestra un ligero aumento para las muestras codopadas con óxido de samario. Este efecto sugiere que las adiciones de Sm3+ cambian el equilibrio de carga en la capa de carga espacial favoreciendo una mejor distribución del dopante

A conserved role for Syntaxin-1 in pre- and post-commissural midline axonal guidance in fly, chick, and mouse

Axonal growth and guidance rely on correct growth cone responses to guidance cues. Unlike the signaling cascades that link axonal growth to cytoskeletal dynamics, little is known about the crosstalk mechanisms between guidance and membrane dynamics and turnover. Recent studies indicate that whereas axonal attraction requires exocytosis, chemorepulsion relies on endocytosis. Indeed, our own studies have shown that Netrin-1/Deleted in Colorectal Cancer (DCC) signaling triggers exocytosis through the SNARE Syntaxin-1 (STX1). However, limited in vivo evidence is available about the role of SNARE proteins in axonal guidance. To address this issue, here we systematically deleted SNARE genes in three species. We show that loss-of-function of STX1 results in pre- and post-commissural axonal guidance defects in the midline of fly, chick, and mouse embryos. Inactivation of VAMP2, Ti-VAMP, and SNAP25 led to additional abnormalities in axonal guidance. We also confirmed that STX1 loss-of-function results in reduced sensitivity of commissural axons to Slit-2 and Netrin-1. Finally, genetic interaction studies in Drosophila show that STX1 interacts with both the Netrin-1/DCC and Robo/Slit pathways. Our data provide evidence of an evolutionarily conserved role of STX1 and SNARE proteins in midline axonal guidance in vivo, by regulating both pre- and post-commissural guidance mechanisms