Micromechanics analysis of particulate-reinforced composites and their failure mechanisms

Stress fields and failure mechanisms have been investigated in composites with particles either surface treated or untreated under uniaxial tension. Previous experimental observation of failure mechanisms in a composite with untreated particles showed that tensile cracks occurred mostly at the polar region of the particle and grew into interfacial debonding. In a composite with surface-treated particles, however, shear yielding and shear cracking proceeded along the interphase-matrix interface at the polar area of the matrix and thus may improve the mechanical behaviour of the material. The finite element calculations showed that octahedral shear stress at the polar and longitudinal areas of the particle treated by coupling agents is much larger than that of materials with untreated particles, and the shear stress distribution around the interface is sensitive to the interphase property. The results suggest that a th ree-phase model can describe the composites with surface-treated fillers.

The interlaminar fracture-behavior and toughening mechanisms of new carbon-fiber-reinforced bismaleimide composites

The interlaminar fracture behaviour of carbon fibre-reinforced bismaleimide (BMI) composites prepared by using a new modified BMI matrix has been investigated by various methods. Laminates of three typical stacking sequences were evaluated. Double cantilever beam, end-notch flexure and edge-delamination tension tests were conducted under conventional conditions and in a scanning electron microscope. The strain energy release rates in Mode I and Mode III G(lc) and G(llc), as well as the total strain energy release rate, G(mc), have been determined and found to be higher than those for laminates with an epoxy matrix. Dynamic delamination propagation was also studied. The toughening mechanisms are discussed.

Mixed-mode fracture mechanisms near the fatigue threshold of aisi-316 stainless-steel

Near threshold, mixed mode (I and II), fatigue crack growth occurs mainly by two mechanisms, coplanar (or shear) mode and branch (or tensile) mode. For a constant ratio of ΔK_I/ΔK_II the shear mode growth shows a self-arrest character and it would only start again when ΔK_I and ΔK_II are increased. Both shear crack growth and the early stages of tensile crack growth, are of a crystallographic nature; the fatigue crack proceeds along slip planes or grain boundaries. The appearance of the fracture surfaces suggest that the mechanism of crack extension is by developing slip band microcracks which join up to form a macrocrack. This process is thought to be assisted by the nature of the plastic deformation within the reversed plastic zone where high back stresses are set up by dislocation pile-ups against grain boundaries. The interaction of the crack tip stress field with that of the dislocation pile-ups leads to the formation of slip band microcracks and subsequent crack extension. The change from shear mode to tensile mode growth probably occurs when the maximum tensile stress and the microcrack density in the maximum tensile plane direction attain critical values.

Accountability na outorga e renovação de contratos de radiodifusão : aumenta a demanda por participação social

A outorga e renovação de concessão, permissão ou autorização de serviço de radiodifusão sonora e de sons e imagens é um conjunto de decisões políticas do Poder Público que está no cerne da questão ou da problematização da comunicação no Brasil. O modelo adotado no Brasil desde cedo concentrou o poder concedente no Executivo Federal. Além de não haver uma forte accountability institucional, a população não é consultada durante o processo e não há mecanismos estabelecidos de fiscalização e controle social sobre o serviço prestado. Esse estudo tem por finalidade oferecer elementos para que se fortaleça a accountability, notadamente a social, para o exame das concessões à luz dos capítulos da comunicação na Constituição Federal. Levanta-se, como hipótese, a possibilidade de que seja falsa a dicotomia participação social versus liberdade de manifestação e de imprensa. A excessiva centralização ou a falta de participação social na outorga e renovação conduz a uma associação entre o poder concedente e os concessionários, permissionários e autorizados na radiodifusão. Os mecanismos de accountability multiplicar-se-iam com o que é chamado aqui de popularização do poder concedente e do poder concedido. E desses mecanismos poderia se servir o poder público ao examinar a eficiência e a eficácia dos "proprietários" da radiodifusão.