This work is focused on a creep formulation in bending for timber beams, of glue laminated (glulam) type, which is suited to be\ud implemented in both geometrical and material non-linear finite element method (FEM) models. It studies the performance of timber beams by accounting the deformation under increments of time and stress in the long term. The proposed creep formulation uses a power-law function expressing the creep strain by means of one coefficient and two exponents. This function needs a fewer number of material coefficients than a classical mathematical analysis of non-linear viscoelasticity, so design applications, such as FEM codes, may more simply describe the deformations in the long term. The formulation has been developed for cantilever beams made of glulam timber using experimental tests in bending and accounting the relations between strains, and both time and tension progresses. Also, the timber creep law has been numerically tested by FEM code runs, with large displacements and large strains assumptions, and further compared with a Navier analytical model. The scatters between experimental and analytical values, and FEM numerical values have reached a standard deviation of 4.06 and of 7.66%, respectively. The acceptable agreement in the comparison between experimental, analytical, and FEM numerical results have demonstrated the capability of the creep formulation to model the behavior in bending of a glulam timber. Also, the proposed creep model has shown an interesting ability to be employed in a FEM code to analyze a timber beam in bending, along time and load evolutions
The market for recycled thermoplastic polymers is undergoing a sharp increase, although recyclate polymers are regarded as materials with inferior properties when compared to the virgin material. This study investigates the behavior of non-contaminated scraps of low density polyethylene (LDPE) from the plastic packaging industry after single processing stage, in terms of the productivity and the thermal and rheological properties. The LDPE used was reprocessed on a single-screw extruder (Miotto) of 90 mm screw diameter and L/D = 25. Three screw speeds (80, 90 and 100 min−1) were investigated at three processing temperatures (200, 250 and 300°C). The reprocessed materials were characterized by thermogravimetric analysis TGA, parallel plates rheometry and productivity. The results obtained by TGA showed a typical range of polyolefin degradation (350 to 450°C). The viscoelastic properties did not show significant changes in relation to the rheological behavior. Increasing the temperature and screw speed promoted a productivity gain of approximately 30%. This suggests that under the conditions studied it is possible to reprocess this material with good productivity ensuring its thermal, mechanical and rheological properties.
Resumo: O mercado de reciclagem para os polímeros termoplásticos encontra-se atualmente em acentuada ascensão. Os materiais reciclados são vistos como materiais de propriedades inferiores em relação ao material virgem. O presente trabalho avaliou as características viscoelásticas e térmicas do polietileno de baixa densidade (PEBD) reciclado. As amostras foram reprocessadas até dez vezes em condições extremas de processamento (300 °C/80 rpm) em uma extrusora monorosca, a fim de avaliar modificações estruturais nas suas propriedades. Foram realizadas análises de reometria oscilatória de placas paralelas e calorimetria exploratória de varredura (DSC). A partir das análises de reometria oscilatória foram calculados espectros de relaxação e retardação pelo programa de regularização não-linear (NLREG) e através das análises de DSC no primeiro aquecimento foram calculados os parâmetros cinéticos pelos métodos de Avrami e Freeman-Carroll. Os resultados do estudo reológico demonstraram que as amostras reprocessadas acima de quatro vezes apresentaram aumento da viscosidade complexa e dos módulos de armazenamento e perda, além de fenômenos de relaxação e retardação mais largos. Entretanto, os termogramas de DSC e os parâmetros cinéticos de fusão demonstraram que o PEBD estudado manteve sua estabilidade térmica, independentemente da modificação de suas características viscoelásticas. Palavras-chave: PEBD, reciclagem, reprocessamento, viscoelasticidade, cinética. Study of the Reprocessing of Low Density Polyethylene (LDPE) Recycled from Extruded Blown FilmsAbstract: The recycling market of thermoplastic polymers is currently in a sharp increase. Recycled materials are regarded as having inferior properties compared to the virgin material. Here we investigated the viscoelastic and thermal properties of the recycled low density polyethylene (LDPE). The samples were reprocessed up to ten times under extreme processing conditions (300 °C/80 rpm) in a single screw extruder in order to evaluate structural changes in their properties. Analyses were performed in an oscillatory rheometer with parallel plates and in a differential scanning calorimeter (DSC). From the analysis of the oscillatory rheometry, relaxation and retardation spectra were obtained upon applying a non-linear adjustment using a computational program (NLREG), while the kinetics parameters were estimated from the results of the first DSC heating using the Avrami and Freeman-Carroll methods. From the rheological study we inferred that the samples reprocessed more than four times had an increase in complex viscosity, storage and loss moduli, also displaying wider relaxation and retardation phenomena. However, the DSC thermograms and the melt kinetic parameters showed that the LDPE studied maintained their thermal stability, regardless of the modification in their viscoelastic properties. Keyworks: LDPE, recycling, reprocessing, viscoelasticity, kinetic. IntroduçãoOs polietilenos (PE) são materiais commodities amplamente utilizados. Para a reciclagem destes polímer...
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