Viscoelastic behavior of HDPE polymer using tensile and compressive loading

Large containers for liquids, exposed to different static loadings, are mainly made of high-density polyethylene (HDPE). The viscoelastic response of HDPE under long-term tensile and compressive creep is investigated. Monotonic experiments under tension are performed over a wide range of strain rates. In these experiments, the transition in the damage mechanisms (development and propagation of contraction in the HDPE specimen) is analyzed. The monotonic tensile behavior of the HDPE is found to be nonlinear and depends on the strain rates. It is observed that both elastic modulus and plastic flow stress present an increase with displacement speed due to the viscoelastic behavior of HDPE. A similar observation can be made for monotonic compressive tests by developing a new experimental device that ensures accurate measurement of the strain. Such a device makes use of an extensometer of compressive displacement of the specimen. In addition, the long-term behavior of HDPE is evidenced through creep and relaxation tests at an imposed range respectively of lower stresses and strains. It is shown that the normalized curves, associated with these tests, can be represented by a single curve characterizing the compressive creep compliance or relaxation stresses versus time. The linearity of the viscoelastic behavior is confirmed within the linear domain of the monotonic compressive and tensile tests.     

Fe modelling of CFRP-concrete interface subjected to cyclic temperature,humidity and mechanical stress

Modelling of environmental effects on the bond surface between FRP composites and concrete is crucial for the purpose of life prediction of strengthened members. The service performance of composite systems depends on the degree of chemical and physical aging of the system. Physical aging is a reversible process which is related mainly to temperature fluctuations. Chemical aging is the permanent molecular level degradation of fibres, resin or interfaces due to long-term exposure to the environment. The resulting strength reduction and the system behaviour due to aging are important in evaluating the strengthening of civil engineering infrastructure. In this paper, a finite element model is developed to predict the interface properties after long-term exposure to the cyclic temperature and the constant humidity. Effects of sustained loading are also taken into account. The predicted strength, cracks pattern, failure mode and strain variation along the bond line agree well with test results.     

Mechanical characterization of steel/CFRP double strap joints at elevated temperatures

This paper examines the mechanical performance of steel/CFRP adhesively-bonded double strap joints at elevated temperatures around the glass transition temperature (T_g, 42 °C) of the adhesive. A series of joints with different bond lengths were tested to failure at temperatures between 20 °C and 60 °C. It was found that the joint failure mode changed from adherend failure to debonding failure as the temperature approached T_g. In addition, the ultimate load and joint stiffness decreased significantly at temperatures near to and greater than T_g, while the effective bond length increased with temperature. Based on the ultimate load prediction model developed by Hart-Smith for double lap joints and kinetic modelling of the mechanical degradation of the adhesive, a mechanism-based model is proposed to describe the change of effective bond length, stiffness and strength degradation for steel/CFRP double strap joints at elevated temperatures. The modelling results were validated by the corresponding experimental measurements.     

Calculation of Photo-Ionisation Cross Sections and Radiative Recombination Rate Coefficients for CO and CO+ Molecules

A method based upon the weighted total cross section （WTCS） theory is proposed to calculate the photo-ionisation cross sections and the radiative recombination rate coefficients between the fundamental level of CO and the main electronic states of its corresponding ion. Total photo-ionisation cross sections and radiative recombination rate coefficients are determined from the calculation of elementary vibrational photo-ionisation cross sections. Transitions be- tween CO＋（X, A and B） and CO（X） are considered. Total photo-ionisation cross sections and recombination coefficients are computed in the temperature interval 500-15000 K.     

Fermentation of date palm juice by curdlan gum production from Rhizobium radiobacter ATCC 6466™: Purification, rheological and physico-chemical characterization

The present study was undertaken to investigate the possibility of using date palm juice byproducts for curdlan production by Rhizobium radiobacter ATCC 6466™ in batch experiments. A number of operational parameters, namely pH value, temperature range, inoculum ratio, agitation speed, carbon concentration, nitrogen source, and fermentation time, were investigated in terms of their optimal values for as well as individual and synergistic effects on curdlan production. The findings indicated that the strain exhibited a high ability to use the natural substrate under investigation. A curdlan production yield of 22.83 g/l was obtained in 500-ml agitated flasks (50 ml) when the strain was cultivated in the optimal medium (pH, 7; ammonium sulphate concentration, 2 g/l; date glucose juice concentration, 120 g/l) operating at 30 °C with an inoculum ratio of 5 ml/100 ml, an agitation speed of 180 rpm, and a fermentation period of 51 h. The purified date byproducts-curdlan (DBP-curdlan) had a molecular weight of 180 kDa, a linear structure composed exclusively of β-(1,3)-glucosidic linkages, a melting temperature (T_m) and glass transition temperature (T_g) of 1.24 and −3.55 °C, respectively. The average measured heights of its molecules were noted to fluctuate between 14.1 ± 0.07 and 211.73 ± 0.6 μm.     

Investigation of CNT modification of epoxy resin in CFRP strengthening systems

The use of carbon fiber‐reinforced polymers (CFRPs) to reinforce old structures has become popular in recent years. In this study, the chemical structure of the epoxy resin used as the bonding agent in the CFRP strengthening system was modified by dispersing multi‐walled carbon nanotubes (MWCNTs) in order to improve the performance of the strengthening system. Composites were fabricated with different mixing orders employing the solvent‐assisted dispersion method and ultrasonic mixing. Thermogravimetric analysis, dynamic mechanical analysis, and tensile tests were conducted to investigate the effect of CNT dispersion and fabrication method on the thermal and mechanical properties of epoxy composite. In addition, the temperature‐dependent tensile behavior of fabricated composites was studied by performing tensile tests at elevated temperatures. The morphology of CNT/epoxy composites was characterized using scanning electron microscopy (SEM). Fourier transform infrared (FTIR) was also used to show the influence of solvent on the molecular structure of composites. Based on the experimental results, the decomposition temperature of the epoxy resin was heightened by 15°C as a result of solvent‐assisted dispersion of nanotubes. However, the glass transition temperature (T_g) was slightly reduced due to the solvent effect. FTIR analysis revealed that the solvent negatively affects the curing process of epoxy composite. A considerable enhancement was recorded in the tensile properties as a result of CNT infusion. This was attributed to the homogeneous dispersion of nanotubes which was shown by SEM images. Using solvent to disperse nanotubes led to the reduction of tensile strength of the epoxy composite at elevated temperature due to the lower T_g. POLYM. COMPOS. 37:1021–1033, 2016. © 2014 Society of Plastics Engineers