جستجوی مقالات مرتبط با کلیدواژه "چقرمگی شکست" در نشریات گروه "مهندسی شیمی، نفت و پلیمر"

Extensive efforts have been made by scientists and researchers in polymer science tounderstand the mechanism underlying the failure of polymer materials. Complexbehavior of polymeric materials compared to other conventional materials (metals andceramics) makes it difficult to study this behavior. The fracture of polymeric materials,which is classified into brittle or ductile, also depends on the mechanism governing theirfracture. From the material science point of view, the fracture mechanics of polymermaterials discusses the relationships and equations that govern fracture. These relationshipslead to a deeper understanding and obtain the values of stress concentration, criteria, andmodes of fracture and express their relationship to the stress intensity index. However,from the point of view of polymer science, not only the mechanics of fracture, but also themechanisms governing fracture are of great importance. The three mechanisms of cracking,shear yielding and cavitation are described as the mechanisms governing the fracture ofpolymeric materials. Understanding which mechanism governs a polymer piece requires athorough understanding of that piece as well as the operating conditions. In this paper, themechanics of fracture and the mechanisms that govern it are discussed, as well as how totransfer the mechanism to another one.

An epoxy resin was modified by silica nanoparticles and cured with an anhydride. The particles with different batches of 12, 20, and 40 nm sizes were each distributed into the epoxy resin ultrasonically. Electron microscopy images showed that the silica particles were well dispersed throughout the resin. Tensile test results showed that Young’s modulus and tensile strength increased with the volume fraction and surface area of the silica particles. The simultaneous use of two average sizes of 20 and 40 nm diameter silica particles still increased these mechanical properties but other combinations of silica particles were unsuccessful. A three-point bending test on each pre-cracked specimen was performed to measure the mode I fracture toughness energy. The fracture energy increased from 283 J/m2 for the unmodified epoxy to about 740 J/m2 for the epoxy with 4.5 wt% of 12 nm diameter silica nanoparticles. The fracture energy of smaller particles was greater because of their higher surface to volume ratio. The fracture energy results showed also that the combined nanoparticles has a synergic effect on the fracture toughness of nanocomposites. Simultaneous use of 10 and 20 nm particles increased the fracture energy to about 770 J/m2. Finally, crack-opening displacement was calculated and found to be in the range of several micrometers which was much larger than the sizes of particles studied. Thus, the toughening mechanisms of crack pinning and crack deflection have a negligible effect on improvement of toughness, nevertheless, the plastic deformation and plastic void growth are dominant mechanisms in epoxy toughening by nanoparticles.

Thermoset polymers have excellent properties of high dimensional stability at elevated temperatures, low creep and good resistance against solvents due to their three-dimensional crosslinking structure. Proper thermal-mechanical properties for these polymers are based on such cured structures with high crosslink density. Due to its excellent mechanical properties, epoxy resin has been widely used in coatings, adhesives and composites. In recent years, a variety of modification procedures have been introduced for toughening of cured resin structure in order to minimize crack formation and improve their impact resistance. Chemical modification of epoxy resins with polyurethanes is one of the most efficient procedures to achieve this goal. Polyurethanes have unique properties including good abrasion resistance, ease of processing and high rupture strength. After a brief introduction of epoxy and polyurethane resins, the procedures and the mechanisms of fracture toughness in polyurethane-modified epoxy are reviewed in present article. Different polyurethanes with wide range of chemical structures are able to toughen the epoxy structure efficiently and provide improved resins for use indifferent applications.

Epoxy resins as one of the most important thermoset materials have shown various applications in production of composites، adhesives and surface coatings. Due to formation of the network structure، the cured epoxy resin shows brittleness and low impact resistance. The mechanical properties in the epoxy resin (especially the fracture toughness) can be modified by adding a softer phase preferably the rubber phase، thermoplastic fillers and even rigid particles. In comparison with other methods، the inclusion of rigid particles into epoxy resin improved the fracture toughness of epoxy resin without sacrificing the other main properties of epoxy resin such as the modulus (E) and glass transmission temperature (Tg). In recent years، the introduction of nanotechnology has shown some possibilities in toughening improvement of nanomaterials. Due to the special properties and different types of nanoparticles، a review study on effective nanoparticles would be useful in utilizing nanomaterials in toughening of the epoxy resin.

Fast fracture is one of the major failure modes in brittle polymers. Generally, crack growth in these polymers may occur under combined tensile-shear loading. However, the fracture test data reported previously for some dental restorative resins can not be predicted by using the available fracture criteria.In this paper, a modified fracture criterion is used for estimating the fracture load for the mentioned resin samples. It is shown that by considering a more accurate description for the crack tip stresses and also by accounting for the craze zone effects in front of the crack tip, the modified criterion presents significantly better predictions for the fracture resistance in such brittle polymers.