جستجوی مقالات مرتبط با کلیدواژه « پلاستیک » در نشریات گروه « مکانیک »

Food packaging ensures long-term storage and protection of all types of food against physical, chemical and biological factors. In some food packaging, one of the purposes of packaging is to keep its contents away from oxygen and external contaminants. Today, it is an important part of the materials used to package food from petroleum products. This has caused many environmental problems. One of the proposed solutions for this issue is the use of materials with photocatalytic activity along with synthetic (petroleum) polymers. Minerals that have this property include titanium dioxide and zinc oxide. These light-sensitive materials are excited by receiving light. Hence, a hole (h*) in the valence band and an electron (e-) in the conduction band of these semiconductor compounds are created. The electron-hole pair leads to the production of active radicals which are responsible for attacking the plastic polymer chains. Consequently, changes in weight, mechanical properties, and plastic morphology are observed by realizing carbon dioxide and carbonyl groups. These changes demonstrate the decomposition of petroleum based polymers in the environment.

The packaging and design on the box, depending on the type and nature of the product, can greatly affect the sales of these products such as cakes, cookies, biscuits, crackers, etc.Packaging materials include flexible films with offering moisture-proof properties and heat sealability, paper, trays, cartons and corrugated paper contributing to the moisture barrier properties of the pack, tins, cartons and cases outside the moisture-proof pack, and shrink wrapping. Regenerated cellulose and plastic based (usually polypropylene) films are used as heat sealable and moisture-proof packaging films. Regenerated cellulose films are available in various thicknesses ranging from about 0.06-1.5 mm and the 0.08 mm is most commonly used for biscuit packaging. Polypropylene is a naturally good moisture barrier and its strength is good to achieve similar performance to coated cellulose films; it can be used at much thinner gauge at competitive prices. Aluminum foil is used in packaging as amultilayer coating with waxed paper, polyethylene or polypropylene. The film is a metallized, roll or polymer sheet which is coated with a thin layer of metal (usually aluminum). This metallic and shiny product is in the form of aluminum foil with lower cost and weight. This paper introduces the materials used in the packaging cakes, cookies, biscuits, crackers, etc., their specification, advantages, disadvantages, and labeling criteria.

The term “migration” refers to the diffusion of substances from a zone of higher concentration (the food-contact layer) to one of a lower concentration (usually the food surface). Diffusion of chemical substances from polymers is a very complex process, and is dependent on several parameters, such as concentration of substances in the packaging film and food, the nature of the food, temperature, and the time period over which duration of contact occurs. The likelihood of monomer and oligomer migration increases when a plastic is exposed to high temperatures during thermal processing or when food is stored for extended periods. Transfer of chemical compounds from plastics to food has raised concerns about the potentially adverse effects of food products on human health. Appropriate analytical methods are needed to determine migration from plastic packaging into food. Regulatory agencies have specified analytical methods for some migrant materials. A number of gas chromatography methods have been used for various plasticizers. The first step in analyzing the antioxidant migration is to determine the level of primary and secondary antioxidants in the plastic. High pressure liquid chromatography with a light-sensitive diode detector was used to analyze the antioxidants. Determination of styrene monomer in food and food simulators using common gas chromatography methods. For example, in one study, an HP-FFAP column for Styrene and a Rtx-5 column for oligomers were used. This study is a comprehensive review of chromatographic methods for the detection of polymer materials from migratory materials.

Sheet Metals are widely used in different industries such as ship building. One important subject in these industries is to create the desired sheets through line heating process. In this paper, at first, the simulation of heat transfer between a gas torch and a plate during the line heating process is investigated. Impingement jet model is used to simulate the effect of a heat source (flame) and air cooling on the plate by using the commercial engineering software, FLUENT. Then, the computed temperature distribution by FLUENT is fed into the ANSYS FEM package for thermo Elasto-Plastic deformation analysis and the results are validated. Process execution needs heat paths and heat conditions. For this purpose heat paths of the cylindrical shape was obtained based on the Strain-Based Method. For thermal conditions a neural network was trained. In this regard, close to 63 different situations in different powers and torch speeds were run. Finally, to verify the thermal characteristics obtained for the cylindrical shape, paths and thermal conditions obtained was passed on a flat sheet metal by simulation and the result was compared with the desired shapes. It was shown that the Strain-Based Method in determining the thermal paths is very practical.

For ductile fracture, the toughness can be measured as a single parameter value or in a resistance curve format (J-R curve) and is often characterized by the J-integral for elastic-plastic materials. Because of their effectiveness in measuring toughness, the J-integral and J-R curve have become the most important material parameters in elastic-plastic fracture mechanics, and have been applied widely in practical engineering. Polymeric materials are widely being used for load-bearing structural applications and, therefore, understanding of their fracture properties is becoming more important. In this study, mixed-mode I/II stable crack growth experiments were carried out on a widely used polymeric material, polypropylene, using recently modified fixture. Multi-specimen R-curve method were used for obtaining J-R curves of different states of mixed-mode loading conditions from pure mode-I to pure mode-II by varying the loading angle by 15° steps accordance with the standard ASTM-D6068 and then the resulting R-curves have been evaluated to determine the values of initiation toughness, JIC, following the schemes of the E813 and E1820 standard procedures. Finite-element analyses were done by ABAQUS and mode-I and mode-II non-dimensional stress intensity factors and geometric work factors of elastic-plastic fracture were obtained for different conditions. Results show that for this material the value of JIC is much more than the value obtained for the JIIC. This material also exhibited a greater resistance to ductile crack growth in mode-I.

When a dynamic load passes a control volume of material as a shock wave, passing this wave through the control volume could cause different phases such as elastic and plastic. From the microscopic view, during phase change, material flow would be taken in control volume which includes mass, heat, energy, and momentum transport. Phase change in material causes a material discontinuity in the control volume. During the phase change process, mass, heat, energy, momentum transport and etc will occur and the equations governing these phenomena are called transport equations. In this article, for the first time, the governing equations of elastoplastic behavior of beam under dynamic load are extracted by using mass, energy and momentum transport equations. Using transport equations with non-physical variables in integral form will cause in employing discontinuity conditions in governing equations and eliminates the discontinuity condition. These equations are also used in continuously modeling of beam elastoplastic behavior under dynamic loading and a continuous model is presented. Finite element method is used to solve the transport equation with non-physical variable. Finally, the time history of stress, strain and velocity wave propagation along beam are presented in elastic and elastoplastic phases.

An appropriate joining technology is the laser beam welding process, because of its low localized energy input leading to low distortion, high strength of the joint and high processing speeds. The growing of aircraft industry in reducing the weight of aerospace structure has led the introduction of laser beam welding into the fabrication of aerospace structure with stiffeners, instead of riveted joints. In the present paper an analytical model for calculation of the residual stresses induced by laser welding is proposed and numerically validated. The aim of this work is to study the transverse residual stress distribution of plates made of an aluminum alloy 6061-T6, which is used for fabrication of fuselage panels. The results show that, if the maximum stress amount induced in the workpiece does not exceed the material yield stress, no plastic penetration will occur and there will be no residual stress. It was found that high stresses are developing close to the plate surface while these stresses decrease rapidly to almost zero values at the lower surface. An ideal elastic-plastic material curve was assumed. Good accordance is found between the calculated and numerical results.

In this paper, elastic-plastic buckling of a thick rectangular plate has been investigated based on both Incremental (IT) and Deformation (DT) plasticity theories. Uniform biaxial edge traction was assumed as the plate loading while simply supported as the boundary conditions. Integral uniqueness criterion has been minimized to determine the critical buckling traction. Based on Rayleigh-Ritz method, a linear combination of polynomial base functions, which satisfy the geometrical boundary conditions, has been used as the trial functions for rotations and transverse deflection. To validate the analysis, the results for the Mindlin plate theory have been compared with the previously published results and a very close agreement has been observed. Then the effects of thickness ratio, aspect ratio and also different biaxial traction ratios on the buckling traction have been investigated. The results show that for the problem considered here, very close critical buckling traction is predicted by the both Mindlin and sinusoidal plate theories. This implies that Mindlin plate theory is sufficiently accurate to predict critical buckling traction in this problem. Moreover when the loading is gradually changed from biaxial into uniaxial compression or when the thickness-ratio is increased, the difference between the two theories is also increased. Also for compression-tension loading case, the critical buckling traction predicted by deformation theory is much less than the incremental theory.

Axial compression behavior of foam materials can be explained by two ideal deformation scenarios: discrete crush band process and progressive collapse. In this paper، a perfectly new model for strength assessment and quantitative/qualitative description of one-dimensional progressive collapse of aluminum foams under impulsive loadings is presented and its capability to split this way of crushing into two distinct regimes of shock wave and elastic- plasic wave propagation is highlighted. Then، using conservation relations and the new introduced model، the analytical solution of dynamic deformation of aluminum foams in the two mentioned regimes is developed. Regime 2 considers the case when the crushing front velocity is lower than the linear sound velocity of the foam; but remains higher than the effective sound velocity for a perturbation in which the amplitude lies in the so-called “plateau region’ of the static stress-strain diagram. The physical difference between this regime and the fiest one entails not only the creation a shock front associated with the collapsing foam، but also an acoustic precursor in the case of second regime. Finite element simulation is also performed to validate the analytical procedure. The numerical prediction is found to be in very good agreement with the analytical results.

The sheet bending on elastic foundation is a relatively new method in recent years. In this work, the plastic deformation of sheet is studied. The governing differential equations for the sheet deformation zones were obtained. Then, the equations were analytically-numerically solved for a given problem and the effect of indentation depth on the sheet bending radius was studied. A relationship was obtained to correlate the indentation depth to the sheet bending radius. This relationship can be used in controlling the process and also in process planning. The results were verified by a set of experiments.

In this paper, the mean value of strain energy density criterion has been used to predict the critical fracture load in specimens with U-shaped notch under bending loads. An equation for calculation of averaged strain energy density in this zone has been presented. Bending loads under mode I have been considered. Two cases have been studied: 1) the control volume only includes the semicircular arc of the notch root, and 2) the control volume includes rectilinear edge of the notch. The effect of notch root radius and notch depth on the mean value of strain energy density has been studied in two cases above and the critical fracture load rate has been predicted. Finite Element Analysis has been carried out for verification. The studies show that the critical fracture load increases and then decreases by increasing the notch root radius. Also, this parameter decreases exponentially by increasing the notch depth.

Predicting the status of contact between the pipe and the roll through deformation is considered as one of the problems of reshaping process in non-circular pipe production. In the present study, the elastic-plastic analysis is investigated assuming plane strain condition for reshaping process. One of the problems of the mentioned process is the separation of pipe from roll in some zones. The proposed model has the capability of predicting the separation of the pipe from roll, which results in better prediction of the defect. To verify the reliability of the present contact model, the result of the analysis was compared to those of experimental ones and showed good agreements.

In this work, the elastoplastic torsion of solid sections without work hardening with variable yield stress is examined. In similar previous works with constant yield stress assumption, exact numerical or analytical approaches were utilized. In view of the new assumptions, the coupled elastoplastic differential equations were decoupled. In variable yield stress assumption, a yield stress gradient from surface to core is assumed, which is a valid and reasonable assumption in cast and heat treated parts. Thus, in elastoplastic torsion of prismatic bars with the aforementioned yield stress gradient assumption, the above mentioned method was used. To carry out the computations, a new software with coupled programming approach was used. A few examples were run both on our developed software and on an accredited finite element software (ABAQUS). Comparison of the results affirms the benefit of our approach.