Application of a modified strain transient dip test in the determination of the internal stresses of PVC under tension

A modified strain transient dip test which involves the design of a load reduction apparatus to perform rapid step unloading and extrapolating to zero extension rate has been developed to measure the internal stresses (recovery and effective stress) of PVC under uniaxial tension. This simple technique appears to be consistent with other internal stress measurement techniques. It was found that the effective stress approaches a limiting value with applied strain and an extrapolated yield point could be defined. The limiting value is a function of the strain rate during the initial load application. The general increase in applied stress (at fixed applied strain) with crosshead speed was attributed to the increase in magnitude of the effective stress. The maximum peak ratio of effective over recovery stress, at each crosshead speed, could indicate that it was the energy-dissipating part of the material that played a dominant role in the early stages of the deformation while the energy-storage part dominated the latter stages.     

Structural-mechanical and thermophysical properties of modified polyvinyl chloride

On the basis of a systematic investigation of the viscoelastic properties of polyvinyl chloride filled with waste from mineral fertilizer and phosphoric acid production in the form of highly dispersed phospho-gypsum, the specific heat is calculated and the energy of interaction of the structural elements of the system matrix is determined, together with the effective thermal conductivity coefficient, with allowance for energy dissipation by the elements of the structure.Translated from Inzhenerno-fizicheskii Zhurnal, Vol. 60, No. 6, pp. 987–994, June, 1991.     

Some studies of deformation processes in fully-spherulitic polypropylene

The deformation of isotactic polypropylene has been studied by optical microscopy of thin sections cut from bulk tensile specimens. An attempt has been made to relate the three different deformation zones within a spherulite to the cross-hatched structure peculiar to polypropylene.     

Internal and effective stresses in high-temperature creep evaluated from transient dip tests and dislocation bowing

Because creep of metals and alloys is modelled on the basis of microstructural observations, it has been shown that there is a difference between the mathematical treatment of high-temperature deformation and the real material behaviour. One idea to consider is to split the applied stress into a part depending on the substructure (the internal stress which has to be reached to start dislocation motion) and a part describing the resistance to the glide motion of dislocations (the effective stress). For ferritic chromium steel these quantities have been measured by means of the stress transient dip test technique. This leads to mean values of internal and effective stresses for the whole specimen. Additionally, local stresses acting on individual dislocations are evaluated from dislocation bowing for a wide range of applied stresses. The results show that the ratio of internal to applied stress decreases with increasing applied stresses, which, on the other hand, causes a large increase of effective stresses. Dislocation bowing stresses show a similar dependence. Compared to the results of dip tests, the determination of local stresses leads to less accurate results and to a large deviation of results within small regions of one specimen. Therefore, it is only valuable for comparison purposes.     

Finite element analysis of time-dependent inelastic deformation in the presence of transient thermal stresses

A finite element formulation for the solution of time-dependent inelastic deformation problems for metallic structures, in the presence of transient thermal stresses, is presented in this paper. A rate formulation of the equations is used and any of a number of recently proposed combined creep-plasticity constitutive models with state variables can be adopted to describe material behaviour. The computer program developed can solve planar (plane strain and stress) and axisymmetric problems. Using one of the above-mentioned constitutive models, numerical results are presented for several illustrative problems, and comparisons of results, using either the quasi-steady or the unsteady diffusion equation for the determination of the temperature field, are carried out.     

Role of recovery in high temperature constant strain rate deformation

A model based on the three-dimensional distribution of dislocations is used to delineate the role of recovery during high temperature constant strain rate deformation. The model provides a good semi-quantitative explanation for classical work-hardening as well as for high temperature work-softening resulting from rapid recovery. It predicts linear work-hardening, whereby the ratio of the work-hardening rate,H, to the shear modulus,G, is constant when a crystal is tested in the absence of recovery. The slope of the stress-strain curve, θ, for high temperature deformation is related to the low temperature work-hardening rateH; the dislocation annihilation rate , the flow stress a, the free dislocation density ρ, the strain rate , and a parameter which is sensitive to the dislocation distribution. A modified version of the Bailey-Orowan equation for simultaneous work-hardening and recovery during constant strain rate deformation which is derived from the model takes the form

Evaluation of permanent deformation characteristics of unmodified and Polyethylene Terephthalate modified asphalt mixtures using dynamic creep test

One of the major types of plastics that can be found in Municipal Solid Waste (MSW) is Polyethylene Terephthalate (PET) which is a non-biodegradable semi-crystalline thermoplastic polymer, and is considered as polyester material. Generating large amount of waste PET, mainly as bottles, would cause environmental hazards by disposing in landfills. This paper aims to evaluate effects of utilizing waste PET flakes as modifier in asphalt mixture as an alternative solution to overcome the potential risks arise from producing large amount of waste PET as well as evaluating the deformation characteristics of unmodified and PET modified asphalt mixtures. To achieve this aim, different percentages of PET were designated for this investigation, namely: 0%, 0.2%, 0.4%, 0.6%, 0.8% and 1% by weight of aggregate particles, and dynamic creep test was performed at different stress levels (300 kPa and 400 kPa) and temperatures (10 °C, 25 °C and 40 °C). Consequently, Zhou three-stage model was developed. The results showed that permanent deformation characteristics of asphalt mixture were considerably improved by utilization of PET modification, when the permanent strain was remarkably decreased in PET modified mixture compared to the conventional mixture at all stress levels and temperatures. Besides, based on Zhou model, it was concluded that elastic and visco-elastic properties of asphalt mixture were improved by application of PET modification.     

On the quantification of quenching transient thermal stresses in brittle solids using Vickers indentations

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Influence of additives on the global mechanical behavior and the microscopic strain localization in wood reinforced polypropylene composites during tensile deformation investigated using digital image correlation

The structural integrity of polypropylene (PP) matrix composites reinforced by natural wood fibers is investigated by digital image correlation (DIC) coupled with tensile tests. The use of the material as an alternative construction material requires extensive understanding of its micromechanical properties, which primarily define its performance. Addition of several additives such as coupling agents is common practice for such materials. These ingredients improve the performance of these materials mainly by improvement of the chemical and physical interactions between the nonpolar matrix and the polar wood fibers. These interactions facilitate the transfer of the applied deformation particularly in the interphase region between the polymer matrix and the reinforcing fibers. Such localized changes can influence the performance of the material specially its micromechanical behavior. The DIC via photogrammetry was used to study the spatial distribution of the accumulated plastic surface strain, which is based on pattern recognition of the surface before and after straining. The heterogeneous strain distribution reveals a structural inhomogeneity of the material. The magnitude of local strain was much higher than the global strain, suggesting preferred regions for plastic deformation formed by the microstructure.     

Antiplasticization of polyvinyl chloride in relation to crazing and fracture behaviour

Experiments were carried out to study the effects of small amounts (10 p.h.r.) of tricresylphosphate in polyvinylchloride on the crazing behaviour and fracture toughness over a wide range of temperatures. Crazing was induced by water/methanol mixtures and the critical crazing strain was measured as a function of immersion time and methanol concentration. Fracture toughness was measured on single-edge notched specimens in tension over three decades of strain rate in the temperature range −80 to 60°C. The 60 sec isochronous critical crazing strain displayed a minimum at around room temperature in an approximately parabolic fashion; the plasticized polymer exhibiting lower trough values, while the two curves intersected at both low and high temperatures. The fracture toughness curves, on the other hand, exhibited intermediate peaks, associated with β relaxations. The addition of plasticizers to the polymer reduced considerably the heights of the peaks and, once more, the two curves intersected at high temperature and merged at low temperature. From a comparison of the two sets of data, it is suggested that embrittlement due to antiplasticization is associated with a reduction in stability of the crazes as a result of the depression of β relaxations.     

A comparison of transport properties for concrete using the ponding test and the accelerated chloride migration test

In order to develop a better understanding of the relationship between 90-day salt ponding test and accelerated chloride migration test (ACMT; the electrochemical technique is applied to accelerate chloride ion migration), the transport properties for concrete obtained from ACMT are compared to the diffusion coefficient from ponding test. The plain cement concrete, fly ash concrete, and slag concrete with different w/b ratios (0.35, 0.45, 0.55, and 0.65) were used. In this study, the total chloride content and penetration depth of concretes were measured after the ponding test, and the Fick's second law of diffusion was fitted to the data from experiment to determine the diffusion coefficient. The non-steady-state diffusion coefficient, the migration coefficient, and the current corresponding to the 30000 coulomb, charge passed obtained from ACMT in the previous works were compared with the diffusion coefficients obtained from ponding test. Parallel tests show that the diffusion coefficients obtained from ponding test correspond well with the non-steady-state diffusion coefficient, the migration coefficient, and the current corresponding to the 30000 coulomb charge passed obtained from ACMT, although the diffusion coefficient measured by ponding test is different from that measured by the ACMT in non-steady state and steady state.

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Résumé Pour mieux comprendre la relation entre l'essai de trempage dans l'eau salée pendant 90 jours et l'essai accéléré de migration de chlorure (ACMT; la technique électro-chimique est employée pour accélérer lamigration des ions chlorure), les propriétés de transport du béton obtenues par l'ACMT sont comparées au coefficient de diffusion de l'essai de trempage. Du ciment simple, des cendres volantes ainsi que des scories avec des rapports eau/liant différents (0.35, 0.45, 0.55 et 0.65) ont été utilisés. Dans cette étude, la teneur totale en chlorure et la profondeur de pénétration des ciments ont été mesurées après l'essai de trempage, et la seconde loi de diffusion de Fick correspond aux données tirées, d'expériences visant à déterminer le coefficient de diffusion. Le coefficient de diffusion de l'état non stationnaire, le coefficient de migration, et le courant de charge passé correspondant à 30000 coulombs obtenu à partir de l'ACMT d'expériences précédentes ont l'eté comparés aves les coefficients obtenus à partir de l'essai de trempage. Des essais parallèles montrent que les coefficients obtenus à partir de l'essai de trempage correspondent bien au coefficient de diffusion de l'état non stationnaire, le coefficient de migration, et le courant de charge passé correspondant à 30000 coulombs après l'ACMT, bien que le coefficient de diffusion mesuré par l'essai de trempage soit différent, dans les états non stationnaire et stationnaire de celui mesuré par l'ACMT.

     

The relationship between charge passed and the chloride concentrations in anode and cathode cells using the accelerated chloride migration test

In this study the electrochemical technique is applied to accelerate chloride ion migration in cement-based material to estimate its permeability. Specimens with six fine aggregate volume fractions were cast and tested. For all test series, the chloride concentrations both in anode and cathode cells as well as the electrical current were simultaneously monitored in the accelerated chloride migration test. The chloride migration rates of anode and chloride cells were used to assess the permeability of cement-based materials. A good correlation was observed between the chloride migration rate of anode cell and cathode cell. And, a good correlation was also observed between the chloride concentration in anode cell, the chloride concentration in cathode cell and charge passed obtained from the accelerated chloride migration test.

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Résumé La technique electrochimique est appliquée pour accélérer la migration des ions chlorure dans des matériaux à base de ciment pour estimer leur perméabilité. Des échantillons avec six granulats fins ont été préparés et testés. Dans tous ces tests, les concentrations dans les cellules anode et cathode ainsi que les courants, électriques ont été simultanément suivis. Le taux de migration dans les cellules anode et cathode a été utilisé pour évaleur la perméabilité des matériaux testés. Une bonne corrélation a été observée entre le taux de migration de chlorure dans la cellule anode et dans la cellule cathode. Une bonne corrélation a aussi été observée entre ces concentrations et la charge passée obtenue de ces tests de migration de chlorure accélérés.

     

Solution of the slump test using a finite deformation elasto-plastic Drucker-Prager model

Development of a finite deformation elasto-plasticity model based on the multiplicative decomposition of the deformation gradient is presented and discussed in detail. The formulation presented in this paper includes the derivation of the full set of equations for the Drucker-Prager yield criterion. The equations, which are not available elsewhere, are developed within a framework using a spectral decomposition approach. Further, expressions for the consistent (algorithmic) tangent moduli in the finite strain regime are developed. Since the finite deformation framework employed to obtain the expressions presented here collapses to the classical infinitesimal plasticity framework when the finite strain assumption is no longer necessary, the finite deformation consistent tangent moduli are compared to the consistent tangent moduli valid for use with infinitesimal plasticity. Validation of the implemented finite deformation elasto-plastic Drucker-Prager model is performed through the solution of the concrete slump test. Comparisons between an existing approximate analytical solution and experimental data are presented, and results are discussed in detail.     

Safety and reliability analysis in a polyvinyl chloride batch process using dynamic simulator-case study: Loss of containment incident

In this paper, a novel methodology in batch plant safety and reliability analysis is proposed using a dynamic simulator. A batch process involving several safety objects (e.g. sensors, controller, valves, etc.) is activated during the operational stage. The performance of the safety objects is evaluated by the dynamic simulation and a fault propagation model is generated. By using the fault propagation model, an improved fault tree analysis (FTA) method using switching signal mode (SSM) is developed for estimating the probability of failures. The timely dependent failures can be considered as unavailability of safety objects that can cause the accidents in a plant. Finally, the rank of safety object is formulated as performance index (PI) and can be estimated using the importance measures. PI shows the prioritization of safety objects that should be investigated for safety improvement program in the plants. The output of this method can be used for optimal policy in safety object improvement and maintenance. The dynamic simulator was constructed using Visual Modeler (VM, the plant simulator, developed by Omega Simulation Corp., Japan). A case study is focused on the loss of containment (LOC) incident at polyvinyl chloride (PVC) batch process which is consumed the hazardous material, vinyl chloride monomer (VCM).     

Hyper-elastic constitutive equations of conjugate stresses and strain tensors for the Seth-Hill strain measures

The use of hypo-elastic constitutive equations for large strains in nonlinear finite element applications usually requires special considerations. For example, the strain does not tend to zero upon unloading in some elastic loading-unloading closed cycles. Furthermore, these equations are based on objective material time rate tensors, which require incrementally objective algorithms for numerical applications and integration. Hyper-elastic constitutive equations on the other hand do not require such considerations. However, their behaviour for large elastic strains is important and may differ in tension and compression. In the present work, Hyper-elastic constitutive equations for the Seth-Hill strains and their conjugate stresses are explored as a natural generalisation of Hook’s law for finite elastic deformations. Based on the uniaxial and simple shear tests, the response of the material for different constitutive equations is examined. Together with an objective rate model, the effect of different constitutive laws on Cauchy stress components is compared. It is shown that the constitutive equation based on logarithmic strain and its conjugate stress gives results closer to that of the rate model. In addition, the use of Biot stress-strain pairs for a bar element results in an elastic spring which obeys the Hook’s law even for large deformations and has the same behaviour in both tension and compression. The effect of the constitutive equation on the volume change of the material has also been considered here.     

Direct observation of the deformation of isolated huge spherulites in isotactic polypropylene

In situ observation of the uniaxial deformation of isolated individual large spherulites of isotactic polypropylene was performed using films with a few large spherulites embedded in a smectic matrix. The effects of the variation of sign of birefringence in spherulites, which reflects the difference in lamellar morphology within spherulites were examined. In the case of a spherulite with few tangential lamellae, arcshaped cracks appear in the polar zone of the spherulite and then the radial craze-like fracture begins in the equatorial region perpendicular to the stretching axis. In the case of a spherulite with a large number of tangential lamellae, however, the radial crazing appears in the equatorial zone and then straight dark cracks, rather than arcs, running perpendicular to the stretching direction are initiated in the polar region. These results can be explained by considering the mechanical anisotropy in polypropylene lamellae.