Optimizing the design and impact behavior of a polymeric enclosure

This paper focuses on the application of finite element analysis to design an electronic enclosure with improved impact resistance properties. With the growing push towards miniaturization there is a constant decrease in the wall thickness of the enclosure applications. This necessitates use of ribs to enhance the impact resistance. This study aims at investigating optimal design of ribs for improving impact resistance. The ‘DSGZ’ phenomenological constitutive model, which uniformly describes the entire range of stress–strain constitutive relationship of polymers under any monotonic loading mode is used to predict the plastic failure energies. Several simulation runs were performed based on the design parameters using a 2³ factorial design of experiments. The results from these simulations were used to analyze and study the various design parameters and its influence on the impact energy. It was found that when designing enclosures with ribs with an objective to maximize the impact failure energy, stress should be laid on optimizing the ratio of wall thickness to rib height within permissible limits while center-to-center spacing between the ribs and rib thickness do not have a significant effect.     

Impact behavior of recycled core composite polymeric enclosures

The growing awareness about the impact of non-biodegradable polymeric waste on the environment and the associated cost benefits have led to extensive use of recycled materials. The properties of polymers degrade once they are recycled. This paper presents a comparison of the impact behavior of virgin and recycled polymers as a function of thickness. The thickness is an important parameter in design of polymeric enclosures and as such, the impact energy as a function of thickness needs to be optimized. The ‘DSGZ’ phenomenological constitutive model, developed at Tufts University, uniformly describes the entire range of stress–strain constitutive relationship of polymers under any monotonic loading mode, and is used to predict the plastic failure energies. This paper compares the impact behavior and impact energies of a monolithic virgin PBT (polybutylene-terephthalates) to that of recycled ABS (acrylonitrile–butadiene–styrene)/ASA (acrylic–styrene–acrylonitrile) composite enclosures. The composite enclosures consist of skin layers of ASA at the top and bottom with a center core of recycled ABS. ABAQUS/Explicit and finite element analysis are used to model the enclosures, calculate the plastic failure energies and develop a better understanding of the impact behavior of polymers for enclosure applications.     

Characterization and modeling of stain-rate-dependent behavior of polymeric foams

A polymeric foam was characterized under quasi-static and dynamic loading and a constitutive model was proposed to describe its nonlinear behavior at varying strain rates. Four characteristic properties were identified in the compressive stress–strain curves: (1) yield stress, (2) peak or “critical” stress corresponding to collapse initiation of the cells, (3) plateau stress following the initial collapse of the cells, and (4) strain hardening stress at the end of the plateau region and before the onset of densification. All of the above characteristic stresses vary linearly with the logarithm of strain rate. A strain-based nonlinear constitutive model was proposed. A unified (master) constitutive model with built-in strain rate dependence was formulated and was shown to be in very good agreement with experimental results. The master stress–strain response was modeled in two parts, a power law and one consisting of two exponential terms.     

The effect of temperature on the behavior of the interphase in polymeric composites

The single-filament fragmentation method for measuring the fiber/matrix stress transfer was used for the identification of interphase perturbations. This technique is based on the measurement of the fiber length resulting from the multiple fracture of a single fiber embedded in a resin specimen during tensile loading. A series of single-fiber fragmentation experiments was conducted over a wide range of temperatures on the AS4-carbon-fiber/Epon-828/PACM20-epoxy-resin system. Critical aspect ratios, the magnitude of which is considered to be inversely proportional to the square root of the matrix modulus, showed a significant increase from ambient to elevated temperatures, at temperature levels much lower than the glass transition point of the bulk matrix. This increase was consistent with the existence of an interphase of lower glass transition temperature than the bulk matrix. A three-concentric-cylinder elastic model was employed to correlated the effect of material properties.     

On the polymeric foams: modeling and properties

Polymeric foams are now widely used and researched. The physical properties of polymeric foam can be related to a set of independent structural parameters or variables of the foam. Study of these variables and correlation with commercial FE packages is essential for reliable and faster product development. Some aspects of foam behavior are widely studied while some are little less, like correlation of physical unloading behavior. For example, a lot of work in the area of phenomenological constitutive modeling of uniaxial loading was done, though research in areas of unloading–reloading and their correlation still demands more attention. Increasing number of OEMs and suppliers are moving to computer simulations in the design phase to assess their future products. Hence, different parameters within FE packages play a significant role and also affect the results. Appropriate use of these parameters will narrow down error band and automatically reduce the cycle time and development cost. This brief review is expected to set the perspective for major research work done so far in terms of FE modeling correlation and constitutive modeling of polymeric foam vis-a-vis to its properties.     

Models of thermomechanical behavior of polymeric materials undergoing glass transition

The phenomenological models of the thermomechanical behavior of polymeric materials in a temperature range including the relaxation transition from the highly elastic to the glassy state (vitrification) and the reverse transition (softening) are considered. A model based on the interpretation of the glass transition as a process of gradual increase in intermolecular bonds in the polymer network, “freezing” the current strain with decreasing temperature is developed. A scalar parameter is introduced—the “degree of vitrification,” to establish the quantitative dependence of the relaxation transition completion by temperature. Constitutive relations of thermomechanical behavior of vitrifying polymers in uniaxial and complicated stress states in the “elastic approximation” simplification are obtained. A system of experiments for the identification of the proposed model material functions and constants is formulated and implemented. Analytical model problems are solved, clearly illustrating the mechanism for generation of technological and residual stresses in glass polymers in non-uniform cooling.     

Positron annihilation behavior in several corrosion protective polymeric coatings

Positron annihilation was studied in a vinyl ester and four epoxy coatings on steel. Measurements were made on coatings in equilibrium with air at approximately 50% relative humidity and on coatings immersed in liquid water for 24 h. Three spectral components were identified: a short-lived component associated with annihilation largely in the steel substrate; a medium-lived component associated with annihilation in small voids in the coating; and a long-lived component associated with annihilation of o-positronium in the polymer matrix and within large voids in the polymer and at the polymer/metal interface. An excellent correlation was observed between the effect of water exposure on the annihilation spectra and the protective properties of the coating in an aggressive H₂SO₄ environment. Other correlations between the annihilation spectra and the protective properties were also identified.     

Hybrid nonlinear surrogate models for fracture behavior of polymeric nanocomposites

We present a hybrid nonlinear surrogate model for fracture in polymeric nanocomposites. The phase field method is employed to model fracture in the polymer matrix. Since the stochastic analysis on the output of the mechanical model is prohibitively expensive, surrogate models (SM) are very attractive alternatives. In order to get an optimal and robust solution, we propose a hybrid nonlinear surrogate model (HSM) for the prediction of the fracture toughness of PNC. It is constructed with the use of the polynomial regression and the Kriging interpolation. The support data for such HSM is generated by a phase-field model for brittle fracture with six chosen input parameters. The validation of the surrogate model and by this its qualitative assessment is done based on a scanning test set algorithm. The constructed and assessed HSM is then used to present the behavior of fracture toughness of PNC with respect to various input parameters with very low computational costs and high accuracy. Within the domain of interest, the analysis shows that Young’s modulus of the matrix has no optimum value, in which, the higher input value causes higher response. On the other hand the volume fraction of clay platelets at about 5% showed stability of the response, in which, the higher input value leads to no change in the response.     

Investigating the quasi-static axial crushing behavior of polymeric foam-filled composite pultrusion square tubes

The capability of structures to absorb large amounts of energy is a crucial factor, particularly for structural components of vehicles, in reducing injury in case of collision. In this study, an experimental investigation was conducted to study the crashworthiness of polymeric foam-filled structures to the pultruded square cross-section E-Glass fiber-reinforced polyester composite tube profiles. Quasi-static compression was applied axially to composite tubes to determine the response of the quasi-static load displacement curve during progressive damage. Three pultruded composite tube wall thicknesses at different sizes were examined, and the effects of crushing behavior and failure modes were analyzed and discussed. Experimental results indicated that the foam-filled profile is superior to the non-filled foam composite tube profile in terms of the capacity to absorb specific energy.     

On the relationship between wetting and adsorption of polymeric resins

A general relationship between wetting and adsorption of polymeric resins is derived from the Gibbs and Young's equations and its implications discussed.     

Predictions of viscoelastic strain rate dependent behavior of fiber-reinforced polymeric composites

A viscoelastic damage model for aligned and 3D randomly oriented discontinuous fiber-reinforced polymeric composites is proposed. The model, which predicts the effective viscoelastic stress-strain behavior of the composites, is based on a combination of the Laplace-transformed superposition principle and the ensemble-averaged micromechanics. The Weibull’s damage function is incorporated into the model for the modeling of the evolution of damaged fibers. An inverse analysis based on experimental data is adopted to simulate the strain rate sensitivity of the model. A series of numerical simulations based on the proposed model are performed to examine the influence of damage parameters, fiber orientations, strain rates, and the aspect ratio of discontinuous fibers on the behavior of the composites. In addition, experimental comparisons are made to illustrate and assess the predictive capability of the proposed model.     

Investigation of the laserlike behavior of polymeric scattering gain media under subpicosecond laser excitation

The narrowing effects of scatterers on the lifetime and the spectral width of the laser-induced fluorescence of organic dyes hosted in poly(methyl methacrylate) polymer sheets were studied. The excitation source was a distributed-feedback dye laser emitting 0.5-ps pulses at 496 nm. Spectral and temporal features were recorded simultaneously on a spectrograph-streak-camera detection system. The results were then compared with those obtained from dye solutions in methanol that were recorded in previous studies. The effects of the different host environments on the fluorescence characteristics of the dye were thus investigated. These effects are currently studied when the dye is inserted into human tissue in an attempt to boost tumor detection and photodynamic-therapy efficiency. Some initial results are presented.     

Bending behavior of mortar reinforced with steel meshes and polymeric fibers

The main purpose of this research was to study the effects of combining reinforcing steel meshes with discontinuous fibers as reinforcement in thin walled Portland cement based mortar beams. The term ‘thin’ implies thicknesses of less than about 25 mm. The underlying idea behind this combination is to satisfy the ultimate strength limit state through the steel mesh reinforcement (main reinforcement) and to control cracking under service loads through fiber reinforcement (secondary reinforcement).

An extensive experimental program with bending tests was undertaken. Specimens were 127 × 457 × 12.7 mm. The following variables were investigated: (a) the reference mesh size — 25.4 × 25.4 mm and 50.8 × 50.8 mm; (b) the transverse wire spacing — 25.4 mm, 50.8 mm, and no transverse wires; (c) the type of fibers — polyvinylalcohol (PVA) and polypropylene (PP); and (d) the fiber volume fraction — 1 and 2% for PVA fibers, and 0.5 and 1% for PP fibers.

Some of the main conclusions are: (a) for the same fiber volume fraction, the use of PVA fibers led to a better overall performance than that of PP fibers; (b) an increase in cracking moment and a decrease in crack spacing was observed when 1% PVA, 2% PVA, and 1% PP fibers were used; (c) when 0.5% PP fiber was used, no noticeable change in behavior was observed in comparison to specimens without fibers; and (d) for 1% PVA fibers the transverse wire spacing had little effect on the crack spacing and for 2% PVA fibers, the transverse wire had no influence.     

本征型自修复高聚物粘结炸药的制备与损伤愈合性能

为减轻高聚物粘结炸药(PBX)由于力、热等环境因素所产生的微裂纹等损伤对于其性能与使用寿命的影响,根据颗粒填充高分子复合材料的结构特性,设计合成了含DA键的本征型自修复高聚物粘结剂,以期实现PBX内部损伤的自主修复。研究结果表明,采用含可逆DA共价键的TAPE-DAPU为粘结剂,设计制备的PBX材料具有较强的损伤愈合能力,当损伤较轻时,该PBX的强度恢复率超过95%,对于较严重的贯穿性损伤,其修复效率也在65%以上。      

A continuous threshold model for the visco-elasto-plastic behavior of PET based multi-layer polymeric films

The envelopes of the super-pressure balloons fabricated by the French space agency (CNES) are made of a multi-layer polymeric film that shows substantial viscoelastic and viscoplastic behavior, both depending nonlinearly on stress. A model is presented that takes into account stress depending viscoelastic and viscoplastic strain response functions observed in uniaxial creep experiments. For easy numerical implementation, the strain response functions are represented by a Prony series, whose coefficients form a continuous spectrum on the logarithmic retardation time scale. The observed response functions are generated by an exponential power law distribution of the Prony coefficients with exponent 3. The distribution is fully characterized by three stress dependent parameters: its center, width, and an intensity factor, corresponding to the maximum coefficient. Creep and recovery experiments show that both viscoelastic and viscoplastic strain are highly stress dependent over a limited stress range and are approximately linear at low stresses and around the maximum stress reached during flight. A continuous threshold function is proposed that approximates well the observed stress dependence of the intensities. It is assumed that the other viscoelastic (viscoplastic) parameters change around the same threshold as the viscoelastic (viscoplastic) intensity and are approximately constant elsewhere. The model reproduces very well the strain response observed in creep and recovery experiments with different creep stresses.     

Effect of sea environment on interfacial delamination behavior of polymeric sandwich structures

Sandwich structures are utilized in naval craft and thereby are exposed to sea water environment and temperature fluctuations over extended periods. The sandwich layup consists of a closed cell polymeric foam layer placed between thin carbon or glass fiber reinforced polymeric composite facings. Attention in this paper is focused on sea water effects on the interfacial mechanical response between foam and facing due to sustained sea water exposure using carefully controlled laboratory conditions. Pre-cracked sandwich composite samples are soaked in sea water for extended periods and interfacial fracture behavior compared against dry specimens. Results indicate that the delamination crack propagates close to the interface in the wet case, while it stays within the foam in the dry case. Significant reduction in fracture toughness due to sea water exposure is observed and needs to be considered in the design of ship structures. The effect of sea water on values of energy release rate are determined experimentally and predicted using the J-integral concept. A good agreement between data and predictions is achieved, indicating a reduction in fracture toughness by 30% due to sustained exposure to sea water.     

仿生各向异性高分子人工瓣膜制备及性能研究（英文）

人体心脏瓣膜具有力学各向异性特征,使其能够承受长期的开合循环负荷.本文采用静电纺丝法分别制备了具有各向异性(anisotropic silk fibroin,ASF)和各向同性(isotropic silk fibroin,ISF)的丝素蛋白纤维膜,并进一步与聚乙二醇二丙烯酸酯(poly(ethylene glycol)diacrylate,PEGDA)水凝胶结合,作为人工高分子瓣膜材料(PEGDA-ASF和PEGDA-ISF).有限元分析结果表明PEGDA-ASF瓣膜在心脏舒张期的最大主应力值(2.20 MPa)低于PEGDA-ISF瓣膜(2.37 MPa).与人体瓣膜相似,收缩期时PEGDA-ASF瓣膜在瓣叶根部附近会产生一个弯折区域,而PEGDA-ISF瓣膜的弯折区却接近瓣叶的自由边缘.此外,PEGDA-ASF瓣膜在打开过程中,能够通过动态调整弯折区域获得较为平滑的表面形貌.因此,人工高分子瓣膜的各向异性特征对于实现其与人体瓣膜相似的力学和流体动力学行为起着至关重要的作用.     

Investigation of the adhesion toughness of lacquer to zinc-phosphated steel

A shear test under uniform plane stress loading conditions at the interface has been used to evaluate the adhesion toughness of the bond of a fast-cure thermosetting powder coating to zinc-phosphated cold-rolled steel surfaces of two different origins. The shear test is designed to measure the nominal and the net ultimate shear stress (USS) values, as well as the critical stress intensity factor of interfaces. The experimentally measured critical load at which the adhesion failed, together with the ultimate shear stress values calculated and fractographic analysis of shear-failed specimens on mating surfaces, were used to quantify the bond toughness along the interfaces of zinc-phosphated cold-rolled steel-lacquer specimens, and to provide an insight into fracture phenomena and adhesion mechanisms. It was shown that differences between these two layers manifested by change in the USS net value are due to variation in the morphology of the two zinc phosphate conversion layers studied.     

Fatigue behavior of rigid polymeric materials and composites at room and elevated temperature

In order to characterize the fatigue behavior of rigid polymeric materials, various thermoplastics and glassfiber reinforced plastics (thermoplastics and thermosets) were tested under alternating plane bending stresses at room (20°C) and elevated (50°C) temperatures. Increase in surface temperature and decrease in flexural rigidity of specimens were also measured during the fatigue testing. The results revealed that

1. Rigid polymeric materials can be classified into 3 groups according to their temperature rise characteristics (ΔT) and rigidity decrease (ΔE) during fatigue testing as follows: Group I. lower ΔT and ΔE: for unreinforced thermoplastics and bulk molding compound. Group II. higher ΔT and ΔE: for glassfiber reinforced thermoplastics and thermosets. Group III. higher ΔT and ΔE: for unreinforced thermoplastics.
2. Fatigue strengths of 10⁷ cycles of these materials of groups I～III are well correlated with their elastic moduli, respectively.