Birefringence measurement for validation of simulation of precision glass molding process

During fabrication of glass lens by precision glass molding (PGM), residual stresses are setup, which adversely affect the optical performance of lens. Residual stresses can be obtained by measuring the residual birefringence. Numerical simulation is used in the industry to optimize the manufacturing process. Material properties of glass, contact conductance and friction coefficient at the glass‐mold interface are important parameters needed for simulations. In literature, these values are usually assumed without enough experimental justifications. Here, the viscoelastic thermo‐rheological simple (TRS) behavior of glass is experimentally characterized by the four‐point bending test. Contact conductance and friction coefficient at P‐SK57? glass and Pt‐Ir coated WC mold interface are experimentally measured. A plano‐convex lens of P‐SK57? glass is fabricated by PGM for two different cooling rates and whole field birefringence of the finished lens is measured by digital photoelasticity. The fabrication process is simulated using finite element method. The simulation is validated, for different stages of PGM process, by comparing the load acting on the mold and displacement of the molds. At the end of the process, the birefringence distribution is compared with the experimental data. A novel plotting scheme is developed for computing birefringence from FE simulation for any shape of lens.     

Influence of variation in the local interface fracture properties on shear debonding of CFRP composite from concrete

Abstract

The debonding mode of failure, which is observed in concrete beams strengthened using externally attached CFRP composite sheets, is investigated using the direct shear test. The Mode II, cohesive stress-crack relative slip relationship is established using full-field displacements obtained from digital image correlation. The interface crack is associated with a cohesive stress-transfer zone of fixed length. The load capacity of the CFRP composite bonded to concrete is attained when the cohesive crack is fully established. The acoustic emission monitored during the interface fracture initiation and propagation indicates that microcracking events accumulate at a constant rate up to failure. The variations in the local fracture parameters are quantified and are adequately represented using the normal probability distribution. A numerical analysis of the direct-shear debonding response of CFRP composite attached to a concrete substrate is performed to study the influence of the variability of the local fracture parameters on the load-carrying capacity and the ultimate failure. An instability associated with a snapback in the load response resulting from a decrease in both load and displacement, is predicted close to failure. The variation in the local fracture properties does not influence the load-carrying capacity or the intensity of snapback instability at ultimate failure.     

Numerical simulation of cement-to-formation interface debonding during hydraulic fracturing of shale gas wells

Abstract

The hydraulic fracturing process involves high pump pressure and large displacement, which increase the risk of debonding on the interface of the cement sheath and rock formation. Therefore, in this study, a three-dimensional numerical calculation model of casing-cement sheath-formation assembly was established, and the failure mechanism of the bonding surface caused by fracturing due to fluid migration was investigated. The effects of the cement sheath’s Young’s modulus and Poisson’s ratio, bonding strength, perforation azimuth angle, and non-uniform in-situ stress on the anti-debonding ability of the cement-to-formation interface were analyzed. Finally, the following conclusions were drawn: The calculation results show that the failure of the bonding surface is greatly affected by the bonding strength, and its anti-debonding ability significantly increases linearly with increasing bonding strength. Furthermore, while increasing the Young’s modulus improves the anti-debonding ability, the Poisson’s ratio of the cement sheath and the perforation azimuth angle have little effect. Under non-uniform in-situ stress, the anti-debonding ability decreases with the increase of the difference between the horizontal and vertical in-situ stress. Thus, the non-uniform in-situ stress accelerates the failure of the bonding surface.     

Fabrication of chalcogenide microlens arrays by femtosecond laser writing and precision molding

In this study, convex microlens arrays (MLAs) on chalcogenide glass (ChG) surface were fabricated through femtosecond laser direct writing and precision molding. Femtosecond laser writing was first employed to induce regularly arranged damage craters on the surface of silica glass. Then, the surface of silica was etched with hydrofluoric acid to obtain a smooth concave MLA. Finally, the concave microlens of silica was replicated on the surface of ChG by precision molding to obtain ChG MLA. The resulting ChG MLA had a uniform structure, clear image, and good focusing effect. By optimising the parameters of laser direct writing and chemical etching, we produced 1600 rectangular and hexagonal microlenses from As₂Se₃.The optical performances of MLAs were demonstrated by their excellent imaging and focusing capabilities. The method applied provides an efficient way to prepare large-scale MLA masks and MLAs.     

Study on the interfacial interactions and adhesion behaviors of various polymer-metal interfaces in nano molding

This study focuses on investigating interfacial interactions and the adhesion mechanism of polymer-metal interfaces in nano-molding. Polyphenylene sulfide (PPS), polyamide 6 (PA6), and isotactic polypropylene (iPP) were chosen as candidate polymers, and aluminum (Al), and copper (Cu) were used as metal substrates. By establishing the metal matrix composed of a rectangular pit with length, width, and depth of 4.5, 4.5, and 2.0 nm, respectively, six paired polymer-metal interfacial systems in a cuboid of 7.5 × 7.5 × 11.5 nm, consisting of metal, polymer, and vacuum layer (from bottom to top) were constructed. Molecular dynamics simulations were performed to calculate interfacial interactions and bonding processes. Results showed that wall-slip behavior was pronounced in nano-molding. Viscoelasticity and polarity of the polymers played a crucial role in interfacial interactions, which guided the wall-slip behavior and greatly affected the bolt performance. PA6 and PPS were more suitable for molding than iPP on both Al and Cu substrates. PA6 showed the best filling and bonding performances, followed by PPS, while iPP revealed the poorest performances. The Cu substrate exhibited better anchor strength and filling rate than Al substrates with the same polymer.     

玻纤增强酚醛注塑料的制备技术和性能

研究了玻纤增强酚醛注塑料制备过程中基质树脂的选择、固化作用与交联结构的控制及玻纤分散技术,考察了不同基质树脂制备的酚醛注塑料的固化成型结构形态和固化流变特性.进一步采用热固性与热塑性酚醛树脂相复配的基质树脂体系,经配方和制备工艺的优化,制备了高填充量玻纤增强酚醛注塑料.该注塑料具有良好的注塑成型性能,注塑制品具有高强度, 冲击强度达到4.3 kJ•m^-2,弯曲强度137.4 MPa,同时热变形温度为 245 ℃,阻燃性通过美国UL 94 V-0级认证,并具有优良的尺寸稳定性、电绝缘性能和低成本优势.     

The generation mechanism of demolding force based on the mold-part interface contact mode in micro-injection molding

Demolding is the key step of micro-injection molding (μIM), which influences the molding quality of polymer parts to a great extent. The essence of demolding is to overcome interface interactions between the mold cavity surface and the part surface, which is closely related to factors such as the cavity surface topography and process parameters. However, a theoretical model predicting the demolding force of μIM is still lacking, resulting in the difficulty of improving the quality of demolding in μIM. Therefore, a comprehensive demolding force model is proposed to clarify the demolding mechanism of μIM. The model focuses on the adhesion behavior and friction behavior in the special contact mode formed by the replication effect of μIM. The theory proof and computation results indicate that the cavity surface topography determines the contact mode of the mold-part interface (MPI), which in turn changes the real interface contact area, the composition mode of adhesion stress, and the friction forms, thereby affecting the adhesion and friction force during demolding. In addition, the process parameters can also change the contact mode of MPI as the process parameters have a great effect on the filling capacity and the degree of replication, thereby influencing the demolding process.     

Study on shape deviation and crack of ultra-thin glass molding process for curved surface

The demands towards high precision and surface quality of ultra-thin glass for curved screens are continuously rising in the field of smart mobile terminals. Although the ultra-thin glass molding process (UTGMP) has the advantage of the shorter production cycle and higher efficiency, there are still typical forming defects in the molding process, namely crack, shape deviation, and large surface roughness. This paper aimed to investigate the influence mechanism of UTGMP molding temperature and pressure on the shape deviation, crack area, and surface quality of ultra-thin glass. In this study, a finite element model (FEM) was established to study typical forming defects of curved surfaces, and the effects of molding temperature and pressure on the shape deviation and crack area for ultra-thin glass were studied by the FEM simulation method. The simulation results revealed the molding temperature has a significant effect on the shape deviation, crack area and surface quality, while the molding pressure is only strongly correlated with shape deviation and crack area. In addition, the reliability of the model was verified by a series of five-level single factor experiments, and the shape deviation and crack area of ultra-thin glass were discussed in detail. Under the appropriate molding pressure and temperature range (0.45 MPa, 802–806 °C), the accuracy of curvature was improved by 33%, the roughness was reduced by 21%, and the probability of crack was also reduced. Thus, this study contributes to improving UTGMP's molding accuracy and reducing molding defects, and plays a positive role in reducing production costs and improving production efficiency.     

Numerical and experimental determinations of contact heat transfer coefficients in nonisothermal glass molding

Heat transfer at the interfacial contact is a dominant factor in the thermal behavior of glass during nonisothermal glass molding process. Recent research is developing reliable numerical approaches to quantify contact heat transfer coefficients. In most previous studies, however, both theoretical and numerical models of thermal contact conductance in glass molding attempted to investigate this factor by either omitting surface topography or simplifying the nature of contact surfaces. In fact, the determination of the contact heat transfer coefficient demands a detailed characterization of the contact interface including the surface topography and the thermomechanical behavior of the contact pair. This paper introduces a numerical approach to quantify the contact heat transfer by means of a microscale simulation at the glass-mold interface. The simulation successfully incorporates modeling of the thermomechanical behaviors and the three-dimensional topographies from actual surface measurements of the contact pair. The presented numerical model enables the derivation of contact heat transfer coefficients from various contact pressures and surface finishes. Numerical predictions of these coefficients are validated by transient contact heat transfer experiments using infrared thermography to verify the model.     

Silane and silazane surface modification of recycled glass fibers for polypropylene composites

Glass fiber (GF) composites are one of the significant challenges in recycling thermoset materials. After pyrolysis, the glass fibers lack sufficient strength and show poor matrix compatibility. Here we have investigated a series of multifunctional silane and silazane agents for surface modification of recycled glass fibers that provide a combination of hydrophobic properties and residual reactive groups on the surface. This allowed testing of interfacial effects from the surface modification as well as a potential synergistic compatibilization using maleated PP (MAPP). The treated GFs were used to prepare new polypropylene (PP) composites by multiple extrusion steps, resulting in a series of composites where the dispersion efficiency was attributed mainly to the surface chemistry and compatibilization effects. The amino-silane modifications of the recycled fibers resulted in further improvements in the mechanical properties of the PP composites in comparison with the hydrophobized GFs. Moreover, synergistic effects from the addition of MAPP were observed with scanning electron microscopy. The results clearly demonstrate that the surface modifications were effective and good alternatives to currently used methods.     

Use of the Blister Test to Study the Adhesion of Brittle Materials. Part II. Application

Using a modified form of the blister test, where the adhesive layer was between the substrate and a massive base, instead of as a continuous sheet on top of the substrate, we determined the interfacial fracture energy F for a series of interfaces where a brittle material (ice) was adhering to various substrates. Fracture energies obtained were compared with work of adhesion values measured for water on the same substrates. Fracture energy, which contains within it both a reversible contribution due to intermolecular interactions across the interface (work of adhesion) and an irreversible contribution due to collective dissipative processes, was found to rise rapidly with modest increases in work of adhesion. The observed relation suggests that the irreversible contribution to fracture energy is influenced strongly by the intermolecular interactions at the interface.     

Use of the Blister Test to Study the Adhesion of Brittle Materials. Part II. Application

Using a modified form of the blister test, where the adhesive layer was between the substrate and a massive base, instead of as a continuous sheet on top of the substrate, we determined the interfacial fracture energy F for a series of interfaces where a brittle material (ice) was adhering to various substrates. Fracture energies obtained were compared with work of adhesion values measured for water on the same substrates. Fracture energy, which contains within it both a reversible contribution due to intermolecular interactions across the interface (work of adhesion) and an irreversible contribution due to collective dissipative processes, was found to rise rapidly with modest increases in work of adhesion. The observed relation suggests that the irreversible contribution to fracture energy is influenced strongly by the intermolecular interactions at the interface.     

Influences of debonding rate and temperature on tack properties and peel behavior of polyacrylic block copolymer/tackifier system

The influences of debonding rate and temperature on the peel behavior of polyacrylic block copolymer/tackifier system were investigated. Poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate) triblock copolymer (MAM) with hard block contents of 23 (MAM-23) and 16 wt.% (MAM-16) and a 1/1 blend with a diblock copolymer (MA) consisting of the same components (MAM-23/MA, total hard block content of 15 wt.%) were used as the base polymer. A special rosin ester was used as a tackifier at various contents in the block copolymer/tackifier system. The peeling process at the probe/adhesive interface during probe tack testing was observed using a high-speed microscope at 23 °C with debonding rate of 10 mm/s. Three different peeling mechanisms were observed. Type A, where peeling progressed linearly from the edge to the center of the probe without cavitation (MAM-23). Type B, where peeling progressed linearly from the edge to the center of the probe with cavitation (MAM-16). Type C, where cavitation occurred over the entire adhesive layer, and peeling initiation was delayed (MAM-23/MA). The peel behavior of MAM-23 changed from Type A to Type B with a decrease of the debonding rate (1 mm/s) or increase of the temperature (40 °C). In contrast, there was no change for MAM-16 and MAM-23/MA. Cavity formation in an adhesive layer restrains peeling; therefore, it is desirable for improvement of the adhesion strength. The tack properties increased with the tackifier content, and the formation of cavitation was less than that for the systems without the tackifier.     

Adhesion-induced deformations of polyurethane substrates in contact with spherical glass particles: the effect of particle size on the radius of contact

Spherical glass particles having radii between approximately 0.5 and 100 *m were deposited onto a polyurethane substrate and the radii of contact, resulting from the adhesion forces between the particles and the substrate, were determined using SEM. For particles having radii less than approximately 5 μm, it was found that the contact radius varied as the particle radius to the 0.75 power. In addition, large menisci, presumably resulting from tensile interations, were observed. For particles having radii between 5 and 60 μm, the contact radius varied as the particle raidus to the 2/3 power. Stretching of the substrate was also observed for particles having radii of approximately 100 μm. This is probably a harbinger of the impending separation of the particle from the substrate, due to gravitational forces. The thermodynamic work of adhesion was calculated from the data and the results were compared with the predictions of several theories of particle adhesion.     

Effects of interfacial adhesion on microdamage and rheological behaviour of glass bead filled nylon 6

Model composites consisting of glass beads dispersed in a matrix of nylon 6 with different degrees of interfacial adhesion were prepared. The effects of interfacial adhesion on damage generation in the glass bead filled nylon 6 were studied by acoustic emission monitoring and scanning electron microscopy, and the critical damage stress was measured. Improvement of the interfacial adhesion enhanced the tensile strength of the composite. The melt rheological behaviour of the glass bead filled nylon 6 was also investigated. It was found that the interfacial adhesion strength significantly affected the rheological behaviour of the glass bead filled nylon 6.     

UP/PU嵌段共聚树脂/玻璃纤维界面粘结性的研究

合成了一种异氰酸酯嵌段共聚改性不饱和聚酯树脂(UP/PU),并以玻纤增强制备了复合材料(GFRP)。通过接触角、拉伸性能、弯曲性能测定和扫描电镜观察研究了UP/PU GFRP界面的粘结性能。结果表明:UP/PU树脂与玻璃表面的接触角为20°,对玻璃表面的润湿性较通用邻苯型UP好;GFRP拉伸强度1 050 MPa,弯曲强度1 220 MPa,较通用邻苯型UP的GFRP分别提高了145%和78%,说明UP/PU与玻纤的界面粘结性能较好。     

Development of rubber pressure molding technique using butyl rubber to fabricate fiber reinforced plastic components based on glass fiber and epoxy resin

A rubber pressure molding (RPM) technique is developed to prepare fiber reinforced plastic components (FRP) using glass fiber and epoxy resin. The technique is based on the matching die set, where the die is made of hard metal like steel and the punch from flexible rubber like materials. The use of flexible rubber punch helps to intensify and uniformly redistribute pressure (both operating pressure and developed hydrostatic pressure due to the flexible rubber punch) on the surface of the product. A split steel die and rubber punch were designed and fabricated to prepare the FRP components. The same split die was also used to cast the rubber punch. Butyl rubber was used to prepare a rubber punch in this investigation. Burn test, coin test, scanning electron microscopy and mechanical tests like interlaminar fracture toughness, interlaminar shear test, tension test, etc were carried out to know the fiber content, void content, presence of delamination, bonding between fiber and resin, microstructure, and mechanical properties of the composite materials. These properties were also compared with FRP components made by the conventional technique to evaluate its performance in the structural applications. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 1095–1102, 2006     

Effect of the interface with solid on the interfacial region in the blends of linear polymers formed in situ

Blends of polyurethane and poly (methyl methacrylate) of various compositions were synthesized in situ in presence of various amounts of nanoparticles (fumed silica). From thermo‐ physical measurements it was found that the reaction is accompanied by the phase separation and evolution of two phases. The temperature transitions in the systems and their positions depend on the blend composition and on the filler amount. Using scanning differential calorimetry, the fraction of an intermediate region between two main phases has been estimated from the changing of heat capacity increments. It was observed that in filled polymer blends in the temperature region between two main relaxation transitions, there appears the third transition. This transition is supposed to be the result of the formation of adsorption layer at the interface with solid. The appearance of such an intermediate regions increases essentially the total fraction of an interfacial region in the system. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4646–4651, 2006     

Rheological, Interfacial and Thermal Control of Polymer Adhesion. I. Isothermal Theoryf

We have developed an isothermal theory of separation in polymer-solid adhering systems. The model used is based on the (observed) drawing of filaments between a bulk polymer and a solid. In the isothermal theory, a criterion is set up, demarcating filament elongation vs. detachment of the filament base from the solid. It employs a dimensionless parameter, ω, that relates free energy of adhesion, elongational viscosity or yield strength of the polymer, and filament size, to adhesive performance. The isothermal theory can be applied directly to the separation processes that occur with pressure-sensitive adhesives. Certain observations by Aubrey and Sherriff, by Gardon and by Kaelble are explained. The validity of the demarcation is believed to extend beyond pressure-sensitive systems, to all thermoplastic adhesives and/or coatings.