Instantaneous crack resistance during crack propagation in a viscoelastic solid

Instantaneous crack resistance values during the mode I stable crack propagation in poly(methyl methacrylate) (PMMA) were investigated with the aid of the sector area method at different test temperatures. The crack resistance during stable crack propagation is a gradually decreasing function of crack passage at all temperatures. The rate decreases as the test temperature decreases, down to ?30°C, irrespective of high initial crack resistance. The crack propagation velocity profiles, obtained through velocity gages, also show the decreasing function of crack passage. Both crack resistance R and its gradient with respect to the crack propagation velocity, dR/d?, become greater as the temperature decreases. R becomes greater as ? increases, contrary to the usual crack resistance behavior in metals.     

Microscopic approach to energy dissipation during viscoelastic crack propagation

The microscopic factors relating to the energy dissipation increase in the viscoelastic dynamic crack propagation such as in PMMA [poly(methyl methacrylate)] were investigated through the observation on the microfracture process ahead of the crack tip up to the microcrack formation. It is found that the energy dissipation remains almost constant in a certain low velocity range but increases sharply in the higher velocity range for the PMMA case. A cleavagelike fracture mode works in the former, and both the cleavagelike and a ductile-fracture mode work in the latter. The increase in energy dissipation is attributed to the secondary cracks which remarkably increase in number depending on the crack velocity. The energy dissipation is proved to be independent of the crack velocity in cases where the crack is propagated by the cleavagelike fracture mode.     

Phenomenological aspects of viscoelastic crack propagation

The strain rate dependence on dynamic crack propagation in viscoelastic solids was experimentally verified for PMMA by the velocity gauge method and fracture surface observation. It was found that the dependence was mainly due to the viscous term effect, and this was supported by the observation of deviation of the stress-strain curve from an elastic case. The elastic brittle crack propagation can be realized only in the high-strain rate case for PMMA.     

Strain disturbances due to viscoelastic crack propagation

Local strain disturbances near a running crack in a viscoelastic material were investigated in PMMA. Specimens having different initiation crack tip radii and under different tensile strain rates were examined by use of a series of strain gauges placed parallel and close to the expected crack path, and by velocity gauges. The strain disturbance, which was observed ahead of the running crack front, diminished gradually owing to the viscoelastic damping. The maximum strain disturbances increased with increase in gross breaking load.     

Prefatigue hysteresis effects on viscoelastic crack propagation

Prefatigue hysteresis effects on viscoelastic crack-propagation velocity were investigated through velocity-gauge techniques at different temperature levels. It was found that prefatigue effects accelerate elastic predominance in viscoelastic dynamic crack propagation, approaching elastic brittle fracture as shown by Berry's equation. This results in greater dynamic crack-propagation velocity at any given test temperature level.     

Dynamic stress intensity factors during viscoelastic crack propagation at various strain rates

Dynamic stress intensity factors K_D were measured by the caustic method and crack propagation velocity ? by the velocity gauge techniques for PMMA [poly(methyl methacrylate)] during dynamic crack propagation at various strain rates \documentclass{article}\pagestyle{empty}\begin{document}$ \rm \dot \varepsilon $\end{document} . No definite applied strain rate effects on the dynamic stress intensity factor were observed for applied strain rates ranging from 8.33 × 10^?4 to 30/sec; however, the test results do show crack propagation velocity dependency in K_D ～ ? relations. The high local strain rate region may be realized at the running crack tip even under the quasi-static loading case of \documentclass{article}\pagestyle{empty}\begin{document}$ \rm \dot \varepsilon $\end{document} = 8.33 × 10^?4/sec, since all the crack propagation velocities obtained were greater than 50 m/sec even up to 450 m/sec.     

Fatigue crack propagation and damage evolution in PBT-GF and SAN-GF

Our earlier investigations of fatigue behavior in PBT-GF and SAN-GF with different fiber lengths have shown that fatigue crack propagation (FCP) can be described in terms of elastic-plastic fracture mechanics. In this work it is shown that the influence of structural material parameters on the resistance to FCP correlates with the extent of energy dissipation at the crack tip. With increasing fiber length, the zone of energy dissipation is increased. By means of microscopic investigations, the prevailing damage in the zone of energy dissipation is identified as micro cracks in the matrix.     

Birefringence measurement on the liquid crystal by phase modulation technique

A convenient and accurate technique for measuring the birefringence of liquid crystal (LC) is developed. The birefringence of LC was obtained with only one experimental run by using phase modulation technique, which is an expansion of PMFB (Phase Modulated Flow Birefringence) technique developed elsewhere [1], The basic principle of the method is based on the determination of the phase differences which occur when a phase modulated monochromatic polarized light propagates through a medium with an anisotropic refractive index. Birefringence measurements with red light (wavelength: 0.6328μm) for commercial liquid crystal material (Merck, ZLI 3201-000) are reported.     

Preparation and electrooptical properties of liquid crystal dispersed film by electron-beam irradiation

Polymer films consisting of nematic liquid crystal (LC) droplets and polymer networks were prepared by using a low-energy electron beam to irradiate a homogeneous mixture of nematic LC and bifunctional methacrylate monomer. Influences of such polymerization conditions as polymerization temperature, monomer concentration, and radiation energy on electrooptical properties of the compound films were examined. The polymer yield, affecting to a large extent the film properties, depended on the monomer concentration and the radiation energy. Compound films, which have a switching function from the scattering state to transparency by applying approximately 20–30 V between the two sides of the film, were obtained. In addition, it was found that a compound film with excellent electrooptical properties was prepared by changing impure LC in the droplets into pure LC. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 65:1675–1681, 1997     

An electrical method of measuring crack length during crack propagation tests of polymers

Measurement of fatigue crack propagation rate (da/dn) in polymers is normally done optically with a travelling microscope. Electrical methods have not been very successful due to lack of sensitivity and to inaccuracies caused by factors such as plastic deformation and temperature variation. Use of electrically conductive surface grids eliminates these problems. An electrically conductive grid pattern is vapor-deposited onto the surface of a compact tension specimen. The electrically conductive grid pattern is vapor-deposited onto the surface of a compact tension specimen. The electrically conductive grid lines run across the expected path of the crack. With crack advancement, the grid lines are successively torn apart. Proper interpretation of the discontinuous steps that result when electrical current is plotted against time directly generates da/dn data.     

Zenithal alignment of liquid crystal on homeotropic polyimide film irradiated by ion beam

We investigate the pretilt characteristics of a nematic liquid crystal [LC] in terms of ion beam exposure conditions on the homeotropic polyimide alignment layer. The pretilt angle of LCs in the case of high-energy ion beam treatment was decreased considerably almost the same to that of the homogenous alignment layer though we used homeotropic polyimide film at first. Increasing irradiating energy, we could control the pretilt from 90° to 1° with several steps. We believe that this is because the side chain with hydrophobicity in the used polyimide is broken by ion beam exposure. To confirm it, contact angle measurement was carried out. With this result, we can easily control the LC pretilt in the pixel with appropriate exposure conditions which is critical to achieve excellent electrooptic characteristics and good image quality.     

Grafting of side chain liquid crystal monomers onto polyethylene film

In order to obtain films of liquid crystalline polymers, a commercial polyethylene film was grafted with acryloyl chloride using γ-irradiation, and two tolane-containing alcohols were reacted with the chloride to obtain the corresponding acrylates. The morphology changes were studied by DSC, X-ray diffractometry and optical microscopy. The crystallinity of polyethylene films decreased by grafting, and mesophase with spherulites was observed over a wide temperature range. Received: 18 May 1997/Revised: 11 August 1997/Accepted: 13 August 1997     

A simple viscoelastic model for fatigue crack propagation in polymers as a function of molecular weight

A simple theory is presented to explain the strong influence of molecular weight (M) on rates of fatigue crack propagation (FCP) in amorphous polymers. It is proposed that the equation describing FCP rates may be expressed as the product of two functions, one involving the stress intensity factor (ΔK), and the other characterizing the relaxation process occurring in the plastic zone. To provide a physical network in the plastic zone that can sustain fatigue loading, it is proposed that one needs a sufficient fraction of molecular fibrils per unit area ($\text{W1}$) whose lengths are greater than M_c, the critical value of M required for entanglement. This effect can be summarized as a generalized rate process (confined at the plastic zone) expressed by A exp (Bσ) where σ is a stress and A and B are constants (B including the volume of activation). It is deduced that M influences the activation volume through the values of $\text{W1}$ and W, the weight fraction of molecules whose M>M_c. Using the equation developed it was possible to correlate FCP data of PVC and PMMA as a function of M with a high degree of confidence. Also, the value of activation volumes obtained compared favourably with those in the literature for static tests. The complementary value of $\text{W1}$ for these polymers was also seen to approximate closely to the void fraction in a craze. Extension to other cases such as semi-crystalline materials also seems possible.     

Modification of ultrasonic transducers to study crack propagation in vinyl polymers,supported by SEM technique

Detecting defects in various industrial products remains a challenging task in the industry. Researchers are constantly working to improve detection techniques and tools for various defects, particularly cracks. Many industrial structures suffer from cracks. The selection of a suitable technique and/or tool is based upon the tested structures and the accuracy of the technique and/or tool. The work's novelty is the development of a fully accurate, simple, and safe ultrasonic tool for precisely scanning cracks. A pen-shaped cone was added to the ultrasonic transducers to improve their performance. Different crosshead speeds and annealing techniques were used to cause cracks on medium density polyethylene (MDPE) and poly-methyl-methacrylate (PMMA) polymer plates with specific dimensions. Cone reduced the front diameter of the transducers from 12 to 2 mm (cone tip diameter). It improved the transducers by giving them new properties such as a small near field, a collimated beam, high sensitivity, and high wave reflection. The modified transducers tracked the cracks at discrete sequential sites, where the ultrasonic velocity was measured to determine the crack speed, critical crack speed, dynamic stress intensity factor, and crack branching phenomenon. Additionally, ultrasonic attenuation was measured in order to accurately determine crack growth behavior, the crack's neck zone, and the crack growth dependency on both plate thickness and annealing. Scanning electron microscope (SEM) images revealed crack propagation in polymer plates. The results of ultrasonic testing and SEM evaluations were consistent. This guaranteed that the modified transducers could scan cracks accurately.     

Photoelectrochemical oxygen evolution from water by a manganese chlorophyll—liquid crystal electrode

In order to accomplish the photoelectrochemical evolution of oxygen from water, manganese chlorophyll was incorporated in a thin liquid crystal membrane attached to a platinum plate. The manganese chlorophyll—liquid crystal electrode reacted to water on visible light irradiation. The potential of the platinum markedly depended on the states of the immobilized chlorophyll. Visible light irradiation induced a negative shift in potential. The incorporation of a nematic liquid crystal was found to considerably enhance the potential shift on light irradiation. The negative potential shift accounted for the light-induced electron flow from water to the platinum phase. The photoelectrochemical reaction of the immobilized manganese chlorophyll was performed in parallel with the direct determination of the oxygen evolution. The molecular oxygen evolution was associated with the photocurrent.     

Response time measurements of liquid crystal dispersed in polyester resin film

New material consisting of nematic liquid crystal droplets from 2-30 μm in radius in a crosslinked UV-cured polyester resin binder was prepared. Micrographs of the samples were taken in the optical microscope with cross-polaroids. Reorientation of the droplets by application and removal of an electric field is discussed. Electrooptic properties of the material were measured using polarized light from a helium-neon laser. Transmittance and response time vs. voltage applied with driving frequency 2 kHz were detected on the oscilloscope and photographed. The response times were: rise time τ^on shorter than 20 ms at applied voltage higher than 50 V, and decay time τ^off = 80 ms. They are a promising feature for this material for a display application.     

Structure and physical properties evolution of ITO film during amorphous-crystalline transition using a highly effective annealing technique

Three kinds of amorphous ITO (a-ITO) films were deposited on chemically strengthened glass by DC magnetron sputtering at room temperature. A rapid infrared annealing (RIA) device was built, and an infrared fast crystallization process was developed. Not only the effects of RIA on the structural, optical, and electrical properties of ITO films, but also the effects of RIA on the mechanical properties of chemically strengthened glass substrates were investigated compared with those of conventional furnace annealing (CFA). The rules of the structural and physical properties during amorphous-crystalline transition are basically the same by RIA and CFA. However, the RIA time is only 10% of the CFA time, and the compressive stress reduction of chemically strengthened glass is only 4.2% at 500 °C, which is 38.5% by CFA. These results demonstrate that very effective RIA technology could replace the traditional annealing method (CFA), which can greatly reduce production costs and significantly improve production efficiency.     

Simultaneous heat and mass transfer with liquid film vacuum distillation

The problem of simultaneous heat and mass transfer associated with the vacuum distillation of a thin liquid film is solved by analyzing the system as a conjugated boundary value problem. The solution is developed in terms of well-known tabulated functions using a Green's function approach. The effects of heat transfer, liquid phase diffusion and effusion at the surface on the rate processes are elucidated, and separation efficiencies are predicted as functions of the length of the heated wall over which the film flows. The theoretical analysis is applied to calculate mass transfer rates for the separation of fatty acids from tall oil.     

Preparation and characterization of liquid crystal composite film using photosensitive hydrophilic matrix

Methoxybenzylidene n-butylaniline (MBBA) and ethoxybenzylidene n-butylaniline (EBBA) liquid crystal were synthesized. To investigate the applications of hydrophilic monomers and polymers on the field of liquid crystal composite film, 2-hydroxyethyl methacrylate (HEMA) and acrylic acid (Aa) were selected as monomers for the polymer matrix. To prepare the liquid crystal composite film, MBBA, EBBA, and commercial liquid crystals ZLI-2444, 2452, 2459, and BDH-E7 were mixed with HEMA, Aa, and photosensitive materials and then irradiated by UV light. The electro-optical behavior and the microstructure of the liquid crystal composite films were investigated by He-Ne laser and scanning electron microscopy, respectively. The effects of UV light irradiation and monomer ratio on the electro-optical behavior of liquid crystal composite films were investigated. The reproducibility and storage stability of the liquid crystal composite films were investigated. © 1996 John Wiley & Sons, Inc.