The plastic yield behaviour of polymethylmethacrylate

The yield behaviour of an amorphous glassy polymer has been investigated under a system of combined stress in an attempt to define a criterion for yield. Sheets of polymethylmethacrylate were compressed in plane strain and the compressive yield stress was determined as a function of the tension applied in the plane of the sheet. The compressive yield stress was found to decrease with applied tension more rapidly than would be expected if the shear yield stress of the material were independent of pressure. The results have been analysed in terms of a Coulomb yield criterion where the shear yield stress is expressed as a constant plus a friction term proportional to the pressure on the shear plane. Birefringent shear zones were observed in the deformed region after the load was removed and these zones were inclined at 52.9° to the plane of the sheet. It was found that if the stresses at yield were expressed as nominal stresses then the inclination of the shear planes predicted by the yield stress data coincided with the observed inclination. It also appears that it may be possible to define a fracture criterion in terms of the applied stress system.     

The fracture behaviour of notched specimens of polymethylmethacrylate

The fracture behaviour of notched specimens of polymethylmethacrylate has been examined for a wide range of geometries in Charpy impact tests, and in tensile and slow bend fracture tests. It was found that the failure of the very sharply notched specimens was consistent with linear elastic fracture mechanics and defined a constant fracture toughness K _IC for a constant notch tip radius, whereas the blunt notched specimens failed at a constant critical stress at the root of the notch.     

Rain erosion damage in brittle materials

In order to understand the processes involved in the high-velocity rain erosion of brittle materials the impact damage produced in soda-lime-silica glass by single and multiple jet impact was studied. The damage was quantified by measuring the post-impact strength of specimens. It is shown that the impact damage depends on the impact velocity, the number of impacts and the specimen dimensions. A new analysis for calculating the velocity dependence of jet/drop impact damage in brittle materials is presented. The model is based on Hertzian contact analysis and dynamic fracture mechanics and takes into account the statistical nature of the flaws in the specimen. A good qualitative agreement with experimental results is obtained.     

Wet erosion behaviour of low SiC alumina-SiC nanocomposites

We have investigated the erosive wear behaviour of alumina and Al₂O₃-SiC nanocomposites with SiC content between 1 and 5%. Nanocomposites (grain sizes between 3.15 and 7.16 m) and alumina (grain size 4.43 m) were eroded with SiC particles using a custom-built erosive slurry wear tester. The erosion resistance of the nanocomposites increased slightly with decreasing grain size. Nanocomposites of all grain sizes showed better wear resistance than the alumina. Erosion resistance increases with SiC content, though this effect is not strong for SiC contents greater than 2%. These results are compared with related results from the literature.     

The erosion behaviour of TiN coatings on steels

The erosion behaviour of physical vapour-deposited titanium nitride has been studied using both blunt particle and angular particle erodent streams. The mechanisms of erosive loss have been identified and related to the microstructure and internal stress state of the coatings. High levels of internally stored energy (which scales with coating thickness and internal stress) induce spalling in erosion testing using blunt erodents. Consequently, it is found that thick coatings are more resistant to angular particle erosion whilst thin coatings have longer lifetimes when exposed to blunt erodents. Scratch adhesion testing has been performed on all erosion specimens and attempts made to correlate the critical load for coating failure, with the failure mechanisms observed in erosive wear. Whilst no correlation can be found between erosion resistance and critical load, good correlations between the failure mechanisms found in erosion and scratch testing can be made.     

Microstructural influence on erosion behaviour of thermal spray coatings

The influence of structure on erosion performance of thermally sprayed Cr₃C₂–NiCr coatings under industrial turbine conditions has been investigated. Thermal spraying of these materials results in substantial variation in composition and microstructure due to exposure of the coating powders to the high temperature accelerating gas.Coatings were characterised using Back Scatter Electron imaging in conjunction with X-ray diffraction which showed carbide dissolution into the matrix of varying extent depending on deposition technique. Heat treatment at 900 °C caused carbide precipitation and matrix refinement. Erosion testing of as-sprayed and heat treated coatings was conducted at ambient and elevated temperature. Single impacts were characterised using Scanning Electron Microscopy in order to determine the erosion mechanism.At ambient temperature the single impacts caused a brittle response with both carbide grains and matrix being cleaved by the erodent particle. Brittle cracks surrounded each impact and intersected with splat boundaries leading to a significant contribution to erosion rate from splat structure. Following heat treatment the erosion response of the coatings was more ductile with mounds of plastically deformed material surrounding each impact, this significantly reduced erosion rate.     

Long-time stregth of polymethylmethacrylate

Experimental data produced by short and long-time strength tests on polymethylmethacrylate (PMMA) specimens supported the view that the time-to-rupture of this material under stress is the time required for a defect (crack) to be formed and grow to a critical size as a result of which instantaneous fracture of the specimen takes place.     

Analytical model for erosion behaviour of impacted fly-ash particles on coal-fired boiler components

Fly ash particles entrained in the flue gas from boiler furnaces in coal-fired power stations can cause serious erosive wear on steel surfaces along the flow path. Such erosion can significantly reduce the operational life of the boiler components. A mathematical model embodying the mechanisms of erosion on behaviour, has been developed to predict erosion rates of coal-fired boiler components at different temperatures. Various grades of steels used in fabrication of boiler components and published data pertaining to boiler fly ash have been used for the modelling. The model incorporates high temperature tensile properties of the target metal surface at room and elevated temperatures and has been implemented in an user-interactive in-house computer code (EROSIM-1), to predict the erosion rates of various grades of steel. Predictions have been found to be in good agreement with the published data. The model is calibrated with plant and experimental data generated from a high temperature air-jet erosion-testing facility. It is hoped that the calibrated model will be useful for erosion analysis of boiler components.     

Dielectric properties of doped polymethylmethacrylate

Permittivity of about 1 μm thick films prepared from polymethylmetactrylate (PMMA) solution doped with 20% of diphenyl sulfoxide was studied. The film structure was characterized by XRD, RBS, DSC and SEM techniques. The dopant was found to be distributed homogenously in the film volume. Permittivity of the films was measured as a function of the temperature. The measurement of the dependence of polarization on electrical field was performed using a standard Sawyer-Tower circuit. The presence of the dopant increases the composite permittivity namely above the PMMA glass transition temperature. Hysteresis loops observed on the measured polarization vs. electrical field dependence indicate easier and more pronounced polarizability of the composite comparing to pristine PMMA.     

The hydrostatic extrusion of polymethylmethacrylate

The hydrostatic extrusion behaviour of two grades of polymethylmethacrylate (PMMA) is reported and an analysis of the mechanics of the extrusion process is presented. Although the maximum degree of deformation achieved is considerably lower than that obtained for crystalline polymers, the process mechanics are controlled by the same factors (i.e. the billet-die friction, and the effects of strain rate and pressure on the material flow stress). A method for equating the effects of pressure and friction is described, following the work of Tabor on the adhesive mechanism of friction in polymers. This method gives a friction coefficient in the range 0.1 to 0.2 for the hydrostatic extrusion of PMMA, compared to values in the region 0.03 to 0.08 for crystalline polymers, suggesting conditions of boundary lubrication. The relatively high values of friction for PMMA are consistent with the requirement for careful preparation of the billet surface prior to extrusion, and the observation of the stick-slip phenomenon during extrusion. The differences between the grades of PMMA in both extrusion behaviour and tensile drawing behaviour are explicable in terms of their different glass transition temperatures.     

Influence of solid solution and dispersion strengthening meehanismson room temperature erosion behaviour of niekel

Abstract

The effect of solid solution and dispersion strengthening of Ni on the room temperature erosion behaviour has been evaluated. Commercially pure Ni, Ni–20Cr, and MA 754 alloys were tested at two different impact angles and two different impact velocities. It was observed that dispersion strengthening increases the erosion rate while solid solution strengthening decreases the erosion rate. The observed behaviour is explained using the localisation model in conjunction with the dynamic flow stress–strain data of the test materials.

MST/2088     

Transient volume holograms in polymethylmethacrylate

New transient volume holographic recording materials are described in this report. These consist of either coronene or 1,2;5,6-dibenzanthracene incorporated into polymethylmethacrylate. The largely amplitude, volume holograms formed as a result of triplet-state absorption have lifetimes of 6.4 and 0.9 s respectively and diffraction efficiencies close to the predicted values of 10⁻⁴. Production of permanent holograms has been prevented by careful control of the polymerization conditions to produce a matrix of sufficient rigidity to prevent reactions between the triplet states and the polymer. By using spectrally well separated write and read wavelengths, erasure of the holograms by the read beam is avoided.     

Space-charge-limited currents in polymethylmethacrylate films

An investigation was undertaken to examine the conduction mechanism in polymethylmethacrylate (PMMA) films. The current was measured in Al/PMMA/Al capacitors in the voltage range 4–300 V.     

On the environmental fracture of polymethylmethacrylate

Crack propagation of PMMA in some liquid environments is described for various testing conditions, such as fixed load, fixed displacement and monotonically increasing displacement. Fracture mechanics concepts have been used successfully in analysing the results. When continuous stable cracking is achieved, values of fracture toughness (R) for PMMA under these loading conditions are obtained as a function of crack velocity () using the method of Gurney and Hunt [11]. For crack velocities greater than 10^–2 mm sec^–1, the fracture toughness values in the environments are increased when compared with the corresponding air results. Unique relationships betweenR and have been shown to exist for cracking in ethanol and carbon tetrachloride.     

Dielectric properties of doped polystyrene and polymethylmethacrylate

About 1 μm thick films of polystyrene (PS) and polymethylmethacrylate (PMMA) doped with diphenylsulfoxide (DS) up to 40 wt.% were prepared from solutions using spin-coating method. Glass transition temperature (T _g) of doped polymer films was determined by DSC technique. The depth profile and surface concentration of DS dopant were measured by RBS and XPS methods, respectively. The temperature dependence of relative permittivity of the films was determined from capacitance measurement. The dependence of polarization (P) on electric field (E) was measured using a standard Sawyer–Tower circuit. The glass transition temperature T _g of both composites was found to be decreasing function of the DS concentration. The DS doping leads to an increase of relative permittivity of the PS and PMMA films. RBS and XPS measurements reveal an outward diffusion of DS dopant in PS/DS films at elevated temperature. No such effect was observed in PMMA/DS films. PMMA/DS layers were found to be more thermally stable comparing to PS/DS.