The prediction of crack growth in bonded joints under cyclic-fatigue loading I. Experimental studies

The performance of adhesively-bonded joints under monotonic and cyclic-fatigue loading has been investigated using a fracture-mechanics approach. The joints consisted of an epoxy film adhesive which was employed to bond aluminium-alloy substrates. The effects of undertaking cyclic-fatigue tests in (a) a ‘dry’ environment of 55% relative humidity at 23°C, and (b) a ‘wet’ environment of immersion in distilled water at 28°C were investigated. In particular, the influence of employing different surface pretreatments for the aluminium-alloy substrates was examined. In addition, single-lap joints were tested under cyclic fatigue loading in the two test environments, and a back-face strain technique has been used which revealed that crack propagation, rather than crack initiation, occupied the dominant proportion of the fatigue lifetime of the single-lap joints. In Part II, the data obtained in the present Part I paper will be employed to predict theoretically the lifetime of the adhesively-bonded single-lap joint specimens.     

Synthetic hydrogels: 3. Hydroxyalkyl acrylate and methacrylate copolymers: surface and mechanical properties

The surface and mechanical properties of copolymers of hydroxyalkyl acrylates and methacrylates have been examined by a variety of techniques. This work is complementary to earlier parts of this series which describe the effect of copolymer structure on water binding properties. Water structure has been demonstrated to exert a profound effect upon mechanical properties whether measured in compression or in tension. In particular, water that is characterized by differential scanning calorimetry as ‘freezing’ water is observed to have a marked plasticizing effect upon the gel, whereas ‘non-freezing’ water has little such effect. Similarly, the ‘freezing’ water produces a more marked effect on thermally induced transitions. Two distinct transition points are observed as a result of its presence. One corresponds to the freezing point of water and the other to a glass transition temperature, whose value depends upon the proportion of ‘freezing’ or ‘plasticizing’ water in the gel. Several predictive and direct measurement techniques have been used to study the surface properties of the copolymers in both hydrated and dehydrated states. Taken together they have established a sound understanding of the way in which polar and dispersive components of surface free energy vary as a function of copolymer composition and water content. Use of protein adsorption and fibroblast cell interaction techniques demonstrate that biological phenomena respond to changes at a molecular level which current macroscopic surface energy techniques are unable to discern.     

Relations between structure and properties in bisphenol A polyester carbonates

A range of hydroxy-terminated polybisphenol A terephthalate and isophthalate blocks have been prepared with molecular weights 800–5000, which were then coupled with phosgene to give alternating polyester copolycarbonates. These materials have been characterized by their physical, thermal and mechanical properties. The thermal properties have been investigated using differential scanning calorimetry and the glass transition temperatures, specific heats and Δcps′ (at the T_g) values obtained. A relationship appears to exist between Δcp and the reciprocal molecular weight of the polyester blocks, and the molar ratio of ‘ester’ to carbonate of the copolycarbonates. A maximum in the glass transition temperature has also been observed in the copolycarbonates, corresponding to a certain ‘ester’: CO₃ ratio. Tensile mechanical analyses have been performed on cast or moulded films of the copolycarbonates. The terephthalates can give films which extend uniformly but the isophthalates always neck.     

The interplay of physical aging and degradation during weathering for two crosslinked coatings

Polymer molecular relaxation, or ‘physical aging’, is a very important influence on permeability and mechanical properties of any polymer below its glass transition. ‘Physical aging’ occurs as even an unstressed polymer gradually relaxes towards its equilibrium conformation. This and the shorter term response to stress happen over periods much longer than the typical cycle of an accelerated weathering test, thus important properties of a polymeric coating may be affected by the difference in frequency between natural and artificial exposures, in addition to other factors. Further, ‘physical aging’ is affected by chemical changes to the polymer network caused by the degradation during a weathering exposure. In this investigation, purely physical aging was compared with the effect of concurrent chemical degradation by measuring ‘enthalpy recovery’ and mechanical stress relaxation at a variety of temperatures and at various stages during accelerated weathering exposure. The effect of physical aging was quite apparent in both an epoxy-polyamide coating and a polyester-urethane coating. Changes in physical aging behaviour during degradation were different for the two coatings, which points to further reasons for discrepancy between accelerated weathering and natural exposure.     

Scale effects in fluidized multiphase reactors

It is shown that the two-phase model for bubbling gas—solid fluidized beds can be extended to bubble column slurry reactors operating in the heterogeneous flow regime by proper definition of the ‘dilute’ and ‘dense’ phases. The ‘dilute’ phase in a bubble column slurry reactor is to be identified with the fast-rising ‘large’ bubbles. The ‘dense’ phase consists of the slurry phase in which ‘small’ bubbles are finely dispersed. With the aid of extensive experimental data obtained in columns of 0.1, 0.19 and 0.38 m diameter it is shown that the rise velocity of the ‘dilute’ phase for gas—solid fluid beds and slurry reactors show analogous scale dependencies and can be modelled in a similar manner. It is also demonstrated that fluidized multiphase reactors can be modelled in a common manner using Computational Fluid Dynamics (CFD) within the Eulerian framework. It is concluded that CFD is an invaluable tool for scaling up of fluidized multiphase reactors.     

A study of the suitability of the KENTORT II eastern shale oil for asphalt paving applications

The uncertain future of petroleum reserves has fuelled the search for alternative resources. A feasibility study was conducted to determine potential paving applications of the oil extracted from eastern shale by the KENTORT II process. The eastern shale oil (ESO) in this study was separated into two drastically different viscosity portions, designated as ‘hard’ and ‘soft’ ESO. It was hypothesized that the ‘hard’ portion might enhance the asphalt performance by increasing the stiffness. It was discovered that the ‘hard’ ESO modified asphalt properties deteriorate significantly with time. On the other hand, the ‘soft’ ESO was found to exhibit desirable properties in an asphalt recycling application. Further studies are recommended to fully characterize the binder and mixture properties of ESO modified/rejuvenated asphalts.     

Catalytic reduction of nitric oxide on Pt and Rh catalysts supported on alumina and titania synthesized by the sol-gel method

Platinum and rhodium supported on alumina and titania were synthesized by the sol-gel method. Characterization and catalytic activity for the reduction of nitric oxide by carbon monoxide was performed. In sol-gel ‘sintered/reduced catalysts’ a redispersion of the metal phase and higher resistance to sintering was observed. On the other hand, in the impregnated ‘sintered/reduced catalysts’, an important sintering effect was observed. The sol-gel ‘reduced catalysts’ and ‘sintered/reduced catalysts’ showed higher activity than that of impregnated reference catalysts, mainly when the titania is the support. Moreover, sol-gel preparations are more selective to N₂, whereas impregnated reference catalysts are selective to N₂O. The increase in dispersion and high resistance to sintering on sol-gel ‘sintered/reduced catalysts’ was interpreted as a surface migration effect of the metal particles buried in alumina and titania gels.     

A polarised μ-FTIR study on a model system for nylon 6 6: implications for the nylon Brill structure

Polarised transmission FTIR microscopy studies (μ-FTIR) have been performed on a monodisperse 3-amide oligomer. The oligomer is a model compound for nylon 6 6; it has essentially the same room temperature crystal structure, and it undergoes the same high temperature transition, the Brill transition, prior to melting. However, the oligoamide forms extended chain, rather than chain-folded, crystals, and so crystals are produced that are essentially 100% crystalline, and of μm–mm size. Consequently, this material is ideally suited for polarised μ-FTIR single crystal studies. The thermal polarised FTIR behaviour of this material provides definitive proof that the Brill transition does not involve major rearrangement of hydrogen bonds, since the strong parallel polarisation of both the NH stretch and amide I bands are retained right up to melting. Quantitative infrared dichroism measurements indicate that a maximum of 5° rotation of the N–H bonds about the extended chain axis occurs prior to melting. These results strongly suggests that the equivalent Brill transition in nylon 6 6 also proceeds without significant hydrogen bond rearrangement. In addition we have investigated the behaviour of designated ‘Brill’, ‘crystalline’, ‘amorphous’ and ‘fold’ bands that are present in our spectra.     

Capture and rebound of dust in granular bed gas filters

Experimental data for the efficiency of filtration of gases by fixed beds of granular solids are used to evaluate the reliability of the ‘cell’ and ‘constricted tube’ models for gas flow and aerosol transport. The dominant capture mechanisms are Brownian diffusion and inertial deposition. For Brownian diffusion, both models give sensible estimates for capture efficiency, but this process is shown to be insensitive to the model assumptions. Inertial deposition provides a much more sensitive test, and it is shown that neither model gives satisfactory predictions for the efficiency of inertial capture. Whether a dust particle adheres or rebounds on contacting a filter granule depends on the relative importance of kinetic and adhesion energies. An approach is proposed which enables the theoretical analyses to be applied to predict the limits of adhesion.     

Local measurements of void fraction and liquid holdup in packed columns using X-ray computed tomography

A ‘tailor made’ computed X-ray tomographic scanner has been developed as a tool for the analysis of the distribution of gas, liquid and solid phases in packed columns. The very good spatial resolution of the scanner has first been assessed by the imaging of objects of known shape and size, called ‘physical’ phantoms. Images have then been realized on a 0.6 m diameter and 2 m height column packed with Cascade Mini-Ring 1A packing elements, which is a random polypropylene packing designed to be used in absorption columns. The solid phase distribution, leading to the void fraction distribution, has been analyzed on cross-section images of the dry packed column (without any liquid flowrate). The measured value of the bed void fraction is equal to the value provided by the manufacturer. The analysis of the axial profile of void fraction shows that the ‘end effect’ may be neglected, whereas the analysis of the radial profile evidences the existence of a non negligible ‘wall effect’. Images have then been carried out on the packed column irrigated by a liquid flowrate ranging between 0 and 6000 l h⁻¹ (0–6.10⁻³ m s⁻¹). Hold-up values have been measured in different cross-sections of the column and averaged in order to obtain the total hold-up value in the bed. The computed values are in very good agreement with those reported in the literature for similar packing. The dependence of the liquid hold-up on the liquid superficial velocity can be expressed in terms of a power law. The fitted value of the exponent, equal to 0.65, is in the range of exponent values found in correlations of the literature.     

Unifying concepts in non-linear unsteady processes Part I: Solitary travelling waves

Non-linear unsteady processes, as different as car traffic, adsorption, sedimentation, packed-bed heat transfer and hydraulic waves, can be analysed in a unified fashion by using the concept of the travelling wave. The fundamental aspects of the ‘solitary’ waves are discussed, as well as their distinctive features, such as overall and local propagation velocities, shape modifications, appearance of shocks, and ‘coherent’ asymptotic shape. These features are governed by the velocity distribution along the wave, itself determined by the relationship between the flux and the concentration variables of the process. Qualitative rules result from the analysis of ‘operating lines’ and ‘equilibrium lines’ in the flux versus concentration diagram, in a way somewhat similar to the McCabe-Thiele analysis for steady counter-current operations.     

Group transfer polymerization and its use in water based pigment dispersants and emulsion stabilizers

Group transfer polymerization (GTP) can be used to make AB diblock acrylic polymers. It provides excellent control of the structure of these polymers. With a hydrophilic B block, these polymers have been used to prepare water based emulsions, pigment dispersions, and slurries. These systems have property advantages over emulsions and dispersions made with conventional stabilizers. These advantages include increased stability, smaller particle sizes, lower viscosities, and less moisture sensitivity. The structure of the AB diblock polymer affects the properties of both the pigment dispersion and the emulsion particle. The composition, size and ratio of each block affect the overall quality. For aqueous systems, a balance of hydrophobic ‘A’ blocks and very hydrophilic ‘B’ blocks is needed for optimum properties. The hydrophobic ‘A’ blocks, which are homo or copolymers of methacrylate monomers (such as butyl or ethylhexyl methacrylate), are surface active and can associate with either pigment or emulsion polymer surfaces. The hydrophilic ‘B’ blocks, which are neutralized acid or amine containing copolymers, provide both ionic as well as steric stabilization in water-borne systems.     

Continuous plasticized melt-extrusion of polyacrylonitrile homopolymer

In this work, it is shown that polyacrylonitrile (PAN) powder which had been plasticized by propylene carbonate (PC) could be ‘melted’ and extruded continuously into filaments. The ‘molten’ extrudate solidified on-line as a result of cooling, without the need for coagulation. In this respect, PAN-PC solutions behaved differently when compared with other solutions commonly used in the wet-spinning of PAN fibres. Spontaneous solidification on cooling meant that the filament could be wound-up on a take-up system without the need to pass it through a coagulation bath. The solidification occurred as a result of the rapid crystallization of the PAN from the propylene carbonate solution. The X-ray diffraction pattern of the solidified filament (which contained 40–50 wt% of solvent) was different from that of the dry PAN powder, indicating that a different polymorph had been formed. Drawing of the filaments, however, led to the normal hexagonal polymorph. A PAN:PC (50:50 by wt) ‘melt’ had a shear viscosity comparable with that of a conventional thermoplastic such as extrusion-grade polyethylene. The plasticized melt showed shear-thinning behaviour.     

A novel system for self-validating adhesive joints

A self-validating adhesive system is proposed in which ‘zero volume unbonds’ or ‘kissing’ bonds, which are undetectable by NDE techniques, may be filled and bonded by a secondary component in the adhesive during cure. The system shows the principal criteria required for such a smart system, and has been shown to enhance the strength of a bonded joint without undue deterioration in durability or water uptake.     

Distribution of chemisorbed oxygen in mildly oxidized caking coal

A high-volatile A bituminous coal was oxidized with ¹⁸O₂ at 100 °C for 72 h and the resultant ‘oxycoal’ heat-treated at atmospheric pressure and in vacuo at temperatures up to 430 °C. The ¹⁸O content was determined in the ‘oxycoal’ before and after heat treatment, and in extracts and residues obtained from chloroform extraction of heat-treated ‘oxycoal’. In addition, unheated ‘oxycoal’ was extracted with pyridine and chloroform, and [¹⁸O] was measured in the -, β- and γ-fractions. The results support the view that decreased yields of chloroform-soluble matter (and corresponding loss of plasticity) associated with oxidation are mainly due to condensation reactions which reactive oxygen-bearing groups undergo during heat treatment, and that the extent of these reactions is markedly sensitive to pressure.     

Radiation chemistry aspects of polymerization and crosslinking. A review and future environmental trends in ‘non-acrylate’ chemistry

Radiation induced polymerization of acrylate based materials has been in industrial use for almost twenty years. This field is still growing rapidly and the advantages of the technology, e.g. being solvent free and rapid ‘cure’, are very attractive properties from an industrial and environmental point of view. Mixtures of mono- and multifunctional acrylates are today the ‘heart’ of radiation curable systems. However, serious concerns about the health hazards connected with the handling of the liquid coating is an important issue. Different approaches to overcome this problem have been addressed and the obvious thing to start with was the investigation of a great number of acrylates regarding their potential health effects. Acrylates do exhibit some specific unattractive properties, regarding their toxicology profile. Sensitization and skin irritation are considered to be limiting factors for a continuing rapid expansion of this successful technology. A further development and improvement of the photoinitiator chemistry together with a more efficient use of high powered irradiators with a narrow bandwidth distribution, will improve still the acrylate technology. The importance of degree of conversion of carbon-carbon double bonds as a function of dose rates, i.e. residual unpolymerized acrylic monomers, will be discussed. An additional approach, since the acrylate technology is well established, is to further optimize the ‘reactivity’ and conversion on ‘new’ acrylates in order to obtain maximum conversion, thus minimizing the residual amount of monomers that can migrate out of the coating. The increasing functionality of the acrylate/oligomer will of course result in an increasing probability of monomer/oligomer being attached to the crosslinked network. Furthermore, this paper emphasizes potential acrylate replacements, and ‘new’ or alternative chemistry for acrylates will be introduced. The state of the art and the associated problems for cationically induced polymerization, free radical alternating copolymerization, hybrid systems and direct photolysis of donor/acceptor pairs are discussed.     

Properties of some blended high-alumina cements

The compressive strength of concretes made from mixtures of ‘Ciment Fondu’ high-alumina cement and granulated blastfurnace slag has shown an increasing trend up to 5 years when kept under water at 20° and 38°C. Concretes made from neat HAC have shown much reduced strength at 38°C after only one week, following the well known ‘conversion’ reactions. The presence of slag and of pozzolanas such as microsilica and metakaolin in blended HAC is responsible for the formation of strätlingite in the hydrated product and the opportunity for reactions leading to the ‘conversion’ phenomenon to proceed is severely limited. The sulphate resistance of mixtures of HAC and ggbs is excellent. Concretes made from blended HAC containing ggbs are expected to show a much lower rise in temperature when placed than those made from HAC alone. The hydration chemistry of blends of Ciment Fondu and several mineral admixtures has been studied using conduction calorimetry, x-ray diffraction, thermogravimetry and electron beam microanalysis.     

Correction for sampling errors due to coagulation and wall loss in laminar and turbulent tube flow: direct solution of canonical ‘inverse’ problem for log-normal size distributions

Aerosol sampling from industrial environments (e.g. combustion engines) or natural environments (e.g. the troposphere) frequently involves conveying the sample to a downstream (‘sheltered’) instrument via an upstream tube or duct. While the instrument may be capable of characterizing, say, the particle size distribution (PSD) of the aerosol actually presented to it, the investigator is, of course, usually more interested in the PSD of the aerosol entering the upstream sampling tube. Invariably, this differs from that measured because of several systematic phenomena—perhaps the two most obvious of which are particle size-dependent losses to the tube walls (i.e. incomplete ‘penetration’) and PSD distortion due to suspended particle-particle coagulation when the particle concentrations are sufficiently high. We show here how recent research on the use of ‘moment methods’ to predict the effects of size-dependent walls loss and/or Brownian coagulation in flow systems can now be brought to bear to conveniently solve this ‘inverse’ problem by numerically integrating the quasi-one-dimensional coupled moment equations in the upstream direction, using downstream (measured) aerosol properties in the definitions of all dimensionless dependent variables and parameters. Illustrative ‘universal’ graphs are presented here for the sampling of log-normally distributed ‘inertialess’ (Brownian) aerosols in long straight adiabatic ducts for both commonly encountered extremes of particle Knudsen number Kn_p 1(free molecule) or Kn_p 1 (continuum), as well as convenient rational approximations derived from the leading terms of a Taylor series expansion of the above-mentioned dimensionless moment equations.