We present results from a computer modelling study of fracture propagation in a rubber-toughened polymer subject to external uniaxial tension. Using a simple lattice model comprising two bond types (one for rubber and one for polymer), simulations have been performed on both 2- and 3-dimensional systems. Each bond is characterised by two parameters : an elastic constant and a mechanical strength. All bonds behave in a purely elastic manner up to fracture; this limits the model's applicability to impact failure of brittle glassy polymers. We employ a stochastic breaking process based on Eyring's reaction rate theory, and allow the system to equilibrate fully after each bond breakage. It is found that the Conjugate Gradient method is the most efficient technique for calculating these equilibria. The model allows straightforward measurement of the total damage at failure, which is a crude approximation of the material toughness, and the stress distribution during the damage evolution. We show that, for a wide range of bond parameters, as the rubber loading is increased the damage goes through a sequence of behaviour : embrittlement; toughening; saturation; and weakening. This sequence is consistent with experimental data for rubber-toughened Poly (methyl methacrylate) (RT-PMMA). Our results show that the embrittlement and saturation stages are sensitive to the ratio of the two elastic constants but that they disappear as this ratio approaches unity. The toughening achieved is sensitive to both the concentration and the mechanical strength of the rubber bonds, the most damaged systems being those with large rubber mechanical strength at approximately 50% concentration. We also find that the damage is dependent on the spatial distribution of the rubber bonds, the greatest toughness being achieved using a homogeneous rubber distribution. We also find that the evolution of the system's failure goes through four regimes. In the first of these, the damage is evenly distributed throughout the system but showsa propensity for occuring adjacent to rubber bonds. Subsequently, short cracks develop between some of these weakly damaged sites. As more of these develop, a number of long rubber-bridged cracks appear. In the third regime, the stabilising effect of the bridging rubber bonds forces the damage to extend throughout the system, leading to branching and interconnecting of rubber-bridged cracks. Finally, the stress experienced by the crack-bridging rubber bonds becomes unsustainable and catastrophic failure occurs. Zones of material consistent with each of these four regimes can be observed in experimentally fractured RT-PMMA. The main toughening mechanisms in operation in our model are, therefore, crazing (which occurs mainly in the second regime) and rubber-bridging (which occurs in the third). The former is enhanced by using a lower rubber elastic constant, whereas the latter is prolonged by a large rubber mechanical strength. We also apply the model to simulations of 2D and 3D systems containing rubber macro-particles, i.e. large volumes of connected rubber bonds, and we present visual results of the damage and the stress field evolution in such systems. In these, the damage appears to develop in accordance with the four regimes described above. The stress maps of multi-particle 2D systems show the gradual stress transfer which takes place as cracks develop, from initially high-stressed inter-particle polymer regions to the crack tips and, then, to the rubber particles along the crack lines. These visual results also highlight three possible modes of particle-crack interaction : particle debonding, particle splitting and crack reflection.

In recent years, the number of reinforced concrete structures requiring repair and rehabilitation has increased considerably. This is due mainly to the corrosion of the reinforcing steel within the concrete which has become contaminated with a highly corrosive environment containing chlorides and carbon dioxide. Specification of repair materials at present, in accordance with BD 27/86, is based on an inadequate understanding of the interaction between the substrate concrete and repair material. Cracking may occur in the early ages after application of the repair material due to the restraint to shrinkage provided by the substrate concrete and steel reinforcement. This would have serious durability consequences which would aid the onset of further corrosion. Furthermore, it is not clear at present if an efficient composite interaction is developed between the repair patch and substrate concrete in the long term to enable the repair patch to carry external load, or if the repair patch just acts as a barrier to protect the steel reinforcement from further corrosion. This research relates the basic properties of repair materials (elastic modulus, shrinkage and creep) to the long term performance of repair materials applied to actual concrete highway structures and simulated laboratory repairs. The research forms part of a LINK project sponsored by three companies and the Department of Transport. Three deteriorating highway structures were selected for concrete repairs. The repairs were monitored over the long term to gain an understanding of the interaction between the repair material, substrate concrete and steel reinforcement. These were Gunthorpe Bridge, a three span reinforced concrete arch bridge, crossing the River Trent to the east of Nottingham, Lawns Lane Bridge, carrying part of the Ml near Wakefield in West Yorkshire, and finally, Sutherland Street Bridge, carrying the B6080 in Sheffield, South Yorkshire. In order to gain information on the effectiveness of the repair materials in developing an efficient composite action with the substrate concrete, vibrating wire strain gauges were attached to the substrate concrete and steel reinforcement after the deteriorated concrete was removed. Embedment and surface gauges were installed in the repair patches either during or after repair material was applied. Laboratory investigations consisted of repairs to simply supported reinforced concrete beams. Repair patches were applied centrally at mid-span to beams which were subjected to two point loading at one-third span points. The interaction between the repair materials, steel reinforcement and concrete substrates were monitored by a combination of a demec extensometer and electrical resistance strain gauges. A theory is presented which predicts the long term interaction between the concrete substrate and repair materials to unpropped compression members. The analysis is divided into two stages. The first part deals with the distribution of shrinkage strain within the repair patch; the second part deals with the redistribution of external load from the substrate concrete into the repair patch. The distribution of shrinkage strain in the repair patch is based on the analogy of a bi-metallic strip contracting due to adrop in temperature (similar to a repair material contracting due to shrinkage). It has been observed that a repair material with an elastic modulus greater than the elastic modulus of the substrate concrete is able to transfer some shrinkage strain to the substrate concrete. This analysis, therefore, enables the tensile stress in the repair material, due to the partial restraint to shrinkage provided by the substrate concrete, to be predicted. Similarly, the restrained shrinkage strain that is transferred to the substrate concrete is also predicted as an increase in compressive stress in the substrate concrete. The transfer of external load into the repair patch is based on analysing the distribution of stress in a compound member when subjected to an axial force. The compound member consists of two different materials - the substrate concrete and repair material. The analytical expressions require basic properties of the repair material (elastic modulus, shrinkage and creep) and substrate concrete (elastic modulus) to enable a prediction of the stress across the repair patch in the long term to be made. The validity of the model is verified on the basis of the field data from the spray applied repair materials on two of the highway structures (Lawns Lane Bridge and Gunthorpe Bridge).

A mathematical model was used to calculate the internal stresses and strain produced when plates of a high hardenability steel (835M30) were quenched in water, polymer and oil. A prerequisite for the calculations was the experimental determination of relevant physical and mechanical property data not available from other sources? this included the surface heat transfer coefficient during quenching and the mechanical properties of 835M30 steel in the metastable austenitic condition. The results of the thermal stress calculations showed that the production of residual strain was markedly dependent on both the surface heat transfer coefficient and the flow strength of the steel during the early stages of the quench. It was predicted that the low stability of the vapour blanket, found during the early stages of a water quench, gave large variations in distortion. The more consistent values of the surface heat transfer coefficient, obtained during oil quenching, were associated with more reproducible levels of distortion. The predicted residual stress distributions were less sensitive to variations in either the surface heat transfer coefficient or the flow strength of the steel during the early stages of the quench. However, the residual stress distributions were strongly dependent on the flow strength of the material during the martensite transformation. A correlation of the results of the calculations with experimental behaviour showed that agreement was good in the cases of both the residual stresses and strains obtained after a water quench. However, in the case of oil quenched plates, a similar correlation was only obtained with residual strains. A possible explanation for the lack of good correlation in the case of oil quenched plates may be found by consideration of the effect of strain rate on the mechanical properties of the material, particularly during the martensitic transformation.

It is important to know the lifespan of a component, thus services can be planed. At Siemens Industrial Turbomachinery AB (SIT AB) develop and produce gas and steam turbines, the turbines are heavily strained by thermal and centrifugal forces during their life span which lead to cracking. Today there is no good crack propagation method for materials with large amount of plastic deformation and creep. At SIT a new method, ΔKBDCPP, has been developed that takes these phenomenon in consideration. It is assumed that the whole model has plastic material properties except circular elements at the crack front which have elastic properties. The new method is reviewed by doing simulations on a a specimen model in Franc3D together with the FE program Abaqus. There have been conducted thermo-mechanical fatigue (TMF) tests and 2D simulations on specimen to measure the crack propagation. The TMF tests where immitaded in new simulations with the method ΔKBDCPP. Forces, creep times, cycles and material data where emulated in the model and then simulated. The number of cycles per crack length for the isothermal simulations became higher then for the TMF tests. This is a expected result because the conditions are optimal during simulations, which they may not be during experimental test. The result of ΔKBDCPP are closer to the TMF test results then the 2D simulated ΔKeffective, another crack propagation calculation method. Additionally, out-of-phase (OP) simulations where carried out, in OP the maximum temperature and maximum stress do not coincide. When the temperature is at max the component is held in the minimum stress. For OP simulations the results became irregular, for one of the simulations the number of cycles per crack length became lower for the simulated specimen than for the TMF test, and contrariwise for the other OP simulation. However, it was concluded that the later was correct because the majority of the simulations, and all the isothermal simulations, gave this result. it also was the result for the isothermal simulations. One OP simulation was compared to a 2D simulation and here the method of ΔKBDCPP was also more accurate to the TMF tests. The method of ΔKBDCPP seem to function well however, the solving time for models with creep is long. A model with a small crack and creep is time consuming to simulate, which is a disadvantages. / Det är viktigt att veta livslängden på en komponent, då kan det planeras för en service. På Siemens Industrial Turbomachinery AB (SIT AB) utvecklas och produceras det gas- och ångturbiner, turbiner utsätts för höga temperaturer och krafter under dess livstid vilket resulterar i sprickor. Idag finns det inte en bra sprickpropageringsteori för material i ett stort plasticerat tillstånd och kryp, SIT har därför tagit fram en ny metod som ska ta hänsyn till dessa parametrar. I den nya beräkningsmetoden, ΔKBDCPP, antas det att hela modellen har en plastisk materialmodell förutom ett antal ringelement vid sprickspetsen som antas ha en elastisk materialmodell. Den nya metoden undersöks genom att provstavsmodell simuleras i programmet Franc3D tillsammans med Abaqus. Det har tidigare gjorts termodynamiska utmattningsprov (TMF), samt simuleringar i 2D, på provstavar för att mäta sprickpropageringen. Därför imiterades dessa TMF prov i nya simuleringar med metoden för ΔKBDCPP. Krafter, hålltider, cykler och materialdata efterliknades i modellen och sedan kördes simuleringarna. För de isoterma körningarna blev resultatet att antalet cykler per spricklängd är högre för simulerade värden än för TMF prover, vilket var väntat. Vid simulationer är det optimala förhållanden utan avvikelser, vilket kan finnas vid experimentella tester. De simulerade värdena för ΔKBDCPP stämmer bättre överens med TMF proverna än de 2D simulerade resultaten för ΔKeffective, en annan beräkningsmodell för sprickpropagering. Även out-of-phase (OP) simuleringar genomfördes, vid OP sker inte temperatur max och spännings max samtidigt. När det är max temperatur hålls komponenten i den minimala spänningen. För OP simuleringarna blev resultatet annorlunda, ena körningen blev antalet cykler per spricklängd lägre för de simulerade värden än för TMF prover och i den andra blev det tvärt om. Dock drogs slutsatsen att den senare stämde då majoriteten av körningarna, och alla isoterma körningar, gav det resultatet. En OP körning jämfördes med en 2D körning och även där var metoden för ΔKBDCPP närmare TMF provsresultatet. ΔKBDCPP metoden verkar fungera bra, dock är simuleringstiden för att lösa körningar med kryp väldigt lång. En modell med en liten spricka och kryp är mycket tidskrävande att lösa, vilket är en nackdel.

The purpose of this study was to assess the bioprotectant performance of chemical modification wi th three different isocyanates (n-butyl, hexyl and l,6-diisocyanatohexane, BuNCO, HeNCO and HOI respectively) in Corsican pine (Pinus nigra Schneid) sapwood. Wood-isocyanate bond formation was verified by the increase in sample weight, volume and by infrared spectroscopy. Basidiomycete (Coniophora puteana, Gloeophyllum trabeum, Coriolus versicolor, Pycnoporus sanguineus) decay tests demonstrated protection by chemical modification. The relationships of fungal species, weight percent gain (WPG), decay induced weight loss and moisture content were examined. One of the brown rot fungi, ~. puteana, showed higher threshold protection values than the other fungi tested and the di isocyanate showed better perf ormance. Scanning Electron Microscopy and enzyme detection tests (cellulase and phenol oxidases) have been carried out in an attempt to gain a better understanding of the treatment performance. Chemical characteristics of the sound and brown rotted wood (~. puteana) have been examined using sulphuric acid, sodium chlori te and high performance liquid chromatography-HPLC (gel permeation chromatography-GPC) procedures to clarify the principles which govern isocyanate modifications and restrict fungal decay. A number of parameters were examined including lignin and holocellulose contents, holocellulose molecular weight and degree of polymerisation (OP and dispersity). These demonstrated that chemical modification changed the configuration of the original wood polymers. Al though preferential modification occurred at lower weight percent gains in the lignin fraction appreciable wood protection against C. puteana only occurred when the holocellulose fraction showed substantial changes due to chemical modification. To examine further the effect of moisture and loading of substituent groups within the outer layers of wood after chemical modification (BuNCO & HOI), tensile strength resistance to surface colonisation by soft rot fungi was undertaken utilizing thin wood strips after unsterile soil tests. Less modification was necessary to achieve protection against soft rot in this test.

No description available.

Titanium aluminides are being developed to expand the temperature capability of titanium alloys with maximum service temperatures around 700°C. These materials also have the ability to replace nickel superalloys with potential applications in the high pressure compressor, and in the 4th stages of the low pressure turbine. The above applications place these alloys in environments not previously considered. Within the compressor hot salt corrosion may be a problem with salt ingested from the atmospheric aerosol. While in the turbine the combination of salt ingestion, and SO, from the burning of fossil fuels, results in hot corrosion being a potential problem. In this study the individual effects of salt and So2 were investigated, with corrosion mechanisms being proposed using kinetic, metallographic and thermodynamic data. Understanding these effects enabled both the hot salt corrosion and hot corrosion behaviour of TiAl alloys to be evaluated. In air alone continuous alumina layers, within a mixed alumina/rutile scale, provide the oxidation resistance of TiAlNb alloys. Logarithmic kinetics operated for 100 hours at 700°C and for 13 hours at 750°C. Parabolic kinetics then operated out to 100 hours at 900°C. Mass gains ranged from 0.06 to 2.1mg/cO after 100 hours at 700 and 900°C respectively. This situation changes in bi-oxidant, air/S02, atmospheres where increased growth rates are linked to the formation of a continuous sulphide layer at the scale/substratien terface. Below 800°C logarithmic/parabolic kinetics operate. At and above 800°C initial logarithmic kinetics change to near linear/breakaway kinetics with spallation becoming a problem. Mass gains, after 100 hours, ranged from 0.2 mg/cM2 at 700°C up to 6.4 mg/cm2 at 900°C. The presence of low salt concentrations [<0.05mg/cm²] resulted in severe substrate degradation, with preferential attack down a2 lathes.The first 10-20 hours were shown to be the most important with low melting point salt mixtures spreading across the surface, increasing the rate of attack. The evolution of HCI/Cl2 during initial substrate attack leads to the Vapour Phase Transport of aluminium and manganes chlorides resulting in whisker growth over a porous rutile scale. The presence of salt modified the diffusion controlled kinetics under purely oxidising conditions. Chlorine was shown to promote the vapour phase transport mechanism which resulted in the initial accelerated logarithmic kinetics. A change to parabolic type kinetics occurred due to the loss of chlorine to the atmosphere. The mass gains, after 100 hours, ranged from 0.06 to 1.1rn g/cm² between 500 and 800°C. The combination of salt deposits and S02 bearing environments resulted in severe substrate degradation. Salt played a dominant role during the early stages of corrosion, whilst low partial pressures of S02 affected the later stages of corrosion. Non protective oxide scales were developed with low melting point MnSO4-Na2SO4 mixtures forming at salt deposits and a continuous sulphide layer at the scale/substrate interface. Rapid scale growth resulted in severe scale spallation. The initial stages of hot corrosion followed rapid logarithmic type kinetics. Further increases in the corrosion rate where promoted by the formation of continuous sulphide layers at the scale/substrate interface. Parabolic kinetics, at this stage, were followed by linear growth rates once scale spalling occurred. Mass gains, after 100 hours, ranged from 0.52 to 3.89 mg/cm² between 650 and 800°C.

No description available.

No description available.

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