Concern for durability of reinforced concrete structures has, in recent years, coincided with an increasing interest in the use of steel fibre reinforcement. In this investigation the corrosion behaviour of conventional and steel fibre reinforcement in concrete under long term marine curing have been studied. The corrosion behaviour of reinforcement has been assessed visually, and by using electrochemical techniques. Three types of steel fibre were investigated namely low carbon steel, stainless steel melt extract, and galvanised steel. Previous studies have shown that steel fibres exhibit good corrosion resistance in concrete exposed to marine curing. It has been suggested that this is due to the discrete nature of the individual steel fibres which prevents the development of electrochemical cells with large cathode/anode area ratios. In order to determine, therefore, whether a 'size-effect' phenomenon influences steel fibre corrosion rates, concrete specimens were cast with different lengths and diameters of steel wire and subsequently exposed to marine curing. Parallel concrete specimens containing samples of conventional reinforcing bar were also manufactured. Cement replacement materials such as pulverised fuel ash, ground granulated blast furnace slag and microsilica are widely used in order to enhance specific properties of fresh or hardened concrete. In this investigation durability characteristics of concrete containing cement replacement materials were studied. These characteristics, including alkalinity of pore fluid and diffusion rates of chloride ions are of importance in relation to the passivation or corrosion of steel reinforcement.

No description available.

This thesis addresses the problem of detecting and positively identifying the approximately known acoustic emission signatures produced through fatigue crack propagation in power station steam chests. This work includes extensive laboratory fatigue testing to produce and record signatures in specimens fabricated from the steam chest steel, on-site recording of the ambience noise levels from a fully operating oil-burning power station and develops and demonstrates the effectiveness of various signal processing techniques at extracting the signatures embedded in the noise. This noise is high amplitude, giving us a low signal to noise ratio, and is broad in the frequency domain, with both regular and irregular high-amplitude metallic noise transients that cover the entire frequency range of interest. It is therefore essential to use sophisticated signal post-processing techniques to detect and to identify the crack signatures. The post-processing techniques developed and employed include time-frequency transformations, matched filters and signal expansion filters implemented in both in the time domain and in various two-dimensional time-frequency domains. From a performance comparison, both on the experimentally recorded data and on data digitally generated for the purpose, we determine the optimum signal processing method for our requirements and provide an assessment of the relative computational efficiencies. Generated for comparison are spurious but similar signatures characteristic of the power station steam chest environment; oxide crushing within an existing dormant crack and stress corrosion cracking signatures produced by the same steel constantly loaded in a corrosive environment. It is demonstrated that there is sufficient distinction between these signatures and those produced by crack propagation.

In the introduction to a recent symposium on rolling contact fatigue, R.A.Smith stated that it was difficult to apply our greatly increased understanding of metal fatigue, to rolling contact fatigue, because of "the apparent lack of alternating tensile stresses to drive the cracks." He went on to say "alternating shear stresses are easily found, but the reproduction of continuous crack growth controlled by shear (Mode II in fracture mechanics terms), has proved to be near impossible." This project has demonstrated that under specific conditions this mode of growth does occur. The project began by studying rolling contact fatigue defects, in particular the 'squat' defect in railway lines, and the stress analyses that have been performed on them. It was concluded that the largest stress cycle experienced by the cracks must be a shear stress. It. series of tests were then performed that loaded a crack in pure shear, or a mixture of tension and shear, looking at the effects of using fully reversed shear loading, and the effects of applying tensile mean stresses to reduce the friction on the crack flanks. However these tests all produced less than one millimetre of mode II growth, before the cracks arrested or branched. The final series of tests however applied a tensile load cycle before each shear load cycle. This time coplanar growth was produced, that is the crack grew in the direction of the maximum shear stress. This type of load cycle is a simplification of the load cycle calculated by Bower and Johnson of Cambridge University, where the tensile load is produced by fluid trapped in the crack. Two crack growth rate formulae were produced that fitted the data, indicating that the growth rate was dependent on both the tensile and the shear parts of the cycle.

The primary objective of this study is to test the applicability to plastics of a fracture toughness testing tool developed for metals. The intent is to study pre-test conditioning of several plastic materials and the effect of the depth of the razor notch cut in the chevron notched fracture toughness test specimens. The study includes the careful preparation of samples followed by conditioning in various environments. Samples were subjected to laboratory air for a specific duration or to a controlled temperature-humidity condition as per the ASTM D1870. Some of the samples were subjected to vacuum conditioning under standard test specifications. Testing was conducted using the conventional three-point bend test as per ASTM D5045-95. ASTM E1304, which sets a standard for short rod and bar testing of metals and ceramics provides some basis for conducting chevron notched four-point bend tests to duplicate the toughness tool. Correlation of these results with the ASTM test samples is determined. The four-point bend test involves less specimen machining as well as time to perform the fracture toughness tests. This study of fracture toughness testing has potential for quality control as well as the fracture property determination.

Under certain loading conditions, surfaces topography coupled with materials degree of brittleness can significantly compromise the mechanical performance of structures. The foregoing remains valid even if roughness is intentionally introduced for engineering reasons. In either case, stress can concentrate. The case of the stress concentration in surfaces having randomly distributed pits is a problem that, although being very practical, yet it remains unsolved. The complexity of a random configuration renders difficult the problem of analytically finding relationships between surface parameters and markers indicative of mechanical failure. Another difficulty is the reproducibility of replicates of specimens possessing random rough surfaces, for destructive testing followed by statistical analysis. An experimental technique to produce highly controlled replicates of random rough surfaces (including modeling of degradation growth) was developed. This method was used to experimentally and statistically study the effects on fracture of early randomly degraded surfaces of poly methyl methacrylate (PMMA) versus topographical parameters. Growth of degradation was assumed to go from an engineering surface to one whose heights are normally distributed. (Early stage of degradation is meant to be that level of roughness which is in the neighborhood of the critical flaw size for a given material). Among other findings, it was found that neither stress nor strain alone can be used to predict fracture at this early stage of degradation. However, fracture location was found to be strongly correlated to the ratio of the root-mean square roughness (RMS) to auto correlation length (ACL), above some RMS threshold. This correlation decreases as the material becomes less brittle (i.e., decrease of Young’s modulus or increase of percent of elongation). Simultaneously, a boundary value problem involving traction-free random rough surfaces was solved using a perturbation method, assuming elastic and isotropic conditions. For small RMS/ACL ratio, the solution for the RMS stress concentration factor, kt was found to be: kt = 1 + 2*SQRT(2)*(RMS/ACL), which agrees very well with the experimental work. Finally, a generalization of stress concentration factor formulas for several geometrical configurations and loading conditions into the Modified Inglis Formula was proposed. Finite element analysis was carried out and comparison was made with both experimental and analytical results. Applications of these results are broad. In surface engineering, for example, our analytical solution can be coupled with Fick’s Law to find critical conditions under which a film could become unstable to random roughness. Additionally, in design and maintenance of surfaces in service, it can be used to preliminarily assess how stress concentrates in surfaces where well defined notches cannot be used as an approximation.

The erosion resistance of 50 m metal-ceramic multilayer coatings has been investigated under impact conditions comparable to those in a gas turbine compressor cascade. lt was possible to improve upon the erosion resistance of Ti-6Al-4V by a significant margin. The influence of layer mechanical properties, layer thickness, ceramic content and coating process on erosion resistance has been studied over a range of impact conditions. The most suitable coating formulation for maximum erosion resistance changed with particle impact energy. Under low energy impact conditions (<55 joules) coatings with a high ceramic content demonstrated the highest erosion resistance. As particle impact energy increased, coatings with a high ceramic content perfonned poorly, and those containing a high volume fraction (50%) ductile metal layer, with thin metal and ceramic layers become more successful. Three principal damage types were observed: lateral fracture, tensile fracture and plastic definition. The most severe coating losses resulted from spallation due to lateral fracture. Coatings containing a high proportion of ductile metal with thin metal and ceramic layers were successful because such coatings had a high resistance to lateral fracture. Erosion resistance was greatest when the metal layer had a high yield strength and elastic modulus; such a combination of properties also resisted plastic definition. Scratch testing was investigated as a simple alterative technique for assessing coating erosion resistance. Repeated pass scratch testing generated similar damage modes to those of particle impact, but there was poor correlation between coating erosion rate and the threshold load for scratch damage.

The fracture of water ballast tank (WBT) coatings due to thermal stresses is widely recognised as an issue. Upon coating fracture, rapid corrosion of the tanker steel structure will occur, leading to expensive structure repairs or even tanker scrapping. In this project, the fracture behaviour of two experimental WBT coatings, referred to as A and B, in the forms of free film and substrated coatings was investigated. Static tensile tests and fatigue tests of the substrated coatings were performed. A finite element model of coating cracking was developed. Thermal stress and J-integral of surface cracking defects in substrated coatings were calculated using the model, in which the effects of defect size, coating thickness, and thermal strain on coating fracture were investigated. For the first time, fracture mechanics was used to explain WBT coating fracture behaviour. The J-integral of surface defects was used to predict the onset strain of coating cracking under mechanical strains in laboratory and under thermal strains in service. A theoretical comparison between the cracking drive forces in terms of J - integrals in WBT coatings under thermal strains and mechanical strains was performed.

Analytical, experimental and numerical work is reported on failure and fatigue crack growth in rotary shouldered drillstring threaded connections, where fatigue failure occurs at critical (last engaged) threads. A comprehensive review is made of drillstring failure data in the period 1989 to 1994, where connection failure accounted for 58% of the total, confirming it as the principal cause of drillstring equipment failure over the period. Approximately equal proportions of pin and box failures occurred. An extensive programme of 2-D elastostatic finite element analyses was undertaken on drilling motor and drillcollar connections investigating peak stress response at the critical threads to preload (from connection make-up) and applied loads. A pronounced effect of preload on these peak notch stresses was found, particularly at the pin, revealing the stress concentration factor (based upon remote nominal pipe stress) to be inconstant and a strong function of applied load, its value reducing with increasing load. Fracture mechanics data in the form of finite element derived stress intensity factor solutions were produced. New K solutions in the form of Y functions were derived for tension loaded connections containing cracks: in a drilling motor box at its last engaged thread (without preload): and in an API NC-61 nonmagnetic drillcollar pin and box at their last engaged thread sites, both with preload. These solutions are all conveniently referenced to nominal stresses in the uniform pipe making their use in crack growth studies straightforward. A new solution is also given for a fully circumferential internal surface crack in a tube under axial tension. The growing fatigue crack is also shown to cause load to be shed from the last engaged thread onto adjacement threads. The dominance of the thread root notch singularity at short crack lengths (a/T<0.1) means that equations fitted to the majority of the Y function data using linear regression analysis are done so for two separate regions of a/ T.

No description available.