No description available.

No description available.

No description available.

No description available.

The most commonly used method for non-destructive testing (NDT) of welded tubulars in underwater locations is magnetic particle inspection (MPI). This method is effective in terms of crack or defect detection, but requires much diver effort. This work examines the use of Hall effect probes for crack detection and measurement in steel specimens and underwater pipelines and structures. A simple theory of magnetic leakage fields is developed, and how such fields relate to crack characteristics. The finite sizes of the Hall probes employed are taken into account, and an analytic expression for the field from a tapered crack is developed. Practical magnetic signals from a cracked Y-jointed tubular are taken, and shown to be consistent with MPI indications. A double probe system is proposed which enables crack depth measurement to be made irrespective of a knowledge of the crack width or level of magnetisation in the specimen. Experiments using a prototype double probe system show encouraging results on artificial cracks in small specimens, though there is a troubling unknown background bias effect in the measured signals. An instrument using a time differentiated probe signal has been developed which is capable of detecting a crack in a Y-joint at a scan height of up to 5mm with a level of magnetisation rather less than that used by MPI. A method of continuously monitoring a crack in a Y-joint is also described, using multiple differential pairs of probes. The method is found to give indications consistent and comparable with MPI.

Benefiting from good specific mechanical properties, exceptional oxidation resistance, and high temperature capability, Titanium Alloys are used in Gas Turbine Engines, especially in the early stages of the compressor. However they are subject to stresscorrosion cracking in the laboratory when subjected to stresses and contaminated with salts at elevated temperatures. The lack of in-service failures of titanium components due to Hot-Salt Stress-Corrosion Cracking (HSSCC) is not yet understood. The parameters influencing the HSSCC of titanium alloys (temperature, load, stress and temperature cycling, quantity and kind of salt, air velocity, water vapour or oxygen content of the atmosphere, composition, texture, and microstructure of the alloy, surtace conditions), cannot account for the lack of in-service failure. After an examination of the service conditions within a typical gas turbine engine compressor, it was considered that the high pressures prevailing may extend the life of titanium alloys subjected to HSSCC. This work used a unique high temperature, high pressure, servo-hydraulic facility in order to carry out hot-salt stress-corrosion testing on titanium alloy 1M! 834 at high pressure. The results obtained show that high oxygen partial pressures extend significantly the life of 1M! 834 subjected to HSSCC. Continuous thermogravimetric measurements both in oxidising and salt-corroding environments were carried out to study the kinetics of the hot-salt attack of IMI 834. Basic metallography revealed the formation of channels which extend deep into the metal during the initial stages of hot-salt-corrosion. Theoretical thermodynamic studies highlighted the role of alloying elements and vapour phase metallic chlorides in the mechanisms of the HSSCC of titanium alloys. A new model for the hot-salt stress-corrosion of titanium alloys is proposed. It is based on the establishment of a self sustaining cycle where vapour phase metallic chlorides act as hydrogen carriers and can diffuse quickly into the material through channels.

Extruders are used in a wide range of process industries and high reliability is essential if cost effective manufacturing is to be maintained. A critical part of twin-screw extruders is the barrel that must withstand many different wear and corrosion environments depending on the end user. For many applications the extruder barrel is a critical component and it is essential that it performs in a predictable manner, providing the necessary design life-time. This project has addressed these aims by considering the wear/corrosion behaviour of current and potential extruder barrel materials from which a life prediction model has been developed. A wide range of engineering materials has been evaluated in the laboratory for abrasive wear resistance using a dry sand abrasive wear test according to ASTM G 65-93. An appraisal of the tests and the applicability of the results to the in-service conditions of an extruder has lead to further testing for abrasion and abrasion-corrosion resistance of four materials, namely Mild Steel, 440C, N18 and N18+5%TiC+5%TiN. Plastic deformation was the main feature of the damaged surfaces in the form of ploughing which has been modelled in terms of a low-cycle fatigue process. The relative hardness between material and abrasive was found to be an important parameter in controlling the rate of material removal. It has also been shown that the synergistic effect of abrasion-corrosion results in an accelerated material removal rate. The information from these tests has been used to develop a model of the wear of extruder barrels by abrasive particles. It is shown that there is a correlation between the particle size, wear debris size and wear groove size distributions. From a knowledge of the particle flux, the particle size distribution and the loading conditions, metal recession is predicted based on a low-cycle fatigue process. The wear rates for a wide range of Fe- and Ni-based materials are predicted to better than a factor of two. When corrosion is also present, the mechanism of metal recession depends on whether passive surface films are formed. For the Fe-based materials which exhibit direct dissolution of material, the wear/corrosion rate can be estimated by combining the metal loss rate under pure wear and pure corrosion conditions only. For the Ni-base alloys, thin passive films form in all the aqueous environments studied and corrosion rates are extremely low. However, during abrasive wear the passive films are removed and the overall metal recession rate is a combination of metal loss due to abrasive wear of the substrate and the continual formation and removal of surface passive films.

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Metallic structures dominated by tensile loads are vulnerable to fatigue and fracture. Fatigue is produced by oscillatory loads. Quasi-static brittle or ductile fracture can result from a "large" load in the random sequence. Moreover, a fatigue or fracture failure in a member of a redundant structure produces impulsive redistributed loads to the intact members. These transient loads could produce a sequence of failures resulting in progressive collapse of the system. Fatigue and fracture design factors are subject to considerable uncertainty. Therefore, a probabilistic approach, which includes a system reliability assessment, is appropriate for design purposes. But system reliability can be improved by a maintenance program of periodic inspection with repair and/or replacement of damaged members. However, a maintenance program can be expensive. The ultimate goal of the engineer is to specify a design, inspection, and repair strategy to minimize life cycle costs. The fatigue/fracture reliability and maintainability (FRM) process for redundant structure can be a complicated random process. The structural model considered series, parallel, and parallel/series systems of elements. Applied to the system are fatigue loads including mean stress, an extreme load, as well as impulsive loads in parallel member systems. The failure modes are fatigue, brittle and ductile fracture. A refined fatigue model is employed which includes both the crack initiation and propagation phases. The FRM process cannot be solved easily using recently developed advanced structural reliability techniques. A "hybrid" simulation method which combines modified importance sampling (MIS) with inflated stress extrapolation (ISE) is proposed. MIS and ISE methods are developed and demonstrated using numerous examples which include series, parallel and series/parallel systems. Not only reasonable estimates of the probability of system failure but also an estimate of the distribution of time to system failure can be obtained. The time to failure distribution can be used to estimate the reliability function, hazard function, conditional reliability given survival at any time, etc. The demonstration cases illustrate how reliability of a system having given material properties is influenced by the number of series and parallel elements, stress level, mean stress, and various inspection/repair policies.

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