The development of condition monitoring strategies and techniques appropriate to mechanical structures
Recent legislation, LOLER, removed the compulsion of periodical proof testing of lifting equipment to ascertain its "fitness for purpose". It has become the responsibility of a competent person to assess equipment's fitness for purpose and ability for continued safe use. This thesis reviews the technologies available to the competent person to enable him/her to come to an informed decision regarding the condition of mechanical structures. It was identified that an optimal methodology would interrogate structural integrity whilst the equipment performed its intended function. Coupling a means of assessment with the equipment's operation allows the investigator to focus on only defective conditions that will limit the future operation. Such an approach of condition monitoring structural integrity as opposed to employing traditional methods of inspection that are essentially failure finding tasks permits the discrimination between benign and malignant defects. Restorative and replacement activities can therefore take place based upon the likelihood of equipment's functional failure. The supplementary monitoring of Acoustic Emission (AE), with the established industrial practice of proof testing, was considered to provide data to monitor structural integrity and provide the basis upon which a structure can be re-qualified for future service. The nature of failure of engineering materials was examined which identified failure modes such as corrosion, creep and fatigue resulted in a progressive degradation of a localised area. The AE is a proportion of energy released during such deterioration. Further it was determined that the rate at which the deterioration increased was non-linear. Within a laboratory environment wire ropes with seeded faults were subjected to a simulated life during which the qualitative and quantitative nature of the AE was investigated.It was found that the quantity of the emission generated during proof tests was indicative of the severity of the induced defect. This substantiated the claim that AE could be used to enhance the proof test and provide a means by which a condition assessment could be made at intervals throughout the life of a structure. A series of five case studies explored the use of AE on a variety of differing in-service mechanical structures, mostly lifting equipmept. The case studies were conducted on pad-eyes,link-plates, cranes, both Electrical Overhead Travelling (EOT) and pedestal cranes and finally, an underwater vehicle pressure hull. The approach of using the combination of AE with a proof test was verified in the cases of pad-eyes and EOT cranes. In the instance of link plates, simultaneous measurement of strain and AE during a load test demonstrated the ability of AE to detect localised yielding. During the destruction test of a pedestal crane boom section, various conventional methods of AE evaluation were utilised to investigate which would provide the most reliable condition indicator; it was found that Intensity Analysis generated the most effective trendable measurement. A study on a pressure hull with known fatigue cracks that were subjected to both static and dynamic testing whilst monitoring with AE was conducted. The fatigue cracks were sized pre and post the trial using Time of Flight Diffraction (ToFD). During the trial Alternating Current Potential Drop (ACPD) was used to detect any growth as it occurred. Such techniques were used to substantiate claims AE could detect a propagating defect. When the AE is viewed in conjunction with ACPD results and the measurements attained with the ToFD it was clear that all three techniques concluded that crack growth occurred at two sites. Finally the investigation returns to a laboratory to exarnine the robustness of the technique through the life of a mechanical structure. The objective being to identify if periodical measurement of AE taken during the course of the life of the structure would repetitively generate information pertaining to the identification of the flaw as well as the severity of the flaw as it initiates and propagates through to failure. A power law was fitted to the data acquired during the proof tests. The use of a power law was considered appropriate due to the previously identified non-linear nature of material failure. A Scanning Electron Microscope was used to visually examine the fracture surfaces. It was found that increasing increments between striations on the fracture surface illustrated the non-linear increase of crack extensions during fatigue and corroborated the appropriateness of fitting a power law to the proof test data. Such an investigation permitted the conclusion that the approach of fitting a power law to the discrete energies from sequential proof tests is an appropriate method of attaining a trendable condition indicator. The competent person could employ such a methodology for the purposes of attaining information upon which an informed decision can be made on the continued safe use of mechanical structures.