Abstract:
Modern aircraft structures are designed to be damage tolerant and to have an adequate durability. The Damage Tolerance provides structural safety in case of inadvertent cracking, the Durability determines the time that the structure can be kept in service in an economically acceptable way.
Many aircraft are planned today to be kept in service considerably longer than the original design life, and knowledge of the actual durability of the aircraft becomes important. Unfortunately, only a lower limit of that durability, determined in a full scale durability test is-known, however.
The present report investigates how estimates of the actual durability can be made on the basis of the absence of cracks at a certain lifetime.
Durability is loosely defined: In the present study, two Durability Criteria are investigated. In the one, it is assumed that the Durability Life ends when a specific number of cracks has occurred. The other criterion assumes that Durability life ends when a certain crack rate, that is the number of new cracks developing per flight hour, is reached.
Using a simple statistical model, the concept is quantified and the influence of the various parameters involved is investigated.
Some examples quantify the predictions that can be made and indicate that the methodology defined can be applied in practical cases.Keywords:
Service life
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Life extension
Aircraft Maintenance
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Reinforced concrete is one of the most durable, versatile, and widely used construction materials. It can be used to make small posts, vast bridges, and tall buildings as well as lining tunnels and constructing pipeline. Generally, this composite material is capable of withstanding a wide range of environments, from oil rigs in the North Sea to deserts. However, occasionally it does not give the low maintenance life expected of it. Sometimes this is due to more adverse condition than initially expected. Consequently, there are many structures in the built environment suffering from corrosion induced damage.
During the last few years, infrastructure deterioration caused by corrosion has escalated, warranting serious consideration. Among the different distressing consequences of reinforcing-bar corrosion, the most common is concrete cover cracking. When a reinforcing steel bar corrodes in concrete, a surface layer of steel is consumed and a layer of corrosion products rust forms on the perimeter of the bar. The rust that forms occupies a larger volume than the consumed steel layer, the increased volume creates internal high pressure against the surrounding concrete, and cracking and spalling result. Thus, steel corrosion may cause damage in steel, concrete, and the bond between them.
Investigations have been conducted during the last three decades regarding chloride penetration and prediction of corrosion initiation. However, few investigations have dealt with corrosion propagation and/or residual life predictions, which are also needed for durability forecasting. Therefore, the aim of this investigation is to discuss, based on experimental information from previous investigations, the possibility of linking the degree of degradation (from a load-capacity reduction point of view) to the surface distress (for example, crack width opening) of a corroding reinforced concrete element in a marine environment.
This dissertation presents a laboratory study, applied to three different type of concrete, in order to analyze and monitor the material behavior under accelerated ageing. The purpose of an accelerated test is to cause, degradation, corrosion or failure in a shorter time period than under normal conditions without change in failure mechanism. The studyoutline is based on different reference, but basically it was carried out performing accelerated corrosion test and using knowledge related to mechanical, chemical and electrochemical engineering. The analysis provided information concerning durability and service life of material.
This information were applied to investigate on the possibility of on-field analysis and the correlation among traditional techniques of monitoring, service life modeling and increasing of acoustic emission technology for health monitoring.
Spall
Rust (programming language)
Corrosion monitoring
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Brittleness
Service life
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Flammable liquid
Service life
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Mechanical products occupy great proportion in weaponry equipment, the faults of which often resulted from some wearing out processes such as fatigue, abrasion, erosion and aging. Service life is one of the most important problems in mechanical reliability. Durability is set forth to solve the problem associated with service life of mechanical product. In this paper probe the definition of durability. the limiting conditions, the parameters of durability, the allocation and prediction of durability and provide engineering examples of the application of durability allocation and prediction.
Service life
Abrasion (mechanical)
Limiting
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Modern technology places great demands on structural material performance. Until the 1950s, structures were invariably designed according to safe-life principles, for which a crack-free service life was required and assumed. However, the tendency to much longer utilisation of aircraft, and latterly the introduction of high strength materials, have focused attention on damage tolerant design in which allowance must be made for possible cracking and its certain detection before catastrophic failure. Especially for high strength materials is this change in philosophy necessary, because the improved static strengths (and hence potentially increased structural efficiency) rarely result in comparable improvements in fatigue properties, and are usually accompanied by a decline in fracture toughness and in greater susceptibility to environment-induced cracking.
Allowance (engineering)
Service life
Catastrophic failure
Structural material
Paris' law
Fatigue limit
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Obtaining durability in concrete structures over a long service life in chloride exposures requires knowledge of the concrete properties, relevant transport processes, depths of cover as well as minimization of cracking and construction defects. For example, imperfect curing can result in depth-dependent effects of the concrete cover’s resistance to chloride ingress. Several service life models with various levels of sophistication exist for prediction of time-to-corrosion of concrete structures exposed to chlorides. The model inputs have uncertainty associated with them such as boundary conditions (level of saturation and temperature), cover depths, diffusion coefficients, time-dependent changes, and rates of buildup of chlorides at the surface. The performance test methods used to obtain predictive model inputs as well as how models handle these properties have a dramatic impact on predicted service lives. Very few models deal with the influence of cracks or the fact that concrete in the cover zone will almost certainly have a higher diffusion coefficient than the bulk concrete as the result of imperfect curing or compaction. While many models account for variability in input properties, they will never be able to account for extremes in construction defects. Therefore, to ensure the reliability of service life predictions and to attain a concrete structure that achieves its predicted potential, designers, contractors and suppliers need to work together, using proper inspection, to ensure proper detailing, minimize defects, and adopt adequate, yet achievable, curing procedures. As well, concrete structures are often exposed to other destructive elements in addition to chlorides (eg. freezing or ASR) and this adds another level of complexity since regardless of cause, cracks will accelerate the ingress of chlorides. These issues are discussed along with the need to use performance-based specifications together with predictive models.
Service life
Concrete cover
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thawing, alkali-silica reactivity (ASR), and sulfate attack because, in each case, water or aggressive solution entering the concrete is part of the problem. For damage due to cycles of freezing and thawing, another important factor is the proper entrainment of air voids in the concrete. However, low permeability and a proper air-void system do not always ensure durability if the concrete contains excessive cracks that facilitate the intrusion of aggressive solutions. This cracking can be due to many factors related to both environmental effects and structural loads (TR Circular E-C107, 2006). Building durable bridge structures requires innovation and the use of available resources in an efficient manner. An ideal durable structure needs to have a low permeability concrete with a proper air-void system, no cracks, and not be subject to deleterious chemical reactions. These characteristics are discussed below in relation to design practices, material selection, construction practices, and specifications.
Air entrainment
Void (composites)
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