Eighth DOD/NASA/FAA Conference on Fibrous Composites in Structural Design, Part 2

Papers presented at the conference are compiled. The conference provided a forum for the scientific community to exchange composite structures design information and an opportunity to observe recent progress in composite structures design and technology. Part 2 contains papers related to the following subject areas: the application in design; methodology in design; and reliability in design

Research and Technology

Langley Research Center is engaged in the basic an applied research necessary for the advancement of aeronautics and space flight, generating advanced concepts for the accomplishment of related national goals, and provding research advice, technological support, and assistance to other NASA installations, other government agencies, and industry. Highlights of major accomplishments and applications are presented

Fatigue Enhancement of Undersized, Drilled Crack-Arrest Holes

Fatigue cracks occur in steel bridges from repeated loads. If allowed to continue to grow, eventually the fatigue cracks will require either expensive repairs or reduction of traffic loads on the bridge, or they may lead to the failure of the bridge. The objective of this research was to develop a new, cost-effective technique that can be used to arrest the growth of fatigue cracks before they develop to an extent that more expensive repairs are required. It is well known that drilling a hole (crack-stop hole) at each end of a fatigue crack can arrest the growth of the fatigue crack. This new technique consisted of cold-expanding a crack-arrest hole thereby cold-working the material around the hole and subjecting the cold-worked material to ultrasonic vibration. It was hypothesized that this process would increase fatigue crack initiation life three ways. First, the compressive force used to cold-expand the hole would result in residual compressive stress fields around the hole when the radial force was removed. Second, the cold-expansion would cause strain-hardening and cold-working with a concomitant increase in the yield and ultimate strengths of the steel. Third, the ultrasonic vibration from the PICK treatment would further increase the resistance to fatigue propagation by increasing the yield and ultimate strengths and increasing the radial extent of the residual compressive stress field. These expected results and their effects on fatigue crack initiation were investigated through a proof-of-concept testing program using reduced-scale, laboratory models and by mathematical modeling

Probabilistic Risk Assessment Procedures Guide for NASA Managers and Practitioners (Second Edition)

Probabilistic Risk Assessment (PRA) is a comprehensive, structured, and logical analysis method aimed at identifying and assessing risks in complex technological systems for the purpose of cost-effectively improving their safety and performance. NASA's objective is to better understand and effectively manage risk, and thus more effectively ensure mission and programmatic success, and to achieve and maintain high safety standards at NASA. NASA intends to use risk assessment in its programs and projects to support optimal management decision making for the improvement of safety and program performance. In addition to using quantitative/probabilistic risk assessment to improve safety and enhance the safety decision process, NASA has incorporated quantitative risk assessment into its system safety assessment process, which until now has relied primarily on a qualitative representation of risk. Also, NASA has recently adopted the Risk-Informed Decision Making (RIDM) process [1-1] as a valuable addition to supplement existing deterministic and experience-based engineering methods and tools. Over the years, NASA has been a leader in most of the technologies it has employed in its programs. One would think that PRA should be no exception. In fact, it would be natural for NASA to be a leader in PRA because, as a technology pioneer, NASA uses risk assessment and management implicitly or explicitly on a daily basis. NASA has probabilistic safety requirements (thresholds and goals) for crew transportation system missions to the International Space Station (ISS) [1-2]. NASA intends to have probabilistic requirements for any new human spaceflight transportation system acquisition. Methods to perform risk and reliability assessment in the early 1960s originated in U.S. aerospace and missile programs. Fault tree analysis (FTA) is an example. It would have been a reasonable extrapolation to expect that NASA would also become the world leader in the application of PRA. That was, however, not to happen. Early in the Apollo program, estimates of the probability for a successful roundtrip human mission to the moon yielded disappointingly low (and suspect) values and NASA became discouraged from further performing quantitative risk analyses until some two decades later when the methods were more refined, rigorous, and repeatable. Instead, NASA decided to rely primarily on the Hazard Analysis (HA) and Failure Modes and Effects Analysis (FMEA) methods for system safety assessment

Durability of bonded crack retarders for aerospace

The increase in demand for aircraft focuses the structural designers and manufacturers towards the reduction of manufacturing cost and structural weight while maintaining high safety, damage tolerance, and structural integrity. These weight savings can be achieved by using large integral structures. However, large integral structures show reduced performance with respect to damage tolerant design due to lack of physical barriers that can arrest a growing crack such as presently exist in structures joined with rivets and bolts. Fibre metal laminates such as Glass Laminated Aluminium Reinforced Epoxy (GLARE) have been proven effective as bonded crack retarders (BCR) in reducing the fatigue crack growth rate and improving the life of metallic aircraft structures. A major problem associated with bonded crack retarders is the development of thermal residual stresses which may have negative impact on the performance of the structure. Hence, the objectives of this research are to investigate the thermal residual stress developed during the strap bonding process and the fatigue durability of bonded crack retarders. Extensive research performed in this dissertation covers the detailed investigation of thermal residual stresses and the fatigue durability of GLARE bonded crack retarders when incorporated onto different structural coupons and on an aircraft mock-up panel. Thermal residual stresses developed during the strap bonding process are very low and the application of a bonded crack retarder improved the fatigue performance of the specimen. The experimental data on residual stress measurements and fatigue testing provides information for researchers and aircraft structural designers to improve the performance and life of critical aircraft structures and the possibilities of incorporating the bonded crack retarder concept in the initial design and fabrication stages

30th International Conference on Condition Monitoring and Diagnostic Engineering Management (COMADEM 2017)

Proceedings of COMADEM 201

Load sequence effects and mixed-mode fatigue crack growth in offshore structures

PhD thesis in Offshore technologyAn increasing number of the offshore structures in the North Sea are being operated in a life extension phase, which means that their original design life has been exceeded. As the structures age, they are deteriorating, mainly due to fatigue and corrosion. In order to ensure that the structural integrity is maintained during the life extension phase, the remaining fatigue life of the structures needs to be assessed. While simple and conservative approximations are used in fatigue design, it will often be necessary to use more advanced and accurate models when assessing the remaining fatigue life of an existing structure. Among the different effects that influence the fatigue life, load sequence effects and mixed-mode conditions are currently not explicitly included in relevant fatigue life assessment standards for offshore structures. Therefore, this thesis addresses how these effects can be accounted for in fatigue life evaluation. The following three topics are investigated: – Load sequence effects in variable amplitude fatigue crack initiation – Mixed-mode fatigue crack propagation – Load sequence effects in variable amplitude mixed-mode fatigue crack propagation A number of experimental studies have previously demonstrated the influence of load sequence effects in variable amplitude fatigue crack initiation. In the present work, a model is proposed for estimating the fatigue life, in which the load sequence effects are taken into account. The model contains one material parameter, but a single value for this parameter is found to give reasonable agreement between predicted and experimental fatigue lives for four different steels from three different experimental studies. This indicates that the proposed model is able to predict the observed load sequence effects quite well and easily. Mixed-mode fatigue crack propagation usually needs to be modelled using a computational method. Currently, the finite element method seems to be the most easily applicable method for practicing engineers. One of the main tasks when modelling mixed-mode fatigue crack propagation is the determination of the stress intensity factor, which is a measure of how severely a crack is loaded. Here, seven different techniques for obtaining the stress intensity factor from a finite element model are compared. It is found that the domain integral method is the most reliable technique. If this method is not readily available, the displacement extrapolation technique can be used as an alternative for plane cracks. Models for estimating the fatigue crack propagation rate and crack path of mixed-mode cracks are also evaluated. It is shown that the Richard effective stress intensity factor is more conservative than the Tanaka effective stress intensity factor for estimating the fatigue crack propagation rate in most practical cases. Furthermore, most of the existing criteria for predicting the crack path work reasonably well, but the criterion of maximum tangential stress is the easiest one to apply. Very few experimental studies have previously considered variable amplitude mixed-mode fatigue crack propagation. In the present work, fatigue crack propagation tests have been carried out, in which the specimens were subjected to a mixed-mode overload, followed by mixed-mode constant amplitude cyclic loading. It is found that the fatigue crack growth retardation caused by the overload lasted longer than predicted by the models proposed in the literature. This demonstrates the need for new models, and the experimental results presented here may be used in the development of such a model

Loads and aeroelasticity division research and technology accomplishments for FY 1985 and plans for FY 1986

The Langley Research Center Loads and Aeroelasticity Division's research accomplishments for FY85 and research plans for FY86 are presented. The rk under each branch (technical area) will be described in terms of highlights of accomplishments during the past year and highlights of plans for the current year as they relate to five year plans for each technical area. This information will be useful in program coordination with other government organizations and industry in areas of mutual interest

Potential structural materials and design concepts for light airplanes Final report

Potential structural materials and design concepts evaluated for light aircraft application