Defense Technical Information Center Compilation Part Notice ADP 014084 TITLE : Recognition and Correction of Sonic Fatigue Damage in Fighter Aircraft DISTRIBUTION : Approved for public release , distribution unlimited
High performance aircraft that fly aggressive low altitude or high-angle of attack maneuvers will experience high acoustic loading, especially in the presence of external stores. These high acoustic loadscan lead to rapid sonic fatigue in thin gage metallic structure. As a means of prolonging fatigue life, while at the same time restoring structural load carrying capability in damaged structure, composite bonded repairs which incorporate damping layers have been developed. These damped composite bonded repairs are installed on F-15 flight test aircraft. This paper describes ways to recognize sonic fatigue damage, and the design and flight test results of represeintative damped patches.
Durability Patch: Repair and Life Extension of High Cycle Fatigue Damage on Secondary Structure of Aging Aircraft
Abstract : The Durability Patch Program addresses the repair and life enhancement of nuisance cracks which have been induced into secondary structure by resonant high cycle fatigue from aerovibroacoustics. For this type of damage, safety of flight concerns are virtually non-existant, but maintenance and repair costs are high. Conventional repair techniques consist of mechanically fastened, single sided doublers. For significant static in-plane loads and/or for significant vibration levels due to out-of-plane dynamic loads, the repair does not last long because new cracks will form and emanate from the repair. Eventually, large areas of skin and substructure will have to be replaced. The Durability Patch consists of a bonded pair region which is an elastic elliptical laminate overlaid by and surrounded by a thoroughly integrated damping treatment. In some configurations the transition from the elastic repair region to the damping region is accomplished by the use of a viscoelastic material instead of a structural adhesive in one layer; thus, the other layers are multi-functional. The bonded repair does not introduce stress concentrations, does reduce static and dynamic stresses, and does reduce crack tip stress intensities. Damping is maximized within thickness and area constraints in order to enhance the life of adjoining structure with undetected damage. The life improvement goal is 600x. Finite element analysis results comparing static and vibratory stresses will be presented. High cycle fatigue and crack growth rates will be compared. The design and use of a miniature autonomous damage dosimeter to obtain service temperature and vibration environmental data at low cost will be described. Selection of structural materials and processes to attain a goal of field installation will be described. Comparison of analysis and laboratory results will be presented. Patch configurations will be described and compared using a numerical measure of merit system.
Quantitative and qualitative relationship between swirl burner operatingconditions and pulverized coal flame length
Abstract Currently, there are used in power plants two types of burners to combustion the pulverized coal: jet burners and swirl burners. Most of the published works in the field of analysis of burners operating conditions are related to jet burners and are limited to initial region of the flame at the outlet of the burner. For jet burners an offset between the burner outlet and the flame beginning point could be analyzed. In the case of swirl burners with the pre-mixing diffuser chamber an offset between flame ignition point and burner outlet is usually not visible. Because already in the pre-mixing diffuser chamber of swirl burner occurs the ignition of coal-dust together with oxidant-gas cloud and thereby the formation of the flame. Therefore, the analysis of operating conditions for this types of burners requires a different approach. There is a lack of researches, in which will be analyzed changes of the flame length, that are caused by changes of design and operating parameters of swirl burner with the pre-mixing diffuser chamber. Therefore the aim of this study was to implement and verify the methodology for estimating the flame length, which will allow monitoring changes of operating conditions, that are caused by changes of design and operating parameters of swirl burner with the pre-mixing diffuser chamber. To carry out the analysis of the image of the coal flame, the two algorithms have been presented. This two algorithms, being based on the video images of the coal flame allows to specify and detect the end point of the flame and on that basis the length of the flame, as well as the fluctuation changes of flame length can be later determined. Both, the change thermal loads of coal burner, streams of secondary air, as well as the constructional change of the angle of swirler assemblies, resulted in distinct changes in the length of the pulverized coal flame.