"Investigation of the weight saving potential of SHM for a damage tolerant design philosophy based on a simple plate with hole"
Investigation of the weight saving potential of SHM for a damage tolerant design philosophy based on a simple plate with hole
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In terms of lightweight construction and economic thought, the design of aeronautical structures is increasingly prone to total failure. However, the risk of such fatal events must be within certain limits and regulations. With conventional maintenance plans and approaches, you quickly reach the boundaries of the feasible. Thus, for over two decades, Structural Health Monitoring (SHM) systems have been researched to integrate into a wide range of areas. Therefore, the basic idea of this work is to show and evaluate the weight-saving potential through SHM.
Modern structures are designed using the Damage Tolerant Design philosophy, which allows the presence of small, non-critical, structural damage. However, this requires the control of those structures at regular intervals. The fundamental task of the maintenance is to ensure save operation and allow extended operation of the machine, as damage to the structure is detected and repaired. In this work, a plate (AL 2024-T3) with a central hole is used as a representative of a more massive structure. The dimensions were taken from an aircraft fuselage frame to maintain a specific relation to a component. Basically, this diploma thesis is divided into a theoretical part, which should provide a basic understanding of applied calculation methods and two calculation blocks. The limits which have to be met according to European Avia-
tion Safety Agency (EASA) were taken into account and complied within all calculations. These include the fatigue strength (3), the static strength (reserve factor 1,5), and the probability of failure in the last hour of operation (p< 10-9). The loads were calculated using standardized load tables from the Handbook ?Fundamentals and Methods of Aeronautical Design and Analyses?.
For damage tolerance evaluation arbitrary operational load blocks were generated for different maintenance approaches. For maintenance-based design, ultrasound inspection was chosen at an annual interval. This serves to determine the maximum forces acting on the component. A complete design carried out including a static strength calculation, a fatigue strength calculation based on Miner, a crack growth calculation according to Forman, and finally, the statistical evaluation of the probability of failure. The aim was to calculate a kind of status quo of the structure in order to show the influence of SHM.
The loads and boundary conditions are transferred and serve as initial values of the calculation. Likewise, in this calculation, all the boundary conditions were met, which are specified by the authorities. In order to achieve a weakening or more critical initial position of the structure, the sample thickness was reduced incrementally. Starting from the minimum value of the sample thickness, the component was equipped with a SHM system that acts with assumed probabilities. Assumptions had to be used out for lack of literary information. This was accomplished by varying the system parameters such as the
sampling rate and probability of detection (POD) of the system. The results show that enormous weight savings can be achieved by using a suitable sensor system (the weight of sensors, cabling and measurement equipment was not considered). However, it should be mentioned here that the optimized structure itself is, of course, much more susceptible to failures. But it is counteracted by the SHM system and the resulting repairs. This raises the question of whether you want to place enough trust in the system at this point to implement it.