Strength computational and experimental analysis of turbojet fan case under the impact of a torn blade

The paper considers the problem of holding a torn blade of a turbojet fan to ensure safety for passengers and crews. It is concerned with one of the main trends in designing fans satisfying this requirement. The experimental and computational analysis of strength for several fan case designs is given. A difference between the calculation results and experimental data for metals with different plasticity is presented. We outlined and described the well-known procedures of calculating metal armoured protection, specified its common disadvantage which is neglecting the material ultimate strain. We suggested the calculation procedure for the metal case which takes the material ultimate strain in account including the boundary value of the material ultimate strain dividing metals on rigid and flexible ones with the calculated formulas of the armoured wall. A good convergence is revealed between the calculation data based on the proposed correlations and the experimental results for Ti and Al alloys. We described the structural design, analyzed strength and the rate of impact absorption by a separated blade using a specially developed goffered hood (a cover) made of the high-strength cloth which can be installed on any fan case made of either metal or polymer composite materials (PCM) without principal changes in the structural configuration. The calculated advantages of using the developed goffered hood design in comparison with the conventional one have been shown. The efficiency has been estimated with respect to applying aluminum alloys as a production material for a fan case being an alternative to the titanium ones. Experimental equipment for testing full-size cases is proposed. The disadvantages of the testing equipment for model cases have been outlined. We analyzed the problem of PCM application for a load-carrying case which is cut by fan blades when one blade is torn away. The engineering estimation of using PCM in a fan case and the solutions aiming to increase the ballistic resistance of PCM cases have been presented.