Marco Giglio

FEM submodelling fatigue analysis of a complex helicopter component

Abstract This paper deals with the analysis of fatigue damage to upper and lower folding beams on the rear fuselage of a naval helicopter which may result from flight and folding loads; an original FEM-based analytical approach is used and is found to be applicable both together with and as an alternative to experimental tests. The finite element model of the helicopter part is created by means of an ABAQUS/Standard finite element programme combined with advanced submodelling techniques, thus enabling assessment of structural stresses. Thanks to submodelling, it is possible to obtain more precise results with only a limited increase in the amount of time required for calculations. The finite element model defines the stress values in points of the beams most affected by flight and folding loads previously applied in experimental tests. Linear damage summation (Miners rule) and multiaxial stress criterion enable us to evaluate the fatigue damage to the beams under investigation.

Bending fatigue tests on a metallic wire rope for aircraft rescue hoists

Abstract During normal operation, the ends of helicopter rescue hoist ropes, to which a hook is attached, can be subject to bending stress caused by vibrations. This happens in the event of partial or total recovery of the hook into its lodging without a spring-loaded blocking system. The swinging of the rope end consequently causes bending of the rope near the terminal, with resultant fatigue stress that can lead quickly to breakage or damage of the rope. A series of alternating bending fatigue tests using a constant load, similar to those known as BoS (bending over sheave) tests, were carried out. These simulate the effective working conditions of a rescue hoist. This paper describes modifications to test machinery, the test method and the data obtained; statistical analysis of this data enabled us to make a rope life prediction which was then experimentally confirmed by run-outs on test ropes.

Fatigue analysis of different types of pressure vessel nozzle

Abstract This research aims at comparing two different methods for the construction of pressure vessel nozzles, designed with the same safety coefficient, according to ASME and VSR 1995 standards. It defines numerical and experimental analysis of behaviour under low-cycle fatigue for pulsating pressure. In particular, a nozzle with integral reinforcement, designed according to ASME standards, is compared to a nozzle with external reinforcement (applied reinforcement plate) designed according to VSR 1995 standards with the same safety coefficient. Strain gauge tests have been carried out on the plastic behaviour of the two structures in order to evaluate the expected fatigue life based on common criteria, using both the local strain and energetic approaches. At the same time, a FEM model of the nozzle with plate has been used to calculate numerically the expected fatigue life based on the same criteria. Finally, in order to identify the best system to exploit for design, comparisons are made of the fatigue life predictions, which are numerically and experimentally obtained and which are determined according to the standards of the two nozzle types to identify the better system.

Life Prediction of Notched Components

In this study, keyhole and smooth specimens, made from a low alloy pressure vessel steel (ASTM A-533 grade B), were subjected to monoaxial fatigue tests. The results show the influence of the stress concentration factor, K[sub t], on the number of cycles to failure, N[sub f]. Total strain energy per cycle, [Delta]W[sub t] = [Delta]W[sub p] + [Delta]W[sub e], was proved to be a good parameter for predicting the life of notched components. Elasto-plastic FEM analysis, utilizing the cyclic and monotonic curve of the material, showed close agreement with the experimental values.

Spherical vessel subjected to explosive detonation loading

The following work utilizes a calculation method for the design of spherical containment vessels for pressure gases likely subjected to internal detonation. A centrally initiated explosion within the vessel is taken into account and the history of pressures on the internal vessel wall is investigated by means of the fundamental compressible fluid dynamics, for which a non-viscous perfect gas is assumed. The obtained differential equations are integrated with explicit finite difference techniques, introducing artificial viscosity. The response of vessel is decoupled from the fluid and it is determined by a dynamic finite element code of explicit type; the investigated material is considered to have an elastic-plastic behaviour. Comparison with data found in references on PBX-9404 high-explosive induced explosions confirms the calculation method developed.

Life prediction of pressure vessel nozzles

Abstract When the material yields local stress and strain behaviour changes, especially if there is a notch, determination of the local strain value can be difficult. Therefore it is not easy to predict the life of mechanical components in the low-cycle region. In the present work pressure vessels are considered and fatigue tests carried out. The most stressed zones, which are at the connections between the nozzles and the vessel, are subjected to cyclic loads with subsequent repeated plastic strains that cause nucleation and propagation of fatigue cracks. The life of such a mechanical component under low cycle fatigue can be predicted by utilizing the life strain curves e − N f or the energy approach (plastic strain and total strain energy density). In the present work, life prediction is performed by considering several different approaches. The results are compared with the experimental results obtained by fatigue testing pressure vessel nozzles. The material considered is Fe 510 B UNI 7070-72 steel.

Experimental and numerical damage evaluation of a lift safety gear

The aim of this paper is the experimental and numerical evaluation of the accumulated damage in a progressive lift safety gear, during its life cycle. In particular this evaluation has been carried out by means of experimental tests that reproduce exactly the emergency condition which activates the safety gear and with a detailed FE model of the component. Another purpose of these tests is to verify that the average deceleration (retardation) during gripping action lies between 0.2g and 1g (permissible limits) according to European Standard UNI EN 81-1 (1999). In particular the experimental tests, consisting in free falls of a test lift car with full load, and the following gripping phase caused by the safety gear activation, have been carried out evaluating the strains in the component and the car frame acceleration. The double approach, by test and with a numerical model, allows a better knowledge of this critical phenomenon.

Residual stress analysis in cold-worked holes with interference bushing

Cold working is widely applied in fatigue design of mechanical and aeronautical components with holes in order to induce a stress compression field in the most loaded zone and thus increase fatigue life. None of the investigations in literature consider the insertion of a bushing in the hole by a cold expansion process: in particular cases may be predicted the presence of tensile circumferential residual stress on hole surface, often loaded by external cyclic forces. The present investigation introduces a closed-form analytical method for simply predict the complete residual stress distribution on a cold-expanded bushing-hole connection commonly used in aerospace structures, under imposed conditions. A comparison of these solutions with 2-D finite element analysis reveals perfect agreement. Finally, the paper discusses the results of the closed-form solution obtained on several bushing-hole combinations, providing interesting indications in the fatigue design in order to obtaining the highest benefits on hole fatigue life.

PLASTIC STRAIN ENERGY IN LOW-CYCLE FATIGUE

Notched specimens, fabricated from a low alloy pressure vessel steel (A-533 B), were subjected to fatigue tests. These tests showed the influence of the theoretical stress concentration factor, Kt, on the fatigue life. A energy-based criterion has been adopted and it is shown that the total strain energy per cycle, ΔWt = ΔW + ΔWe, is a proper damage parameter which can be used for the life prediction of notched elements.