Abstract: The paper deals with the impact of explosion on the damage development in the structure that forms a boundary of closed or semi-closed space – tunnel - the explosion in which is initiated. In the previous publications of the author the penalty formulation of the problem was used; now Uzawa’s algorithm is applied, which appears to be more promising algorithm accelerating the iterative process of damage development in the structure. Shock waves propagating in the airspace can be described by non-linear conservation equations starting with Euler’s equations which are used to be solved by forward differences of the system of pseudo-linear equations, using Arbitrary Lagrangian-Euler Method (ALE). Conservation equations are formulated for two phases: air and solid. In the solid phase, concrete lining, the time-dependent elasticity of free rectangular elements undergoing the damage, is solved by uniformly distributed boundary elements. The paper is focused on damage to the concrete lining due to impact of the explosion in the tunnel.
Abstract: A two-dimensional elastic-plastic finite element model was built to simulate the closure of a long fatigue crack with arbitrarily shaped crack faces. The model growth is simulated by the successive mesh splitting along the crack path defined by element edges. To obtain a realistic morphology of the fracture surface, fatigue crack growth experiments with CT specimen made from AISI 304 stainless steel were performed and fracture surface topology was determined using a single camera and a depth-from-focus method. Simulated closing loads and closure lengths for the cracks with rough and smooth faces and for plane-stress and plane-strain conditions are compared. A mismatch of rough crack faces, resulting in an additional contact, is visualized.
Abstract: Composite materials are increasingly used in those fields where it is necessary to achieve the requirements of lightweight and high mechanical properties. Even though their high specific strength which get these materials very attractive, especially for the transport field, there are several critical issues that still limit their application in primary structures. Among these, dynamic loading conditions play a critical role because they can significantly lower their residual strength. This paper aims to investigate experimentally the structural response of a 25 mm thick Omega composite structure under different impact loading conditions. The investigated test article consists of E-glass fibres (40% volume fraction) reinforced polyester matrix. The structure is covered by a HELIOPOL 1401 M AGC W 11 gelcoat layer and it has been impacted through a drop mass of 3.94 kg, dropped from heights of 50 mm, 75 mm, 100 mm, 150 mm, 200 mm, 250 mm, 350 mm and 500 mm.
Abstract: In the case of catastrophic events, such as an emergency landing, the fuselage structure is demanded to absorb most of the impact energy preserving, at the same time, a survivable space for the passengers. Moreover, the increasing trend of using composites in the aerospace field is pushing the investigation on the passive safety capabilities of such structures in order to get compliance with regulations and crashworthiness requirements. This paper deals with the development of a numerical model, based on the explicit finite element (FE) method, aimed to investigate the energy absorption capability of a full-scale 95% composite made fuselage section of a civil aircraft. A vertical drop test, performed at the Italian Aerospace Research Centre (CIRA), carried out from a height of 14 feet so to achieve a ground contact velocity of 30 feet/s in according to the FAR/CS 25, has been used to assess the prediction capabilities of the developed FE method, allowing verifying the response under dynamic load condition and the energy absorption capabilities of the designed structure. An established finite element model could be used to define the reliable crashworthiness design strategy to improve the survival chance of the passengers in events such as the investigated one.
Abstract: Delamination is a well-known issue in polymeric composite materials. Interlaminar Shear Stress (ILSS) plays an important role in delamination initiation and thus its value needs to be monitored when designing a composite component. In this paper a Short Beam Shear experimental test of a carbon/epoxy specimen reproducing a Formula 1 component laminate is represented through a Finite Element analysis of two different models, one featuring 2D shell elements and the other 3D elements. ILSS results from both models are compared to experimental data in order to assess whether a 2D shell model can be used instead of its 3D counterpart, in favour of design versatility.
Abstract: Composite materials structures are particularly susceptible to the damages induced by low-velocity impacts that may result in Barely Visible Impact Damages (BVIDs), which can hardly be identified through visual inspection. These damages are particularly dangerous, since they can critically reduce the mechanical properties of the impacted structures. In this work, the damage induced in impacted long and short fibre composite specimens has been experimentally evaluated by means of Non-Destructive Technique (NDT) inspections. The damages size and location have been evaluated by means of ultrasonic testing to assess the influence of fibres aspect ratio (long and short fibres), fibres material (carbon and glass), volume fraction, and impact energy for low velocity impacts on composite specimens. Considerations about the failure mechanisms arising as a consequence of the impact event and their interactions have been finally introduced.
Abstract: In this work, an optimization procedure able to determine the optimum design of a stiffened aeronautical panel subjected to low velocity impacts is presented. As design variables, the number of plies and the stacking sequence of the panel have been considered. The optimization is based on a genetic algorithm, while the onset of the impact induced damage is predicted by means of an approach based on the critical impact force threshold. A multi-objective optimization has been carried out to determine, among the configurations able to withstand the low velocity impacts, the ones characterized by the maximum buckling load and the minimum weight.
Abstract: Composite materials damage behaviour is, nowadays, extensively investigated in the frame of aerospace research programmes. Among the several failure mechanisms which can affect composites, delamination can be considered as the most critical one, especially when combined to compressive loading conditions. In this context, nanofillers can represent an effective way to increase the composites fracture toughness with a consequent reduction of the delamination onset and evolution. Hence, in this paper, the toughening effect of the nanofillers on the delamination growth in composite material panels, subject to compressive load, has been numerically studied. A validated robust numerical procedure for the prediction of the delamination growth in composite materials panel, named SMXB and based on the VCCT-Fail release approach, has been used to perform numerical analyses by considering two different types of nanofillers. Reference material, without nanofillers insertion, has been used as benchmark in order to assess the capability of nanofillers to enhance the fracture toughness in composite laminates.
Abstract: In this work, the mechanical behaviour of natural composite fibre sandwich panels is experimentally investigated. The composite sandwich panels are composed of a natural core made of Posidonia dried leaves reinforced with aluminium skins. The introduced material system is characterized by good structural behaviour (due to the aluminium skins) combined with the good thermal and acoustical insulation provided by the Posidonia core. Flexural and impact tests, characterized by different impact energies, have been performed on specimens with and without the aluminium skins, to preliminary assess the influence of the Posidonia core densities on the natural fibre sandwich mechanical behaviour.