Cavity Formation during Asymmetric Water Entry of Rigid Bodies

This work numerically evaluates the role of advancing velocity on the water entry of rigid wedges, highlighting its influence on the development of underpressure at the fluid-structure interface, which can eventually lead to fluid detachment or cavity formation, depending on the geometry. A coupled FEM-SPH numerical model is implemented within LS-DYNA, and three types of asymmetric impacts are treated: (I) symmetric wedges with horizontal velocity component, (II) asymmetric wedges with a pure vertical velocity component, and (III) asymmetric wedges with a horizontal velocity component. Particular attention is given to the evolution of the pressure at the fluid-structure interface and the onset of fluid detachment at the wedge tip and their effect on the rigid body dynamics. Results concerning the tilting moment generated during the water entry are presented, varying entry depth, asymmetry, and entry velocity. The presented results are important for the evaluation of the stability of the body during asymmetric slamming events

Experimental evaluation of the air trapped during the water entry of flexible structures

Deformable structures entering the water might experience several fluid-structure interaction (FSI) phenomena; air trapping is one of these. According to its definition, it consists of air bubbles trapped between the structure and the fluid during the initial stage of the impact. These bubbles might reduce the peak impact force. This phenomenon is characteristic for the water entry of flat-bottom structures. Above a deadrise angle of 10°, air trapping is negligible. In this work, we propose a methodology to evaluate the amount of air trapped in the fluid during the water entry. Experiments are performed on wedges with varying stiffness, entry velocity, and deadrise angle. A digital image post- processing technique is developed and utilized to track the air trapping mechanism and its evolution in time. Interesting results are found on the effect of the impact velocity and the structural deformation on the amount of air trapped during the slamming event

Fracture mechanics of additive manufactured crack-like notches by digital image correlation

The rapid development of a variety of Additive Manufacturing (AM) techniques is witnessed by the large interest of the scientific community. Among the numerous new features offered by these techniques, very lately the possibility of including cracks of any shape, direction, and position inside AM samples to validate FEM models was explored. Therefore, the necessity arose of full-field measurement techniques that would allow the evaluation of the fracture mechanics parameters in the cases of both linear elastic and partially plastic materials, as a function of the fracture modes. Invented in the ’80 of the XX century, the Digital Image Correlation (DIC) in recent years has taken place in nearly every laboratory. It is used to measure displacements, and by numerical differentiation or coupling with Finite Element Method, to calculate strains at different spatial scales. In particular, two procedures that were successfully employed on a specific kind of specimens, based on DIC at a small scale near the crack-tip, in this paper are shown in detail.The authors wish to acknowledge the support of European Commission through the project “Advanced design rules for optimal dynamic properties of additive manufacturing products – A_MADAM”, which has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 734455.Publishe

Influence of electrospun nanofibers on the interlaminar properties of unidirectional epoxy resin/glass fiber composite laminates

Nylon 6,6 nanofibers were interleaved in the mid-plane of glass fiber/epoxy matrix composite laminates for Mode I and II fracture mechanic tests. The present study investigates the effect of the nanofibers on the laminates' mechanical response. Results showed that Nylon 6,6 nanofibers improved specimen's fracture mechanic behavior: the initial energy release rates GIC and GIIC increased 62% and 109%, respectively, when nanofibrous interlayer was used. Scanning electron microscope micrographs showed that nanofiber bridging mechanism enhances performances of the nanomodified specimens, still able to link the layers when the matrix is broken

Multi-Objective Design Optimization of the Reinforced Composite Roof in a Solar Vehicle

Abstract: A multi-step and -objective design approach was used to optimize the photovoltaic roof in a multi-occupant racing vehicle. It permitted to select the best combination of design features (as shapes, widths, angles) in composite structures simultaneously balancing opposite requirements as static strength and dynamic stiffness. An attention to functional requirements, as weight, solar cells cooling and solar energy conversion, was also essential. Alternative carbon fiber-reinforced plastic structures were investigated by finite elements using static and modal analyses in the way to compare several design configurations in terms of natural frequencies, deformations, flexural stiffness, torsional stiffness, and heat exchange surfaces. A representative roof section was manufactured and tested for model validation. A significant improvement respect to the pre-existing solar roof was detected. The final configuration was manufactured and installed on the vehicle

Using Acoustic Emission to Evaluate Fracture Toughness Energy Release Rate (GI) at Mode I Delamination of Composite Materials

Delamination is a critical damage mode in composite structures, not necessarily because it will cause the structure split into two or more pieces at the end of the damaging process, but because it can degrade the laminate strength to such a degree that it becomes useless in service. The design of composite structures to account for delamination and other forms of damage involves two fundamental considerations, namely damage resistance and damage tolerance. Knowledge of a laminated composite material’s resistance to interlaminar fracture is useful for product development and material selection

MEASURING DEFORMATIONS IN THE TELESCOPIC BOOM UNDER STATIC AND DYNAMIC LOAD CONDITIONS

The interest in pushing the mechanical structures closer to their limits of usage makes necessary to combine the traditional design with the implementation of specific tests able to definitely confirm and guarantee their safety. Exploring the case of a large telescopic boom, the present study analyses the response to intense loads prevenient from static and dynamic conditions. The measure of deformations was oriented to validate several design assumptions, but also to investigate the presence of phenomena of local instability, not easily predictable within theoretical formulations

Damage characterization of nano-interleaved CFRP under static and fatigue loading

© 2019 by the authors. The use of high strength-to-weight ratio-laminated fiber-reinforced composites is emerging in engineering sectors such as aerospace, marine and automotive to improve productivity. Nevertheless, delamination between the layers is a limiting factor for the wider application of laminated composites, as it reduces the stiffness and strengths of the structure. Previous studies have proven that ply interface nanofibrous fiber reinforcement has an effective influence on delamination resistance of laminated composite materials. This paper aims to investigate the effect of nanofiber ply interface reinforcement on mode I properties and failure responses when being subjected to static and fatigue loadings. For this purpose, virgin and nanomodified woven laminates were subjected to Double Cantilever Beam (DCB) experiments. Static and fatigue tests were performed in accordance with standards and the Acoustic Emissions (AE) were acquired during these tests. The results showed not only a 130% increase of delamination toughness for nanomodified specimens in the case of static loads, but also a relevant crack growth resistance in the case of fatigue loads. In addition, the AE permitted to relate these improvements to the different failure mechanisms occurring

Buckling analysis of telescopic boom: theoretical and numerical verification of sliding pads

U cilju poboljšanja najviših performansi, materijali u strojarskim konstrukcijama su često vođeni ka sve bližim i bližim kritičnim granicama. Razmotrimo, na primjer, kako progresivno smanjenje debljine može dovesti do nepredviđenih efekata u stabilnosti lima, sve do brzog loma cjelokupne konstrukcije. Ovaj rizik je posebice poznat konstruktorima teleskopskih dizalica, koje se koriste za pokretanje radnih platformi. U ovom radu, numeričkim pristupom i ANSYS kodom, čvrstoća i stabilnost teleskopske dizalice su verificirani. Nakon preliminarne teorijske analize, različite konfiguracije opterećenja i graničnih uvjeta su razmatrane u skladu s uvjetima realne uporabe. Kao opći rezultat, potvrđeno je da su naprezanja bila u okviru elastičnih granica materijala, osim u ograničenom broju kontaktnih površina, gdje su korišteni posebni kontaktni elementi za sprječavanje loma. Osim toga, linearne tehnike izvijanja su pokazale da su kritična opterećenja i odgovarajući moduli izvijanja bili veći od najtežih uvjeta rada; stabilnost je potvrđena. Konačno, FEM simulacije dopuštaju smanjenje brojnih eksperimenata, nudeći time brze metode za poboljšanje konstrukcija.With the aim at improving the highest performances, materials in mechanical structures are constantly pushed closer and closer to their critical limits. Consider, for example, how the progressive reduction in thickness may lead to unforeseen effects in the instability of metal sheets, until the rapid collapse of the whole structure. This risk is specially known by designers of telescopic booms, used for moving aerial platforms. In this paper, by a numerical approach and ANSYS code, structural resistance and stability of a telescopic boom were verified. After a preliminary theoretical analysis, different loads and boundary configurations were considered in accordance with the most common conditions of real utilisation. As general result, it was confirmed that stresses were under the elastic limit of materials, except in a very limited number of contact zones, where specific connecting solutions have to be installed to prevent failures. Furthermore, linear buckling techniques showed that critical loads and corresponding buckling modes were higher than the most extreme working conditions; thus, structural stability was also confirmed. Finally, the large adoption of FEM simulations permitted to reduce the experiments, offering a fast methodology for improvements in design

THE EFFECT OF SUPPORT PLATE ON DRILLING-INDUCED DELAMINATION

Delamination is considered as a major problem in drilling of composite materials, which degrades the mechanical properties of these materials. The thrust force exerted by the drill is considered as the major cause of delamination; and one practical approach to reduce delamination is to use a back-up plate under the specimen. In this paper, the effect of exit support plate on delamination in twist drilling of glass fiber reinforced composites is studied. Firstly, two analytical models based on linear fracture mechanics and elastic bending theory of plates are described to find critical thrust forces at the beginning of crack growth for drilling with and without back-up plate. Secondly, two series of experiments are carried out on glass fiber reinforced composites to determine quantitatively the effect of drilling parameters on the amount of delamination. Experimental findings verify a large reduction in the amount of delaminated area when a back-up plate is placed under the specimen