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

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

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

Toughening behavior of carbon/epoxy laminates interleaved by PSF/PVDF composite nanofibers

This paper presents an investigation on fracture behavior of carbon/epoxy composite laminates interleaved with electrospun nanofibers. Three different mats were manufactured and interleaved, using only polyvinylidene fluoride (PVDF), only polysulfone (PSF), and their combination. Mode-I and Mode-II fracture mechanics tests were conducted on virgin and nanomodified samples, and the results showed that PVDF and PSF nanofibers enhance the Mode-I critical energy release rate (GIC) by 66% and 51%, respectively, while using a combination of the two registered a 78% increment. The same phenomenon occurred under Mode-II loading. SEM micrographs were taken, to investigate the toughening mechanisms provided by the nanofibers

Experimental analysis of GFRP laminates subjected to compression after drilling

This paper presents an experimental study of drilling-induced delamination on the compressive properties of woven glass fiber-reinforced epoxy composites. In the drilling of laminated composites, interlaminar cracking or delamination has a detrimental effect on compressive properties. The onset of delamination and the extent of the damage are governed by the cutting forces developed during the drilling process. High cutting forces, in turn, result from the use of improper drilling parameters. This study investigates the effects of feed rate and spindle speed on delamination and residual compressive strength. The composite laminates were cut into the standard dimensions of compression after impact specimens. The drilling of composite specimens was conducted at three different levels of spindle speed and feed rate based on general full factorial design. Analysis of variance was used to find the percentage contribution of the drilling parameters and it was found that feed rate has the most significant influence on the residual compressive strength. A polynomial regression model was also developed to express the residual compressive strength as a function of the selected process parameter

Managing heat phenomena in epoxy composites production via graphenic derivatives: synthesis, properties and industrial production simulation of graphene and graphene oxide containing composites

A commercial two-components epoxy resin formulation was successfully modified by adding graphene and related materials (GRMs) and the effect of these nanofillers was assessed on their thermomechanical properties as well as on the simulation of their industrial application for the production of thick composites objects with interesting results. GMRs were added in different concentrations in order to improve thermo-mechanical properties of the nano-composite thermoset. Different dispersion methods were taken into account in order to produce stable long-lasting dispersion of the GRMs, that can withstand a commercial shelf life. Addition of the GRMs improves the glass transition temperature of the nanocomposite up to 20 °C with respect to the plain commercial formulation, and both stress and elongation at break increase up to almost 4 times the original values. Moreover, the industrial curing of some of the more promising modified resins was computer-simulated when the two-components resins are used to produce a carbon-fibre reinforced thick composite beam. Simulation results show that some of the applied GRMs helps reducing or even completely preventing the overheat phenomena that are well renown to induce significant thermal stresses negatively affecting the final object performances. These interesting effects would contribute reducing the time required for a single industrial production cycle, since no time for overheat dispersion is required, thus helping increasing the production rate