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Blast Resistant Composite Structures_3

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Energy Dissipating Sandwich Structure for Blast Resistance

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Energy Dissipating Sandwich Structure for Blast Resistance

Problem statement Naval and defense industries are more and more involved in the development of structures that guarantee safety of personnel and systems in extreme attack threat. One of the priorities of modern engineering is the creation of structures able to sustain heavy loads derived from blast. These blast resistant structures must withstand high stresses produced by shock waves, sudden increase in static pressure and ballistic impacts. Blast wind (dynamic pressure and overpressure are usually modeled as distributed loads and concern both structural design and single panel stiffness. !hen a structure incurs overpressure, panels and beams are forced to bend to e"uilibrate the pressure gradient between external and internal conditions, possibly leading to catastrophic collapse. #igid materials, with high flexural stiffness, can contribute positively to the overall safety of the structure in this case. $nother dangerous ha%ards produced by blast is high speed flying ob&ects of various dimensions, which can sensibly weaken the structure, perforate the external shell or locally induce buckling. 'n this picture new materials able to absorb high energy impacts have a fundamental role. (olymeric composite materials have been studied for years showing excellent impacts characteristics. )oreover polymers and foam materials shows in general impact energy absorption properties. *evelopment of bulletproof structures shows that high static strength of materials is not providing high dynamic resistance. Only combination of the strong (but brittle materials,

Energy Dissipating Sandwich Structure for Blast Resistance

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such as ceramic and strong metals, with stiff and strong, viscoelastic polymer fibers gives good effect. ,ombination of damping effect, providing ways of impact-blast energy dissipation, and overall structural stiffness and strength is the key in making blast resistant structure. Blast resistant sandwich structures (olymeric sandwich structures might be a suitable solution for resisting impact loads and bending. .andwich structures are popular due to their high bending stiffness per unit of mass. /owever, their resistance to transversal loads and shear loads is relatively low. 'n the case of blast, the transversal loads are high and classical sandwich structures become very vulnerable. The improvement of transversal static and dynamic resistance of sandwich structures is one of the main goals of the pro&ect. $ new concept of high resistance sandwich structures include new design of face sheets and core materials, where 0evlar1 honeycomb and viscoelastic foams are incorporated to form and incredibly responsive structure. 'n addition a new class of reinforcement made of 2ltra /igh )olecular !eight (olyethylene (2/)!(3 shows extremely promising properties in terms of energy dissipation. 0evlar1 fibers are light (about +.45+.6 g-cm4 , have good combination of high strength, high stiffness, and they have nonlinear creep, which takes place only when stresses are high. This creep provides energy dissipation at impact loading. 2/)!(3 fibers have low density (about 7.89 57.8: g-cm4 , high specific strength and stiffness. /owever, their properties drop with rising temperature very fast and the creep of these fibers is more linear, taking place at relatively low static loads. 2/)!(3 fibers are made by $llied.ignal (.pectra 1 , *.) (*yneema1 , Tenfor (.nia1 , and )itsui (Tekmilon1 . The composition is the same of commercial polyethylene molding resin, but with up to one hundred times higher molecular weight.

Energy Dissipating Sandwich Structure for Blast Resistance

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)echanical and physical properties are comparable with commonly used reinforcement, such as carbon and glass fibers. To improve the bonding between this class of fibers and resin matrix systems, such as epoxy, polyester and vinylester, a surface treatment is usually re"uired. Other advantages are resistance to water-moisture related effects and it is light weight relative to water, which make the use of 2/)!(3 fibers a material of choice in marine industry. These fibers are often used for armoring and shielding, because of their excellent impact properties. Objectives $ new sandwich structure with polymeric skin with glass, carbon and 2/)!(3 and 0evlar1 reinforcement, and 0evlar1 and Nomex1 honeycomb viscoelastic foam filled core will be developed and tested. The ability of absorbing and dissipating high energy impact will be evaluated and optimi%ed, together with the maximi%ation of its flexural stiffness. The effect of polymer modification by different nano particles will be studied. $ series of physical and mechanical tests together with flammability tests will be performed. Experimental procedure $ new sandwich structure will be developed and manufactured. (reliminary impact and static tests will be performed to optimi%e configuration and composition. #esistance versus manufacturing and compositional parameters will be analy%ed. The data received will be then widely analy%ed and discussed. $n optimum configuration will be proposed. $ study on the effects of the materials used on blast resistance will be carried out extensively. The analysis of blast resistance is very complex and re"uires multiple tests of various kinds. $n exhaustive preliminary analysis on marine <#( composite panels was made in 20 in +886+. Below there is a list of typical tests performed to evaluate blast resistance.

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+. Bending ;. 'mpact 4. .hock tube 6. =arge scale blast 9. /igh speed ballistic impacts >. ?lammability @. 2nderwater explosion Three and four point bending tests are important because the structure under blast is sub&ected to loads which tent to bend panels and transversal elements. This happens because of two main reasons. The shock wave produced by the blast act as an impulsive distributed force on the part of the structure directly facing the blast, producing loads, which initially bends panels and beams in the direction opposite the blast. $fter that, the structure reacts to a temporal relative decompression, bending in the other direction. This phenomenon, occurring at the initial state of the blast, produces oscillation of the structure around its neutral position, which can be very dangerous for &oints and connecting elements. )oreover, the structure tent to bend to e"uilibrate the pressure gradient created by overpressure. ?or example, if the blast occurs externally the hull of a ship, difference in pressure will be between external (high pressure and internal (low pressure part of the hull. To e"uilibrate the difference in pressure, the structure will tent to implode with bending of the elements of the hull. There are some publications made by research institution and private companies on bending tests for blast resistant structures; . Three point bending tests will be carried out on specimen of the proposed modified sandwich structure. The flexural stiffness will be evaluated for different configurations and material compositions. The optimal solution in terms of bending behavior will be proposed, considering flexural stiffness, weight, cost and manufacturability. 'mpact tests are important for blast resistance due to many flying ob&ects produced by the blast. These ob&ects are part of the shell of the explosive element, part of the structure

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sub&ected to blast, and part of the surrounding environment which is sub&ected to extremely strong wind. $lso the shock wave acts partially as a hypothetical impact ob&ect. $ blast resistant structure must be able to withstand these kinds of impacts, absorbing and internally dissipating the energy transmitted. )oreover the structure must be able to redistribute the load on a bigger surface. ?racture toughness and crack propagation is also a primary concern. 'nformation about this can be found in literature4. 'mpacts test will be performed with ,harpy pendulum and falling mass impact machine. The ability of dissipating impact energy will be evaluated for the specimen produced. $lso the momentum of pro&ectile, which creates failure, will be evaluated with the missile falling mass machine. )odes of failure will be discussed for various specimens with different compositions. ?racture toughness and crack propagation will also be evaluated and discussed. ?lammability is a phenomenon usually conse"uent to a blast. Burning or very hot particles and mid5si%e ob&ects are impacting the structure after explosion and they can ignite structural elements. This combustion can propagate and affects the overall resistance of the structure, creating serious damages and eventually catastrophic failure. $ flame retardant outer layer is always advisable, especially in ships, where the overall integrity of the vessel is always connected to the safety of all personnel. )odern building and military vehicles consider flammability a key point for safety and effectiveness. (olymeric-nano5particles coating were widely tested in recent times. 'n5house it will be possible to evaluate flame resistance. $n advanced flammability machine (cone calorimeter was purchased and used extensively in the last months. Nanocomposites were studied, showing promising flame retardation properties. ?or the proposed blast resistant structure, time of ignition, heat release rate, burning time, total heat released, smoke produced will be evaluated and discussed. The insertion of relatively light weight components (for example nanoclay will be considered for

Energy Dissipating Sandwich Structure for Blast Resistance

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improving the flammability behavior and the flame retardation properties of the sandwich.

To study the interaction between the shock wave and the structure produced by the blast, a shock tube is re"uired. $ shock tube is a steel hollow component with circular of rectangular cross5section. 't is divided in two partitions separated by a diaphragm. 'n one of the two partitions, air is pressuri%ed and then suddenly released. The pressuri%ed air ApushesB like a cylinder, producing a high pressure front moving ahead with speed of sound, which develops in a shock wave. .amples of panels are fixed and a series of strain gages are attached to various points of the surface. 'n this way it is possible to record the deflection and mode of failure of the structure for various shock wave intensities. $lso the interaction of material and high pressure front can be understood, considering that modeling is very complex for this kind of phenomena, and not always faithful to the reality. There are other design concepts to produce shock wave loading utili%ing explosives, or electromagnetic accelerators. To perform shock wave-material interaction tests, a shock tube is needed. Test specimens will be sent to companies that will perform the tests. The data about deflection and mode of failure will be examined and correlated with the tests performed in5house. $lso a video camera for rapid filming will be mounted to record the experiment. The video taken will be viewed in slow motion to understand the dynamic of the phenomenon. $ll the results of the tests will be widely discussed and multiple designs will be proposed. The selected structures can be then further tested at large scale, through real air blast exposure and underwater explosion by speciali%ed agencies.

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C. 3. .later, “Selection of a Blast-Resistant GRP Composite Panel Design for Naval Ship Structures”, )arine .tructures @ (+886 [email protected] ; Cohn 3. ,rawford, “Mo eling Blast-Resistant Protection S!stems Compose of Pol!mers an "a#ric”, 0arago%ian D ,ase 4 $. (. )ourit%, “Ballistic impact an e$plosive #last resistance of stitche composites”, ,ompositesE (art B 4; (;77+ 64+5 648

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