| Over the past three decades,developments in the field of design solutions and technologies in the aviation and shipboard industry,as well as in the sports and entertainment fields have demanded improvements in well-known high-performance materials and structures.The increasing number of composite materials allows to choose the right material taking into account a number of their advantages,such as:· Composites are incredibly light and therefore find increasing use in internal connection systems(connectors)for which low weight is decisive.For most of these applications,the typical weight reduction with composites is approximately 40%,and 80% compared to aluminum,brass and stainless-steel parts.· Composite materials are extremely durable.As an example,high strength fiberstructured composites are widely used in body armor.Due to the high strength of such composite materials,soldiers are well protected from fragments and bullets.· Composites are very resistant to aggressive chemicals;they will never rust or corrode.This is why the maritime industry was one of the first to start to use them.· Some composites are non-conductive.This is important because often composite materials are needed where durability and high electrical insulating properties are needed.· Composites can weaken magnetic fields,reduce the effect of magnetic fields,and dampen the so-called “acoustic signature”,that is,acoustic radiation characteristic of each device,which is a very important feature in the development of products for which low probability of their detection is important.Parts made of composites will collapse under stress with a much lower degree of probability than parts made of metal.A small crack in a metal part can develop into a catastrophic one very quickly and with very serious consequences.Fibrous materials in their complex composite structure can distribute internal stress and block the expansion of small cracks.The load in any composite is distributed along its fibers that carry the entire load,so their type,quantity,orientation and linearity determine their effectiveness.Fiberglass composites are used for applications that simultaneously require rigidity,high electrical insulating properties and abrasion resistance.Carbon fibers in composite materials are used for applications requiring high strength and stiffness.The resin matrix in the composite,distributed between the fibers,protects them and keeps the fibers in their proper location and orientation.In addition,the type of resin determines the method of manufacturing the final product and its cost relative to alternative types of resin and methods of manufacturing.The most important of all the advantages of composite materials is their strength and rigidity combined with a low specific weight.It is most difficult to design complex parts from composites,which use the listed properties for their own purposes,but at the same time have to fulfill the necessary requirements for geometric dimensions,installation and functional use.But,choosing the appropriate combination of reinforcing material and matrix material,manufacturers can provide all the necessary characteristics of the product that will meet the requirements for its specific design,and for the specific purpose of its use.Composites are the basis of many modern projects in the field of device development with a minimally noticeable effect.One of them are unmanned aerial vehicles(UAV).Composite materials were very actively used in their construction,the result of it was the possibility of detection UAV only from close range.Composites provide high durability and rigidity,making them suitable materials for systems used in avionics.These materials give a reduction in weight,high strength and operational stability,which greatly exceeds the similar characteristics of many metals and non-composite thermosetting materials.The special condition of the environment in space requires special assemblies that can be used in outer space,in addition,they must meet the requirements for the absence of toxic gases and be made of non-magnetic materials.Carbon-based composites are the main material in modern launch vehicles and heat shields of reusable spacecraft.They are also widely used in antenna reflectors,spacecraft traverses,adapters to the payload compartment,inter-unit designs and heat shields of reusable spacecraft.The undoubted fact that composite materials are increasingly being developed for the specific requirements of internal connection systems,despite the complexity of both their design and the manufacturing process,these materials,due to their properties,are worth using.Without a doubt,we need to know about all the possibilities that composites can give us.Scientists continue to look for methods to improve existing technologies.One of these solutions is to replace standard sandwich panels on panels with pyramidal truss core,providing a combination of high rigidity and strength with significant weight savings.Such truss structures have higher porosity than foam sandwich panels,high specific strength and rigidity,better mechanical properties and high energy absorption.One of the most important advantages of sandwich truss panels is the presence of empty space inside the panels,which can meet other requirements,such as sound absorption,heat dissipation,electromagnetic shielding,use of space for laying control systems or fuel,and so on.In recent years,a large number of experiments have been carried out,indicating that truss structures with variable geometric parameters,called graded truss core panels,show the excellent mechanical properties.The chapter 1 gives information on the selection of gradations of truss core composite panels.Example of the use of composite panels in aviation as an example of increasing economic profit,safety and environmental friendliness when using composite parts instead of metal parts is given.Conducted review of the research history of composite sandwich panels with truss cores of various types.The characteristics of various types of core for sandwich panels: crystal core,cellular cores of various shapes,gofer cores,truss cores of various shapes.The advantages and disadvantages of the truss cores are discussed.Scientific works,published in leading journals are surveyed.The large-scale research conducted by scientists from Europe,the USA,South Korea,and China also described.Information about the most important and currently used mechanical theory are surveyed.Existing problems of describing models using present theories are considered.In the second chapter of this study,a general theoretical approach is used to describe the behavior of a real composite panel through an equivalent simplified model.The advantage of this theory is the ability to express the mechanical characteristics of the material in any direction as a dependence on the properties in the main directions and the geometric shape of the structure.In this model,a single cell is a representative element of the model.The mechanical properties of a single unit are described by using tensor calculation.Chapter 3 provides information on the general method for constructing models in ABAQUS software.Provided information on modeling the properties of carbon-epoxy prepreg,on the basis of which the behavior of the models is studied.Contains additional information about the labeling of models within this study.Due to the large number of investigated models with different geometrical parameters,the method of labeling panels based on the inclination angles of the core struts is presented.Using the model of a composite panel without a core gradient as an example,the method of constructing models is described step by step.Additionally,specified features of building models with a core gradient.There are fifteen panels with different inclination angle gradient of the struts in the longitudinal direction and the transverse direction.Geometrical overall and internal parameters of the models are described,characteristics of parameters common to all panels are given,as well as variable parameters are indicated and the laws of which they obey.For panels with a gradient of the core,a gradation parameter is selected,a law is presented according to which the geometric parameters change in panels with a gradient of the core.All investigated panels are divided into four categories,according to the nature of core grading.Among them are the following categories: models with a constant increasing gradient of the inclination angle of the rods in the longitudinal direction and different angles of inclination in the transverse direction,panels with different increasing gradients of the inclination angle in the longitudinal direction and a constant angle of inclination in the transverse direction,models with a decreasing gradient of the inclination angle in the longitudinal direction and constant in the transverse direction and models with different thickness of rods.Information on marking each category or graded panels is provided.Geometric variable parameters,as well as step-by-step instructions on the construction of the full geometry of the structures with gradient also described.In Chapter 4,based on the mechanical properties of carbon fiber,a comparative analysis of the bearing capacity of structures with different gradient options for the core was carried out.The properties of the sandwich panels under the action of four types of loading were studied and compared with each other.The four types of loading include distributed pressure,three-point bending,lateral pressure and shear are considered.There was made an attempt to determine the most appropriate gradation combinations for each type of loading.Recommendations on the design of structures for operation under the action of each type of loading were made,and also suggestions on the most promising options for grading panels for each type of loading were made.The damage and strength of graded sandwich panels were studied.A number of panels with different inclination angle gradient of the struts in the longitudinal direction and the transverse direction were analyzed.A series of reverse inclination angle gradient in the longitudinal direction with a constant angle in the transverse direction are presented.Two options for truss cores with graded thickness at constant inclination angles are considered.This study provides general information about the behavior of composite panels with different core structures under the action of different types of loading.By comparing the mechanical properties of the panels with the gradient of the core and the panel without a gradient,it is possible to determine if there are positive effect of the panel's gradation on their properties,and also to establish in which cases it leads to deterioration or does not affect the mechanical properties of the structure.This information will allow to determine exactly how the change in the geometric parameters of the core of the truss graded panels affects their bearing capacity.It will allow to get recommendations on the most promising directions for grading cores to improve the mechanical properties of panels.Such information will allow in the future to improve the designs used now in avionics and cosmonautics,advanced industrial areas.Innovations in these areas have long-lasting consequences,such as cheaper flights into space,increased stability and safety of structures,the emergence of new technological solutions in other areas of industry.The considered method of designing a composite panel model in Abaqus software can speed up the process of researching new gradation options and will help attract more graduate students to research this and similar problems.Information of this kind often remains inside a research institute where experiments are conducted.Data dissemination,and the use of simplified models that available not only to highly qualified specialists obtain results will speed up the development of the theory of graded panels and increase the amount of information on similar studies. |
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