Previous Definition (Better)
High volume production of FRP component demands automation of the molding process for efficiency, component quality, and safety issues. Automated high volume production of processes due to the simplicity and maturity of such techniques, SMC and BMC, for example, are used extensively inthe automotive industry. A disadvantage of these methods is that short fibre orientation is difficult to control during injection, causing a wide variation in the component mechanical properties. As a result, short fibre reinforced components are limited to non-structural application.
Rtm is a closed-mould FRP process which can utilize low moulding pressures and low cost flexible tooling. RTM can be used to produce complex components with high fibre volume fraction and a good surface finish. The process involves the placement of dry reinforcement in closed matched moulds followed by the injection of a liquid thermosetting resin. The resin is polymerized either at ambient or elevated temperatures and the component is removed with a smooth surface on both sides. Several types of resin system are available including polyesters, vinyl esters, urethane methacrylates, phenolic resin and epoxy resins. The choice depends on both cost and application. Prior to moulding the reinforcement is often shaped to produce a fibre preform in a seperate process. The preform can consist of different hypes of fibres such as glass, polyester, aramid or carbon in several forms such as woven fabric, continuous filament random mat, non-crimp fabric or chopped strand mat.
(P.W. Dufton,Lightweight Thermoset Composites: Materials in Use, Their Processing and Application, p. 151)
New Definition
To define Resin Transfer Molding the process is described: Fiber reinforcement is placed in a mold set; The mold is closed and clamped; The resin is injected into the mold cavity under pressure. The motive force in RTM is pressure. Therefore, the pressure in the mold cavity will be higher than atmospheric pressure. In contrast, vacuum infusion methods use vacuum as the motive force, and the pressure in the mold cavity is lower than atmospheric pressure.
(C. Lee, P. Rice, Resin Transfer Molding Process Monitoring And Control, p.1)
4) Resin Film Infusion (Manufacturing Technique)
Previous Definition
4) Resin Film Infusion (Manufacturing Technique)
Previous Definition
In the resin film infusion (RFI) process, dry- fiber performs are placed into the mold with precast
or extruded resign films. Controlled heat and pressure are applied, generally in an autoclave.
Infusion parameters are established by a combination of experimental and modeling approaches.
This process result in high-quality, near- net- shaped parts that require only cleanup and limited
trim.
(National Research Council (U.S.). Committee on New Materials for Advanced Civil Aircraft,
New materials for next-generation commercial transports, p.84)
New Definition (Better)
The resinfilminfusion is a promising and low cost technique for the manufacturing of high performance polymer-based composite materials. The processing cycle is usually performed in the autoclave where, under temperature and pressure application, a bagged dry fiber mat is infiltrated by a thermoset polymer resinwhich is pre-shaped in film form under the fibers. After complete infusion, the consolidation step behaves as in the conventional autoclave process. The infusion process is strongly affected by the permeability of the fiber mat that determines processing issues, such as the cycle time and the complete fiber impregnation and, as a consequence, the performance of the final part. Therefore, the permeability measurement represents one of the most crucial requirements for the design and the optimization of the composite manufacturing by RFI.
(V. Antonucci, Resin Flow Monitoring in Resin Film Infusion Process, p.1)
5) Sandwich Structured Composites (Material)
Previous Definition
Sandwich composites are a special class of composite materials which are widely used because of their high specific strength and bending stiffness. Lower density of these materials makes them well suited for marine and aerospace applications. Developments in aviation posed requirements of lightweight, high strength and highly damage tolerant materials. Sandwich structured composites, fulfilling these requirements became the first choice for many applications including structural components for ground transport and marine vessels. The use of cores such as closed cell structured foam in sandwich structures gives some distinct advantages over open cell structured foams and cores. The specific compressive strength of close cell structured foams is much higher and absorbs less moisture than open cell structured foam.
(E.E. Gdoutos, Experimental Analysis of Nano And Engineering Materials and Structres,p.761)
New Definition (Better)
Sandwich structured composites are a special class of composite materials which have become very popular due to high specific strength and bending stiffness. Low density of these materials makes them especially suitable for use in aeronautical, space and marine applications.
Concept of sandwich structured composite materials can be traced back to as early as the year 1849 AD [5] but potential of this construction could be realized only during Second World War. Developments in aviation posed requirement of lightweight, high strength and highly damage tolerant materials. Sandwich structured composites, fulfilling these requirements became the first choice for many applications including structural components. Now their structural applications spread even to the ground transport and marine vessels.
Sandwich composites comprise of two thin but stiff face sheets attached on either side of a
lightweight, thick slab known as core. Many variations of this definition are available but the
key factor in making this type of materials remains the lightweight core, which reduces the overall density of the material and stiff skins providing strength. The structure of sandwich
composites is shown in Figure 1.
Integral bonding between skins and core prevents the interfacial failure under the applied load
enhancing the flexural properties of sandwich composites. There is no general rule about the
relationship between the thickness of skin and core. It depends on the application and required properties. Major advantage of sandwich structured composites is the possibility of tailoring properties by choosing appropriate constituting materials and their volume fractions.
Components in Sandwich Composites
Sandwich composites primarily have two components namely, skin and core as shown in Figure 1. If an adhesive is used to bind skins with the core, the adhesive layer can also be considered as an additional component in the structure. The thickness of the adhesive layer is generally neglected because it is much smaller than the thickness of skins or the core. The properties of sandwich composites depend upon properties of the core and skins, their relative thickness and the bonding characteristics between them.
Core
Based on the performance requirements, large numbers of materials are used as core [6]. Popular
core materials can be divided into three classes as described below.
1. Low density solid materials: open and closed cell structured foams, balsa and other types of
wood.
2. Expanded high-density materials in cellular form: honeycomb, web core.
3. Expanded high-density materials in corrugated form: truss, corrugated sheets.
High-density materials used for the purpose of making expanded core include aluminum,
titanium and various polymers. The structure of the core material affects the interfacial contact
area between skins and the core. Expanded high density materials normally provide much
smaller contact area compared to the solid low density materials. The choice of appropriate
structure for core provides additional parameter to design a sandwich composite as per given
specifications or service conditions.
The use of cores like closed cell structured foam gives some distinct advantages over open cell structured foams and cores. The specific compressive strength of close cell structured foams is much higher. They also absorb less moisture than open cell structured foam.
Skins
A wide variety of materials are available for use as skins. Sheets of metals like aluminum,
titanium and steel and fiber reinforced plastics are some of the common examples of skin
materials. In case of fiber reinforced skins, the material properties can be controlled
directionally in order to tailor the properties of the sandwich composite. Fiber reinforced
polymers are used widely as skins due to their low density and high specific strength. Another
advantage offered by the use of polymer composites in skins is that the same polymer can be
used to make the skin and the core. Cross-linking of polymer between core and skin would
provide adhesion strength level equal to the strength of the polymer. This provides possibility of making the skin an integral part of the structure eliminating the requirement of the adhesive.
When an adhesive is used to bond the skin and the core together, selection of adhesives becomes very important, as they should be compatible with both the skin and the core materials. The adhesion must have desired strength level and should remain unaffected by the working environment.
In case of metallic components, welding or brazing is used as a means of binding the core and
skins together. Use of adhesives is also possible but is limited to such cases where one or more of the components cannot withstand heat.
Choice of skins is important from the point of view of the work environment as this part of the
structure comes in direct contact with the environment. Corrosion, heat transfer characteristics,
thermal expansion characteristics, moisture absorption and other properties of the whole
sandwich composite can be controlled by proper choice of skin material. In most cases both
skins of the sandwich are of the same type, but could be of different type depending upon
specific requirements. Difference may be in terms of materials, thickness, fiber orientation, fiber volume fraction or in any other possible form.
Properties of Sandwich Composites
Main advantage of any type of composite material is the possibility of tailoring their properties
according to the application. The same advantage also applies to sandwich composites. Proper choice of core and skins makes sandwich composites adaptive to a large number of applications and environmental conditions. Some general characteristics of sandwich composites are described below:
1. Low density: choice of lightweight core or expanded structures of high-density materials
decrease the overall density of the sandwich composite. Volume of core is considerably
higher in the sandwich composite compared to the volume of skins so any decrease in the
density of the core material has significant effect on the overall sandwich density.
2. Bending stiffness: this property comes from the skin part of the sandwich. Due to a higher
specific stiffness sandwich composites result in lower lateral deformation, higher buckling
resistance and higher natural frequencies compared to other structures.
3. Tensile and compressive strength: the z-direction (Figure 2) properties are controlled by the
properties of core and x and y directions (Figure 2) properties are controlled by properties of
skins.
4. Damage tolerance: use of flexible foam or crushable material as core makes sandwich
material highly damage tolerant structure. For this reason foam core or corrugated core
sandwich structured materials are popular materials in packaging applications.
Advantages of Sandwich Composites
Some of the advantages of sandwich composites are:
• Tailoring of properties according to requirements.
• Large available choice of constituents for core and skins.
• Low density leading to saving of weight.
• High bending stiffness.
• Higher damage tolerance.
• In-situ fabrication.
• Good vibration damping capacity.
Limitations of Sandwich Composites
There are current limitations that can be overcome through the development of new materials
and manufacturing methods. Some of these are:
• Higher thickness of the sandwich composites.
• Higher cost of sandwich composites compared to conventional materials.
• Processing is expensive.
• Difficult to join.
• Difficult to repair, if damaged.
Applications of Sandwich Composites
There are several applications that require materials of low density, high strength and high
damage tolerance. Due to their lightweight, sandwich composites are widely used in various
kinds of vehicles used for air, ground or sea transportation. Some of the main areas of
applications of sandwich composites are listed below.
1. Structural applications: aircraft, spacecraft, submarine, ships and boats, surface transport
vehicles, building materials etc.
2. Packaging materials.
3. Thermal and electrical insulation.
4. Storage tanks.
(N.Gupta, Characterization of Syntactic Foams And Their Sandwich Composites: Medeling And Experimental Approaches, p.1-4)
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