Casting, Semi-Solid Forming and Hot Metal Forming

P. Kapranos , ... T. Haga , in Comprehensive Materials Processing, 2014

5.02.4.4.1 Continuous Fibers

Continuous fibers exhibit the highest strength when they are oriented unidirectionally, but the composite exhibits low strength in the direction perpendicular to the fiber orientation. Carbon (C), boron (B), silicon carbide (SiC), and alumina (Al 2O3) are the most researched continuous reinforcements (34). As the density of carbon fiber is the lowest, it can offer significant weight savings. Boron fibers show the greatest strength in comparison with other fibers; however, the cost of these fibers is very high.

Compared to other types of reinforcement, the continuous fiber reinforced composites offer the best combination of strength and stiffness, increased strength with increased temperature being among some of the benefits. Aluminum-based fiber MMCs have useful strengths up to 400 °C (74); however, the costs of these systems are very high, mainly because of the high costs of the continuous fibers and composite production costs (75). These expensive materials have military applications, where weight saving is of greater importance than the production cost. However, continuous fibers can suffer from fiber damage, especially during secondary processing (76), such as rolling and extrusion. At present these materials are nonrecyclable.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780080965321005033

INTRODUCTION TO COMPOSITE MATERIALS

George H. Staab , in Laminar Composites, 1999

Fibrous.

A fibrous composite consists of either continuous (long) or chopped (whiskers) fibers suspended in a matrix material. Both continuous fibers and whiskers can be identified from a geometric viewpoint:

Continuous Fibers. A continuous fiber is geometrically characterized as having a very high length-to-diameter ratio. They are generally stronger and stiffer than bulk material. Fiber diameters generally range between 0.00012 and 0.0074 μin (3-200 μm), depending upon the fiber [3].

Whiskers. A whisker is generally considered to be a short, stubby fiber. It can be broadly defined as having a length-to-diameter ratio of 5 < l/d < 1000 and beyond [4]. Whisker diameters generally range between 0.787 and 3937 μin (0.02-100 μm).

Composites in which the reinforcements are discontinuous fibers or whiskers can be produced so that the reinforcements have either random or biased orientation. Material systems composed of discontinuous reinforcements are considered single layer composites. The discontinuities can produce a material response that is anisotropic, but in many instances the random reinforcements produce nearly isotropic composites. Continuous fiber composites can be either single layer or multilayered. The single layer continuous fiber composites can be either unidirectional or woven, and multilayered composites are generally referred to as laminates. The material response of a continuous fiber composite is generally orthotropic. Schematics of both types of fibrous composites are shown in Figure 1.3.

Figure 1.3. Schematic representation of fibrous composites.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780750671248500011

Carbon/Carbon, Cement, and Ceramic Matrix Composites

EDGAR LARA-CURZIO , in Comprehensive Composite Materials, 2000

4.18.7 Summary

Continuous fiber-reinforced CVI–SiC matrix composites exhibit remarkable physical and mechanical properties that are attractive for use in aerospace and many other industrial applications. Although substantial developments have been achieved since the 1980s particularly in the area of SiC-based fibers, there is still considerable work needed, particularly in the areas of resistance to aggressive industrial environments, durability, and reliability. This includes the development of corrosion/oxidation-resistant fiber coatings, oxide fibers with better dimensional and microstructural stability, and environmental barrier coatings.

As a result of substantial efforts, standardized test methods for the measurement of physical and mechanical properties are now available in the US, Europe, and Japan, and work continues towards the international harmonization of these test standards. These tests methods will be essential for building databases once these materials achieve a maturation stage.

It is expected that work will continue to be focused on the development and implementation of design codes and life-prediction analyses that will include consideration of multiaxial states of stress.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B0080429939001121

Metal Matrix Composites

J.E. SPOWART , H.E. DÈVE , in Comprehensive Composite Materials, 2000

3.09.4.2 Transverse Compressive Loading

Continuous fiber MMCs can be substantially stronger than the unreinforced matrix under compressive transverse loading. Results on Al/Al 2O3 and Ti/SiC composites show strength improvement regardless of interfacial properties. The compressive response of composite reinforced with weakly bonded fibers is therefore in sharp contrast with the tensile response where interface debonding is responsible for severe strength reduction.

The failure in compression is governed by matrix shear along a maximum shear stress plane; the strengthening is attributed to the difficulty for matrix shear bands to cut across fiber networks. Therefore, the magnitude of the strengthening is largely dependent on the fiber volume fraction; for example with 35 vol.% fibers the strength improvement is only 1.15 that of the matrix, however with 55 vol.% fiber, the strength can be as high as three times that of the matrix. Expectedly, the fiber–spatial arrangement influences strengthening as well as the matrix strain hardening rate. Finally, thermal residual stresses mainly influence the transient stress–strain response but have a minor effect on the ultimate strength.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B0080429939001248

Volume 1

Min Wang , Qilong Zhao , in Encyclopedia of Biomedical Engineering, 2019

Long fibers

Continuous fiber reinforced composites (i.e., long fibers as reinforcement) are also employed in the biomedical field. In general, long fibers are either polymer or ceramic fibers and include aramid fibers, UHMWPE fiber, cellulose fibers, CFs, glass fibers, and Ca-P fibers. CFs are made from a variety of precursor fibers (polymer, mesophase pitch, etc.) and are usually graphitized PAN fibers for use in the medical field. PAN-derived CFs consist of crystallites of "turbostratic" graphite whose basal planes are parallel to the fiber axis which produces high axial modulus and strength for CFs. Manufacturing of CFs from PAN fibers requires high-temperature processing. The modulus of resultant CFs is affected by the processing temperature, with standard (230–240  GPa), intermediate (250–300   GPa), and high (350–500   GPa) modulus values being achieved after treatments at 1200°C–1400°C, 1400°C–2200°C, and above 2200°C, respectively. CFs are regarded as biocompatible and therefore were employed for investigations to, for example, reinforce porous PTFE for soft tissue augmentation, reinforce UHMWPE as bearing surface in total joint prostheses, reinforce epoxy resins as fracture fixation devices, and be braided into artificial ligament/tendon. Bioactive glass fibers are also used in studies of long fiber reinforced biomedical composites. As amorphous materials, mechanical properties of glass fibers are normally isotropic, which is different from CFs. As they are made of ductile materials, polymer fibers such as aramid fibers and UHMWPE fibers exhibit different mechanical behaviors as compared to CFs and glass fibers. In contrast to the brittle manner of ceramic fibers under tensile stress to fracture, polymer fibers usually show ductile fracture. Consequently, polymer fiber reinforced composites have high toughness, providing application examples in the orthopedics and dentistry. Furthermore, by using biodegradable polymer fibers such as PLA, PCL, and PLGA fibers as reinforcements, various composite-based fully biodegradable/resorbable medical devices have been fabricated.

As stated earlier, mechanical properties, such as strength, of continuous fiber reinforced composites depend on those of the reinforcing fibers and the strength of brittle fibers can be treated statistically. For example, the strength of ceramic fibers (e.g., CF and glass fibers) can be described by the three-parameter Weibull distribution equation:

(1) P σ = 1 − exp − l l 0 σ − σ 0 β a

where σ is applied stress, P is probability of fiber failure, l is fiber length, l 0 is reference length, α is shape parameter, β is scale parameter, and σ 0 is location parameter. Once Weibull distribution parameters (α, β, σ 0 ) are known, the strength of a fiber of a given length can be predicted. Single fiber tensile tests are the most direct experimental method to determine Weibull distribution parameters. Alternatively, Weibull distribution parameters are obtained from the bundle tensile tests on the assumption that there is no friction between the fibers.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128012383998684

Introduction to Composite Materials and Processes: Unique Materials that Require Unique Processes

F.C. Campbell , in Manufacturing Processes for Advanced Composites, 2004

1.1 Laminates

Continuous-fiber composites are laminated materials (Fig. 2) in which the individual layers, plies or laminae are oriented in directions that enhance the strength in the primary load direction. Unidirectional (0°) laminates are extremely strong and stiff in the 0° direction; however, they are also very weak in the 90° direction because the load must be carried by the much weaker polymeric matrix. While a high-strength fiber can have a tensile strength of 500 ksi or more, a typical polymeric matrix normally has a tensile strength of only 5–10 ksi (Fig. 3). The longitudinal tension and compression loads are carried by the fibers, while the matrix distributes the loads between the fibers in tension and stabilizes and prevents the fibers from buckling in compression. The matrix is also the primary load carrier for interlaminar shear (i.e., shear between the layers) and transverse (90°) tension. The relative roles of the fiber and the matrix in determining the mechanical properties are summarized in Table 1.

Fig. 2. Quasi-Isotropic Laminate Lay-Up

Fig. 3. Tensile Properties of Fiber, Matrix and Composite

Table 1.1. Effect of Fiber and Matrix on Mechanical Properties

Mechanical Property Dominating Composite Constituent
Fiber Matrix
Unidirectional
0° Tension ✓
0° Compression ✓ ✓
Shear ✓
90° Tension ✓
Laminate
Tension ✓
Compression ✓ ✓
In-Plane Shear ✓
Interlaminar Shear ✓

Since the fiber orientation directly impacts the mechanical properties, it would seem logical to orient as many layers as possible in the main load-carrying direction. While this approach may work for some structures, it is usually necessary to balance the load-carrying capability in a number of different directions, such as the 0°, +45°, −45° and 90° directions. Fig. 4 shows a photomicrograph of a cross-plied continuous carbon fiber reinforcement in an epoxy resin matrix. A balanced laminate with equal numbers of plies in the 0°, +45°, −45° and 90° directions is called a quasi-isotropic laminate, since it carries equal loads in all four directions. Fig. 5 provides a graphical presentation of the preferred laminate orientations. These are preferred orientations because they are fairly balanced laminates that carry loads in multiple directions.

Fig. 4. Laminate Construction

Fig. 5. Preferred Laminate Orientations

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9781856174152500022

Crack Propagation in Continuous Glass Fiber/Polypropylene Composites: Matrix Microstructure Effect

M.N. Bureau , ... J.I. Dickson , in Plastics Failure Analysis and Prevention, 2001

INTRODUCTION

Continuous fiber composites with commodity thermoplastic matrix systems of relatively low glass transition temperature, such as polypropylene/glass fiber (PP/GF), are increasingly used in various applications, namely in the automotive sector. These relatively new composites combine the general advantages of semi-crystalline thermoplastic matrix composites with respect to molding processes, damage tolerance, chemical and environmental resistance and recycling possibilities, with the need for lower costs of production and higher production rates in large volume markets such as the construction, transport and automotive industries. 1

Impact studies on discontinuous GF composites with a PP matrix have suggested 1 that the use of a tough thermoplastic matrix, instead of a thermoset matrix with glass transition well above room temperature, in composites may result in significant toughness improvement, especially at low temperatures (e.g., −40°C). It has been established that the fracture behavior of GF composites with a PP matrix or a polyethylene terephthalate (PET) matrix is influenced by the matrix crystalline structure 2–3 as well as the laminate configuration. 4 Among several processing parameters affecting the mechanical behavior of thermoplastic GF composites, the cooling rate employed during the molding process, by modification of the matrix morphology, appears to be critical. 2–4

In continuous GF composites with a PP or a PET matrix, mode I quasi-static fracture studies showed that the fracture behavior of is dominated by fiber pullout, whereas mode II quasi-static fracture studies showed that their fracture behavior is matrix dominated. 3, 5 Moreover, fatigue crack propagation studies 6 have shown that interlaminar fracture, mostly delamination under mode II conditions, plays a key role in the fatigue fracture of continuous GF composites. Fatigue crack propagation is also generally very sensitive to the morphology and the microstructure of the materials tested. Mode I fracture testing is thus appropriate to study the influence of GF/matrix interactions, and mode II fracture testing appropriate to study the influence of the matrix itself on the fracture process in these composites.

The objective of this paper is to study the effect of the matrix morphology, by using two different cooling rates in the molding process, on the mechanical behavior of a PP/GF composite.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9781884207921500196

Introduction to Carbon Composites

Deborah D.L. Chung , in Carbon Composites (Second Edition), 2017

2.1.1 Assemblies of Continuous Carbon Fibers

Continuous fibers are available in the form of bundles, which are known as tows. The number of fibers in a tow varies, but is typically in the thousands or tens of thousands. For example, tows with 12,000 fibers in a tow are designated 12K. A tow may have its fibers spread out and aligned mechanically to form a sheet, such that the number of fibers stacked along the thickness of the sheet (typically 20–50) is much smaller than the number of fibers in a tow. Tows with the fibers spread out and aligned can be used to form a ply (known as a lamina) in the resulting laminate. The larger is the number of fibers in a tow, the thicker tend to be a lamina. Alternately, tows can have its fibers similarly spread out and aligned to form a ribbon (known as a tape), which can be wound on an object to form a composite in the form of a cylinder or hollow rod with a selected cross-sectional shape ( Fig. 2.1).

Figure 2.1. A carbon fiber composite made by the winding of fiber tape.

This process of composite fabrication is known as filament winding.

 http://www.longwin.com/english/news/filament-winding-machine.html, public domain.

The filament winding process may be applied to a cylindrical substrate, such as a steel gun barrel. However, the process is complicated by the difference in CTE between the composite and the substrate. Due to the elevated temperature used in the composite fabrication, the CTE mismatch causes poor bonding between the composite and the substrate. This problem can be alleviated by using a thermoplastic resin, a cure on the fly process, and winding under tension (Littlefield and Hyland, 2012).

Tows (without the fibers in a tow spread out) can be woven together to form cloth or fabric (Fig. 2.2), or be braided to form a tube (Fig. 2.3). The weaving involves the fibers in two or more directions (Fig. 2.4), so that the cloth has adequate mechanical properties in multiple directions. Weaving is commonly conducted in two dimensions. However, three-dimensional (3D) weaving is increasingly common. A composite made of a stack of two-dimensionally woven fabric is strong in the plane of the fabric, but is weak in the direction perpendicular to this plane. In contrast, a three-dimensionally woven structure is mechanically good in all three directions in terms of the mechanical properties and the thermal conductivity ( Richardson, 2010; McHugh, 2009, 2010 Richardson, 2010 McHugh, 2009 McHugh, 2010 ).

Figure 2.2. Carbon fiber cloth or fabric.

 http://ponero-bikes.com/technology, public domain.

Figure 2.3. Carbon fiber braided tube.

 http://www.easycomposites.co.uk/carbon-fibre-reinforcement/braid/carbon-fibre-braided-sleeve-40mm.aspx, public domain. For the braiding process, please view the video at the following link: http://giphy.com/gifs/tech-engineering-mechanical-12DHWQlHbb3PnG?utm_source=iframe&amp;utm_medium=embed&amp;utm_campaign=tag_click

Figure 2.4. Weaving involving fibers in two, three, or four directions.

The uniaxial geometry involves all the fibers in the same direction, such that they are held together by a relatively small number of fibers that are spaced relatively far apart from one another and are oriented in the transverse direction.

 http://www.sgl-kuempers.com/cms/international/products/multiaxial-fabrics/index.html?__locale=en, public domain.

The tow may break if its radius of curvature during the weaving is too small (Ramirez and Chung, 2016). Nevertheless, the fabric form is convenient to handle, as it is mechanical more sturdy than a sheet of aligned fibers. However, the fibers in a fabric are necessarily bent, since weaving involves having fibers overlapping and bending around one another. Due to the fact that the fiber properties are superior along the fiber axis than the transverse direction, the bending negatively affects the composite properties. Therefore, fabric is typically not used for high-performance structural composites, which use sheets of aligned fibers instead.

A laminate with layers of metal sheet and continuous fiber polymer–matrix composite stacked up and bonded together is known as a fiber reinforced metal laminate (FRML), also known as a hybrid fiber-metal laminate (FML). The metal and composite layers are commonly in an alternate arrangement. The attractions of FRMLs include fatigue resistance, fracture toughness, impact resistance (particularly in the through-thickness direction), erosion resistance, flame resistance, corrosion resistance, machinability,and plasticity. An FRML is commonly made by stacking the metal sheets with the fiber prepreg, and subsequent consolidation of the stack and curing of the resin in the prepreg under pressure and heat. The most common type of FRML involves aluminum and a glass fiber polymer–matrix composite. Aluminum and glass fibers are both inexpensive. For aerospace application, low density is important for fuel saving. The density of magnesium (1.74 g/cm3) is substantially lower than that of aluminum (2.70 g/cm3) and the density of carbon fibers is lower than that of glass fibers. Moreover, carbon fibers exhibit higher modulus than glass fibers. Thus, an FRML involving magnesium and a carbon fiber polymer–matrix composite is increasingly important (Zhu et al., 2011c). For high-temperature airframes (for temperatures up to 180°C, as needed for supersonic jetliners), an FRML involving titanium (with melting point much higher than aluminum or magnesium) and carbon fiber PEEK–matrix composite (PEEK being more resistant to elevated temperatures than epoxy) is attractive for its high propagation toughness; without the titanium, the carbon fiber composite is good for its damage initiation toughness, but not the propagation toughness (Tarpani et al., 2009).

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9780128044599000026

Compounds and Composite Materials

Stuart Green Ph.D. , in PEEK Biomaterials Handbook, 2012

3.4.2 Continuous CFR PEEK

Continuous fiber-reinforced materials cannot be manufactured using conventional polymer processing equipment such as compounding extruders described earlier. The long fibers and high fiber loading (usually around 60–65% by volume) preclude any type of screw-based processing equipment in the conventional sense. Instead, continuous carbon fiber-reinforced materials have to be produced by a process in which the carbon fibers and polymer are combined with minimal amounts of shear, because this would disrupt the fibers and lead to significant fiber breakage. In essence, in the case of continuous reinforcement, the polymer is melted and pressed (impregnated) among the carbon fibers that are either woven in a fabric form, or are arranged axially in parallel.

The polymer may be in the form of powder, film, or fibers. There are examples of PEEK fibers blended with carbon fibers in woven products commercially available. The advantage of such an arrangement is that the two types of fiber are intimately blended (meaning that the flow path is short when the polymer is later melted, which assists polymer impregnation of the fabric) and the fabric construction is flexible as opposed to rigid. This enables the material to be bent and folded into the mold to create complex shapes, as may be required.

Polymer films can also be used in combination with fabrics in an interleaved sense and these can be hot pressed some way above the melting point of the polymer (360–380 °C) to force the molten polymer between the fibers to achieve full impregnation of the fabric. This can be a convenient process because it avoids the need for specialist fiber blending as required with polymer/carbon fiber mixes and conventional high-temperature-capable carbon fabrics may be employed.

The third method of creating pre-impregnated fibrous material involves the use of polymer powder. In this case the powder is mixed in between the reinforcing fibers such that it is evenly dispersed at the appropriate volume ratio. Conventionally, the mixture of powder and reinforcing fibers is heated such that the powder melts, coating and binding together the fibers. At room temperature this forms a solid pre-preg sheet of aligned unidirectional fibers, as illustrated for ENDOLIGN pre-preg in Fig. 3.23.

Figure 3.23. ENDOLIGN pre-preg tapes (unidirectional carbon fibers).

 Photo courtesy of Invibio.

With all of these methods there are optimized conditions that manufacturers use to achieve the best possible pre-preg material. The diameter of PEEK fibers in relation to the carbon fibers is important for the blended fiber route and an optimized powder particle size distribution is important for the powder impregnation route, including the specific method of powder impregnation, which remain trade secrets.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B978143774463710003X

Durability of external fiber-reinforced polymer strengthening systems

J.J. MYERS , in Durability of Composites for Civil Structural Applications, 2007

Fatigue

In continuous fiber-reinforced composites the fatigue process is characterized by the initiation and multiplication of cracks, rather than initiation and propagation. Crack initiation occurs early in the fatigue life, and coincides with cracking of the weakest ply ( Pritchard, 1999). However, fibers in unidirectional composites have relatively few defects, and any crack that forms does not travel across the matrix easily, which contributes to the fatigue resistance of FRPs (Barnes and Mays, 1999). In a research study conducted by Curtis (1989), the fatigue life or endurance limit of unidirectional carbon fiber/epoxy strips at 2 million cycles was found to be 1.5 GPa. Curtis claimed that the fatigue resistance of unidirectional composites might be expected to depend solely on fibers. Hence, carbon fiber composites are believed to have higher resistance to fatigue as compared with metals.

Read full chapter

URL:

https://www.sciencedirect.com/science/article/pii/B9781845690359500128