Hierarchical Structures in Biology as a Guide for New Materials Technology (1994) / Chapter Skim
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3 SYNTHETIC HIERARCHICAL SYSTEMS
3 SYNTHETIC HIERARCHICAL SYSTEMS
Pages 39-72
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From page 39... ...
In rigid composites, these tend to be calcium carbonates, calcium phosphates, and silica. In mollusk shells, thin layers that surround the stiff ceramic constituents, such as those 39
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From page 40... ...
Finally, the influence of moisture upon the mechanical behavior of rigid composites has been noted by Vincent (1990~. Stiffness in natural rigid composite materials, such as horn and mollusk shells, diminishes with increasing moisture.
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From page 41... ...
The toughening mechanisms revealed by fractographic analysis of fracture surfaces and indentation cracks include crack blunting and branching; microcrack formation; sliding and pullout of aragonite plates; polymeric ligament formation, akin to crazing, which bridges cracks; and possible strain hardening and shearing of the organic material. The challenge is to design synthetic discontinuous laminates that use as an example the architecture of nacre; that is, (1)
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From page 42... ...
For instance, the mechanical properties of most metallic materials are controlled through the manipulation of dislocations at the nanometer length scale, whereas the mechanical properties of ceramic materials are controlled through the propagation of cracks that are initiated from defects of micrometer length scales. For composites that are composed of two constituents, often of quite different character, the controls are much more complex even though mechanistic understanding in many instances is reasonably well in hand.
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From page 43... ...
Synthetic fibers make up the largest group of oriented polymers, and all possess a hierarchical structure in the sense that they possess a repeat of fiber symmetry structure from the molecular to the macroscopic. The observation of hierarchical structure in synthetic fibers, as manifested in a series of district fibrillar substructures that have characteristic diameters in range from Angstroms to microns, is analogous to naturally occurring systems such as tendon, as described in Chapter 2 (Figure 2-2~.
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From page 44... ...
The noncrystalline portion of the fibril is characterized by an orientation parameter and the concentration of chains that serve as interfibrillar tie molecules. For convenience and utility in property correlations, the noncrystalline portion of the fibril is often divided into two parts, an unoriented fraction (chains having end-to-end distance associated with the unoriented chain)
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From page 45... ...
Synthetic Hierarchical Systems 45 TABLE 3-1 Microfibrillar and Hierarchical Dimensions in Angstrom Units in Synthetic Fibers (Tucker and George, 1972) Basic Fibrin Microfibril Fibril Macrofibul Cellulose and derivatives 3~75 Protein fibers 3~100 Polyacryloni~ile 50 Polyamides 30 Polyester Polyethylene 100 3SO, 35~500 800 4,000 100 250, 28~500 800-1,200 36,000 150 450 3,000 4,000 30,000 40,000 100 200, 40~500 2,000-5,000 30,000 40,000 100 200 400 1,200 5,000 30,000 40,000 100 200, 400 500 2,000-3,000 ~7 .
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From page 46... ...
A schematic representation of the undulating ribbon model is shown in Figure 3-5. As a result of the structure of the precursor, PAN-based carbon fibers have a relatively low degree of axial orientation and low graphitization compared with pitch fibers.
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From page 47... ...
Source: Edie and Stoner, logs. Figure 3-5 Schematic representation of undulating ribbon model of P~-b~ed capon fiber.
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From page 48... ...
Fibers from both PAN and pitch have poor compressive strength due to the low transverse strength, the deleterious effect of defects, and buckling instability. There are indications that irregularly shaped fibers could enhance compressive strength by increasing buckling stability and improve adhesion in composite applications (Edie and Stoner, 1993~.
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From page 49... ...
Early classical work on steel laminates was the basis for Damascus steels, which were developed in the Middle East during the late iron age, more than 2000 years ago. The metallurgy that produced Damascus steel is based on a simple thermomechanical cycle that forms a composite microstructure of tough martensite containing ultrafine participates with hard strings of carbides decorating prior austenite grain boundaries.
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From page 50... ...
with mild steel, processed by low-temperature roll-bonding procedures, exhibit remarkable notch-impact properties (Kum et al., 1983~. This result is attributable to notch blunting by delamination at the interfaces of the dissimilar layers during impact testing.
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From page 51... ...
SAN crazed or cracked, while shear bands initiated in polycarbonate from the craze tips (Figure 3-~. As the layer thickness decreased, crazing or cracking of the SAN was suppressed, and shear bands that extended through several layers produced shear yielding of both polycarbonate and SAN (Figure 3-9~.
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From page 52... ...
Improved toughness has been observed in layered biological materials, such as insect cuticle (Vincent, 1990) , and crazing, microcracking, and other failure modes during deformation have been observed in other natural materials, such as woods and mollusk shells (Mayer, 1992~.
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From page 53... ...
Source: Ma et al., 1990a. Metallic Composites Many examples exist of the strengthening of metallic materials by addition of second phases.
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From page 54... ...
This plot shows critical roles of reinforcement size, aspect ratio, and volume fraction in determining yield strength of a variety of metallic composites that are reinforced by ceramic and metallic particles and fibers. More-recent work shows higher levels of strengthening that can be achieved by these methods, but the trends were demonstrated early in the evolution of composite materials.
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From page 55... ...
Lood Is applied 2 3 parallel to the fiber orientation. FIGURE 3-11 Effect of particles and fibers on composite strengthening, oq/a,,,,, at room temperature.
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Growth direction 11 [101]
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From page 57... ...
it L Multifilament Monofilament Watt Warp Biaxial Triaxial 1 1 I- 1 1 Flat Twisted Textured Woven Knit Braided Nonwoven ,~L Angle I I interlock Triaxisl \ Biaxial Triaxial At ire ~|~3-D ~ 2 Stop I XYZ ~Multiaxial MWK fully fashioned watt knit 6-ply 4-ply impaled unimpaled FIGURE 3-12 Classification of fiber architecture. Source: Ko, 1989.
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From page 58... ...
, which restrict radial tire deformation under load. Tires are hierarchical in the sense that they represent a multilayer composite construction, with the reinforcing layers comprising fabrics woven from twisted (usually three ply)
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From page 59... ...
ADJUSTABLE VARIABLES AND THEIR INFLUENCE ON MECHANICAL BEHAVIOR OF SYNTHETIC MATERIALS Due to the complexity of composite and heterophase materials. issues of scale, interaction, and architecture must be considered to obtain an adequate description of the solid-state structure to describe or predict end-use properties.
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From page 60... ...
Novel materials synthesis and processing, as discussed in the next chapter, has played a key role in the development of modern synthetic materials. Variables critical to material properties include: · atomic lattice design; · nanostructures and boundaries; · cells and other substructure (size, morphology, structure, orientation)
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From page 61... ...
Some simple approaches to analysis of synthetic composites, along with the beginnings of application of these analyses to natural composites, are described in the following sections. Mechanical Behavior of Rigid Composite Materials The main purposes of stiff structural natural composites are to provide protection, shape, and support and to serve as jointed limbs and weapons.
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From page 62... ...
Similarly, in natural biological materials, many diverse examples exist of rigid composites. Examples discussed in Chapter 2 include wood (Figure 2-4)
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From page 63... ...
Synthetic Hierarchical Systems Elastic Response of Rigid Composite Materials 63 In order to compare the behavior of some biological materials to synthetic materials, it is useful to look at functions of simple shapes. For example, if the goal is to minimize weight for a given stiffness for the buckling of a slender column or a tube, a plot such as that shown in Figure 3-16 can be useful.
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From page 64... ...
3-2) where: Ec is the Young's modulus of the composite Em is the Young's modulus of the matrix material Vm is the volume fraction of the matrix material EF is the Young's modulus of the fiber VF is the volume fraction of the fiber phase In the case of discontinuous reinforced composites, mixing rules are significantly more complex.
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From page 65... ...
Synthetic Hierarchical Systems 65 To account for directionality in laminates, two limiting cases that bound the elastic properties of two-phase composite systems are as shown in Figure 3-17.
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From page 66... ...
~}'h S is the aspect ratio of the nacre platelet Vp~ is the volume fraction of the nacre platelet Ep, is the Young's modulus of the nacre platelet M is the shear modulus of the matrix This prediction and another shear lag model by Riley (1968) follow the trend of the actual mechanical behavior of nacre better than the predictions of the Voigt or Reuss models.
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From page 67... ...
Similar effects, though perhaps not as dramatic, are observed in synthetic resin-matrix composites that take up moisture. Toughening Mechanisms for Rigid Composites Examination of the mechanical behavior of both synthetic and natural composites generally shows higher levels of toughness and resistance to cracking in carefully designed composites than in monolithic materials (Clegg et al., 1990~.
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From page 68... ...
all need to be studied more closely in order to arrive at directions for improvement of synthetic composites.
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From page 69... ...
In addition to crack blunting at interfaces, other energy-absorbing mechanisms can operate in both synthetic and natural composites. Shear yielding and microstructural defects that may create complex stress fields and cleflect propagating cracks can be effective energy dissipators.
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From page 70... ...
The dominant failure modes for composite structures include matrix cracking, delamination, tensile fiber failure, microbuckling, and global instability. Also to be considered is the effect that service environmental factors such as moisture, temperature, chemical or electrochemical interactions, and radiation have in altering the mechanical properties and hence the mechanical response of the composite system.
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From page 71... ...
S5nthcac Hierarchical Systems 71 they have resulted in limited success. A better understanding of the micromechanisms of deformation and failure in complex systems and of the application of analysis methods on multiple size scales is needed before additional progress is made in analytical mechanics and in the development of suitable analogues of such natural materials in synthetic composites.
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