Matrix plastic deformation

Plasticity The ability to deform plastically. Ability to irreversibly deform under an applied stress by motion within the tape matrix. Plastic deformation causes rearrangement within the tape structure, and thus the tape does not revert to its original shape when the applied stress is removed. 

If Poisson s ratio v is taken to be 0.43 (as for a semicrystalline, rubber-modified polypropylene), the term in parentheses amoimts to 0.20 in plane strain and to 0.14 in plane stress. This means that the critical distance where y > vc is by a factor 17 (0.20/0.14) = 2 larger in plane strain than in plane stress. The same applies for the size of the plastic zone if matrix plastic deformation is triggered by particle cavitation. This is a remarkable result since it is exactly the opposite of what the von Mises criterion would predict. 

In summary, even though an increase in toughness of some polymer/clay iianocomposites was achieved, the results are not always consistent. Especially, the role of polymer matrix plastic deformation versus nano-filler mobility is a critical issue that needs further detailed studies. 

Two approaches have been taken to produce metal-matrix composites (qv) incorporation of fibers into a matrix by mechanical means and in situ preparation of a two-phase fibrous or lamellar material by controlled solidification or heat treatment. The principles of strengthening for alloys prepared by the former technique are well estabUshed (24), primarily because yielding and even fracture of these materials occurs while the reinforcing phase is elastically deformed. Under these conditions both strength and modulus increase linearly with volume fraction of reinforcement. However, the deformation of in situ, ie, eutectic, eutectoid, peritectic, or peritectoid, composites usually involves some plastic deformation of the reinforcing phase, and this presents many complexities in analysis and prediction of properties. 

Table 13 is a representative Hst of nickel and cobalt-base eutectics for which mechanical properties data are available. In most eutectics the matrix phase is ductile and the reinforcement is britde or semibritde, but this is not invariably so. The strongest of the aHoys Hsted in Table 13 exhibit ultimate tensile strengths of 1300—1550 MPa. Appreciable ductiHty can be attained in many fibrous eutectics even when the fibers themselves are quite britde. However, some lamellar eutectics, notably y/y —5, reveal Htde plastic deformation prior to fracture. 

A series of events can take place in response to the thermal stresses (/) plastic deformation of the ductile metal matrix (sHp, twinning, cavitation, grain boundary sliding, and/or migration) (2) cracking and failure of the brittle fiber (5) an adverse reaction at the interface and (4) failure of the fiber—matrix interface (17—20). 

Binders improve the strength of compacts through increased plastic deformation or chemical bonding. They may be classified as matrix type, film type, and chemical. Komarek [Chem. Eng., 74(25), 154 (1967)] provides a classification of binders and lubricants used in the tableting of various materials. 

Al-Mg (5000 Series) and Al-Mg-Si (6000 Series) In the binary alloy system strength is obtained mainly by strain hardening. Stress corrosion is thought to be associated with a continuous grain boundary film of Mg,Alg which is anodic to the matrix . Air cooling prevents the immediate formation of such precipitates, but they form slowly at ambient temperatures. Thus only low Mg alloys are non-susceptible (Al-3% Mg). Widespread precipitation arising from plastic deformation with carefully controlled heat-treatment conditions can lower susceptibility. Al-5Mg alloys of relatively low susceptibility are subjected to such treatments. Mn and Cr... 

Sandorf, 1980 Whitney, 1985 Whitney and Browning, 1985). According to the classical beam theory, the shear stress distribution along the thickness of the specimen is a parabolic function that is symmetrical about the neutral axis where it is at its maximum and decreases toward zero at the compressive and tensile faces. In reality, however, the stress field is dominated by the stress concentration near the loading nose, which completely destroys the parabolic shear distribution used to calculate the apparent ILSS, as illustrated in Fig 3.18. The stress concentration is even more pronounced with a smaller radius of the loading nose (Cui and Wisnom, 1992) and for non-linear materials displaying substantial plastic deformation, such as Kevlar fiber-epoxy matrix composites (Davidovitz et al., 1984 Fisher et al., 1986), which require an elasto-plastic analysis (Fisher and Marom, 1984) to interpret the experimental results properly. 

The descriptions presented in the foregoing sections are concerned mainly with composites containing brittle fibers and brittle matrices. If the composite contains ductile fibers or matrix material, the work of plastic deformation of the composite constituents must also be taken into account in the total fracture toughness equation. If a composite contains a brittle matrix reinforced with ductile libers, such as steel wire-cement matrix systems, the fracture toughness of the composite is derived significantly from the work done in plastically shearing the fiber as it is extracted from the cracked matrix. The work done due to the plastic flow of fiber over a distance on either side of the matrix fracture plane, which is of the order of the fiber diameter d, is given by (Tetelman, 1969)... 

Also, mechanical data on the influence of low volume fractions (0.03-0.05) of rigid filler particles provide evidence of a localized plastic deformation which would not seem understandable by reference to a uniformly crosslinked network. A non-uniformly crosslinked matrix might also be invoked to account for insensitivity of the rate of diffusion of water on the apparent degree of crosslinking. However, an observed increase in the uptake of water with apparent degree of crosslinking remains unexplained. 

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