Composite materials, brittle

This experience so clearly demonstrated on the brittle composite materials can be applied immediately to the deformation of fully drawn fibrous material which may be extended up to fracture. If one stops a little before rupture and removes the load completely, the sample will slowly approach almost zero strain. The deformation is nearly completely recoverable. The next loading yields a lower load-elongation curve. One... 

On the other hand, ultimate properties are largely used to evaluate the adhesion and the chemical treatment efficiency but, again, the relevant theories are in an elementary stage they often consider the inclusion of one filler only and assume isostrain behaviour of the two phases and a a, e linear behaviour until the ultimate properties are reached in the case of brittle composite materials. In the case of thermoplastics, the yield point appears to substitute adequately, with the same approximation, the break point. 

The fundamental goal in the production and appHcation of composite materials is to achieve a performance from the composite that is not available from the separate constituents or from other materials. The concept of improved performance is broad and includes increased strength or reinforcement of one material by the addition of another material. This is the well-known purpose in the alloying of metals and in the incorporation of chopped straw into clay for bricks by the ancient Egyptians and plant fibers into pottery by the Incas and Mayans. These ancient productions of composite materials consisted of reinforcing britde materials with fibrous substances. In both cases the mechanics of the reinforcement was such as to reduce and control the production of cracks in the brittle material during fabrication or drying (2). 

The modem interest in composite materials can be traced to the development of BakeHte, or phenoHc resin, in 1906. BakeHte was a hard, brittle material that had few if any mechanical appHcations on its own. However, the addition of a filler— the eadiest appHcations used short cellulose fibers (2)—yielded BakeHte mol ding compounds that were strong and tough and found eady appHcations in mass-produced automobile components. The wood dour additive improved BakeHte s processibiHty and physical, chemical, and electrical properties, as weU as reducing its cost (3,4). 

Many metals are naturally brittle at room temperature, so must be machined when hot. However, particles of these metals, such as tungsten, chromium, molybdenum, etc., can be suspended in a ductile matrix. The resulting composite material is ductile, yet has the elevated-temperature properties of the brittle constituents. The actual process used to suspend the brittle particles is called liquid sintering and involves infiltration of the matrix material around the brittle particles. Fortunately, In the liquid sintering process, the brittle particles become rounded and therefore naturally more ductile. 

For E-glass-epoxy, the Tsai-Hill failure criterion seems the most accurate of the criteria discussed. However, the applicability of a particular failure criterion depends on whether the material being studied is ductile or brittle. Other composite materials might be better treated with the maximum stress or the maximum strain criteria or even some other criterion. 

Oscar Hoffman, The Brittle Strength of Orthotropic Materials, Journal of Composite Materials, April 1967, pp. 200-206. 

A unidirectional fiber-reinforced composite material deforms as the load increases in the following four stages, more or less, depending on the relative brittleness or ductility of the fibers and the matrix ... 

Consider fibers that all have the same strength and are relatively brittle in comparison to the matrix as studied by Kelly and Davies [3-26]. Moreover, both the fibers and matrix are active only in the linear elastic range (stage 1 in Figure 3-46). If the composite material has more than a certain minimum volume fraction of fibers, V, the ultimate strength is achieved when the fibers are strained to correspond to their maximum (ultimate) stress. That is, in terms of strains. 

Because the fibers are more brittle than the matrix, they cannot elongate as much as the matrix. Thus, the fibers are the weak link, from the strain viewpoint, in the strength chain that the composite material comprises. 

Net-tension failures can be avoided or delayed by increased joint flexibility to spread the load transfer over several lines of bolts. Composite materials are generally more brittle than conventional metals, so loads are not easily redistributed around a stress concentration such as a bolt hole. Simultaneously, shear-lag effects caused by discontinuous fibers lead to difficult design problems around bolt holes. A possible solution is to put a relatively ductile composite material such as S-glass-epoxy in a strip of several times the bolt diameter in line with the bolt rows. This approach is called the softening-strip concept, and was addressed in Section 6.4. 

The mechanical properties of Nafion materials have not been of the most critical importance, as in the case of commercial thermoplastics or composite materials that are expected to be load-bearing. Rather, the primary focus has been on transport properties. To be sure, the mechanical integrity of membranes as mounted in cells, and under the perturbation of pressure gradients, swelling-dehydration cycles, mechanical creep, extreme temperatures, and the onset of brittleness and tear resistance, is important and must be taken into consideration. 

Lee, J.W. and Daniel, I.M. (1992). Deformation and failure of longitudinally loaded brittle matrix composites. In Composite materials Testing and Design (Tenth Voi). ASTM STP 1120, (G.C. Grimes, ed.), ASTM, Philadelphia, P.A., pp. 204-221. 

Nanocrystalline materials form an exciting area of materials research because bulk materials with grain size less than 100 nm have properties not seen in their microcrystalline counterparts. But the brittleness of nanoceramics has limited their potential for use in structural applications, namely, research on nanoceramics shows that they are not inherently tougher than their microcrystalline counterparts. Many strategies have been proposed to improve the mechanical properties of nanoceramics by using reinforcement by a second-phase addition and hybridization to develop nanocrystalline matrix composite materials. 

Hansson, T., Warren, R. (2000), Particle and whisker reinforced brittle matrix composites , in Kelly, A., Zweben, C. (editors-in-chief), Comprehensive Composite Materials, Vol. 6 (R Warren, editor), Elsevier. 

Epoxies are especially reliable when used with epoxy-based composites because they have similar chemical characteristics and physical properties. Room temperature curing adhesives are often used to bond large composite structures to eliminate expensive fixtur-ing tools and curing equipment required of higher-temperature cure adhesives. However, room temperature epoxies require long cure times, so they are not suitable for large, highspeed production runs. Some of the lower-temperature composite materials are sensitive to the heat required to cure many epoxies. Epoxies are too stiff and brittle to use with flexible composites. 

As previously noted, this chapter has been concerned mainly with those models for the creep of ceramic matrix composite materials which feature some novelty that cannot be represented simply by taking models for the linear elastic properties of a composite and, through transformation, turning the model into a linear viscoelastic one. If this were done, the coverage of models would be much more comprehensive since elastic models for composites abound. Instead, it was decided to concentrate mainly on phenomena which cannot be treated in this manner. However, it was necessary to introduce a few models for materials with linear matrices which could have been developed by the transformation route. Otherwise, the discussion of some novel aspects such as fiber brittle failure or the comparison of non-linear materials with linear ones would have been incomprehensible. To summarize those models which could have been introduced by the transformation route, it can be stated that the inverse of the composite linear elastic modulus can be used to represent a linear steady-state creep coefficient when the kinematics are switched from strain to strain rate in the relevant model. 

Alternatively, if one of the component UV curable materials is soft and elastic and the other is hard and brittle. Digital Materials with intermediate or varying mechanical properties can also be produced. In addition, because Digital Materials are not simply a homogeneous combination of two different materials, but Composite Materials, where each component material maintains its own properties within a microscopic phase range, their properties are not just the average of the properties of the component materials, but much more. Because Digital Materials are defined by means of precise software... 

It is assumed that the tube is made of a composite material which is composed of stiff duetile fibers arranged in a parallel uniform array in a ductile matrix. The eomposites of metal-metal type are eonsidered, first of all. However, the combination of a ductile matrix with brittle fibers can be easily accounted for, as well. As an example of the latter ease serves the aluminium alloy reinforced with boron fibers. 

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