Explain the limitations on the statement: Decreasing grain size increases toughn

Question

Explain the limitations on the statement: Decreasing grain size increases toughness in a material. A crystalline material is 95 % dense and the average size of uniformly distributed pores is 20 microns. The MOR of the fully dense material is 500 MPa, the theoretical strength is 20 GPa. The MOR of the fully dense material is 500 MPa, the theoretical strength is 20 GPa. What is the Young's modulus of the porous material If a crack starts will it stop? Discuss in detail how the pores will alter the toughness. Will the current microstructure allow crack arrest if one begins to propagate? Show how you would calculate this (or actually calculate) if you can from the given information. A fibers with a strength of 1 GPa reinforce a material. The fibers are oriented Perpendicular to the direction of the crack propagation. Show how you would estimate the reduction in stress at the crack tip PER FIBER (you do not need to calculate)

Explanation / Answer

1. Decreasing grain size has an advantage of both increasing the strength and improving the Impact toughness. Increase in strength is evident from Hall Patch Equation but what is reason for lowering of the transition temperature.

2.

3.It is reasonable to begin an introduction to composite materials by defining just what these materials are. It turns out, however, that materials technologists are always arguing about such definitions. What is a ceramic, for instance? Ceramists, like most of us always wanting as much turf as possible, sometimes say a ceramic is anything that isn’t a metal or an organic. They call silicon carbide (SiC) a ceramic, and most engineers agree – it’s hard, brittle, and infusible: these are properties we associate with ceramics. But it’s full of carbon. Does this make it an organic? No; even organic chemists who define their field as the chemistry of carbon call SiC a ceramic, feeling in this case that properties outweigh chemical composition in assigning titles. There are a lot of gray areas in materials nomenclature.
Nowhere is this ambiguity more evident than in the modern materials category titled “composites.” The name implies that the material is composed of dissimilar constituents, and that is true of composites. But isn’t it true of all materials? Even a material as simple as pure hydrogen has a composite chemical constitution of protons and electrons, which in turn are composed of still smaller and dissimilar entities. A certain degree of arbitrariness is required in settling on a working definition for most materials classes, and certainly for composites.   
In this text, we will follow a common though far from universal convention that takes “composites” to be materials in which a homogeneous “matrix” component is “reinforced” by a stronger and stiffer constituent that is usually fibrous but may have a particulate or other shape. For instance, the term “FRP” (for Fiber Reinforced Plastic) usually indicates a thermosetting polyester matrix containing glass fibers1, and this particular composite has the lion's share of today's commercial market. Figure 1 shows a FRP laminate fabricated by “crossplying” unidirectionally-reinforced layers in a 0o-900 stacking sequence.

This text will concentrate primarily on fiber-reinforced polymer-matrix composites, with less attention to materials such as rubber reinforced with carbon black or Portland cement reinforced with rock or steel. Both of these are definitely considered to be composites by the communities dealing with them, but they lie outside the arena of composite materials as the term has come to be used today by many practitioners.
As seen in Table 12, the fibers used in modern composites have strengths and stiffnesses far above those of traditional bulk materials. The high strengths of the glass fibers are due to processing that avoids the internal or surface flaws which normally weaken glass, and the strength and stiffness of the polymeric aramid fiber is a consequence of the nearly perfect alignment of the molecular chains with the fiber axis.
Table 1 - Properties of Composite Reinforcing Fibers. Material E, GPa b, GPa , kg/m3 E/ , MJ/kg b/ , MJ/kg cost, $/kg E-glass 72.4 2.4 2,540 28.5 0.95 1.1 S-glass 85.5 4.5 2,490 34.3 1.8 22-33 aramid 124 3.6 1,440 86 2.5 22-33 boron 400 3.5 2,450 163 1.43 330-440 HS graphite 253 4.5 1,800 140 2.5 66-110 HM graphite 520 2.4 1,850 281 1.3 220-660

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