Tuesday, May 25, 2010

Harder, stronger, stiffer, tougher.

Today I’m taking a quick look at material properties and what exactly they mean in terms of the masonry system I’ve been describing.

Strength is defined as the ability of a material to withstand applied stress without suffering failure. As stated before on this blog, ceramics, concrete and glass have a high compressive strength.

Stiffness is defined as a measure of resistance of a material to deformation under an applied stress. Concrete and ceramics generally have a high stiffness. Stiffness is characterized by Young’s Modulus.

Hardness is defined as the measure of how resistant a material is to permanent shape change under an applied stress.

Toughness is defined as how resistant a material is to crack propagation. It is the resistance to fracture under applied stress; it is the amount of energy per unit volume that a material can absorb before rupturing.

Concrete and ceramics have a high compressive strength, a high stiffness and hardness. Concrete and ceramics are generally not very tough: they will tend to crack because of their somewhat brittle nature. The larger the item, the greater tendency to suffer cracking from applied stress. As the size increases, this tendency toward cracking increases exponentially.

The masonry system I’ve been describing is strong, stiff, hard and tough. This system gains its toughness by virtue of the fact that it is composed of relatively small unit shapes. In this sense, it is “pre-fractured” and is held together by the interlocking features. That is, a large structure is less likely to break if it is already “broken” into component parts.

The component parts in this case are masonry units which are free to move (strain) under an applied force (stress). Strain relieves stress. This results in a strong, hard and tough structure.

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