Composite

By ۩ INO ۩ on Nov 8, 2009 | Reply

Basically i should mension that stiffness and strength are two of the mechanical propewrties of any material.Material properties are classified into physical , chemical , mechanical , thermal … etc.

In our case we have stifness and strenght, these are ‘mechanical properties of a material (any material)

Stifness (K) is defined mathematically as :Force per deflection

K = F / delta , where

If a force is applied to a material to make deformation , it deflects by an amount proportional to the stifness of the material. High stifness material undergoes less deformation than a material with less stifness for the same load.

Strength of a material is represented by different parameters, most common is the "yield strenght" or yield stress , which is the stress that causes the material to yield (i.e. to flow , and deform loosely plastically)

Other forms of strenght are shear strenght , impact strenght … etc.

Hope this helps

Good Luck
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By Scorpio9 on Nov 8, 2009 | Reply

The terms strength and stiffness are almost have the same meaning when pertaining to capability of materials to carry loadings. Strength of materials usually denotes as a capability of materials to carry the loads without having failure in shear, elongation or yield, while stiffness is a capability of materials to resist against bending, buckling or deflection.
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By Scott H on Nov 8, 2009 | Reply

Strength refers to load carrying capability and stiffness refers to deflection capability. Structures are designed for either strength (load) or stiffness (deflection or natural frequency).

For composite materials, strength is more of a structural component property (i.e. dependent on the design) rather than material property.
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By alpha b on Nov 8, 2009 | Reply

*Strength and stiffness are not the same thing. Here’s an analogy: A rubber band is stretched to failure. The rubber band failed at five pounds of force, but it stretched more than double its length before failure. The rubber band was not very stiff. In fact, it was elastic. Next, we stretch a kite string and find that it also fails at five pounds. It only stretched five percent before failure. It is very stiff. Both the rubber band and the kite string have the same ultimate strength. However, one is very stiff and the other is very flexible. This should demonstrate that strength and stiffness are not the same thing, and they are dependent upon the chosen material. Furthermore, the shape of the material also determines its stiffness without affecting its ultimate strength. For instance, if we take a plastic ruler that is 1/8" thick and 1" wide and bend it in the flat direction it is obvious that it is flexible. However, if we try to bend across the 1" thickness we find that it is very stiff. This demonstrates that the shape of the material causes the stiffness to change. We can take a piece of metal with a given weight and length and change its stiffness by making it narrower and thicker. Conversely, we can make it more flexible by making it wider and thinner.

*Composite structures have some other unique characteristics. There is no yielding or bending point. When the ultimate strength is reached, it simply explodes or disintegrates. Metal alloys on the other hand usually exhibit a definite yield point or point at which some bending occurs before final failure. This allows us to utilize a smaller safety margin when using metals than is possible with composite structures. In other words, we must make the composite structure bigger and heavier to account for the extra safety margin. Composite materials allow engineers to create very complicated structures with reduced weight and increased strength at a lower-cost than can be achieved with some metal structures, but the limitation is usually in the manufacturing or machining methods available and not in the strength to weight ratio. In fact, it is usually possible to create a simple metal structure that is just as strong, stiff, and lightweight as is possible with most composites.
http://www.usace.army.mil/publications/eng-tech-ltrs/etl1110-2-548/c-4.pdf
References :
http://www.bledsoebrace.com/education/cp030004.htm

By Adam W on Nov 8, 2009 | Reply

Strength is the ability to resist stress (pressure). It is a property of the materials involved and the direction of the stress.

Stiffness is a derived property involving not only strength but the shape of the material (section). For example, 2 beams of different shapes made of reinforced concrete (a composite) will have different stiffness even though they are the same material.
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