Lezione 4: Stiffness and weight Flashcards
What is the material’s response to stress?
A materials responses to stress (loading) with strain (change of shape)
What is stiffness?
The resistance to change of shape that is elastic: the material will return to its original shape when unloaded
What is strength?
The resistance to permanent distortion or total failure
Are stress and strain material properties?
No they’re not, they describe a stimulus and a response
Are stiffness and strength material properties?
Yes, they are measured by:
- STIFFNESS: elastic modulus E, elastic limit sigma y
- STRENGTH: tensile strength sigma ts
Which are the main modes of loading?
- Axial tension (causes tensile strain)
- Compression (causes compressive strain)
- Bending
- Torsion
- Bi-axial tension or compression
Describe stress-strain curve for brittle response
The entire response is elastic, we have no plastic deformation. The yield strength is not reached before failure. Young’s modulus is determined by calculating the slope of the region under the line
Describe stress-strain curve for ductile response
The tensile strength is the maximum stress on the curve- The yield strength is determined by standard offset methods. Permanent deformation occurs at stresses beyond the yield strength, and past this point the material will not return to its original shape
Is there another way to cause strain?
Yes, strain can be caused not only from stress; however, stresses can develop if strain is constrained.
Which chart can help us find materials that are both stiff and light?
The modulus-density chart
Which chart can help us find materials that are both stiff and cheap?
The modulus-relative cost chart
What is meant by anisotropy/anisotropic materials?
Materials that are considered anisotropic have their properties dependant upon which direction in the material they are being measured. For example, woods are stiffer along the grain; fiber composites are stronger and stiffer parallel to the direction of the fibers than perpendicular to them
Talk about atomic packing for different materials
-Crystalline structures: most materials are crystalline, meaning they have a regularly repeating pattern of structural units. Atoms behave as if they are hard spheres.
In glasses we find ad amorphous structure that is maintained and allowed by rapid cooling.
For polymers, we find a carbon-carbon backbone with different side groups. Polymer chains bond to each other through weak hydrogen bonds. We can find 4 different situations:
-no regular repeating of polymer chains, resulting in a glassy or amorphous structure.
-regions in which polymer chains line up and form crystalline patches
-occasinal cross-linking allowing the polymer to stretch, typical of elastomers
-heavily cross-linked polymers exhibit chain sliding, typical of epoxy.
What is meant by cohesive energy and what is its correlation to the elastic modulus E?
Atoms are held together by bonds that behave like springs. The strength of these bonds is measured by cohesive energy.
Bond stiffness largely determines the value of the elastic modulus (see formula)
Talk about the elastic moduli of elastomers
Undeformed polymer chains have high randomness, so high entropy. When the polymer chain gets stretched, it resembles a crystalline structure, therefore the entropy is decreased. For elastomers E is generally low and increases with temperature.
