1.5 Solids and Stresses (Materials) Flashcards
What is the 3 classes of a solid?
Crystalline (Arranged in a regular way)
Amorphous (Non-regular way)
Polymeric (Long molecules of repeating monomers)
What is a Crystal solid?
A material where the atoms are arranged in a regular crystal lattice with repeating structures
What is an Amorphous solid?
A material where the atoms are arranged in a non-ordered structure
E.g. Glasses and ceramics. Ceramics
What is a polymeric solid and its properties?
A material that contains long chains of polymer molecules which are usually strong and can rotate.
Therefore, polymers are strong and flexible. Cross-linkages between chains can make a polymer more stiff.
What is a crystalline solid
A materials with which consists of a 3-D repeating lattice of atoms
What is Hooke’s Law?
The tension (force) on a spring or wire is proportional to its extension from its natural length, provided the extension isn’t too great and the material deforms.
F = kx
E = ½kx²
E = ½Fx
What is spring constant, k?
Force per unit length extension
k = F/x
What is stress (and pressure), σ?
Force per unit cross-sectional area when equal opposing forces act on a body
Unit = Nm^-2 or Pa (Pascal)
σ = F/A
What is strain, ε?
Defined as the extension per unit length due to an applied stress. Effectively how much the material has extended compared to the original length as a fraction. (original length x ε = extension)
ε = x/L
ε = Extended length due to stress/ Original Length
unit = no units, it only what you times the original length by to get the extension
What is Young’s Modulus, E?
E = tensile stress/ tensile strain
It is the ratio between stress and strain.
A stress (Nm^-2 or Pa) of E is needed to increase the length of an object by 1m
E.g.
For mild steal, E = 210 G Nm^-2
It takes a stress of 210 Giga Nm^-2 to increase the length by 1 m- a lot of stress.
What is a ductile material?
A material which can be drawn out into a wire. This implies that plastic stain occurs under enough stress
What is plastic (inelastic) strain?
Strain that decreases only slightly when the stress is removed (slightly moves back to original length). In metals this happens due to the movement of dislocations within the crystal structure
What is elastic strain?
Strain that disappears when the stress is removed and the material returns to its original shape and size.
What is the elastic limit?
The point at which deformation (strain) stops becoming elastic and plastic stain occurs. For a specimen it is measured by the maximum force and for a material by the maximum stress before the stain ceases to be elastic.
