4/5 -Materials Flashcards
Define Stiff
Small extension per unit of force (high Young Modulus)
Define Elastic.
Returns to unstretched form when stresses are removed
Plastic
Permanent deformation.
Define ductile.
Can be drawn into wires
Define a hard material.
A material that resists indentation on impact.
Define Tough
Absorbs a lot of energy before fracture (deforms plastically)
Define a brittle material.
A material that undergoes little to no plastic deformation before fracture.
Define strong
Can withstand high stresses.
What is tension?
When two forces act on an object to make it stretch.
What is compression
When two forces act in opposite directions on an object to make it squash.
What is yield stress?
The stress at which a specimen begins to plastically deform (yield).
What is fracture stress?
The stress at which fracture occurs.
What is tensile strength?
The fracture stress of a material in tension.
Explain the graph of extension against force up until the fracture point for a wire or spring.
For small extensions, the graph is linear as the wire stretches or deforms elastically. Just before the elastic limit (the limit of proportionality), the gradient of the graph decreases and no longer follows Hooke’s law. At this point, the bonds between the molecules start to break and the extra force required to stretch the wire the same amount begins to decrease. After the elastic limit, the wire begins to stretch permanently as the bonds break and atoms rearrange. Extension causes the cross sectional area to decrease, meaning a lower force is required to stretch the wire. The gradient is then increasingly more negative up until the fracture point.
Equation for energy stored in a spring?
E=0.5Fx= 0.5kx^2
What is stress,σ?
Force per unit area.
What is strain,ε?
The fractional increase in length.
What is an amorphous material?
Where the atoms within a material form no regular structure and have no long range order.
How can you make polymers stiffer?
Adding cross linkages between polymer chains.
