Materials

• F=k*change in L


  Force=spring constant (Nm^-1) x extension(m)
• Stress


  The force (N) per cross sectional area (m^2)

  stress=force/area

  measured in Nm^-2 or pascals (pa)
• Breaking stress


  Maximum stress that can be exerted on material before it breaks
• Elastic behaviour


  material returns to it‘s original length when tension is released (obeys Hooke‘s law)
• Plastic behaviour


  Material can be stretched beyond the point where they continue to obey Hooke‘s law -material is permanently deformed
• Brittle behaviour


  Likely to fracture under stress -little or no deformation
• Ductile behaviour 


  Can be drawn into a wire
• Strain 


  the extension per unit length

  strain= extension/original length= change in L/L

  strain has no units

  may be given as a percentage
• Young’s modulus


  Stress/ strain

  assuming limit of proportionality hasn’t been exceeded
• Elastic limit


  Point beyond which a wire is permanently deformed (stretched)
• Plastic deformation


  Deformation of a solid beyond its elastic limit
• Elasticity of an object


  The ability to regain it’s shape after it has been deformed or distorted once the forces causing the deformation are released
• Density 


  The mass per unit volume
• equation for density of an alloy


  density=p1v1+…pnvn/v
• Hooke's law 


  The extension of a spring from its natural length is proportional to the force needed to extend it
• The effective spring constant for a spring in series


  1/k=1/k1+1/k2+….+1/kn
• The effective spring constant for a spring in parallel 


  K=k1+k2+….kn
• How can energy in a stretched spring be determined 


  The area under a force extension graph

  Energy=1/2 x force x extension
• Ultimate tensile stress


  The stress needed to break a solid material
• Brittle material


  A material will snap without stretching or bending when subject to stress
• describe a stiff material


  A material that requires a lot of force to extend
• Limit of proportionality 


  The limit beyond which, when a wire or a spring is stretched, the extension is no longer proportional to the force that stretches it
• Describe a strong material

  Material with a high ultimate tensile stress.