3.4 - Materials

• compression:

  The result of two coplanar forces acting into an object Compression usually results in a reduction in the length of the object
• Compressive Deformation:

  The changing of an object's shape due to compressive forces
• Polymeric:

  A material made from polymers
• Tensile Deformation:

  The changing of an object's shape due to tensile forces
• Breaking stress

  The amount of stress a material can take without it breaking
• Brittle

  A material that does not behave plastically snapping whe it exceeds its ultimate tensile stress
• Brittle

  A material that does not behave plastically snapping whe it exceeds its ultimate tensile stress
• Compressive strain

  The compression to unit original length ratio
• Describe the energy changes that occur during plastic deformation

  The material is stretched and the energy from the work done is used to break the bonds between the molecules. This causes permanent deformation
• Describe the energy changes that occur during plastic deformation

  The material is stretched and the energy from the work done is used to break the bonds between the molecules. This causes permanent deformation
• Ductile

  A material that can behave plastically Can deform without fracturing
• Elastic deformation

  When the force is removed the object will return to its original shape
• Elastic limit on a Stress-strain graph for a ductile material

  Material starts to behave plastically No longer returns to its original shape
• Hooke's law

  The extension of a material is proportional to the force applied unless the elastic limit is exceeded
• How do you find the young's modulus from a stress-strain graph


  Gradient
• How is energy stored during elastic deformation

  The work done is transferred and stored as elastic potential energy
• How to work out total sping constant when springs are in parallel

  K=K1+K2+....

  Add spring constants
• How to work out total sping constant when springs are in series

  1/K=1/K1+1/K2+....

  add reciprocal of spring constant
• Limit of proportionality on a Stress-strain graph for a ductile material

  Graph is no longer straight starts to bend Stops obeying hooke's law Will still return to its original shape
• Plastic deformation

  After the force is removed the object will not return to its original shape (limit of proportionality has been exceeded)
• Plastic deformation

  After the force is removed the object will not return to its original shape (limit of proportionality has been exceeded)
• Spring constant

  Force per unit extension
• Strain energy

  Energy stored within a stretched or compressed material
• Stress-strain graph for ductile materials

  Straight line through the origin (obeys hooke's law) until the limit of proportionality where it starts to bend After the limit of proportionality the material no longer obeys hookes law but would still return back to its original shape Then it reaches the elastic limit where the material starts to behave plastically meaning beyond this point the material will no longer return to its original shape Then it reach the yield point where the material starts to stretch without any extra load

  
• Stress-strain graph for polythene

  Polythene behaves plastically so applying a stress to it stretches it into a new shape Polythene is a ductile material

  
• Stress-strain graph for a brittle material

  Straight line through the origin It obeys hooke's law However when stress reaches a certain point the material fractures (snaps)

  
• Stress-strain graph for rubber

  Rubber returns to its original length when the load is removed- it behaves elastically Loading and unloading curves for rubber are different . The energy released when the rubber is unloaded is less than the work done to stretch the rubber. This is because some of the elastic potential energy is stored in the stretched rubber is converted to heat The amount of energy converted to heat per unit volume is given by the area between the loading and unloading curves

  
• Tensile strain

  A measure of how the material stretches. The ratio of extension to original length (extension per unit length of a material)
• Tensile strain

  A measure of how the material stretches. The ratio of extension to original length (extension per unit length of a material)
• Tensile stress

  The tensile force applied per unit area
• The elastic limit

  The load/point at which the material starts to behave plastically
• The limit of proportionality

  The largest value of extension where we can apply the spring constant
• Tough


  Materials that can withstand a large (impact) energy without breaking
• Ultimate tensile stress

  The maximum stress that a material can sustain
• What does the area underneath a force extension graph represent

  What does the area underneath a force extension graph represent
• What does the start of a Stress-strain graph look like for a ductile material

  Straight line through the origin Obeys hooke's law
• What does young's modulus measure 


  It measures how stiff a material is If a material has a high young's modulus it is stiffer
• What is stiffness

  A measure of a material's resistance to change in size and/or shape Stiff - not easily bent or changed shape
• What is the yield point on a stress-strain graph for a ductile material

  Point where the material stretches without any extra added load
• Young's modulus

  Tensile stress / tensile strain The ratio of stress to strain of a material