3.4 Materials

• a tensile force is equal and opposite forces acting on an object to stretch it

  eg pulling an elastic band
• compressive forces are two or more forces that reduce the length or volume of an object

  eg sitting on a chair
• Hooke's law

  The force applied is directly proportional to the extension of the spring unless the limit of proportionality (elastic limit) is exceeded
• Force = spring constant x extension
• springs in series

  $1/k=$$(1/k_1)+$$(1/k_2)+$$....(1/k_n)$
• Elastic limits

  The value of stress or force beyond which elastic deformation becomes plastic deformation and the material or object will no longer return to its original shape and size when the force is removed
• Plastic deformation

  Permanent structural changes to the spring so it does not return to its original length when the force is removed
• The area under a force-extension graph is the elastic potential energy
• elastic potential energy is directly proportional to the square of the extension
• A hysteresis loop is the loop formed by the loading and unloading force-extension graphs of a material
• The area of a hysteresis loop is the thermal energy lost to the surroundings.
• Molecular structure of a rubber band

  Rubber consists of squashed and tangled long chain molecules which can be untangled easily with small forces but once straightened they require large forces to extend further
• Molecular chains are easier to untangle for smaller forces so the graph has a smaller gradient

  The molecular chains are difficult to extend further past their fully extended length so the gradient grows steeper

  
• Tensile stress is the force applied per cross sectional area of the wire
• Tensile strain is the fractional change in the original length of the wire
• the ultimate tensile strength is the maximum stress which an object can withstand before breaking (the highest point of a stress-strain graph) and is independent of shape and size of the material
• Young modulus is a constant for a given material and is independent of shape or size of an object
• Limit of proportionality is the point until which Hooke's law is true
• beyond a yield point of a graph, the material enters a phase of non linear pattern and irrecoverable strain or plastic deformation
• ultimate tensile strength is the maximum stress an object can take whereas breaking strength is the stress value at the point of breaking