6-Materials

6-Materials

1. 61-springs and Hooke’s Law

   • You need a pair of equal and opposite forces to alter the shape of an object
   • Forces that produce extension are tensile forces
   • Forces that shorten objects are compressive forces
   • On an F-E graph up to the elastic limit it will be a straight line
     • Elastic deformation means that there are no permanent structural changes
   • Plastic deformation is when there is permanent structural changes
   • Hooke’s law is when the extension of the spring is directly proportional to the force applied
   • Force constant is the measure of the stiffness of a spring
   • F=Kx

2. 6.4-Stress,strain and Young’s modulus

   • Stress
     • the force applied per unit cross sectional area
     • force/CS area
     • sigma
     • unit=Pa
   • Strain
     • Extension/Original length
     • Epsilon
     • unit= none (it's a ratio)
     • Fractional change in the original length of the wire
   • Stress-strain graph for a metal
     • v steep then small hemisphere then another big curve upwards, then drop to breaking point
       • UTS-Highest point on the graph
     • Ultimate Tensile Strength-max stress that a material can withstand before it breaks
     • The stress at the point of fracture is known as the braking fracture
   • Young Modulus
     • stress is directly proportional to strain
     • Represented to E and Y
     • Sigma/Epsilon
     • unit=Nm^-2 or Pa

3. 6.2-Elastic potential energy

   • W is roughly equal to Force x extension
   • The area under an F-E graph = work done
   • E=1/2FX
   • E=1/2KX^2
   • E is directly proportional to x^2

4. 6.3-Deforming materials

   • Metal wire
     • loading diagonal up to elastic limit
     • deformation-small change in f = big change in x
     • unloading- parallel but not identical to loading
   • Rubber
     • Doesn't obey Hooke's Law
     • Loading-steep gradient,shallow then steep again(curved)
     • Unloading-Steep,shallow then steep (below the former)
     • Hysteresis loop
   • Polythene
     • not obey Hooke's Law
     • same loading curve as rubber
     • unloading straight steep gradient back to Extension axis