Testing Materials

Testing Materials

1. Properties definition

   • Hard - difficult to scratch
   • Brittle - easy to shatter
   • Stiff - hard to stretch or bend
   • Malleable - can be shaped easily
   • Tough - can undergo a lot of deformation before breaking
   • Ductile - can be drawn into wires

2. Hookes law

   • The force F, is proportional to the extension x, for a weight hanging off a spring
   • F = kx
   • k = spring constant
   • Every spring or wire has its own value for the spring constant
   • k depends on the material, its length and its cross-sectional area
   • The first part of graph is where it deforms elastically
   • Then it curves at its elastic limit
   • And in the next curve it deforms plastically until it reaches its fracture point where it breaks
   • When deformed plastically it doesnt return to its original length
   • Energy stored in a spring
     • E = 1/2 Fx
     • Also written as E= 1/2 kx²

3. Stress

   • Stress (Pa) = F/A
   • Fracture stress is the stress at which the material breaks
   • The yield stress is the point where the material begins to deform plastically and becomes permanently deformed

4. Strain

   • Strain ε = x/L
   • L = original length

5. Young Modulus

   • Young modulus E = stress/strain
   • E = FL/xA
   • Young modulus units [Pa]
   • The young modulus is the gradient of a stress-strain graph where it is elastically deforming

6. Choosing materials

   • A material is in tension when a force is acting on it in a direction to stretch the material
   • Other important properties when choosing a material is density and toughness and also cost and availability
   • Selection charts show certain materials and their properites

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