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Material Testing

Jamie Z 2025-08-19

Material Testing

  • Engineers need to understand the mechanical properties of materials and behaviours they can exhibit during loading

Examples

  • Tensile Strength - The maximum stress a material can withstand while being pulled or stretched before breaking or fracturing
  • Yield Strength - The point at which a material will begin to deform permanently (plastically) under stress and not return to its original shape when the stress is removed
  • Youngs modulus - A mechanical property of solid materials that describes their stiffness or their ability to resist deformation when subjected to stress
  • Elasticity - A material's ability to return to its original shape after being deformed by an applied force.

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\[ \sigma=\frac{Force}{Area} \]

  • Also, \(1Mpa=1N/mm^2\)

Strain

  • Strain, refers to the deformation of a material in response to applied stress
  • Stress is an internal force within a material caused by an external force, while strain is the resulting deformation

\[ \epsilon = \frac{\Delta \ length}{original\ length} \]

Stress Strain Diagrams

  • Stress strain diagrams are used to identify the elastic deformation limit, maximum tensile strength and fracture point

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  • A ductile material can withstand the most inelastic strain (can withstand a lot before fracture) (opposite of brittle)
  • The highest modulus (gradient) of elasticity means that it has stiffness and resistant to deformation
  • The toughness is the ability of a material to absorb energy and deform plastically before fracturing. This is the area under the graph.
  • The highest tensile strength (strain before necking) is the highest point of the graph

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Young’s Modulus

  • The measure of stiffness of a material
  • Young’s modulus is found by the gradient of the linear part of a stress strain graph

\[ E=\frac{\sigma}{\epsilon} \]

Factor Of Safety

Materials loaded within their elastic limits do not suffer permanent deformation. However, maximum stress that is allowed to develop is considerably less than the yield stress. The reasons are:

  • To allow of material imperfections
  • Manufacturing imperfections and error
  • the difficulty of determining elastic limit
  • the need to limit deformation or deflection in members
  • uncertainty in service loads or unforeseen usages