Technical Objects: Materials, Constraints, Deformations

12.1 Materials

• All technical objects have have one or more materials
• Every technical object experiences stresses which can deform them
• Two things to consider when manufacturing technical objects:
  • The stress the objects exposed to
  • The possible resulting deformations
• Different material types may react differently to the same stress and have different deformations

12.1A Constraints and Deformations

• Different parts of a technical object can be subjected to one or more external forces
• The forces deform the parts
• A constraint describes the effect of external forces on a material

The five main types of constraints

• Compression: Material subjected to forces that tend to crush it
• Tension: Material subjected to forces that tend to stretch it
• Torsion: Material subjected to forces that tend to twist it
• Deflection: Material subjected to forces that tend to bend it
• Shearing: Material subjected to forces that tend to cut it

Deformation

• Depending on the type of constraint (stress) on an object, three deformation types can occur:
  • Elastic deformation: Change is temporary, and the material returns to its original form
  • Plastic deformation: Change is permanent
  • Fracture: Material breaks
• The same constraint may result in different deformation types

Examples of Constraints and Deformations

• Deflection constraint and elastic deformation is exemplified by shoes
• Deflection constraint and plastic deformation is exemplified by a spoon
• Deflection constraint and fracture deformation is exemplified by pencils

Example Question

• A clothespin is used to hang a towel on a clothesline, the spring is subjected to torsion, and undergoes elastic deformation

Mechanical Properties

• The reaction of a material under stress depends on its properties
• The mechanical properties of a material describe how it reacts when subjected to one or more constraints
• Some examples of mechanical properties:
  • Hardness: Ability to resist dents or abrasion
  • Elasticity: Ability to return to their original shapes after undergoing a constraint
  • Resilience: Ability to resist shocks without breaking
  • Ductility: Ability to be stretched without breaking
  • Malleability: Ability to be flattened or bent without breaking
  • Stiffness: Ability to retain shape when subjected to various constraints

Other desirable property examples

• Corrosion resistance: Ability to resist corrosive substances like water, acids, and salts, which can cause a chemical reaction with the material
• Electrical conductivity: Ability to carry an electric current
• Thermal conductivity: Ability to transmit heat

12.1C Degradation and Protection

• All materials degrade, albeit at varying rates
• Material degradation is the decline in some of its properties due to the surrounding environment
• Materials can be protected to counteract the damaging effects of degradation
• Material protection involves applying procedures to prevent or delay degradation