Materials

• Tensile test


  A test that provides information about the relationship between the loading (stress) and the extension (strain) of a material, its strength in tension, and how much the material will have been extended by the loading

  
• Tensile test specimen


  • Has wider ends which are easier to grip and a uniform cross section in the middle with a gauge length identified
• Performing a tensile test

  1. Measure the initial cross-sectional area of the test specimen

  2. Load the specimen in tension

  3. Measure the extension of the gauge length

  
• Engineering stress


  The load divided by the original cross-sectional area of the test specimen
• Engineering strain


  The change in length of the gauge length divided by the original gauge length
• Young's modulus


  The ratio of stress to strain in the elastic region

  
• Poisson's ratio


  The ratio of the transverse strain to the axial strain
• Yield stress


  The stress at which plastic deformation begins

  
• Proof stress


  The stress at which a small amount of permanent deformation occurs (usually 0.2%)

  
• Tensile strength


  The maximum stress the material can withstand before fracture
• Ductility


  The ability of a material to undergo plastic deformation before fracture

  
• Elastic deformation


  Deformation that is recoverable when the load is removed
• Plastic deformation


  Permanent, non-recoverable deformation

  
• Work hardening


  The increase in a material's strength and hardness that occurs due to plastic deformation

  
• Tensile test


  Provides useful information about how the material might perform under different types of loading

  
• How the tensile test is performed

  1. Get a representative piece of the material called the test specimen

  2. Attach the specimen to the test frame using machine grips

  3. Pull the specimen to failure
• Tensile test specimen


  • Has wider ends which are easier to grip
  • Has a length in the middle with a uniform cross section
  • Has a gauge length identified within the uniform region
• Measuring during the tensile test

  • Load cell measures the applied force, P
  • Extensometer measures the extension of the gauge length, x

  
• Electrical signals from the load cell and extensometer are recorded by a data acquisition system
• It is usually unwise to rely on a single test measurement, so a number of nominally identical specimens are tested
• The distanced moved by the crosshead will be greater than the extension of the gauge length
• Engineering (or nominal) stress

  The force/original cross-sectional area

  
• Engineering (or nominal) strain

  The extension of the gauge length/original gauge length
• Young's modulus, E


  The gradient of the linear elastic region of the stress-strain curve
• Yield strength


  The stress at the first deviation from linearity in the stress-strain curve

  
• Proof strength


  The stress at a certain strain offset

  
• Ultimate tensile strength (UTS)


  The maximum stress attained
• Poisson's ratio, ν


  The ratio of transverse strain to axial strain
• For uniaxial deformation, typical values for Poisson's ratio are: Ceramics ~0.2; Metals ~0.3-0.35; Polymers ~0.35-0.4
• Plastic deformation


  Deformation that is not recoverable after the force is removed
• Work hardening


  A higher stress is required to continue to deform a plastically deformed material

  
• Ductility


  The mechanical property that quantifies how much plastic deformation a material will sustain at fracture

  
• Percentage elongation


  A measure of the plastic strain at fracture

  
• Percentage reduction in area (%RDA)


  A geometry independent measure of ductility
• For polymers, the percentage elongation is defined at the strain at fracture (but prior to elastic unloading)

  
• True stress


  Instantaneous and local to where the cross-sectional area A is specified

  
• Engineering stress


  Historic (based on the original area) and averaged over the gauge length
• Elastic deformation occurs at increasing volume (ε<0.5)

  
• Plastic deformation occurs at constant volume (ε=0.5)

  
• If pl>>el (correct for strains significantly beyond yield point) then deformation occurs at approximately constant volume