Midterms

• Fracture toughness


  The resistance of materials to the propagation of flaws under an applied stress
• Longer the flaw


  Lower is the stress needed to cause fracture
• Critical stress-intensity factor (KC)

  A parameter used to represent the fracture toughness of most materials
• KI

  Stress intensity factor in MPa/m or psi/in
• σ

  Applied stress in MPa or psi
• β

  Dimensionless correction factor depends on specimen geometry
• a


  Crack length in meters or inches
• Mode I fracture


  • Fracture plane is perpendicular to the normal force
• Mode II fracture


  • Fracture occurs under the action of shear stress and propagates in the direction of shear
• Mode III fracture


  • Fracture occurs by shear mode but it propagates in a direction perpendicular to the direction of shear
• Fatigue in engineering materials


  Materials fail under repeated or otherwise varying loads which are generally significantly below the maximum static stress
• 1. N curve


  Represents the relationship between stress and the number of cycles to failure in a material
• Endurance limit


  The maximum stress that a material can endure indefinitely without showing any signs of fatigue
• Miner's rule


  The cumulative fatigue damage in a material is equal to the sum of the ratio of the actual stress cycles to the total life cycles
• High-cycle fatigue


  • Occurs when materials are subjected to stresses much lower than their yield strength, over a high number of cycles
• Low-cycle fatigue


  • Occurs when materials are subjected to higher stresses, typically exceeding the yield strength over a smaller number of cycles
• Thermal fatigue


  • A specific type of fatigue caused by cyclic thermal loads, usually as a result of fluctuating temperatures
• Fatigue strength

  The highest stress that a material can withstand for a specific number of cycles without failure
• Fatigue limit or endurance limit


  The maximum stress level a material can undergo an infinite number of times without showing signs of fatigue
• Fatigue life

  The number of stress cycles a material can endure before failure occurs
• Impact testing


  A technique used to determine a material's ability to resist deformation when subjected to a sudden shock or impulse load
• Impact testing


  • It is a critical property that determines the material's ability to withstand sudden forces
• Different types of impact testing


  • Charpy
  • Izod
  • Drop-weight impact test
  • Dynamic tear test
• Charpy impact test


  A type of impact test where a weighted pendulum hammer is released from a specified height and strikes the part
• Izod impact test


  Similar to the Charpy test in that a weighted pendulum hammer strikes a test specimen containing a V- shaped notch. The primary differences are the size of the test specimen, how it is restrained, and which side is struck by the pendulum hammer
• Drop-weight impact test


  Uses a weight suspended over a simply supported horizontal test specimen and then dropped to produce the impact
• Dynamic tear test


  Similar to the drop-weight impact test, but often used for test specimens with a thickness less than 5/8" while drop-weight impact testing is for test specimens thicker than 5/8"
• Standards for impact testing

  • ASTM E23 / ISO 148-1 (for metals)
  • ASTM D256 / ISO 180 (for plastics)
  • ASTM A370 / ASTM E208 (for steel materials)
• Destructive testing


  Test method conducted to find the exact point of failure of materials, components, or machines. During the process, the tested item undergoes stress that eventually deforms or destroys the material. Generally conducted before a component enters mass production.
• Professionals who perform destructive testing


  • Material scientists
  • Metallurgical and polymer engineers
  • Chemistry and electrochemical process experts
  • Failure analysis experts
  • Quality control analysts
  • Regulatory compliance experts
• Destructive testing and using materials of specific characteristics come as a regulatory requirement. The purpose of destructive testing is ultimately to understand how a material will react to extreme stress or loads.
• Application of destructive testing


  • Testing the strength of safety glass. Sandbags can be dropped at specified heights to simulate impactful forces for failure analysis, and fire can also be applied to determine flame resistance.
• Destructive testing methods

  • Corrosion testing
  • Hardness testing
  • Tensile elongation test
  • Torsion testing
• Corrosion testing


  Corrosion decreases the tensile strength and life of metals. Test method done to test the effectiveness of applied corrosion resistance measures. Measuring the characteristics and the rate of corrosion can also be considered to be part of corrosion testing.
• Hardness testing


  Material hardness determines whether components undergo permanent deformation due to stress. Hardness shows how effectively a material resists indentation.
• Tensile elongation test


  Tensile testing is conducted by applying controlled force across test material till it fails (crack, breakage, etc). The measured properties are ultimate tensile strength, breaking strength, and maximum elongation or reduction.
• Types of torsion testing

  • Torsion only
  • Axial-Torsion
  • Failure Testing
  • Proof Testing
  • Functional Testing
• Torsion only


  Applying only torsional loads to the test specimen.
• Axial-Torsion


  Applying both axial (tension or compression) and torsional forces to the test specimen.
• Failure Testing


  Twisting the product, component, or specimen until failure. Failure can be classified as either a physical break or a kink/defect in the specimen.