L6

• Three different classes of failure based on component/system performance


  • The part is inoperable
  • The part no longer performs its intended function satisfactorily
  • Deterioration has made the part unreliable or unsafe for continual use
• The part is inoperable


  • A computer that doesn't work
  • An automobile engine that won't start
  • An airplane missile launcher that will not launch any of its missiles
• The part no longer performs its intended function satisfactorily


  • A computer printer that keeps jamming the paper
  • An automobile engine that stalls when it is put into gear
  • An airplane missile launcher that will launch some, but not all, of its missiles
• Deterioration has made the part unreliable or unsafe for continual use


  • A computer printer that has a loose wire that may short-circuit and cause electrical shock or ignition
  • An automobile engine that stalls after operating at 65 mph on an interstate highway
  • A missile launcher that may activate the missile but not launch
• Classification of defects by failure mode


  • Excessive elastic deformation
  • Excessive plastic deformation
  • Fracture
  • Corrosion and wear of parts resulting in loss of part geometry
  • Corrosion and wear of products that prevents the operation of other parts
• Excessive elastic deformation


  • Deformation is too high even though loading is below yield point
  • Implies the modulus of elasticity is too low, so wrong material was selected
• Excessive plastic deformation


  • Most undesirable failure as it is frequently catastrophic
  • Excessive elastic or plastic deformation usually won't cause complete system failure but fracture can damage other parts
• Fracture


  • Pipelines used to transport coal slurries can fracture due to abrasive wear
• Corrosion and wear of parts resulting in loss of part geometry


  • Corrosion in automobile cooling systems can cause wear of the water pump
• Corrosion and wear of products that prevents the operation of other parts


  • Products of corrosion in cooling system can cause wear of water pump
• Sources of failure


  • Design deficiencies
  • Material deficiencies
  • Processing deficiencies
  • Assembly errors
  • Improper operation
  • Neglect (inadequate maintenance and repair)
  • Service conditions
  • Management attitudes
• Design deficiencies


  • Failure to consider stress raisers
  • Inadequate knowledge of service loads and environment
  • Difficulty of stress analysis in complex parts and loadings
• Material deficiencies


  • Poor match between service conditions and selection criteria
  • Inadequate material data
  • Incomplete material specifications
• Processing deficiencies


  • Improper processing can lead to cracks or other defects
• Assembly errors


  • Loose bolts, misalignment, and loose tolerances
• Improper operation


  • Not following proper startup or shutdown procedures
• Neglect (inadequate maintenance and repair)


  • Failure to perform regular maintenance
• Service conditions


  • Using product beyond design limits
  • General deterioration over time
• Management attitudes


  • Excess focus on cost leading to improper material substitutions
  • Excess focus on production leading to hasty decisions
• Types of failures


  • Overload
  • Fatigue
  • Corrosion-influenced fatigue
  • Corrosion
  • Wear

  
• Overload


  • Ductile or brittle fracture when stresses exceed load-bearing capacity

  
• Fatigue


  • Progressive localized plastic deformation from cyclic stresses and strains leading to cracks or fracture

  
• Corrosion-influenced fatigue


  • Combination of cyclic loading and aggressive environment substantially reduces fatigue strength
• Corrosion


  • Chemical or electrochemical reaction causing material deterioration
• Wear


  • Damaging, gradual removal or deformation of material at solid surfaces
• When a part or device can no longer perform its intended function, the part has failed
• Failure analysis


  A systematic, science-based method employed for investigation of failures occurring during tests or in service
• Objectives of failure analysis


  • Determine the primary cause of a failure
  • Suggest corrective actions to prevent similar failures
• Impact of failure analysis on society


  • Cost of failure
  • Cost of failure analysis
  • Improvement of products
• Types of analysis


  • Failure analysis (how the material failed)
  • Root cause analysis (how it could have been prevented)
• Failure


  When a part or device can no longer perform its intended function
• Impact of Failure Analysis on Society


  • Cost of failure
  • Cost of failure analysis
  • Improvement of products
• Types of Analysis


  • Failure analysis (how the material failed)
  • Root cause analysis (how it could have been prevented)
• Failure Analysis


  Determining: 1. How did the material fail? 2. Why did it fail? 3. Who or which party is responsible?
• Root Cause Analysis


  Emphasizes managerial role in failures, how managerial techniques can be improved, focus on prevention, not failure determination, used in large plants, construction sites, manufacturing facilities
• Stages of Failure Analysis


  • Background data
  • Preliminary exam
  • Non destructive tests
  • Mechanical tests
  • Sample selection
  • Macroscopic exam
  • Microscopic exam
  • Metallography
  • Chemical analysis
  • 10. Fracture mechanics
  • 11. Simulated tests
  • 12. Analysis and report
  • 13. Recommendations
• Background data


  Collection of manufacturing history, service history, photographic records, samples, wreckage analysis, inventory of parts, abnormal conditions, sequence of fractures, sample selection
• Preliminary Examinations


  Visual inspection of all parts, detailed photography, study of the fractures

  
• Nondestructive Inspections


  • Magnetic particle inspection
  • Liquid penetrant inspection
  • Electromagnetic inspection (Eddy current)
  • Ultrasonic inspection
  • Radiography
  • Residual stress analysis (X-ray diffraction (XRD)

  
• Mechanical Testing


  • Hardness testing
  • Tensile testing
  • Shear testing
  • Impact testing
  • Fatigue testing
  • Fracture mechanics testing