Introduction to Fatigue

1
Introduction to Fatigue
The failure of the Boeing 737-200 Aloha Airlines
due to fatigue damage, corrosion low bonding
durability. (28 April 1988)
2
Overview of Fatigue

• Many different mechanical failure modes exist in
  all fields of engineering.
• These failures can occur in simple, complex,
  inexpensive, or expensive
• components or structures.
• Failure due to fatigue, i.e., repeated loading,
  is multidisciplinary and is
• the most common cause of mechanical failure.
• Even though the number of mechanical failures
  compared
• to successes is minimal, the cost in lives,
  injuries, and dollars is too large.
• Proper fatigue design can reduce these
  undesirable losses.
• Proper fatigue design includes synthesis,
  analysis, and testing.
• The closer the simulated analysis and testing are
  to
• the real product and its usage, the greater
  confidence
• in the engineering results.

3
FATIGUE ANALYSIS NEEDS

• The principles of fatigue behaviour and
  fatigue design have been developed, used, and
  tested by engineers and scientists in all
  disciplines and in many countries.
• The current capability of computers and
  simulated testing has a pronounced influence on
  the efficiency and quality of today's fatigue
  design procedures.
• However, in proper fatigue design, both
  computer synthesis and analysis must be
  integrated with proper simulated and field
  testing, along with continued evaluation of
  product usage and maintenance, including
  non-destructive inspection.

4
Tips in Design for Fatigue

• 1. Do recognize that fatigue failures are the
  most common cause of mechanical failure in
  components, vehicles, and structures and that
  these failures occur in all fields of
  engineering.
• 2. Do recognize that proper fatigue design
  methods exist and must he incorporated into the
  overall design process when cyclic loadings are
  involved.
• 3. Do not rely on safety factors in attempting to
  overcome poor design procedures.
• 4. Do consider that good fatigue design, with or
  without computer-aided design, incorporates
  synthesis, analysis, and testing.
• 5. Do consider that fatigue durability testing
  should be used as a design verification tool
  rather than as a design development tool.
• 6. Do not overlook the additive or synergistic
  effects of load, environment, geometry, residual
  stress, time, and material microstructure

5
STRATEGIES IN FATIGUE DESIGN

• Fatigue design methods have many similarities
  but also differences.
• The differences exist because a component,
  structure, or vehicle may be safety critical or
  non-safety critical, simple or complex, expensive
  or inexpensive, and failures may be a nuisance or
  catastrophic.
• The product may be a modification of a current
  model or a new product. Significant
  computer-aided engineering (CAE) and
  computer-aided manufacturing, CAM) capabilities
  may or may not be available to the design
  engineer.

6
FLOW CHART FOR STRATEGIES IN FATIGUE DESIGN
Fatigue design flow chart originated by H. S.
Reemsnyder from Bethlehem Steel Corp. and
slightly modified by H. 0. Fuchs, It was created
for use by the Society of Automotive Engineers
Fatigue Design and Evaluation (SAEFDE) Committee
University of Iowa's annual short course on
Fatigue Concepts in Design.
7
Choosing the fatigue life model

• Choosing the fatigue life model is a significant
  decision.
• Currently four such models exist for design
  engineers. These are
• The nominal stress-life (S-N) model, first
  formulated
• between the 1850s and 1870s.
• 2. The local strain-life (?-N) model, first
  formulated in the 1960s.
• 3. The fatigue crack growth (da/dN-?K) model,
  first formulated in the 1960s.
• 4. The two-stage model, which consists of
  combining models 2 and 3
• to incorporate both macroscopic fatigue crack
  formation (nucleation)
• and fatigue crack growth.

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Purposes of Design

• 1. Designing a device, perhaps a special bending
  tool or a test rig, to be used in the plant where
  it was designed. It is called by an "in-house
  tool."
• 2. Changing an existing product by making it
  larger or smaller than previously, using a
  different material or different shapes, perhaps a
  linkage and coil spring in place of a leaf
  spring. It is called by a "new model."
• 3. Setting up a major project that is quite
  different from past practice. A spacecraft or an
  ocean drilling rig or a new type of tree
  harvester is example. It is called by a "new
  product."
• 4. Designing a highway bridge or a steam boiler.
  The expected loads, acceptable methods of
  analysis, and permissible stresses are specified
  by the customer or by a code authority. It is
  called by "design to code."

9
Tips in Design Related to Crack Initiation

• 1. Do recognize that fatigue is a localized,
  progressive, and permanent behaviour involving
  the nucleation and growth of cracks to final,
  usually sudden fracture.
• 2. Do recognize that fatigue cracks nucleate
  primarily on planes of maximum shear and usually
  grow on the plane of maximum tensile stress.
• 3. Do examine fracture surfaces as part of a
  post-failure analysis, since substantial
  information concerning the cause of the fracture
  can be gained. The examination can involve a
  small magnifying glass or greater magnification
  up to that of the electron microscope.
• Do not put fracture surfaces back together again
  to see if they fit or allow corrosive
  environments (including rain and moisture from
  fingers) to reach the fracture surface.

10
Tips in Design Related to Crack Initiation (cont.)

• 5. Do consider that stress-strain behaviour at
  notches or cracks under repeated loading may not
  be the same as that observed under monotonic
  tensile or compressive loading.
• 6. Do take into consideration that your product
  will very likely contain cracks during its design
  lifetime.
• 7. Do recognize that most fatigue cracks nucleate
  at the surface, and therefore that surface and
  manufacturing effects are extremely important.
• 8. Do not assume that a metal that has good
  resistance to crack nucleation also has good
  resistance to crack growth and vice versa.

11
Fatigue Loading
Constant amplitude cyclic loading.
Schematic ground-air-ground flight spectrum.
12
Tips in Design for Fatigue Test and the
Stress-Life (S-N) Approach

• 1. Do consider the wide range of test systems and
  specimens available for fatigue testing. Tests
  can range from those performed on small, highly
  polished specimens for material characterization
  to full-scale durability tests of large
  structures.
• 2. Do not neglect to refer to ASTM, ISO, or
  similar standards on fatigue testing and data
  reduction techniques.
• 3. Do consider that the fully reversed fatigue
  strength, Sp at 106 to 108 cycles for components
  can vary from about 1 to 70 percent of the
  ultimate tensile strength and that the engineer
  can substantially influence this value by proper
  design and manufacturing decisions.
• 4. Do note that cleaner metals, and generally
  smaller grain size for ambient temperature, have
  better fatigue resistance.

13
Tips in Design for Fatigue Test and the
Stress-Life (S-N) Approach (cont.)

• 5. Do recognize that frequency effects are
  generally small only when corrosion, temperature,
  or other aggressive environmental effects are
  absent.
• 6. Do consider that surface finish can have a
  substantial influence on fatigue resistance,
  particularly at longer lives.
• 7. Do not neglect the advantages of compressive
  mean or compressive residual stresses in
  improving fatigue life and the detrimental effect
  of tensile mean or tensile residual stresses in
  decreasing fatigue life, and that models are
  available to account for these effects.
• 8. Do attempt to use actual fatigue data in
  design however, if this is not possible or
  reasonable, approximate estimates of median
  fatigue behaviour can be made.

14
Tips in Design for the Strain-Life (?-N) Approach

• 1. Do consider that inelastic stress-strain
  behaviour under repeated loading is not the same
  as that determined under monotonic tensile or
  compressive loading. Under repeated loading the
  difference between materials is less than that
  under monotonic loading.
• 2. Do not ignore the role of material hardening
  or softening in cyclic loading applications.
  Using a monotonic stress-strain curve of a cyclic
  softening material in a cyclic loading
  application can significantly underestimate the
  extent of plastic deformation present.
• 3. Do consider the importance of material
  ductility on low-cycle or plastic strain
  dominated fatigue resistance and the importance
  of material strength on the high-cycle or elastic
  strain dominated fatigue resistance.
• 4. Do recognize that strain-life fatigue data of
  smooth uni-axial specimens are based on cycles to
  failure, where failure represents the formation
  of cracks on the order of 1 mm in depth, which
  may or may not have caused fracture.
• 5. Do recognize that mean strains generally
  affect fatigue resistance only if they produce a
  non-relaxing mean stress. The greatest effect of
  mean stress is in the high-cycle fatigue regime.

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End