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Fracture mechanics and fatigue

Which pairing of a feature seen when observing a fracture surface in a scanning electron microscope with the type of fracture is correct?

  1. AA fracture surface covered with many small hollows (dimples) indicates ductile fracture by the nucleation and coalescence of voids
  2. BA fracture surface that broke along flat crystal planes and shows river patterns indicates ductile fracture accompanied by extensive plastic deformation
  3. CA fracture surface on which the shapes of the grains stand out as they are indicates cleavage fracture that ran through the grains (transgranular fracture)
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Answer: A. A fracture surface covered with many small hollows (dimples) indicates ductile fracture by the nucleation and coalescence of voids

In ductile fracture, voids form around inclusions and precipitates, then grow and coalesce, so the fracture surface is covered with many hollows (dimples). Cleavage fracture is a brittle fracture that splits through the grains along specific crystal planes; its surface is flat and shows river patterns, where steps merge like streams flowing together. Calling this ductile fracture gets it backwards. A fracture surface on which the grain shapes stand out like piled stones is intergranular fracture along grain boundaries, caused by temper embrittlement, hydrogen embrittlement, precipitation at grain boundaries and so on. Observing fracture surfaces (fractography) is the basis of failure analysis to find the cause of an accident.

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More Fracture mechanics and fatigue questions

  1. Which statement correctly describes the initiation of fatigue cracks when a stress amplitude below the yield stress is repeatedly applied to a metal with a smooth surface?
  2. The fracture surface of a part that failed by fatigue shows shell-like patterns visible to the naked eye (beach marks) and fine lines visible in an electron microscope (striations). Which statement about them is correct?
  3. A cyclic stress with a mean stress σm = 200 MPa is applied to a steel with a fully reversed fatigue limit σw = 300 MPa and a tensile strength σB = 800 MPa. Using the modified Goodman relation σa = σw(1 − σm/σB), which value is closest to the stress amplitude corresponding to the fatigue limit?
  4. Which surface treatment can be expected to improve the fatigue strength of a steel part?
  5. A steel part fails by fatigue after 1.0×10⁵ cycles at stress amplitude σ₁ and after 1.0×10⁶ cycles at σ₂. After it is used for 4.0×10⁴ cycles at σ₁, it continues to be used at σ₂. Which value is closest to the remaining life estimated with Miner's rule (failure when Σn/N = 1)?