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Fracture mechanics and fatigue

Which surface treatment can be expected to improve the fatigue strength of a steel part?

  1. AFinishing the surface rough so that paint and lubricants adhere to it more easily
  2. BGrinding the surface so that tensile residual stress remains in the surface layer
  3. CIntroducing compressive residual stress into the surface layer by shot peening
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Answer: C. Introducing compressive residual stress into the surface layer by shot peening

Most fatigue cracks start at the surface and open and grow under tensile stress. Shot peening fires small steel balls or similar media at the surface at high speed, plastically deforming the surface layer and leaving compressive residual stress; this offsets the tensile stress at the surface and suppresses crack initiation and early growth. Carburizing, nitriding and induction hardening also harden the surface layer and introduce compressive residual stress, so they raise fatigue strength. Surface roughness acts as small notches, so a rough finish lowers fatigue strength. Tensile residual stress is equivalent to shifting the mean stress toward tension and is harmful in fatigue. Poor grinding conditions can leave tensile residual stress in the surface layer.

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More Fracture mechanics and fatigue questions

  1. 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)?
  2. Which statement about low-cycle fatigue (roughly 10⁴ cycles or fewer to failure) is correct?
  3. Which statement correctly describes the curve of fatigue crack growth rate da/dN plotted against the stress intensity factor range ΔK on log-log axes?
  4. Fatigue crack growth in a steel plate is described by the Paris law da/dN = C(ΔK)ᵐ (C = 1.0×10⁻¹², with da/dN in m/cycle, ΔK in MPa·√m, and m = 4), and ΔK = Δσ√(πa) (geometry factor 1). Under cycling with a stress range Δσ = 200 MPa, which value is closest to the number of cycles for the crack to grow from 1.0 mm to 5.0 mm? Assume that ΔK over this range (about 11–25 MPa·√m) is well above the threshold and within the range where the Paris law holds.
  5. The theoretical strength of a perfect crystal is estimated at roughly one tenth of the Young's modulus, yet the actual fracture strength of brittle materials such as glass is often only a few tens to a few hundreds of times smaller than that. What is the main reason?