Development of an engineering assessment procedure for predicting notch fracture toughness using a constraint-based approach.

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Modern defect assessment procedures assume flaws that cannot be considered as a reduction in section are a sharp crack, a conservative assumption that can result in pessimistic evaluations. These may result in a defect prematurely failing a structural integrity assessment due to worst case assumptions about material properties, structure geometry and defect size. This study has developed an engineering assessment approach to estimate the resistance to fracture of structures containing non-sharp defects. The new procedure builds on a two-parameter constraint-based approach used to assess low constraint sharp defects.
The new methodology has been developed and validated using testing and finite element analysis. An S355J2+N structural steel was used for the experimental work composing of a large test programme of Single Edge Notch Bend (SEN(B)) specimens with deep sharp cracks and U-shaped notches. These were used to measure the increase in effective fracture toughness that accompanies increasing notch tip radius. Additional tests of SEN(B) specimens with shallow sharp cracks were used to quantify the standard constraint behaviour of the material.
In comparison to the methodology used to allow for constraint in fracture assessments in BS 7910/R6, a notch corrected constraint method based on a J-Q approach is proposed. A notch constraint relationship is defined between the level of notch constraint loss due to increasing crack tip acuity and resulting increase in fracture toughness.
The approach is validated using notched C(T) experimental data from open literature for the same material. A two-parameter Weibull model is used to describe the crack and notch toughness probability of failure to incorporate different confidence levels into the analysis. The methodology shows to be a promising method for assessment of non-sharp defects where benefit may be taken from resolving crack tip acuity.
Date of Award23 Jan 2024
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorNicolas O Larrosa (Supervisor), Isabel Hadley (Supervisor) & Harry Coules (Supervisor)

Keywords

  • Elastic-plastic fracture mechanics,
  • Constraint
  • J-integral
  • Q-factor
  • Notch effects
  • Non-sharp defects
  • Fracture toughness

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