Full-field strain measurements near fatigue crack tip using digital image correlation

  • Shaher Alshammrei

Student thesis: Doctoral Thesis

Abstract

The phenomenon of fatigue of materials, whereby cyclic loading causes failures in structures, can be traced back to the nineteenth century and beyond. Damage often occurs at a stress range well below the strength of the material. In ductile solids, cyclic load may lead to the development of plastic deformation near a crack tip. Damage tolerance of engineering components is generally treated by linear elastic fracture mechanics under small scale yielding (SSY) conditions. There are occasions where significant plastic deformation can occur. Additionally, the influence of premature contact behind the crack tip, known as crack closure, on fatigue crack growth has not been comprehensively studied.
In this research, Digital Image Correlation (DIC) was utilised to characterise the near crack tip field in stainless steel 316L of compact tension specimen under mode I loading conditions. Three stress intensity factor ranges were used, ΔK=15, 20, and 25 MPa√𝑚 (R=0.1) to investigate the material responses behind and ahead of a fatigue crack tip. Baseline error of DIC assessment was carried out using three pattern techniques and selected measurement window sizes and positions. In situ full-field surface measurements for crack tip strains ahead of the crack tip and crack opening displacement (COD) were carried out, from which a crack opening loads (Pop) was identified at characteristic distances behind the crack tip. Strain evolution and critical strains near a fatigue crack tip were measured in both a stationary and a growing fatigue crack. In addition, stress intensity factor K and J-integral, were extracted from the displacement maps obtained from the DIC measurements. Attempts to map the deformation field for extracting K values using the Williams’ series expansion proved to be unsuccessful due to the presence of significant plasticity beyond the SSY regime for the selected material.
J-integral was extracted from the displacement maps around the crack obtained using DIC at selected load increments. The displacements were introduced as boundary conditions into the finite element (FE) models to obtain an effective J-integral, and the results were compared with those nominal from the standard FE analysis. Both visual observation and compliance curves were used to determine the crack opening levels; whilst the impacts of the crack opening on the J and the normal strains ahead of the crack tip were evaluated. The results present a complete picture of the crack tip field at the selected load levels, where events both ahead of and behind the crack tip were studied for the first time in terms of crack driving force and attenuation effects. The results from the study indicate that crack closure, although clearly identifiable in the compliance curves and visual observation, does not appear to impact on global crack driving force, such as J-integral, or on strains ahead of the crack tip.
Date of AwardJan 2020
Original languageEnglish
Awarding Institution
  • University of Portsmouth
SupervisorJie Tong (Supervisor) & Sarinova Simandjuntak (Supervisor)

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