Developing the Theory of Critical Distances for practical integrity assessment of real-life structural components

This PhD thesis details the research conducted to answer three questions in the field of fracture and
fatigue engineering. The opening chapters provide fracture and fatigue background theory as well as
a more comprehensive review of the Theory of Critical Distances (TCD), a theory proven to be
successful in the assessment of components containing stress concentration features.
Chapter 4 details an engineering approach based on the TCD for the static assessment of
engineering components containing stress concentrators made of brittle, quasi-brittle and ductile
materials; and loaded by any combination of static forces. To validate the method, 1744
experimental data was taken from technical literature is provided in Annex A. Each data was
modelled using FE software, the extracted stress data was then post-processed using this
reformulation of the TCD. The results obtained were compared to the commonly used Hot-Spot
Stress-Method, across the same set of data there was an order of magnitude improvement in
accuracy, the TCD Point Method giving an average error less than 30% whilst the HSSM gave an
average error greater than 300%.
Chapter 5 is concerned with the use of the linear-elastic TCD to assess notched metallic components
in the high-cycle fatigue regime at elevated temperatures. Full details of two experimental
programmes are provided, notched samples of a low carbon steel C45 and an aluminium alloy A319
T7 was tested, the results are provided in Annex C. Additional experimental data was taken from
technical literature to further validate the method. The results showed that the approach was highly
accurate with errors falling within ±20%.
The 6th Chapter gives account of a study into the combined use of the TCD and the Modified Wöhler
Curve Method (MWCM to accurately and efficiently assess metal engineering components
IV

containing complex 3D stress raisers experiencing complex load histories that resulted in fatigue
failures in the medium- and high-cycle fatigue regime. The method is based on critical plane theory
which assumes that fatigue cracks initiate on the material plane experiencing the maximum shear
stress amplitude. The method was proven to be successful independent of the stress raiser
geometry and the complexity of the load history, typically returning errors of ±20%.
Chapters 4-6 each have their individual conclusions and suggestions for further work, chapter 7 gives
a summary of the conclusions and chapter 8 provides some suggestions for further work.