The influence of interacting small defects on the fatigue limit of steels

Research output: ThesisDoctoral ThesisCollection of Articles


Metallic engineering components possess numerous small natural defects, which can potentially become sites for fatigue crack initiation. Although the influence of a single small crack on the fatigue strength is well studied, knowledge of the behavior of interacting defects is more limited. Current design rules and standards for the interaction effect, such as British standard BS 7910, are based on material-independent criteria. Consequently, this thesis examines experimentally the effect of the material on defect interaction. In addition, this work introduces an efficient finite element-based method for stress intensity factor (SIF) evaluation of interacting arbitrarily shaped 3D cracks. In experimental investigations, fully-reversed tension-compression fatigue tests were performed using different steel grades. Small artificial defects were manufactured onto the specimen surface with an accurately determined distance between them. Fatigue limits were determined by the non-propagating condition of cracks that had emanated from artificial defects. The size of natural non-propagating cracks (NPC's) varies among different materials. Thus, the influence of the material's effect is determined on both NPC characteristics and defect interaction on the fatigue limit. In order to evaluate SIF's of arbitrarily shaped 3D cracks, this thesis develops the Stress Component Division Method (SCDM), which utilizes the theory of elasticity, the superposition principle, and stress component division. The key point of the SCDM is that it separates the total stress in an element into two components, singular and non-singular terms, the former of which is associated with the SIF. This study showed that the behavior of interacting defects varied greatly among different steels. In low strength steel, the defects coalesced at the fatigue limit regardless of the spacing between them, whereas defects never coalesced at the fatigue limit in high strength steel regardless of the spacing between the defects. In case of moderate strength steels, the interaction phenomena were more complicated and the coalescence at the fatigue limit depended on the materials hardness, the relative size of NPC's, microstructure and the spacing between original defects. The numerical SIF solutions for arbitrarily shaped 3D cracks computed by SCDM are in good agreement with the known numerical solutions even when a coarse mesh is employed. It was concluded that, by itself, the commonly used material-independent interaction criterion is insufficient in the evaluation of fatigue strength for interacting crack problems in different steel grades. The experimental results presented here provide new information and guidance to improve the interaction criteria in real engineering structures. It was also shown that in addition to the aspect ratio of the defects and the spacing between defects, the defect shape must also be considered in SIF evaluation of interacting defects.
Translated title of the contributionVuorovaikuttavien pienten vikojen vaikutus terästen väsymisrajaan
Original languageEnglish
QualificationDoctor's degree
Awarding Institution
  • Aalto University
  • Remes, Heikki, Supervising Professor
  • Matsunaga, Hisao, Thesis Advisor
  • Marquis, Gary, Thesis Advisor
Print ISBNs978-952-64-0199-7
Electronic ISBNs978-952-64-0200-0
Publication statusPublished - 2020
MoE publication typeG5 Doctoral dissertation (article)


  • small crack
  • interaction effect
  • defect interaction
  • fatigue limit
  • non-propagating crack

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