VERY HIGH CYCLE FATIGUE OF ENGINEERING ALLOYS

Abstract

Fatigue crack initiation and growth at extremely low stresses is a concern for structural integrity of aerospace, locomotive and automotive structures where cracks are initiated well below the fatigue limit of the materials. Fatigue Fracture is the most common failure phenomenon in materials and structures under cyclic loading. Fatigue cracks initiate from stress concentration regions at stresses lower than the static strength of the materials. The typical fatigue S-N or Wohler curve of many materials is available in open literature up to 107 cycles and fatigue life beyond these cycles is usually predicted by statistical methods. However, with the invention of piezoelectric fatigue machines vibrating at 20kHz, a new concept has recently been emerged. It has been found that statistical prediction of fatigue life beyond 107 cycles is not true and metals and alloys may show fatigue fracture even at very low stresses up to 1012 cycles. In addition, a very new failure mechanism in fatigue has been observed where the materials and alloys show fracture from the sub-surface of the material and show a step-wise S-N curve. Although many studies focused to develop a thorough understanding of the crack initiation up to 1012 cycles or Very High Cycle Fatigue (VHCF), yet the complete understanding has not been established yet. There is no comprehensive model for explanation of the fatigue crack initiation phenomenon beyond 107 cycles of service life of materials. In this monograph, the basic principles of Very High Cycle Fatigue (VHCF) and the development of equipment required for testing specimens up to VHCF have been discussed. In addition, typical transition of crack initiation site from surface to sub-surface of the materials and their deformation mechanisms has been investigated. The process of local plasticity accumulation around the inclusion required for initiation of fatigue cracks has been discussed in detail. The current understanding of the VHCF phenomenon and fatigue life improvement techniques has also been discussed. The concept and understanding of VHCF are very important for postgraduate and PhD students to understand the importance of the field. The VHCF field is now termed as the highest contributing subject in the top 100 most cited papers in the field of fatigue. The application of the field is widespread in Mechanical, Materials, Aerospace Civil and Structural Engineering. Many mechanical, aerospace, automobile and wind energy structures under dynamic loading suffer fatigue fracture up to 1012 cycles. Owing to economic concerns, it is now desired that structures and alloys perform service up to at least giga cycles (109 cycles). However, the transition of fracture mechanism from surface to subsurface of many engineering alloys is the main hurdle in establishing the same. Hence, it is very important to understand this critical deformation mechanism for engineering community. The VHCF field is relatively new and most of the development has been made in last 15 years or so. The fatigue and fracture mechanics course is a compulsory subject in the field of Postgraduate programs of mechanical/ materials/ aerospace/ civil engineering. However, the VHCF topic is still not covered in the syllabus. This may be attributed to the unavailability of the text books about the subject in Pakistan. Globally, only one book related to VHCF has been published so far authored by Prof. Claude Bathias and Prof. Paul Paris of France. Apart from this book, there is no other comprehensive book available and large proportion of the experimental data is only available in research papers. The access to these research journals and papers is based on costly subscriptions and access for students studying the topic is difficult. It is envisaged that the monograph will be of substantial importance for engineering and scientific community at national and international level, and will be well cited in near future by students.