doi: 10.1111/j.1460-2695.2008.01307.x Changes in magnetic flux density around fatigue crack tips K. KIDA 1 , H. TANABE 2 and H. OKANO 3 * 1 ‘Fundamental Studies on Technologies for Steel Materials with Enhanced Strength and Functions’ Consortium of the Japan Research and Development Centre for Metals (JRCM), Kyushu University, Department of Mechanical Engineering, Division of Engineering, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka Prefecture, 819-0395, Japan, 2 Department of Mechanical Systems Engineering, School of Engineering, The University of Shiga Prefecture, 2500 Hassaka-cyo, Hikone, Shiga Prefecture, 522-8533, Japan, 3 Osaka University, Graduate School of Engineering Science, 1-3, Machikaneyama-cho, Toyonaka, Osaka Prefecture, 560-8531, Japan Received in final form 20 November 2008 ABSTRACT Fatigue failure of steel occurs when small cracks form in a component and then continue to grow to a size large enough to cause failure. In order to understand the strength of steel components, it is important to find the cracks, which eventually grow to cause failures. However, at present, it is not easy to distinguish, in the early stages of growth, the cracks that will grow fast and cause failure. We hypothesized that it may be possible to distinguish them by comparing changes in the magnetic flux density around the tips of those cracks that grew to failure. In order to measure these changes in magnetic flux density, we developed a scanning Hall probe microscope and observed the fatigue cracks growing from artificial slits in soft bearing steels. Note that we did not magnetize the specimens artificially but succeeded to measure the changes in magnetic flux density during the fatigue tests. We also compared the changes in magnetic flux density around crack tips, which grew under different loads, and found that there is a strong correlation between the magnetic flux density, crack growth and stress intensity factors. In order to understand this, we looked into the relation between stress field, residual strain and magnetic flux density, and concluded that the changes in magnetic flux density are caused not only by the residual strain occurring around the crack tips but also by the increase in the elastic stress. Keywords bearing steel; fatigue crack growth; magnetic flux density; non-destructive method; scanning Hall-probe microscope; stress intensity factor. NOMENCLATURE a = crack length B = magnetic flux density Bn = normalized value of magnetic flux density FWHM = full width at half maximum K = stress intensity factor P = load W = height of bar specimen Ya = y-coordinate of apex of the magnetic flux density distribution Yb = y-coordinate where the values of magnetic flux density distributions become the same as the values when their distribution curves converge. INTRODUCTION Since Orowan and Irwin applied Griffith’s theories con- cerning the cracking of glass to the study of steel, the field of fracture mechanics has expanded greatly. Following these pioneering studies, there have been many investiga- Correspondance: K. Kida. E-mail: kida@mech.kyushu-u.ac.jp ∗ Former student, Osaka University tions into the nature of fatigue and fracture of materials. One important thing coming out of these studies is the significance of crack tips, and many investigations have been done into stress around crack tips. We used a magnetic technique for observing the phe- nomena occurring around crack tips. Up to now, many methods have been developed for observing phenomena such as the plastic deformation, residual stress and grain structure around crack tips. The most popular method 180 c  2009 The Authors. Journal compilation c  2009 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 32, 180–188 Fatigue & Fracture of Engineering Materials & Structures