AFM observation of crack initiation by high cycle fatigue in low carbon steels

E. ABE; M.D. CHAPETTI; T. TAGAWA; T. MIYATA

Mar del Plata

Congreso; SAM/CONAMET/MEMAT 2005; 2005

INTEMA - Sociedad Argentina de Materiales (SAM)

      Atomic Force Microscopy (AFM) provides three dimensional digitized images of surface morphological features and thus is one of the most useful tools to analyze the mechanisms of high cycle fatigue crack initiation. In this work AFM is used to study the surface slip evolution in polycrystalline low carbon steels tested under cyclic load at relatively low stress ranges (high cycle fatigue). The steels were also tested under monotonic tensile load for comparison purposes. The steels have ferritic/perlitic structure with an average grain size of about 45 mm. Specimens were 1 mm thick plates and were tested under strain controlled fatigue tests performed at stress ratio R = -1 in a out-plane bending machine. Applied strain range yielded fatigue lives between 105 and 106 cycles. Interrupting the fatigue tests properly, periodic AFM observations were carried out in specific surface positions.    AFM observations showed clear differences in damage patterns. Tensile specimens show stairs-like features while fatigue specimens show distinct extrusions and intrusions. Slip bands are early observed on fatigue specimens (at 103 cycles, N/NF = 0.5%), and extrusions and intrusions grow linearly as the number of cycles increases. Then the growth is saturated after about 104 cycles, while cracks cannot still be found. The growth rate before saturation is about 0.02 nm/cycle and the highest extrusion was found to be about 600 nm high. Crack initiation can occur at both grow-up intrusions and extrusions.