| A series of fatigue tests were performed on two different types of steels: A (annealed and decarbed AISI-1018 steel), and B (annealed and polished AISI-1018 steel). Tests were carried out to separation or to a maximum of 10,000,000 cycles which was taken to be equivalent to infinite life. Strain levels ranging from 0.0014 in/in (0.0014mm/mm) down to 0.0008in/in (0.0008mm/mm) were used to execute experimental load tests at a stress ratio R=-1. An MTS machine was used for these trials. At the same time, magnetic fields and piezo-Barkhausen pulses were recorded by means of a flux gate magnetometer and a copper coil connected to a series of signal filters and amplifiers. Results were used to construct the classical S-N Wholer curve for both steels, as well as in exhibiting the behavior of the magnetic parameters coupled to the fatigue lives of the samples tested.;Also, a fractography analysis of the crack patterns using a scanning electron microscope was performed to represent statistically the geometry of fatigue "striations" from the inception of the crack, until the formation of "dimples" at the shear lip at the final stages of the crack. The Color Code for crack identification, based on eight types of micro-cracks, is introduced and cracked surfaces are interpreted according to such Code. It was found that a correlation exists between applied strain and geometry of micro-crack formations.;A joint analysis of the amplitudes of the magnetic excursions recorded in time, and the dominant frequencies of the magnetic signals were found to be discriminators of the elastic and plastic behavior of both types of steel. Furthermore the observed magnetic parameter variations when correlated as a function of applied strain, determined in a clear way the endurance limit for each type of steel; all proposed correlations showed extreme (minima or maxima) at the strain at which samples reached more than 10,000,000 cycles. Based on these results a "bell analogy" for interpreting the fatigue behavior is introduced. Bridge engineering applications and further research is also discussed. |
