A series of fatigue tests were performed on two different types of steels named Steel A (annealed and decarbed AISI-1018 steel), and Steel B ... Show moreA series of fatigue tests were performed on two different types of steels named Steel A (annealed and decarbed AISI-1018 steel), and Steel B (annealed and polished AISI-1018 steel), 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 all experimental load tests at a stress ratio R=-1 (complete reversal). 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 Whöler curve for both steels, as well as in exhibiting the behavior of the magnetic parameters (magnetic excursions, dominant frequencies of the magnetic signals) coupled to the fatigue lives of the samples tested. To describe such couplings, a set of correlations were introduced among the monitored variables as functions of testing time and applied strain. Also, a fractography analysis of the crack patterns using a scanning electron microscope was performed to represent statistically the geometry of “dimples” and fatigue striations from the inception of the crack, until the formation of a shear lip at the final stages of the crack. It was found that the magnitude of the mean amplitude of the piezo-Barkhausen pulses at early stages of the test is correlated to a mid-level energy of cracking, and thus explains the geometry of fatigue striations near the initiation of the crack at higher strain levels, compared to the geometry of the fatigue striations at lower strain levels. x xvi 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 determined in a clear way the endurance limit for each type of steel; these values also are in agreement with the strain level for which fatigue lives were greater 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. PH.D in Civil Engineering, May 2014 Show less
