Uso del ensayo small punch (SPT) en la caracterización a fatiga de un acero estructural S355

Abstract

The Small Punch Test (SPT) is a test that allows characterizing the mechanical behaviour of materials subjected to quasistatic stresses from very small samples (10x10x0.50mm). Under these conditions, the SPT has demonstrated a great applicability in the determination of tensile parameters, such as the yield strength (ys) or the tensile strength (u) of steels, having already a European standard that supports it. This work develops and proposes a methodology that allows the application of this test under cyclic loading conditions, seeking the determination of the fatigue limit of a structural steel S355. For this purpose, fatigue tests have been carried out, setting the maximum load at a certain percentage of the Py load that is usually used in the determination of t material yield stress (ys = 1Py/t2 ) and using a load ratio R = 0.1. In these tests it was found that damage starts in the central zone of the face of the specimen subjected to tension, forming several cracks, which subsequently grow radially, until they stopped at a certain distance from the embedment, without on any occasion producing a catastrophic rupture. On this basis, the SPT fatigue failure (SPT-N curve) is defined at the moment of damage initiation, which is detected from flexibility measurements. The SPT-N curve thus obtained has been compared with the one coming from rotatory fatigue tests (R = -1), justifying the differences between both curves based on all those variables that affect fatigue behaviour, such as load ratio (R), stress distribution or specimen size. On this basis, a correlation between both tests is proposed.