Issue34

Y. Hos et alii, Frattura ed Integrità Strutturale, 34 (2015) 133-141; DOI: 10.3221/IGF-ESIS.34.14 134 D ESCRIPTION OF THE PROBLEM onstant amplitude fatigue tests have been performed using thin-walled tubes under tension-compression (force F) and torsion (moment M ). The specimen geometry is shown in Fig. 1. The specimens were machined from longitudinally welded tubes. The individual specimens were saw-cut and the notches were milled. Two holes, diameter 4mm, were drilled with the distance of 10mm of the centres of the holes (length of an arc measured at the outer surface). The notch was positioned opposite to the longitudinal weld. The material is the construction steel S235 with mechanical properties as given in Tab. 1. Besides Young’s modulus, E , plastic offset stress, R p0.2 , ultimate tensile strength, R m , the parameters of the cyclic stress-strain curve, 1 ' n a a a E K             , (1) are given. For use in finite element calculations the cyclic stress-strain curve is reformulated in terms of Chaboche’s model [2],     5 pl 1 1 exp i y i i C                 , (2) with the parameters listed in Tab. 2. Figure 1 : Specimen geometry. E in MPa R p0.2 in MPa R m in MPa K’ in MPa n  214000 310 435 1170 0.239 Table 1 : Mechanical properties of S235 [3].  y in MPa C 1 in MPa  1 C 2 in MPa  2 C 3 in MPa   3  C 4 in MPa   4  C 5 in MPa   5  58.2 13442 46 39134 400 72245 1904 603494 7404 800707 104861 Table 2 : Chaboche parameters for representing the cyclic stress strain curve of S235. Two series of experiments have been performed. In the first series of experiments, the fatigue crack lengths were monitored automatically by taking photographs of the specimen at predefined numbers of applied cycles. Three cameras were installed, one directly facing the slit of width 14 mm from front, one with 45° angle on one side and one with 45° angle on the other side. At regular, pre-defined intervals the test was interrupted automatically. The specimen was C