Issue34

Y. Hos et alii, Frattura ed Integrità Strutturale, 34 (2015) 133-141; DOI: 10.3221/IGF-ESIS.34.14 135 unloaded completely. Then, a static load of 90% of the maximum fatigue load was applied. During the short hold time the cameras took pictures of the specimen’s surface. The specimen was unloaded again and the cyclic loading continued. The pictures were inspected after the test. In order to ease the optical evaluation of the crack tip coordinates the specimens were branded with a 1mm Laser-dot pattern prior to testing. The five different loading sequences have been applied: pure tension-compression loading, pure torsion loading, proportional loading resulting from the superposition of these two and out-of-phase loading with phase angles of 45° and 90°. The load ratio was R F = R M = -1. The experiments have been conducted under load control and moment control, respectively, using a servo-hydraulic, four-pillar tension-torsion testing machine with frequencies between 0.25 Hz and 2 Hz. In the air conditioned laboratory, a temperature of 21°C and a relative air humidity of 50% were kept constant. The cracks were assumed to be through-wall cracks with a straight crack front. The crack length is defined as the arc length with the arc starting at the crack initiation location. Here, the scheme of presenting the results was adopted from references [4] an [5] with crack 1 being left and crack 3 being right. In the second series of experiments the tests were occasionally interrupted and the deformation field was measured applying the digital image correlation (DIC) technique. More details on the measurement technique can be found in references [6,7]. Some results of these investigations are used later when dealing with crack closure. Uniaxial Loading The specimen R-001 has been tested under pure tension-compression loading with max 45kN F  and 1 F R   . Two symmetric cracks grew in the centre cross section plane, Fig. 2. The crack growth curve is shown in Fig. 3. Figure 2 : Cracks in the specimen R-001, pure tension-compression with max 45kN F  and 1 F R   , steel S235. Figure 3 : Crack growth curve of specimen R-001, pure tension-compression with max 45kN F  and 1 F R   , steel S235. The results of this experiment serve as a reference for determining the basic material crack growth properties and for calibrating numerical procedures.