PSI - Issue 41

Hendrik Baarssen et al. / Procedia Structural Integrity 41 (2022) 183–191 Baarsen et al. / Structural Integrity Procedia 00 (2022) 000–000

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Moreover the critical defects have been assessed also using the FAD in agreement with the provisions of the British Standard BS7910 BSI (2013), considering the e ff ect of reduced constraint at the crack tip.

2. Methods

2.1. Experimental program

The present investigation has been carried out using specimens made of S275JR structural steel grade. In particular, 3 types of tests have been carried out: • Characterization of monotonic tensile properties using standard test coupons; • Tensile tests on notched plate specimens, see Figure 1a; • Tensile tests on notched plate specimens in which cracks have nucleated due to cyclic loading. In the first part of the experimental program, specimens A1 to A4 are those tested to determine the monotonic tensile properties. Then specimen B1 to B4 are used to characterise the monotonic strength and to have a reference for the determination of the stress range to be applied for the fatigue pre-cracking. Lastly, specimen B5 to B12 are cyclically loaded under tension, i.e. with load ratio R > 0, until cracks are detected. When a crack is detected the cyclic loading stops and the pre-cracked specimen is monotonically loaded until failure occurs. For both the fatigue loading and the tensile tests on notched plate specimens, a Schenck hydraulic universal testing machine with a capacity of 400 kN is used, see Figure 1b. The machine is equipped with mechanical wedge grips, therefore it is suitable for cyclic loading with R > 0. The tensile tests are executed with a displacement-controlled closed loop with a speed of 1.2 mm / min until there is evidence of plastic deformation. From this moment on, the speed is increased to 3.6 mm / min. The elongation of the specimen is measured over a length of 100mm, with the hole in the middle. The force is acquired by a load cell of the same nominal capacity as the machine. The cyclic loading of specimen B5 to B12 is applied in a force-controlled closed loop with the force following a sine-wave with P min = 10 kN and P max = 200 kN, with a loading frequency of 2.75 Hz. This was selected so that the time for crack nucleation is minimized, without causing excessive plastic deformation at the notch location. Specimen B5 served to define the most optimal way for crack detection. For this reason, this specimen was equipped with: (1) four strain gauges with a gauge length of 6 mm and 2% detection range glued on the specimen besides the hole; (2) a strain gauge of 3 mm gauge length and 2% detection range is glued inside the hole, in correspondence with the net section; (3) a COD-meter was used to monitor the hole elongation, see Figure 1c. After carrying out the first test it was concluded that the COD-meter was su ffi cient to detect the presence of the crack. Therefore, this device was selected to be used in the other tests. For all sensors the data is gathered with a frequency of 100 Hz. In addition, all specimens are regularly visually inspected for cracks using a optical hand microscope with a magnification of 20X. The specimen are inspected whenever the COD-meter showed a 10% increase in the peak value. The failure assessment diagram is used to assess the failure load of the pre-cracked plate specimens. The assessment is graphically shown in Figure 2, and it consists of a curve indicating the locus of failure in the K r - L r plane. When the assessment point is inside the locus of failure, failure is not deemed to occur, i.e. the inner area denotes a safe zone. Vice-versa, when the assessment point is outside then failure is assumed to have occurred, i.e. the outer area denotes an unsafe zone. The brittle fracture ratio, K r , is defined as the ratio between the stress intensity factor, K , and the material fracture toughness in terms of stress intensity factor including constraint correction K c mat , which is derived from the plain strain fracture toughness K mat , taking into account the level of constraint at the crack tip. The plastic collapse ratio, L r , is defined as the ratio between the applied load, P , and the plastic collapse load, P c , defined by a limit load analysis on the cracked section. The stress intensity factor is calculated considering the measured geometry 2.2. Test set-up 2.3. The failure assessment diagram