PSI - Issue 23

Golta Khatibi et al. / Procedia Structural Integrity 23 (2019) 475–480 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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a voltage of 27 – 29V, current of 320-345 A and a wire speed 2.5 – 3.0 m/min and keeping an interpass temperature of max 150°C . Two series of fatigue samples, hereafter called C-series and E-series were cut out of the welded steel plates as schematically shown in Fig. 2c. While presence of different cracks in weld overlays are not uncommon and in some cases desirable, such imperfections may result in early failure during the fatigue testing. Thus in this study it was attempted to select defect free welded samples for testing, to ovoid affecting the intrinsic fatigue properties of the two types of welding consumables.

Fig. 2. Sample preparation technique for welded joints: (a) steel base material with a U-groove, (b) steel disc after filling the groove with the welding consumable and grinding, (c) schematic drawing showing the sample preparation concept.

2.3. Testing procedures

Fatigue tests were performed using an ultrasonic resonance testing system consisting of a power supply, a piezoelectric transducer, an acoustic horn and the sample. The mechanical part of the system including the sample is excited to symmetrical longitudinal push-pull vibrations at a resonance frequency of about 20 kHz. During the excitation, due to resonance conditions the maximum strain occurs in the mid-section of the sample (Roth 1985), ( Puškár 1993). At this location a V-groove was machined into the welded area. The notch which serves as a stress concentration site results in inducing the fatigue crack in the defined midsection area of the sample. Strain was measured by using two strain gauges, which were placed at the locations of the maximum strain of the acoustic horn and the sample. A strain calibration factor (K 1 ) was obtained by determination of the ratio of the strain amplitude of the sample to that of the horn at different excitation amplitudes Ǥ —„•‡“—‡– ‡š’‡”‹‡–• ™‡”‡ …‘†—…–‡† „› —•‹‰ –Š‡ •–”ƒ‹ ‰ƒ—‰‡ ’Žƒ…‡† ‘ –Š‡ ‹†Ǧ•‡…–‹‘ ‘ˆ –Š‡ Š‘”Ǥ Assuming that high cycle fatigue experiments are conducted at amplitudes fairly below the yield strength of the material, the stress amplitude (∆σ/2) can be calculated based on the Hooke’s law, according to the following equation (Eq 1). ∆ 2 = 1 . . . ∆ ℎ 2 (1) E=240 GPa is the average Young’s modulus of the weld area as measured by nanoindentation test, K t =3.48 is the theoretical stress concentration factor for the V-notch according to (Peterson 1974). A water circulating system containing a corrosion inhibitor was used to keep the sample temperature constant at ~25°C during the testing. The ultr asonic fatigue testing system used in this study is shown in Fig. 3a. The geometry of the fatigue samples is given in the drawing in Fig. 3b and an image of the prepared sample including the coupling tip can be seen in Fig. 3c. Wear tests were conducted by using an impeller-tumbler test equipment which is specially designed for continuous impact abrasion testing (Kirchgassner 2008), (Franek, 2009). The samples were mounted on the inner impeller of the test equipment which worked at rotation speeds of 60 and 600 rpm for the tumbler and impeller, respectively. During the experiments 1 kg of coarse corundum particles (5 – 10 mm) was used for impact loading with a duration of 20 minutes. The exposed surface of the samples was 2.5 × 1.0 cm and the impact velocity of the corundum particles was about 10 m/s. The tests were repeated tree times for each type of weld overlay sample.