Longitudinal attachment test specimens were fabricated from hot-rolled (EN 10025-2) S355J0 general structural steel, which is currently marketed under the trade name Multisteel. Both 5 mm and 8 mm thick plates were used for the main plate while the attachment thickness was 8 mm. Overall specimen length was 600 mm and the width was 40 mm at gripping section and 34 mm in the narrowed portion of the specimen. The attachments of 100 by 40 mm were welded to along one side of base plate. The test specimen geometry and dimensions are shown in Fig. 5.1.
t
40
8 34 R25
300 600
100 40
Figure 5.1 Test specimen of structural steel S355 longitudinal attachment.
Specimen thickness t = 5 mm or t = 8 mm
This longitudinal attachment specimen type is used extensively in laboratory fatigue experiments. The relatively small specimens are known to have high tensile residual stresses similar to those found in larger and more complex structures. Additionally, the specimen has a relatively small region of high stress concentration so the location of expected fatigue crack initiation is more predictable than for many other weld geometries. The longitudinal welded attachment specimen has been used in many investigations and a large database is available.
For example, the fatigue strength of ultrasonic impact treated longitudinal attachments has earlier been presented by Haagensen (1998).
Chemical analysis and mechanical properties of material S355J0 are shown in Tables 5.1 and 5.2 (Rautaruukki 1996). The gas metal arc (MAG, GMAW) welding and UIT treatment parameters for the one-sided longitudinal attachment specimen are given in Table 5.3.
Table 5.1 Chemical analysis of material S355J0 [%].
C Si Mn Al Nb S P
<0,18 <0,50 <1,60 >0,02 <0,05 <0,02 <0,025 Table 5.2 Mechanical properties of material S355J0.
Yield strength
Table 5.3 Welding and UI -treatment parameters.
Welding parameters
Welding process MAG
Current [A] 280
Voltage [V] 29
Shielding gas: Argon/CO2 [%] 90 / 10
Filler material OK 12.51
Travel speed [cm/min] 34
UI -treatment parameters
Depth of treated groove [approximate,mm] 0,5
Diameter of UIT intender end [mm] 3
Carrier frequency of transducer vibrations [nominal, kHz]
27
Ultrasonic vibration amplitude/power [µm/W] 30 / 1100*
Ultrasonic impact frequency [kHz] 350 - 400*
Amplitude of ultrasonic impacts, rebounds [mm] 1,5 - 1,8*
Press force on the tool without its weight [kgf] Up to 2,5*
Groove roughness < 2,5 µm*
Treatment speed [cm/min] 42
Number of passes 3
*Data provided by the Northern Scientific and Technology Company, Severodvinsk, Russian Federation.
The two plates needed to fabricate the specimen, i.e., the base plate and the attachment, were removed from large steel plates using laser cutting. The quality of the plate edges was high and no specimens failed from the base material. The MAG welding process was used with the filler material OK 12.51, see Table 5.4.
Table 5.4 Composition of filler material OK 12.51 [%] (ESAB 2000).
C Si Mn 0,1 0,85 1,5
The leg length of the fillet welds was from 8 to 10 mm. Start and stop positions of welds were not located at the corners of attachment. UIT was applied in the weld toe region at the attachment ends. This area is the most sensitive to the fatigue cracks for non-load carrying longitudinal attachment specimens.
In the IIW recommendations on post weld improvement methods (Haagensen and Maddox 2006), burr grinding to the end of longitudinal attachment should extend around end of attachment and continuously along both sides of the attachment. The length of treatment along the attachment sides should reach approximately four times of plate thickness, see Fig.
5.2. In this research, the extent of treatment recommended for burr grinding was used as a guideline for determining the extent of ultrasonic impact treatment. Start and stop position of treatment along the weld line was 50 mm from the original weld toe at the attachment end.
Treatment around the end of attachment was repeated 3 times and the treatment speed was about 42 cm/min. Visual inspection were executed after treatment and the specimens were accepted if the original weld toe was completely removed and if the material in the UIT grooves had a smooth appearance.
Figure 5.2 Recommendation for burr grinding weld toe technique (Haagensen and Maddox 2006).
In total 33 specimens were fabricated. Of these, 13 remained in the as-welded condition and 20 were treated using UI -treatment.
5.1.2 Two-sided longitudinal attachment
Six additional test specimens with longitudinal attachments were fabricated specifically with the intent of trying to measure the fatigue crack initiation period observed in ultrasonic impact treated specimens. Material for specimens, S355J0, was the same as that used in the original series. Material thickness and overall specimen length were 8 and 650 mm, respectively. Base plate width in the gripping region was 60 mm and 48 mm in the narrowed part of specimen where the gusset was attached. Dimensions of the welded attachments for both sides of the specimen were 100 x 40 x 8 mm. A specimen drawing is shown in Fig 5.3. Welding and treatment parameters of these specimens are presented in Table 5.5.
307 650 R40 48
60
8 40
100
8
Figure 5.3 Geometry of specimens of two-sided longitudinal attachment.
All six specimens were UI -treated. Welding and treatment of additional specimen were prepared based on same principles and recommendation as in original series. Composition of filler material OK 12.50 is same as that of filler material OK 12.51 used in original specimens.
Table 5.5 Welding and UI - treatment parameters Welding parameters
Welding process MAG
Current [A] 245
Voltage [V] 28,4
Shielding gas: Argon/CO2 [%] 92 / 8
Filler material OK 12.50
Wire feed rate [m/min] 11,7
Travel speed [cm/min] 37
UI -treatment parameters Depth of treated groove [approximate,mm] 0,5
Diameter of UIT intender end [mm] 3
Carrier frequency of transducer vibrations [nominal, kHz]
27
Treatment speed [cm/min] 40
Number of passes 3