Laboratory tests included quasi-static tests for six test specimen. Besides quasi-static tests, also other measurements were performed, for example, hardness measurements. The tested specimen consisted two butt welded (BW) joints, two non-load carrying X-joints, and also two arrow plate (AP) specimen. The dimension drawings of the specimen are shown in figure 26.
Figure 26. Dimension drawings of the butt weld, X-joint and one-sided arrow plate joint specimen. Note that the dimensions are after removing the weld ignition and ending points.
The point of laboratory tests was to examine the effect of the joint shape on the ultimate static capacity, thus, one specimen of each type was grounded flat before testing. In table 1 is shown the used test specimen IDs and respective joint types.
Table 1 Test specimen IDs and joint types.
N2_2 Arrow plate Ground flat
N2_4 Arrow plate Arrow plate only on one side
The material of every test specimen is SSAB Strenx® S960MC UHSS, and the used weld metal is Voestalpine Böhler Union X96 with a wire diameter of 1 mm. The test specimen were manufactured in the year 2012 when the corresponding material notation was Ruukki Optim S960QC. The chemical compositions of both materials, along with mechanical prop-erties are shown in table 2. Values for base material S960MC are obtained from the test report of the specific plate, delivered by SSAB.
Table 2 Base material and weld material mechanical properties and chemical compositions (Voestalpine Böhler 2013, p. 250).
All specimen were welded by using gas metal arc welding (GMAW). The butt weld and X-joint specimen were welded manually, but for the arrow plate specimen, welding robot was used. The butt weld specimen were welded with two passes into 50° V-groove, and both
passes were welded from the same side. Fillet welds for X-joint specimen were welded in one pass for each side. The arrow plate specimen have slightly different welding parameters for arrowheads and sides because the head is of greater importance, thus, the melting of attached plate edge needs to be avoided especially near arrow plate ends. The arrow plate specimen welds ignition and ending points are located on the sides of the arrow plate. The arrow plate specimen had the attachment only on one side. In table 3 is shown the main welding parameters for all specimen. The values were obtained from corresponding welding procedure specifications (WPS).
Table 3 Welding parameters for test specimen.
Specimen Current
The test specimen dimensions and shapes were also measured before the tensile tests were performed. For butt weld and X-joint specimen, 2D laser measuring was sufficient in order to obtain the geometry of the joint. The measuring was carried out over 100 mm span over the joint in such way, that the joint is located in the middle of the span. Also, the shape of the weld at arrow plate tip was measured by using the same procedure with measuring the span of 50 mm along the direction of the test specimen. The specimen effective weld throat dimensions were defined from the shape measurements. In addition, the X-joint specimen weld start and end parts were sawed off from the specimen and polished, thus the effective weld throat dimension for X-joints was also defined from the cross section. The polished
cross section also shows the shape of the HAZ. In figure 27 is shown the nominal and effec-tive weld throat dimensions for X-joint specimen. The laser shape measurements are shown in appendix I.
Figure 27. X-joint specimen weld throat dimensions and HAZ shape.
As observed from figure 27, the nominal and effective throat dimensions distinguish signif-icantly, thus there is notable penetration on every weld bead. The amount of penetration affects the shape of HAZ and due to greater penetration, the width of unaffected base mate-rial becomes narrower. Especially on right-hand side of the polished section in figure 27, the width of unaffected base material is very narrow. In table 4 is shown the nominal weld throat dimensions obtained from laser shape measuring.
Table 4 Specimen nominal weld throat dimensions from laser measurements.
Specimen Weld 1 [mm] Weld 2 [mm] Weld 3 [mm] Weld 4 [mm]
X1_3 5.27 5.00 5.20 5.34
X2_2 5.21 5.09 5.19 5.14
A2_1 10.709 - - -
A2_3 10.710 - - -
N2_2 3.86 3.92 - -
N2_4 3.85 3.78 - -
The nominal throat thicknesses of the arrow plate specimen are notably smaller than the manufacturing drawing indicates. This is due to the fact that the maximum allowable throat dimension, in order to avoid melting of the arrow plate edge, is 5 mm, which was in the drawing. In reality, the actual throat thickness is smaller in order to ensure the edge will not melt.
In addition, the hardness was measured from the same polished sections, where the throat dimensions were measured. A used method for hardness measuring was Vickers hardness with 5 kg weight (HV 5). In figure 28 is shown the hardness distributions across fillet weld from test specimen X1_3 and X2_2. Red and blue curves represent specimen X1_3 and X2_2, respectively. Even though the hardness distributions are measured across two differ-ent welds, they are comparable, because the measuring points are corresponding to each other.
Figure 28. Hardness distributions for test specimen X1_3 (red) and X2_2 (blue).
Figure 28 shows similar hardness distribution as presented by Björk et al. (2012, p. 72), and both distributions show the soft zone at the boundary of HAZ and BM. The figure also shows the zone with a smaller decline in hardness near the fusion line. In table 5 is shown the average hardness values for different zones.
Table 5 Average hardness values for X-joint specimen zones.
Specimen Average Vickers hardness (HV)
WM CGHAZ FGHAZ SCHAZ BM
X1_3 403 324 352 281 362
X2_2 381 314 347 277 342
Based on hardness measurements, material yield, and tensile strengths can be estimated. The estimations are not exact, because the hardness measurements include some deviations, and the hardness values are dependent on the exact location, for example, the hardness is differ-ent at a grain boundary and in the middle of a grain, or when the measuring tool encounters hard phases in the microstructure, the measures might deviate. Pavlina & Van Tyne (2008, p. 888–889) states that there is a relation between Vickers hardness and both yield and tensile strength of the material. The equations presented by Pavlina and Van Tyne (2008, p. 888–
889) are obtained by using least squares linear regression, so they produce approximate re-sults for yield and tensile strengths. In equations 3 and 4 are shown the relation between Vickers hardness and material yield and tensile strength.
𝑓𝑦 = −90.7 + 2.876𝐻𝑣 (3)
𝑓𝑢 = −99.8 + 3.734𝐻𝑣 (4)
In equations 3 and 4 Hv is Vickers hardness. By utilizing previous equations and average results from hardness measurements, the approximations for yield and tensile strengths of different zones can be estimated. All test specimen are from the same plate, thus, the me-chanical properties of the base material are identical, regardless of joint type. Also, the weld metal is same for each specimen, so following strength estimations are valid for every spec-imen. In table 6 is shown the strength estimations based on hardness measurements.
Table 6. Yield and tensile strengths based on hardness measurements.
The weld metal strength values based on hardness measures gives slightly higher results that are reasonable, especially for specimen X1_3. For example, Amraei et al. (2016a, p. 4), Am-raei et al. (2016b, p. 232) and Björk et al. (2012, p. 72) have reported that WM hardness for the same material pair (S960 and Union X96) was approximately 360 HV. Based on that, the hardness measurement for specimen X2_2 should provide more accurate results. There are many reasons why the hardness at WM of specimen X1_3 is higher than usually, for example, the weld might have cooled down more rapidly than normally, thus, martensite forming and hardening occurs. Also, the alloying near the fusion line might affect the local hardness values. Nevertheless, the values for other zones, from both curves, seem reasonable when compared to previously mentioned references and material certificates.
The test specimen were tensile loaded by using displacement control with approximate draw-ing speed of 2 mm/min. The speed was decreased, when plastic deformations and neckdraw-ing occurred in the specimen, in order to obtain more accurate results near the maximum load.
Stresses and strains of the specimen were measured by using digital image correlation (DIC), and no strain gages were used. The DIC camera lens diameter was 75 mm, shutter time 22.8 ms, and the used facet size was 18*14. In addition, the test rig had force and displacement sensors, which reported the force acting to the specimen, and the total displacement of the rig’s hydraulic cylinder. In figure 29 is shown the DIC camera setup, and the used test rig with specimen A2_3 attached.
Figure 29. Test rig with specimen A2_3 and the DIC camera setup.
The specimen were fastened to the rig by using pins on both ends of the specimen. The pin was then fastened by a bolt, so the specimen is firmly attached to the rig. As seen from figure 29, the forced displacement is applied on the right-hand side of the rig.