Group B Dynamic Tests

These welded specimens were preliminary tests leading up to this thesis. Of the 34 tests performed, 22 provided successful results while 12 yielded unsuccessful results.

The square mild steel tubes making up these specimens had the same cross sectional dimensions as the tubes used in master’s testing, however the wall thickness was closer to 1.5mm instead of 1.6mm. This explains the larger crush distances measured in these specimens compared to the crush distances measured in the master’s specimens. The welded specimens were within the progressive buckling region with original lengths of 350mm, 400mm and 450mm and were labelled Series A, B and C for these lengths, respectively. Four different weld configurations were tested. These included four-sided seam weld, corner spot welds, side spot welds and two sided seam welds. Three side-slit specimens and four single tube specimens were also tested.

Side-split specimens are single tubes in which opposite sides are cut into with a hacksaw. Table 4.3 summarises the results obtained from the tests. Successful results are ones in which specimens buckled progressively while failed tests are ones in which bending, splitting or tearing occurs. Table 4.8 shows schematic diagrams of the weld configurations and summarises the test results in terms of number of successful and failed tests.

Table 4.8 indicates that the four-sided seam welded specimens are the most reliable as all the tests performed on these types of specimens provided desirable results. Of the

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4. EXPERIMENTAL RESULTS 58 welded specimens, the corner spot welded and two-sided seam welded specimens were the least reliable, with each configuration only having yielding four out of seven useful results. All three of the side-slit specimens failed disastrously.

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4. EXPERIMENTAL RESULTS 59 Table 4.8 Successful and failed results summary of different weld types in welded specimens

4-sided

As indicated in Table 4.3, two drop heights of 3.26m and 3.6m were used. These drop heights correspond to impact velocities of 8m/s and 8.4m/s, respectively. Figure 4.3 shows original lengths and crushed profiles of 400mm specimens tested at a drop height of 3.26m. Results for these tests are tabulated in Table 4.9. Table 4.9 indicates that the corner spot-welded specimen is the most efficient as it crushed the most but this behaviour was not consistent in other sets tested. Figure 4.4 shows original lengths and crushed profiles of 400mm specimens tested at a drop height of 3.6m.

Results for these tests are tabulated in Table 4.10. Although Table 4.8 suggests that the weld type has a significant effect on how the system behaves, from Figure 4.4 and the results in Table 4.10 it is clear that the weld type makes very little difference to the deformation mode and efficiency in terms of crushed distance in a welded tube. The weld type has a notable effect on a system in terms of the reliability and predictability of the crush mode. There is only a 5mm difference in crushed distance between the four different welded specimens.

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4. EXPERIMENTAL RESULTS 60 Figure 4.3: 400mm specimens tested with h = 3.26m (50mm clamp length is shown).

Table 4.9 Results of tests performed on 400mm welded specimens with h = 3.26m

Specimen Weld type

L1 (mm)

L2

(mm) δt (mm) Epd

(kJ) P^dm (kN)

B1a 200 200 286 11.82 41.31

B1b 200 200 300 11.87 39.58

B1c 200 200 253 11.71 46.27

B1d 200 200 273 11.79 43.12

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4. EXPERIMENTAL RESULTS 61 Figure 4.4: 400mm specimens tested with h = 3.60m (50mm clamp length is shown).

Table 4.10 Results of tests performed on 400mm welded specimens with h = 3.60m

Specimen Weld type

L1 (mm)

L2

(mm) δt (mm) Epd

(kJ) P^dm (kN)

B2a 200 200 318 13.06 41.00

B2b 200 200 321 13.07 40.73

B2c 200 200 316 13.07 41.30

B2d 200 200 317 13.05 41.07

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4. EXPERIMENTAL RESULTS 62 Highspeed camera footage was obtained for some welded specimens. Figures 4.5 -4.8 show the progressive buckling sequences for specimens B2a, B2b, B2c and B2d respectively. Each frame in a sequence is numbered from (a) to (j) and the corresponding point is shown in the specimen length and axial displacement graph in Figure 4.9. The four specimens display very similar trends in terms of their crushing profiles. The graph shows that all four specimens are crushed by the same amount and that the time taken to stop the drop mass is approximately the same in each specimen (70ms). This in turn indicates that the average deceleration of the impact mass is the same in each specimen. All four specimens decelerated the drop mass by about 110m/s².

Figure 4.5: Progressive buckling sequence for specimen B2a

Figure 4.6: Progressive buckling sequence for specimen B2b

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4. EXPERIMENTAL RESULTS 63 Figure 4.7: Progressive buckling sequence for specimen B2c

Figure 4.8: Progressive buckling sequence for specimen B2d

Figure 4.9: Specimen length (left) and axial displacement (right) vs time taken to crush specimen graph for Series B with h=3.6m

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4. EXPERIMENTAL RESULTS 64 Recall that geometric efficiency is a measure of how well an absorber compresses [8]

and is given by:

eG = δ/L (4.4)

where δ is the axial displacement of the tube and L is the original length of the tube.

Geometric efficiency has been used for the basis of comparison in this section. Figure 4.10 shows the geometric efficiency chart for specimens tested at a drop height of 3.26m while Figure 4.6 shows the geometric efficiency chart for specimens tested at a drop height of 3.60m.

Figure 4.10 Geometric efficiencies of specimens tested with h = 3.26m.

The graph in Figure 4.10 shows that at a drop height of 3.26m, specimens in series A and B exhibit a fairly large difference in terms of geometric efficiency. In series A there is a difference of 15% between the four-sided seam weld specimen and the single tube specimen. In series B there is a 12% difference between the corner spot weld and the side spot weld specimens. However, there is no consistency in terms of

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4. EXPERIMENTAL RESULTS 65 which weld is more efficient in Series A and B. In series C however, the specimens have very similar geometric efficiencies with the single tube only being 3.5% more efficient than the least efficient specimen (four-sided seam weld) in the series.

Figure 4.11 Geometric efficiencies of specimens tested with h = 3.60m.

At a drop height of 3.60m, the graph in Figure 4.11 shows that in all three series, the differences in geometric efficiencies between each weld type specimen is much smaller. Series B has a difference of less than 1% between the four weld types tested in that series. Single tubes were not tested at this drop height. These results show that there is no superior weld type in terms of geometric efficiency but that in terms of reliability (least failures), the four sided seam weld performed best.

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4. EXPERIMENTAL RESULTS 66