Simple Design Model
To understand how critical geometry is in compression, we constructed a very simple model of a beam and evaluated it in both column strength (straight compression) and in bending (combination tension and compression). For the large open section (t = 2.0 mm, b = 6.0 in.) t/b = .013. This section would be good for about 180 MPa compressive stress before buckling. Now if we take that same over-all section size and split it in two (assume 2 hydroform tubes side by side) we have in essence increased the t/b ratio by a factor of 2 to t/b = .026 (assuming a gage of 2.0 mm). This would give you an allowable buckling stress of 650 MPa. This is 3.6 times higher than the base structure. Now what can be done is down gaging to get equivalent performance between the two sections. Down gaging to 1.4 mm ended up giving equivalent performance but provided a weight reduction of approximately 15%. This shows how improving the section geometry can allow for weight reduction.
Results (designs with similar performance):
- Single tube (baseline) weight (2.0mm wall) 7.4 kg
- Duel tube weight (1.4mm wall) 6.3 kg
- Delta 1.1 kg (15% reduction)