6.4: Composite Vaulting Poles – Why Don´t They Break?
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Stresses in a Vaulting Pole
A composite vaulting pole, of the type shown in the video, typically has a diameter of about 50 mm and the pole is being bent to a radius of curvature, R , of about 1 m. The calculations below show how the peak stress on the outer surface of a 50 % fibre pole can be estimated.
Why are they Strong Enough?
Since σ1u ~ f σfu the strength of the fibres, σfu , must therefore be at least about 2 GPa. In fact, the glass fibres used in composites, which are about 7 µm in diameter, have strengths of about 2-3 GPa. Using the Griffith criterion and assuming brittle fracture with a surface energy, γ, of 1 J m -2, the maximum flaw size must be less than about 10 nm. The key to their strengths is therefore the absence of large flaws.
Why are they Tough Enough?
It is not immediately clear why the material as a whole should be sufficiently tough, since both constituents are individually very brittle. A repeatedly loaded component will acquire many, relatively large, surface defects, so the fact that it can sustain such high stresses without fracturing indicates that, as a material, the composite has a high toughness. We will explore this in detail in the next section.