From: del@branta.demon.co.uk   
      
   On Tue, 29 Mar 2005, in sci.space.tech,   
   Craig Fink  said:   
      
   >Henry Spencer wrote:   
   >>>Why would a titanium structre weight 2 tons *more* than an aluminum   
   >>>one?Titanium has a higher strength/weight ratio, and although aluminum   
   >>>has a better stiffness/weight ration, that does not seem as relevent.   
   >>   
   >> In real structures, surprisingly often, the structural design is driven by   
   >> stiffness requirements, not strength.   
      
   >Modulus of elasticity,   
   >Aluminum 10,000,000 psi   
   >Titanium 16,000,000 psi   
   >   
   >Titanium's stiffness is 60% greater than Aluminum.   
      
   Yes but that's a comparison between two metals of equal volume, and   
   takes no account of the weight of the two structures.  Ti is also about   
   60% denser than Al, so weight-for-weight, the tensile moduli are about   
   the same.  Furthermore, the greater density of Ti is a disadvantage to   
   it in resisting bending forces, since the lower-density Al will be on   
   average further from the centre of bending.   
      
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   Del Cotter   
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