The material selected for the sheath 18 may be the same as or different from {HEAD ~ the material} chosen for {TAIL ~ the core tube} 12, 
The material selected for the sheath 18 may be the same as or different from { HEAD ~ the material } chosen for { TAIL ~ the core tube } 12 ,
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH