I tried:

Select DamageNo,Cast(RequisitionNo AS Integer) As RequisitionNo From
(Select  DamageNo,
   Case ISNUMERIC(DamageNo) 
      When 1 Then DamageNo
   Else Null
   End As RequisitionNo
from dbo.DamageInserts) T

The column DamageNo is of type varchar and contains values such as '1542A111','0455E204','104702'...

Isnumeric filters out '1542A111' as not convertible, but leaves '0455E204' because it can can be converted to float, that is, a numeric.

Which is all Isnumeric promises to determine.

Since I've got a legacy table to convert, I need is something like

-- pseudo code

Select DamageNo,
   Case Cast(DamageNo As Integer) 
      When -- no error occurred return the integer
   Else    -- Return Null
   End As RequisitionNo
from dbo.DamageInserts

Help is deeply appreciated.