Geometry of Deformation Equation  
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The figure shows the brass link labeled 1 and the steel rod labeled 2. The reference datum is a vertical line passing through the left inside edge of the brass link. The brass link and the steel rod are elongated by delta subscript 2.

Looking at the lengths of the link and the rod, it is evident that the link has not been stretched far enough so that rod (2) will fit in the link as it must.