Strain indicator

Claim 1

An optical stress-strain indicator comprising in combination: a first member subject to stress-strain and having a recess extending from an external surface of said member into said member; a pin member received within said recess and having a first end and a second end, said first end being secured to said first member at an interior portion thereof; a fluid holding member having a wall portion and a window; said fluid holding member being at least partially received within said first member and secured to said first member at said wall portion with said window positioned with respect to said first member so as to be visible; said second end of said pin member received in said fluid holding member; a light absorbing indicator fluid in said fluid holding member substantially filling said fluid holding member while remaining out of contact with said first member; an indicator member in said fluid holding member having an indicator area disposed substantially adjacent said window; sealing means between said pin member and said fluid holding member, said sealing means comprising a sealing member having a portion thereof secured to said fluid holding member with another portion thereof secured to said pin member, said sealing member being flexible to permit relative movement of said pin member with respect to said window; means permitting the cross-sectional shape of said sealing member to change upon relative movement of said pin member with respect to said window such that the internal space occupied by said fluid in said fluid holding member is always constant with or without movement of said pin member; means operatively connecting said indicator member with said second end of said pin member, whereby; upon application of stress to said first member there is accomplished a change in length of said first member with the result that said pin is caused to move axially thereby causing said second end and said indicator member to move simultaneously in the same direction with the consequence that said indicator area moves from said window and indicator fluid is caused to flow between said indicator area and said window, the volume of the space within said fluid holding member containing said fluid remaining constant.