Non-contact strain measurement with a video extensometer in tensile tests

Need

A contact extensometer scratches a thin wire, is damaged at the moment of fracture and runs out of range at large elongations. The wire sways under load; the diameter change is only a few percent and perspective in an ordinary lens distorts this small difference.

Solution approach

A telecentric lens removes perspective: the silhouette width does not change even if the wire moves towards the camera. The backlight turns the wire into a sharp silhouette; the two gauge marks are tracked as edges, elongation is computed from their distance and the diameter from the silhouette width. Every frame is time-stamped and matched with the force signal of the machine.

Process steps

1. 01

   Clamping

   The wire is clamped; the camera locates the gauge marks.

2. 02

   Pulling

   The upper grip rises at constant speed; mark distance and diameter are measured in every frame.

3. 03

   Necking

   The cross-section narrows locally; the necking curve is matched with the load data.

4. 04

   Decide

   Elongation at break is compared with the lower limit.