Application of digital speckle photography to measure thickness variations in the vacuum infusion process

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Abstract
A new method to measure the movement of the flexible bag used in vacuum infusion is presented. The method is based on an in‐house developed stereoscopic digital speckle photography system (DSP). The advantage with this optical method, which is based on cross‐correlation, is that the deflection of a large area can be continuously measured with a great accuracy (down to 10 μm). The method is at this stage most suited for research but can in the long run also be adopted in production control and optimization. By use of the method it was confirmed that a ditch is formed at the resin flow front and that there can be a considerable and seemingly perpetual compaction after complete filling. The existence of the ditch demonstrates that the stiffness of the reinforcement can be considerably reduced when it is wetted. Hence, the maximum fiber volume fraction can be larger than predicted from dry measurements of preform elasticity. It is likely that the overall thickness reduction after complete filling emanates from lubrication of the fibers combined with an outflow of the resin. Besides, the cross‐linking starts and the polymer shrinks. Hence, the alteration in height will continue until complete cross‐linking is reached.

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