You can (and IMHO) should use a variety of materials to create inclusions, delams, porosity, etc., for testing NDT apparatii.
Backing film, teflon tape, bagging film, flash breaker tape, release film, pieces of latex gloves and even masking tape are good candidates. I suggest using multiple layers in different spots as you will find different materials interact with your prepreg creating different conditions and results for your apparatus to see.
eg. (1) one .25"x.25" layer of backing film in one location and (2) two .25"x.25" layers stacked in another. (Size to vary based on sensitivty of evaluating method)
My personal experience is two layers of Teflon will easily create a classic NDT Delam detectable with ultrasonic techniques in both honeycomb and solid laminate constructions in the size shown above. I have had to use bigger pieces for some techniques due to probe size, location, etc.
Also consider other contaminants such as oils, lotions, foaming adhesives, powder from latex gloves, debris from teflon coated gloves, etc. to create other types of issues. Powdered teflon gives great porosity results particularly if you get it to clump in your laminate.
Please note: My recommendations here are to show what is likely to find its way into a part during fabrication and create a defect described as a "delamination" by an NDT technician. In fact every one of the artificial methods really creates an inclusion/contaminant that is detectable due to the delamination signal. Consider putting in an inclusion and then fatiguing the part to "grow a delam" through interlaminar stresses.
Bets of luck.
Composites and Airplanes - what was I thinking?
There are gremlins in the autoclave!
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