I invite you to come and 'crack' my plastic covers. I have used them for 15 years and they are still flexible and very difficult to cut. I suspect you are thinking of another plastic product, not these mats I bought from Office Depot. They are translucent; I did not see any transparent ones when I bought them. They do let light inside but, as my son says, don't plan on doing any bird watching. :)
If yours are older and just translucent then they are PVC. They won’t crack, just bend or deform a bit when a flying object hits them. The polycarbonate is transparent, and it cracks. Not suitable. They probably didn’t have polycarbonate 15 years ago.