I live in a condo with a back doorwall, but no patio. The doorwall opens directly onto lawn. In summer, the grounds crew comes through with a weed trimmer and (since no patio) trims right up the to house. In doing so, they repeatedly cut a slit in the bottom of the screen door, allowing bugs and dirt to enter the house. I've repaired the screen twice, but they just cut through again. Short of installing a patio that I probably won't use very much, is there any cheap way to fix/prevent this problem? I've googled for this problem, but it returns no references at all. I find it hard to believe that I have encountered something completely unique.
I solved a similar problem (cat scratching a low screen) by attaching a piece of clear acrylic plastic to the frame, just high enough to prevent damage, and still allowing ventilation through the upper part of the screen. I would guess 8 - 10 inches across the bottom would be sufficient to guard against string trimmers.
My frame was wood, so drilling some holes for screws was easy. Might be harder if you have aluminum. Also, the cut edge of the acrylic was not especially nice looking.
A steel pet protector should be sufficient. I would personally add a little edge to it to trim the weedeater lines if they get too close.