Tried hitting a carbon fibre spacer with a hammer (as you do); after 11 quite hard taps the spacer seemed to suffer no damage and appeared as sound as ever. Terrific, I thought, just what I was hoping for; however, the 12th tap saw the spacer crumple flat like a piece of paper mache!
Now, I'm no structural engineer (as you can probably tell!), but I found this slightly worrying. How many small hits can my carbon fibre road bike absorp without any apparent damage before it suddenly collapses underneath me when I ride over a discarded Macdonalds wrapper?
Probably worrying unnecessarily, but it wouldn't seem a normal day if I didn't have something to worry about!