Dynamic fracture toughness of PMMA decreases with crack velocity

Dear Alls;

I am loking for the paper which said the Dynamic fracture toughness of PMMA decreases with crack velocity under impact loading, but after crack starts propagation, crack velocity is a decreasing function with crack propagation length.

So, if you know the paper which related to that problem, Could you please show me its title?

Thank you very much,

Best Regards,

 Chau