Simultaneous p a i r s of values of K ( s t r e s s i n t e n s i t y f ac to r ) and R ( f r a c t u r e toughness , or e q u i v a l e n t l y s t r a i n energy r e l e a s e r a t e G) fo r q u a s i s t a t i c a l l y running cracks in compact t ens ion t e s t p i e c e s of po lymethylmethacry la te have been determined a t var ious crack v e l o c i t i e s (~) and temperatures (T) [1 ,2] . K were a r r i ved a t from the Gross-Srawley express ion (see, fo r example [3]) us ing cur ren t crack lengths and loads , and R was measured by Gurney's segmental area methods [4].