Abstract The influence of specimen size on the crack resistance ( K R -curve) behavior of a coarse-grained carbon is investigated using compact tension samples of varying size. The results are compared to previous measurements on asbestos cement. In order to analyse and estimate the toughening contribution appearing due to crack-wake related processes, the unloading compliance is measured during the K R curve test. Subsequent renotching of the extended crack and comparison with the behavior of an ideal linear-elastic body enables the crack-closure forces to be estimated.