( ) ( ( ) ( )) I i r t E t t (1) T( ) ( ) t t E t (2) where E is Young's modulus of incident and transmitted bars; ( ) r t and ( ) t t are reflected and transmitted waves, respectively. Aluminum bars 15 mm diameter were used. Impact velocities of the striker tube were applied by controlling the pressure of the gas gun to achieve desired loading rates. Specimens were designed to suit the Hopkinson bars and essentially single-edge notched tension (SENT) geometry (see Figure 4). The specimen, with the size of 4.5mm×14mm×65mm, was cemented and embedded 20 mm into the bars. The strain gauge glued near the crack-tip was used to ascertain the crack initiation time [8]. The pulse shaping technique was employed to trim and optimize the incident stress, and to achieve stress equilibrium in the specimen in a short time [9]. At least 3 repeated tests were conducted for each material. 4. Results and Discussion 4.1 Calculation of fracture toughness For the CT tests, the fracture toughness is calculated by [5] ( / ) IC P K f a W B W (3) where P is fracture initiation load, and 2 3 4 3/2 2 ( ) ( ) (0.886 4.64 13.32 14.73 5.6 ) (1 ) a f f W (4) For the SENT specimen under dynamic loading, the fracture toughness is obtained from [10] ( ), IC I a K a F W (5) where σ is fracture stress on the specimen width, and 2 3 4 ( ) 1.12 0.231 10.55 21.72 30.39 IF (6) The fracture stress σ is calculated from the transmitted stress T : (a) Lateral view of fixed specimen (b) Top view of fixed specimen Figure 4 Fixture of dynamic toughness specimen and arrangements of strain gauge and crack
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