13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 5/2 5/2 0 2 2 2 1/2 1 2 2 2 2 2 2 1 2 2 2 2 ( ) ( ) ( ) , 3(3.17) [4 (1 ) ] ( ) ( ) , (1 ) ( 1,2) [( )(1 )] j j F v KI t D t z cd v t F v j c π αα α α α α α = − + = = − = − + (1), where KI is the stress intensity factor of a crack in tension; D(t) is the change in the diameter of the caustic; z0 is the distance from the object to the image; c is the shadow optical constant; d is the thickness of the sample; с1 and c2 are the speeds of longitudinal and transverse waves respectively; v(t) is the speed of the crack. 3. Results and Discussion 3.1. Quasi-static Tests 1,0x10-5 2,0x10-5 3,0x10-5 4,0x10-5 5,0x10-5 6,0x10-5 0 5 10 15 20 25 30 35 crack length (mm) time (s) a 1,0x10-5 2,0x10-5 3,0x10-5 4,0x10-5 5,0x10-5 6,0x10-5 0 200 400 600 800 1000 1200 1400 crack speed (m/s) time (s) b Figure 1. Crack propagation under quasi-static tension of plane samples of PMMA: a) slit scan of the crack tip trajectory; b) fluctuation of the crack front speed; dashed line is the mean speed (polynomial fitting). Fig. 1a shows the typical trajectory of the crack front under quasi-static tensile of PMMA plates
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