Effect of Crack Propagation Velocity on the Fracture Surface Energy of Poly(methyl methacrylate)

WE have measured values of the fracture surface energy (γ) at + 20° C of a sample of high molecular weight poly(methyl methacrylate) sheet (‘Perspex’, manufactured by Imperial Chemical Industries, Ltd., Plastics Division). In order to measure γ over a wide range of crack propagation velocities (ċ), we used three different experimental techniques: (1) A modification of the static loading method described by Van den Boogaart and Turner1 in which the crack length was measured as a function of time. From the results it was possible to deduce the relation between γ and ċ by assuming a value for Young's modulus (E = 2.85 × 1010 dynes/cm2). This technique covered the range of velocities from comparatively slow rates (≃ 10−4 cm/sec) up to about 1 cm/sec. Beyond this the crack propagation rate increased rapidly and the specimen failed catastrophically. (2) A modification of the cleavage technique described by Broutman and McGarry2 which produced controlled crack growth at various lead screw speeds. (3) Charpy impact tests on sharply notched specimens. γ was taken as the ratio of the energy to break to the area of the new surfaces created during crack growth. Although ċ was not measured accurately, it is between the pendulum velocity at the point of impact (240 cm/sec) and tho maximum possible crack propagation velocity (∼ 105 cm/sec). Thus it is substantially higher than in the other experiments.