Fig. A1. Plots used in experimental determination of critical strain energy release rate. (A) Load–displacement plots for specimens with various initial crack lengths. Solid circles indicate points at which unstable tearing occurred. Lower curves are for specimens with longer initial cracks. ^* represents a displacement chosen for generation of plot (B). Area 4, A₄ (hatched), indicates strain energy present in specimen 4 when it is pulled to displacement ^*; specimen 4's load–displacement curve must be extrapolated. _breaking,1 indicates the displacement at which specimen 1 tore unstably, which is used for plot (C). (B) A plot, derived from (A), of strain energy versus initial crack length at a selected value of specimen displacement, ^*. For example, strain energy A₄ is plotted against the length of specimen 4's initial crack, as depicted by the open circle 4. (C) A plot, derived from (A), of displacement at tearing versus initial crack length. For example, _breaking,1 is plotted against specimen 1's initial crack length, as indicated by grey circle 1. From this plot, initial crack length, a^*, corresponding to ^*, is determined. At this value of a^*, the tangent to plot (B) is found, which gives (dU/da).