with t 0 , the thickness of the test specimen and G being expressed in J/m 2 . Since crack propagation is hard to control and often results in sudden catastrophic failure of the sample, multiple samples with varying crack lengths c are measured consecutively (Figure ). In such a scenario, each sample exhibits a different elastic energy U, which is quantified by integrating the area under the curve until a selected displacement length L. Next, the calculated elastic energies U 1 to U 4 (unit Joule J) are plotted against the crack length c to deliver the energy release rate G for the sample to be extended to this particular L value (Figure , inset). Finally, when the displacement length L is chosen as such that at least one sample fractures (L = L c ), the energy release rate becomes equal to the critical energy release rate G c also known as fracture energy or fracture toughness Γ.