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The healing efficiency can be calculated as the ratio between the tensile strength results for the healed and original sample according to Equation (3.1) [35–38]:

(3.1)

This relation also works for others mechanical properties such as failure stress, failure strain, energy at break or J-integral at crack initiation [36].

Raman Spectroscopy can be used following the evolution of the three main spectral bands: and C–C stretching (υ C–C = 1,590 cm⁻¹), C−S stretching (υ C–S = 650 cm⁻¹), S−S stretching (υ S−S = 500 cm⁻¹) [35], which are related to the chemical structure of the crosslinked material.

In addition, there are optical methods to characterize the healed sample. Optical microscope and digital cameras are used to analyze the healing process [39] and also scanning electron microscopy (SEM) to study the healing of the scratches [40] as can be seen in Figure 3.4. The figure shows two samples of a compound, which were cut and submitted at different conditions to recover the mechanical properties: room temperature (that take 17 h) and 60 °C (that take 7 h).

Figure 3.4 SEM images to trace the healing process of the scratch. (Reused with permission and modified after Peng et al. [40]).