###### Abstract
In more complex settings, scattering in the infrared imaging can serve as a measure of strain energy fluctuations in the vicinity of the fatigue crack tip, thus, providing the possibility to characterise the \(J\)-integral or the stress intensity factor \(K\) during fatigue crack growth [7] from non-contact thermographic measurements [8]. Amongst other appealing applications of IRT, the possibility proposed by [PERSON] to estimate the fatigue limit rapidly with a small number of samples is to be mentioned [9]. The further elaboration of this approach allowed not only the fatigue limit to be estimated, but the entire fatigue life curve to be recovered in a cost-effective way [10]. Another common use of the IRT method relies on the comparison of the temperature response with the load signal. If the adiabatic conditions are met and no cyclic plastic deformation occurs, the temperature change should be proportional to the change in the load signal (with a negative proportionality constant). This behaviour changes when plasticity is present [7]. Being a dissipative process by nature, plastic deformation affects the temperature response. Based on this premise, [PERSON] et al. [11] used a second order Fourier series to investigate the first- and second-order harmonics of the temperature, which allows to perform thermoelastic stress analysis, locate the crack tip, and gain information regarding plastic deformation. Other authors have focused on the energy sources that cause the dissipation of the thermal energy at the crack tip, and its relation to damage [12]. Many of these analytical approaches can be combined into a common workflow. RVI has been proven to be a technique enabling direct assessment of the dynamic effects related to plastic deformation and fracture observable at the surface. The rapid observation of the deformation-induced surface morphology permits for the in-depth characterisation of the failure mechanisms occurring at time scales ranging broadly from seconds to micro-seconds. For instance, [PERSON] et al. [13] investigated the effect of the crack velocity in the vicinity of the crack tip, which has not been well understood as yet. The challenges faced frequently by the RVI users include, but are not limited to, (i) adequate illumination, which is particularly important for high shutter speeds; (ii) limited recording time [14]; and (iii) duration of an event, which can be too short to be captured with sufficient resolution [15]. These commonly known issues highlight the significance of an automated triggering system allowing to capture only the moments of interest with a proper reference to external conditions such as load, strain, temperature, etc. The successful attempt to use the acoustic emission technique to trigger the rapid video camera in a mechanical testing setup has been reported in [16]. DIC is a modern non-contact method of local displacement measurements which is gaining greater and greater popularity due to its versatility and high accuracy results of the measurement of the strain tensor components [1], which is comparable to or exceeds that of other conservative methods available to researchers [17]. The DIC technique enables the full-field displacement and strain maps to be obtained in a set of photographs taken at different stages of deformation by tracking unique features in the images. DIC is an undemanding method that uses a simple set-up permitting for a widely variable sensitivity and resolution, and for high automation of displacement and strain measurements [15]. The report by [PERSON] et al. [15] is one of multiple examples of the extensive use of RVI in conjunction with DIC to obtain displacement and strain fields, as well as to estimate the strain distribution in the vicinity of the crack tip, and the size of the plastic zone. The outstanding sub-micrometre (per pixel) resolution has been achieved with the DIC method in [18], where the plastic strain field was studied near a growing fatigue crack in a nickel-based superalloy. [PERSON] et al.