## 2 Methods and Materials
### Experimental Setup  
Using the same design philosophy, two slightly different experimental setups have been developed for the axial monotonic and cyclic testing under load or displacement control and for fatigue crack growth (FCG) tests. The schematics of both setups are shown in Figure 1. Since the involved key techniques are essentially the same in both arrangements, the tensile setup can be considered as a simplified version of the one for FCG. In addition to the testing machine and the three independent measurement systems representing IRT, AE, and RVI/DIC techniques, the entire setup does, however, require additional instrumentation and preparation to turn it working as desired. Specifically, depending on the testing machine and the grips, measures to reduce the machine noise, which is particularly severe on most commercially available servo-hydraulic frames, is required for the AE acquisition. The AE unit often needs to be galvanically isolated from all other devices to eliminate electrical interference and improve the signal-to-noise ratio (SNR): the sensors with conductive contact surfaces have to be isolated from the specimen, and triggering circuits have to be designed with optical decoupling of all signal connections. In the setup used in the present work, the high-speed CMOS-based camera PHOTRON SA-Z maintaining a square aspect ratio of 1024 \(\times\) 1024 pixels for frame rates up to 20,000 fps (up to 1,000,000 fps at a reduced resolution) with 64 GB on-board memory was  
Figure 1: Schematics (**a**,**b**) and photograph of the setup (**c**). used. A versatile modular microscopic and microscopic long-focal zoom lens Navitar-6000 with the through-the-lens coaxial high-power Hayashi LA HDF7010 RL LED illumination unit was attached to the camera for high-speed shooting. The maximal spatial resolution with this lens system is approximately 0.7 \(\upmu\)m. The camera was mounted on a motorised computer-controlled linear stage LNTR502E (Thorlabs, Newton, NJ, USA), which allows the camera to move together with the crack tip to measure the crack length precisely even when the surface is polished, and the camera's position and relative motion would be difficult to track. IRT measurements were performed using a Telops FAST M350 mid-wave, high-speed camera, with an operating spectral range of 1.5 \(\upmu\)m to 5.4 \(\upmu\)m and the indium antimonide (InSb) cooled sensor. The camera has a typical in the class noise equivalent temperature difference (NETD) value of 20 mK (referring to 30 \({}^{\circ}\)C background temperature). Even though, NETD is a quantity that might characterise the inherent property of the sensor at a given reference temperature, when it comes to practical measurements, the NETD parameter represents the upper bound estimate of the thermal resolution of the entire thermographic system. The NETD value depends on the settings, most notably on the temperature of the tested object, the spectral transmittance of the camera optics, \(f\)-number characterising the lens aperture (a low \(f\)-number means a larger aperture), exposure time, etc. [65, 66]. Therefore, the actual NETD value of the experimental setup can be considerably higher. The sensor has a 640 \(\times\) 512 pixels resolution and the maximum frame rate is 355 fps at full resolution. The internal memory of the camera was 16 GB and a CameraLink interface enabling direct data transfer to an external drive. The camera was firmly mounted on a tripod as shown in Figure 1c. The IRC-LENS-G1-MW 1X microscopic lens (\(f/3\)) with a working distance of 260 mm was used in the FCG setup. The sensor size is 15 \(\upmu\)m, which, in combination with the 1X lens, renders a spatial resolution of 15 \(\upmu\)m. The Janos Technology 40,494 25 mm lens (\(f/2.3\)) with \(1/4^{\prime\prime}\) extension ring was used in the axial setup, although both lens sets can be used interchangeably, depending on the dimensions of the test specimens.