## 2 Noncontact Mechanical Property Testing Methods
### Photothermal (PT) Techniques
The PT testing modality is local, quantitative, and absolutely noncontact. Photothermal radiometry (PTR) is an active infrared emission testing modality. High power lasers are typically used to generate transient or harmonic local heating in solid and liquid samples, followed by free diffusion or convection. Monitoring the behavior of the diffusing field, the thermophysical properties can be evaluated. Since the 1980s, [PERSON] et al. [87; 88], [PERSON] et al. [89; 90], [PERSON] et al. [91] and [PERSON] et al. [92; 93; 94] have reported on a series of PT-NDT techniques and applications, mainly for measuring thermophysical properties. The first report on mechanical performance monitoring published by [PERSON] et al. [95] used a pyroelectric device to capture the thermal field and evaluate the residual stress. The system was simple but could not achieve fully noncontact detection. [PERSON] et al. [96; 97] constructed a totally noncontact system using an Ar-ion laser and an infrared camera to generate and record the PT field on a stressed sample as shown in Figure 7. The beam splitter in that figure ensured most of the laser power was directed to the sample while an optical detector provided a reference signal for amplitude and phase data acquisition. The relative change in thermal conductivity was calculated based on the diffusion field amplitude and phase images. For a carbon steel sample, a few percent change in its thermal conductivity was found in the direction paralleled to the uniaxial loading within the elastic regime [96]. Based on a similar setup, [PERSON] et al. examined the PT signal of a plastically deformed metal sample [98]. The sample was stretched to fracture so that a high level of residual stress was distributed non-uniformly around the failure surface. The diffusivity changed as much as 65% between the fracture region and the surrounding intact part. [PERSON] et al. used a halogen lamp instead of a laser to test the change in thermal properties of a plastically deformed metal subjected to tensile loading [99]. Since the test was continuous, a thermocouple was attached to the sample surface to measure the temperature. The contact measurement makes the system inflexible and rigid. An elastic-to-plastic full-range tensile photo-thermo-mechanical radiometry (PTMR) test was carried out by [PERSON] et al. using both a Mercury Cadmium Telluride (MCT) detector (single point measurement, shown in Figure 8 [100] and an infrared camera for imaging [84]. In those systems, a fiber-coupled high power diode laser was used to provide more operational flexibility. The pump laser was configured at oblique incidence and the tested data were processed with a normalization algorithm. The results are shown in Figure 9. The stress-strain relation of full-range tensile loading was plotted as a function of the thermal parameter \(\kappa\) which is defined as the sample thickness over square-root of thermal diffusivity. The curves show a good analogy with the conventional stress-strain relation and the relative change of thermal conductivity is at the same level as [97]. The aforementioned PTMR development shows the possibility of using a PTR setup as a non-contact strain gauge. In addition to regular metallic materials, polymers and composites are also treated with PT techniques to assess their mechanical performance, especially after plastic deformation occurs. [PERSON] and [PERSON] studied the change of polymer thermal diffusivity in the plastically deformed range [101]. They derived the principal diffusivity tensor as a function of the Cauchy-Green deformation tensor. A much more conspicuous change in diffusivity was observed in polymers compared with metals as expected. However, such change is reversible because of the hyperelasticity of polymers. [PERSON] et al. further used a single point PTR configuration to test nano-coated aluminum composites [102].