## 2 Noncontact Mechanical Property Testing Methods
### Optical/Radiation Techniques
A narrow collimated visible laser beam impinges perpendicularly on a sample surface, while the camera monitors and captures scattered laser light from the incident spot at a non-normal angle in a scanning patterns over the surface so as to determine roughness mappings. This method is usually named laser triangulation. A series of tests with laser triangulation scanning were carried out [28; 29; 30] to systematically quantify mechanical bonding strength between different concrete layers. The correlation between adhesive performance of concrete and roughness, grain size, texture etc. was investigated. Optical non-contact measurements represent an accurate and effective modality for evaluating mechanical strength of inelastic building materials. DIC is a successful approach to remotely detect small displacements and reconstruct strain fields. It relies heavily on the resolution, processing speed and aberration performance of the camera. However, there are some limitations with respect to its applications. The processing algorithm requires input of  
Figure 1: The schematic of digital-image correlation (DIC) strain evaluation. Figure 2: Strain measurement configuration with blue light source, redrawn from [24]. speckle images, therefore, the signal-to-noise ratio from highly reflective surfaces can easily deteriorate. Furthermore, the strain field inside the sample cannot be visualized with purely optical means. The existence of local stress/strain should be justified with other techniques to yield a comprehensive stress-strain evaluation. Ionizing radiative electromagnetic waves have deep penetration and thus can be used for internal stress-strain evaluation. Among the established methods, X-ray diffraction is regarded as a classic and powerful tool for diagnosing subsurface or internal stress without contact. Having very small wavelength, X-ray radiation penetrates metallic materials and diffracts from irregular-lattice area caused by internal stress/strain, a common case in the production of composites consisting of non-metal and metal layers with different thermal expansion/contraction properties [31, 32]. X-rays have also been shown to be capable of evaluating the stress-strain relation from mechanical tests by single-angle, two-angle and \(\sin^{2}\Psi\) scattering modes [33, 34, 35]. The schematic of X-ray diffraction and the associated relevant coordinates is shown in Figure 3. The basic principle for evaluating strain due to external load can be represented as a function of latitudinal lattice spacing \(d_{\Psi\partial}\) as [32, 33]:  
\[\varepsilon_{\psi\phi}=\frac{d_{\psi\phi}-d_{0}}{d_{0}} \tag{2}\]  
where, \(d_{0}\) is the lattice spacing before deformation. The strain component can be determined by analyzing the X-ray radiation signal in different directions, i.e., at angles \(\Psi\) and \(\Psi\). Based on this basic principle, a numerical analysis was developed to obtain tomographic images of stressed materials in recent reports [36, 37, 38, 39]. X-ray computed tomography was further extended to evaluate mechanical performance and defects. [PERSON] et al. [40] and [PERSON] et al. [36] investigated the stress-strain property of polymers by X-ray tomography. They used a finite element method to model and in-situ test to validate elastic and hyper-elastic deformation properties of polymers under compression; [PERSON] et al. focused on the new technology of 3D printing and used X-ray to test the failure possibility of printed parts [37]; [PERSON] et al. inspected pointed rocks with X-ray and scanning electron microscope to find evidence of cracks under mechanical compression [39]. Although X-ray diffraction provides sensitive, reliable and quantitative stress-strain measurements and usually acts as a reference, such radiographic NDT has the obvious disadvantage of complicated instrumentation and harmful ionizing radiation which requires high level of operation and testing standards.