## 2 Noncontact Mechanical Property Testing Methods  
### Optical/Radiation Techniques  
The stress-strain relation of elastic materials is important because it is not only a way to characterize the hardness and brittleness of raw materials, but also reveals the mechanical status of a sample under test [14; 15]. Normally, the strain value is measured with a strain gauge affixed on the sample surface. Under extreme conditions such as high temperature, strain sensors along with special designs of insulating and high-Curie-temperature materials, are used for stress-strain measurements in contact configurations [16; 17; 18; 19]. Such a scheme cannot satisfy the requirements of many in-field applications due to difficulties with transducer bonding. To perform totally noncontact measurements, digital-image correlation (DIC), a.k.a. digital laser speckle (DLS), a purely optical imaging technique, was devised. This method was first proposed by [PERSON] et al. [20] with the basic principle shown in Figure 1. To avoid blackbody radiation from a high temperature surface, a visible-range expanded-beam laser is usually adopted in DIC measurement to generate diffuse-reflection-induced speckle patterns. A camera (with optical filter) is placed in front of the sample to record speckle images during tensile tests. By cross-correlating the digital images before and after tensile loading, the displacement field can be determined using the following equation [21]:  
\[\left( \begin{array}{c}u_{x}\\ u_{y}\end{array} \right)=\frac{1}{V_{i}}\left( \begin{array}{c}\Delta mP_{x}\\ \Delta nP_{y}\end{array} \right) \tag{1}\]  
where \(u_{x}\) and \(u_{y}\) represent the two orthogonal displacement fields in the camera, \(V_{i}\) is the image size and \(\Delta m\), \(\Delta n\) are the respective cross-correlation values. \(P\) stands for the pixel pitch which is determined by the resolution of camera. The ultimate strain field of interest can be further derived from the gradients of \(u_{x}\) and \(u_{y}\). Using the proposed method, [PERSON] et al. achieved strain measurements in tensile tests at 1200 Celsius degree with aluminum [21]. Four years later, [PERSON] et al. further increased the temperature to 3000 degree with an Ohmic heater and conducted strain field measurements using a similar DIC schematic [22]. [PERSON] et al. developed a time-dependent DIC system and dynamically recorded the change of strain field under a constant loading rate with respect to non-metallic composites [23]. By using laser extensometry, the local strain rate value can be characterized by time-dependent phase shift of correlated signal thus real-time measurement is possible. Another interesting topic is exploring the stress-strain behavior of alloys under high temperature which can reveal the strength of materials under extreme conditions, and also investigating how they react to thermal stress at such high temperatures. Such a thermal reactive character can be used to determine thermal compatibility with other materials in a heterogeneous structure [24]. [PERSON] et al. introduced an optical bandpass imaging system shown in Figure 2 to evaluate surface thermal strain without a surface irradiating laser source [25]. The bandpass filter allows only the violet-to-blue light to pass and avoids interferences from blackbody radiation at high temperatures. The use of an incoherent light source makes the system less costly but still sensitive [26]. The DIC of incoherent speckle imaging technique can also be used for real-time strain recording, as proposed by [PERSON] et al., to assist optimization of supersonic aircraft coating-material thermal shock processes [27]. Apart from industrial applications, noncontact optical measurements can also be used to perform strength evaluation of concrete in civil engineering using an alternative configuration of DLS.