# Full-Field Vibration Measurements by Using High-Speed Two-Dimensional Digital Image Correlation
## 1 Introduction  
Vibration measurement provides an efficient way of characterizing structural safety and durability, which has attracted much attention in engineering and industry [1; 2; 3]. Therefore, many vibration measurement methods, including accelerometers and displacement transducers, have been developed [4; 5]. However, these early methods are generally contact-type ones, which can inevitably affect the natural frequencies of structures during the measurement and hardly be applied for characterizing lightweight and lightly damped structures. Laser Doppler vibrometers (LDVs) and continuous scanning LDVs (CSLDVs) emerged later [6; 7]. Benefitting from their wide frequency range, high spatial resolution, and non-contact measurement features, LDVs and CSLDVs have started to be used in many applications [8; 9; 10]. Nevertheless, during these applications, researchers found that LDVs and CSLDVs still face two problems [11]: (1) the measurement accuracy is easily affected by the movement of LDV or CSLDV devices during a test; (2) the scanning parameters of a CSLDV are hard to select when measuring the structures subjected to a random load. For these reasons, an alternative non-contact method is still desirable to characterize full-field vibrations conveniently and effectively. Digital image correlation (DIC) is an image-based full-field displacement and deformation measurement method developed in the early 1980s [12]. Since the previously developed two-dimensional DIC (2D-DIC) uses a single camera to capture specimen images and it measures only in-plane displacement on the specimen surface [13; 14; 15]. The method of three-dimensional DIC (3D-DIC) based on binocular vision was developed to overcome this limitation. Although the 3D-DIC requires two synchronized cameras and a relativelycomplex calibration process, it can measure both in-plane and out-of-plane displacements, which has allowed it to be used in an increasing number of applications in current experimental mechanics and material testing [16; 17; 18; 19; 20]. Recently, researchers discovered that a 3D-DIC could also be applied for full-field vibration measurements after the installation of synchronized high-speed cameras [21]. Due to its simple optical arrangement and low requirement of the experimental environment, the high-speed 3D-DIC (HS-3D-DIC) has rapidly become a competitive vibration measurement method, and many HS-3D-DIC-based vibration investigations have been subsequently carried out. For example, [PERSON] et al. investigated the dynamic behavior of rotating structures by using an HS-3D-DIC [22]; [PERSON] et al. used an HS-3D-DIC to characterize the vibro-impact absorber to help understand the interactions occurring during impact [23]; [PERSON] et al. studied the 3D dynamic deformation of the underwater rotor blades with the aid of an HS-3D-DIC [24]; and [PERSON] et al. realized the vibration-based damage identification in composite plates by using an HS-3D-DIC [25]. Very recently, [PERSON] and [PERSON] discovered that full-field vibration measurements using an HS-3D-DIC still have some problems [11]: (1) the method requires a pair of high-speed cameras, which significantly increases hardware investment; (2) the precise synchronization of high-speed cameras is very complicated; and (3) the intensity variations and non-linear geometric distortion between the image pairs decrease the measurement accuracy. They introduced a single-camera HS-3D-DIC method for full-field vibration measurements to address these problems. With a specially designed light split device, generally a four-mirror adaptor, two different views of the specimen surface can be projected onto two halves of the same camera sensor target. Therefore, complicated camera synchronization is no longer needed, and only one high-speed camera is required to measure vibrations. Based on this method, [PERSON] et al. studied the vibrations on a cantilever plate and a turbine blade [26]; and [PERSON] et al. investigated the flow-induced vibration of a flexible splitter plate [27].