Nondestructive assessment of cross-laminated timber using non-contact transverse vibration and ultrasonic testing

Cross-laminated timber (CLT) is a new generation of engineered wood product developed initially in Europe and has been used in North America for various applications. CLT is composed of at least three layers of lumber boards stacked crosswise in orthogonally alternating orientation to the adjacent layers. CLT has been gaining popularity and increasing application areas in the construction industry, such as building systems, mats for construction and temporary road applications as well as temporary bridges. It is especially identified as the prevailing material in modular construction for the future building industry. However, CLT as a relatively new structural material has limited studies related to their durability and long-term performance. Moreover, nondestructive evaluation methods to understand their mechanical behavior after exposure to harsh environment are lacking. This paper contributes to the development of a rapid nondestructive evaluation method to assess the mechanical properties using transverse vibration method and ultrasonic testing. The free vibration response triggered by the impulse excitation is captured using contact accelerometer and non-contact laser Doppler vibrometer (LDV). The LDV measurement is proven to respond similar to conventional accelerometers but more convenient as a rapid assessment method. While the longitudinal wave speed is measured by ultrasonic testing (UT) in a through-transmission mode, the modulus of elasticity was calculated in terms of measured wave speed. By investigating the different structural states and dimensions of CLT samples, the laboratory and the manufacturing facility, the influences of aging on effective dynamic modulus of elasticity (MOE) and damping are shown. The numerical and experimental results indicate the increase in moisture content decreases the fundamental frequency. Therefore, moisture content should be measured in order to examine the coupled effects of moisture content and aging on the fundamental frequency. The damping ratio extracted using half-power bandwidth method is shown as an additional information related to the moisture content. The non-contact transverse vibration method and UT are shown as an efficient nondestructive evaluation method for assessing the CLT structural state right after the manufacturing as a quality assurance method or after their service in field as a diagnostic tool.