zyx zyxwvutsr General Technical Report FPL-GTR-239 • Proceedings: 19th International Nondestructive Testing and Evaluation of Wood Symposium The influence of sensor placement on in-situ ultrasound wave velocity measurement Francisco Arriaga Department of Forest and Environmental Engineering and Management. Universidad Politécnica de Madrid, Madrid, Spain, francisco.arriaga@upm.es Daniel F. Llana Department of Forest and Environmental Engineering and Management. Universidad Politécnica de Madrid, Madrid, Spain, danielfllana@gmail.com Roberto Martínez Department of Agroforestry Engineering. Universidad Santiago de Compostela, Lugo, Spain, robertodmartinezlopez@gmail.com Miguel Esteban Department of Forest and Environmental Engineering and Management. Universidad Politécnica de Madrid, Madrid, Spain, miguel.esteban@upm.es Guillermo Íñiguez-González Department of Forest and Environmental Engineering and Management. Universidad Politécnica de Madrid, Madrid, Spain, guillermo.iniguez@upm.es Abstract Ultrasound wave velocity was measured in 30 pieces of Spanish Scots pine (Pinus sylvestris L.), 90 x 140 mm in cross-section and 4 m long. Five different sensor placement arrangements were used: end to end (V 0 ), face to opposite face, edge to opposite edge, face to same face and edge to same edge. The pieces were successively shortened to 3, 2 and 1 m, in order to obtain these velocities and their ratios to reference value V 0 for different lengths and angles with respect to the piece axis for the crossed measurements. The velocity obtained in crossed measurements is lower than V 0 . A correction coefficient for crossed velocities is proposed, depending on the angle, to adjust them to the V 0 benchmark. The velocities measured on a surface, are also lower than V 0 , and their ratio with respect to V 0 is close to 0.97 for distances equal to or greater than 18 times the depth of the beam. Keywords: nondestructive techniques, sensors positioning, ultrasonic wave, wave velocity Introduction Non-destructive methods are used in the evaluation of existing timber structures, among other uses. For example, determination of wave transmission velocity makes it possible to estimate the mechanical properties of structural elements. Studies and laboratory research in this field are usually performed by measuring the Time-of-Flight (ToF) of the wave between the ends of the pieces. This is the best way to measure ToF, in a direction parallel to the axis of the piece and approximately parallel to the grain. But in practice, during the in situ inspection of timber structures this is not possible, 443