Detection of shear crack propagation on timber elements using acoustic emission tests

Timber elements often present significant cracks. Although some do not represent a reason for alarm, gross cracks have to be monitored and require an assessment of the structural soundness of the elements. For that purpose, an exploratory study was carried out to evaluate the use of Acoustic Emission, as a damage monitoring method, to validate the decision about the crack stability and integrity state of timber elements with shear cracks. An experimental campaign was made considering elements from two different species, pine and spruce. The small scale elements differ in growth ring direction (either radial or tangential) and different length of cracks were artificially made in the elements. The number of accumulated events was registered while the elements were subjected to 3-point bending tests. With consideration to the Kaiser effect, this work verified if by unloading the element and after loading it back to the previous load level, the number of accumulated events increased. This work also confirmed that near failure the number of registered acoustic emission events increases significantly. The aim of this work is to verify the adequacy of this process for the safety assessment of elements evidencing crack damage by monitoring the propagation of shear cracks.This work was partly financed by FEDER funds through the Competitivity Factors Operational Programme - COMPETE and by national funds through FCT within the scope of the project POCI-01-0145-FEDER-007633. The financial support of FCT, through national funds, within the scope of the project Protimber, PTDC/ECM-EST/1072/2014, is acknowledged.