The aim of this research was to find relationships between the structure and fracture and fatigue behaviour of wood, and to investigate the micromechanisms of wood deformation and fracture using acoustic emission (AE). Matched-pair samples of clear Scots pine were prepared, using one half as a control and the other half as the test sample. Test samples were subjected to fatigue loading in tension, bending and torsion at various load simplitudes. Mechanical properties and AE of samples previously loaded in fatigue were investigated by monotonic loading to failure, and compared with the controls. It was found that cyclic loading caused no reduction in residual strength, and in some cases increases in strength relative to the controls were observed. A general decrease in stiffness after fatigue in bending was found to occur, and was related to compressive damage in the cell wall. Appreciable decreases in total work of fracture were found after cyclic loading in bending, primarily as a result of the decrease in the amount of deformation after initial fracture. Uniform increases in the logarithmic decrement of damping during fatigue indicated the progressive nature of damage ocurring during fatigue. AE recorded from samples loaded to failure after fatigue revealed a progressive displacement of the onset of flaw growth to higher levels of strain, and was attributed to the activation and development of microflaws during fatigue and the redistribution of stress and alleviation of stress-concentrating effects around flaws. Amplitude distribution analysis of the acoustic emission signals revealed small increases in the energy of events immediately prior to failure in fatigued samples, indicating an increase in high energy fracture events such as interlaminar shear. The strain energy release from various micromechanisms of failure was calculated and related to the electromechanical energy of the AE stress wave as measured at the transducer, Microcracks, such as those ocurring in the S2 reinforcement of the ceil wall were correlated with low amplitude AE events, whilst the energy release from intercellular cracks was related to higher amplitude events. AE recorded during load cycling indicates most flaw development occuring in the first half-cycle. In most cases the AE rate decreased rapidly over the next few tens of cycles, and stabilised to almost zero rate. Those that stabilised did not fail within the test period, whilst in those samples which failed the AE rate continued to rise sporadically.
|Date of Award||1983|