13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- the AE counts until the maximum force was reached are much higher for the softwoods supporting the interpretation that the softwoods behave more ductile and therefore build a process zone containing much more microcracks. It was also shown that the differences in macrocrack formation and propagation can be visible in the shape of the cumulated AE counts and the AE amplitudes. More recently, Ando et al., [6] used AET to examine the microscopic process of shearing fracture of old wood. In conclusion, the potential of laboratory AET to examine the fracture process in wood has been clearly demonstrated in the literature review. Simple AE approaches were used, for example: recording the events by expressing them in the form of event rate or cumulative events; study of amplitude distributions; investigation of frequency characteristics of emission events. All these approaches were performed by considering AE events without determination of source locations. The purpose of this study is to investigate mode I fracture of wood using determination of the position of developing cracks with AE measurements. The AE source is determined from the time differential of AE signals among two piezoelectric transducers (linear location). The determination of source locations associated to the attenuation curve in the tested sample allows correction of the output signal amplitude of the AE transducer. In this study, AE measurements were associated to digital image acquisition during the tensile test. 2. Experimental Procedure For the present study specimens were cut from softwood (Douglas) and conditioned, in climate-controlled room, to constant moisture content for about 10%. The temperature and relative humidity were regulated at 20 °C and 40% RH, respectively. The specimens had the following dimensions: W = 80 mm; L = 170 mm; T = 15 mm (Fig. 1). Next, a 50 mm notch was cut along the grain to create a mode I double cantilever configuration. The starter notch was introduced with a band saw (3mm) and the notch orientation was chosen such that crack propagation took place in longitudinal direction. Figure 1. Photography of mode I fracture specimen.
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