Issue 40

K. Kaklis et alii, Frattura ed Integrità Strutturale, 40 (2017) 1-17; DOI: 10.3221/IGF-ESIS.40.01 11 Figure 12: A typical CCNBD specimen fractured under mode II loading. At this point it should be mentioned that investigation of the closed vicinity of the ‘crack tip’, particularly in the case of mode-II loading, reveals that crack growth seems to start from the inner edge of the chevron slit and then propagates towards the corner points of the slit that are nearer to the points of application of the externally applied load (Fig 12).That seems to be the case even for mode-I loading (Fig.11), due, however, to reasonable imperfections detected in the final shape of the chevron slit. That phenomenon may, in a first approximation, be attributed to the fact that the corner points nearer to the points of application of the externally applied load are under a combined tensile and shearing stress field probably enhancing fracture, while the opposite ones are under a combined compressive and shearing stress field [13], probably not enchasing fracture. In this context, the SIF concept referring to mathematical cracks, should be considered with extra care, when it comes to slits. In addition, it should also be mentioned that the fracture patterns detected in the vicinity of the disc-jaw contact region (visible at the bottom areas of contact in Figs. 11, 12) and the possibility of premature fracture in these areas (even in the present case of the pre-CCNBD specimen), are probably attributed to frictional stresses developed at the contact areas to counterbalance the rotation tendency of the disc as a whole due to the presence of the slope slit [24]. This may be an area to be closely monitored in subsequent experiments by potentially using additional piezoelectric sensors at these regions of the specimen. Acoustic emission results The time evolution of various acoustic emission parameters during the CCNBD test of a representative specimen (specimen 2.3) subjected to mode I fracture (see Fig. 11) is shown in Fig. 13a-d. Observable AE activity starts at the time t=30 s of the test, i.e., just before the occurrence of the first macroscopic crack at the load of 4.3 kN. The final rupture of the specimen occurs at around 54 s, as it is evident from the corresponding drop in the load which is also accompanied with relatively high amplitudes of the recorded signals in all channels (see Fig. 13a). It is noteworthy that the first macro-crack is clearly distinguished in the recorded parameters since it is associated with an abrupt increase of recorded hits with high amplitudes (see Fig. 13a, c) and the corresponding observed peak of high hit rate at around 33 s (see Fig. 13b). In contrast, the recorded relatively high mean hit rate and the corresponding uniform increase of cumulative hits after 52 s are not sufficient to correlate them with the final rupture of the specimen. An important AE parameter which has also to be considered is the rise time (RT) of the recorded waveforms that gives us information about the fracture process [25]. It is evident from Fig. 13d that high values of RT are observed at around t = 32.5 s due to the formation of the first macroscopic crack and afterwards during the stage of the final rupture (53 - 54 s). In the intermediate region, the mean value of RT remains almost constant (RT < 250 μs), while the AE activity increases continuously. These low values of RT are related to the formation of tensile micro-cracks while, the high recorded values are due to the formation of mixed mode cracks (tensile and shear) observed either solely during the formation of the macroscopic crack at the initial stage of loading (around 32.5 s), or at the final stage where the coalescence of the existent cracks takes place, leading to the ultimate rupture of the specimen [25]. Notably, during the final rupture of the specimen (at 53-54 s), the load does not decrease abruptly as it could be expected, while, large values of amplitudes are still observed afterwards. This is attributed to the fact that although the specimen was separated into two parts (see Fig. 11), it retained its stability, while considerable AE activity is produced due to the friction between, either the two separated parts of the fractured specimen, or the mounting jaws and the surface of the specimen.