Table 1 indicates the dimensions of the test specimen and the maximum compressive stress (fc) obtained during the short-term creep tests.
In parallel with the deformation measurements, the acoustic emission activity was detected by means of two AE sensors, one on each side of the specimen. The sensors are attached to the masonry by means of a thin metal plate which is carefully glued on the surface. A vacuum grease is used as a couplant in between the sensor and the metal plate. The preamplifier gain is set to 49 dB and a threshold level of 38 dB is applied. The measurements were carried out with equipment from Vallen Systeme, type AMS3.
Figures 9-10 shows a clear relation between the acoustic emission measurement and the stress path followed during the accelerated creep tests as well as during the cyclic accelerated creep tests. At every stress increase, a peak in acoustic emission detection is noticeable. After this peak value, the AE detection drops to a much lower level. During the first steps of constant stress, this AE detection level is very low and fades away after about half an hour. As the stress level increases, the AE detection during constant stress also increases, which results in an AE rate increase between successive load steps. This is clearly visible through the increasing slope of the cumulative curve and can be related to the strain rate increase, noticed from the
Figure 10. Acoustic emission measurement during cyclic accelerated creep test (CACT), events indicated in time by differential bars and cumulative (above). Load increment steps during CACT (below).
graphs which indicate the evolution of strain in time (Figs 7b-8b).
During the CACT's, it was noticed that a negligible amount of AE events were detected during the reloading step until the stress level, reached during the preceding load step, was exceeded. This phenomenon is in literature described as the Kaiser effect, named after Joseph Kaiser who first investigated this phenomenon in the 1950's.
To enable a closer analysis of the AE data, the instantaneous and the time-dependent damage detection will be split by separating the emissions during stress increase from the ones during constant stress intervals. In terms of strain, this can be seen as the damage occurring during elastic deformation and the damage during time-dependent deformations. Only the CACT is included in this analysis, as a clearer distinction can be made between both phases for this type of test.
3.2.1 AE detection during stress increase Figures 11-12 show results for CACT on two masonry cores. The elastic modulus first increases, due to the
compaction of the material, until a maximum value is reached and then decreases due to the damage increase.
As the number of detected counts is proportional to the total emitted energy, and therefore related to the total damage, the AE measurement is presented in terms of AE counts. The detected AE counts during stress increase appear to evolve inverse compared to the Young's modulus, with a pronounced increase during the last load steps. This indicates that much more damage is growing during these last stress increases.
As not all stress increase steps were equally large, the AE counts indicated are calculated per unity of stress. And to take into account the Kaiser effect, only that part of the stress increase is considered which exceeds the maximum stress level of the previous step.
3.2.2 AE detection during constant stress level As the strain rate increased for the successive load steps during both ACT and CACT, clearly showing a stress-dependent behaviour, this behaviour is also expected for the AE count rate. Figure 13 confirms
this behaviour. During the last steps, the AE count increases for both cyclic accelerated creep tests presented. This means that the damage accumulation rate during constant stress depends on the load level. The stress in Figure 13 is indicated relatively in relation to the maximum stress obtained during the creep tests, which is indicated as 1.
As not all constant stress intervals had an equal duration, the AE count rate (counts per second) is indicated in stead of the total amount of counts during a period of constant stress.
During the last load step, a very high, unstable damage growth is detected which leads to failure of the specimens.
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