# Acoustic Emission and Modal Frequency Variation in Concrete Specimens under Four-Point Bending
## Acoustic Emission Monitoring
### Results from Previous TPB Tests and Aim of Present Study  
In a recent paper [7], the AE parameters acquired during three-point bending (TPB) tests on notched concrete beams and a compression test on a cylindrical specimen were analyzed. These analyses were performed in order to identify the dominant fracture mode and to investigate the evolution of the released, dissipated, and emitted energies during the test and their mutual correlations. While the dissipated energy was determined directly from the experimental load-displacement curves, the emitted energy was obtained from the AE signals energy detected by the sensors. The proposed comparisons between the dissipated and emitted energies for three different specimen sizes--carried out both in terms of their cumulative values at the end of the tests and of the evolution of their rates during the loading process--suggest that there is no direct proportionality between them. In particular, a considerable increase in the fracture energy was evidenced by increasing the specimen size, whereas an opposite trend was obtained for the AE energy per unit surface [7]. According to the interpretation proposed in [25], the AE energy should be correlated to the surplus of elastic energy with respect to the dissipated one that is emitted in correspondence of unstable behaviors. The aim of the present study is to survey the damage process in notched concrete specimens by analyzing both the AE signals and the modal frequency variations. The specimens have the same  
Figure 2: Load–displacement curves representing (**a**) a catastrophic failure (single snap-back) and (**b**) a global softening behavior perturbed by multiple local instabilities (snap-backs). The grey areas identify the dissipated energy, \(D\), whereas the red (dashed) ones represent the emitted energy, \(E\). The total released energy, \(R\), is the summation of the two previous areas: \(D+E=R\). Reproduced with permission from [7], Elsevier, 2016.  
geometry and material properties as one of the beams considered in [7]. Here, they were tested in a four-point bending configuration instead of a three-point one. The former configuration, compared to the three-point bending scheme, may result in a more brittle behavior of the specimen, with unstable crack growth [28]. However, the four-point configuration is more suitable for the dynamic testing of a specimen notched at midlength; in fact, this avoids having a support--and therefore a fixed node in the modal shapes--in correspondence to the weakened section. As it is well known from structural dynamics, a decrease in the structural stiffness will result in decreased frequencies [29; 30]. Therefore, extracting the natural frequencies under loading can directly give information about the reduction in the beam global stiffness produced by crack development and propagation; this may be indirectly brought back to the crack advancement by means, for example, of numerical models [31; 32; 33; 34]. In the case under investigation, cracks would preferably develop near the notch. More in general, provided a sufficient number of AE sensors is mounted (the number adopted here was insufficient), AE signal analysis could allow for localizing cracks. Moreover, in a case such as the one under investigation, localization of AE sources could also allow for more information about the crack advancement. However, in complex full-scale structures (bridges, dams, buildings, etc. ), localization of cracks is not straightforward due to strong signal attenuation (especially for the higher frequencies), which would require a great number of sensors to detect each cracking event. Therefore, specific information about crack advancement are rather difficult to obtain from AE. On the other hand, information on the crack propagation may be obtained by analyzing modal parameters, especially if the damage location is known. Therefore, it results that combining AE and DI techniques may be of interest in the monitoring of large structures.