The mechanical behavior of a fiber-reinforced concrete after extensive thermal damage is studied in this paper. Undulated steel fibers have been used for reinforcement. After being exposed to direct fire action at the temperature of 850°C, specimens were subjected to bending and compression in order to determine the loss of strength and stiffness in comparison to intact specimens and between the two types. The fire damage was assessed using nondestructive evaluation techniques, specifically ultrasonic pulse velocity (UPV) and acoustic emission (AE). Apart from the strong, well known, correlation of UPV to strength (both bending and compressive), AE parameters based mainly on the frequency and duration of the emitted signals after cracking events showed a similar or, in certain cases, better correlation with the mechanical parameters and temperature. This demonstrates the sensitivity of AE to the fracture incidents which eventually lead to failure of the material and it is encouraging for potential in-situ use of the technique, where it could provide indices with additional characterization capability concerning the mechanical performance of concrete after it subjected to fire.
In construction sector marble and granite are widespread because of their unique properties through the centuries. The issue of repair in these materials is crucial in structural integrity and maintenance of the monuments through the world, as well as in modern buildings. In this study fracture experiments on granite specimens are conducted. The goal is to compare the typical acoustic emission (AE) signals from different modes (namely bending and shear) in plain granite and marble specimens as well as repaired in the crack surface with polyester adhesive. The distinct signature of the cracking modes is reflected on acoustic waveform parameters like the amplitude, rise time and frequency. Conclusions about how the repair affects the mechanical properties as well as the acoustic waveform parameters are drawn. Results show that AE helps to characterize the shift between dominant fracture modes using a simple analysis of AE descriptors as well as the integrity of the specimen (plain or repaired). This offers the potential for in-situ application mainly in the maintenance of the monuments where the need for continuous and nondestructive monitoring is imperative, but always care should be taken for the distortion of the signal, which increases with the propagation distance and can seriously mask the results in an actual case.
Current work deals with the non-destructive evaluation (NDE) of the fatigue behavior of metal matrix composites (MMCs) materials using Infrared Thermography (IRT) and Acoustic Emission (AE). AE monitoring was employed to record a wide spectrum of cracking events enabling the characterization of the severity of fracture in relation to the applied load. IR thermography as a non-destructive, real-time and non-contact technique, allows the detection of heat waves generated by the thermo-mechanical coupling during mechanical loading of the sample. In this study an IR methodology, based on the monitoring of the intrinsically dissipated energy, was applied for the determination of the fatigue limit of A359/SiCp composites. The thermographic monitoring is in agreement with the AE results enabling the reliable monitoring of the MMCs’ fatigue behavior.
The characterization of the dominant fracture mode may assist in the prediction of the remaining life of a concrete structure due to the sequence between successive tensile and shear mechanisms. Acoustic emission sensors record the elastic responses after any fracture event converting them into electric waveforms. The characteristics of the waveforms vary according to the movement of the crack tips, enabling characterization of the original mode. In this study fracture experiments on concrete beams are conducted. The aim is to examine the typical acoustic signals emitted by different fracture modes (namely tension due to bending and shear) in a concrete matrix. This is an advancement of a recent study focusing on smaller scale mortar and marble specimens. The dominant stress field and ultimate fracture mode is controlled by modification of the four-point bending setup while acoustic emission is monitored by six sensors at fixed locations. Conclusions about how to distinguish the sources based on waveform parameters of time domain (duration, rise time) and frequency are drawn. Specifically, emissions during the shear loading exhibit lower frequencies and longer duration than tensile. Results show that, combination of AE features may help to characterize the shift between dominant fracture modes and contribute to the structural health monitoring of concrete. This offers the basis for in-situ application provided that the distortion of the signal due to heterogeneous wave path is accounted for.
The prediction of the remaining life of a structure can be assisted by the characterization of the current cracking mode.
Usually tensile phenomena precede shear fracture. Due to the different movement of the crack sides according to the
dominant mode, the emitted elastic energy possesses waveforms with different characteristics. These are captured by
acoustic emission sensors and analyzed for their frequency content and waveform parameters. In this study fracture
experiments on structural materials are conducted. The goal is to check the typical acoustic signals emitted by different
modes as well as to estimate the effect of microstructure in the emitted wave as it propagates from the source to the
receivers. The dominant fracture mode is controlled by modification of the setup and acoustic emission is monitored by
two sensors at fixed locations. Signals belonging to tensile events acquire higher frequency and shorter duration than
shear ones. The influence of heterogeneity is also obvious since waveforms of the same source event acquired at
different distances exhibit shifted characteristics due to damping and scattering. The materials tested were cement
mortar, as a material with microstructure, and granite as representative of more homogeneous materials. Results show
that in most cases, AE leads to characterization of the dominant fracture mode using a simple analysis of few AE
descriptors. This offers the potential for in-situ application provided that care is taken for the distortion of the signal,
which increases with the propagation distance and can seriously mask the results in an actual case.