WANG Na, WANG Yan, SU Yihui, ZHAO Haitao
(College of Civil Engineering and Transportation, Hohai University, Nanjing 210024, China)
Abstract: We completed the uniaxial tensile test of mortar in the range of strain rate from 10-6 to 10-4 s-1 in the section containing softening, and carried out acoustic emission monitoring (AE) simultaneously. A series of AE parameters and spectrum analysis methods were used to identify the damage evolution process and cracking mechanism of mortar at different strain rates. The results show that, with the increase of strain rate,the peak stress and tensile elastic modulus of mortar increase obviously, and the stress level corresponding to the starting point of AE activity increases significantly as well, which indicates that the mechanical properties and AE characteristics of mortar have obvious strain rate effect. With the increase of strain rate, the cumulative AE hit decreases gradually, while the average AE hit rate increases significantly, indicating that the increase of strain rate reduces the damage degree of internal microstructure of the specimen, but the crack propagation speed increases. In the pre-peak stress stage, the average of AE ringing count and signal energy decreases with the increase of strain rate, while the average of duration increases; in the post-peak stress stage(ft - 30% ft), the average of the three AE parameters all increase with the increase of strain rate, indicating that the strain rate effect on the damage process of mortar is different before and after peak stress, and the damage mechanism represented by different parameters is also different. In the whole process of uniaxial tensile of mortar, with the increase of strain rate, the scatter distribution of AE frequency-amplitude becomes more discrete, and the b-value shows a decreasing trend. In addition, the average level of AE peak frequency decreases with the increase of strain rate, while that of ca8 band wavelet energy spectrum coefficient increases. It is indicated that the increase of strain rate enables the crack propagation state of mortar specimen to become unstable, and the width of macrocrack increases but the proportion decreases.
Key words: mortar; acoustic emission; uniaxial tension; strain rate
As an important component of concrete, mortar is also a widely used building material in masonry structure. Its microstructure properties often play a crucial role in the performance and reliability of concrete and masonry structures. However, due to the existence of bubbles in mortar, the bond strength between cement and sand is weak[1], and the tensile strength and ultimate deformation of mortar are small[2], thus the mortar structure is often damaged due to overextension of tensile stress.
In recent years, the mortar strain rate has been discussed due to the increasing application of mortar in civil infrastructure and several new fields[3]. In fact, it has been experimentally revealed that the mechanical behavior of mortar depends on strain rate over a wide range of conditions[4], while the dynamic scenarios of building materials often needs to be considered in structural design and critical infrastructures evaluation[5]. At present, the research on dynamic mechanical properties of mortar materials has become an important subject in structural engineering.
Despite the large number of loading tests on the mechanical properties of mortar fracture behavior, the tests aiming to investigate the fracture damage mechanism are scarce. As a typical quasibrittle material, the heterogeneity is one of the main characteristics of mortar, therefore, deformation and crack propagation often occur under the action of stress[6].AE technology, as one of the main techniques for monitoring crack development, draws a great attention to the application of detection and diagnosis in material testing[7]. Therefore, it has great potential in characterizing the damage and crack behavior of engineering materials such as concrete[8,9],, mortar[10,11,-12]and granite[13,14]. Stergiopoulos C[15]measured the pressure stimulated currents and acoustic emission of cement mortar beams under three-point bending test at the same time, proving that the AE technology can well characterize the propagation process of tensile microcrack in mortar. Sagar R V[6]made clear that the similarities and differences of micro-fracture activities in each stage of the whole fracture process of the mortar and concrete specimens. Rouchier S[16]found that scattered AE signals were generated along with the distribution of micro-cracks in the mortar, and then the strong AE activities were generated during the expansion of macro-cracks. Wang Y[17]used AE energy,RA value, AF value and b value to analyze the damage evolution process, failure mode and damage degree of polypropylene fiber reinforced mortar under uniaxial compression, found that the mortar specimens cracks appeared in the initial stage, then developed further in the middle stage, and the fibers were pulled out on the later stage. Wang Y[18]found that the AE wavelet energy spectrum coefficients of different energy bands can be used to evaluate the anti cracking effect of basalt fiber under axial compression, to warn the failure and instability of specimens, and to identify the damage degree of specimens.
In order to directly reflect the dynamic tensile properties and damage mechanism of mortar and explore the influence of different strain rates on the damage process of mortar, the dynamic uniaxial tensile tests of mortar specimens under three different strain rates are designed, and the whole damage process of each specimen is monitored by acoustic emission technology. For unification of the loading process of the specimens under three strain rates, the data of the stress drop section after 30% of peak stress is eliminated.By using AE analysis methods, like that time-history normalized curve of cumulative AE hits and hit rate,analysis of typical AE characteristic parameters,amplitude distribution and spectrum characteristics,the strain rate sensitivity of mortar is thoroughly and meticulously discussed, the dynamic uniaxial tensile damage mechanism is further explored, and the damage characteristics are identified.
2.1 Specimens preparation
In this experiment, ordinary portland cement CEM I 42.5N and river sand with fineness modulus of 2.1 were used for preparation of cylindrical mortar specimens, and the mix proportion of mortar specimens was cement/sand/ water: 1/2/0.5. First, the cement and sand were mixed evenly, then water was added to mix the mixture. In order to ensure the uniformity of the specimen, the mixture was first poured into a steel mold with the dimension of 300 mm × 300 mm × 1100 mm,vibrated close-grained and placed at room temperature for 24 hours before removing the mold. Then, after 28 days of spraying water curing, core samples with a diameter of 74 mm were drilled by coring machine,and the laitance coating and uneven parts at both ends of the core samples were cut off by cutting machine to obtain a 150 mm high specimen with parallel and flat ends. Finally, FC-SRS adhesive was used to bond the force transfer steel plates (Φ74 mm × H40 mm) at both ends of the cylinder specimen. In order to reduce the experimental error caused by the age of materials and specimens, the test materials were purchased from the same batch at one time, the specimens were all prepared in the same period, and the test was completed in a short time.
2.2 Equipment
The loading device was an electro-hydraulic servo material testing machine with a multi-channel Flex Test GT digital controller. Three extensometers which could be adjusted in real time were used to control the displacement, so as to avoid the influence of eccentric force and achieve the desired effect.
AE signal acquisition equipment was the PCI-2TMAE acquisition system of PAC company, and the matching AE winTMsoftware was mainly used for data acquisition and preprocessing. The bandwidth of preamplifier (model: PAC-2/4/6) was 10 kHz – 2 MHz,the gain was 40 dB, the threshold value was 35 dB,the sampling frequency was 5 MSPS, and the range of filter bandwidth was 1 kHz - 3 MHz.
2.3 Test procedures
Before the test, the surface of the pre-placed sensor should be polished and smoothed first, and an appropriate amount of Vaseline was coated to ensure the coupling effect between the specimen and the probe sensor. Then a wide-band sensor (model: PAC-WD)with a frequency bandwidth of 100 kHz to 1.0 MHz was selected to fix in the middle of the specimen with a rubber belt. When fixing the sample, the force transfer steel plates should keep specimens at both ends as parallel as possible to avoid excessive eccentricity during loading. Finally, lubricating grease was applied to the spherical hinge and other parts to reduce friction noise, and the rotation ability of spherical hinges could be increased to further reduce the effect of eccentricity during testing.
The mortar specimens were loaded by means of displacement control of the three loading rates shown in Table 1, and the AE acquisition system was monitored synchronously until the loading was completed. During the test, the load and its corresponding time can be recorded by the machine simultaneously, and the AE signals can be collected and recorded by the AE acquisition system automatically.
Table 1 Specimen number and loading system in uniaxial tensile test
3.1 Analysis of AE signal parameters
AE parameters are numerous, and the effectiveness varies greatly. At present, scholars have certain subjectivity in the selection of characteristic parameters, and there is no uniform regulation.Therefore, the appropriate parameters and analysis methods should be selected based on the damage phenomenon of materials to analyze their damage mechanism.
3.1.1 Time-history curve analysis of cumulative AE hits and hit rate
AE hit refers to the transient AE signal detected by a single channel, while AE hit rate refers to the number of hits per unit time, which can reflect the total amount and frequency of signals respectively.To identify the expansion scale and velocity of cracks in the whole process of uniaxial tension of mortar specimens at different strain rates, in current paper, the cumulative AE hits and AE hit rate were selected for time-history curve analysis. The cumulative AE hits time-history curve was normalized,i e, the ratio of loading time to total loading time was taken as the time coordinate.
3.1.2 Analysis of typical AE characteristic parameters
In the AE testing, AE parameters such as ringing count, energy, duration, amplitude and threshold value are used to evaluate the damage process of materials, as shown in Fig.1. Based on the reference of domestic and foreign literature, aiming at the characteristics of mortar materials, three typical AE characteristic parameters of the AE ringing count, duration and signal energy,were selected and analyzed to reflect the effect of strain rate on the number of cracks before and after the peak stress of the specimen, as well as the generation and propagation rate.
Fig.1 AE signal characteristic parameters
3.1.3 Amplitude distribution analysis
Different fracture patterns will produce different types of AE signals with different frequency range and amplitude, which may be related to the damage degree of the structure. A large number of small amplitude AE events are generated by microcracks, while few large amplitude AE events are generated by macro cracks[19].According to the basic knowledge of seismology,as can be seen in Eq.(1), the slope of the amplitude distribution can be defined as b-value[20], which is known as an effective index related to the fracture state of the materials, commonly used to identify macro- and micro-cracks.
whereAdBis the peak amplitude of AE events in decibels,Nis the increment frequency, which is the number of AE events with amplitude greater than the threshold,ais the empirical constant andbis theb-value based on AE.
3.2 Analysis of spectrum characteristics of AE signals
3.2.1 AE peak frequency analysis
The spectrum characteristic of the AE signal is an important content of AE monitoring signal analysis.The power spectrum characteristics can be obtained by Fourier transform of AE waveform, and the peak frequency is the frequency corresponding to the peak value of power spectrum. The ringing count, energy and amplitude of AE signals emitted by different damage sources may be the same, but their peak frequencies are generally different. Therefore, the AE peak frequency can be used as a parameter to analyze the failure process of materials, which can achieve good results[21,22].
3.2.2 AE wavelet energy spectrum coefficient analysis
The AE signal can be decomposed into different frequency bands by wavelet transform technique[23],and the energy components of signal in different frequency bands are called a wavelet energy spectrum coefficient. The energy distribution of signals generated by different AE sources is diverse in varied frequency bands, which can be characterized by an AE wavelet energy spectrum coefficients. According to previous studies[8],andare recorded as the AE signal energy of the low-frequency and high-frequency corresponding to the decomposition scale, respectively,andEf(n) is the total energy of the signal. The equations are as follows:
Then, the wavelet energy spectrum coefficients of each frequency band can be expressed asrEJAandrEjD,and the equations are as follows:
The sampling frequency of this experiment was 5 MHz, according to previous analysis and the characteristics of the concrete and its constituent material test. The wavelet basis coif5 was selected to carry out the wavelet threshold denoising of AE waveforms, which were decomposed to eighth scales.The frequency ranges of cd1-ca8 frequency bands are(in kHz): [1 250-2 500], [625-1 250], [312.5-625],[156.5-312.5], [78.5-156.5], [39.5-78.5], [19.5-39.5],[9.5-19.5] and [0-9.5].
4.1 Strain rate effect of mechanical properties and AE activity characteristics
The average of peak stress of mortar specimens at different strain rates is shown in Table 2 and is plotted in Fig.2. It can be seen that the peak stress of mortar increases with the increase of strain rate, as expected.The tensile elastic modulus of mortar is defined as the secant modulus at the 50% peak stress. And the tensile elastic modulus of mortar defined as secant modulus at 50% peak stress also increases with the increase of strain rate, as shown in Table 2. It indicates that the mechanical properties of mortar have obvious strain rate sensitivity. As a kind of heterogeneous material with internal damage, mortar is generally considered to be more brittle than concrete[6], thus the change degree of its microstructure decreases with the increase of strain rate[24]. As a result, the tensile strength increases and deformation capacity decreases. This strain rate sensitive behavior is not only related to the constitutive characteristics of materials, but also to the evolution of internal damage of specimens.
Fig.2 The peak stress and the initial tensile stress level at different strain rates
Table 2 Mechanical properties and AE activity characteristics of mortar
In addition, it can be seen from Table 2 and Fig.2 that the time when the AE activity starts to appear always lags behind the starting point of specimen loading, and the average of stress level at the AE signal starting points, which is called the initial tensile stress level, increases significantly with the increase of strain rate. It can be inferred that at the initial loading stage of the specimen, the stress level is low, and the mortar is mainly cracked by the original cracks, so the activity of AE is very weak, and no AE signal is detected by the sensor[25,26]. Subsequently, the microcracks develop further, producing AE signals that are higher than the threshold. The higher the strain rate, the later the concentration of AE activities begins, indicating that the internal deformation of mortar and the development of cracks lag behind.
4.2 Time-history curve analysis of cumulative AE hits and hit rate
4.2.1 The normalized time-history curve analysis of cumulative AE hits
For visual comparison of the cumulative AE hits of the mortar specimens at different strain rate in the process of loading, the loading time of 30% peak stress of mortar specimens for post-peak stress was normalized, and then the average value of cumulative AE hits of mortar specimens under different strain rates was calculated. The normalized time-history curve of the cumulative AE hits of mortar specimens at three different strain rates was drawn as shown in Fig.3. The average of AE hits at the strain rates of 10-6s-1, 10-5s-1, and 10-4s-1are 4 375, 2 230 and 828, respectively.The cumulative AE hits become less with the increase of strain rate, which shows that the internal defects of the specimen at high strain rate are not fully developed.In addition, the stress peak of mortar specimen is relatively delayed with the increase of strain rate,which further indicates that the lag of the deformation and cracks develop inside the mortar.
Fig.3 The normalized time-history curve of AE cumulative hits at different strain rates
By analyzing the variation trend of each curve separately, it can be seen that a sudden increase in the curve of cumulative AE hits at the strain rate of 10-6s-1has a sudden increase, while that at the strain rates of 10-5s-1and 10-4s-1are flat. The reason is that the mortar specimen under low strain rate completely loses bearing capacity after sufficient damage evolution,resulting in more small microcracks. However, under high strain rate, the cracks develop rapidly in a short time, and the damage of the specimen is relatively uniform under high strain rate.
4.2.2 Time-history curve analysis of AE hit rate
The time-history curves of AE hit rate and tensile stress of mortar specimens are given in Fig.4. It can be seen that the variation trend of AE hit rate is consistent with the stress curve, the AE hit rate increases sharply when the stress drops abruptly, and the AE hit rate is more uniform when the stress changes gently.According to the calculation, the peaks of AE hit rate at the strain rate of 10-6, 10-5and 10-4s-1are 66, 440 and 640 s-1, while the mean values are 15.1, 81.6 and 281.2 s-1, and the standard deviations are 14.6, 81.6 and 175.7 s-1respectively. It can be concluded that the AE hit rate increases significantly with the increase of strain rate, and the fluctuation range of hit rate curve also increases significantly, indicating that the higher the strain rate is, the faster the crack is generated and expanded[27,28].Under the condition of high strain rate,the stress of the mortar specimen has reached a higher level before the merger of defects, leading to more defects which will generate more intense AE signals.This AE characteristic is also fully reflected in the concrete materials[29].
Fig.4 Tensile stress and AE hit rate as functions of time for mortar at different strain rates
In addition, it can be seen that the AE hit rate curve increases gradually before the peak stress,especially near the peak stress, which indicates that a large number of AE occur near the peak stress. After the peak stress, the AE rate reaches the maximum quickly. It is pointed out in the literature[16]that high AE rate occurs during the sudden decrease of stress caused by macro crack growth, which is fully confirmed in this paper. After that, the hit rate curve enters into the descending stage. The descending trend of stress curve is obviously different under different strain rates, so it can be seen that the trends of AE hit rate curve vary considerably: When the strain rate is 10-6s-1, the stress curve first drops abruptly in stages, then gently falls,and the corresponding AE hit rate shows an obvious peak at the moment of stress sudden drop, and keeps a low value; when the strain rate is 10-5s-1, the stress curve first declines rapidly and then declines gently.The AE hit rate curve shows a slow decline trend after reaching the peak value at the inflection point of the stress curve; When the strain rate is 10-4s-1, the stress curve shows a zigzag downward trend and the AE hit rate curve fluctuates greatly, indicating that the failure pattern of specimen may change greatly with the increase of strain rate.
4.3 Variation of AE typical characteristic parameters with strain rate
Here, three typical AE characteristic parameters of AE ringing count, duration and energy are selected,and their cumulative and average values pre- and postpeak stress at different strain rates are calculated, which are summarized in Table 3 and Table 4. It can be seen that with the increase of strain rate, the loading time of the pre-peak stress stage decreases linearly, which indicates that the loading rate of the specimen is well controlled during the test process. But the loading time of the post-peak stress stage is affected by the brittleness and failure pattern of mortar, and no longer decreases in proportion.
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Table 3 Typical AE characteristic parameters in the pre-peak stress stage of mortar
Table 4 Typical AE characteristic parameters in the post-peak stress stage of mortar
In addition, it also can be seen that with the increase of strain rate, the average of AE ringing count and energy decreases gradually in the pre-stress stage,and increases gradually in the post-stress stage. It indicates that at the high strain rate, the microcracks of mortar specimen in the pre-stress stage cannot fully crack, and energy accumulates inside the specimen,resulting in the rapid expansion of the cracks and the release of energy in the post-stress stage. However,the average of AE duration increases both before and after the peak stress, and increases significantly when the strain rate is 10-4s-1, which is similar to the law of concrete materials[30]. The reason is that when the strain rate is increased to a certain extent,the formation and expansion of cracks in the whole process of mortar loading are significantly accelerated,and the damage evolution of microstructure is more continuous. Moreover, the values of the three typical AE characteristic parameters in the post-peak stress stage are always lower than those in the pre-peak stress stage at the strain rate of 10-6s-1, but they are all significantly higher than those in the pre-peak stress stage at the strain rate of 10-4s-1, which indicates that the damage evolution of mortar specimens lags behind obviously under the condition of high strain rate.
4.4 Distribution analysis of AE amplitude
Different patterns of fracture damage are accompanied by distinct AE signals, whose frequency and amplitude distributions are also diverse. The AE frequency-amplitude distribution and its fitting curve of AE signals associated with the mortar damage and failure process at different strain rates is shown in Fig.5. It can be observed that the scatter points of frequency-amplitude at different strain rates are all distributed in bands, and the distribution range can be distinguished clearly. And the AE hits with low amplitude are mainly distributed above the curve, while that with high amplitude are usually scattered below the curve. The higher the strain rate, the smaller the negative gradient of frequency-amplitude, that is, the smaller theb-value. As is well-known, higherb-value is always accompanied by plentiful lower-amplitude AE signals, which represents the formation of new cracks and the slow growth of cracks. Otherwise, it represents the unstable and rapid growth of cracks[31].
Fig.5 AE frequency-amplitude distribution and fitting curve at different strain rates
Table 5 lists the overallb-value and its goodness of fitR2of mortar under uniaxial tensile at different strain rates. It can be seen that as the strain rate increases from 10-6to 10-4s-1, theb-values are 0.94, 0.89 and 0.85, respectively. It indicates that the microcracks have not enough time to evolve slowly, but directly produce larger cracks and release more energy at higher strain rate. Furthermore, with the increase of strain rate, the goodness of fitR2has a tendency to decrease. It can be concluded that the microcracks propagation is a slow and gradual process at low strain rate, and as the strain rate becomes higher, the microcracks propagation in different directions will merge and form local macro cracks, which tends to be a sudden crack propagation process, thus the AE frequency-amplitude curve gradually deviates from the linear.
Table 5 b-value and its goodness of fit R2 of mortar
4.5 Analysis of spectrum characteristics of AE signals
In order to reveal the damage characteristics of different stress stages in the whole process of mortar uniaxial tension under different strain rates more carefully and accurately, two stress stages of pre-peak and post-peak of mortar were divided into corresponding small sections according to the section of 10% peak stress, and the small sections before peak stress were numbered by 1, 2...10 in turn, while that after peak stress were numbered by 11, 12... Taking mortar specimen M01 as an example, the division result is shown in Fig.6.
Fig.6 Division and numbering of stress sections in full curve of M01 specimen
4.5.1 AE peak frequency analysis
The AE peak frequency curve of each stress section of mortar under uniaxial tension with different strain rates is shown in Fig.7. It can be seen that with the increase of the stress level, the AE peak frequency in the pre-peak stress stage increases gradually, while in the post-peak stress stage, it basically keeps a stable and constant trend. Moreover, the higher the strain rate is, the lower the peak frequency is. Relevant research points out that high-frequency AE signals are usually produced by small-scale fracture, andvice versa[32], which means that during the pre-peak stress stage, the internal damage of the specimen is mainly the generation of microcracks. After the peak stress,new microcracks are generated, while the existing microcracks continue to expand into macro cracks,resulting in the final complete failure of the specimen.In addition, with the increase of strain rate, the mortar is not able to get sufficient damage evolution and is directly damaged, resulting in the increase of crack width.
Fig.7 The average of AE peak frequency at different strain rates
4.5.2 AE wavelet energy spectrum coefficients analysis
After wavelet decomposition of AE signals, the energy information components of frequency bands in the signals will be expressed in each decomposition scale component. The average wavelet energy spectrum coefficient of each section of mortar was calculated,and the wavelet energy spectrum coefficient of ca8 band (0-9.5 kHz) was selected for analysis, as shown in Fig.8. It can be seen that, with the increase of stress level, the ca8 band wavelet energy spectrum coefficient of mortar increases gradually in the pre-peak stress stage, while in the post-peak stress stage, it basically maintains a stable and constant trend.
Fig.8 The average of ca8 band wavelet energy spectrum coefficient at different strain rates
Compared with the published research results of uniaxial tension test of concrete based on AE technology[29,33], it can be seen that the AE characteristics of mortar during uniaxial tension at different strain rates have some commonness and difference with concrete. The commonality lies in:First, the initial fracture starting point of mortar and concrete specimens in the loading process can be effectively obtained by AE technology, and has a significant strain rate effect. Second, with the increase of strain rate, the cumulative AE hits of mortar and concrete arerdecrease, while the hit rate increases,that is, the amount of the AE activities of concretelike materials decreases and the frequency increases.The reason is that the AE response of concrete-like materials is dependent on the process of time, when the strain rate is low, the development of microcrack has more sufficient time, and the damage evolution of the specimen is gentle. When the strain rate is high,the microcrack is not fully developed, and the damage evolution is rapid.
The differences between mortar and concrete are as follows: First of all, with the increase of strain rate,the average of AE count and energy in the pre-peak stress stage of mortar decreases, while that in the prepeak stress stage of concrete increases obviously, which indicates that the damage evolution of the specimens in the pre-peak stress stage is influenced not only by the strain rate, but also by its own internal structure and composition. Secondly, with the increase of strain rate,the average of AE count, signal energy and duration of mortar in the post-peak stress stage changes from lower than to higher than the pre-peak stress stage,while the average of three AE parameters of concrete in the post-peak stress stage is always higher than that of the pre-peak stress stage. It indicates that the mortar is greatly influenced by the strain rate owing to its uniform material and the consistent crack development path. However, the cracks of concrete mainly develop along the interface between mortar and aggregate in the pre-peak stress stage, and pass through the aggregate directly in the post-peak stress stage, which depends more on the microstructure of concrete besides the effect of strain rate. Thirdly, with the increase of strain rate, the ca8 band wavelet energy spectrum coefficient of mortar obviously decreases, while that of concrete obviously increases. It indicates that with the increase of strain rate, the proportion of macro cracks in mortar decreases, while that in concrete increases, which further illustrates the differences in the strain rate effect of their damage characteristics as a result of the distinction between the microstructures of mortar and concrete.
The AE signals associated with the damage process of mortar under uniaxial tension load at different strain rates are analyzed, it is found that the strain rate effect of AE characteristics of mortar is closely related to its damage evolution process. The main conclusions are as follows:
a) With the increase of strain rate, the initial time of AE signals of mortar is delayed，and the cumulative AE hits decreases, while the peak and mean value of AE hit rate significantly increases. It indicates that the increase of strain rate leads to the lagging of the the deformation and cracking of mortar, and the damage degree of mortar internal microstructure decreases,a large amount of energy accumulates, resulting in the acceleration of crack propagation speed, which is consistent with the performance of concrete AE characteristics.
b) With the increase of strain rate, the average of AE ringing count and energy decreases, and the average of duration increases in the pre-peak stress stage, while the three AE parameters all increase in the post-peak stress stage. It indicates that the strain rate effect on different AE parameters is different, and the strain rate effect on the AE characteristics of mortar before and after the peak stress is also different.
c) With the increase of strain rate, the distribution range of AE ringing count, energy and durationversusAE amplitude shows regular changes throughout the loading process, theb-value decreases gradually, and the scatter of AE frequency-amplitude is more and more discrete. The results show that the AE parameters can be used to identify the damage degree of mortar at different strain rates, and the increase of strain rate makes the propagation state change of microcrack change from gradual to sudden.
d) With the increase of strain rate, the average level of AE peak frequency decreases, while the overall level of ca8 band wavelet energy spectrum coefficient increases, which indicates that the increase of strain rate increases the width of macro cracks in mortar, but decreases the proportion of macro cracks in mortar.
Conflict of interest
All authors declare that there are no competing interests.
