PSI - Issue 64

Hernán Xargay et al. / Procedia Structural Integrity 64 (2024) 1790–1797 Hernán Xargay / Structural Integrity Procedia 00 (2019) 000 – 000

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AE energy is a parameter derived from the integral of the rectified voltage signal over the duration of the AE waveform, expressed in voltage-time units. By its definition, it offers the advantage of providing a clear indication of the relative magnitude among AE events. Similar trends were observed in other typical cumulative AE parameters, such as amplitude, duration, and signal strength, as in the case of cumulative AE energy. Typically, only a small amount of cumulative AE energy was recorded during the pre-peak branch relative to the post-peak branch. This initial phase is attributed to the initiation and growth of micro-cracks in the cement paste, releasing lower energy compared to the subsequent phase characterized by the localization and development of macro-cracks. Across all temperature scenarios, the AE energy rate becomes higher approximately between 90% of peak load at the pre-peak branch and 50% of peak load in the post-peak branch, which is consistent with observations made by other studies as in Alam and Loukili (2017). During this stage, the internal cohesive forces become activated, leading to a transition in the material from a continuum to a discontinuum state, characterized by the development of discrete macro-crack openings. Afterwards, energy release rate decreases and the slope of the cumulative AE energy curve flattens as the macro-cracks evolve primarily in width. However, lesser AE events continue to be recorded during the discontinuity progresses. In this stage, the AE events mainly result from friction caused by the interlocking of aggregates and the interaction between the surfaces of crack surfaces, in addition to micro-cracks at the fracture tip. An increasing dispersion in AE results with the increase of temperature was observed, attributed to the randomly distributed thermal damage along specimens affecting the development of load-induced crack paths. This aspect could potentially be improved using notched beams. Additionally, the natural heterogeneity of mortars is another contributing factor to dispersion. Based on the experimental results, it is interpreted that elevated temperatures weaken the matrix and interfaces, leading to irreversible damage such as micro-cracks with random direction but uniformly distributed. This results in the formation of a new fracture path that connects weakened areas in a more tortuous manner. It is conceivable that this new fracture path may experience an increased number of AE events, albeit with potentially reduced energy due to the damaged state of the material. Additionally, the presence thermal defects could contribute to greater attenuation of acoustic waves when propagating through the material. Being able to identify the fracture mode is valuable for estimating the root cause of cracks and conducting pathology analysis. Several studies, such as those by Ohno and Ohtsu (2010) and Aggelis (2011), have reported experimental findings evidencing the close correlation between the waveform of AE signals and the cracking mode. Furthermore, according to the recommendation of RILEM TC 212-ACD (2010), AE events can be graphically represented in a two-dimensional representation defined by the combined parameters of RA value (rise time over amplitude) and Average Frequency (AF, counts over duration) in order to assess the cracking modes. It has been observed that tensile fracture modes result in AE signals characterized by higher frequencies and shorter rise times. Conversely, signals associated with shear fracture modes exhibit lower frequencies, as well as longer rise times and durations. Fig. 4 depicts the graphs of RA value versus AF of AE hits for each thermal treatment. The moving average of RA value and AF of 50 hits have been calculated to mitigate scattering, consistent with the guidelines outlined in RILEM TC 212-ACD (2010). As the heating temperature increases, the AF tends to decrease while the RA value tends to increase. This trend is visually represented by the gradual displacement of the AE cluster from the upper-left corner to the lower-right corner of the plots. This shift suggests that as mortar undergoes more severe thermal degradation, the cracking behavior tends to transition towards more shear and mixed fracture modes. This remark aligns with the aforementioned increasingly eccentric paths and tortuous morphologies of cracks observed as temperature rises. Additionally, it can be observed from the time color scale that at the beginning of the pre-peak curve for lower temperatures, the events are mostly tensile in nature. Later, as the peak load is reached, shear/mixed modes become activated and continue to develop during the post-peak curve until nearing the end, where they tend to transition back to being more tensile. It is worth highlighting that, during the process of macro-crack formation, shear/mixed modes are activated. 3.3. Fracture mode assessment by AE