**4. Discussion about the simplification**

#### **4.1 Applicability of peak picking**

The applicability of peak picking, one of the simplest methods for identifying third bending mode frequencies, was investigated. A Fourier spectrum was created by a 0.5-s acceleration at the midspan of sleepers just after an impact excitation. Peak frequencies corresponding to the third bending mode were picked up from the range of 500–1000 Hz frequency according to the aforementioned results discussed in this study.

**Figure 17** presents identified third bending mode frequencies by ERA and peak picking. Both sets of results show excellent agreement; thus, peak picking can effectively identify the third bending mode frequency (i.e., suitable detection indicator) based on a simple measurement using only a single accelerometer.

**Figure 18** presents measured FFT spectra of three tests, which translates to a certain reliability level within the peak picking method. Peaks of three measured spectra were in good agreement for each test sleeper. It should be noted that only damaged Sleeper Nos. 3, 4, 12, and 14 have some minor peaks and discrepancies at frequency ranges other than the peaks between test cases. This complex influence might be caused by nonlinearity or nonstationarity due to damage; thus, there is some degree

**89**

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers*

of possibility that such damage can be detected by focusing on those influences. The peak frequency of concrete sleepers is, however, reduced because of damage and can be easily and reliably found by the peak picking method in the absence of information regarding nonlinearity or nonstationarity. Thus, it can be concluded that peak picking is sufficient for damaged concrete sleeper detection as maintained in this study.

*Reliability of measured spectrum based on TEST I: (a) Sleeper No. 1; (b) Sleeper No. 3; (c) Sleeper No. 4; (d)* 

In the quest for additional simple measurement techniques, damage detection by sound-level meters that do not require the installation of accelerometers was

Structural vibrations can be propagated to peripheral regions as acoustic radiation via the air. Thus, sound pressure caused by impulse hummer test has possibility to be used for indirectly identification of concrete sleepers. Considering the modal characteristics and convenience of practical use, this study sets the sound-level meter position to above the midspan of the sleepers, which can match the antinode positions of the third bending modal shape. In addition, this feasibility study experimentally investigated the effect of each mode on measured sound pressure and then provided an optimized measurement method to ultimately obtain the third bending mode frequency of test sleepers to apply it for practical uses.

**Figure 19** shows Fourier spectra of acceleration responses and sound pressure. The acceleration spectra at the midspan and near the rail seats of a sleeper, in addition to sound pressure, are depicted in the **Figure 19**. **Figure 19(a)** presents the results when an impulse hammer excitement occurs at the midspan, and **Figure 19(b)** corresponds to when the hammer is excited at the rail seat. **Figure 19** indicates good agreement between spectra of acceleration and sound pressure around the frequency peak corresponding to the third mode (750–800 Hz). This fact supports the applicability of sound pressure measurement as a robust tool for damaged-sleeper detection. Another peak of sound pressure(s) in the realm of 350–400 Hz corresponds to the accelerations not at the midspan, but exclusively at the rail seat. Thus, these peaks are caused by second bending vibrations of sleepers. The sound pressure can thus surveil not only the third mode but also the second mode. However, when comparing

experimentally investigated focusing on Sleeper Nos. 7, 8, and 16.

*DOI: http://dx.doi.org/10.5772/intechopen.82711*

**4.2 Applicability of hammering sound**

*Sleeper No. 9; (e) Sleeper No. 12; and (f) Sleeper No. 14.*

**Figure 18.**

**Figure 17.** *Comparison of identification methods of third bending mode frequency based on TEST I and TEST II.*

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers DOI: http://dx.doi.org/10.5772/intechopen.82711*

#### **Figure 18.**

*Advances in Structural Health Monitoring*

historically measured in large variations [33]. Thus, a superior detection indicator should not only be sensitive to damage but also "insensitive" to the states of other track members such as ballast and pads. In order to validate the feasibility of frequency-based damaged-sleeper detection in the actual field, potential impacts from the external environment were investigated. Vibration measurement TEST II and modal identification were hence conducted on intact concrete Sleeper Nos. 17–22, which were on a test line within Railway Technical Research Institute premises. **Figure 16** presents the identified natural frequencies of Sleeper Nos. 17–22. A large variation could be confirmed in the first and second bending modes. It can thus be asserted that the variation of specifications in other track members causes this large variation because completely intact concrete sleepers themselves all display the same properties. On the other hand, the variations in the third bending mode are small. These empirical results are consistent with the trends pointed out in the existing literature [22], in which the variation of ballast-supporting stiffness mainly affects low-order modes, such as first and second bending. These results therefore imply that the third bending mode frequency is a suitable detection indicator, which consistently exhibits desirable

characteristics for efficient damaged-sleeper detection, as described above.

based on a simple measurement using only a single accelerometer.

The applicability of peak picking, one of the simplest methods for identifying third bending mode frequencies, was investigated. A Fourier spectrum was created by a 0.5-s acceleration at the midspan of sleepers just after an impact excitation. Peak frequencies corresponding to the third bending mode were picked up from the range of 500–1000 Hz frequency according to the aforementioned results discussed

**Figure 17** presents identified third bending mode frequencies by ERA and peak picking. Both sets of results show excellent agreement; thus, peak picking can effectively identify the third bending mode frequency (i.e., suitable detection indicator)

**Figure 18** presents measured FFT spectra of three tests, which translates to a certain reliability level within the peak picking method. Peaks of three measured spectra were in good agreement for each test sleeper. It should be noted that only damaged Sleeper Nos. 3, 4, 12, and 14 have some minor peaks and discrepancies at frequency ranges other than the peaks between test cases. This complex influence might be caused by nonlinearity or nonstationarity due to damage; thus, there is some degree

*Comparison of identification methods of third bending mode frequency based on TEST I and TEST II.*

**4. Discussion about the simplification**

**4.1 Applicability of peak picking**

in this study.

**88**

**Figure 17.**

*Reliability of measured spectrum based on TEST I: (a) Sleeper No. 1; (b) Sleeper No. 3; (c) Sleeper No. 4; (d) Sleeper No. 9; (e) Sleeper No. 12; and (f) Sleeper No. 14.*

of possibility that such damage can be detected by focusing on those influences. The peak frequency of concrete sleepers is, however, reduced because of damage and can be easily and reliably found by the peak picking method in the absence of information regarding nonlinearity or nonstationarity. Thus, it can be concluded that peak picking is sufficient for damaged concrete sleeper detection as maintained in this study.

#### **4.2 Applicability of hammering sound**

In the quest for additional simple measurement techniques, damage detection by sound-level meters that do not require the installation of accelerometers was experimentally investigated focusing on Sleeper Nos. 7, 8, and 16.

Structural vibrations can be propagated to peripheral regions as acoustic radiation via the air. Thus, sound pressure caused by impulse hummer test has possibility to be used for indirectly identification of concrete sleepers. Considering the modal characteristics and convenience of practical use, this study sets the sound-level meter position to above the midspan of the sleepers, which can match the antinode positions of the third bending modal shape. In addition, this feasibility study experimentally investigated the effect of each mode on measured sound pressure and then provided an optimized measurement method to ultimately obtain the third bending mode frequency of test sleepers to apply it for practical uses.

**Figure 19** shows Fourier spectra of acceleration responses and sound pressure. The acceleration spectra at the midspan and near the rail seats of a sleeper, in addition to sound pressure, are depicted in the **Figure 19**. **Figure 19(a)** presents the results when an impulse hammer excitement occurs at the midspan, and **Figure 19(b)** corresponds to when the hammer is excited at the rail seat. **Figure 19** indicates good agreement between spectra of acceleration and sound pressure around the frequency peak corresponding to the third mode (750–800 Hz). This fact supports the applicability of sound pressure measurement as a robust tool for damaged-sleeper detection. Another peak of sound pressure(s) in the realm of 350–400 Hz corresponds to the accelerations not at the midspan, but exclusively at the rail seat. Thus, these peaks are caused by second bending vibrations of sleepers. The sound pressure can thus surveil not only the third mode but also the second mode. However, when comparing

#### **Figure 19.**

*Comparison between acceleration and sound pressure based on Fourier spectrum in TEST III: (a) Sleeper No. 7 with midspan excitation and (b) Sleeper No. 7 with rail seat excitation.*

between midspan and rail seat excitation, the midspan excitation can ultimately reduce the second mode vibration level and hence make it easier to identify the third mode frequency by peak picking. It should be noted that the first mode (150–200 Hz) has little impact on the sound pressure because the vibration magnitude is significantly smaller than those of the second and third modes.

**Figure 20(a)** and **(b)** shows the impulse hammer test scheme to investigate the influence of sound observation positions. As shown in **Figure 20(a)**, the position of the sound-level meter was varied among 0.1, 0.3, and 0.5 m from the top surface of the sleeper. In addition, a convenient method of wearing the sound-level meter around the worker's neck (as shown in **Figure 20(c)**) was performed. The peak frequencies of the third mode for Sleeper No. 16 were extracted by peak picking. **Figure 20(d)** shows the extracted peak frequencies of measured acceleration and sound pressure corresponding to the third mode. It was confirmed that the peak frequency of sound pressure can estimate the third mode at the same value for all positions in this test and that these were consistent with the peak frequencies of

#### **Figure 20.**

*Influence of distance of sound-level meter from excitation point: (a) test position; (b) example of sound-level meter condition; (c) wearing around the worker's neck and (d) comparison of peak frequencies.*

**91**

**5. Conclusions**

**Figure 21.**

*TEST IV with ballast support.*

conclusions are summarized as follows:

1.2 times the cracking load is applied.

damage and measurement tests on full-scale test lines.

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers*

acceleration at the midspan. Thus, appropriate positioning of the sound-level meter is required to set it in the vicinity of the excitation point. **Figure 20(d)** illustrates that the same peak frequencies are obtained by a worker who performs an impulse excitation measurement by wearing a sound-level meter about the neck. This demonstrates an efficient damaged-sleeper detection protocol that can excite, measure,

*Peak frequencies of acceleration and sound pressure based on (a) TEST III with support at both ends and (b)* 

**Figure 21(a)** and **(b)** summarizes the peak frequencies of acceleration and sound pressure obtained based on TEST III and TEST IV, which were performed on concrete Sleeper Nos. 7, 8, and 16. The excitation points were at the sleeper midspans, and sound-level meters were worn around the worker's neck. Peak frequencies of acceleration and sound pressure were all found to be in good agreement. In addition, the peak frequency of damaged Sleeper No. 7 was clearly less than that of intact Sleeper No. 8 in both TEST III and IV. Therefore, it can be empirically verified that induced damages decrease frequencies within the third mode and that such frequencies can be accurately estimated via sound pressure measurements, even if the supporting method is changed to ballast (Test IV) from a soft urethane mattress.

In order to validate the feasibility of a frequency-based damage detection method, which is a well-known concept but has seen minimal practical application within the realm of railway concrete sleepers, this study experimentally investigated the impacts of artificial or actual damage on the modal characteristics of such sleeper. In addition, an efficient detection method based on sound pressure and its applicability for practical use was empirically validated. The associated resulting

1.Based on vibration measurements performed in parallel with bending tests, it was confirmed that natural frequencies start to be reduced when greater than

2.Via numerical study, it was confirmed that reductions in natural frequency are caused by open cracks which remain open after unloading has occurred.

3.It was verified empirically that the natural frequency of the third mode (which is not only sensitive against damage but also less influenced by pad stiffness and ballast-supporting stiffness) is a suitable indicator for damaged-sleeper detection based on acceleration measurement tests of sleepers with actual

*DOI: http://dx.doi.org/10.5772/intechopen.82711*

and provide a determination all via a single worker.

*Application of a Frequency-Based Detection Method for Evaluating Damaged Concrete Sleepers DOI: http://dx.doi.org/10.5772/intechopen.82711*

#### **Figure 21.**

*Advances in Structural Health Monitoring*

**Figure 19.**

between midspan and rail seat excitation, the midspan excitation can ultimately reduce the second mode vibration level and hence make it easier to identify the third mode frequency by peak picking. It should be noted that the first mode (150–200 Hz) has little impact on the sound pressure because the vibration magnitude is signifi-

*Comparison between acceleration and sound pressure based on Fourier spectrum in TEST III: (a) Sleeper No. 7* 

**Figure 20(a)** and **(b)** shows the impulse hammer test scheme to investigate the influence of sound observation positions. As shown in **Figure 20(a)**, the position of the sound-level meter was varied among 0.1, 0.3, and 0.5 m from the top surface of the sleeper. In addition, a convenient method of wearing the sound-level meter around the worker's neck (as shown in **Figure 20(c)**) was performed. The peak frequencies of the third mode for Sleeper No. 16 were extracted by peak picking. **Figure 20(d)** shows the extracted peak frequencies of measured acceleration and sound pressure corresponding to the third mode. It was confirmed that the peak frequency of sound pressure can estimate the third mode at the same value for all positions in this test and that these were consistent with the peak frequencies of

*Influence of distance of sound-level meter from excitation point: (a) test position; (b) example of sound-level* 

*meter condition; (c) wearing around the worker's neck and (d) comparison of peak frequencies.*

cantly smaller than those of the second and third modes.

*with midspan excitation and (b) Sleeper No. 7 with rail seat excitation.*

**90**

**Figure 20.**

*Peak frequencies of acceleration and sound pressure based on (a) TEST III with support at both ends and (b) TEST IV with ballast support.*

acceleration at the midspan. Thus, appropriate positioning of the sound-level meter is required to set it in the vicinity of the excitation point. **Figure 20(d)** illustrates that the same peak frequencies are obtained by a worker who performs an impulse excitation measurement by wearing a sound-level meter about the neck. This demonstrates an efficient damaged-sleeper detection protocol that can excite, measure, and provide a determination all via a single worker.

**Figure 21(a)** and **(b)** summarizes the peak frequencies of acceleration and sound pressure obtained based on TEST III and TEST IV, which were performed on concrete Sleeper Nos. 7, 8, and 16. The excitation points were at the sleeper midspans, and sound-level meters were worn around the worker's neck. Peak frequencies of acceleration and sound pressure were all found to be in good agreement. In addition, the peak frequency of damaged Sleeper No. 7 was clearly less than that of intact Sleeper No. 8 in both TEST III and IV. Therefore, it can be empirically verified that induced damages decrease frequencies within the third mode and that such frequencies can be accurately estimated via sound pressure measurements, even if the supporting method is changed to ballast (Test IV) from a soft urethane mattress.
