Sealant Delamination Detection of Structural Sealant Glazing Systems Based on Driving-Point Accelerance

Department of Civil Engineering, University of Science and Technology Beijing, Beijing 100083, China Beijing Key Laboratory of Urban Underground Space Engineering, School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China Center for Curtain Wall and Windows & Doors Technology, China Academy of Building Research, Beijing 100013, China Department of Civil and Environmental Engineering, North Dakota State University, Fargo, ND 58108-6050, USA


Introduction
e structural sealant glazing system (SSGS) is widely used in the high-rise buildings and commercial structures since it provides a beautiful appearance for the building. However, due to the aging and fatigue in the service life, the structural sealant may delaminate and fall off resulting in damages in the supporting system of SSGS [1]. To avoid the detachment of the glass curtain wall panel, it is of great importance to detect the sealant delamination and repair it timely [2][3][4][5].
To detect the sealant delamination, there are two types of methods available, including static-based and vibrationbased method. e static-based detection method is used relatively earlier, which includes appearance visual inspection, knock detection, cutting and drawing method, shore hardness method, suction lifter, air bag, and pulse echo ultrasonic testing [6][7][8][9][10]. e visual and knock methods are simple but can only be used as the preliminary testing of the delamination of structural sealant, whose accuracy depends on the experiences of the inspectors [11]. Cutting and drawing method is to bring the cut part of the sample back to the laboratory for testing.
is method would induce damage to the structure which is a destructive testing method. Shore hardness method detects the degree of hardening of structural sealant, which needs to be combined with other methods together to identify damages [12]. Air bag method needs the displacement measuring equipment to detect. Suction lifter method belongs to outdoor detection, which is usually used to detect the adhesion of the glass curtain wall just for double glazing [13]. Recently, Hong et al. [14][15][16] used the thermal wave and ultrasonic wave to detect structural sealant damage directly and visually. Static-based detection method usually requires location where workers can reach, and the testing labor is huge.
For the vibration-based method, which was firstly developed in bridge engineering, delamination is detected by measuring the changes in dynamic characteristics or dynamic response of the system, such as natural frequency, mode shapes, mode shape curvature, waveform chain code, and signature assurance criteria [17][18][19][20][21][22][23][24]. Memari et al. [25] developed kinematic-based models to predict earthquakeinduce failure of the SSGS. Puga et al. [26] studied the glass failure mechanisms of glass panels with different thicknesses, as well as assessed their probability of failure by estimating fragility curves. Carolina et al. [27] analysed the cyclic response of a glass curtain wall for seismic performance assessment. Xu et al. [28] used the dynamic response parameters and dynamic characteristics parameters as the input of artificial neural network to identify debonding situation, which can be used as the damage monitoring of several important unit of SSGS. Liu et al. [29,30] detected natural frequency reduction of the glass curtain wall and related that to the weakening of the boundary conditions. Miao et al. [31] provided a relationship between the ratio of the main peak frequency of the fast Fourier transform to the total power of the signal and damage length of the SSGS. Fang and Luo [32] identified the damage of perimeter sealant by the change in mode shape curvature.
Although delamination of the structural sealant can be identified using the change of predamage and postdamage dynamic characteristics, there are dozens of taps in experimental modal analysis which spend long time to get mode shapes of only one unit of SSGS. For a glass curtain wall building, it usually contains hundreds of independent units, and different parts of the glass curtain wall have different degrees of aging and delamination. It is a tedious work to detect the safety of glass curtain wall. Detecting the delamination fast is a challenge in practical detection. Recently, Huang et al. [33][34][35] validated in experiments that the change of the fundamental natural frequency can be used to detect damage of the structural sealant and proposed the first peak of acceleration Fourier spectrum as the approximate fundamental frequency to significantly reduce the test time. It is well known that fundamental frequency reflects the overall characters of structures, and it is suitable to identify relatively large delamination; however, local small damage is not easy to be identified [36].
In this paper, a new vibration-based method is developed to identify the delamination of sealants of the SSGS quickly during manual inspection process, even if in the case of small delamination. In this method, a new delamination index is established by using driving-point accelerance (DA) to identify the sealant delamination and approximately estimate the severity of damage. Only one acceleration sensor on a glass panel and a rubber hammer are required and only one driving point should be tapped to measure DA, so the operation of this method is very simple and fast. To improve the effect of sealants delamination on the DA, the optimal sensor location was analysed by the finite element method, and the effect of tapping point from the sensor on the DA was investigated by experiments. en, nine different delamination cases were carried out in the laboratory to verify the rationality and accuracy of the proposed method. Because each glass panel only needs to be tapped one point, it is very suitable to detect the delamination of SSGS with thousands of units for its less detection time.

Delamination Index Based on Driving-Point Accelerance
When the sealants of the SSGS is delamination, the boundary conditions of the curtain wall are changed, which cause the corresponding change of dynamic characteristics and dynamic response of the system. DA, which is sensitive to the change of boundary conditions, is used for the delamination of sealants. Considering the impact hammer load applied to a specific degree of freedom (DOF), the forced vibration equation of an n-degree-of-freedom system is [37] [m] a Let the first n natural frequencies and associated mode shapes be ω j and φ j (j � 1, 2, . . . , n). e driving-point accelerance (DA), which is the acceleration frequency response function (FRF) of the rth DOF, can be expressed as follows: where i � �� � −1 √ , P(θ) and A r (θ) are the Fourier transforms of p(t) and a r (t), respectively; a r (t) is the acceleration time φ j is the modal damping coefficient; and k j � φ j T [k] φ j is the modal stiffness coefficient; and φ rj is the rth element of φ j . Once the sealants have some delamination, the dynamic characteristics of panel will be changed, leading to the corresponding change of DA generated by the hammer load. erefore, based on the relative changing of predamage and postdamage of DA, the delamination index, called relative accumulation difference (E a ) of DA, is proposed to detect the delamination as follow: where the superscript u and d denote the undamaged and damaged case, respectively. In the practical test, the signals of acceleration and load are discrete signals so that DA is calculated by the discrete Fourier transform (DFT). From equation (3), it can be seen that the location of acceleration sensor significantly affects DA. us, the hammer impact location must be optimized for less error because the theoretical tap location is occupied by the sensor. erefore, first, investigate the optimized location of acceleration sensor and the effect of the error of hammer impact location and then investigate the accuracy of the delamination index, E a .

Optimization of Acceleration Sensor Location
In order to optimize the sensor location, numerical analysis was performed using ANSYS on a typical panel, as shown in Figure 1(a), to obtain the mode shapes and frequency responses. e size of the unit was 1585 mm in height, 985 mm in width, and 6 mm in thickness. As shown in Figure 1( Figure 1(a) were selected as sensor locations to compare the difference of DA with varying sensor locations. Location A is the intersection of the quarter of long-side and short-side; location B is the center of the panel; location C is the intersection between the midpoint of the long-side direction and the quarter of the short side. Figure 2 shows the first 10 modes of vibration from the modal analysis. e DA of the three locations A, B, and C are shown in Figure 3. It can be seen that the DA of location A had six resonance peaks between the frequency range of 0 to 105 Hz, however, that of locations B and C only had three peaks. Comparing with the mode analysis results, it can be seen that the eighth (95.982 Hz) and ninth (103.05 Hz) mode shapes did not appear as resonance peaks of the DA of location A in the first ten natural frequency range, and the resonance peaks of third and fourth and sixth and seventh modes were very close and merge to one. However, only the responses of the first (19. the first (19.120 Hz), fourth (55.099 Hz), and seventh (80.749 Hz) mode shapes were excited to lead three resonance peaks in the DA of location C. Since the acceleration sensor should be installed at a location where the most numbers of DA natural mode shapes are obtained, location A is a relative optimal location to install the acceleration sensor. e DA at location A was used to identify the sealant delamination in the further experiments. e panel was double glazing comprised of two panels of 6 mm tempered glass and a gap with 12 mm, and the size of the panel also was 1585 mm in height and 985 mm in width. e sealant and the unit should be constructed for three months before test. is was an indoor experiment to research the index itself, so the changes of the external factor were very small that the temperature effect was not considered.

Experimental Equipment.
e type of equipment used in the experiment is shown in Table 1.
e equipment was connected together, as shown in Figure 5. e acceleration sensor was installed at location A using the hot melt adhesive to ensure a proper bonding between glass and sensor. e sampling frequency of test was 2048 Hz. Although the DA calculation requires the impact hammer tapping position and the sensor position to coincide, in the actual test, the impact hammer can only tap the locations near the sensor instead, which would induce systematic errors. At the same time, the arbitrary tapping by operators at the aim point might vary a little bit every time while tapping, which would cause the random error. Figure 6 shows the difference of DA between two times taps for the same aim point. e E a between the first and second tap is 2.84%, which was denoted that the random error was very small and can be ignored.
An impact hammer generated impacts on several tapping positions, as shown in Figure 7. e tapping locations were designed to be close to the acceleration sensor (location A, in Figure 7) in the four directions. In each direction, there were six tapping points. e first five e result is shown in Figure 8. It can be seen that the E a along the long-side direction is smaller than the short-side direction due to the fact that the long-side direction is far from the boundary and the short-side boundary is relatively close, which has more influence on the E a . When the excitation point of the impact hammer is controlled within 3 cm of the distance from the acceleration sensor and is along the long-side direction, the error E a is lower than 5%.

Experimental Setup.
To validate that the developed technique can detect delamination for the SSGS, nine cases, as shown in Figure 9, were tested. e delamination of the sealant was simulated by cutting. e severity of delamination to the sealant is defined as the ratio between the failure length l d and the total length l t of the sealant, that is, e severity of structural sealant delamination in each case is shown in Table 2.
To potentially detect the severity and delamination locations, two acceleration sensors were installed at location A and location D, respectively, which are the intersection of the quarter. en, the location A or D was used as the driving point to illustrate the effect of delamination location on the delamination index. e sampling frequency of the test is 2048 Hz, and then the glass panel was tapped by the impact hammer in the vicinity of the acceleration sensor away from the constraint direction (within 3 cm) to control detection error from hammer locations.      Figure 10. In each case, the hammer is tapped for three times. e two lines in Figure 10(a) almost coincide for the undamaged case (case 1) DA of arbitrary two test. In Figures 10(b)-10(i), the dashed line is the average DA of the three tests of delaminated case, also shown in solid lines, are the DA of the undamaged case. It can be seen that the differences of DA between damaged cases and undamaged case is more significant with the increasing severity of damage. e DA of case 2 and case 3 is almost the same as that of the case when the frequency is lower than 60 Hz. It means that the effect of the tiny damage on response of low-   Shock and Vibration    order vibration modes is small since the low-order modal reacts to the overall macroscopic property and is not sensitive to the local damage. When the frequency is higher than 60 Hz, the difference of DA between the damaged and undamaged case is significant, even though the damage is small. It is because the higher mode reflects the local information so that it is sensitive to the local damage. erefore, when using the E a algorithm, the analysis frequency should reach the modal order that can fully reflect the structural dynamic response. e interval of frequency for calculating the E a is specified between 0 Hz to 128 Hz in the test. e DAs of the nine cases of driving point D are shown in Figure 11, which has similar responses as in Figure 10.
It was tapped three times under every case. According to the experimental data, the results of E a for various cases are listed in Figure 12. Figure 12 shows the value of E a is positively correlated with the severity of delamination, that is, E a increases with the increase of the severity. In the test, the results of tapings for the same case are all smaller than 5%, which means that the random error of hammer taping location will have a slight effect.

Discussion.
e average value of E a of three taps are shown in Table 3. e fundamental frequency reduction [38] E ω1 and modal assurance criterion (MAC) [18] are also listed in Table 3 for comparison. e E ω1 can be calculated by where ω u 1 is the undamaged fundamental frequency of the SSGS and ω d 1 is the fundamental frequency of damaged SSGS. e ith predamage mode shape is φ u i , and the ith postdamage one is φ d i ; then, MAC of φ u i and φ d i is

Shock and Vibration
For each case, the natural frequency and mode shapes are obtained by experimental modal analysis. In the modal experiment, the glass panel is divided into 32 cells and 45 tap points, as shown in Figure 13. e eigensystem realization algorithm (ERA) [39,40] is used to identify the natural frequencies and mode shapes. e identified fundamental frequencies are shown in Figure 14. e identified first mode shapes were shown in Figure 15.
In Figure 15, colors represent the relative amplitude of the mode shapes with colors from purple to green showing the decrease of the amplitude. A block is installed at the bottom of the glass curtain wall panel. While the block helped supporting the vertical gravity of the panel, and it also restricted the horizontal displacement. erefore, the mode shape of case 1 is unsymmetrical and the mode amplitude of the top was larger than that of the bottom. e block also effectively restricted all of other cases which cause the mode amplitude of bottom was relative less. It can be seen that the differences of mode shapes between case 1, case 2, and case 3 were small. For case 4 and case 5, the change in the mode shapes was obvious, but the MAC of the first mode shape was still almost unchanged. In fact, MAC of the first two modes was larger than 0.95 until the delamination severity is 25.55% (case 5). e low sensitivity of fundamental frequency and mode shape shifts to delamination required either in very precise measurements or more serious  damages [17,35]. However, the MAC of the 3rd mode was 0.91 for case 2, which was relative sensitive. In addition, it can be seen from Figure 15 that the change of fundamental frequency is small until case 6, that is, the fundamental frequency reduction E ω1 is less than 5% until the delamination severity is around 32%. From Figures 10 and 11, together with Table 3, it can be seen that the E a leads to increase with the increase of damage severity. For case 2, the delamination severity is only 6.39%, and the E a is 18.10% and 18.02% for driving points A and D, respectively. When comparing the E a of location A and location D in Table 3, it is clear that the E a is very sensitive to delamination, even though delamination is small and the distance between the driving points and the delamination will also change the detected E a . e difference between the E a of driving points A and D increases as the damage severity increases. Especially, for case 7 to 9, the E a of location A and location D are significantly different. e comparisons between the E a of location A and location D indicates that even if the E a is very sensitive to both delamination severity and locations, it is more sensitive to severity than to locations. In addition, even if the relative location between the driving point and delamination location is very far, the changes of E a is significant for small delamination. e high sensitivity of DA is a result of that fact that the higher natural frequencies and mode shapes are more sensitive to small and local delamination. e DA includes more than 10 mode shapes responses in the test; therefore, the small delamination would induce significant changes of DA. For practical applications, if both delamination location and severity are needed, two or more DA locations may be required and other damage index may be needed to be developed in  addition to E a for the accurate measurements on both parameters. According to the results of test, when the E a is larger than 10%, the existence of sealants delamination can be identified with confidence. In practical application, the measured natural frequency will change because of the environmental influence [41]. It is difficult to work out damage identification when the damage index is less than 5%. erefore, E ω1 and MAC of the lower mode shapes are only suitable for damage identification of large delamination. To get higher modal shape, it needs relative long time to finish much more point taps. However, only one point is required to measure DA. erefore, it is more convenient for engineering applications to detect the damages by DA.

Conclusions
Based on the difference of predamage and postdamage DA, a new delamination index, which calculates the relative accumulation difference of predamage and postdamage DA, is proposed to identify the delamination of the sealant in SSGS.
eoretical and experimental results show that (1) To measure DA, only one acceleration sensor and a rubber hammer are required, so the test operation is extremely simple. e relative accumulative difference of DA increases with the increase of the delamination of the sealant. e relative accumulation difference of DA is very sensitive to delamination because it includes much information of higher modes; thus, it is applicative to identify the delamination of sealant in SSGS. It can be seen that the sealants have delamination when the E a is larger than 10%. (2) e location of acceleration sensor installment will influence the value of DA. Usually, the relative optimal place to install the sensor is at the intersection of the long side and the quarter of the short side of the glass panel where it would cause symmetric and antisymmetric mode shapes. To decrease the effect of the error of hammer impact location, the impact hammer taps in the direction of the long side, and the tapping point should be controlled within 3 cm far from the sensor. (3) e fundamental frequency reduction is suitable to identify relatively large delamination. e MAC of higher mode shape is sensitive to local damage and can be used to delamination identification of SSGS, but it will spend relative long time.
In the future, more research studies are needed to analyse the performance of this method with the changes of temperatures, detect small delamination using the relative accumulation difference of DA under different sizes of panels and different damage sites, combine other damage index to identify the damage locations, and perform actual field testing.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.