Volume 2025, Issue 1 5587918
Research Article
Open Access

Damage Identification in Bridge Structures Based on a Novel Whale-Sand Cat Swarm Optimization Algorithm and an Improved Objective Function

Zhen Chen

Zhen Chen

School of Civil Engineering and Communication , North China University of Water Resources and Electric Power , Zhengzhou , 450045 , China , ncwu.edu.cn

School of Civil and Environmental Engineering , Queensland University of Technology (QUT) , Brisbane , 4000 , Australia , qut.edu.au

Key Laboratory of Geotechnical Mechanics and Engineering of the Ministry of Water Resources , Changjiang River Scientific Research Institute , Wuhan , 430010 , China , crsri.cn

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Yikai Wang

Yikai Wang

School of Civil Engineering and Communication , North China University of Water Resources and Electric Power , Zhengzhou , 450045 , China , ncwu.edu.cn

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Hui Wang

Hui Wang

School of Civil Engineering and Communication , North China University of Water Resources and Electric Power , Zhengzhou , 450045 , China , ncwu.edu.cn

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Shiming Liu

Corresponding Author

Shiming Liu

School of Civil Engineering and Communication , North China University of Water Resources and Electric Power , Zhengzhou , 450045 , China , ncwu.edu.cn

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Tommy H. T. Chan

Tommy H. T. Chan

School of Civil and Environmental Engineering , Queensland University of Technology (QUT) , Brisbane , 4000 , Australia , qut.edu.au

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First published: 01 April 2025
Academic Editor: Jun Li

Abstract

Structural damage identification (SDI) serves as an indirect approach that has the potential to meet real-time monitoring of structures. However, the identification accuracy and efficiency of some methods need to be improved, especially when there are some uncertain interfering factors or noise. This paper presents a new optimization algorithm and an improved objective function for inverse problems of SDI, offering an effective solution for bridge damage identification under uncertain noise interference and incomplete modal data. In this study, by hybridizing the whale optimization algorithm and the sand cat swarm optimization, a novel whale-sand cat swarm optimization (W-SCSO) method is proposed for SDI. The cubic chaotic mapping is introduced for initialization of the W-SCSO method, and then the lens opposition-based learning and the stochastic differential mutation are employed to enhance the search capability and convergence accuracy of the proposed algorithm. Besides, the mode shape curvature, the frequency change ratio, and the L1/2 sparse regularization are used to improve the objective function. Four other existing state-of-the-art methods are used to verify the performance of the proposed W-SCSO method by the CEC2017 benchmark functions and a simply supported beam finite model. The comparative analysis highlights the feasibility and effectiveness of the proposed method in the considered cases. Moreover, an aluminum alloy simply supported beam was conducted for the SDI experiment to further prove the effectiveness of the improved method in practice. Simulation and experimental results show that the proposed method effectively locates and quantifies stiffness reduction in bridge structures, which maintains high accuracy in damage identification despite potential modal incompleteness and uncertain measurement noise interference.

1. Introduction

Bridge structures are easy to be damaged owing to excessive loading, temperature changes, reinforcement corrosion, vehicle impacts, etc. The occurrence of sudden and high-risk structural failures can be avoided if the damage is detected and addressed in a timely manner. Hence, structural health monitoring systems play a crucial role in ensuring the safety and durability requirements throughout the entire service life of structures [13].

Structural damage identification (SDI) methods can be divided into two categories: nondestructive testing methods and vibration-based methods [2]. The majority of nondestructive testing methods are used for localized identification, which are more sensitive to localized damage but cannot easily detect the damage located within structures [46]. Vibration-based methods are capable of examining changes in the overall vibration characteristics of a structure and are considered to be global identification methods, which contain both static and dynamic methods [7, 8]. Static methods commonly utilize static displacement or strain changes for structural condition assessment [9, 10]. Dynamic methods include modal parameter-based methods [11], Bayesian methods [12], and finite element (FE) model updating methods [13]. Modal parameters, including natural frequencies and mode shapes, have proven effective for damage identification [1416]. However, complete modal information needs to be acquired to ensure the accuracy of modal parameter-based methods, which are susceptible to environmental factors and difficult to realize in practical engineering structures [17]. Modal parameters are nonlinear functions of the damage index, which presents challenges in the computational processes of Bayesian methods [18].

FE model updating methods can quantitatively identify the location and degree of damage by establishing an accurate calculation model. With FE model updating methods, the SDI process can be transformed into solving the minimum value of an objective function, which can evaluate the difference between the measured and calculated dynamic responses. The objective function can be composed of different modal parameters, such as modal assurance criterion (MAC), frequency change ratio (FCR), and mode shape curvature (MSC) [19, 20]. However, the objective function typically exhibits ill-posedness when formulated based on the alteration of vibration properties with incomplete modal data [21]. There are often two strategies to overcome this specific challenge. The first strategy is introducing the proper approach to circumvent the issues of ill-posedness and uncertainty within the function [22]. Sparse regularization approaches can convert the ill-posed function to be well-posed, and sparsity can be used as a priori information in favor of practical damage identification [2325]. Another strategy is developing more efficient calculation methods to optimize the objective function, such as swarm intelligence optimization algorithms [26]. These algorithms are not only independent of gradient estimation and specific formulas but they also demonstrate a remarkable effect on solving inverse issues with ill-posedness. Minh et al. [27] proposed a new K-means Optimizer and applied the method to SDI in a complex 3D concrete structure. Zhang et al. [28] proposed a hybrid algorithm based on Jaya and a differential evolution algorithm for SDI. Ding, Li, and Hao [29] put forward a new objective function for SDI and employed an improved Jaya algorithm for model updating and optimization analysis. Chen et al. [30] developed a novel hybrid whale-chimp optimization algorithm for SDI based on the whale optimization algorithm (WOA) [31] and the chimp optimization algorithm, which could identify single and multiple damages in the presence of noise interference. Huang et al. [32, 33] employed a genetic algorithm and shuffled frog leaping algorithm for damage identification in bridge structures while taking into account environmental uncertainty factors. Although many algorithms have proven successful in SDI, some algorithms have the problem of intricate parameters and convoluted calculations, which are not conducive to their application in engineering. In addition, the FE model updating methods still have some drawbacks, such as high measurement noise and temperature changes that can affect the reliability of the model update [34]. Furthermore, during the practical mode identification process, only a few low-order modes can be acquired via a limited number of sensors. Hence, the issue of structural modal incompleteness needs to be addressed through the application of model expansion or model reduction techniques. Modal expansion broadens the error into whole freedom, while model reduction diminishes the error only for the master degree of freedoms (DOFs) such as the Guyan reduction scheme. Thus, the latter technique is more widely employed than the former [35].

In addition to model-based SDI methods, data-driven approaches, particularly machine learning techniques, have also gained significant attention in SDI [36]. Machine learning techniques can learn complex patterns from datasets without relying on physical models of the structure [37]. For example, convolutional neural network (CNN) [38] and long short-term memory neural network (LSTM) [39] have been proven to be effective in feature extraction for SDI, which significantly enhances the accuracy and reliability of damage identification. Swarm intelligence optimization algorithms can play an important role in deep learning-based SDI methods, which are often used to optimize machine learning models by fine-tuning hyperparameters and improving iteration efficiency [40, 41]. However, machine learning techniques require large datasets for training; otherwise, these techniques are prone to overfitting when the data are limited [42]. Optimization algorithms also need to exhibit strong optimization performance to avoid obtaining local optima in the convergence result.

With the development of optimization techniques, many advanced algorithms demonstrate the potential to efficiently optimize ill-posed objective functions. Thereinto, the sand cat swarm optimization (SCSO) algorithm is a recently developed swarm intelligence algorithm [43]. The optimization algorithm possesses several advantages, including a simple structure, few numbers of parameters, and the ability to adaptively balance the exploration and exploitation phases, which has been applied and enhanced by researchers to address optimization problems [4446]. However, it still has some disadvantages, such as the tendency to fall into local search stagnation and low convergence accuracy.

To overcome the challenges mentioned above in swarm intelligence algorithms and FE model updating methods, a novel whale-SCSO (W-SCSO) algorithm and an improved objective function have been developed for damage identification in bridge structures. This approach provides an effective solution for SDI, particularly under conditions of uncertain noise interference and incomplete modal data. In Section 2, the definition of structural damage is briefly introduced. Additionally, an improved objective function for SDI is established by MSC, FCR, and L1/2 sparse regularization. In Section 3, several improvements are applied based on the original SCSO algorithm, and the performance of the proposed W-SCSO algorithm is confirmed utilizing CEC2017 benchmark functions. In Section 4, numerical simulations of a simply supported beam are performed using incomplete modal data with Gaussian white noise. In Section 5, an experimental beam was established to validate the proposed SDI method. Finally, the conclusions of this study are briefly summarized in Section 6.

2. Theory of Structural Damage

2.1. Structural Damage Definition

The index values of the structural dynamic response are changed along with the intrinsic parameters of the structure [47]. Considering the stiffness and mass of the structure, the differential equation for the free undamped vibration of the structural system can be established as
()
where K and M represent the stiffness and mass matrices of the system, respectively; ωi refers to the ith circular frequency and Φi denotes the ith mode shape vector. Thus, the modal parameters of the structure can be obtained by solving eigenvalue problems.
Changes in stiffness or mass within the structural system can reflect structural damage [29]. Therefore, a decrease in stiffness is presumed to express the structural damage in both simulations and experimental verification. Considering the issue of structural modal incompleteness in the measured modal parameters, the Guyan reduction method is adopted to condense the mass and stiffness matrices, effectively addressing this problem [48]. The Guyan reduction scheme is a simple and cost-effective condensation technique, which ignores the inertia terms on slave coordinates and provides a good approximation for low-order modes. Assuming that the practical FE model of a structure is discretized into Q elements, the scalar variables of stiffness reduction represent a set of SDI factors αj(j = 1, 2, …, Q) to analyze structural damage in each element, shown as follows:
()
where denotes the global stiffness matrix in the damaged state of the structure; Q means the total number of elements in the structure; and represent the stiffness matrix of the element j in the damaged and intact states, respectively; and αj indicates the SDI factor of the jth element. Considering the practical structural damage, the values of αj are chosen between 0 and 1.0. For example, αj = 0.1 indicates a 10% decrease in the stiffness of the jth element.

2.2. Improved Objective Function of Damage Identification

The objective function based on MSC is sensitive to damage within bridge structures [19]. The MSC at one node can be calculated using the central difference method. Besides, the L1/2 sparse regularization can efficiently contribute to the sparsity of SDI consequences, which provides a valid trade-off between maintaining damage characteristics and avoiding the over-smoothing [49]. Therefore, in this study, the improved objective function is constructed based on MSC, FCR, and L1/2 sparse regularization to accurately reflect structural damage, which can be defined as obj 1:
()
where and represent the calculated and measured circular frequencies of the ith order, respectively; and are the calculated and measured MSC of the ith order at the zth node, respectively; and present the calculated and measured mode shape values, respectively; OM and OF denote the order of the selected mode shape and circular frequency, respectively; MN denotes the number of measured nodes; ϵα(1/2) denotes the L1/2 sparsity regularization item, which ϵ signifies the regularization parameter; l refers to the distance between two measured nodes, and the notation |⋯| means calculating the absolute value. The calculated modal data are acquired by damage identification factor α = [α1, α2, …, αQ] in the FE analysis. The objective function is commonly considered to be ill-posed, and it has multiple locally optimal solutions [29]. Hence, the swarm intelligence algorithm technique is employed to determine the optimal set of parameters for minimizing the ill-posed objective function.

3. W-SCSO Algorithm

3.1. SCSO

The SCSO algorithm is inspired by the natural behavior of sand cats [43]. Sand cats are capable of detecting noise below 2 kHz. Exploration and exploitation phases represent the two main actions of sand cats in the algorithm. Sand cats choose whether to attack or search for prey according to the frequency of the emitted sound. Optimal values are regarded as prey within the search space, and the values obtained from the objective function are used to ascertain whether the position of the sand cat should be updated during each iteration.

During the initial stage, sand cats and their associated parameters are initialized within the search space. In the context of a d-dimensional optimization problem, each sand cat xk refers to a 1 × d array representing the positions of the kth sand cat, i.e., the solution of the problem xk = [x1, x2, …, xd]. The initial values in each dimension are randomly defined as
()
where xk,q is the qth variable of xk; and are the upper and lower bounds of the variable xk,q, respectively; and rand(0, 1) denotes a random number within the range of 0-1.
The general sensitivity range rG and the adaptive parameter R of xk are used to allow the algorithm to seamlessly switch between the exploration and the exploitation phases. rG decreases linearly from SM to 0, where SM is the maximum sensitivity of sand cats with a default value of 2. The r is the sensitivity range of xk. Parameters rG, R, and r can be defined as
()
where t and itermax denote the current and maximum number of iterations, respectively.
In the exploration phase, the position of the sand cat in the qth dimension is updated based on three factors: the current position xk,q, the best-candidate position xbest,q, and the parameter r. The exploration phase can be mathematically represented as
()
where xk,q and denote the current and the next updated positions of the kth individual in the qth dimension, respectively. According to the exploration mechanism, the position of each individual is randomized, which has the characteristics of low calculation costs.
In the exploitation phase, the algorithm selects a random angle θk,q for each sand cat in the qth dimension through the roulette wheel selection algorithm [43], where θk,q varies between [0, 360], resulting in cos(θk,q) changes between [−1, 1]. Parameters of the sensitivity range allow individuals to move in different circular directions and converge toward the optimal position. The exploitation phase can be expressed as
()
()
where xrand,q denotes the position of a randomly selected individual in the qth dimension, which facilitates the proximity of sand cats to each other.
R is regulated by rG for phase transition control. If |R| ≤ 1, sand cats attack the prey, i.e., the exploitation phase. Otherwise, sand cats are forced to globally search for the prey, i.e., the exploration phase. The phases of the SCSO algorithm can be summarized as in equation (9)
()

3.2. Improved Algorithm

3.2.1. Cubic Chaotic Mapping

To obtain a better effect of identifying minor structural damage, the algorithm incorporates the utilization of cubic chaotic mapping for initialization [50]. This method enables more diverse population ranges with better chaotic traversal, which can be expressed as
()
where βk,q and βk+1,q denote the kth and (k + 1)-th values in the qth dimension, respectively. k takes a value within the range of [1, n], where n indicates the total number of individuals in each dimension. σ is the control parameter of cubic chaotic mapping.

Assuming an initial value of β1 = 0.1, the value of the parameter σ is determined using the bifurcation diagram of the cubic chaotic mapping. Taking σ as the horizontal axis, the parameter space is divided into 1000 segments. For each σ within the range [1.8, 2.6], the value of β is iterated 100 times. By sweeping through the entire parameter range, the bifurcation diagram of the cubic chaotic mapping is obtained, as shown in Figure 1(a). Figure 1(b) presents a local zoomed-in bifurcation diagram for σ within the range [2.4, 2.6]. It can be observed that when σ = 2.59, the generated values lie within the range [0, 1], which aligns with the search space of the SDI problem. Upon testing, it is found that the values yielded by the cubic chaotic mapping in one dimension exhibit good chaotic traversal properties when β1 = 0.1 and σ = 2.59.

Details are in the caption following the image
Bifurcation diagram of cubic chaotic mapping. (a) Global view. (b) Local view.
Details are in the caption following the image
Bifurcation diagram of cubic chaotic mapping. (a) Global view. (b) Local view.

The distributions of the cubic chaotic mapping sequence and pseudo-random sequence are given in Figure 2. The number of iterations for both methods is set as 100, and the search space is between [0, 1]. Compared with the pseudo-random sequence, the cubic chaotic mapping sequence is unevenly distributed and most of the generated values tend to concentrate near the lower bound of the search space. This kind of distribution is more applicable to optimizing the problem of identifying minor damages. The β1 is iterated (n − 1) times in each dimension according to equation (10) to accomplish the initialization in the improved algorithm.

Details are in the caption following the image
Comparison of one-dimensional distributions between two sequences. (a) Pseudo-random sequence. (b) Cubic chaotic mapping sequence.
Details are in the caption following the image
Comparison of one-dimensional distributions between two sequences. (a) Pseudo-random sequence. (b) Cubic chaotic mapping sequence.

3.2.2. WOA

The WOA is a swarm intelligence optimization algorithm that simulates the foraging behavior of humpback whales [31]. A random search mechanism in the WOA is employed in the exploration phase of the SCSO algorithm to enhance the global search ability. During the search phase, humpback whales conduct random searches based on each other’s position, which can be expressed as
()
()
where yk,q and denote the current and the next updated positions of the kth individual in the qth dimension, respectively; yrand,q represents a randomly selected individual in the qth dimension; a decreases linearly from 2 to 0 during the iteration; and A is a random value between [−a, a] and the value of C is between [0, 2].
For better controlling the iteration process, a proper inertia weight is added to the random search mechanism [51]. Based on the sigmoid function, a curvilinear decreasing inertia weight w is adopted to the position updating equation. Moreover, the number of variables in the improved algorithm is reduced by substituting a and C, utilizing the parameters in the SCSO algorithm. The improved random search mechanism can be given as follows:
()
()
where Yk,q and denote the current and the updated positions of the kth individual in the qth dimension, respectively; Yrand,q is a randomly selected individual in the qth dimension; t and itermax represent the current and maximum numbers of iterations, respectively; r and rG are the sensitivity parameters; and w decreases in the range of (0, 1).

3.2.3. Lens Opposition-Based Learning

The lens opposition-based learning (LOBL) [52] is applied in the exploration phase of the SCSO algorithm to overcome the disappearance of population diversity and search stagnation. The strategy is used to determine the individual opposite position based on the principle of convex lens imaging. The LOBL in the one-dimensional spatial is shown in Figure 3, and the search range is assumed to be [, ]. The height of an individual P is h, and the projection of P on the x-axis is the individual current position Xk,q. A convex lens is placed at the center of the search range and oriented parallel to the y-axis. An inverted solid image P of height h is formed on the opposite side of the convex lens. The projection of P on the x-axis is the generated opposite position . Let the parameter λ = h/h. When λ < 1, is closer to the convex lens; conversely, is farther from the convex lens. In the study, λ is set as 1.2 to assist the algorithm in escaping local optima. If Xk,q and the middle value of the search space γ are used as boundaries of the search range, a new LOBL strategy can be developed as
()
()
where the new position facilitates the algorithm to jump out of search stagnation. During the exploration phase, the strategy for updating the position is determined by a random number rand between [0, 1]. In the exploration phase, the position update method mainly uses the improved random search mechanism, while the LOBL performs an auxiliary search process. For simulating this search strategy, a random parameter rand with a value chosen from [0, 1] is utilized to determine the method to be adopted in each iteration. If rand ≤ 0.8, the position is updated by equation (14). Otherwise, the position can be updated according to equation (16).
Details are in the caption following the image
Lens opposition-based learning in one-dimensional space.

3.2.4. Stochastic Differential Mutation

Inspired by the mutation strategy of the differential evolution algorithm [53], a stochastic differential mutation strategy is proposed to overcome the issue of premature convergence in the exploitation phase of the SCSO algorithm. In the strategy, a new position of a sand cat in the qth dimension is generated by performing an operation of stochastic differential mutation utilizing the current sand cat Zk,q, the current best individual Zbest,q, and a randomly selected individual in the population Zrand,q. The mutation strategy helps the algorithm have better population diversity and improve its convergence accuracy. The developed strategy for stochastic differential mutation can be expressed as
()
where Zbest,q facilitates the algorithm accurately converging to the optimal solution; Zrand,q is utilized to prevent search stagnation and expand the exploitation area; and Vk,q is a random value between [0, 1]. The influence of Zbest,q and Zrand,q is randomly modulated by Vk,q obtained from the pseudo-random distribution, which can aid in achieving a balance between accurately converging and avoiding stagnation. During the exploitation phase, the strategy for updating the position is selected by a random number rand between 0 and 1. In line with the exploration phase, in the exploitation phase, a parameter rand with a random value generated from [0, 1] regulates the process of method selection in each iteration between the exploitation strategy of the SCSO and the stochastic differential mutation. If rand ≤ 0.8, the position is updated by equation (8). Otherwise, the position can be updated according to equation (17).

Through these improvements mentioned above, a new algorithm is developed named the W-SCSO algorithm based on the SCSO algorithm. The flowchart of the new algorithm is shown in Figure 4. It is worth noting that the algorithm chooses only one of the methods to update the position of individuals through simple judgments in each iteration. The iterative strategy does not generate much complexity in the calculation.

Details are in the caption following the image
Flowchart of the W-SCSO algorithm.

3.3. Benchmark Function Test

Five multimodal benchmark functions from CEC2017 have been selected for testing the exploration and exploitation ability of the W-SCSO algorithm [54]. The search domain and global minimum value fmin are presented in Table 1.

Table 1. Details of five multimodal functions.
F Benchmark function Domain fmin
1 Shifted and rotated expanded Scaffer’s F6 function [−100, 100] 600
2 Shifted and rotated Lunacek Bi_Rastrigin function 700
3 Shifted and rotated noncontinuous Rastrigin’s function 800
4 Shifted and rotated Levy function 900
5 Shifted and rotated Schwefel’s function 1000

In the study, simulations are performed using MATLAB 2020b software (MathWorks, US) on the Core i7-8700 processor with 32 GB of RAM computer. The proposed algorithm is compared with the SCSO algorithm [43], Jaya algorithm (Jaya) [55], WOA [31], and Fox optimizer (FOX) [56] to show its superior optimization performance. The parameters for each optimization algorithm are shown in Table 2. The number of individuals for all algorithms is set as 30, and the maximum of iterations is set as 500. The dimension of each benchmark function is set to 10, with additional tests conducted for F1, F2, and F3 at a dimension of 30. Both algorithms are calculated 10 times for each benchmark function to acquire the best value, mean, and standard deviation of the results, and the better values between algorithms are highlighted in bold in Table 3. The convergence process of five algorithms for three typical benchmark functions in a 10-dimensional space is shown in Figure 5. According to Table 3 and Figure 5, the best value and mean of the results obtained by the proposed algorithm are lower than those of other algorithms, and the W-SCSO algorithm maintains a low standard deviation, indicating better computational accuracy and stability. Considering that the proposed algorithm maintains good accuracy and stability without significantly increasing computational complexity, it demonstrates higher efficiency compared to other algorithms. The simulation results demonstrate that the new algorithm outperforms the other four algorithms in terms of both convergence efficiency and calculation accuracy.

Table 2. Parameters of each optimization algorithm.
Algorithm Parameter Value
W-SCSO Sensitivity range (rG) [0, 2]
Random angle (θ) [0, 360]
A [0, 2]
λ 1.2
  
SCSO Sensitivity range (rG) [0, 2]
Random angle (θ) [0, 360]
  
Jaya Without algorithm-specific control parameters
  
WOA a [0, 2]
A [0, 2]
l [−1, 1]
C 2·rand (0, 1)
b 1
  
FOX c1 0.18
c2 0.19
Time_S_Tit [0, 1]
Table 3. Simulation results of five multimodal functions.
F Dimension Result W-SCSO SCSO Jaya WOA FOX
1 10 Best 602.95 609.37 620.88 625.71 651.57
Mean 605.08 620.86 625.21 641.50 658.52
Standard deviation 1.77 7.43 5.41 10.01 5.13
30 Best 637.46 648.25 674.18 652.96 664.09
Mean 639.72 662.73 689.63 672.54 666.59
Standard deviation 1.32 8.72 14.78 18.74 1.68
  
2 10 Best 733.37 785.68 764.44 755.31 811.27
Mean 738.29 790.61 783.16 768.48 818.79
Standard deviation 4.10 3.71 11.73 8.91 8.92
30 Best 957.99 1170.7 1284.9 1332 1283.1
Mean 986.91 1259.23 1417.08 1359.43 1311.78
Standard deviation 22.89 85.07 93.88 20.14 23.41
  
3 10 Best 811.28 826.37 838.77 830.63 833.83
Mean 813.71 827.53 853.20 843.06 848.75
Standard deviation 1.94 0.73 13.32 7.74 21.95
30 Best 941.76 964.04 1126.10 999.59 990.04
Mean 953.33 1002.06 1157.20 1012.47 1010.93
Standard deviation 15.27 26.56 19.49 13.49 26.11
  
4 10 Best 900.56 1008.00 1063.20 1136.40 1742.20
Mean 913.33 1185.73 1088.98 1256.68 1781.65
Standard deviation 20.57 200.69 32.85 123.63 34.34
  
5 10 Best 1172.10 1522.70 2301.40 1835.60 1934.70
Mean 1328.58 1749.03 2388.98 2275.03 2535.98
Standard deviation 96.53 210.75 54.54 363.54 443.86
Details are in the caption following the image
Statistical results of five algorithms on multimodal functions in a 10-dimensional space. (a) Landscape of F1. (b) Convergence process of F1. (c) Landscape of F4. (d) Convergence process of F4. (e) Landscape of F5. (f) Convergence process of F5.
Details are in the caption following the image
Statistical results of five algorithms on multimodal functions in a 10-dimensional space. (a) Landscape of F1. (b) Convergence process of F1. (c) Landscape of F4. (d) Convergence process of F4. (e) Landscape of F5. (f) Convergence process of F5.
Details are in the caption following the image
Statistical results of five algorithms on multimodal functions in a 10-dimensional space. (a) Landscape of F1. (b) Convergence process of F1. (c) Landscape of F4. (d) Convergence process of F4. (e) Landscape of F5. (f) Convergence process of F5.
Details are in the caption following the image
Statistical results of five algorithms on multimodal functions in a 10-dimensional space. (a) Landscape of F1. (b) Convergence process of F1. (c) Landscape of F4. (d) Convergence process of F4. (e) Landscape of F5. (f) Convergence process of F5.
Details are in the caption following the image
Statistical results of five algorithms on multimodal functions in a 10-dimensional space. (a) Landscape of F1. (b) Convergence process of F1. (c) Landscape of F4. (d) Convergence process of F4. (e) Landscape of F5. (f) Convergence process of F5.
Details are in the caption following the image
Statistical results of five algorithms on multimodal functions in a 10-dimensional space. (a) Landscape of F1. (b) Convergence process of F1. (c) Landscape of F4. (d) Convergence process of F4. (e) Landscape of F5. (f) Convergence process of F5.

4. Numerical Simulations

In this section, a simply supported beam is employed for numerical simulations to assess the effectiveness of the proposed SDI method in bridge structure. Gaussian white noise is randomly added to interfere with the modal data for accurately simulating the impact of uncertain measurement noise on practical measurement data [57, 58]. The interference can give randomness to the influence of noise appearing at any moment, which can be expressed as
()
where τnoise and τ0 represent the modal parameter value with and without noise interference, respectively; ε denotes the degree of noise interference; and Rτ is a generated random number subjected to the standard normal distribution.
To illustrate the advantages of the improved objective function, it will be compared to the traditional objective function constructed based on FCR and MAC [29], defined as obj 2:
()
where and represent the calculated and measured mode shape vectors of the ith order, respectively. The superscript T indicates the transpose operation applied to the vector; and MN denotes the number of measured nodes.

The FE model of a simply supported beam is composed of 15 elements and 14 measured nodes, as shown in Figure 6. The left support of the beam is completely fixed, and the right support is constrained in vertical freedom. Deformation along the axial direction of the beam is neglected in the simulation. Vertical DOFs are measured at the nodes on the beam. Two scenarios with different sensor configurations were considered. When 14 DOFs are measured, sensors are placed at all nodes, while for 7 DOFs, sensors are positioned at nodes 1, 3, 5, 7, 9, 11, and 13. The length and Young’s modulus of the beam are set as 6.00 m and 70 GPa, respectively. The rectangular sectional area, density, and moment of sectional inertia are defined as 3.00 × 10−2 m2, 2750 kg/m3, and 1.10 × 10−5 m4, respectively. Figure 7 illustrates the first-order MSC of the simply supported beam in different damage elements with a 10% degree of damage in a noise-free condition. It can be found that the MSC at the location of the intact element remains smooth, whereas the MSC at the damaged element changes significantly. In order to locate and quantify structural damage effectively, the first five modal data of the structure are selected for both obj 1 and obj 2 in simulations [20]. The W-SCSO algorithm, SCSO algorithm [43], WOA [31], and FOX [56] are applied in simulations. The initial number of individuals for four algorithms is set as 30, and the maximum number of iterations is set as 100. Each damage case is calculated 30 times to avoid singularity results. Subsequently, the result with the lowest fitness value is obtained to determine the effective SDI consequence.

Details are in the caption following the image
Finite element model of the simply supported beam (unit: mm).
Details are in the caption following the image
First-order mode shape curvature of simply supported beam with a 10% degree of damage.

To acquire the optimal performance of the improved objective function, different regularization parameters ϵ are selected for analysis. The triple damage of the simply supported beam is used as a case study, and the W-SCSO algorithm along with obj 1 is employed for computation. Table 4 shows the SDI results with different regularization parameters. The symbol “5@10%” in Table 4 indicates that the stiffness of element “5” has been decreased by “10%.” The symbol “0.15%&5%” represents a noise level of ±0.15% in natural frequencies and ±5% in mode shape data [58]. Results show that when ϵ = 0.001, the SDI result achieves the highest accuracy without any misidentifications compared to other parameters. Therefore, ϵ = 0.001 is used to improve the robustness of both obj 1 and obj 2 in the simulations.

Table 4. Damage identification results with different regularization parameters.
ϵ Damage element Noise level Identified damage Misidentification
0 5@10%, 8@10%, 13@10% 0.15%&5% [email protected]%, [email protected]%, [email protected]% [email protected]%
0.0001 [email protected]%, [email protected]%, [email protected]% [email protected]%
0.001 [email protected]%, [email protected]%, [email protected]% None
0.01 [email protected]%, [email protected]%, [email protected]% None
0.1 [email protected]%, [email protected]%, [email protected]% [email protected]%
1 [email protected]%, [email protected]%, [email protected]% [email protected]%

As shown in Table 5, six distinct cases are applied for the simulations of single, double, triple, and symmetrical damages. In cases 1–3, obj 1 is used for SDI, while obj 2 is employed in cases 4–6. The SDI results are shown in Figures 8 and 9. The labels of the actual damaged elements are framed, and the actual degree of damage is marked with a dashed line. On the one hand, the proposed W-SCSO algorithm produces higher precision and less degree of misidentifications compared to the other algorithms in these simulations. On the other hand, compared to obj 2, the proposed obj 1 can more accurately locate and quantify structural damages, which exhibits stronger noise robustness. Especially in triple damage cases, obj 2 struggles to accurately quantify the damage degree of multiple damaged elements, while obj 1 is sensitive to multiple damages, even under high level noise interference. The accuracy of the identification using obj 2 in symmetrical damage cases is reduced because the natural frequency, or circular frequency, sensitivities between symmetric parameters are always equal when damages occur in a symmetric structure with symmetrical form and degree [59]. On the contrary, obj 1 can effectively restrict the adverse effect of natural frequency sensitivities in symmetrical damage identification. Based on the W-SCSO algorithm and the improved objective function, the proposed method has been successfully validated for accurately identifying structural damage in both single and multiple damages using incomplete modal data, even with strong noise interference.

Table 5. Different cases of damage identification for simply supported beam.
Case Measured DOFs Objective function Noise level Damage degree
1 14 obj 1 (MSC & FCR & L1/2 sparse regularization) 0.15%&5% 10%
2 14 0.15%&10%
3 7 0.15%&10%
4 14 obj 2 (MAC & FCR & L1/2 sparse regularization) 0.15%&5%
5 14 0.15%&10%
6 7 0.15%&10%
Details are in the caption following the image
Damage identification results of case 2 using different algorithms. (a) Single damage. (b) Double damage. (c) Triple damage. (d) Symmetrical damage.
Details are in the caption following the image
Damage identification results of case 2 using different algorithms. (a) Single damage. (b) Double damage. (c) Triple damage. (d) Symmetrical damage.
Details are in the caption following the image
Damage identification results of case 2 using different algorithms. (a) Single damage. (b) Double damage. (c) Triple damage. (d) Symmetrical damage.
Details are in the caption following the image
Damage identification results of case 2 using different algorithms. (a) Single damage. (b) Double damage. (c) Triple damage. (d) Symmetrical damage.
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Damage identification results of different cases using W-SCSO algorithm. (a) Single damage. (b) Double damage. (c) Triple damage. (d) Symmetrical damage.
Details are in the caption following the image
Damage identification results of different cases using W-SCSO algorithm. (a) Single damage. (b) Double damage. (c) Triple damage. (d) Symmetrical damage.
Details are in the caption following the image
Damage identification results of different cases using W-SCSO algorithm. (a) Single damage. (b) Double damage. (c) Triple damage. (d) Symmetrical damage.
Details are in the caption following the image
Damage identification results of different cases using W-SCSO algorithm. (a) Single damage. (b) Double damage. (c) Triple damage. (d) Symmetrical damage.

5. Experimental Verification

5.1. Experimental Setup and Finite Element Model Updating

A simply supported beam model made of aluminum alloy was constructed in the laboratory to evaluate the effectiveness of the proposed method. The geometric parameters and boundary conditions of the beam model are shown in Figure 10, and the experimental model and instruments applied in the experiment are shown in Figure 11. The acceleration responses are collected using the DH5922 dynamic signal test system. The data collection method is preset to continuous sampling, and the signal sampling frequency, the number of time domain points, and the sampling time of each case are 1000 Hz, 2048, and 60 s, respectively. The length and moment of sectional inertia of the beam are defined as 2.46 m and 1.00 × 10−7 m4, respectively. The initial sectional area, density, and Young’s modulus of the beam are 3.00 × 10−3 m2, 2750 kg/m3, and 70 GPa, respectively.

Details are in the caption following the image
Schematic diagram and cutting damage of experimental beam (unit: mm).
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Experimental beam and instruments.

The modal information of the beam is obtained by hammer impact testing for FE model updating and SDI. Three piezoelectric acceleration sensors with the INV9828 specification and a voltage sensitivity of 50.047 mV/ms−2 are placed at nodes to measure the vertical acceleration responses. The sensor located at the first node is fixed, and the remaining sensors are moved in turn from the second node to the ninth node to measure the response data. Subsequently, acceleration responses of the beam are obtained by conducting normalization and the Fast Fourier Transform method is utilized to precondition the responses [60]. Then, the data analysis system is used to obtain the mode shapes and natural frequencies of the experimental beam.

The intact experimental beam is measured five times, and the average of the measured data serves as the measured response for model updating. The limited number of sensors causes the issue of structural modal incompleteness; thereby, Guyan reduction scheme is utilized to acquire low-order modes of the FE model. The W-SCSO algorithm and the improved objective function are utilized for model updating. The updated sectional area, density, moment of sectional inertia, and Young’s modulus of the beam are 2.81 × 10−3 m2, 2683 kg/m3, 1.002 × 10−7 m4, and 7.32 × 1010 N/m2, respectively. The accuracy of the updated FE model can be verified by comparing the measured and calculated MAC, natural frequencies, and mode shapes from the first one to the five orders [20], as shown in Figures 12 and 13. It is obvious that, after model updating, the correlation values of the same orders along the diagonal line in the MAC tend to be 1 and the other values tend to be 0, indicating a strong correlation between modes of the same order in the measured and calculated data. Furthermore, the measured and calculated natural frequencies and mode shapes exhibit similarities. Hence, the calculated results of the updated model have been validated to be consistent with the experimental measurements.

Details are in the caption following the image
Comparison of measured and calculated results under intact state. (a) MAC. (b) Natural frequencies.
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Comparison of measured and calculated results under intact state. (a) MAC. (b) Natural frequencies.
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Comparison of measured and calculated mode shapes. (a) First order. (b) Second order. (c) Third order. (d) Fourth order. (e) Fifth order.
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Comparison of measured and calculated mode shapes. (a) First order. (b) Second order. (c) Third order. (d) Fourth order. (e) Fifth order.
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Comparison of measured and calculated mode shapes. (a) First order. (b) Second order. (c) Third order. (d) Fourth order. (e) Fifth order.
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Comparison of measured and calculated mode shapes. (a) First order. (b) Second order. (c) Third order. (d) Fourth order. (e) Fifth order.
Details are in the caption following the image
Comparison of measured and calculated mode shapes. (a) First order. (b) Second order. (c) Third order. (d) Fourth order. (e) Fifth order.

5.2. Experimental Process and Damage Identification

Symmetric cutting damage is employed to simulate the reduction of stiffness within the specified element, as shown in Figure 10 [37, 61]. Without taking into account the loss of mass, the change in flexural stiffness is simulated by reducing the cross-sectional moment of inertia. The SDI factor α in the experiment is calculated as
()
where and represent the moment of inertia of the cross section in the intact and damaged states, respectively; be and bd represent the widths of the cross section before and after cutting, respectively; and he and hd represent the height of the cross section before and after cutting, respectively.

In the experiment, three damage cases are set up, as detailed in Table 6. SDI is performed based on the first four orders of modal information. Each damage case was measured five times, and the average of the measured data was calculated to serve as the reference results. It is worth noting that the measurement data were inevitably interfered with noise, which can lead to uncertain measurement errors and impact the accuracy of the SDI. The reference results are inputted into the W-SCSO and SCSO algorithms to independently calculate the consequence based on obj 1 and obj 2 separately. In this section, the number of individuals for both algorithms is set as 30, with a maximum of 100 iterations.

Table 6. Damage cases of W-SCSO and SCSO algorithms for experimental beam.
Case Damage element Depth of cut (cm) Damage degree (%)
1 4 4 26.67
  
2 4 8 53.33
7 6 40.00
  
3 4 8 53.33
7 8 53.33
9 8 53.33

To select an appropriate regularization parameter ϵ and compare the numerical and experimental results, SDI is conducted using the updated FE model of the experimental beam, with damage cases consistent with those presented in Table 6. Table 7 shows the SDI results for case 3 under different regularization parameters, where the W-SCSO algorithm with obj 1 is applied for computation. The results indicate that when ϵ = 0.05, the SDI results achieve the highest accuracy compared to other parameter values. Thus, the regularization parameter ϵ is set as 0.05 for obj 1 and obj 2 in the subsequent numerical simulations and experiments.

Table 7. Damage identification results of experimental beam with different regularization parameters.
ϵ Noise level Identified damage Misidentification
0 0.15%&5% [email protected]%, [email protected]%, [email protected]% [email protected]%, [email protected]%
0.005 [email protected]%, [email protected]%, [email protected]% [email protected]%
0.01 [email protected]%, [email protected]%, [email protected]% [email protected]%
0.05 [email protected]%, [email protected]%, [email protected]% None
0.1 [email protected]%, [email protected]%, [email protected]% None
0.5 [email protected]%, [email protected]%, [email protected]% None
1 [email protected]%, [email protected]%, [email protected]% None

In both the numerical model and experimental beam, the W-SCSO and SCSO algorithms are, respectively, applied to compute each damage case 20 times, with the SDI results corresponding to the lowest fitness values shown in Figures 14 and 15. Specially, in the numerical simulation, a noise level of ±0.15% in natural frequencies and ±5% in mode shape data are added. The simulation results demonstrate that the use of obj 1 enables accurate damage identification of the updated FE model, exhibiting stronger robustness compared to obj 2. Furthermore, the proposed W-SCSO algorithm outperforms the SCSO algorithm, achieving fewer misidentifications and higher accuracy in SDI. When using the experimental beam model for damage identification, both algorithms struggle to accurately identify the actual triple damage when employing obj 2. Nevertheless, the proposed obj 1 can be used to identify single and multiple damages. The W-SCSO algorithm can effectively optimize the objective function for locating and quantifying actual damages, whereas the SCSO algorithm tends to produce more errors and misidentifications. The reasons for the imprecise SDI results lie in the random noise disturbance, measurement and model errors, and modal incompleteness. Comparative outcomes indicate that the proposed method possesses strong robustness and satisfactory sensitivity to damage. Experimental results proved that the proposed SDI method, which combines the W-SCSO algorithm and the improved objective function, effectively use the first four orders of incomplete modal information to identify single and multiple damages in the experimental beam. Therefore, the developed approach can be considered to have good applicability in addressing damage identification in bridge structures.

Details are in the caption following the image
Damage identification results of finite element model based on W-SCSO and SCSO algorithms. (a) Case 1. (b) Case 2. (c) Case 3.
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Damage identification results of finite element model based on W-SCSO and SCSO algorithms. (a) Case 1. (b) Case 2. (c) Case 3.
Details are in the caption following the image
Damage identification results of finite element model based on W-SCSO and SCSO algorithms. (a) Case 1. (b) Case 2. (c) Case 3.
Details are in the caption following the image
Damage identification results of experimental beam based on W-SCSO and SCSO algorithms. (a) Case 1. (b) Case 2. (c) Case 3.
Details are in the caption following the image
Damage identification results of experimental beam based on W-SCSO and SCSO algorithms. (a) Case 1. (b) Case 2. (c) Case 3.
Details are in the caption following the image
Damage identification results of experimental beam based on W-SCSO and SCSO algorithms. (a) Case 1. (b) Case 2. (c) Case 3.

6. Conclusions

In this study, a W-SCSO method is proposed for damage identification. Through simulations and experimental studies, some of the conclusions can be drawn as follows:
  • 1.

    Better optimization ability and stability of the W-SCSO algorithm are demonstrated compared with the other four algorithms through the CEC2017 benchmark function test. Results indicate that the W-SCSO algorithm exhibits both computational stability and high accuracy.

  • 2.

    Numerical simulations are performed to verify the effect of the proposed SDI method based on the W-SCSO algorithm and the improved objective function. The simulation results demonstrate that this method can effectively address the SDI problem using incomplete modal data, which represents stronger robustness in the presence of high noise interference.

  • 3.

    Damage identification results from both numerical and experimental studies show that the proposed method exhibits excellent accuracy and effectiveness in locating and quantifying damages within bridge structures, even with modal incompleteness and strong noise interference.

Conflicts of Interest

The authors declare no conflicts of interest.

Funding

This work was jointly supported by the National Natural Science Foundation of China (Grant Number U2004184), China Scholarship Council Fund as a Senior Research Fellow (Grant Number 202308410481), Training Plan for Young Key Teachers in Colleges and Universities in Henan Province, China (Grant Number 2021GGJS078), CRSRI Open Research Program (Grant Number CKWV20241182/KY), Key Scientific and Technological Research Projects of Henan Province, China (242102241015), and Henan Provincial Natural Science Foundation Project (Grant Number 252300420048).

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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