3.2. AE Parameter Analysis
Before the acid aging test, the specimen underwent loading to various degrees of damage, during which acoustic emission (AE) signals were collected. It was observed that the AE hits decreased significantly after approximately 18 hours. Subsequently, the collected AE signals predominantly reflected the aging effects of the acid solution. Therefore, the entire procedure was divided into two stages: the stress damage stage and the aging stage. Signals collected from stage 2 to stage 5 were categorized as aging AE signals.
Figure 6 illustrates the AE hits of the GFRP specimens at different aging stages over time. It is evident that there is no significant variation in the number of AE hits for GFRP specimens across the four stages. The AE hits consistently remain at a relatively low level, consistently below 20. This observation reaffirms the accelerating effect of the acidic solution during the aging stage.
In
Figure 7, the time-amplitude relationship of the AE signal reveals the progression of specimen damage during the initial loading stage, where high signal amplitudes peak at 99 dB. During the first stage, AE signals indicating GFRP damage gradually diminish as mechanical stress-induced damage subsides within an hour. Concurrently, the aging effect of the acid solution on the GFRP specimen begins to manifest, characterized by AE signal amplitudes ranging mainly from 40 to 80 dB. Subsequently, AE signals during the aging stages (from stage 2 to stage 5) stabilize, with amplitudes consistently falling within the range of 40 to 70 dB. This pattern suggests a relatively stable state of aging-induced AE signals in the materials as the acid solution continues to exert its effects over time.
As shown in
Figure 8, the time-peak frequency relationship of the signal reveals that the peak frequency of the A1 specimen signal is distributed within the range of 70-180 kHz, primarily concentrated between 70-120 kHz and 140-180 kHz, with most signals falling within the 70-120 kHz range. Additionally, the peak frequency of the damage signal during the stress loading period is also distributed in the 200-380 kHz range. Based on the analysis of extensive data accumulated previously, this part of the signal corresponds to the fiber fracture damage of the material.
A2 is the pipe specimen treated with a 5mm deep pre-damage. The bending section under pressure has reduced matrix resin and glass fiber content. Stress primarily induces delamination as the main damage pattern. The amplitude ranges mainly between 40~70 dB. Compared to the A1 specimen, signals above 70 dB are less frequent. From stage 2 to stage 5, signal amplitudes are predominantly distributed between 40~75 dB. Although the aging signal is reduced compared to the A1 specimen, the overall amplitude remains higher. Specific amplitude distribution is depicted in
Figure 9.
As shown in Figure10, the time-peak frequency distribution characteristics of the aging signal of the A2 specimen are pronounced. Initially, during Phase 1, the peak frequency ranges predominantly between 75~120 kHz and 140~200 kHz, with a notable concentration in the 140~200 kHz range. Subsequently, from the 2nd stage to the 5th stage, frequencies above 150 kHz prevail, although the number of hits diminishes significantly. This suggests that the acidic solution with pH=5.0 enhances debonding and delamination damage in GFRP pipes to a certain extent, whereas the aging effects on the A1 specimen, characterized by matrix crack damage, are more pronounced.
3.3. Cluster Analysis
From the time-amplitude and time-peak frequency analyses of the specimens mentioned earlier, it is evident that the acidic solution exerts varying effects on the aging of GFRP materials under different damage conditions. To further verify the damage types of the A1 and A2 specimens after stress loading, the AE signals from the specimens within the first 10 minutes of loading were intercepted and analyzed using K-means clustering analysis.
Through correlation analysis of the data, the parameters of amplitude, peak frequency, energy, count, and duration of the GFRP pipe specimens were selected. To reduce the complexity of the calculations, principal component analysis (PCA) was employed to reduce the dimensionality of these high-dimensional characteristic parameters. PCA is a powerful algorithm for data dimension reduction, converting a group of correlated data into several linearly independent variables through orthogonal transformation. These linearly independent variables are known as principal components [
26]. Using PCA, two principal components, F1 and F2, were obtained for both specimens. The sampling suitability of the KMO (Kaiser-Meyer-Olkin) measure was greater than 0.6, with values of 0.742, 0.796, 0.716, and 0.774, respectively. The cumulative contribution rate was above 80%, indicating that the data was suitable for principal component analysis [
27].
The damage signals of each specimen can be categorized into four types using the principal components that yield the best effect. These types are named type 1, type 2, type 3, and type 4. Based on the characteristics of these damage signals, the damage evolution process of specimens under stress loading, and microscopic morphology observations, it is found that the four signal types correspond to matrix damage, debonding, delamination, and fiber fracture of GFRP materials. The clustering effect is displayed in two dimensions, and the damage signals are classified on the peak frequency-amplitude scale, as shown in
Figure 11. The number of AE hits on the 2 mm deep V-shaped pre-damaged specimen (A1 specimen) within 10 minutes is significantly greater than that on the 5 mm deep V-shaped pre-damaged specimen (A2 specimen).
As the loading stress gradually increases, the specimen initially exhibits bending deformation. Subsequently, the stress concentration at the V-shaped defect leads to matrix damage and partial fiber fracture. Delamination damage then begins to occur at the layer where the tip of the V-shaped defect is located, extending to one or both sides. Based on
Table 4, it is evident that the damage in the A1 specimen is primarily characterized by matrix cracking and debonding. In contrast, in the A2 specimen, matrix damage is no longer the predominant type, and the proportion of delamination damage has significantly increased.
Similarly, cluster analysis was conducted on the aging signal data of two GFRP materials from Phase 2 to Phase 5 to study the damage types and aging signal characteristics promoted by the acidic aging solution. K-means clustering analysis was performed on the aging signals of the A1 and A2 specimens by selecting amplitude, peak frequency, duration, and center frequency as parameters. It was found that the clustering effect was optimal when the signals were divided into three types. The clustering results are shown in
Figure 12.
The amplitude of the aging signal is concentrated between 40 and 80 dB, while the peak frequency values are concentrated between 70 and 180 kHz. Specifically, Class 1 signals are low-frequency, low-amplitude signals with peak frequencies between 70 and 110 kHz, corresponding to matrix crack damage. Class 2 signals are intermediate-frequency, low-amplitude signals with peak frequencies between 140 and 180 kHz, corresponding to fiber/matrix debonding damage. Class 3 signals are medium-to-low-frequency, high-amplitude signals with peak frequencies between 70 and 170 kHz, corresponding to delamination damage.
The cluster analysis reveals distinct damage characteristics between the A1 and A2 specimens during the stress loading stage. For the A1 specimen, the damage is primarily characterized by matrix cracking and delamination. At this damage level, the aging signals are mainly due to matrix aging, with additional contributions from debonding and delamination. Conversely, for the A2 specimen, the damage is predominantly due to debonding and delamination. At this more severe damage level, the aging signals are primarily due to delamination, with debonding as a secondary contributor, and matrix aging being the least significant.
As shown in
Table 5, the number of fiber/matrix debonding signals is greater for the pre-damaged specimens with a 5 mm deep V-shaped defect compared to those with a 2 mm deep V-shaped defect. This can be attributed to two main factors. First, the larger pretreatment depth of the 5 mm groove increases the contact surface area between the aging solution and the interior of the material. Second, due to the severe delamination damage, the aging solution penetrates and diffuses into the material through multiple channels, such as the delaminated areas, which reduces the mechanical properties and accelerates the aging process.
3.4. Least Squares Support Vector Machine Algorithm for Classified Prediction
The Least Squares Support Vector Machine (LSSVM) algorithm, proposed by Su, is an enhancement of the traditional support vector machine (SVM) algorithm. While the SVM algorithm yields good prediction results in classification modeling, it suffers from complex models, large computational requirements, and low efficiency in solving convex quadratic programming problems [
28]. LSSVM retains the powerful learning and generalization capabilities of the SVM algorithm but improves efficiency by converting the convex quadratic programming problem into a linear solution problem. This is achieved by defining the Lagrange function and employing a least squares algorithm, thereby reducing model complexity and calculation time [
29].
Assuming the data set
, where
represents the output data, the calculation process for high-dimensional mapping of feature data is:
where
is a normal vector,
is the nonlinear mapping function,
is the amount of displacement and
is relaxation variables.
Equation (1) is the constraint condition under which LSSVM controls all samples and errors. The optimization problem is:
where
is a regularization parameter,
is an optimization function.
Therefore, for the above optimization problem, the Lagrange function set by LSSVM is:
where
is Lagrange multiplier. Respectively solve partial derivative for
and eliminate
to obtain a linear equation set:
where
,
,
According to the Mercer condition,
It can be obtained through formulas (3) and (5),
Based on the above analysis, the LSSVM model calculation is straightforward. In this paper, the Gaussian kernel function is selected for model calculation.
The LSSVM model requires the determination of the kernel function and the normalized parameter
C. Both the normalized parameter and the penalty factor serve similar roles, balancing the error size of training results against the complexity of the model. The kernel function choice influences the number of support vectors; an excessively large value can lead to model simplicity, resulting in under-fitting and reduced prediction accuracy. Before modeling, it is crucial to select appropriate kernel functions and normalized parameter
C values, as these parameters significantly impact the accuracy of model training. Intelligent algorithms are increasingly employed to find optimal parameters. In this study, a swarm intelligence optimization technique is proposed which called sparrow search algorithm (SSA). The SSA is utilized to identify optimal penalty factors and kernel function parameters. Introduced by Xue in 2020 [
30], the SSA is a group-based intelligent optimization algorithm known for its rapid convergence and robust local search capabilities. SSA has found applications in various domains including logistics site selection, rolling bearing fault diagnosis, and agricultural network detection.
In the sparrow search algorithm, the population can be represented by
X, which represents n sparrows in the d-dimension space.
Population fitness can be expressed as F, which is the individual fitness value of sparrows:
The finder usually finds the food with the highest fitness value, accounting for about 20% of the population. Its location iterative formula is shown in (9). When
, it means that no danger is detected within the population, and individuals within the population can update their positions; When
, it indicates the presence of danger such as predators within the population, prompting the birds to migrate for self-preservation.
where
is iteration,
is Position of the ith individual of the jth dimension at iteration t,
is a constant between
,
is Pre-Alarm value between
,
is maximum iteration times,
is safety value between
,
is 1×d matrix with all elements equal to 1.
The sparrow assigned the role of follower within the population can continually monitor the location of the finder. When the finder discovers better food, the follower promptly seizes the opportunity. The iterative formula governing the follower’s position is as follows, highlighting that if the fitness of the ith follower is low, it must relocate to another location:
where
is the worst-case global location,
is the best position for the finder at iteration t+1,
is a 1 × d matrix with random elements of 1 or – 1, A+=AT(AAT)-1.
The sparrow designated as the scout within the population, responsible for detecting danger and issuing warnings, operates according to the following location iterative formula:
where
is the global optimum location,
is the step control parameter,
is a constant to guarantee that the molecule is not 0,
is the fitness value of the current individual, and
is the global optimum and worst fitness value.
The detailed process of optimizing the LSSVM algorithm using the SSA includes the following steps: (1) Establishing tagged stress damage-oriented data sample sets and aging-oriented data sample sets. (2) Normalizing the sample data and selecting a training set and a test set. (3) Determining penalty factors and kernel function parameters of the SSA optimization algorithm using the training samples. (4) Evaluating the accuracy of the LSSVM algorithm modeling and model recognition.
To summarize, this paper adopts the SSA-LSSVM algorithm for modeling and predicting data classification. The specific algorithm flow is illustrated in
Figure 13.
Taking the A1 specimen as an example, the AE signal is segmented into two parts: stress damage and aging. A prediction model is established using the LSSVM. For the stress damage part: Training Set: 100 signals of matrix cracking, debonding, and delamination after clustering, along with 50 signals of fiber fracture. Test Set: 30 signals of matrix cracking, debonding, and delamination, along with 20 signals of fiber fracture after clustering. For the aging failure part: Training Set: 60 matrix crack signals after clustering, 20 debonding signals, and 20 delamination signals. Test Set: 30 matrix crack signals, 10 debonding signals, and 10 delamination signals after clustering. The composition and quantity of acoustic emission data samples used for model training and testing are detailed in
Table 6.
Furthermore, characteristic parameters such as amplitude, peak frequency, energy, count, duration, and center frequency are selected as input features for the LSSVM. SVM characteristic parameters are initialized, and both damage signal samples and aging signal samples are trained and classified using these parameters. The prediction results are illustrated in
Figure 14.