Applicability of a new composite amusement ride safety device based on acoustic emission monitoring technology

Under the premise of ensuring safety, it is of great significance to realize the lightweight of the non-main load-bearing parts of amusement facilities. The purpose of this study is to study the failure process of a newly designed carbon fiber bumper by using acoustic emission technology. First of all, the design of carbon fiber anti-collision bar can effectively restrain passengers, and its weight is reduced by nearly two-thirds compared to traditional metal materials. Subsequently, the load-bearing capacity of the bumper was tested and acoustic emission monitoring was carried out. The test results show that this new type of combined structure of amusement facility has high reliability, which exceeds the safety factor of 3.5 required by the steel structure of amusement facility. In addition, Renyi entropy was used to select the best window function of short-time Fourier transform, and the frequency domain characteristics of acoustic emission signals of typical damage modes were discussed through appropriately selected windows function. A classifier based on supervised machine learning is established by combining frequency features and acoustic emission feature parameters. Furthermore, the use of classifiers helps to understand the damage behaviour of composite structures.


Introduction
With the development of technology, composite materials are increasingly integrated in various fields to meet the requirements of high performance, light weight and unique functional characteristics.For example, composite materials are widely used in aerospace because of their strength, stiffness, low density, high temperature resistance, corrosion resistance and fatigue resistance.Research on composite materials in automobile field has also increased, aiming at reducing fuel consumption and emissions and improving the safety and comfort of vehicles.However, in the field of entertainment rides, the research on using composite materials to meet the requirements of high strength and light weight is limited.It is important to note that the structural design of amusement rides must take into account the use of materials with high strength and low density, such as aluminium alloys and fibre-reinforced plastics At the same time, the structural design must be optimized to ensure the stability and safety of the facilities while reducing the amount of materials.In addition, the appropriate manufacturing technology is also one of the necessary conditions to ensure the quality and stability of equipment.In a word, the demand for light entertainment facilities is a comprehensive problem, which needs to be considered and solved comprehensively in structural design, material selection, manufacturing technology and structural strength.Through sensible design and technological choice, it is possible to achieve the light-weighting of amusement rides, leading to enhanced portability and economic advantages.
is crucial for cost-sensitive or real-time feedback required scenarios in fault diagnosis and damage assessment [40].
In this paper, a novel CFRP lap bar for amusement rides was designed with portability, low delivery and installation expenses, which address the issue of lightweight constructions.Furthermore simulated service-state lap bar tests was conducted to evaluate the performance of composite structures in practice.The findings of the investigation are valuable for the assessment of structural resilience and improvement of design methodology to ensure the safe and stable operation.Comprehensive analyses were executed through appropriate tests and monitoring of the damage process to authenticate structural strength of the facility and damage mechanisms.AE technology is a powerful tool for understanding damage evolution of composite structures during test.First, the characterization of AE signals associated with typical damage was investigated.The initial classification of signal categories is completed using the short-time Fourier transform of the renyi entropy optimization plus window function.Subsequently, a supervised signal classification model was established using Support Vector Machines (SVM) to determine and classify the complete AE signals during testing process.In summary, The new CFRP lap bar proposed in this paper achieves the goal of light weight while guaranteeing a high safety factor.Additionally, the combination of machine learning algorithms and AE technology is of great significance in distinguishing damage patterns, clarifying damage mechanisms, and realizing real-time monitoring in composite structural.

New design
Lap bar is a significant element of amusement rides on a large scale, and it also represents a fundamental stress component of passenger restraints in general leisure attractions.Usually, the lap bar is made of metal, but the quality, corrosion resistance and comfort of metal lap bars are not ideal.Particularly after using the soft outer wrapping layer of the metal lap bar, its internal corrosion is sometimes difficult to detect and poses safety hazards.An innovative design to address these issues is to use a high-strength, corrosion-resistant, lightweight carbon fibre composite structure for the lap bar.Unlike the traditional steel frame lap bar assembly structure with soft polyurethane foam cushion wrapped around the exterior, the new structure made with advanced composite materials offers the outstanding advantage of its light weight and easy-to-disassemble parts.The weight of the lever assembly made from conventional 40 Cr metal material is approximately 37 kg.However, the use of T300 carbon fibre as the main material has significantly reduced the weight of the redesigned lever assembly to 12.6 kg, 33.2% of the original weight.

Structure preparation
These rides are usually characterized by their rapidly fluctuating loads and accelerated movements.The use of advanced composite materials in amusement rides must take into account the unique characteristics of such rides.Additionally, the anisotropic nature of composite materials means that mechanical properties such as modulus, tensile strength, compression strength, and shear strength vary depending on the direction of the fibres.This variation must be taken into account before determining the safety margin.Therefore, it is important to balance the economy with a reasonable level of margin of safety.A structure of CFRP lap bar consists of T300 carbon fibre pipe and T300 lever arm and metal connectors nemed 40 Cr.Toray T300 unidirectional carbon fiber and carbon fiber twill were used to create a laminate with 40 symmetric layers and a paving ratio of 30/40/ 30, each with a thickness of 0.15 mm.The composite layer has a thickness of 6 mm and the laminate was prepared using the [±45/0/90/0] method.The epoxy resin used was 5222.
The carbon fiber rod part consists of a carbon fiber tube and metal components nested at both ends of the tube.The metal parts are affixed to the tube using embedding and adhesive, while the soft external wrapping acts as a cushioning layer to provide additional comfort and prevent structural damage.The preparation process is described as follow, discharge of carbon fiber tubes, opening holes to connect pre-pregs, gluing pre-pregs onto carbon fiber tubes, and connecting them with bolts.The final step is curing and molding.Another measure to reduce weight is to fill the internal hollow structure of the lever arm with vibration damping material.
The lever arm is manufactured through a process of mould forming and hot press tank preparation.The carbon fibre prepreg is applied to the mould according to laying requirements and placed in a vacuum bag.The component is then cured by withstanding the uniform temperature and lap generated within the hot press tank.As a result, a lever arm made of high-quality carbon fibre composite material is obtained, displaying both impressive surface and internal characteristics.In addition, lever arm may be manufactured by carbon fiber stacking fabrication and molding processes.The standard preparation process for the carbon fibre lever arm serves as the foundation to ensure compliance with product quality and mechanical property specifications.The new structure designed and manufactured is shown in figure 1.

Mechanical testing and AE monitoring
To comprehensively investigate the practicality and performance assessment of CFRP in the context of largescale amusement rides, CFRP lap bar were evaluated under simulated service conditions through testing and state inspections.It is worth noting that in the area of large-scale amusement facilities, the design of metal parts requires safety coefficients of 3.5.This means that the design of parts must include a certain amount of safety margin to ensure reliability.However, an excessively large safety margin may enhance the quality of the parts at the cost of negating the lightweight advantage of composite parts, leading to an increase in production expenses.Conversely, an insufficiently small safety margin does not meet the safety criteria of amusement rides and pose a safety hazard.

Loading settings
The lever was tested according to its force state and frequency of use under normal conditions.A single passenger with a weight of 500 N is used as the standard load, with the loading direction being vertical.An independent loading test bench that simulates the service state of CFRP lap bar for amusement rides was constructed to confirm the safety coefficient.A force transfer system powered by cylinder pressure is utilized to apply target loads to the CFRP lap bar.Table 1 contains the actual load data and the parameters of the cylinders provided by our counterpart.

Testing programme
A metal connector situated at the base of the composite CFRP lap bar establishes a secure link to the testing apparatus.The safety belt uniformly distributes the load necessary for the examination over the CFRP rod.Starting with an initial increment of 0.1 Mpa, the load was increased until the structural failure.To vary the test  load, adjustments were made to the air pressure.During this period, a loading plateau period was established to stabilise the pressure and testing environment for accurate data collection.The experimental set-up is presented in figure 2. To ensure data consistency, point A on one side of the test platform footing was defined as the origin of a coordinate system.Deformation marking point 1 through 14 were then placed at 100 mm intervals along the CFRP rod to record the test results.Additionally, four marking points were placed at the tension end of the bar.On the lever arm, markings are made every 100 units from the A end upwards with a marking point from 25 to 17. Point 24 represents the position where the joint size changes.
The CFRP rod undergo spatial deformation during stress application, resulting in uneven stresses at the left and right ends.Therefore, the central linkage of the metal pre-embedded cylinders at both point B and C is taken as the reference axis.The actual deformation of the CFRP rod during loading was further analysed by the distance of the measurement point relative to this axis.It is of greater value to measure the deformation at these points when unstressed (as a benchmark for comparison and to assess the permanent deformation or rebound following loading), as opposed to measuring the deformation solely at these points.For the lever arm, the centre of the lower square head serves as a basis for the origin of the coordinates.The Y-axis extends in the vertical direction, while the centres of the two square heads function as the reference Z-axis.The X-axis corresponds to the remaining direction.The deformation of the entire lever arm can be reasonably assessed in the XY-plane, which corresponds to the plane of the external load pulling force during stressing.
According to the preliminary stress analysis, the stress measurement point P1 means the rod on the embedded parts and composite material bonding position.The stress measurement point P2 located at outer side of lever arm represents the preliminary analysis of the maximum shear stress point.The stress measurement point P3 located at the corner of the lever arm indicates the maximum shear stress point for preliminary analysis.Stress measurement point P4 at the inside of the lever arm stands for the larger surface to monitor whether the separation phenomenon occurs in the direction of the thickness of the induced point.

AE monitoring system
The AE transducer is affixed to the critical points of the structure being tested using vacuum silicone grease to gather the signal effectively.Figure 3 illustrates the AE apparatus used in the study, which includes the VS900-RIC AE sensor.The AE sensors were installed at positions 1, 2, 3, and 4 and had a frequency range of 100-900 kHz with a threshold of 34 dB.Additionally, a threshold voltage of 5 mV was employed, and the AE signals were captured during the loading test with a sampling frequency of 1 MHz.

Mechanical behavior
During the loading process, the pressure in the cylinder will increase from 0-0.1 MPa, 0.1-0.2MPa, 0.2-0.3MPa K gradually increasing to structural failure.The load on the CFRP lap bar rises with increasing cylinder pressure.In the early stages, In the early stages, there were fewer AE signals, but after loading to the 0.4 Mpa stage, a large number of AE signals appeared.This phenomenon indicates that composite extrusion deformation and fracture damage within the structure.AE event activity increases as the crack expands.In the test, when the pressure was increased to 0.48 MPa, a combination of pre-embedded parts and carbon fibre structure fractured on the right side of the lever arm square head, while on the left side, the lever arm square head cracked, and there was a diagonal fracture on the lower two parts, as shown in figure 4. With the pressure rod remaining intact, the cylinder stroke remains at 100 mm.The minimum difference between the fracture and patch positions is 10 mm, whereas the furthest difference is 100 mm.These results are consistent with the previous simulation experiments using finite element analysis and stress guidelines.

AE characteristic parameters analysis
Several studies have shown that the extent and pattern of damage occurring in composites throughout the testing process may be characterized by the AE characteristic parameters.Joint analysis of single or multiple parameters offers insights into damage evolution and mechanism in a complex state.In the cylinder pressurised from 0-0.1 Mpa, and finally up to 0.5 Mpa, the CFRP lap bar sustained damage.Ultimately, the lever arm at the metal pre-embedded parts fractured near the end and the lever arm itself did not exhibit any apparent damage.The AE signals were recorded during incremental loading stages.The parameter separation method used the   it is unlikely that significant damage is generated during these stages.Although some plastic deformation of the substrate occurs, it has not yet progressed to the point of early cracking.Therefore, it can be considered a safe stage until the load reaches 13.25 kN.However, during the stage III, the growth rate of signals is much smaller compared to the subsequent two stages.Nevertheless, there is a significant increase in the number and amplitude of the signal sets compared to those in the first two phases.The significant increase in signals during Stage IV is indicative of an expansion of the internal damage in structurally weak areas.Therefore, the third stage represents the initial stage of micro-cracking emergence.The analysis further explores the progression of damage.
Figures 5 and 6 also show that signal characteristics remain stable at the end of the third stage until a force increase.This means that the entire structure is stable and without catastrophic damage.Nevertheless, a small number of 300 kHz signals emerged, potentially indicating minor deformation and damage within the carbon fibre structure near the metal inserts.This signal signature typically associated with fibre breakage or pull-out.It is noteworthy that, after the third stage of loading, the load-holding process also generates a few mediumamplitude, medium-frequency, and high-frequency signals, which are markedly different from the initial two stages.Damage evolution patterns in complex components may be difficult to investigate qualitatively, making it challenging to accurately identify internal damage using only a small number of signals.However, high amplitude and high frequency AE events did not appear consecutively.The member was not catastrophically damaged in the third stage.From a safety standpoint, there was no notable damage, demonstrating the safety and dependability of the structure.
In Stage IV, numerous AE signals arise, indicating the occurrence of Macro-impairment events, matrix cracking, fibre breakage, and delamination.Signals at medium and high frequency become particularly prominent.Additionally, signal generation beyond the band three portion occurs during the load-holding phase.During loading from the Stage IV to the Stage V , the composite structure failed and the left arm fractured.There was no significant damage to the CFRP rod or the other side arm.This failure may be caused by incomplete balancing of the loads on the two lever arms.This scenario is reasonable to consider in a test that simulates field conditions.At this juncture, a substantial signal was detected subsequent to the rupture at approximately 4.5 MPa.The significant enhancement of the high-frequency signal is particularly noticeable, implying that as the surface fibres break and the fibres continue to peel away from the matrix until structural failure.

AE signal waveform processing
Although the characteristic parameters of AE systems provide a wide range of information, it is difficult to characterize the various damage modes by relying only on time-domain parameters.Therefore, the information of waveform was extracted.However, approximately 20,000 AE signals were recorded during the entire testing process, it is a challenge to interpret them in terms of the whole time domain.Therefore, the frequency domain characteristics of individual waveform were analyzed and then intrinsic connections related to the parameters were established.In this study, the frequency domain of the AE signal associated with a single damage pattern serves as the baseline for the analysis, and known damage pattern modes include matrix cracking, debonding, and fibre breakage.Figures 7 and 8   higher than before and maintains a stable frequency range.The high-frequency signals are indicative of reinforced fibre damage.It can be used as a reference for AE signals acquired during testing.Subsequently, labelling of waveform features to establish links between labels and parameters as the foundation of the classifier.This method achieves precise and efficient representation of the general attributes of the signal set, and was ultimately used for the identification of damage patterns in the CFRP lap bar set over the entire time domain.

Micro-CT visualization characterization of composite material lap bar damage
Micro Computed Tomography (Micro-CT) is an invaluable tool for examining the microstructural damage evolution in carbon fiber composite lap bars subjected to external forces.In these images, the dark gray regions indicate the presence of cracks and damage, while the light gray areas denote high-density materials, specifically carbon fiber bundles.The detailed insights gathered from this analysis are instrumental for understanding damage mechanisms and assessing structural performance.
The internal damage assessment of the sample was conducted using the American PE ray CT system.The sample was positioned on a rotating table, which was then spun 180°along the Y-axis in increments of 0.5°.At each rotation step, a two-dimensional x-ray image was captured by scanning through the sample layer by layer.A spatial resolution of 200 μm was chosen for the test, with an acceleration voltage set at 70 kV and a current of 3 mA.Each scanning session lasted about 1.5 h.The data obtained was reconstructed using NRecon software, which employs the Feldkamp algorithm, to build a detailed 3D model of the specimen.This model provides a comprehensive view of the sample's internal structure, facilitating thorough damage analysis and research into the specimen's structural integrity.
Import the reconstructed image of the center position of the lap bar arm into DataView, and reconstruct 1884 th, 1884 th, 2036 th layer images in the x, y, and z directions, as shown in figure 14.The interior of the lap bar arm is a hollow filling structure.From figure 13, it can be observed that there are layering, cracks, and holes in the laying layer of the lap bar arm, mainly distributed on the left and right sides of the lap bar arm.
In order to scan the overall damage situation of the center of the lap bar arm, the CT detection results were characterized in three dimensions.The continuous CT reconstruction images were reconstructed in three dimensions using CT vox software, as shown in figure 13. 3D density imaging can achieve visualization of internal damage.Through inspection, it was found that cracks along the arm direction appeared on both sides of the lap bar arm, which occurred inside the layer and could not be observed on the surface of the lap bar arm.The crack may be caused by fiber cracking or delamination.
To further clarify the damage characteristics, clearer 2D reconstruction images are required for analysis.The 2D CT reconstructed images of the 826th, 1040th, and 1258th layers were cut along the x-direction as shown in figure 14(a).Cross-sectional views at both ends reveal interlayer gaps consistent with the results of the 3D reconstruction model and can be identified as delamination.This damage extends throughout the scanning position with varying degrees of delamination likely due to defects during structure preparation, paving, and hot pressing injection.Under external loading, delamination gradually expands.No layering phenomenon was observed in upper or lower parts mainly because these surfaces are not primarily subjected to force.The observed layering phenomenon confirms accuracy in damage pattern classification patterns for analyzing damage signal recognition.'The 2D CT reconstructed images of the 724th, 818th, and 904th layers were segmented along the y-direction, as depicted in figure 14(b).It can be observed that the upper and lower boundaries exhibit uniform layering, with only layered cracks observed at the edges, indicating their distribution throughout the thickness direction.On the 818th layer, significant delamination and debonding were observed on both sides, consistent with findings in the x direction.The increased presence of layering on both sides may be attributed to a significantly lower thickness of fiber layers compared to other directions, making damages more pronounced.
Cut the 2D CT reconstructed images of the 218th, 1410th, and 2002 layers along the z-direction, as shown in figure 14(c).Through observation, it was found that the gap left by the layering was most obvious at the 1410th layer, where the damage was most severe in the middle of the hollow position, indicating that the stress capacity here was the weakest.However, after actual testing, the lap bar arm did not break at this position.This is because this delamination is along the longitudinal direction of the lap bar arm, and there is almost no transverse cracking.Therefore, although delamination has occurred, the fibers can still play a bearing role.
Using CT to observe the internal damage morphology of the lap bar arm, the fracture interface showed damage patterns of matrix cracking, fiber/matrix delamination, and fiber fracture, verifying the correspondence between the related signals identified by the proposed classification model and the acoustic emission signals collected from static failure experiments.

Conclusion
In this paper, a new type of lightweight and portable safety bumper is designed by using carbon fiber reinforced composite materials.The ultimate load test of the lap bar in the operating state is also simulated.Acoustic emission monitoring system is used to analyze the damage evolution and identify damage mode.Finally, it has been proved that the advanced composite structure is safe and reliable as a key part of passenger restraint.Furthermore, damage signal features are extracted by utilizing short-time Fourier transform with Renyi entropy optimization to build a damage classification model.
1.The results show the remarkable advantages of this new structure, including its light weight and easy disassembly of component.The newly developed structure is mainly composed of T300 carbon fiber, and some metal connectors are used as fixed interfaces.Upon testing, one side of the lever arm broke under the ultimate load (13.25 kN).The new structure has a safety factor exceeding by far 3.5 with a weight reduction of 33.7%.
2. After the signal features associated with typical composite damage patterns were investigated, Renyi entropy was used to select a window function suitable for the current acquisition mode.Relatively large time windows were used to obtain distinct frequency-domain features, Frequency domain features of AE signals associated with the three damages varied widely using STFT.
3. AE signature parameters were combined with waveform frequency domain features and used to establish an intrinsic link between the signal and the type of damage.The prediction models for AE signals accurately classified performed on composite structures for matrix cracking, fibre breakage, and debonding with an expected accuracy of 99% for the SVM classifier.The frequency ranges for matrix cracking, debonding, and fibre breakage damage were also found to be <200 kHz, 200-320 kHz, and >320 kHz respectively.
4. The observation results of micro damage morphology are consistent with the acoustic emission characteristics.Micro-CT can be used to further observe the morphology of internal damage.The combination of acoustic emission and micro-CT can provide reference for the detection of damage process and failure discrimination, verifying the accuracy of the proposed spectral feature extraction method and the SVM based composite material damage pattern recognition model in classifying damage signals.

Figure 1 .
Figure 1.Dimensions of the new structure of the composite press rod.

Figure 2 .
Figure 2. Test system setup and coordinate establishment.

Figure 5 .
Figure 5. Variation of AE characteristic parameters during structural testing.

Figure 6 .
Figure 6.Frequency and amplitude of signal parameters during structural testing as a function of time.
displays both the raw waveforms of the AE signals and the Fourier transform results.Waveform were analyzed using the window method of the STFT to check the frequency domain characteristics of the AE signal.The definition of the window function should take into account the time-frame and properties of frequency domain.A too-narrow window function diminishes resolution in the frequency domain, which is not helpful in analyzing the damage information contained in the waveform.The signal characteristics of the waveforms in the frequency and time domains are presented by merging their dynamic spectrograms.Figures 7-9 depicts the raw signals and their corresponding frequency domain characteristics, revealing distinct frequency components extracted by STFT.Signal a signifies matrix cracking, signal b indicates debonding, and signal c demonstrates fibre fracture.The spectral features successfully distinguish the characteristics of the AE signals associated with various damage.The signal revealed a single, distinctly cracked feature in the frequency domain following the fast Fourier transform.A noticeable low-frequency elastic wave at 150 kHz was observed with no indication of multiple peaks.In contrast, signal b presented a frequency domain peak exceeding 200 kHz, showing multi-peak characteristics compared to the remaining two damage modes.Compared to the simple pattern of matrix cracking in the initial stage, this signal was created during the expansion stage of damage.The peak in the frequency domain is notably above 300 kHz, which is significantly

Figure 7 .
Figure 7. Raw signals of typical damage in composite materials.

Figure 10
shows the performance of the classifier in different dimensions.The distribution of signals associated with each damage type is evident from the confusion matrix.The SVM classifiers using linear kernel functions found to perform satisfactory by 10-fold cross-validation.It is worth noting that the AE signals captured in Stage IV were used as the training set for the classification model.As signals related to matrix cracking, delamination, and fibre breakage were found in Stage IV.In other words, Since Stage IV represents the crack expansion phase, the characterization of typical damage signals allows a thorough investigation.Additionally, the quantity is smaller than in Stage V, which facilitates a swift establishment of a reliable damage classification model.

Figure 11
illustrates the classification results of the AE signals at each stage using the SVM model.During the initial loading stages, solely the a-type signal was recorded, indicating that the early deformation of the structure

Figure 8 .
Figure 8. Signal characteristics and renyi entropy of typical damage in composites.

Figure 9 .
Figure 9. Short time fourier variation of signals for typical damage in composites.

Figure 10 .
Figure 10.Confusion matrix for supervised classifiers of damage signals.
generates a distinct AE signal associated with plastic deformation of the substrate.In contrast with the preceding analysis, there are a few class B and C signals observed in the second stage, indicating matrix cracks extending into the interlayer connecting the fibres and matrix.However, the rather limited number of signals does not indicate the presence of severe damage.The signal appears to correlate with localised microdamage at the junction of composite and metal connectors.Once more, this kind of behaviour is repeated following three stages.In the previous analysis, Stage IV indicates the onset of structural failure.The presence of a large number of c type signals indicates an increase in the extent of fibre damage, leading to separation of matrix and fibres and an irreversible destabilisation of the structure.Catastrophic damage to the composite structure occurred at Stage V, before cylinder pressure reached 0.5 MPa.A large number of AE signals associated with matrix and fibre breakage are generated in CFRP lap bar at this stage, as shown in figure 12.In addition the projection on the YZplane represents the accumulation of various damages, which provides an intuitive view of the evolution of the damage process.In summary, the AE signals excited by damage to composite structures under different loading conditions were successfully distinguished by the supervised classifier.Distinct frequency ranges are distinguishable for different damage modes.In this study, the frequency ranges for matrix cracking, debonding and, fibre fracture were <200 kHz, 200-320 kHz, and >320, respectively.

Figure 11 .
Figure 11.AE signal classification results and damage evolution during structural testing.

Figure 13 .
Figure 13.Cross section image in x,y and z directions and 3D density projection image of the center of the lap bar arm.

Figure 14 .
Figure 14.CT reconstruction image of testing structural arm.(a) x direction, (b)y direction,(c) z direction.