The Influence of Long-Term Service on the Mechanical Properties and Energy Dissipation Capacity of Flexible Anti-Collision Rings

Abstract

This study investigates the long-term performance of flexible anti-collision rings after 12 years of service on the Xiangshan Port Highway Bridge. Stepwise loading–unloading tests at multiple loading rates (0.8–80 mm/s) were performed on the anti-collision rings, with full-field strain measurement via digital image correlation (DIC) technology. The results show that: The mechanical response of the anti-collision ring shows significant asymmetric tension–compression, with the tensile peak force being 6.8 times that of compression. A modified Johnson–Cook model was developed to accurately characterize the tension–compression force–displacement behavior across varying strain rates (0.001–0.1 s⁻¹). The DIC full-field strain analysis reveals that the clamping fixture significantly influences the tensile deformation mode of the anti-collision ring by constraining its inner wall movement, thereby altering strain distribution patterns. Despite exhibiting a corrosion gradient from severe underwater degradation to minimal surface weathering, all tested rings demonstrated consistent mechanical performance, verifying the robust protective capability of the rubber coating in marine service conditions.

1. Introduction

With the rapid development of land and maritime transportation, the number of cross-channel Bridges has been increasing, and the risk of ships colliding with Bridges has also increased significantly. Once a ship–bridge collision occurs, it can not only cause damage to ships and casualties, but also lead to the collapse of Bridges, resulting in huge economic losses and even catastrophic environmental pollution [1,2,3]. In February 2024, a container ship accidentally collided with the piers of the Lixinsha Bridge in Nansha, Guangzhou while passing through the Hongqili Waterway, causing the vessel’s sinking and multiple fatalities [4]. In March 2024, the Francis Scott Key Bridge in Baltimore, Maryland, USA, was struck by a vessel, resulting in the complete collapse of the structure and leaving six individuals unaccounted for [5]. Given the severity and frequency of ship–bridge collisions, the Department of Transportation has requested the installation of anti-collision facilities on high-grade channel Bridges [6].

In recent years, experts and scholars have proposed a variety of anti-collision devices for ships. Fan et al. [7,8,9] proposed a combined anti-collision structure of corrugated steel plate and ultra-high performance fiber-reinforced concrete (UHPFRC), and conducted drop hammer impact tests and numerical simulations. The results showed that this combined anti-collision device could significantly reduce the impact force and prolong the duration of the impact force during ship collisions. Fang et al. [10] developed a new type of self-floating steel box-soft multi-stage energy dissipation ship collision prevention device based on steel structure casings and fiber-reinforced rubber materials, which can significantly reduce the peak force of ship collision. Xu et al. [11,12] applied the sandwich structure to the bridge pier anti-collision facility. This structure has better energy absorption and buffering effect compared to the single-layer structure, and the impact force has less influence on the position of the impact point, showing higher stability and reliability. Bao et al. [13] proposed a corrugated steel sandwich energy absorption structure and conducted a numerical simulation study on the impact of structural design parameters such as the arrangement direction of the corrugated plates, the thickness of the corrugated plates and the thickness of the panels on the crashworthiness of the sandwich structure.

Yang and Lv et al. [14,15,16,17] proposed a ship collision prevention device with a combination of rigidity and flexibility and flexible guidance based on the principle of achieving comprehensive protection of the bridge, the ship, and the anti-collision device itself. The core advantage of this device lies in its ability to significantly extend the impact time, effectively reduce the peak impact force of a large tonnage vessel on the bridge pier, and divert the vessel’s course, providing sufficient space for the bow to turn and deviate from the bridge pier, while taking away most of the vessel’s kinetic energy. The flexible anti-collision ring, in combination with the outer steel enclosure, can effectively divert the ship’s course, retain most of the ship’s kinetic energy, and significantly reduce energy conversion. Therefore, the quality of the mechanical properties of the flexible energy dissipation anti-collision ring is directly related to the performance of the entire anti-collision device in ship collision events. At present, the existing literature has studied the mechanical properties of anti-collision rings, and these studies mainly focus on the material strength, toughness, and impact resistance of anti-collision rings in their initial state. It is worth noting that most of the existing studies are limited to ideal conditions, and there are no reports on the long-term performance evolution of anti-collision rings under actual service conditions. Anti-collision rings that have been in service for many years may experience a series of performance degradation phenomena, such as material aging, crack initiation and propagation, and decreased hardness and toughness, due to the long-term effects of environmental factors (such as temperature, humidity, ultraviolet radiation, etc.) and frequent or high-intensity impact loads. Therefore, an in-depth exploration of the mechanism of mechanical performance degradation of anti-collision rings that have been in service for a long time is not only important for assessing their remaining service life and predicting potential safety hazards, but also can provide a reference for formulating maintenance and replacement strategies for anti-collision rings.

To reveal the influence of long-term service on the mechanical performance and energy dissipation capacity of flexible anti-collision rings, this study takes the anti-ship collision device of the main pier of Xiangshan Port Highway Bridge as the research object and conducts mechanical performance analysis on the flexible anti-collision ring that has been in service for 12 years. Based on anti-collision ring specimens in different corrosion environments (below water surface, at the air–water interface, and above water surface), the influences of corrosion degree on material constitutive relationship and energy dissipation capacity were systematically studied through quasi-static compression and tensile tests as well as dynamic multi-rate loading tests, providing a basis for the safety assessment and performance optimization of anti-collision devices in long-term service.

2. Engineering Background

The main piers of Xiangshan Port Highway Bridge are equipped with flexible guided anti-collision devices as a preventive measure against vessel impacts (Figure 1). The defense grade is designed to accommodate a speed of 3.7 m/s for a 50,000-ton bulk carrier. Through preliminary numerical simulation calculations, when the main piers of the Xiangshan Port Highway Cross-sea Bridge have no anti-collision devices, the instantaneous maximum ship impact force is as high as 140 MN, and the average ship impact force is about 110 MN, exceeding the designed anti-collision capacity of the main piers (85 MN). When there is no anti-collision device, the ship’s impact force will cause serious damage to the pier, and the bow of the ship will also be severely damaged. Therefore, effective anti-collision facilities must be adopted to reduce the ship’s impact force. According to the anti-collision design requirements, flexible anti-collision devices are installed on the main piers and auxiliary piers near the navigable holes. The main pier’s flexible guide anti-collision device and its structure are shown in Figure 1 and Figure 2.

3. Materials and Methods

3.1. The Flexible Anti-Collision Ring

A schematic diagram of the structure of the flexible anti-collision ring is shown in Figure 3, consisting of a binding wire rope ring and a highly elastic rubber layer. When making it, the wire rope is coiled and bound to prevent deformation and then vulcanized and covered with neoprene on the outside to form the anti-collision ring. In this test, the outer diameter of the anti-collision ring is 800 mm, the inner diameter is 380 mm, the cross-section is trapezoidal, and the bottom side lengths are 230 mm and 160 mm, respectively.

In the flexible protection device of the main pier of Xiangshan Port Highway Bridge, the flexible anti-collision rings are installed in three layers: upper, middle, and lower (Section A-A of Figure 2), respectively, serving in three different marine environments. Two anti-collision rings are taken from each of the upper, middle, and lower layers of the flexible anti-collision device removed after ten years of service. Among them, the upper anti-collision ring was placed horizontally about 2 m above sea level and was exposed to ultraviolet rays for a long time in the sun environment without obvious damage. The middle layer of the anti-collision ring, placed vertically near the waterline, has been in the alternating dry and wet sea surface environment for a long time under the effect of surging waves, with a small amount of marine organisms attached to the surface, and the rubber has slight damage. The lower layer of the anti-collision ring is placed horizontally about 2 m below the sea level and has been immersed in seawater for a long time, with a large number of marine organisms attached to its surface, and the rubber is severely degraded. There is a large amount of debris on the surface of the clamping device that holds the anti-collision ring, and there is a relatively severe corrosion phenomenon on the surface. The partially removed protective device and the flexible anti-collision rings at different waterline positions are shown in Figure 4 and Figure 5.

3.2. Full-Field Strain Measurement

In order to study the overall mechanical properties of the flexible anti-collision ring during action, digital image correlation (DIC) technology was used in this study to test the full-field strain of the upper flexible anti-collision ring under compression and tensile test conditions. Figure 6 shows the speckle pattern applied to the surface of the test piece to assist in the analysis of the DIC technique. The digital image correlation (DIC) system employed in this study comprised a high-resolution industrial camera system (RICOH FL-BC1220-9M, 9 MP CMOS sensors, Ricoh Company, Ltd., Tokyo, Japan) equipped with 35 mm f/2.8 fixed focal length lenses, with acquired images processed through VIC-2D software (v8.0, Correlated Solutions Inc., Irmo, SC, USA) using a 29 × 29 pixel subset size and 7 pixel step size. To precisely capture the image information during the deformation process of the flexible anti-collision ring, the industrial camera is placed 1 m directly in front of the flexible anti-collision ring. The camera’s frame rate was set at 30 frames per second (30 fps), and the effective resolution of its frame was 1720 × 1324, ensuring high precision and continuity in image acquisition.

3.3. Testing Program

In this test, the MTS 500 kN actuator cylinder (maximum stroke ±250 mm) was used to perform quasi-static loading on the flexible anti-collision ring, and the force–displacement response information was recorded in real time by the embedded sensor. A schematic diagram of the test loading is shown in Figure 7. The anti-collision ring fixture removed from the original anti-collision device was selected as the test fixture to ensure that the constraints in the test are basically consistent with the actual engineering conditions. Meanwhile, in order to study the mechanical response and energy dissipation characteristics of the anti-collision ring under different degrees of deformation, a displacement controlled stepwise loading and unloading method was adopted for the test. The increment of each loading displacement was 20 mm, and the loading was carried out in 10 stages until the final displacement (200 mm) at a loading speed of 0.8 mm/s. The loading time-displacement curve of the test is shown in Figure 8. Two anti-collision rings were collected from each structural layer (upper, middle, and lower), comprising six anti-collision rings in total. For each layer, one ring was subjected to tensile testing while the other underwent compressive testing, and the test conditions are shown in

Table 1. The conditions and results of each test of the flexible energy-consuming anti-collision ring.

Test Number	Anti-Collision Ring Position	Loading Condition	Loading Mode	Displacement Increment (mm)	Loading Speed (mm/s)	Final Dis-Placement (mm)
1 #	Upper layer	Compression	Displacement controls step-by-step loading and unloading	20	0.8	200
2 #	Upper layer	Stretch
3 #	Middle layer	Compression
4 #	Middle layer	Stretch
5 #	Lower floor	Compression
6 #	lower level	Stretch

. The maximum displacement deformation of the compression test and the tensile test is shown in Figure 9.

4. Results and Discussion

4.1. Compression and Tensile Mechanical Properties

The force–displacement curves obtained from the tests of flexible anti-collision rings at different service positions (degree of corrosion) are shown in Figure 10. As shown in Figure 10, the flexible anti-collision ring exhibits significant viscoelastic deformation characteristics under both compressive and tensile loads, that is, there is a distinct envelope region between each loading and unloading curve, with a full envelope shape, indicating good energy dissipation characteristics during the deformation process. Under compressive load, the peak load-bearing capacity of the upper layer (Figure 10a), middle layer (Figure 10c), and lower layer (Figure 10e) of the flexible anti-collision ring is 29.0 kN, 29.6 kN, and 28.1 kN, respectively, showing an approximately linear growth trend with increasing displacement. As shown in Figure 10b,d,f, under tensile load, the peak load-bearing capacity of the upper (Figure 10b), middle (Figure 10d), and lower (Figure 10f) anti-collision rings is 194.7 kN, 201.6 kN, and 185.9 kN, respectively, when the loading displacement ranges from 0 to 100 mm, the bearing capacity demonstrates a nearly linear growth trend as the displacement increases. However, when the loading displacement surpasses 100 mm, the bearing capacity shows a pronounced nonlinear growth trend. In the tensile test, the slope of the force–displacement curve exhibits a marked increase, leading to a rapid rise in the bearing capacity. By comparing the trends of bearing capacity in the compression and tensile test curves, it is known that the mechanical response of the flexible anti-collision ring shows obvious tension–compression asymmetric. Finally, when the loading displacement reaches 200 mm, the bearing capacity value corresponding to the tensile test curve (201.6 kN) is 6.8 times that corresponding to the compression test curve (29.6 kN).

4.2. Theoretical Model Analysis

Flexible anti-collision ring rubber composite material rate, the bearing capacity of rubber material usually varies with the change in loading speed, that is, rubber material has strain rate effect. To systematically study the strain rate effect of the flexible anti-collision ring, a multi-rate loading test was designed. The specimen had an outer diameter of 800 mm (initial gauge length L₀ = 800 mm), and quasi-static to dynamic loading tests were conducted at three levels of displacement rates of 0.8, 8, and 80 mm/s, with corresponding strain rates of 0.001, 0.01, and 0.1 ${\mathrm{s}}^{-1}$ (Figure 11).

It can be observed from Figure 11 that the bearing capacity of the flexible anti-collision ring is significantly affected by the loading rate, while the unloading section is almost unaffected by the loading rate.

For the study of the strain rate effect of rubber composites, this paper draws on the strain rate sensitive term form of the Johnson–Cook model [18] and combines the specific superelastic behavior of rubber to construct a constitutive equation suitable for the material. The Johnson–Cook model describes the strain rate strengthening effect through multiplicative logarithmic terms (1 + $C\ln \left(\dot{\epsilon }/{\dot{\epsilon }}_0\right))$, a form retained due to its simplicity and wide applicability. However, the power function strain hardening term $\left(A+B{\epsilon }^n\right)$ of metallic materials cannot accurately characterize the nonlinear superelastic response of rubber. For this purpose, the strain hardening term in this model is replaced by polynomial expansion $\left(a\epsilon +b{\epsilon }^2+c{\epsilon }^3\right)$, as in Equation (1), to better fit the stress–strain characteristics of rubber under quasi-static conditions. By separating the behavior of the superelastic matrix from the strain rate correction effect, the model not only retains the engineering practicality of the Johnson–Cook logarithmic rate correction but also precisely characterizes the large deformation mechanical response of rubber through polynomial terms. The parameters ($a,b,c$) are calibrated by quasi-static tensile tests, while the strain rate sensitivity coefficient $C$ is fitted through dynamic loading experiments. The normalized reference strain rate was set as, $0.001{\mathrm{s}}^{-1}$ and the strain was normalized and uniformly expressed as displacement to match the service conditions of typical rubber products. The fitting formulas are as in Equation (2) (compression condition) and Equation (3) (tensile condition).

$$\underset{\begin{array}{l} \\ Hyperelastic term\end{array}}{F\left(d,\dot{\epsilon }\right)=\left(ad+b{d}^2+c{d}^3\right)}\cdot \underset{\begin{array}{l} \\ Strain rate correction term\end{array}}{\left[1+C\ln \left({\displaystyle \frac{\dot{\epsilon }}{0.001}}\right)\right]},$$

(1)

$$F\left(d,\dot{\epsilon }\right)=\left(0.19628d-6.72479\times {10}^{-4}{d}^2+5.35088\times {10}^{-5}{d}^3\right)\cdot \left[1+0.04\ln \left({\displaystyle \frac{\dot{\epsilon }}{0.001}}\right)\right],$$

(2)

$$F\left(d,\dot{\epsilon }\right)=\left(0.55067d-0.00906d^2+5.35088\times {10}^{-5}{d}^3\right)\cdot \left[1+0.05\ln \left({\displaystyle \frac{\dot{\epsilon }}{0.001}}\right)\right].$$

(3)

In the formula, $d$ is the loading displacement and $\dot{\epsilon }$ is the strain rate (${\mathrm{s}}^{-1}$).

To verify the authenticity of the model, the model parameters were taken as $\dot{\epsilon }$ 0.001–0.1 ${\mathrm{s}}^{-1}$ and compared with the test curves, respectively. The results are shown in Figure 12 and Figure 13.

As can be seen from Figure 12 and Figure 13, the trend of the fitted response curve considering the strain rate effect is highly consistent with the test curve, and the force error under all conditions is less than 10%, with the maximum error being 9.6%.

4.3. DIC-Based Deformation Field Analysis

The first frame captured by the high-speed camera was set as the reference image for the DIC analysis. The deformation field analysis using Digital Image Correlation (DIC) was performed on upper anti-collision rings positioned above the water surface. Figure 14 and Figure 15 present the characteristic strain distributions under compressive and tensile loading, respectively. Image data at displacements of 0 mm (initial position), 40 mm, 80 mm, 120 mm, 160 mm, and 200 mm (the maximum loading displacement) were extracted to comprehensively reflect the strain conditions of the anti-collision ring at each loading stage. Throughout the test, the distribution of the strain field showed symmetrical characteristics along the loading axis.

Figure 14 shows the dynamic evolution of the strain field on the surface of the flexible anti-collision ring under compression test conditions. When the loading displacement reaches 160 mm, distinct concentration areas of compressive strain begin to appear on the left and right sides of the inner ring of the anti-collision ring and expand rapidly towards the central area. As the displacement further increases to 200 mm, that is, at the maximum displacement of loading, the strain concentration phenomenon is almost entirely distributed along the inner ring, especially in the main deformation areas on the left and right sides of the inner ring, where the strain value reaches the maximum, approximately 17%. Observing from the inner side to the outer side of the anti-collision ring, the compressive strain gradually weakens, and in the outer ring area, the strain nature changes, transitioning from compressive strain to tensile strain. During the process of the flexible anti-collision ring bearing compressive loads, the inner side mainly experiences compressive effects, while the outer side mainly shows tensile effects.

Figure 15 shows the gradual evolution of the strain field on the surface of the flexible anti-collision ring during the tensile test. When the loading displacement reached 40 mm, the flexible anti-collision ring began to form a tensile strain region along the edge of its inner ring. As the loading displacement gradually increases, the area of this tensile strain zone expands, and the tensile strain strength increases accordingly. When the loading displacement reached 160 mm, a significant strain concentration began to appear along the inner ring area at the end of the clamping device. As the loading displacement reaches its maximum value, the strain concentration area of the flexible anti-collision ring is mainly located on the left and right sides at the end of the clamping device, and the strain values in these areas reach the maximum, which is 33.5%. Radial analysis from the inner to outer periphery of the anti-collision ring reveals progressive attenuation of tensile strain. Conversely, compressive strain dominates the outer ring region, with peak compressive strain (approximately 6%) localized at the bilateral extremities of the outer periphery. Under tension, the inner ring experiences direct stretching while the outer ring is compressed radially due to Poisson’s effect and fixture constraints, leading to opposite strain signs. This result indicates that in the tensile test, the strain of the flexible anti-collision ring is mainly concentrated near the inner ring at the end of the clamping device, and the strain intensity and concentration increase significantly with the increase in loading displacement.

By analyzing the areas where strain concentration occurs under compression and tensile conditions, and in combination with the discussion of the difference in bearing capacity under compression and tensile loading modes in Section 4.1 of this paper, the specific effect of fixture confinement on the deformation mode of the anti-collision ring can be inferred: in the tensile test, the clamping effect of the fixture on the inner wall of the anti-collision ring limits the bending deformation of the anti-collision ring in the clamping area. This effect may cause the anti-collision ring to reach its deformation limit state earlier, thereby promoting a rapid growth trend in its load-bearing capacity and energy dissipation rate. In contrast, in the compression test, the fixture mainly acted on the outer wall of the anti-collision ring and had a relatively small effect on the deformation of the anti-collision ring in the clamping area, indicating that under the compression condition, the constraint effect of the fixture on the overall deformation mode of the anti-collision ring is not as significant as that under the tensile condition.

4.4. Service Position Effects on Mechanical Properties

Figure 16 and Figure 17 show the comparison of peak load-bearing capacity of the flexible anti-collision ring under compressive and tensile loads under different service conditions.

As shown in Figure 16 and Figure 17, under the same loading conditions, each layer of the anti-collision ring exhibits basically consistent mechanical properties in terms of step-by-step bearing capacity and loading curves. The peak bearing capacity of the middle and upper fender layers is close, while that of the lower fender layer is slightly lower than that of the middle and upper layers. This indicates that under long-term different marine service environments, the mechanical properties of the flexible energy-absorbing fender layers remain basically consistent and have not been significantly affected.

5. Conclusions

This paper conducts a quasi-static mechanical test study on the flexible energy-consuming anti-collision ring that has been in service for a long time in the flexible protection device of Xiangshan Port Highway Bridge. The mechanical response characteristics of the anti-collision ring under cyclic compressive and tensile loads and the influence of different marine service environments on the mechanical properties of the anti-collision ring were analyzed. The results show that:

(1)

The anti-collision ring exhibits approximately linear mechanical behavior during the initial loading stage. However, as it enters the large deformation stage, its nonlinearity becomes prominent, with the curve slope steeply increasing. The tensile peak force is approximately 6.8 times that of compression, and the tensile-compressive asymmetry becomes more pronounced with increasing displacement. As the loading speed increases, the bearing capacity also increases.

(2)

Based on the explicit constitutive equation constructed by the improved Johnson–Cook model, high-precision prediction of the force–displacement curve was achieved within the strain rate range of 0.001–0.1 s⁻¹, with a maximum error of 9.6%, verifying the engineering applicability of the model.

(3)

DIC full-field strain analysis shows that strain peaks are concentrated in the inner loop region and gradually decay outward. The fixture constraint significantly affects the tensile deformation pattern, resulting in strain concentration at the clamping end, while the constraint effect is weaker under compression conditions.

(4)

In long-term marine environments, the degree of corrosion on the surface of the flexible anti-collision ring varies by location, but its compressive and tensile mechanical properties are basically the same, indicating that the outer rubber effectively protects the internal structure and has excellent mechanical durability.