1. Introduction
With rapid social progress and deepening globalization, governments are focusing on energy consumption and environmental protection. Specific requirements for energy conservation and emission reduction have been proposed with the aim of addressing climate change and promoting the realization of sustainable development. Shipping is one of the main sources of energy consumption and emissions in the transportation industry, thus presenting new requirements and challenges for the ship’s structural design. Lightweight structural design can effectively control the weight and gravity of the ship structure, which can not only improve navigational performance, but also reduce energy consumption and emissions. The Swedish government launched the LASS project and the LASS-C project [
1,
2], which focus on the development and application of composite materials in ship structural design. Composite materials are used in the structural design of ship superstructures such as a high-speed craft, a Ro-Pax ferry, and a cruise ship. The project investigates the application of composite materials in modern ship engineering from the technical, economic feasibility, and safety perspectives. The application of composite materials in ship engineering reduces the weight of hull structures, effectively lowers energy consumption, and enhances operational efficiency, providing new directions for future ship design and construction.
The composite stiffened plate combines the advantages of stiffened plate and composite material, offering significant advantages in lightweight ship design by meeting structural loading requirements while reducing the structural weight. However, the structural weight reduction typically heightens the sensitivity to structural vibration, and the use of composite materials introduces complexity to the vibration characteristics. Extensive research has been conducted on stiffened plate vibration, including the finite difference method [
3], the spline composite strip method [
4], the finite element method (FEM) [
5], and the Rayleigh–Ritz method [
6]. Peng et al. [
7] developed a meshless theoretical model based on the first-order shear deformation theory (FSDT) to analyze the stiffened plate bulking and free vibration characteristics. Cho et al. [
8,
9,
10,
11,
12] developed the VAPS software based on the assumed modal method to simulate the free and forced vibrations of the stiffened plate and explore the impact of different topologies, attachments, and arbitrary boundary conditions. Qin et al. [
13] proposed an analytical model to investigate the static and dynamic characteristics of curvilinearly stiffened plates. Zhang and Lin [
14] investigated Mindlin stiffened plate vibration characteristics based on the modal expansion method. Sahoo [
15,
16] applied the FEM to study the stiffened plate free vibration and the dynamic response under moving loads. Liu [
17] proposed a theoretical model to examine the free vibration in stiffened plates with arbitrary cross-section beams. Qin et al. [
18] established a meshless theoretical analytical model to analyze the circular stiffened plate structure bending and free vibration. Zhang et al. [
19] presented an analytical solution for the rectangular orthogonal stiffened plate vibration response. Liu et al. [
20] introduced a dynamic analysis method for stiffened plates under moving loads utilizing the FSDT along with Chebyshev polynomials for spatial discretization and penalty functions for various boundary conditions. Ko and Boo [
21] proposed a condensed finite element matrix using the iterated improved reduced system (IIRS) method to address modal, frequency, and transient responses of stiffened plates. Guo et al. [
22] investigated the concentric stiffened plate free vibration using a two-dimensional spectral Chebyshev technique. Shen et al. [
23] proposed a meshless approach for analyzing the stiffened plate free and forced vibration and conducted structural vibration experiments. Wang [
24] explored the free vibration of rectangular stiffened plates with cutouts. Gao et al. [
25] developed a semi-analytical approach based on the domain decomposition method (DDM) to study the dynamic behavior of the stiffened plate under various conditions, complemented by experimental validations. Chen et al. [
26] conducted an experimental study on the stiffened plate vibro-acoustic characteristics under various welding conditions.
Composite stiffened plate vibration characteristics have attracted extensive research due to their special structure and complex mechanical properties. Rikards et al. [
27] explored the buckling and vibration characteristics of composite stiffened plates using the FEM. Ahmadian et al. [
28] applied the super elements method to analyze the free vibration characteristics of the laminated stiffened plate with eccentric stiffeners. Qing et al. [
29] developed a semi-analytical solution model for the laminate stiffened plate free vibration. Nayak and Bandyopadhyay [
30] utilized the FEM to study the free vibration of doubly curved stiffened laminated shells. Prusty and Ray [
31,
32] analyzed the free vibration and buckling characteristics of composite stiffened plates and shells with various cross-section stiffeners using the FEM. Thinh and Quoc [
33] used the FEM and experimental methods to study the composite stiffened plate buckling and vibration with various cross-section stiffeners. Mejdi and Atalla [
34] proposed a semi-analytical model based on the modal expansion technique for the vibro-acoustic analysis of the composite stiffened plate. Damnjanović et al. [
35,
36] introduced a dynamic stiffness approach utilizing the FSDT and the HSDT (higher-order shear deformation theory) to analyze the stiffened and cracked laminated plate free vibration. Zhao and Kapania [
37] developed an effective FEM to analyze the pre-stress vibration of curvilinearly stiffened composite plates under in-plane loads. Sinha et al. [
38,
39,
40] conducted numerical and experimental studies on the free vibration of glass–fiber laminated composite stiffened plates. Chandra et al. [
41,
42] investigated the damping and structural dynamic responses of composite stiffened plates in a thermal environment. Wang et al. [
43] formulated an analytical model for the structural dynamic characteristics of stiffened composite sandwich plates embedded with multilayer viscoelastic damping membranes and validated this model through a comparison with FEM results. Peng et al. [
44] introduced a meshless method based on the FSDT to investigate the static and free vibration characteristics of the composite stiffened plate with varying parameters.
The improved Fourier series method (IFSM) is widely used in the vibration analysis of beams, plates, and shells [
45,
46,
47,
48,
49] due to its advantages in handling boundary conditions and achieving efficient computational convergence. Ye et al. [
50,
51] developed a unified model using IFSM for the free vibration of laminated composite shallow shells and moderately thick laminated plates under various boundary conditions. Zhang et al. [
52] derived a series solution for orthogonal rectangular plate in-plane vibration with elastic constraints using IFSM. Wang et al. [
53,
54,
55,
56] applied IFSM to model the vibrations of laminate plates and shells, conducting comprehensive research to explore the effects of material parameters, shell types, and boundary conditions on structural dynamic characteristics. Shi et al. [
57] investigated the free and forced vibration of moderately thick laminated plates under multi-point support boundary conditions through IFSM. Zhang et al. [
58] studied the moderately thick laminated plate free vibration under non-uniform boundary conditions using IFSM. Cao [
59] used IFSM to establish a theoretical model of rectangular stiffened plates with various numbers and lengths of stiffeners, analyzing the dynamic characteristics. Du et al. [
60] formulated theoretical models to analyze the stiffened plate free and forced vibration based on IFSM and conducted experiments to validate the accuracy.
Through the above research, it can be found that the researchers focus on the isotropic stiffened plate and composite laminated shell bulking and free vibration characteristics. The study of the composite stiffened plate forced vibration response is also very important. This paper develops a theoretical model based on the FSDT, Timoshenko beam theory, and IFSM to analyze the free and forced vibration of the composite stiffened plate. Numerical examples demonstrate that the theoretical method presented in this paper has good reliability and accuracy. Utilizing this method, the parametric analysis of the composite stiffened plate is conducted, including geometry parameters, material parameters, ply parameters, stiffener parameters, etc. It aims to understand the vibration mechanisms of the composite stiffened plate and provide theoretical support for low-vibration design.
3. Numerical Results and Discussions
In this section, a numerical procedure is developed using MATLAB 2020 based on the theoretical model in the previous section. Several numerical examples are conducted to demonstrate that the method in this paper exhibits good convergence, accuracy, and reliability. Initially, the convergence of the method is investigated, and a suitable spring stiffness value is selected. Subsequently, the accuracy of the method is verified through comparisons with the FEM results and experimental data from the published literature. Finally, the impact of various design parameters on the vibration characteristics of the composite stiffened plate is discussed.
3.1. Convergency Analysis
This section investigates the convergence of the numerical procedure developed in this paper. The laminate panel dimensions are
a/
b = 1 and
hp/
b = 0.01, and the laminate stiffener are
hs/
hp = 1 and
bs/
b = 0.01. The longitudinal laminate stiffener, aligned parallel to the
y-axis, is positioned at
x =
a/2. The material parameters are
E2 = 10 GPa,
E1 = 20
E2,
G12 = 0.5
E2,
G13 = 0.5
E2,
G23 = 0.33
E2,
μ12 = 0.25, and
ρ = 1500 kg/m
3. The ply schemes of the laminate panel and laminate stiffener are [0/90] and [0]
10, respectively.
Table 1 presents the first eight dimensionless frequency parameters of the composite stiffened plate under fully free boundary conditions. The composite stiffened plate dimensionless frequency parameter is
.
Table 1 reveals that the frequency parameters gradually converge as the mode truncation number increases, indicating that the method exhibits good convergence and accuracy. Considering both computational efficiency and accuracy, the mode truncation number for subsequent numerical calculations is
M =
N = 16.
In this paper, artificial virtual spring stiffness is adjusted to simulate the composite stiffened plate boundary conditions. For the free boundary condition, the spring stiffness value is set to 0. For the clamped boundary condition, the spring stiffness value is ideally set to , but it cannot be achieved in numerical calculations. Therefore, it is necessary to select suitable values for the spring stiffness K (ku, kv, kw, Kx, and Ky) to simulate the clamped boundary condition.
Figure 4 illustrates the first three dimensionless frequency parameters of the composite stiffened plate for varying spring stiffness
K. The geometrical and material parameters of the composite stiffened plate depicted in
Figure 4 are consistent with
Table 1. The boundary conditions of the composite stiffened plate are FCFE, i.e.,
x = 0 and
x =
a are the free boundaries (F),
y = 0 is the clamped support boundary (C), and
y =
b is the elastic support boundary (E). The spring stiffness varies from
K0 (0) to
K15 (10
15). According to
Figure 4, when the spring stiffness value is below
K5, the frequency parameter is minimally influenced by the spring stiffness. As the spring stiffness value increases from
K5 to
K10, the frequency parameter rises rapidly, indicating strengthened boundary constraints. Within this range, the boundary condition is considered as elastic support. Above
K10, the frequency parameter remains essentially unchanged, which can be regarded as clamped support. In this study, the spring stiffness is set to
K13 to simulate the clamped support.
Table 2 lists the spring stiffness values
K for classic boundary conditions at
x = 0.
3.2. Method Verification
In this section, the theoretical results are compared with the FEM results and the experimental results published in the literature to verify the accuracy of the proposed method.
3.2.1. Composite Stiffened Plate Vibration Characteristics Verification
Rikards [
27] explored the free vibration of the composite stiffened plate with a central stiffener under fully clamped boundary conditions. The dimensions are
a = 0.25 m,
b = 0.5 m,
hp = 0.00104 m,
hs = 0.0105 m, and
bs = 0.00364 m. The central stiffener, positioned at
x = 1/2
a, is parallel to the
y-axis. The material parameters are
E1 = 128 GPa,
E2 = 11 GPa,
G12 = 4.48 GPa,
G13 = 4.48 GPa,
G23 = 1.53 GPa,
μ12 = 0.25, and
ρ = 1500 kg/m
3. The ply schemes for the laminate panel and laminate stiffener are [0/ ± 45/90]
s and [0
7/90
7]
s, respectively. Each ply thickness is 0.13 mm. The FEM model of the composite stiffened plate is established using Patran. For the finite element analysis (FEA) of the composite stiffened plate, the upper frequency limit is set at 1000 Hz. Based on the bending wavelength formulation, the maximum element size should not exceed 0.02 m. Both the laminate panel and the laminate stiffener are modeled using shell elements, with a maximum element size of 0.0125 m, and the number of total meshes is 840. Nastran is used to analyze the composite stiffened plate free and forced vibration characteristics.
Table 3 presents the first five natural frequencies of the composite stiffened plate. The average difference between the theoretical results and the FEM results is 2.15% and the maximum error does not exceed 5%.
Figure 5 shows the composite stiffened plate vibration displacement response subjected to single-point excitation. The excitation position is (
a/2,
b/2) and the excitation amplitude is 1 N. It can be found that the theoretical results and the FEM results of displacement response are in good agreement.
3.2.2. Steel Stiffened Plate Vibration Characteristics Verification
GAO [
25] conducted an experimental study on the steel stiffened plate free and forced vibration. The dimensions of the steel stiffened plate are
a = 0.5 m,
b = 0.5 m,
hp = 0.008 m,
hs = 0.01 m, and
bs = 0.01 m. The stiffeners are arranged at
x = 1/4
a and
x = 3/4
a, parallel to the
y-axis. The material parameters are
E = 210 GPa,
μ = 0.3, and
ρ = 7800 kg/m
3. The boundary condition is FCFC, i.e.,
x = 0 and
x =
a are the free boundaries, and
y = 0 and
y =
b are the clamped support boundaries. Patran is used to create the FEM model for the steel stiffened plate. The calculation upper frequency is 1000 Hz. According to the bending wavelength formulation, the maximum element size must not exceed 0.046 m. The panel and stiffeners are modeled using shell elements and beam elements, respectively, with a maximum element size of 0.0125 m. The total number of shell elements is 1600, and there are 80 beam elements. Nastran is employed to analyze the steel stiffened plate vibration characteristics. GAO [
25] performed a modal experiment using the multi-point excitation and single-point pickup techniques. Both sides of the steel stiffened plate are fixed to simulate the clamped boundary conditions. An acceleration sensor is arranged at the center of the steel stiffened plate. A force hammer is used to knock the excitation points one by one, and the natural frequencies and the mode shapes are obtained through frequency analysis. Furthermore, GAO also conducted a forced vibration experiment of the steel stiffened plate under single-point excitation. A vibration exciter, which induces a linear sweep excitation at an amplitude of 1 N, is positioned beneath the center of the stiffened plate and is connected to the stiffened plate through a force sensor. The force sensor is used to record the excitation information from the vibration exciter. Acceleration sensors, arranged at the excitation point (
a/2,
b/2) and the observation point (
a/2,
b/5), are used to measure the vibration acceleration response through the data acquisition system post-processing.
Figure 6 shows the experiment arrangements of the vibration exciter and sensors on the steel stiffened plate.
Table 4 displays the first three modes of the theoretical, FEM, and experimental results and shows that the mode shapes are consistent. The maximum discrepancy between the theoretical and experimental results is less than 5%.
Figure 7 illustrates the steel stiffened plate vibration acceleration levels of the excitation and observation points. From
Figure 7, it is obvious that the theoretical curves are in good agreement with the FEM curves. Both theoretical and experimental curves show similar trends, with relatively minor deviations at single peaks. It demonstrates that the theoretical method presented in this paper effectively analyzes the stiffened plate free and forced vibration.
3.3. Parameter Analysis
The previous section demonstrated that the theoretical method presented in this paper has good reliability and accuracy in studying the composite stiffened plate vibration characteristics. This section explores the impact of the geometric, material, and laminate ply parameters on the free and forced vibration characteristics of the composite stiffened plate. In this section, a composite stiffened plate with two stiffeners is taken as the research object. The stiffeners are located at x = 1/4 a and x = 3/4 a, parallel to the y-axis. Unless specially stated, the laminate material parameters are E2 = 10 GPa, E1 = 20 E2, G12 = 0.6 E2, G13 = 0.6 E2, G23 = 0.5 E2, μ12 = 0.25, and ρ = 1600 kg/m3, while the isotropic parameters are E = 210 GPa, μ = 0.3, and ρ = 7850 kg/m3. The boundary conditions are simply supported.
3.3.1. Free Vibration of Composite Stiffened Plate
Table 5 presents the first eight dimensionless frequency parameters of the composite stiffened plate with various geometric parameters. The laminate panel aspect ratio
a/
b is 1, 1.5, and 2, and the laminate panel thickness–length ratio
hp/
b is 0.004, 0.006, 0.008, and 0.01. The stiffener dimension is
hs = 0.01
b and
bs = 0.01
b. The ply schemes of the laminate panel and stiffeners are [0/90] and [0]
10, respectively. According to
Table 5, it is evident that the composite stiffened plate frequency parameter decreases as the aspect ratio and panel thickness increase.
Table 6 presents the dimensionless frequency parameters of the composite stiffened plate with different anisotropic ratios. The dimensions are
a/
b = 1, 1.5, 2,
hp = 0.01
b,
hs = 0.01
b, and
bs = 0.01
b. The laminate panel material anisotropic ratios are 5, 10, 20, and 30. The laminate panel ply scheme is [45/−45]
s, while the stiffener material is made from isotropic material. As indicated in
Table 6, both the stiffness and frequency parameters of the composite stiffened plate increase as the anisotropic ratio increases.
During designing the laminated composite structure, the ply scheme, ply angle, and ply number are the key design parameters. Reasonable ply design solutions can meet the special functional requirements of the actual project.
Figure 8 illustrates the variation curves of the fundamental frequency parameters of the composite stiffened plates with different ply angles, which vary from 0° to 180° in 5° increments. The dimensions are
a/
b = 1, 1.5, 2,
hp/
b = 0.01,
hs = 0.01
b, and
bs = 0.01
b. The ply schemes of the laminate panel are [
θ]
4, [0/
θ]
2, [0/
θ]
s, and [
θ/0]
s. The ply scheme of the laminate stiffener is [0]
10. According to
Figure 8, the fundamental frequency parameter is symmetrical with respect to 90° across different geometries and ply schemes. The subsequent study on the ply angle can be discussed in the range of 0° to 90°. For the square composite stiffened plates, the fundamental frequency parameter initially increases and then decreases with the change in the ply angle. For the rectangular composite stiffened panels, the fundamental frequency parameter increases gradually. As the plate thickness increases, the trend of the fundamental frequency parameter with the changing ply angle remains consistent, although the value of the frequency parameter decreases.
Figure 9 explores the impact of the ply number on the composite stiffened plate frequency parameter. The ply schemes are [0/90]
n, [30/−30]
n, [45/−45]
n, and [75/−75]
n. The thickness and material of each layer is the same. The results from
Figure 9 indicate that the frequency parameter increases as the ply numbers increase. However, once the ply number reaches a certain level, the frequency parameter tends to stabilize and remain essentially unchanged.
Stiffener structural parameters are crucial for the design of stiffened plate vibrations.
Figure 10 shows the natural frequency variation in the composite stiffened plate with different stiffener heights. The dimensions are
b = 0.5 m,
a = 2
b,
hp = 0.01
b,
bs = 0.01
b, and
hs/
hp = 2, 4, 6, 8, 10. The ply schemes of the laminate panel and laminate stiffener are [0/90]
10 and [0]
10. According to
Figure 10, the natural frequency of the composite stiffened plate increases as the stiffener height increases.
Figure 11 investigates the influences of stiffener space on the natural frequency of the composite stiffened plate. The dimensions are
b = 0.5 m,
a = 2
b,
hp = 0.01
b,
hs = 2
hp, and
bs = 0.01
b. The stiffener space
c is 0.3 m, 0.4 m, 0.5 m, and 0.6 m. As depicted in
Figure 11, the fundamental frequency of the composite stiffened plate decreases as the stiffener space increases.
Figure 12 explores the impact of the stiffener numbers on the composite stiffened plate natural frequency. The number of stiffeners is 2, 4, 6, and 8. The results, as shown in
Figure 12, indicate that the frequency parameter increases with the stiffener numbers.
3.3.2. Forced Vibration of Composite Stiffened Plate
This section studies the vibration displacement response of the composite stiffened plate under single-point excitation, utilizing the theoretical method presented in this paper. The excitation position is (a/2, b/2) and the excitation amplitude is 1 N.
Figure 13 displays the displacement response curves of the composite stiffened plate with varying thicknesses. The dimensions are
b = 0.5 m,
a = 2
b,
hp/
b = 0.004, 0.006, 0.008, 0.01,
hs = 0.01
b, and
bs = 0.01
b. The laminate ply schemes are consistent with
Table 5. According to
Figure 13, as the laminate panel thickness increases, the displacement response curve shifts to the higher frequency region, and the resonant peak value decreases. As the laminate panel thickness increases, the natural frequency and stiffness of the composite stiffened plate also increase.
Figure 14 illustrates the displacement response of the composite stiffened plate with different anisotropic ratios. The dimensions are
b = 0.5 m,
a = 2
b,
hp = 0.01
b,
hs = 0.01
b, and
bs = 0.01
b. The material and ply scheme parameters are consistent with
Table 6. According to
Figure 14, as the anisotropic ratio increases, the resonance peak of the displacement response shifts to a higher frequency, and its amplitude decreases. It is attributed to the improved structural stiffness of the composite stiffened plate, which leads to an increase in the natural frequency.
Figure 15 investigates the influence of ply angle on the displacement response of the composite stiffened plate. The dimensions are
b = 0.5 m,
a = 2
b,
hp = 0.01
b,
hs = 0.01
b, and
bs = 0.01
b. The ply scheme for the laminate panel is [
θ]
4, with the ply angles varying from 0° to 90° in 15° increments. According to
Figure 15, as the ply angle increases, the first-order resonance frequency shifts to a higher frequency region and the corresponding amplitude decreases. The increase in the ply angle significantly affects the higher-order modes, and there is uncertainty in the displacement response curve of the composite stiffened plate.
Figure 16 presents the effect of the ply number on the vibration displacement response of the composite stiffened plate. The ply schemes are [0/90], [0/90]
2, [0/90]
3, [0/90]
5, [0/90]
10, and [0/90]
20. As illustrated in
Figure 16, the displacement response curve shifts to a higher frequency region as the ply number increases. However, as the ply number increases to a certain level, the displacement response curves gradually converge and coincide.
Figure 17 illustrates the influence of the stiffener height on the displacement response of the composite stiffened plate. As the stiffener height increases, the displacement response curve shifts toward the higher frequency region. However, increasing the stiffener height does not uniformly reduce the structural vibration displacement response across all frequencies. This is because the higher stiffener increases the overall stiffness of the structure while changing the local mass distribution.
Figure 18 shows the vibration displacement response of the composite stiffened plate with different stiffener spaces. It shows that as the stiffener space increases, the first resonance peak moves to a lower frequency region, while the first resonance peak value increases. The stiffener space leads to a change in the local stiffness and mass distribution, which has a complex influence on the composite stiffened plate vibration, with different performances at different frequency regions.
Figure 19 analyzes the impact of varying stiffener numbers on the displacement response of the composite stiffened plate. As the number of stiffeners increases, the displacement response of the composite stiffened plate decreases, and the resonance peak shifts to a higher frequency region.