1. Introduction
Driven by the interdisciplinary development of materials science and structural mechanics, the development of material-structural systems and manufacturing technologies that integrate excellent mechanical properties with specialized functionalities has become a hot research focus [
1,
2]. On one hand, modern engineering has an increasingly urgent demand for “structure-function integration”. For instance, protective engineering requires both load-bearing capacity and impact resistance, while intelligent equipment demands the synergy of structural support and controllable actuation. On the other hand, traditional manufacturing technologies struggle to achieve the precise integration of complex structures and functional materials. As a disruptive additive manufacturing technology, 3D printing converts digital models into physical entities through layer-by-layer deposition, breaking the limitations of traditional manufacturing on complex structures [
3]. It not only enables the fabrication of customized complex structures with reduced material waste but also adapts to the processing of various materials such as metals and composites [
4,
5,
6], providing core technical support for the integrated forming of “functional materials-complex structures”.
Arch structures, as classical mechanical structures, have a design history dating back to the tombs and temples of ancient Egyptian and Greek civilizations [
7]. In modern architecture, they are also widely used in large-scale bridges, opera houses, and other structures. By converting vertical loads into axial compressive forces and transferring them along curved paths to the foundation, such structures offer advantages including high mechanical efficiency, strong stability, broad material adaptability, and high space utilization [
8,
9], making them extensively applicable in construction, transportation, and other fields [
10,
11,
12,
13]. The periodic arch-inspired structures (PAS) developed based on this design, which are composed of regularly repeated arch units, further amplify the advantages of mechanical performance [
14]. For example, they have high load-bearing capacity and can outperform other structures with the same material consumption. Zhang et al. [
15] verified the feasibility of large-span construction using arc-shaped hingeless arch structures through a model of the overlying arch in the shallow buried section of a mountain tunnel. The arch geometry forms a self-stabilization mechanism, and the collaboration of multiple units can absorb energy from dynamic loads. The meta-arch structure (MAS) proposed by Sun et al. [
16] and the straight-arch-straight (SAS) series-connected quasi-zero stiffness (QZS) structure proposed by Liu et al. [
17] both demonstrated vibration reduction and isolation effects. Moreover, energy can be dissipated through unit deformation when subjected to transient loads. Rafiee et al. [
18] conducted numerical studies on masonry arches under seismic loads, and the results showed that arches with relatively weak internal structures could still resist strong dynamic excitations effectively.
Notably, endowing PASs with magnetic functionality can expand their applications in fields such as magnetically controlled protection and intelligent load-bearing. For example, magnetically controlled anti-seismic structures can adjust the horizontal thrust at the arch feet via magnetic fields, while load-bearing components of magnetic soft robots need to balance support and magnetic actuation capabilities [
19,
20]. As a material with high magnetic energy product, neodymium–iron–boron (NdFeB) has been widely used in soft robots, biomedicine, and other fields [
21,
22]. Researchers have also explored the applicability of different 3D printing processes for NdFeB materials, including selective laser sintering [
23], fused deposition modeling (FDM) [
24], and 3D gel printing [
25,
26]. Han et al. [
27] designed a magnetically driven soft crawling robot capable of four-directional movement by combining unidirectional magnetic field actuation and leg friction differences. The robot’s legs were made of a composite material of NdFeB particles and silicone, and its torso was fabricated from polyimide film. Li et al. [
28] dispersed NdFeB magnetic particles in a polydimethylsiloxane (PDMS) matrix to design and manufacture an integrated bionic robot with both actuation and sensing functions, confirming the potential of combining NdFeB with 3D printing. Appiah et al. [
29] used 3D printing technology to prepare various Ti-6Al-4V triply periodic minimal surface (TPMS) scaffolds, and the results showed that these scaffolds met the mechanical performance and cytocompatibility requirements for tibial weight-bearing implants.
However, integrating NdFeB powder into PASs and fabricating them via 3D printing still faces multiple challenges [
30]. NdFeB itself has high hardness and brittleness, which easily leads to cracks and deformation during 3D printing. A study by Slapnik et al. [
31] found that NdFeB/PA12 magnets have significant warpage and fracture risks during printing. In addition, the precise control of the material’s microstructure and magnetic properties during the printing process remains an urgent issue to be addressed. Pigliaru et al. [
32] prepared NdFeB–polyetheretherketone (PEEK) composite filaments with different filling contents (25%, 50%, 75%wt) and used FDM 3D printing for forming. They found that the printed samples contained pores, and some NdFeB powder was oxidized during printing, resulting in reduced magnetic properties. Hajra et al. [
33] discovered that 3D printing introduces high porosity into bonded NdFeB magnets, which in turn leads to decreases in the residual magnetization and magnetic energy product of the magnets. Atomized spherical powder, in particular, is more prone to forming micro-pores, thereby significantly impairing the magnets’ performance. The 3D printing of PASs also presents unique technical difficulties, such as the control of interlayer precision for curved units and the adjustment of unit stress uniformity after powder doping. Currently, relevant research lacks exploration into the combination of PASs and NdFeB doping, and the correlation law between NdFeB doping ratio, microstructure, and the mechanical properties of periodic arches has not been clarified.
In response to the above challenges, subsequent research has achieved certain progress. In terms of process optimization, Sun et al. [
34] improved the refinement degree of raw materials through an ultra-fine powder reduction process and optimized the sintering process to enhance the material’s microstructure and densification, thereby significantly suppressing the occurrence of cracks and deformation. The results showed that this method not only improved the mechanical strength and fracture toughness of the magnets but also maintained their excellent magnetic properties. Wang et al. [
35] applied a local magnetic field during printing to reorient NdFeB particles along the printing path, ultimately achieving alignment between the magnetization direction and the printing trajectory. Compared with previous discrete magnetization designs, this method effectively avoids problems such as poor interface bonding and discontinuous magnetization. Damnjanović et al. [
36] found that plasma treatment can significantly improve the mechanical properties of FDM-printed NdFeB/PA12 magnets while ensuring the magnets maintain good magnetic properties and environmental stability. Pigliaru et al. [
37] developed bonded magnets that are both feasible for 3D printing and possess good magnetic properties. Zhou et al. [
38] successfully prepared NdFeB/AlSi10Mg composites. Their work indicated that in the future, particle orientation can be regulated via magnetic field-assisted printing, and composition can be optimized to reduce Al-Nd reactions, thereby improving both the magnetic and mechanical properties of the material. Therefore, 3D-printed periodic structures doped with NdFeB powder exhibit unique advantages and enormous application potential.
Previous studies on the mechanical properties of periodic structures have mainly focused on structures made of traditional materials, covering mechanical responses under static and dynamic loads, including load-bearing capacity, deformation characteristics, stability, and energy absorption mechanisms [
39,
40,
41,
42,
43]. Through theoretical analysis, numerical simulation, and experimental investigation, researchers have established a series of computational models and theoretical systems for the mechanical properties of periodic structures. Li et al. [
44] established a multi-scale TPMS scaffold design model using signed distance fields, interpolation merging methods, and an improved Allen–Cahn equation, and proposed a volume merging algorithm with low computational complexity. The results showed that their method can generate porous structures with smooth transitions and constant mean curvature, suitable for additive manufacturing and topology optimization. Gawronska et al. [
45] established a numerical model based on strain–stress relationships and geometric conditions and conducted finite element analysis. The results showed that geometric factors of the structure have a significant impact on its stress distribution and thermal conductivity, and such structures exhibit lower stress concentration and better load-bearing performance than cubic reference structures. In terms of numerical simulation, Ghafoorian et al. [
46] investigated the heat transfer performance of four TPMS aluminum structures filled with the organic phase change material in a microgravity environment via numerical simulation. They concluded that the Neovius structure has the highest heat transfer efficiency and strongest energy absorption capacity, making it most suitable for passive thermal management. Zhang et al. [
47] prepared Ti-6Al-4V alloy and its lattice structures using selective laser melting (SLM), and established a finite element model by combining low-cycle fatigue experiments with constitutive and damage models. This method accurately predicted the fatigue life of the material and structure, and it was found that the diamond lattice has better low-cycle fatigue resistance than the gyroid lattice. Xia et al. [
48] performed mechanical simulations on two-dimensional periodic heterogeneous materials using the extended multi-scale isogeometric analysis method (EmsIGA). They found that this method can accurately describe the geometry of smooth inclusions, improve computational efficiency and accuracy, and serve as a new model for the analysis of periodic heterogeneous structures. Liu et al. [
49] prepared TPMS structures from Ti-6Al-4V powder via electron beam melting (EBM), established a theoretical model of the effects of topological structure, surface quality, and defects on the stress distribution and fatigue performance of TPMS scaffolds. They proposed that porous scaffolds with excellent fatigue performance under cyclic compressive loads can be obtained by adjusting the topological structure and increasing buckling components. Zhang et al. [
50] prepared Ti-6Al-4V lattice structures using SLM and investigated the effects of different node reinforcement methods on mechanical properties. They found that variable cross-section rods achieved the best results, significantly improving the elastic modulus and yield strength while achieving more uniform stress distribution. Qi et al. [
51] prepared 3D octahedral trusses and truncated octahedral lattice structures using selective laser melting, and investigated the effects of conical rods on mechanical properties by combining compression experiments, SEM fracture analysis, and finite element simulation. They found that node reinforcement can significantly increase the structural modulus and reduce anisotropy.
In summary, current studies on 3D-printed PASs and NdFeB powder doped PASs lack an understanding of the influence of structural parameters and NdFeB doping on the structural mechanical behaviors. Therefore, this work aims to explore the mechanical responses of 3D-printed PASs and NdFeB powder doped PASs. The work will systematically investigate the effects of NbFeB powder doping, wall thickness, relative density, and other factors on structural mechanical properties (including strength, modulus, yield strength, etc.) through quasi-static compression experiment, dynamic impact experiment, and numerical calculations based on different constitutive models. The relationship and mechanism between each influencing factor and structural mechanical properties are revealed. By predicting the mechanical responses of different structures through unit-cell design, periodic structure design, magnetic powder doping design, etc., it can provide theoretical guidance and technical reference for the integrated design and performance improvement of functional structures of PASs.
2. Material and Methodology
This section elaborates in detail on the design and fabrication (
Section 2.1), material characterization (
Section 2.2), design mechanism (
Section 2.3), constitutive models used (
Section 2.4), and finite element analysis method (
Section 2.5) of PASs. In the study, basic mechanical parameters of the material are obtained through tensile tests, and structural specimens with relative densities ranging from 0.18 to 0.48 are constructed by adjusting the wall thicknesses of the PASs. In the mechanical response tests, the mechanical response characteristics of the PASs are acquired through quasi-static compression test and dynamic impact test.
2.1. Structural Design and Fabrication
The design process of the PAS is illustrated in
Figure 1, with the design steps as follows. Step 1: Construct the basic unit. Taking the structural form of bridges as a design reference, an arch beam unit is built to serve as the core load-bearing unit of the structure. Step 2: Form the connected framework. Four arch beams of the same specification are enclosed to construct a closed four-arch connected framework. Step 3: Optimize the stiffened framework. A “cross-shaped” member is added in the central area of the connected framework to form a stiffened framework, so as to realize the dispersion of node stress and improve the structural stability. Step 4: Assemble the unit cell and cell body. Six identical stiffened frameworks are combined according to the spatial orthogonal relationship to form a cubic unit cell; two cubic unit cells are stacked along the axial axis of the unit cell to form a two-layer cell body. Step 5: Form the two-period structure. With four two-layer cell bodies as the basic units, an array arrangement method is adopted to finally form a PAS with two-period characteristics. In this work, experimental tests are carried out on the unit cell, two-layer cell, and two-period structure, respectively, which are named PAS11, PAS12, and PAS22 correspondingly.
All specimens in this work were fabricated using a YDM-L1001 digital light processing (DLP) stereolithography 3D printer (Yidimu, Shenzhen, China) and rigid photosensitive resin. During the fabrication process, through the selective light transmission control of the LCD screen combined with ultraviolet (UV) irradiation, the liquid photosensitive resin was cured layer by layer and stacked to eventually form a 3D structure. The fabrication process of the magnetic periodic arch-inspired structure (MPAS) doped with neodymium–iron–boron (NdFeB) powder is as follows: 1 g of NdFeB powder was weighed using an electronic balance and added to 600 mL of rigid photosensitive resin in the resin tank. The mixture was then fully stirred using a mechanical stirrer to ensure uniform dispersion of the powder. Finally, the homogeneously mixed NdFeB-resin composite was poured into the 3D printer’s material tank, and the specimen was printed according to the preset model parameters. During the specimen preparation process, the relative density of the structure was adjusted by changing its wall thickness. The relative density is defined as
= ρ/ρ_s, where ρ represents the structural density and ρ_s denotes the density of the solid material. The geometric parameters of the PASs with different wall thicknesses are presented in
Table 1. In addition to the 1 g/600 mL ratio, we also prepared resin composites with NdFeB powder at ratios of 1 g/200 mL, 2 g/200 mL, and 3 g/200 mL. Specimens fabricated from these three composites were characterized using a vibrating sample magnetometer (VSM). When the external magnetic field was increased to 20 kOe, the saturation magnetization (Ms) of the samples increased with the NdFeB content, reaching 0.41 emu/g, 0.62 emu/g, and 0.99 emu/g for the 1 g/200 mL, 2 g/200 mL, and 3 g/200 mL samples, respectively. A quantitative analysis indicates that the saturation magnetization increases nearly linearly with NdFeB content in this range; for example, increasing the NdFeB content from 1 g/200 mL to 2 g/200 mL results in approximately a 51% increase in Ms, while increasing from 2 g/200 mL to 3 g/200 mL leads to approximately a 60% increase. These results demonstrate that the magnetic response of the resin composite can be effectively tuned by adjusting the NdFeB content.
2.2. Material Characterization
To meet the material parameter requirements for subsequent finite element analysis, quasi-static tensile tests were conducted on the cured photosensitive resin to obtain its basic mechanical properties.
Figure 2 shows the quasi-static and dynamic experimental equipment used in this study. The tensile specimens adopted dumbbell-shaped standard parts, with the following dimensional parameters: total length of 75 mm, thickness of 2 mm, and gauge length of 20 mm. The speed of the tensile test was set to be 1 mm/min. During the test, the force-displacement curve was recorded in real time and converted into an engineering strain–stress curve after test. Here, the engineering stress is calculated as
σ = F/S, where F is the load recorded during the test and S is the cross-sectional area of the specimen’s gauge section. The engineering strain is calculated as
ε = Δ
l/
l0, where Δ
l is the tensile deformation of the gauge section and
l0 is the initial length of the gauge section.
Figure 3 shows the tensile test process and strain–stress curves, where the stretching process of Test 3 in
Figure 3b is depicted in
Figure 3a. To ensure the reliability and repeatability of the experimental data, each group of tensile tests was repeated 5 times (
Figure 3b), and the average values of the 5 test results were used as the final material property parameters (
Figure 3c). Based on the average strain–stress curve, the linear elastic stage of the curve was selected, and the slope of this stage was calculated via linear fitting, which is defined as the Young’s modulus (E) of the material. Using the fitted straight line of the elastic stage as a reference, a parallel line shifted 0.2% to the right along the strain axis was drawn. The stress value corresponding to the intersection of this parallel line and the strain–stress curve is defined as the yield stress σ
y of the material. The final material parameters of the photosensitive resin are presented in
Table 2.
2.3. Design Mechanism
To clarify the influence of “cross-shaped” stiffeners on the mechanical responses of the structures, comparative analyses of the connected framework and stiffened framework were conducted using COMSOL Multiphysics 6.2. In the simulation, the bottom surface of the framework was set to be fully fixed, and a specified vertical displacement load was applied to the top surface. For the design mechanism analysis, the Hardening Function Model was adopted to describe the plastic behavior of the material, with model parameters derived from the tensile test of the photosensitive resin in
Section 2.2. Finally, equivalent plastic strains and von Mises stresses were used as core evaluation indices to analyze the stress distribution characteristics and plastic deformations of the two frameworks.
The finite element models of the four-arch connected framework and the stiffened framework, along with their maximum von Mises stress results, are presented in
Table 3. In the finite element analyses, all models were discretized using free tetrahedral meshes with four levels of mesh density: normal, fine, finer, and extra fine. The isotropic material model is used for the finite element analysis of both PAS and MPAS in this work. By comparing the results obtained with different mesh densities, it can be observed that the maximum von Mises stress gradually converges as the mesh is refined, indicating good convergence of the finite element solutions. The results reported in this paper correspond to the extra fine mesh. Since the difference in the maximum von Mises stress between the finer and extra fine meshes is negligible, further mesh refinement has a minimal effect on the results. Therefore, to ensure computational efficiency without compromising accuracy, the finer mesh was adopted for all subsequent finite element analyses of the PASs.
From the perspective of structural mechanics, when the top surface of the stiffened framework is subjected to a compressive load, it can be simplified as a planar force system. In the main force transmission path, the vertically and horizontally distributed “cross-shaped” member form core force transmission channel, through which external pressure can be dispersed and transmitted upward, downward, leftward, and rightward. Due to the geometric mutation between the arc segment and the straight segment, stress concentration tends to occur at their junction. Among them, the stress distribution in the straight members (especially the “cross-shaped” intersection area) is relatively uniform. While the arc segment, affected by the curvature effect, exhibits compressive stress on the inner side and tensile stress on the outer side, which conforms to the mechanical characteristics of curved beams. In addition, the connection nodes between arc segments and straight segments (e.g., arc endpoints) are potential stress concentration areas, which are prone to fatigue damage or plastic deformation preferentially under load. In terms of overall deformation characteristics, the framework tends to “concave downward” and compressively shorten in-plane under compressive loads. Since the flexural stiffness of the arc segment is relatively lower than that of the straight segment, its deformation is more significant. When the material’s own stiffness is insufficient, the structure may experience local buckling failure, such as outward bulging of arc segments and lateral bending of straight segments.
Based on the above analysis,
Figure 4 presents the influence of structures with and without cross-shaped stiffener on the distributions of equivalent plastic strains under different strain conditions. For the connected framework without stiffener, the maximum equivalent plastic strain is concentrated at the vertical arch crown, as indicated by the star in
Figure 4a. When the overall strain reaches 20%, the maximum equivalent plastic strain at this location is 0.46. Under the same strain condition, the equivalent plastic strain at the vertical arch crown of the stiffened framework decreases to 0.36, which is 0.1 lower than that of the connected framework. This indicates that the cross-shaped stiffeners can alleviate the accumulation of plastic deformation in the vertical arch crown. However, the location of the maximum equivalent plastic strain in the stiffened framework shifts significantly, moving to the joint between the straight beam on the upper side of the arch and the arch, as indicated by the star in
Figure 4b. The equivalent plastic strain at this location reaches 0.65, which is 0.19 higher than the maximum equivalent plastic strain of the connected framework, making this location prone to shear failure.
Figure 5 illustrates the evolution of von Mises stress in the two frameworks under different strain levels, while
Figure 6 further presents the relationship between strain and von Mises stress at the locations of maximum stress. The results show that, for both the connected framework and the stiffened framework, the locations of maximum von Mises stress coincide with those of maximum equivalent plastic strain. Although the stiffened and unstiffened frameworks exhibit nearly identical peak von Mises stresses (~71.17 MPa vs. 71.11 MPa)—a reflection of the material’s intrinsic strength limit governed by the hardening–softening constitutive behavior (peak at ~12% strain, per
Figure 3b)—the introduction of the cross-shaped stiffener fundamentally alters the structural response: it does not enhance absolute load capacity, but instead redistributes internal forces and plastic deformation. Specifically, stress and equivalent plastic strain—whose maxima coincide spatially—shift from the vertical arch crowns (in the unstiffened frame) to the straight–arc junctions (in the stiffened frame), suppressing local damage in the former while intensifying plastic strain accumulation in the latter. This subtle 0.06 MPa stress reduction at the global peak belies a significant change in failure mechanism: the stiffened structure exhibits more localized plastic zones and a more controllable, geometry-directed failure pathway. Thus, structural performance cannot be judged by peak stress alone; rather, the spatial evolution, localization, and extent of plasticity govern reliability, energy dissipation, and failure sequence—highlighting a key design principle for PAS optimization: balancing global stiffness gains with targeted mitigation of stress concentrations at geometric singularities.
2.4. Constitutive Models
To accurately describe the mechanical behavior of the material and verify the applicability of the simulation model, four different constitutive models were adopted in this work to simulate the experimental process, namely the Ideal Plastic Model, Bilinear Model, Johnson–Cook Model, and Hardening Function Model. Then, the material parameters of the following constitutive equations were obtained through fitting based on the aforementioned tensile test data, as presented in
Table 2.
2.4.1. Ideal Plastic Model
The Ideal Plastic Model refers to a model where after the material reaches the yield stress, the stress remains constant at the yield limit while the strain can increase continuously. The model is expressed as Equation (1).
where
σ denotes stress,
ε denotes strain,
denotes yield stress, and E denotes Young’s modulus.
2.4.2. Bilinear Model
The Bilinear Model describes the material as exhibiting a two-stage linear behavior. In the elastic stage, the strain–stress curve starts from the origin and increases linearly, with the slope being the Young’s modulus of the material. When the stress reaches the yield stress, the curve continues to increase linearly at a different slope. The model is expressed as Equation (2) below.
where E and E
t represent Young’s modulus and tangent modulus, respectively.
2.4.3. Johnson–Cook Model (J-C Model)
The Johnson–Cook (J-C) Model is a constitutive model suitable for conditions of high strain rate, large deformation, and high temperature. Its plastic flow equation is given by Equation (3).
where A is the yield stress, B and n are the hardening modulus and hardening exponent, and C is the strain rate sensitivity coefficient, obtained by fitting the tensile stress–strain curve. The dimensionless strain rate
, where
is the effective plastic strain rate and
is the reference strain rate, often 1 s
−1. The normalized temperature
, where T
0 is the reference temperature, T
m is the melting temperature, T is the current temperature, m is the thermal softening exponent. When the effect of temperature is not considered, a simplified version of the J-C Model can be used, as shown in Equation (4).
2.4.4. Hardening Function Model
Hardening Function Model is a mathematical expression that describes how the yield stress
changes with plastic deformation after the material enters the plastic stage, and it is given by Equation (5).
where
σy0 denotes the initial yield stress. In this work, after the material reaches the initial yield stress, the measured strain–stress curve of the material was used as the hardening function
and imported into the numerical model for simulation.
2.5. Finite Element Analysis
All finite element simulations in this study were carried out using COMSOL Multiphysics 6.2. For the quasi-static compression simulations, the bottom surface of the arch structure was fully fixed, and a prescribed downward displacement was applied to the top surface. Global engineering strain was defined as the ratio of the applied displacement to the initial specimen height, consistent with the experimental protocol. The global compressive stress was computed as the total axial reaction force on the fixed bottom boundary divided by the nominal cross-sectional area, which is equivalent to area-averaging the normal stress over the bottom surface. This definition ensures force equilibrium and direct comparability with experimental engineering stress–strain curves. The resulting global stress–strain relationship was then constructed by plotting the computed stress against the prescribed strain.
To ensure the accuracy and reliability of the numerical results, a mesh refinement study was conducted by progressively increasing the mesh density from normal and fine to finer and extra fine. The results indicate that key mechanical indicators, such as the maximum von Mises stress, gradually converge with mesh refinement, demonstrating good mesh convergence of the finite element solutions. Considering both computational accuracy and efficiency, the finer mesh was adopted for most simulations, as further refinement to the extra fine mesh resulted in negligible changes in the calculated results.
The boundary conditions employed in the dynamic compression simulations were generally consistent with those used in the quasi-static simulations, with the primary difference being the analysis type. The quasi-static compression simulations were performed using a stationary (steady-state) analysis to represent slow loading under low strain-rate conditions. In contrast, the dynamic compression simulations were conducted using a transient analysis approach. The total simulation time was set to 0.00045 s for PAS11 and 0.00065 s for PAS12 and PAS22, corresponding to loading velocities of 1.6 m/s and 3.0 m/s, respectively. The time domain was discretized using a time step of 5 × 10−5 s to accurately capture the dynamic mechanical responses of the structures under high strain-rate loading. Post-processing of energy histories confirms that kinetic energy accounts for >10% of internal energy at early loading stages, verifying the necessity of transient (inertial) analysis for the given impact velocity (1.6–3.0 m/s).