Flexural performance of strain-hardening cementitious composite joints with bio-inspired jigsaw-shaped interlocking suture interfaces

Abstract

Bio-inspired interlocking sutures, inspired by the diabolical ironclad beetle, exhibit tunable mechanical behavior and are strong candidates for designing structures with enhanced mechanical properties. These interlocking sutures maintain strong flexural strength, absorb energy, and exhibit toughening and abnormal deformation mechanisms. Inspired by the bio-inspired interlocking suture, we designed specimens of Strain-Hardening Cementitious Composite (SHCC) with sutures of various geometries. The effects of interlocking sutures on mechanical and failure characteristics were studied via flexural performance tests. The toughening and abnormal deformation mechanisms of jigsaw-shaped interlocking suture joints were explored. A balance has been discovered between toughness, flexural strength, and energy absorption, allowing for optimal utilization of the performance of the bio-inspired suture joint. The optimal interlocking angle α and a:b ratio for bio-inspired suture joints were developed, achieving 9.24 times greater bearing capacity and 219.3 times greater energy absorption than conventional linear joints. These captivating findings present significant opportunities for revolutionizing the design of bio-inspired jigsaw-shaped interlocking sutures, positioning them as a promising joining technology for concrete structures.

Keywords

bio-inspired interlocking structure strain-hardening cementitious composite (SHCC)flexural performance suture joint failure mode

Introduction

Conventional concrete is a brittle material with high compressive strength and weak tensile strength. Under low tensile loads, cracks can readily occur and extend rapidly, exhibiting the concrete’s brittle failure characteristics (Xu and Šavija, 2019). Therefore, conventional concrete is reinforced with steel bars in most actual projects. Well-designed reinforced concrete structures have a reliable service life due to the high strength and excellent tensile resistance of steel bars (Van Overmeir et al., 2022). However, under extreme loads such as earthquakes, explosions, and continuous vibrations, reinforced concrete structures are still subject to brittle failure, posing great hazards (Yu et al., 2018b). Simultaneously, the frequent demand for repairing concrete structures due to issues such as cracking, surface spalling, and steel corrosion incurs significant costs (Yu et al., 2018a). Effectively improving the performance of concrete infrastructure has become a huge challenge for engineers around the world.

Strain-hardening cementitious composite (SHCC), also known as engineered cementitious composite (ECC), is a relatively new fiber-reinforced material developed in the early 1990s (Chaves Figueiredo et al., 2019; Li et al., 2020; Yu et al., 2021). Compared with conventional concrete, SHCC exhibits higher ductility under uniaxial tensile loading (Alrefaei and Dai, 2018; Yu et al., 2018b, 2020a), cyclic loading (Jun and Mechtcherine, 2010; Yu et al., 2019), fatigue loading (Huang et al., 2018), and impact loading (Lu et al., 2018; Mechtcherine et al., 2011). Once initial micro-cracks appear in the concrete matrix, the fibers can effectively pull the matrix on both sides of the crack, causing the load at the crack to be redistributed, thereby restoring the force balance of the concrete component. As the load increases further, the concrete can crack at different locations where the same mechanism occurs and continuous, multiple, steady-state cracks are formed. This happens repeatedly until the tensile force within the material is greater than the bridging ability of the fibers at the cracks. At this point, concrete experiences local failure and the specimen eventually fails. In this way, SHCC can evenly distribute plastic deformation during an entire loading program and suppress brittle fractures (Yu et al., 2021). Therefore, SHCC exhibits saturated multiple crack behavior, which confers on the specimen higher damage tolerance and ultimate tensile strain capacity, ensuring high tensile ductility and durability (Cai et al., 2021; Lei et al., 2021; Qin et al., 2020; Yang et al., 2009). In fact, SHCC has hundreds of times the tensile ductility and toughness of conventional concrete (Nguyễn et al., 2020; Yoo et al., 2021; J Yu et al., 2020a; K Yu et al., 2020b). Various standards and specifications provide mandatory requirements for the elongation of steel bars, among which the minimum elongation is not less than 9% (Liu et al., 2020; Luo et al., 2019; Zhang et al., 2019). The latest research shows that the tensile ductility performance of the most ductile SHCC is close to the elongation of steel bars. Hence, SHCC can equal or approach the tensile performance requirements of steel bars (Guo et al., 2020; Li et al., 2019; Oh et al., 2022; Yu et al., 2017). Accordingly, SHCC is expected to be used alone, without steel bars, in advanced concrete projects.

In engineering, the connection joints between specimens are often the areas most prone to fracture (Chen et al., 2017; Wang and Bi, 2019; Yang et al., 2023). Suboptimal connection types can reduce the strength of specimens and cause unpredictable, catastrophic failure. Since SHCC can be used as a singular concrete material without steel bars, the connection joints used with it become an important influence on the overall performance of SHCC specimens. Existing methods for connecting structural components mainly include riveting, welding, and adhesion (Hoang et al., 2010; Machalická and Eliášová, 2017; Sun et al., 2018). Based on concrete structural joints, extensive research has revealed the superiority of novel wet joints, including inclined wet joints and sawtooth wet joints, over conventional wet joints in terms of crack resistance (Hoang et al., 2011). The innovative dovetail connection joints offer enhanced strength and stiffness, enabling them to withstand higher loads and exhibit superior earthquake resistance (Centelles et al., 2019). Based on these pivotal findings, our primary emphasis lies in the development of innovative node designs that hold immense potential for enhancing the advantages of SHCC. The goals are to (1) find a type of high-performance connection joint that does not require additional materials and procedures (such as steel bars) and (2) optimize the joint parameters to improve interfacial connection performance and give SHCC joints outstanding mechanical properties.

Optimization algorithms are often the primary method used in structural optimization design problems (Malik and Barthelat, 2016; Malik et al., 2017). Organisms possess highly optimized structures that have evolved over a long history of natural selection. Biologically inspired researchers have explored efficient connection joints inspired by natural structures, offering valuable insights for creating new joint designs in engineering structures. In line with this approach, the suture interface is a fascinating example of a geometrically structured joint found in biology (Figure 1). It consists of an interlocking mechanism where a relatively pliable interface layer connects two rigid components. This unique joint configuration allows for a wide range of properties and functions, which can be achieved through variations in geometry. Surprisingly, the suture interface exhibits enhanced mechanical properties, making it a valuable source of inspiration for connection design in various applications. This type of suture interface has been observed in diverse biological structures, including bone and armored exoskeletons (Barthelat et al., 2007; Dunlop et al., 2011; Ji and Gao, 2004; Tang et al., 2007), craniums (Herring, 2008; Hubbard et al., 1971; Jaslow, 1990; Persson et al., 1978; Pritchard et al., 1956), turtle carapace (Krauss et al., 2009), and even algae (Gebeshuber et al., 2003). The suture interfaces found in these examples demonstrate excellent mechanical properties. For instance, cranial sutures have been found to absorb more energy than the surrounding skull during impact loading (Jaslow, 1990). The teeth-like structures within the suture interfaces contribute to enhanced flexural strength and reduced strain energy (Jasinoski et al., 2010).

Figure 1.

Suture interface in nature. (a) Suture on a woodpecker’s beak (Lee et al., 2014). (b) Sagittal sutures of the human cranium. (c) Sea urchin has an external architecture that force cracks to follow a sinusoidal path (Hosseini et al., 2019). (d) Example of a natural suture interface in a turtle shell (Alheit et al., 2021). (e) Armadillo osteoderm sutures connected with collagen fiber. (f) Marine threespine stickleback (Liu et al., 2017).

Of all the suture joints, the elytra sutures of the newly discovered diabolical ironclad beetle are particularly noteworthy. The diabolical ironclad beetle Phloeodes diabolicus can survive trampling by humans and even cars. In Nature, Kisailus reported on the suture interface of these beetles based on CT scans, microscopy, three-dimensional printed models, and computer simulations (Rivera et al., 2020). The ironclad beetle can withstand a maximum load of 149 N, equivalent to 39,000 times its own weight, which is equivalent to putting two aircraft on a 200-pound person. Microscopic observation of the cross-section of the ironclad beetle shows that the jigsaw unit of the junction of the elytra can absorb energy and buffer impacts, thus preventing damage to biological structures (Figure 2).

Figure 2.

Phloeodes diabolicus. (a) Diabolical ironclad beetle. (b) Cross-section of the elytra (Rivera et al., 2020). (c) Junction of its elytra. (d) Geometric features of the elytra’s jigsaw unit.

Inspired by the interlocking sutures of diabolical ironclad beetles, a suture interface was adapted to engineering structures. Sachini et al. used a fused deposition modeling printing technique to 3D print a bio-inspired suture interface and study its bending response behavior. The bending behavior of several suture designs was analyzed through three-point bending tests, along with digital image correlation and numerical simulations, to understand the deformation process (Wickramasinghe et al., 2023). Subsequently, they developed three different 3D-printed plates to observe the effects of the size and number of interlocking points on fracture behavior. The suture joints helped control crack propagation along the direction of the suture specimens, which helped predict fracture performance (Wickramasinghe et al., 2022). Jie et al. established a theoretical model of the ironclad beetle suture joint, analyzed the mechanical behavior of its jigsaw unit during stretching, and determined the influences of quantity, angle, and geometry on the mechanical properties of the jigsaw unit (Wei and Sun, 2023).

The optimization of suture joints requires a multi-dimensional balance between toughness, flexural strength, and energy absorption. The interlocking suture joints of diabolical ironclad beetles provide a great opportunity for further research on the mechanical behavior of natural suture interfaces. To the best of our knowledge, there are no examples of interlocking suture joints in concrete structures. Applying the interlocking and toughening effects of unique suture joints to building design is a new opportunity for structural engineers. Before considering suture joint applications, researchers need to further evaluate mechanical interlocking sutures; for example, to characterize their structural toughness and failure characteristics under bending and to further understand the mechanical properties of interlocking sutures. To this end, the present study conducted flexural performance tests on the SHCC suture joints with various conbined suture modules and values of the a:b ratio and interlocking angle α. The effects of geometry on the mechanical properties and failure characteristics of the connection interface were studied, and the mechanisms of toughening and abnormal deformation in the sutured joints were explored. By achieving a harmonious combination of toughness, flexural strength, energy absorption, and failure mode, the bio-inspired jigsaw-shaped interlocking sutures demonstrated outstanding performance. This innovative approach to joining concrete structures based on bio-inspired sutures offers a fresh and novel perspective.

Experimental program

Mixture design

Type I Portland cement, Class F fly ash, and microsilica were used as binders. The aggregate used in the SHCC mixture was silica sand with a particle size of 180–425 μm. Nanoclay, a highly purified hydrous magnesium aluminosilicate, was used as a thixotropic agent to improve the rheological properties of the fresh mixture (Xu et al., 2022). Ultra-high-molecular-weight polyethylene (PE) fiber was used to improve the ductility of the SHCC mixture, which had the properties shown in Table 1. The mix proportions of the SHCC mixture were shown in Table 2, in which the mass of nanoclay in the binder was 0.5% and the volumetric content of PE fiber was 1.0%.

Table 1.

Physical properties of PE fiber.

Density (g/cm³)	Length (mm)	Cross section	Diameter (μm)	Tensile strength (GPa)	Elastic modulus (GPa)	Elongation (%)
0.97	12	Nearly round	24	3.0	110	2.0-3.0

Table 2.

Mix proportions of the SHCC mixture.

Binder (B)			Nanoclay/B	Silica Sand/B	Water/B	PE fiber
Portland cement	Fly ash	Microsilica	Nanoclay/B	Silica Sand/B	Water/B	PE fiber
0.6	0.3	0.1	0.5%	0.2	0.35	1.0 vol%

Suture design

Six specimens based on the jigsaw-shaped blade of the diabolical ironclad beetle were designed. Various values of the a:b ratio (1.0:1.0, 1.0:1.4, 1.0:1.8, and 1.0:2.2) and interlocking angle (15°, 25°, and 35°) were used to observe the effects of geometry on flexural performance. The a and b are the small axis and large axis of the ellipse, respectively, as shown in Figure 2. Subsequent to flexural mechanical testing on these specimens featuring diverse suture geometries, the fracture behavior of the suture joints was analyzed, leading to the identification of the optimal interlocking suture. Leveraging this optimal suture design, three suture modules of different sizes were designed, with the number of jigsaw units varied to form four sub-designs. The scheme employed in this study aimed to investigate the ideal suture combination that yields superior performance advantages and mechanical properties. The designs of the specimens are shown in Figure 3, and their geometric parameters are shown in Table 3.

Figure 3.

Geometric designs of the bio-inspired jigsaw-shaped interlocking sutures.

Table 3.

Geometric parameters of the specimens.

Group	Number	Angle α	a:b ratio	b radius (mm)	a radius (mm)	Thickness (mm)	Quantity
Contral group	G-L	/	/	/	/	60	3
Test group	A_15°R_1.8	15°	1.0:1.8	13.88	24.98	60	3
	A_25°R_1.8	25°	1.0:1.8	14.66	29.39	60	3
	A_35°R_1.8	35°	1.0:1.8	20.11	36.20	60	3
	A_25°R_1.0	25°	1.0:1.0	24.83	24.83	60	3
	A_25°R_1.4	25°	1.0:1.4	19.19	26.87	60	3
	A_25°R_2.2	25°	1.0:2.2	14.66	32.25	60	3
	N₃G	25°	1.0:1.8	/	/	60	3
	N_2-LG	25°	1.0:1.8	/	/	60	3
	N_2-SG	25°	1.0:1.8	/	/	60	3

Note: The N₃G specimen comprises three jigsaw units, each with a size of 30 mm. Both the N_2-LG and N_2-SG specimens incorporate a combination of one 30 mm and one 60 mm jigsaw unit. In the bending test, the N_2-LG specimen positions the 60 mm jigsaw unit on the lower surface, while the N_2-SG specimen places the 30 mm jigsaw unit on the lower surface.

Specimen preparation

Molds with different geometric shapes were printed to block the fiber connections at the positions of the jigsaw units in the SHCC specimens. The material used for the molds was Polylactic Acid (PLA) filament, with a diameter of 2.85 mm. A three-dimensional drawing of each mold was generated in Solidworks software before they were prepared by a computer-controlled 3D printer. The printing speed was 150 mm/s, while the printing height was controlled to 60 mm by adjusting the number of repetitions of each pattern. The prepared 3D-printed molds are shown in Figure 4.

Figure 4.

3D-printed molds.

After mixing, the SHCC mixture was poured into the printed molds, which were placed on a vibrating table to eliminate air bubbles. Afterward, the mold was sealed with plastic film and cured for 24 h. The suture geometries and combinations of jigsaw units are shown in Figure 5. Then, each specimen was fixed on one side of a mold with dimensions of 360 mm × 90 mm × 60 mm, and the SHCC mixture was poured on the other side of the mold to make a rectangular specimen. Finally, the specimen was placed in a curing tank at a temperature of 23 ± 2°C for 28 days. Through time-staggered curing of the modules on the left and right sides of each specimen, the fiber connections within the SHCC at the position of the jigsaw unit were blocked.

Figure 5.

Bio-inspired jigsaw-shaped suture joint.

Four point bending tests

A symmetrical loading scheme was utilized to conduct tests on the flexural behavior of the SHCC specimens with suture joints. Before loading, each specimen was accurately positioned on a 500 kN hydraulic testing machine. The lower part of the hydraulic testing machine was placed on a distribution beam with a span of 130 mm, which was used to transfer the load evenly to the specimen. A vertical load was applied to the mid-span position of the specimen via a Material Testing System (MTS). The specific test layout is shown in Figure 6.

Figure 6.

Experimental loading device.

Loading system: First, the specimen was preloaded according to a load-control scheme of applied load = 0.5 kN and loading speed = 0.5 kN/min. In the formal loading stage, the loading program adopted a displacement-control scheme of displacement increment = 0.5 mm/min until the specimen broke or the bearing capacity diminished to 40% of the maximum bearing capacity.

Testing Procedures: A pressure sensor was placed at the mid-span section of the specimen to test the load applied to the specimen. LVDT displacement meters were placed on both sides of the mid-span position of each specimen to test its overall deflection. Four strain gauges were pasted on the side opposite to the mid-span position to test the concrete’s strain changes along the section height during the bending process.

Experimental results and analysis

Load-deflection curve

During the loading process, the specimens experienced three loading stages. The important points of load and deflection in different loading stages are shown in Figure 7.

(1) First stage, from initial loading to the first peak load (P_u1: the maximum load of the specimen due to suture bonding): the specimen had high stiffness, low deflection, and a linear load–deflection curve. The specimen was in the elastic stage and showed strong elastic performance.

(2) Second stage, from the first (P_u1) to the second peak load (P_u2: the maximum load due to the interlocking effect of the jigsaw unit): During this stage, the stress generated by the suture bonding of the specimen gradually transitions to being carried by the interlocking effect of the jigsaw units. As the loading program advanced, cracks persistently propagated along the suture, while the interlocking mechanism of the jigsaw units remained operational. Consequently, the load-bearing capacity steadily increased, eventually leading to the emergence of the second peak load (P_u2). During this phase, the jigsaw unit of the specimen exhibited the most robust interlocking effect. The deflection between the two peak loads is denoted as Δl₂. The greater the deflection, the higher the predictability of the specimen, making it a crucial indicator for our observations.

(3) Third stage, from the second peak load (P_u2) until specimen failure or the load decreased to 40% of the maximum peak load. At this stage, cracks continued to develop in the specimen, deflection continued to increase, and the specimens exhibited different failure modes.

Figure 7.

Load-deflection curve of specimen at different loading stages.

Table 4 presents the load and deflection data for key points obtained from the bending test of the specimens. The P_u2 values of specimens A_25°R_1.8, N₃G, and N_2-LG were respectively 19.7%, 24.6%, and 34.5% higher than their corresponding P_u1 values. This highlights the enhanced load-bearing capacity achieved through the interlocking sutures. Notably, even after suture bonding failure occurs in a specimen, the interlocking effect of the jigsaw unit continues to bear the load, showcasing a strong load reserve. Among the specimens, A_25°R_1.4 displayed the largest deflection deformation between the two peak loads. This observation indicates that the specimen retains substantial deflection even under higher load levels, highlighting its strength and high energy reserve performance. Specimens N_2-LG and N_2-SG exhibited distinct advantages in terms of bearing capacity and deflection deformation. These specimens effectively balance the strength, toughness, and energy absorption performance of the structure, offering a well-rounded performance.

Table 4.

Load and deflection of key points in bending test of specimens.

Specimens	P_u1 (kN)	P_u2 (kN)	Growth rate (%)	∆l₁ (mm)	∆l₁+∆l₂ (mm)	∆l₁+∆l₂+∆l₃ (mm)	∆l₂ (mm)
G-L	1.17	/	/	0.266	0.266	0.332	0
A_15°R_1.8	6.12	6.19	+1.1	1.199	1.466	6.258	0.267
A_25°R_1.8	9.03	10.81	+19.7	0.390	1.592	7.107	1.202
A_35°R_1.8	4.74	4.64	−1.1	0.303	0.820	2.038	0.517
A_25°R_1.0	4.91	5.13	+4.5	0.494	1.287	5.672	0.793
A_25°R_1.4	8.99	8.89	−1.1	1.045	2.932	8.509	1.887
A_25°R_2.2	7.21	7.15	+0.8	1.812	2.376	6.368	0.564
N₃G	6.90	8.60	+24.6	0.498	1.396	5.426	0.898
N_2-LG	7.54	10.14	+34.5	0.420	1.963	6.848	1.543
N_2-SG	7.63	9.11	+19.4	0.445	2.238	8.564	1.793

Crack propagation procedure

Reference specimen G-L

As the load increased, the load of the G-L eventually reached a peak value (P_u1), at which time the maximum load was caused by the suture bonding. After that, the load decreased rapidly and cracks extended rapidly along the height direction and quickly formed a penetrating crack. Finally, the specimen became damaged. An example of crack propagation at different loading stages is shown in Figure 8. The specimen showed brittle failure characteristics; that is, it rapidly underwent plastic deformation and failure after cracks appeared. The specimen had low deflection and poor energy absorption and ductility properties.

Figure 8.

Crack propagation procedure of specimen (G-L) at different loading stages.

Effects of interlocking angle

Specimens with three different interlocking angles showed different failure modes during the loading process. After the cracks in specimen A_15°R_1.8 developed along the interlocking suture to 2/3 of the cross-sectional height, cracks no longer increased and, eventually, an oblique crack appeared at the edge of the jigsaw unit, causing compression-bending failure (Figure 9(a)). After the cracks developed to a certain height in specimen A_25°R_1.8, the interlocking effect of the suture joints began to take effect and the load on the specimen continued to increase. Eventually, Cracks 2 and 3 appeared at the neck and edge positions of the jigsaw unit, respectively, causing a combined failure (Figure 9(b)). Under combined failure, cracks appeared almost simultaneously on the neck and outer edges of the jigsaw unit. The specimen had a high bearing capacity and a large deflection, resulting in strong energy absorption performance when the specimen was damaged. The cracks developed to the neck position in specimen A_35°R_1.8 and then cracked along the neck of the jigsaw unit. Finally, a penetrating crack formed at the neck position of the jigsaw unit, causing a necking failure (Figure 9(c)). The specimen had a relatively low bearing capacity and deflection when it failed.

Figure 9.

Crack propagation procedure of specimens with different interlocking angles. (a) A_15°R_1.8. (b) A_25°R_1.8. (c) A_35°R_1.8.

Effect of minor/major radius ratio

The crack propagation at different loading stages is shown in Figure 10. After the crack in specimen A_25°R_1.0 first appeared, it extended rapidly along the interlocking suture until it penetrated the entire jigsaw unit, and eventually suture failure occurred (Figure 10(a)). The second peak load (P_u2) produced by the interlocking effect of the jigsaw units was smaller than the first (P_u1). The interlocking effect of specimen A_25°R_1.4 has been effectively enhanced, and the corresponding deflection between the two peak loads was relatively large and the specimen had strong ductility performance. Eventually, Cracks 2 and 3 appeared at the edge of the jigsaw unit, causing combined failure (Figure 10(b)). The crack propagation pattern of specimen A_25°R_1.8 can be seen in Section 3.1.1. After the cracks in A_25°R_2.2 appeared, they first extended along the interlocking suture until the neck edge of the jigsaw unit; then, the interlocking effect of the jigsaw unit rapidly came into play, causing the crack to develop slowly along the interlocking suture. Subsequently, Oblique Crack 2 appeared in the weakest section of the jigsaw unit and a through crack formed, resulting in compression-bending failure (Figure 10(c)).

Figure 10.

Crack propagation procedure of specimens with different a:b ratios. (a) A_25°R_1.0. (b) A_25°R_1.4. (c) A_25°R_2.2.

Effect of combined suture modules

Figure 11 shows the crack propagation procedure of the specimens at different loading stages. After the first crack appeared at the interlocking suture on the bottom of the jigsaw unit of specimen N₃G, the interlocking performance of the specimen’s joints quickly took effect, so that the bearing capacity did not decrease rapidly at the loading point. As the cracks extended, the first jigsaw unit on the lower surface underwent necking failure and the cracks penetrated the neck of the first jigsaw unit. Subsequently, the second and third jigsaw units suffered necking failure, causing the specimens to lose their bearing capacity (Figure 11(a)). After specimen N_2-LG reached its first peak load (P_u1), the crack developed along the jigsaw unit. Later, the bottom jigsaw unit plays an interlocking effect, and the bearing capacity gradually reaches the second peak load (P_u2). Subsequently, the crack continued to expand, and eventually penetrated the entire neck of the two jigsaw units, causing necking failure (Figure 11(b)). In specimen N_2-SG, the crack passed through a small jigsaw unit to form penetrating cracks. Subsequently, the interlocking effect of the large jigsaw unit came into play and the specimen gradually reached the second peak load (P_u2). Afterwards, small cracks appeared at the neck position of the large jigsaw unit but a high load was maintained. Eventually, necking failure occurred at the neck position of the second jigsaw unit and the specimen gradually lost its bearing capacity (Figure 11(c)).

Figure 11.

Crack propagation procedure of specimens with different combined suture modules. (a) N₃G. (b) N_2-LG. (c) N_2-SG.

Failure modes

The specimens displayed four distinct failure modes, namely suture failure, compression-bending failure, necking failure, and combined failure (Figure 12). The failure modes and characteristics of the specimens are shown below (Table 5).

Figure 12.

Failure modes. (a) Suture failure. (b) Compression-bending failure. (c) Necking failure. (d) Combined failure.

Table 5.

Failure modes and reasons of specimens.

Failure mode	Number	Reasons and characteristics of failure
Suture failure	A_25°R_1.0	Characteristics: The specimen exhibits low deflection and bearing capacity, along with inadequate energy absorption performance. Consequently, the specimen displays poor predictable performance.
Suture failure	A_25°R_1.0	Possible reason: The a:b ratio of the A_25°R_1.0 is 1:1, the ellipse in the jigsaw unit degenerates into a circle, which reduces its interlocking effect, causing suture failure along the jigsaw unit.
Compression-bending failure	A_15°R_1.8 and A_25°R_2.2	Characteristics: The specimen demonstrates a low deflection between two peak loads and exhibits relatively poor energy absorption performance.
Compression-bending failure	A_15°R_1.8 and A_25°R_2.2	Possible reason: Tensile damage occurs at the outer edge of the suture joint due to the action of the bending moment, which manifests as concrete damage caused by tensile stress.
Necking failure	A_35°R_1.8	Characteristics: The specimen displays low bearing capacity, minimal deflection, poor predictability, and is highly susceptible to brittle failure.
	A_35°R_1.8	Possible reason: The jigsaw unit of the specimen has a small neck cross-sectional area, and cracks can quickly penetrate the neck of the jigsaw unit to form penetrating cracks.
	N₃G, N_2-LG, and N_2-SG	Characteristics: The specimen exhibits high bearing capacity, significant deflection, robust energy absorption performance, and impressive plastic deformation capability.
	N₃G, N_2-LG, and N_2-SG	Possible reason: Since the specimen has relatively small jigsaw units, the jigsaw units close to the lower surface bear a tensile load, causing necking failure of the jigsaw unit.
Combined failure	A_25°R_1.8 and A_25°R_1.4	Characteristics: The specimen has significant characteristics such as high bearing capacity, large deflection and efficient energy consumption.
Combined failure	A_25°R_1.8 and A_25°R_1.4	Possible reason: Under combined failure, cracks appeared almost simultaneously on the neck and outer edges of the jigsaw unit, resulting in strong bearing capacity.

In the event of combined failure, cracks manifested nearly simultaneously at both the necking and outer edges of the jigsaw unit, establishing a robust interlocking effect. This interlocking effect contributed to the specimen’s notable attributes of high bearing capacity, substantial ductility, and efficient energy dissipation performance. More importantly, there was a large deflection deformation between P_u1 and P_u2. When using this type of jigsaw unit in a real project, these cracks can be checked regularly to prevent early brittle fractures. By adjusting the suture geometry, structural components and connectors with different mechanical properties are designed and manufactured, enhancing the fracture behavior of engineering joints and improving the mechanical properties and reliability of structural components. For the specimen with conbined suture modules, since the jigsaw units of the specimens were relatively small, those closest to the lower bottom surface bore the tensile load, which caused them to gradually transform from compression-bending failure of A_25°R_1.8 to necking failure. The specimens could maintain a large deflection at a higher load level (80% of the ultimate load).

Mechanical performance assessment

Effect of interlocking angle α on mechanical properties

Figure 13 shows the load–deflection curves, peak load and energy absorption of specimens with different interlocking angles α. Compared with specimens A_35°R_1.8 and A_15°R_1.8, the bearing capacity of A_25°R_1.8 was 1.75 times and 1.73 times higher, respectively, and the energy consumption was 2.28 times and 7.01 times higher. The likely reason is that A_15°R_1.8 had a low interlocking angle and weak interlocking effect, giving it a relatively low bearing capacity under the action of a bending load. Specimen A_35°R_1.8 had a large interlocking angle; however, its jigsaw unit had a small cross-sectional neck area. Under bending load, the crack directly penetrated the neck of the jigsaw unit, causing neck failure and reducing the specimen’s bearing capacity. A more open and extreme interlocking angle design did not translate into a more favorable combination of strength and toughness. Considering the flexural strength, toughness, energy absorption, and failure mode, specimen A_25°R_1.8 (α = 25°) exhibited an ideal interlocking effect and achieved the expected effect on the suture joint.

Figure 13.

Effect of interlocking angle α on mechanical properties. (a) Load–deflection curves. (b) Peak load and energy absorption of jigsaw units.

Narrow grooves were designed in the SHCC to guide cracking and modify the material’s deformation and fracture characteristics. In comparison to specimen G-L, the bearing capacity and energy absorption performance of specimen A_25°R_1.8 increased by 9.24 times and 182.33 times, respectively. By adjusting the interlocking angle of the jigsaw units, a significantly improved interlocking effect is attained, leading to enhanced flexural strength and energy absorption in the bio-inspired suture joints. This enhancement is vital, especially for mitigating crack propagation in scenarios involving impact and seismic activity.

Effect of minor/major radius ratio on mechanical properties

Figure 14 shows the load–deflection curves, peak load and energy absorption of specimens with jigsaw units with different a:b ratios. As the a:b ratio decreases, the interlocking effect makes it difficult to separate the upper and lower parts of the jigsaw unit. The interlocking effect of specimen A_25°R_1.8 reached its peak, with the load of P_u2 being 19.7% higher than that of P_u1. Upon closer observation, it was noted that among the four specimens, specimen A_25°R_1.4 exhibited the greatest deflection between the two peak loads. This characteristic enables the specimen to sustain higher deflection under a higher load level, ultimately resulting in the strongest energy absorption performance.

Figure 14.

Effect of minor/major radius ratio on mechanical properties. (a) Load–deflection curves. (b) Peak load and energy absorption of jigsaw units.

Taking into account flexural strength, energy absorption, and failure mode, different jigsaw units can be used to achieve the desired performance. For example, the design of A_25°R_1.8 is better than those of A_25°R_1.0, A_25°R_1.4, and A_25°R_2.2, as it has higher flexural strength. Under bending loads, A_25°R_1.4 exhibits the maximum displacement, high ductility, and outstanding energy absorption. The bearing capacity of the specimen A_25°R_1.4 with a bio-inspired jigsaw-shaped suture was 7.68 times higher, while the maximum energy absorption was 209.3 times higher. The suture changes the brittle fracture characteristics of conventional concrete joints and provides excellent energy absorption performance. The toughness and energy absorption of a suture joint can be effectively improved and enhanced by optimizing the suture design.

Effect of conbined suture module on mechanical properties

Figure 15 shows the load-deflection curve, peak load and energy absorption of the specimens with combined suture modules. Specimen N₃G was composed of three jigsaw units, which increased the interlocking effect and enhanced the mutual constraints between jigsaw units. After a crack appeared, rapid load drops disappeared. It was almost impossible to observe the first peak load in N₃G, with the load–deflection curve of the specimen appearing approximately smooth. Compared with N_2-SG, the bearing capacity of N_2-LG was 11.3% higher and the energy absorption performance was 10.5% lower. The likely reason is that the lower surface of specimen N_2-LG was a large jigsaw unit, which provided a strong interlocking effect. Under bending load, the neck of the interlocking unit could bear a larger load before necking failure occurred, giving N_2-LG a relatively high bearing capacity. At the same time, after the small jigsaw units on the lower surface of N_2-SG underwent necking failure, the large jigsaw units on the upper surface are capable of enduring substantial loads even under significant deflection. This specific behavior contributes to the specimen’s strong energy absorption performance. Compared with specimen A_25°R_1.8, the ultimate bearing capacities of N_2-LG and N_2-SG were 6.2% and 15.7% lower, respectively, and the energy absorption performance was 7.6% and 20.3% higher.

Figure 15.

Effect of conbined suture module on mechanical properties. (a) Load–deflection curves. (b) Peak load and energy absorption of jigsaw units.

Compared with the conventional specimens (G-L), the ultimate bearing capacities of N₃G, N_2-LG, and N_2-SG were 7.35, 8.67, and 7.79 times higher, respectively, and the energy absorption performance was 124.9, 196.3, and 219.3 times higher. These sutures provide multiple benefits in structural design. Specimens employing suture combinations demonstrate exceptional energy absorption and high bearing capacity, making them a sturdy option for connection joints in concrete structures. Additionally, the interlocking mechanism facilitates the connection of structural components without requiring adhesives or external joining treatments. More importantly, the interlocking mechanism of bio-inspired jigsaw units seamlessly integrates small modular components into large structures, creating a comprehensive system, which facilitates flexible disassembly, maintenance, and replacement of structural elements.

Discussion

Figure 16 shows the peak load and energy absorption of different specimens. Among the specimens featuring a single jigsaw unit, specimen A_25°R_1.8 demonstrates the highest flexural strength, while specimen A_25°R_1.4 exhibits superior energy absorption performance. Table 6 shows the comparison of the bearing capacity and energy absorption of conventional specimens and specimens with jigsaw units. Compared with G-L, specimen A_25°R_1.8 (α = 25°, a:b ratio = 1:1.8) had the most outstanding performance, with 9.24 times greater bearing capacity and 182.3 times greater energy absorption. Similarly, specimen A_25°R_1.4 (α = 25°, a:b ratio = 1:1.4) outperformed G-L, with a bearing capacity 7.68 times greater and energy absorption 209.31 times greater.

Figure 16.

Peak load and energy absorption of jigsaw units.

Table 6.

Increases in the bearing capacity and energy absorption of specimens with suture joints relative to conventional specimen (G-L).

Specimens	Load (kN)	Increased multiple	Energy absorption (J)	Increased multiple	∆l₂ (mm)
A_15°R_1.8	6.19	5.29	28.21	105.60	0.267
A_25°R_1.8	10.81	9.24	48.71	182.33	1.202
A_35°R_1.8	4.74	4.05	6.95	26.02	0.517
A_25°R_1.0	5.13	4.38	21.86	81.82	0.793
A_25°R_1.4	8.99	7.68	55.92	209.31	1.887
A_25°R_2.2	7.21	6.16	33.14	124.02	0.564
N₃G	8.60	7.35	33.37	124.86	0.898
N_2-LG	10.14	8.67	52.43	196.26	1.543
N_2-SG	9.11	7.79	58.59	219.29	1.793

All specimens featuring combined suture modules demonstrate exceptional mechanical properties. Compared with G-L, the flexural strengths of N₃G, N_2-LG, and N_2-SG were 7.35 times, 8.67 times, and 7.79 times higher, respectively, and the energy absorption was 124.9 times, 196.3 times, and 219.3 times higher. Simultaneously, the deflections between the two peak loads of specimens A_25°R_1.4, N_2-LG and, N_2-SG are 1.887 mm, 1.543 mm, and 1.793 mm respectively. The tests show that suture joints have a flexible structural design that can provide different mechanical properties. For instance, the N_2-SG₃ design offers excellent energy absorption capabilities, while the A_25°R_1.8 design provides significant load-bearing effects. Additionally, A_25°R_1.4, N_2-LG, and N_2-SG exhibit substantial deflection between two peak loads, allowing for regular joint inspection for prompt maintenance and early prediction of structural failure.

The suture interface establishes a predetermined path to guide crack propagation and utilizes an interlocking mechanism to create dual peak points, further enhancing its mechanical performance. This innovative design facilitates strength monitoring and early fracture behavior prediction, offering an effective solution for high-performance connecting joints in engineering, especially under dynamic loads or seismic conditions. Simultaneously, the interlocking mechanism of bio-inspired jigsaw units can connect small modular components into large modular structures to form a comprehensive structural system without the need for external connection procedures. This enables flexible disassembly, maintenance, and replacement of components, particularly beneficial for repairing high-energy engineering joints. Additionally, it streamlines connection processes, fostering modular construction efficiency, cost-effectiveness, and resilience in line with sustainable construction practices. Prefabrication of SHCC components ensures precise assembly, reducing construction timelines, enhancing operational efficiency, and meeting emergency and temporary construction needs.

Conclusions

The study examined the flexural performance of SHCC joints utilizing bio-inspired jigsaw-shaped suture interfaces, which mimic the elytra connection geometry found in the diabolical ironclad beetle. The toughening and abnormal deformation mechanisms of jigsaw-shaped interlocking suture joints were explored. A balance has been discovered between toughness, flexural strength, and energy absorption, allowing for optimal utilization of the performance of the bio-inspired suture joint. The conclusions are as follows:

(1) The jigsaw units in the SHCC specimens exhibited four distinct failure modes: suture failure, compression-bending failure, necking failure, and combined failure. Among these, specimens experiencing combined failure demonstrated remarkable interlocking performance and exhibited significant plastic deformation. It appears that a method has been devised to modify the suture geometry with the aim of directing crack propagation, enhancing structural strengths and fracture properties, thereby creating a SHCC suture joint characterized by superior energy absorption and remarkable deformation attributes.

(2) The joints featuring a single interlocking suture interface demonstrate exceptional mechanical properties. Compared with the conventional specimen (G-L), specimen A_25˚R_1.8 (α = 25°, a:b ratio = 1:1.8) had 9.24 times greater bearing capacity and 182.3 times greater energy absorption. Similarly, specimen A_25°R_1.4 (α = 25°, a:b ratio = 1:1.4) outperformed G-L, with a bearing capacity 7.68 times greater and energy absorption 209.31 times greater. The significant improvements in both bearing capacity and energy absorption highlight the effectiveness and potential of this design approach.

(3) Compared with a single suture interface, SHCC specimens with interlocking jigsaw suture joints enable efficient and seamless load transfer. Simultaneously, specimens with combined suture modules demonstrate excellent bearing capacity and remarkable energy absorption. Compared with the conventional specimen (G-L), the ultimate bearing capacities of specimens N_2-LG and N_2-SG were 8.67 times and 7.79 times greater, respectively, and their energy absorption performance was 196.3 times and 219.3 times greater. These findings offer valuable insights for the development and optimization of more intricate suture interfaces, allowing for the exploration of advanced joint designs and their potential applications.

(4) The suture interface creates a preset path to guide crack development and further enhances the bearing capacity through the interlocking suture mechanism. The P_u-2 values of specimens A_25°R_1.8, N₃G, and N_2-LG were respectively 19.7%, 24.6%, and 34.5% higher than their corresponding P_u-1 values. The specimen still had a high load-bearing capacity after cracking, which is particularly attractive as it provides opportunity for strength inspection and prediction of early fracture behavior.

Footnotes

ORCID iD

Shiping Li

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to acknowledge the Hong Kong Research Grants Council (Project No. 27209020 and 17204322) for the financial support.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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