Influence of Bamboo Fibers Weight Fraction on the Quasi-Static and Dynamic Compressive Responses of Epoxy Matrix Composites

ABSTRACT

The influences of natural bamboo fibers (BFs) weight fraction on the microstructure, quasi-static and dynamic compressive responses of the epoxy resin matrix composites were investigated with the aid of scanning electron microscope, electronic universal testing machine and the modified Split Hopkinson pressure bar (SHPB) setup. The quasi-static compressive properties of the BFs/epoxy resin matrix composites suggested that the compressive strength were strongly improved with the addition of the natural BFs, which was attributed to the excellent strengthening effect. When the BFs content increased up to 15 wt.%, the compression ratio of the BFs/epoxy resin matrix composites showed 11% reduction than that of the unmodified epoxy resin. Moreover, the stress and strain were linear at the initial part but nonlinear when stress exceeds a certain value. SHPB impact tests results indicated that, compared with the pristine epoxy resin, the natural BFs reinforced epoxy matrix composite with 15 wt.% BFs exhibited the highest dynamic compressive strength, compression modulus and the best strengthening effect. Epoxy resin matrix composites also showed an obvious effect rate effective under impact load. Similar to that in the quasi-static compressive test, the addition of natural BFs has a positive effect in improving the compressive strength of the epoxy matrix.

摘要

利用扫描电子显微镜、电子万能试验机和改进的Split Hopkinson压杆装置，研究了天然竹纤维对环氧树脂微观结构、准静态和动态压缩响应的影响. BFs/环氧树脂基复合材料的准静态压缩性能表明，天然BFs的加入大大提高了复合材料的抗压强度，这归因于其优异的强化效果. 当BFs含量增加到15 wt%时，BFs/环氧树脂基复合材料的压缩比比未改性的环氧树脂降低了11%. 此外，应力和应变在初始部分是线性的，但当应力超过一定值时是非线性的. SHPB冲击试验结果表明，与原始环氧树脂相比，添加15 wt%BFs的天然BFs增强环氧基复合材料表现出最高的动态抗压强度、压缩模量和最佳的增强效果. 环氧树脂基复合材料在冲击载荷作用下也表现出明显的效应率. 与准静态抗压试验类似，添加天然BFs对提高环氧树脂基体的抗压强度有积极作用.

KEYWORDS:

• Epoxy matrix composites
• natural bamboo fibers
• dynamic compressive responses
• quasi-static compressive experiment

关键词:

• 环氧树脂基复合材料
• 天然竹纤维
• 动态压缩响应
• 准静态压缩实验

1. Introduction

In the past few decades, fibers reinforced polymer composites (FRPC) have been widely used in various engineering applications, such as civil buildings, bridges, highways, oceans, hydraulic structures, and underground structures due to their remarkable mechanical properties, high ratios of stiffness-to-weight and strength-to-weight, corrosion resistance, lower manufacturing costs, unique self-lubrication capabilities, and low working noise (Zegaoui et al. 2018; Cheng et al. 2023). The most commonly used reinforcement fibers in polymeric matrix composites were synthetic fibers (SF) such as carbon, graphite, glass, aramid fiber, thermoplastic fibers, and other petroleum-based products, which were difficult to decompose in nature causing environmental pollution, and natural fibers (NF) include bamboo, hemp, jute, sisal, and flax fibers (Rahman and Putra 2019; Joshi et al. 2004).

Compared with synthetic fibers, natural fibers were harmless, renewable, biodegradable, ecofriendly, and economical with high specific strength (Selvaraj et al. 2023; Kanaginahal 2022). Besides, natural cellulose fibers are easier to be modified than relatively inert carbon, aromatic polyamide, and glass fiber. Nowadays, the natural fibers can become an economical alternative to synthetic fibers as promising reinforcement material for fiber reinforced polymer composites due to the growing environmental awareness and new regulations (Wasti and Adhikari 2020). Among the natural fiber reinforcements, bamboo having the higher strength and the hugger flexibility, the usage of bamboo fibers was enhancing annually due to its sustainability, higher mechanical strength, fast growth rate, lightweight, less embodied energy, reliable, and low-cost characteristics, and it was considered as an ideal candidate for the reinforced composites (Prabhakar et al. 2021; Li et al. 2015). Fiber surface modification techniques help to improve the fiber-matrix and fiber–fiber interactions within the composites. Surface modification of natural fibers can be generally achieved through a physical or chemical treatment route (Gangadhar, Hebbar, and Kulkarni 2019). However, fiber surface modification using a chemical route was highly preferred over physical treatment. According to the literature, the chemical modification method of NFs can be divided into two groups, namely: alkali and silane coupling agent (SCA). Nevertheless, the modification method using alkali treatment may increase the interfacial adhesion strength but damaging the fiber itself (Xie et al. 2010). The 3-glycyloxypropyltrimethoxysilane (KH560) coupling agent was widely used in polymeric composites to enhance mechanical properties, and its bifunctional structure has attracted interest in reinforcing plant fibers/polymers (Wang et al. 2016). The research showed that the distribution as well as adhesion of NFs within polymeric matrix can be significantly improved after treatment with 3-glycyloxypropyltrimethoxysilane (KH560) coupling agent (Zegaoui et al. 2018). So, it has been used in this research for the experimental investigations. Many researchers studied the effect and properties of bamboo fibers modified polymer composites (Fuentes et al. 2011; Braga and Magalhaes 2015).

The fabricated products can be used for structural works of commercial aircraft, body parts of boats, interiors and exteriors of automotive bodies, and so on (Chenrayan et al. 2023). These automobile outer bodies should have flexibility to withstand the impacts and vibrations. Strain-rate sensitivity was a very important issue in the transport segment with considerable attention devoted to the crashworthiness of systems (Bharath et al. 2016). The mechanical properties of epoxy-based composites were viewed as strain-rate sensitive (Kiran et al. 2022). Many researchers studied the effect of quasi-static on polymer composites and how it led to the degradation of their performance. Their studies illustrated that the damage of thermosetting composites under quasi-static impact also can seriously weaken the residual strength and stiffness properties (Cheng et al. 2023; Kiran et al. 2023). During service, FRPC structures were also subjected to dynamic loadings due to earthquakes, explosions, fast-moving traffics, wind-driven objects, and machine vibrations (Gangadhar et al. 2023). The response of a structure at high strain rate loading differed significantly with that at quasi-static loading conditions as the high strain-rate effect occurs in the case of dynamic loadings (Wang, Smith, and Wang 2019). Under dynamic loading, however, it was not easy to obtain reliable data due to strain and stress of the composite sample was not uniform, mainly at high strain rate levels (Meredith et al. 2013). Therefore, researches on the mechanical behavior and failure mechanisms under high strain rates were comparatively insufficient. Therefore, it was essential to study the properties of materials under different strain rates.

In our current work, we aimed at producing a bamboo-plastic composite with a combination of high strength, satisfactory ductility, and superior impact toughness by the incorporation of natural bamboo fibers into the epoxy resin. The BFs were initially treated by using silane coupling agent, and then systematically incorporated with various contents ranging from 0 to 20 wt.% with an increment of 5 wt.%. The microstructure and hardness of epoxy resin matrix composites were also investigated. Additionally, the effects of bamboo fibers on the quasi-static and dynamic compressive properties of the epoxy resin matrix composites were discussed, respectively.

2. Experimental details

2.1. Materials

The diglycidyl ether of bisphenol A epoxy resin (E51) was obtained from Wuxi Synthetic Resin, China. Polyetheramine D230 was provided by Shanghai Reagent, China. Natural BFs (100 mesh) were provided by the Haibosi, Fujian, China. The chemical constituents of natural bamboo fibers contain 48.78 wt.% cellulose, 18.21 wt.% hemicelluloses, and 15.6 wt.% lignin, contributing significantly to its strength and stiffness. Anhydrous ethanol (C₂H₅OH, AR) and acetic acid (CH₃COOH, AR) were supplied by Tianda Chemical Reagent Co., Ltd., Tianjin, China. The organic silane coupling agent, 3-glycidyloxypropyltrimethoxysilane coupling agent (boiling point: 290°C) was obtained from Yaohua Chemical, Shanghai, China. BFs were modified by silane coupling agent before they were added into the epoxy matrix composites. Deionized water was prepared in our laboratory.

2.2. Fabrication of the epoxy matrix composites

In this paper, the natural BFs were surface modified by the following steps. Firstly, the BFs were firstly oven dried at 100°C for 20 hr to remove the moisture and volatiles, then washed thoroughly with deionized water under mechanical stirring and dried at 60°C. Secondly, 60 ml of anhydrous ethanol and 40 ml of deionized water were mixed uniformly, and acetic acid was added to adjust the pH to 4–5, and then mechanically stirred by using a glass stick for 30 min at a room temperature. Then, 2 wt.% silane coupling agent and appropriate quantities of the dried BFs were added to the solution and stirred mechanically at room temperature for 2 hr. The surface modified BFs were room dried for 12 hr, and systematically followed by a vacuum drying at 60°C for 12 hr (Zegaoui et al. 2018; Wang et al. 2019). The various quantities of the surface modified BFs were put into the epoxy resin solution, and stirred in a water bath at 60°C upon ultrasonic agitation for 2 hr. After cooling to room temperature, the curing agent D230 was added, continuously stirring for additional 15 min, then degassed under a vacuum oven for 2 hr to avoid the presence of unwanted bubbles or voids in the final samples and poured into the preheated PTFE mold. Subsequently, they were cured following the curing schedule of 80°C for 2 hr and 130°C for 2 hr. The resulted cured composites were demolded and labeled. The designation and composition of the polymer composite samples are summarized in Table 1 and the mechanism of the natural bamboo fibers modified by silane coupling agent is illustrated in Figure 1.

Figure 1. Mechanism of the natural bamboo fibers modified by silane-coupling agent.

Table 1. Designation and composition of the polymer composite samples.

Sample	EP	Bamboo fibers	D230
BFs0/EP	75.8	0	24.2
BFs5/EP	72	5	23
BFs10/EP	68.2	10	21.8
BFs15/EP	64.4	15	20.6
BFs20/EP	60.6	20	19.4
BFs100	0	100	0

2.3. Characterization and compressive properties testing

The fracture surface morphologies of the specimens were observed using a scanning electron microscope (SEM, KYKY-EM3200) at an accelerating voltage of 1.00 kV after coated with a thin evaporated layer of gold. Quasi-static uniaxial compressive tests were performed using a CTM® electronic universal testing machine (UTM, XieQiang Instrument Manufacturing Co., Ltd, Shanghai). Schematic illustration of the electronic universal testing machine is shown in Figure 2. The specimens were sandwiched between two steel plates with lubricating fluid on their surfaces to reduce friction. All specimens were compressed up to densification zones at room temperature at the loading speed of 2 mm·min⁻¹.

Figure 2. Schematic diagram of the electronic universal testing machine.

Dynamic compressive responses of the epoxy matrix composites were investigated on a modified Split Hopkinson pressure bar device in the impact dynamics laboratory of the University of Science and Technology of China. The SHPB apparatus consists of a gas ejector, an incident bar, a cone-shaped striker, a transmitted bar, an absorbing bar, a damper, a dynamic strain meter, and a digital oscilloscope, as shown in Figure 3. The incident, transmission, and absorption bars used in the experiment were alloy steel, and their physical parameters are listed in Table 2. During the dynamic loading test, a cylinder specimen was sandwiched between the incident bar and transmitted bar. Then, a gas gun launched the strike bar to impact the incident bar. An elastic compressive wave was generated in the incident bar and propagated to the specimen. Once the compressive stress wave arrived to the interface between the incident bar and the specimen, partial stress wave was reflected back into the incident bar, but the residual stress wave was transmitted to the transmitted bar through the specimen. The experimental samples were processed into cylinders with a diameter of 10 mm and a length of 10 mm. The dynamic compressive tests were carried out at four impact gas pressures, namely, 0.2 MPa, 0.3 MPa, 0.4 MPa, and 0.5 MPa, respectively. Different impact gas pressures corresponded to different strain rates in the SHPB experiments. The dynamic mechanical behaviors of composites often varied at different strain rates. The obtained transient data for each sample tested under different strain rates were recorded and stored. To ensure the good reproducibility of the experiments, each test was repeated three times.

Figure 3. Schematic illustration of the Split Hopkinson pressure bar system.

Table 2. Experimental parameters for dynamic compressive testing.

Parameters	Values
C0 (m·s − 1) ρ0 (g·cm^−3) ρ0C0 (MPa·m^−1·s^−1)	5190 7.8 40.5
E (GPa)	210
A (mm2)	1962.5
As (mm2)	314
ds (mm)	10
ls (mm)	10

Suppose the strain pulses of incident, reflection, and transmission are divided into ε_i, ε_r, ε_t, and the displacement of the interface on the left and right of the sample is denoted by u₁ and u₂. According to the stress wave propagation theory and one-dimensional stress assumption, and the continuity requirements of displacement, then the specific expression of u₁ and u₂ is as follows:

(1) $u_1=C_0{\int }_0^t({\epsilon }_i-{\epsilon }_r)dt$(1)

(2) $u_2=C_0{\int }_0^t{\epsilon }_{t}dt$(2)

where C₀ refers to the P-wave velocity of the bars, t the time duration.

Once dynamic stress equilibrium is reached, the average dynamic compressive strain rate in the specimen, $\dot{\epsilon }\left(t\right)$, can be expressed as:

(3) $\dot{\epsilon }\left(t\right)=\frac{C_0}{l_s}\left[{\epsilon }_i\left(t\right)-{\epsilon }_r\left(t\right)-{\epsilon }_t\left(t\right)\right]$(3)

where l_s represents the length of the specimen. The average strain, ε_s(t), is given by

(4) $\epsilon \left(t\right)=\frac{C_0}{l_s}{\int }_0^t\left[{\epsilon }_i\left(t\right)-{\epsilon }_r\left(t\right)-{\epsilon }_t\left(t\right)\right]dt$(4)

According to the law that action and reaction are equal, the stress at interface 1–1 and interface 2–2 (Figure 4), σ₁(t) and σ₂(t), is accomplished by the following formulas:

(5) $A{\sigma }_1\left(t\right)=A_0{E}_0\left[{\epsilon }_i\left(t\right)+{\epsilon }_r\left(t\right)\right]$(5)

(6) $A{\sigma }_2\left(t\right)=A_0{E}_0{\epsilon }_t$(6)

Figure 4. Theory of Split Hopkinson pressure bar experiment.

The average axial dynamic compressive stress, σ_s(t), of the specimen can be derived as follows:

(7) ${\sigma }_s(t)=\frac{EA}{2A_s}\left[{\epsilon }_i\right.(t)+{\epsilon }_r(t)+\left.{\epsilon }_t(t)\right]$(7)

where E is the elastic modulus of the bar, A and A_s are the cross-sectional area of the bar and the specimen, respectively. The data processing methods of equation (3)(3) $\dot{\epsilon }\left(t\right)=\frac{C_0}{l_s}\left[{\epsilon }_i\left(t\right)-{\epsilon }_r\left(t\right)-{\epsilon }_t\left(t\right)\right]$(3) , (4(4) $\epsilon \left(t\right)=\frac{C_0}{l_s}{\int }_0^t\left[{\epsilon }_i\left(t\right)-{\epsilon }_r\left(t\right)-{\epsilon }_t\left(t\right)\right]dt$(4) ) and (7(7) ${\sigma }_s(t)=\frac{EA}{2A_s}\left[{\epsilon }_i\right.(t)+{\epsilon }_r(t)+\left.{\epsilon }_t(t)\right]$(7) ) are three-wave method. Parameters in the formula are shown in Table 2.

Based on the assumption of stress uniformity within the specimen, stress, and strain fields in the sample are uniform in its axial direction, so

(8) ${\epsilon }_i+{\epsilon }_r={\epsilon }_t$(8)

According to formulas (7), the simplified three-wave formula can be obtained:

(9) $\stackrel{\left(t\right)}{{\epsilon }_s}=\frac{2C_0}{l_s}{\epsilon }_r\left(t\right);{\epsilon }_s\left(t\right)=-\frac{2C_0}{I_s}\underset{0}{\overset{t}{\int }}{\epsilon }_{r}dt;{\sigma }_s\left(t\right)=\frac{EA}{A_s}{\epsilon }_t\left(t\right)=\frac{E{d}^2}{d_s^2}{\epsilon }_t\left(t\right)$(9)

The stress-strain curves of epoxy matrix composites can be obtained according to the data processing of Formula (9).

3. Results and discussion

3.1. Microstructures of the BFs/epoxy matrix composites

Details of the fracture surfaces of the BFs/epoxy resin matrix composites, investigated by SEM, were illustrated in Figure 5. It could be noted that the fracture morphology of the unmodified epoxy was typical brittle fracture, some river-like lines in the same direction and cracks (red arrowhead) appeared on the fracture surface (Figure 5(a)). As shown in Figure 5(b), the bamboo fiber pullout voids (yellow arrowhead) and fiber breakages (pink arrowhead) were observed on the fracture surface of the BFs5/EP composites. Moreover, some crack propagations appeared on the fracture surface of pristine epoxy and BFs5/EP samples. Some researchers believed that the natural bamboo fibers will be pulled out if the interfacial compatibility between fibers and epoxy resin matrix was weak, whereas, the fiber breakage would occurred and the existence of crack pinning effect would be revealed by non-planar fracture surface if the interface bonding was strong (Mi et al. 2020). As illustrated in Figure 5(c), the fracture surface morphologies of BFs10/EP specimen displayed a relatively rough surface with shear deformation, which corresponding to higher energy absorption and tougher matrix (Bai et al. 2017). In addition, fiber pullout, breakages, and parallel were also observed on the fracture surface of the composites. Noted that when BFs content was up to 15 wt.%, more fiber breakage was observed on the fractured surface of the composites. Furthermore, the fracture surface became crimped and disordered which suggests that there was a transfer from brittle fracture to plastic fracture (Figure 5(d)). In this case, much impact energy was dissipated through these new sites of BFs in the composites (Rakesh et al. 2013). That is, the distinct facture graphs give a hint that the composites can absorb more energy during the fracture process, probably leading to higher toughness (Jeyachandran et al. 2020). However, after adding 20 wt.% BFs into epoxy matrix, much more almost intact fibers aligned parallel to the surface (pink arrowhead in Figure 5(e)), which barely played a vital role in arresting crack propagation in the matrix. The surface modified BFs were surrounded by epoxy at the fracture sights, indicating a closer contact between the fibers and epoxy matrix as well as a wetting of the fibers. Based on the above analysis, it can be inferred that there was a better interface interaction between fibers and epoxy matrix due to friction and mechanical interlocking in the structure which resulted in increases in the mechanical properties of composites.

Figure 5. Morphologies of the impact fracture surfaces of: (a) BFs0/EP, (a1) partial magnified image of (a); (b) BFs5/EP composites, (b1) partial magnified image of (b); (c) BFs10/EP composites, (c1) partial magnified image of (c); (d) BFs15/EP composites, (d1) partial magnified image of (d); (e) BFs20/EP composites, (e1) partial magnified image of (e).

3.2. Quasi-static mechanical properties

Figure 6 depicted the quasi-static compression behavior of the BFs/epoxy matrix composites, and the relevant test data are listed in Table 3. The compressive surfaces of all the samples are similar, which is independent of the BFs content. Although the sample was not broken, some macroscopic cracks were apparent on the compressive surface of the specimen, as illustrated in Figure 6(a). As shown in Figure 6(b), all the representative curves climbed up linearly in the initial part, then continue their increase in a slower way until up to the peak point. There is no clearly distinguishable stress plateau region. All curves showed representative three stages in compressive responses of polymer composites. The regions were: (i) initial elastic stage, (ii) subsequent nonlinear yielding and (iii) followed by plastic deformation region with higher and increasing slope that appears after densification, resulting in the unloading of the samples (Bharath et al. 2016; Yu, Zhuang, and Shi 2021). A linear-like elastic stage appeared initially up to the stress peak. When the stress increased up to around the peak point, the failure process entered the unstable crack propagation stage, and many micro-cracks gradually propagated, coalesced and form macroscopic crack locally. Stress of the specimen increased up to the peak value, and then began to decrease. After exceeding the peak stress, the cracks were further propagated and expanded, leading to visible cracks, the sample was considered to have completely failed (Ayswarya, Ajalesh, and Thachil 2021). In addition, the static compressive strength of the BFs/EP matrix composites was higher than that of the unmodified epoxy, especially for the BFs15/EP specimen. When the BFs content increased up to 15 wt.%, the compression ratio of the BFs/epoxy matrix composites showed 11% reduction than that of the pure epoxy, as illustrated in Table 3. This is mainly due to the existence of BFs reduced the elasticity of the polymeric material, thus reducing the deformation capacity of the epoxy matrix (Li et al. 2019).

Figure 6. Quasi-static compressive experiment specimen: (a) compressive sample before test, (b) compressive sample after test and (c) stress–strain relationships of the BFs/epoxy resin matrix composites.

Table 3. Compressive performances of the BFs/epoxy resin matrix composites.

Specimen	Maximum load (N)	Compressive Strength (MPa)	Maximum Compression (mm)	Compression ratio (%)
BFs0/EP	42108.501	134.036	8.224	63.265
BFs5/EP	42709.441	135.948	8.750	67.305
BFs10/EP	42260.913	134.521	8.291	63.779
BFs15/EP	42568.351	135.499	7.332	56.400
BFs20/EP	41989.184	133.656	7.367	56.668

3.3. Dynamic mechanical responses

3.3.1. Dynamic stress balance

Figure 7(a) showed typical impulse waveforms of the BFs/epoxy matrix composites. All the waveforms exhibited almost the same shape regardless of the mechanical strength of the composites, but the peak value of incident and reflected waves appeared earlier for the pristine epoxy specimen. This is because the wave impedance of BFs/epoxy matrix composites improved with increasing strength of the specimen. Dynamic stress balance during the dynamic loading process, especially before crack occurs, was the precondition for the validity of the experimental data (Zou et al. 2018). Peak values of incident, reflected and transmitted wave increased with the increasing of shock pressure (Figure 7(b–f)). This showed that the stress, strain, and strain rate of the BFs/epoxy matrix composites were also increased with the increasing of the impact force. In addition, vibration frequency was generated at the peak amplitude of the incident and reflected wave due to the micro cracks occurred in the material under 0.5 MPa. No apparent signs of macroscopic damage in the BFs/epoxy matrix composite specimens, indicating that there was no damage or only a small amount of damage occurred in the specimen. So, it still maintained a certain degree of load carrying capacity (Khan et al. 2018). Comparing these original waves, it was observed that the raw dynamic signals showed a clear filler content dependence of the specimen reflected signals. Both the reflected pulses and the transmitted pulses exhibited different waveform characteristics. Figure 8 presents the stress histories on the incident and transmitted sides of the specimen in a typical impact test. At the incident interface, the dynamic stress signed as incident + reflected stress in the figure is the representative sum of the incident and reflected stresses, and transmitted stress refers to the dynamic stress at the transmitted interface. This demonstrates that dynamic stress equilibrium was achieved, so, the axial inertial effect was insignificant.

Figure 7. Typical signals recorded by strain gauges on waveguide rods of tested specimens. (a) Impulse waveforms of the BFs/EP matrix composites under 0.5 MPa, (b) impulse waveforms of the unmodified epoxy specimen, (c) impulse waveforms of the BFs5/EP, (d) impulse waveforms of the BFs10/EP composites, (e) impulse waveforms of BFs15/EP composites and (f) impulse waveforms of BFs20/EP composites.

Figure 8. Dynamic stress balance signals of BFs0/EP specimen under 0.2 MPa.

Figure 9 showed the dynamic compression performance curve of pure epoxy resin. It can be seen that the maximum stress and strain rate increase with the increase of impact pressure. Strain of the pure epoxy specimen kept constant with the increase of time in the initial stage (Figure 9(b)). In addition, the initial modules were also different at different strain rates, indicating that the BFs0/EP specimen was sensitive to the strain rate. The compressive stress increased with increasing of the pressure. The compressive strength increases with the increase of pressure from 0.2 to 0.5 MPa. The maximum compressive strength increases by almost 296%. Dynamic mechanical response of the BFs5/EP, BFs10/EP, and BFs15/EP specimens at the different pressures was presented in Figures 10–12. As could be seen, the maximum stress and strain rate also increased with the increase of impact pressure. After the dynamic compression loading, the strain rate evolution vs. time curve vibrated at the pressure of 0.5 MPa, which indicates that the tested sample has obvious damage cracks and lost its bearing capacity (Lu and Yuan 2002), as suggested in Figure 10(c). When the impact pressure was 0.4 MPa, the BFs15/EP composites obtained the maximum compressive strength (Figure 12). It is worth noting that the maximum compressive strength decreased to the lowest with the further increase of impact pressure, as illustrated in Figure 12(d). This indicates that the compressive toughness of the composites was significantly improved, while the strength was reduced when the BFs content was 15 wt. %. Strain rate curve of BFs20/EP composite vibrated violently (Figure 13), indicating that macroscopic damage cracks were produced in the composites under high impact pressure. During the deformation process of bamboo fiber reinforced epoxy resin matrix composites, the insensitivity to the strain rate occurred at the initial stage. During the loading process, elastic deformation, yielding, and strain softening occurred. Finally, the stress was unloaded. When the stress was unloaded, the strain of the composites decreased slightly, showing the toughness of the composites. At the initial stage, the stress increased linearly with the strain, followed by the non-linear stress increment before the peak stress. The internal cracks were initiated and propagated with the increase of strain. Moreover, the dynamic compressive strength of all specimens increased with increasing of strain rate, referring that the natural BFs had obvious strain rate effect under impact load.

Figure 9. (a) stress vs. time curves, (b) strain vs. time curves, (c) strain rate evolution vs. time curves and (d) stress vs. strain curves of the unmodified epoxy resin specimen at various pressures.

Figure 10. (a) stress vs. time curves, (b) strain vs. time curves, (c) strain rate evolution vs. time curves and (d) stress vs. strain curves of the BFs5/EP composites at various pressures.

Figure 11. (a) stress vs. time, (b) strain vs. time, (c) strain rate evolution vs. time and (d) stress vs. strain curves of the BFs10/EP specimen at various pressures.

Figure 12. (a) stress vs. time curves, (b) strain vs. time curves, (c) strain rate evolution vs. time curves and (d) stress vs. strain curves of the BFs15/EP composites at various pressures.

Figure 13. (a) stress vs. time curves, (b) strain vs. time curves, (c) strain rate evolution vs. time curves and (d) stress vs. strain curves of the BFs20/EP composites at various pressures.

The stress-strain response was an important index for characterizing the dynamic properties of polymeric composites. Dynamic impact compressive stress–strain curves of the BFs/EP matrix composites under different impact pressures had similar shapes (Figure 14). It can be seen that the compressive strength of the composites first increases and then decreases under 0.2 MPa (Figure 14 (a)). Among them, epoxy matrix composite with 15 wt.% bamboo fibers has the highest compressive strength, the highest compression modulus, and the best performance. The incorporation of BFs improved the compressive strength of the epoxy (Figures 14(a) & 14(c)).

Figure 14. Dynamic compressive stress–strain curves of the BFs/epoxy resin matrix composites under different impact gas pressures (a) 0.2 MPa, (b) 0.3 MPa, (c) 0.4 MPa and (d) 0.5 MPa.

These results show that when the BFs content increased up to 15 wt.%, much impact energy was dissipated through these new sites of BFs in the composites, the fracture surface became crimped and disordered (Figure 5(d)), the polymeric composites can absorb more energy during the fracture process, probably leading to higher toughness (Jeyachandran et al. 2020). However, after adding 20 wt.% BFs into epoxy matrix, much more almost intact fibers aligned parallel to the surface, which barely played a vital role in arresting crack propagation in the matrix.

4. Conclusions

In this work, epoxy resin matrix composites reinforced by KH560 modified natural BFs were prepared and the compressive behaviors were also studied. The morphologies of fracture surfaces indicated that the BFs were surrounded by epoxy resin at the fracture sights, indicating a closer contact between the fibers and epoxy resin matrix as well as a wetting of the fibers. The quasi-static compressive properties of the BFs/epoxy matrix composites showed that the natural BFs strongly improved the mechanical properties of the epoxy resin matrix which was attributed to the excellent strengthening effect. The obtained results of SHPB impact tests indicated that the BFs/epoxy resin matrix composites were sensitive to the strain rate and exhibited obvious effect rate effective. In summary, the epoxy resin matrix composites with 15 wt.% bamboo fibers had the highest dynamic compressive strength and the compression modulus.

Highlights

• Epoxy matrix composites reinforced by KH560 modified natural bamboo fibers were prepared

• The quasi-static compressive behaviors have been greatly improved

• BFs/epoxy resin matrix composites are sensitive to the strain rate

• The modified natural bamboo fibers have good strengthening effects

• Dynamic compressive properties are also influenced by the natural bamboo fibers

Ethical approval

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Key Program for the Education Department of Anhui Province, Grant number KJ2020A0282; the Natural Science Foundation of Anhui Province Education Department, Grant number KJ2019A0127.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Key Program for the Education Department of Anhui Province, Grant number KJ2020A0282; Natural Science Foundation of Anhui Province Education Department, Grant number KJ2019A0127.