Effects of infill density on mechanical properties of additively manufactured chopped carbon fiber reinforced PLA composites

Tensile, flexural and impact samples were created in Onshape software, following ASTM standards [26]. These designed samples were then produced using Viper Share bot 3D-printing machines. To thoroughly study the properties, each category of testing was replicated with five samples. The designed samples, as depicted in the Figures 1(a) and (b), served as the basis for the subsequent mechanical testing and analysis.

Fig. 1.

The cCFPC was procured from Amazon India and the cost of 3D filament material is 7500rs per 500 grams. The diameter of the cCF 3D filament is 1.75mm. The filament was manufactured by 3DXTECH, an American-based company.

The samples were 3D-printed following the guidelines of ASTM D3039, featuring a rectangular dimension measuring 200 × 20 × 3mm [26]. A set of five samples was selected for the tensile test. The testing was conducted using a universal testing machine and calculations were performed to estimate tensile strength, elongation at break and tensile modulus, based on parameters obtained. The conclusion of the testing process involved the presentation of the average value obtained from the results of the five samples.

The samples were 3D-printed in accordance with ASTM D790 specifications for flexural testing, featuring dimensions of 125.0 × 12.7 × 3.2mm [26]. A total of five samples were prepared for this purpose. The testing was conducted using a universal testing machine, employing a three-point bending test on the flexural samples to determine both flexural strengths and its moduli. The average value of the results obtained from the five samples was then recorded for analysis and documentation. Importantly, the flexural strength and modulus were calculated using Equations (1) and (2), respectively. 1 Flexuralstrength=3FL2bd2 \[\text{Flexural}\,\text{strength}=\frac{3\text{FL}}{2\text{b}\,{{\text{d}}^{2}}}\] 2 Flexuralmodulus=mL34bd3 \[\text{Flexural}\,\text{modulus}=\frac{\text{m}{{\text{L}}^{\text{3}}}}{4\text{b}\,{{\text{d}}^{3}}}\] where F represents the maximum failure load (N), L means the length of span (mm), b and d denote the width and thickness of sample, respectively, and m stands for slope of the load-displacement curve tangent to the initial line.

The samples were 3D-printed according to ASTM D256 without notch for impact test, having a dimension of 63.5 × 12.5 × 3.2mm [26]. The five samples were taken for this test. The test was carried out on an impact testing machine. The average value obtained from the results of the five samples were reported.

A scanning electron microscope (SEM) was employed to investigate into the fractography of the ruptured surfaces of the composite samples. The samples were meticulously cut under machine specifications to ensure precision and accuracy, determining the fiber content within each sample. Subsequently, a uniform coating of golden sputter was applied to the samples before examination under the SEM. Various magnifications were obtained to inspect the tensile samples. The multimagnification approach was used for a detailed and comprehensive analysis of the fractured surfaces, providing valuable insights into the structural characteristics and failure mechanisms of the material. This test was carried out on the CARL ZEISS model of EVO18 SEM with a voltage of 20 kV and a vacuum level of 1.5 × 10⁻³Pa were set in machines at IRC, KARE and Srivilliputtur.

Figure 2 depicts tensile fractured cCFPC samples. Calculations of their tensile strength were conducted for the various varying infill densities of 60%, 80% and 100%. The corresponding tensile strengths and ultimate tensile stress-strain curves are later shown in Figures 3 and 4, respectively. It was observed that tensile strength increased when there was an increase in infill density of composite materials. Therefore, infill density played a significant role in the properties of the cCFPC materials.

Fig. 2.

Fig. 3.

Fig. 4.

Figure 3 shows that the tensile strength of cCFPCs with different infill densities was a main parameter. It was observed that samples with highest infill density produced the highest tensile strength of 34.75±3.2 MPa. Meanwhile, samples with 60% and 80% infill densities yielded the lower values of 25.9±2.3 and 26.9±2.7 MPa, respectively. The lower strengths can be traced to the presence of insufficient fiber content in their composite samples. For the same infill density of 60%, 80%, and 100% without reinforcement by chopped carbon (i.e., Pristine PLA), the tensile strength is measured as 19.2±1.4MPa, 20.6±2.6MPa and 42.9±3.9MPa, respectively, as reported by Mazur et al. [27]. The tensile strength of 100% infilled cCFPCs is decreased by 19% when compared to the 100% infill density of the PLA sample. However, the tensile strength of 60% and 80% infilled cCFPCs has increased with 26% and 23%, respectively, when compared to 60% and 80% infill density of pure PLA samples.

The five samples of each infill density of cCF-PCs were further analyzed. The average value of each category was calculated to obtained stressstrain curves. A typical stress-strain curve of cCF-PCs with different infill densities is depicted in Figure 4. It was observed that the highest tensile stress values were obtained from the sample with a 100% infill density. The cCFPC samples with 80% infill density recorded tensile stress and strain values of 30 MPa and 2.9%, respectively. Similarly, cCFPC samples with 60% produced tensile stress and strain values of 26 MPa and around 3%, respectively. Finally, a curve with an infill density of 100% exhibited maximum tensile stress and strain of 34 MPa and 4.1%, respectively. Evidently, the infill density played a major role on mechanical/tensile properties of 3D-printed cCFPC samples.

The values of tensile moduli from the different infill densities of cCFPCs are shown in Figure 5. A tensile modulus of 1580±85 MPa was obtained from the 60% cCFPC, and the 80% cCFPC produced a value of 1610±105 MPa. Similarly, the highest tensile modulus was produced by the 100% cCFPC and its value was 2160±120 MPa. The optimum tensile modulus value can be associated with more fiber content, as it acts as a load-bearing element within the composite system. Consequently, equally-distributed load within the structure had the capability of withstanding the highest load, compared with the other two cases of lower fiber contents. Moreover, samples with the highest fiber content had less void within their 3D-printed cCFPC samples. The tensile properties of 3D-printed cCFPCs such as modulus and strength are lower when compared with continuous carbon fiber reinforced 3D-printed PLA composite materials [28]. Moreover, the mechanical properties of 3D-printed PLA materials or composite materials could give lower values when compared to injection molded PLA materials or composite materials due to the presence of more microvoids on 3D-printed samples [29, 30].

Fig. 5.

Flexural strengths were determined for all the 3D-printed cCFPC samples with different infill densities. The flexural fractured samples are depicted in Figure 6. The related flexural strengths are shown in Figure 7. It was observed that fiber content in cCFPCs increased with their flexural strengths gradually and significantly with the 100% sample.

Fig. 6.

Fig. 7.

The flexural characteristics demonstrated a positive correlation with the growing infill density. In particular, the flexural strength escalated from 11.8 to 18.4 MPa as the infill density increased by 20%. The trend is clearly depicted in Figure 7, showing that higher infill density was associated with an enhanced resistance to internally applied stress. The flexural strength of cCFPCs is increased by 36% when compared to lower-infilled cCFPCs with high-density filled cCFPCs materials. At the same infill densities of 60%, 80% and 100%, without reinforcement by chopped carbon (i.e., Pristine PLA), Mazur et al. [27] reported flexural strengths of 37±1.8 MPa, 42±2.3 MPa, and 101±3.4 MPa, respectively. The flexural strength of 100% infilled cCFPCs decreases by 81% compared to the 100% infill density of PLA samples. Similarly, the flexural strength of 60% and 80% infilled cCFPCs decrease by 68% and 71%, respectively, compared to the 60% and 80% infill density of pure PLA samples.

The flexural modulus is the ability of plastic material to bend. The flexural modulus of the various 3D-printed cCFPCs is depicted in Figure 8. It was observed that higher fiber content produced higher flexural modulus, as the highest value was observed from the sample with 100% infill density. While 60 and 80% of samples with lower fiber contents produced lower values of 265 and 282 MPa, respectively. The flexural properties gradually increased from the low infill density of cCF-PCs to the high infill density of cCFPCs samples. The same trend is observed by Mazur et al. [27] as they investigated the mechanical properties of 3D-printed composite materials. Flexural strength and flexural modulus values are gradually increased when increasing the fiber content and infill density of the 3D-printed polymer composite materials.

Fig. 8.

Figure 9 shows the flexural stress-strain plots of cCFPCs with different infill densities. It was observed that at 60% infill density, the flexural stress was 4.9MPa with a strain of 10.5%. For 80% infill density, the flexural stress and strain values were 12.5MPa and 4.5%, respectively. The sample with 100% infill density exhibited the highest flexural stress at 17.5MPa with a strain of 4.6%. Specifically, at 100% infill density, the composite demonstrated the highest flexural stress, flexural strength and modulus. In contrast, lower infill densities of 60% and 80% experienced reduced strengths, due to the insufficient fiber content and the presence of voids in their polymer, highlighting the significance of addressing these factors for optimal material performance.

Fig. 9.

Figure 10 shows the fractured impact samples of the various 3D-printed cCFPCs. The impact strengths were determined for all the 3D-printed samples with different infill densities, as shown in Figure 11.

Fig. 10.

Fig. 11.

From Figure 11, the impact strength values were 47.48, 48.45 and 48.96 kJ/m² for samples with 60%, 80% and 100% infill densities, respectively. Significantly, the impact strength of the cCFPC samples improved with their infill densities, as the sample with the highest infill density of 100% recorded the highest impact strength. Therefore, it has the most suitable option for applications in lightweight mechanical scenarios. This further suggested that an optimum 3D-printed cCFPC sample with 100% infill density could serve as a viable alternative to traditional plastic materials, particularly in areas where robust impact resistance is a required critical factor. There was no major changes in the impact strength of cCFPCs for different infilled samples. The impact strength increased by only 3% when compared to lower infill density cCFPCs with high infill density cCF-PCs materials.

Figures 12(a)–(d) depict the SEM images of the 3D-printed tensile fractured samples. Studies on the morphological and surface microstructures of the cCFPC samples with the lowest and highest infill densities were carried out, involving infill densities of 60% at (a) 100x and (b) 500x as well as 100% at (c) 100x and (d) 500x magnifications. Figures 12(a) and (b) show that the 60% infill density with lowest fiber content possessed more uneven voids, leading to earlier mechanical failure. The sample had a major problem regarding the lowest strengths, because its infill concentration contained the lowest cCF. This was previously confirmed, considering its lowest stress-strain curve and other relative tensile properties. This type of 3D-printed cCFPC may not be a suitable lightweight component with responsibility of structural application, but it depends on the required performance.

Fig. 12.

Figures 12(c) and (d) depict the 100% infill density with higher or more chopped fiber content and smooth fractured surface within the PLA matrix when compared with the infill concentrations of other cCFPC samples. The higher fiber content supported strong mechanical properties of the sample. Also, the sample had a cleaner fracture than other lower infill density, confirming the brittle nature of the carbon fiber. These results were previously confirmed, considering their stress-strain curves and other mechanical properties.