Abstract
The progressive damage behaviors of woven fabric-reinforced natural rubber panels under low-velocity impact were studied by the drop-weight tests. The woven fabrics considered include nylon, Kevlar, and carbon. The impact force, impact energy, and delamination area were analyzed, and the damaged samples were observed using an optical microscopy and an X-ray scanner to investigate the damage mechanisms. The results show that the peak force of impact increases while the absorbed energy decreases as the fiber strength/stiffness and fiber content increase. Based on the peak force and the delamination area results, the natural rubber/Carbon/2 specimen, which consists of two carbon fabric layers inside the natural rubber, displays the best impact performance among the composites considered in this study.
Keywords
Introduction
Rubber has been widely used in various industries to make tires, conveyor belts, gaskets, tubes, medical devices, etc. due to its good durability, strength, and large deformation under high stretch loading. However, rubber can be prone to wear and tear over time. As a result, reinforcing rubber with fibers is a common practice to improve its strength and durability while keeping its flexibility. Off-the-road (OTR) tire in mining industry, having much larger dimensions (4.03-m diameter, 1.5-m width, 70-mm depth), 1 is used in the most severe and tortuous conditions involving 400+ US ton payload driving through a mixture of sharp stone edges, ice, and water. Trucks driven in the mining area are so huge that drivers cannot exactly evaluate the distance between the tires and curbs/obstacles. Therefore, driving over curbs/obstacles at excessive speed and/or at the wrong angle happens and the tires are often broken by impact. Moreover, flying debris/stone impacting tires at speed can also cause damage. For conveyor belts used in the mining industry, damage is mainly caused by falling stones and sand. Although impact velocities in these scenarios are generally not high, the invisible damage caused may have extremely negative effects on the post-impact health of the composites.
Extensive research has been reported in the literature on low-velocity impact damage of composite laminates with brittle matrix.2–4 In contrast, researchers have not well-comprehended the impact behaviors and damage mechanisms of fiber-reinforced rubber composites. Anuar et al. 5 investigated the relationship between the content of discontinuous fiber and the impact properties of the fiber-reinforced natural rubber (NR) and found increasing fiber content could improve energy absorption. Taherzadeh-Fard et al. 6 conducted an experimental study on the mechanical properties and energy absorption capacity of chopped fiber-reinforced NR under a dynamic indentation test and found incorporating fibers into elastomeric matrix improves energy absorption in small deformations. Athith and co-workers 7 conducted tensile, flexural, impact, and wear tests to study the effect of tungsten carbide fillers on the mechanical and tribological properties of jute/sisal/E-glass fabrics reinforced NR. Mahesh et al. 8 presented an experimental investigation on low-velocity impact response of jute/rubber composite laminates and demonstrated the effects of stacking sequence on energy absorption and damage resistance. With the increasing industrial applications of continuous fiber-reinforced rubber composites, there is a demand to further understand the damage mechanisms of these materials and to compare the mechanical behaviors of materials reinforced by different fibers under low-velocity impact.
In this study, drop-weight tests of nylon, Kevlar, and carbon fabric-reinforced NR panels are conducted, their impact responses are compared, and damage mechanisms are discussed. The results can provide insights for OTR tire and mining industries to develop effective reinforcement strategies.
Materials and experimental procedure
Materials
In collaboration with Akron Rubber Development Laboratory, we designed the NR compounds listed in Table 1 as the matrix. Three kinds of reinforcement fabrics, nylon, Kevlar, and carbon, are commercially available from Fabric Whole Direct (USA), ACP Composites and Fibreglast®, respectively. All of them are continuous two-dimensional woven fabrics. The fiber length is 125 mm. Table 2 shows their mechanical properties.
Formulation of natural rubber compounds.
Formulation of natural rubber compounds.
Mechanical properties of different fibers.
Parts per hundred rubber (phr) is a unit of measurement that all compounds use. It represents the amount of an additive per hundred parts of base polymer in the compounding mixture.
Specimen processing
The NR compounds were mixed using a two-roll mixing mill with a 7.62-mm distance between two rolls (Figure 1) at speeds from 0.4 to 1.4 rad/s for 15 min at room temperature. 12 The curing time was characterized using a moving die rheometer (MDR) based on ASTM D5289. 13 The results are presented in Table 3. Specimens were cured using a heat press machine as shown in Figure 2 under 17 MPa working pressure at 160 oC for 8.5 min. No adhesives were applied between the rubber matrix and reinforcement fabrics. Therefore, only mechanical interlocking rather than chemical bonding exists between matrix and fibers. The dimensions of the mold were 125 mm×125 mm×3 mm. The warps of the fabric were parallel to the mold width and the wefts of the fabric were parallel to the mold length. The fabrics were adhered to the mold using adhesion tapes. After vulcanization, each specimen was cooled using running water. Eight specimen layouts were considered as listed in Table 4 and shown in Figure 3.
Two-roll mixing mill.
Heat press machine.
Specimen layout: (a) natural rubber (NR), (b) one fabric layer inside NR, (c) two fabric layers inside NR, (d) three fabric layers inside NR.
MDR results.
MDR: moving die rheometer; NR: natural rubber.
Fabricated samples.
Impact test
Low-velocity impact tests were carried out using an Instron CEAST 9350 drop-weight testing machine, Figure 4. A steel hemispherical impactor with a diameter of 16 mm was used and the impact energy was set to 50 J. The specimen was fixed on a cylindrical support frame with a 76-mm diameter window under 100 N clamping force to avoid any tilting during the experiment, as illustrated in Figure 4(b). The fixture was installed on the pedestal as exhibited in Figure 4(c). Two weights were added to the impactor to reach 5.3 kg total mass, and the weighted impactor was raised to a height of 0.95 m to generate an impact velocity of 4.31 m/s. A Strain gauge was embedded in the tup connected to the steel impactor, and the strain measurements over the duration of the impact event provided the displacement of the impactor (deflection of the specimen). An anti-rebound system was used to prevent unintentional multiple impacts on the specimen. At least three specimens were tested for each case.
Impact test setup: (a) Instron 9350, (b) fixture and specimen, (c) pedestal.
After the impact test, the specimens were inspected using a Nikon Metrology XTH 320 LC X-ray micro-computed tomography system, where 3600 images were captured as the sample rotated a full revolution. A 70 kV microfocus X-ray source penetrated the specimen for tomography with X-ray emission scanning at 129 μA. The voxel size was 8.9 μm. The VG Studio Max software was used to reconstruct three-dimensional images.
To further examine the damage mechanisms, an optical microscope is employed to observe fiber breakage because of the insufficient resolution of the X-ray scanner. Due to the lamina structure of the specimen and the symmetry of the experimental setup, a quarter of each specimen was cut with a pair of scissors to inspect damage.
Results and discussion
The Instron CEAST 9350 data acquisition provides the measured force vs. time, displacement vs. time, and impact energy vs. time history. The impact energy equals the change of mechanical energy (kinetic and potential energies) of the impactor, and the energy absorbed by the specimen equals the total energy minus the rebound energy. The delamination area is measured using the X-ray scanned image.
Force-time history
Figure 5 presents the impact force versus time records, showing the process of fall, stop, and rebound experienced by the impactor. The end of contact time is when the impact force reduces to zero. For all specimens, an almost linear increase of impact force is found until a point where a sudden drop in impact force is observed. This drop in impact force indicates a stiffness change of the composite due to damage initiation,2,7 and defines a critical value measuring the material's ability to resist damage initiation. Table 5 lists the critical impact force values of the eight different specimens. The critical impact force increases with fiber strength as well as the number of reinforcement layers.
Contact force vs. time histories: (a) natural rubber (NR), (b) nylon fiber-reinforced composites, (c) Kevlar fiber-reinforced composites, (d) carbon fiber-reinforced composites.
Critical force and peak force.
For the NR specimen, beyond the critical value, the impact force continuously increases to a peak, then gradually decreases as the impactor rebounds. But for composite panels, after damage initiation, the impact force oscillates before monotonically decreasing to zero because of further material damage due to delamination, matrix cracking, and other failure mechanisms. The peak impact force is a measure of the load carry capacity of the material, and the values are given in Table 5 for the eight panels. For NR, NR/Kevlar/1 and all nylon fiber-reinforced composites, the peak force is higher than the critical force value, but for NR/Kevlar/2 and all carbon fiber-reinforced composites, the critical force is the peak force. Among the eight materials tested, no perforation is observed, which means the remaining resistance of the specimens is sufficiently strong.
Figure 6 compares the peak force values of the eight specimens, showing that peak force increases as fiber strength and fiber content increase. Peak force represents the highest load the specimen withstands before suffering significant damage. Bensadoun et al. found composite with high flexural strength displays high peak-impact force. 14 Increasing fiber stiffness and fiber volume ratio results in an increase of flexural strength, and therefore, leads to a higher peak force value. Among the eight specimens, NR/Carbon/2, 2-ply carbon fabrics inside NR, sustain the highest impact force.
Comparison of the peak force values of eight specimens.
Energy-time history
During an impact event, from the moment of contact, the change of the impactor's mechanical energy (kinetic and potential energies) is delivered to the specimen, which is called the impact energy. A portion of the impact energy received by the specimen gets dissipated due to damage formation, interface friction, etc., which is referred to as the absorbed energy, and the other portion is stored in the form of elastic deformation energy. As the downward motion of the impactor continues, its velocity reduces to zero and all its mechanical energy is delivered to the specimen. At this moment, the impact energy reaches the maximum and is called the total energy, after which the stored elastic energy is returned to the impactor and pushes it back. The difference between the total energy and the absorbed energy is the rebound energy. The final energy value corresponds to the energy absorbed by the specimen. Figure 7 shows the energy-time history of the NR/Carbon/2 specimen.
Energy-time history of natural rubber (NR)/Carbon/2.
The energy absorption capability of composites increases as the value of the reinforcement's elongation at break increases. 15 Of the three reinforcement fibers considered in this study, Nylon fiber has the highest elongation at break while carbon fiber has the lowest elevation at break. Moreover, increasing fiber volume ratio reduces fiber spacing, which in turn leads to a higher interply stress and reduces the interply strength. 16 Consequently, the energy absorption capability is reduced.
The total impact energy, absorbed energy, and rebound energy for the eight specimens are listed in Table 6. For the three specimens reinforced by nylon fibers and two specimens reinforced by one layer of Kevlar or carbon fibers, the rebound energy is very small. Figure 8 compares the absorbed energy of the eight specimens, and the difference is not significant. It is seen that the NR specimen has the highest energy absorption capability, and as the increase of fiber stiffness and fiber content, the energy absorption capability decreases. The NR/Carbon/2 panel absorbs the least amount energy.
Comparison of the absorbed energies of eight specimens.
Impact energy.
Delamination area
The delamination area is approximated as a circle and measured using the X-ray image. Figure 9(a) shows the delamination area of the NR/Kevlar/1 specimen. For specimens having two fabric layers, delamination occurs at the interfaces of both reinforcement layers and the delamination area is larger at the interface on the impact (compression) side of the panel, which is denoted as the delamination area of the specimen. Figure 9(b) shows the delamination area of the NR/Kevlar/2 specimen. For the NR/Nylon/3 specimen, the delamination area is the smallest at the interface of the middle reinforcement layer.
X-ray tomography images of the impacted panels: (a) natural rubber (NR)/Kevlar/1, (b) NR/Kevlar/2.
Figure 10 compares the delamination areas of the seven composite panels. For specimens having one layer of reinforcement, higher fiber stiffness results in a larger delamination area. As fiber content is increased with multiple reinforcement layers, the delamination area reduces. The reduction is more significant as carbon fiber is used as the reinforcement.
Comparison of the delamination areas of seven composite panels.
Impact-induced delamination is a process of unstable fracture and subsequent crack arrest, and the delamination size is governed by the strain energy release rate and the crack arrest toughness. The most common experiment to obtain the interfacial strength is the T-peel test, which measures the mode I delamination resistance. We conducted T-peel tests and found that the interfacial strength between carbon fabric and NR matrix is the highest, while it is the lowest between nylon fabric and NR matrix. 17 However, as suggested by Wang and Vu-Khanh, 18 low-velocity impact-induced delamination is usually dominated by mode II fracture. We plan to conduct edge-notched flexure or edge delamination tension tests in the future to quantify mode II delamination resistance. We argue that delamination of composite panels under low-velocity impact is a mixed-mode fracture process.
Damage mechanisms
The damaged specimens were cut along the quarter-symmetric planes to observe damage occurred inside using an optical microscope. Figure 11 shows the images of the damaged region. Fiber breakage and delamination occurred in every specimen. When the fiber content is small (one fabric ply), matrix crack is only observed in the back (tension) side of the NR/Carbon/1 specimen. As the fiber content increases, specimens reinforced with nylon and carbon fibers all developed matrix cracks, however, matrix crack was not observed in specimens reinforced with Kevlar fabrics. The location of matrix cracks in the NR/Carbon/2 and NR/Nylon/2 specimens were at the middle rubber layer. Table 7 lists the damage mechanisms of the composite specimens.
Images of the damaged region: (a) natural rubber (NR)/Nylon/1, (b) NR/Nylon/2, (c) NR/Nylon/3, (d) NR/Kevlar/1, (e) NR/Kevlar/2, (f) NR/Carbon/1, (g) NR/Carbon/2.
Damage mechanisms of the specimens.
Fiber breakage and matrix cracking occur when the deformation exceeds their fracture strains, and delamination initiates when the strain energy release rate reaches the interfacial fracture toughness. Because of load redistribution, fiber breakage increases the deformation of the surrounding matrix and matrix fracture facilitates fiber breakage and interface delamination. Delamination is often associated with transverse cracks. The reason for more matrix cracks in the middle rubber layer of the NR/Carbon/2 and NR/Nylon/2 specimens is due to the higher interply stress caused by the increased fiber volume ratio.
Conclusions
This paper presents an experimental study of the low-velocity impact behavior and damage mechanisms of woven fabric-reinforced NR panels. Some observations and conclusions are summarized as follows:
Reinforcing NR with continuous fibers increases the impact force the composite panel can withstand. The peak force increases as the fiber strength and fiber content increase. The presence of reinforcement fibers reduces the energy absorption capability of NR. As fiber stiffness and fiber content increase, the absorbed energy decreases. Rubber composites having nylon and Kevlar fabrics as reinforcements have better energy absorption capabilities than composites reinforced with carbon fibers. The damage mechanisms of rubber composites reinforced with nylon fabrics, used widely in tire industry, change as the fiber content increases. Based on the peak force and delamination area results, the NR/Carbon/2 composite, which consists of two carbon fabric layers inside NR, displays the best impact performance. Matrix cracks were mitigated with Kevlar reinforcements. Consequently, hybrid Kevlar/carbon reinforcement may be explored for future applications.
Footnotes
Acknowledgments
The authors want to thank Dr Kwek-Tze Tan for helping with impact tests and Dr Lingyan Li for using the X-ray scanner. SCW contributed to this research from 2019 to August 2022.
Declaration of conflicting interests
The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The authors received the following financial support for the research, authorship, and/or publication of this article: This study was funded by the Suncor Energy of Canada.