# Evaluation of Interfacial Fracture Toughness and Interfacial Shear Strength of Typha Spp. Fiber/Polymer Composite by Double Shear Test Method

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

#### 2.1. Materials

#### 2.2. Scanning Electron Microscopy (SEM)

#### 2.3. Mode II Interfacial Fracture Toughness Test

_{i}, U

_{ii}and U

_{iii}are the strain energy of the section (i), (ii), and (iii) in Figure 1. The equations are stated below.

_{min}is the outer fiber bonded with the minimum matrix length. The mean interfacial shear stress can be obtained as:

## 3. Results and Discussion

#### 3.1. Surface Morphology of Typha Spp. Fiber

#### 3.2. Interfacial Fracture Toughness of the Typha Spp. Fiber Model Composite

_{i}of the untreated and 5% alkali-treated Typha spp. fiber/PLLA model composites are presented in Figure 6 as a function of (a) matrix length, L, (b) fiber spacing, t, (c) fiber diameter, D, and bonding area, A. Since the results are not expressed by a single parameter, the least square regression was conducted to fit the following equation:

_{i}of each sample was plotted as a function of the least square regression value of G

_{i}which was calculated from Equation (6) by substituting measured values of L, t, D, and A of each sample in Figure 7. Although a certain amount of scatter exists, no systematic deviation was found. Then, the linear combination of each factor indicated by Equation (6) is considered to be valid in the range of the present experiments, and the real scatter of the interfacial fracture toughness is shown the figure.

_{i}values increased with the increasing matrix length and fiber spacing, and G

_{i}values were found to decrease with increasing fiber diameter and bonding area. In all cases, the untreated Typha spp. fiber model composite had less interfacial fracture toughness value than the alkali-treated fibers. This phenomenon occurs because the alkali treatment changes the surface roughness contour of the Typha spp. fiber. The surface of the alkali-treated fibers is rougher than the raw fiber due to the removal of surface impurities by the alkali treatment [44]. Since the fiber becomes cleaner from the impurities and the surface becomes coarser, the fiber-matrix mechanical interlocking bonding occurs and promotes better adhesion between the fiber and the matrix.

_{i}versus the matrix length plot shown in Figure 6a, greater values of interfacial fracture toughness were observed at longer matrix lengths. The greater the length of the matrix, the higher the interfacial fracture toughness becomes. This finding is similar to the results reported by Chin Von Sia [42]. The highest G

_{i}value obtained for the untreated fiber was only 27.4 J/m

^{2}, which was much smaller than the 1 h, 2 h, 4 h, and 8 h alkali-treated Typha spp. fibers whose G

_{i}value were 78.1, 96.1, 91.4, and 86.2, respectively. The phenomenon of the G

_{i}value increment was also found in the G

_{i}versus fiber spacing plot presented in Figure 6b. The gap of fibers on the Typha spp. fiber/PLLA model composite specimen affects the G

_{i}value. The greater gap between the fibers increased the G

_{i}value of Typha spp. fiber/PLLA model composite. The plotting results of G

_{i}versus the fiber diameter in Figure 6c, and G

_{i}versus the bonding area in Figure 6d showed the distribution of the G

_{i}value decreased.

_{i}values of Typha spp./epoxy model composite on/against matrix length (Figure 8a), fiber spacing (Figure 8b), fiber diameter (Figure 8c), and the bonding area (Figure 8d). The lines in each figure show the relationship obtained using the least square regression employing the same manner as Figure 7. Similar to Figure 7, no systematic deviation was found in the relationship between the measured and least regression values of G

_{i}as shown in Figure 9. This indicates the validity of the linear combination of each factor in Equation (6).

_{i}values were found to be increased against the matrix length and the fiber spacing, and decreased on the fiber diameter and the bonding area. However, it can still be seen that, in general, the G

_{i}of untreated Typha spp. fiber is lower when compared to the alkali-treated Typha spp. fibers. This indicates that the value of the interfacial fracture toughness of the model composite Typha spp./PLLA and Typha spp./epoxy exhibit the same behavior.

^{2}, and the lowest value was on the Typha spp. fiber/PLLA with 14.4 J/m

^{2}.

#### 3.3. Interfacial Shear Strength of the Typha Spp. Fiber Model Composite

_{i}of Typha spp. fiber/PLLA model composite and its relationship with specimen parameters is plotted in Figure 11, where lines indicate the relationship obtained by the least square regression employing average values of other factors of the abscissa. In this case, no systematic deviation was found in the relationship between the measured and the least regression values of τ

_{i}as shown in Figure 12. Figure 11a illustrates the correlation of interfacial shear stress, τ

_{i}with matrix length, L. The results showed that the interfacial shear stress, τ

_{i}of the Typha spp. fiber/PLLA model composite decreased with increasing matrix length. A similar trend was also seen in Figure 11d in which the interfacial shear strength value decreased with the increasing bonding area. Similar results were reported by Day and Cauich Rodrigez in a study on Kevlar fiber [31], Zhandarov and Mader on glass fiber [47], and Gorbatkina et al. on carbon fiber [48] with thermosetting or thermoplastic as the matrix. The increasing bonding area enhanced the occasion probability of the critical flaws, therefore the probability of the initiated fracture upon loading was increasing [49]. Otherwise, the interfacial shear stress values were increased against the average fiber spacing and fiber diameter as shown in Figure 11b,c.

_{i}as shown in Figure 14. On the other hand, the data for interfacial shear stress versus fiber spacing (Figure 13b) and fiber diameter (Figure 13c) increased. In addition, the higher value of the interfacial shear stress was observed on the alkali-treated Typha spp. fiber/epoxy specimens due to the increased surface roughness and the lesser amount of hemicellulose, lignin and wax content on the fiber by alkali treatment. This promotes a better fiber-matrix bonding condition and improves their interface adhesion. Boopathi et al. also reported that strong hydrogen bonds seen in alkali-treated fibers facilitated better mechanical properties for interfacial adhesion between the fiber and the matrix [44].

#### 3.4. Effects of Geometrical Factor on Interfacial Fracture Toughness

## 4. Conclusions

- Alkali treatment on Typha spp. fiber can make the fiber surface coarser due to the removal of impurities, such as fatty substance from the fiber surface, thus increasing the interfacial fracture toughness and interfacial shear strength values.
- The Typha spp. fiber/epoxy has a higher interfacial fracture value than Typha spp. fiber/PLLA because the PLLA-based composite has lower mechanical properties compared to the epoxy, the PLLA has poor melt strength, brittleness and the melt viscosity of PLA has low shear sensitivity and relatively poor strength.
- Interfacial fracture toughness of Typha spp. fiber/PLLA and Typha spp. fiber/epoxy composite model specimens are influenced by the matrix length and the fiber spacing. The longer the matrix length, the higher the value of the interfacial fracture toughness. Meanwhile, the interfacial shear strength decreases with the increasing matrix length and the bonding area. Furthermore, the interfacial fracture toughness and the interfacial shear stress of the composite model increased with the increasing duration of the surface treatment.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 5.**Surface imaging of (

**a**) untreated, (

**b**) Alkali-treated for 1 h, (

**c**) Alkali-treated for 2 h, (

**d**) Alkali-treated for 4 h, and (

**e**) Alkali-treated for 8 h Typha spp. fibers.

**Figure 6.**Interfacial fracture toughness of Typha spp./PLLA, where: (

**a**) Matrix length, (

**b**) Fiber spacing, (

**c**) Fiber Diameter, (

**d**) Bonding area.

**Figure 7.**The comparison between measured and calculated interfacial facture toughness of Typha spp./PLLA.

**Figure 8.**Interfacial fracture toughness of Typha spp./Epoxy, where: (

**a**) Matrix length, (

**b**) Fiber spacing, (

**c**) Fiber Diameter, (

**d**) Bonding area.

**Figure 9.**The comparison between measured and calculated interfacial fracture toughness of Typha spp./Epoxy.

**Figure 10.**The comparison of the mean interfacial fracture toughness Typha spp. fiber/PLLA and Typha spp. fiber/Epoxy.

**Figure 11.**Interfacial shear strength of Typha spp./PLLA, where: (

**a**) Matrix length, (

**b**) Fiber spacing, (

**c**) Fiber Diameter, (

**d**) Bonding area.

**Figure 12.**The comparison between measured and calculated interfacial shear strength of Typha spp./PLLA.

**Figure 13.**Interfacial shear strength of Typha spp./Epoxy, where: (

**a**) Matrix length, (

**b**) Fiber spacing, (

**c**) Fiber Diameter, (

**d**) Bonding area.

**Figure 14.**The comparison between the measured and calculated interfacial shear strength of Typha spp./ Epoxy.

**Figure 15.**The comparison of the mean interfacial shear strength of Typha spp./PLLA and Typha spp./Epoxy.

Alkali Treatment Period | Average Diameter (μm) |
---|---|

0 | 309 |

1 | 286 |

2 | 282 |

4 | 263 |

8 | 226 |

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**MDPI and ACS Style**

Ikramullah; Rizal, S.; Nakai, Y.; Shiozawa, D.; Khalil, H.P.S.A.; Huzni, S.; Thalib, S.
Evaluation of Interfacial Fracture Toughness and Interfacial Shear Strength of Typha Spp. Fiber/Polymer Composite by Double Shear Test Method. *Materials* **2019**, *12*, 2225.
https://doi.org/10.3390/ma12142225

**AMA Style**

Ikramullah, Rizal S, Nakai Y, Shiozawa D, Khalil HPSA, Huzni S, Thalib S.
Evaluation of Interfacial Fracture Toughness and Interfacial Shear Strength of Typha Spp. Fiber/Polymer Composite by Double Shear Test Method. *Materials*. 2019; 12(14):2225.
https://doi.org/10.3390/ma12142225

**Chicago/Turabian Style**

Ikramullah, Samsul Rizal, Yoshikazu Nakai, Daiki Shiozawa, H.P.S. Abdul Khalil, Syifaul Huzni, and Sulaiman Thalib.
2019. "Evaluation of Interfacial Fracture Toughness and Interfacial Shear Strength of Typha Spp. Fiber/Polymer Composite by Double Shear Test Method" *Materials* 12, no. 14: 2225.
https://doi.org/10.3390/ma12142225