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Omar, F. N., Mohammed, M. A. P., and Samsu Baharuddin, A. (2014). "Microstructure modelling of silica bodies from oil palm empty fruit bunch (OPEFB) fibres," BioRes. 9(1), 938-951.

Abstract

Investigating the mechanical behaviour of silica bodies in oil palm empty fruit bunches (OPEFB) is important to improve the process of silica body removal. This study will assist in providing an understanding of the role of OPEFB as a bioresource material for the bioconversion process. The microstructure of silica bodies/protrusions on the OPEFB fibre surface was modelled using the finite element method, based on the information obtained from scanning electron microscopy (SEM). The effects of silica body geometry, possible anisotropy/orthotropy, and debonding between the interface of the silica body and OPEFB fibre were investigated. Agreements were observed between the results using both circular and spiked silica body models with different geometries and volume fractions. In addition, the cohesive debonding modelling results showed that once critical stress was activated, the stress-strain curve deviated from the no-debond model. The results also suggested that the value of cohesive energy should be between 0.5 kN/m and 4 kN/m.


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Microstructure Modelling of Silica Bodies from Oil Palm Empty Fruit Bunch (OPEFB) Fibres

Farah Nadia Omar,a Mohd Afandi P. Mohammed,a,* and Azhari Samsu Baharuddin a,b

Investigating the mechanical behaviour of silica bodies in oil palm empty fruit bunches (OPEFB) is important to improve the process of silica body removal. This study will assist in providing an understanding of the role of OPEFB as a bioresource material for the bioconversion process. The microstructure of silica bodies/protrusions on the OPEFB fibre surface was modelled using the finite element method, based on the information obtained from scanning electron microscopy (SEM). The effects of silica body geometry, possible anisotropy/orthotropy, and debonding between the interface of the silica body and OPEFB fibre were investigated. Agreements were observed between the results using both circular and spiked silica body models with different geometries and volume fractions. In addition, the cohesive debonding modelling results showed that once critical stress was activated, the stress-strain curve deviated from the no-debond model. The results also suggested that the value of cohesive energy should be between 0.5 kN/m and 4 kN/m.

Keywords: Oil palm empty fruit bunch; Microstructure modelling; Silica bodies; Cohesive zone modelling

Contact information: a: Department of Process and Food Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia; b: Institute of Tropical Forestry and Forestry Products (INTROP), Putra Infoport, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor, Malaysia; *Corresponding author: afandi@upm.edu.my

INTRODUCTION

Oil palm empty fruit bunch (OPEFB) consists of silica bodies embedded in fibre, as shown in Fig. 1(a). Silica bodies are observed in natural fibres such as piassava (d’Almeida et al.2006) and oil palm fruit bunch (Law et al. 2007). Investigating the mechanical behaviour of silica bodies in OPEFB is important for several reasons: it will improve the process of silica body removal, so that OPEFB can then be used for biochemical conversion technology (Bahrin et al. 2012) and the pulp production industry (Hubbe and Heitmann 2007; Martin-Sampedro et al. 2012; Ghazali et al. 2009). Additionally, the study will assist in providing an understanding of the role of OPEFB as a bioresource material. One specific application of OPEFB after the silica bodies are removed (using, for example, the stream pretreatment method of Bahrin et al. (2012)) is for biocompost, in which the pretreatment firstly removes the silica bodies, followed by depolymerisation of lignin structures, breakage of the cellulose crystalline structure, and finally, an increase in the porosity of the material (Mosier et al. 2005). The porosity increase allows better bio-degradation of OPEFB. However, a complete physical understanding of the mechanisms in OPEFB during pretreatment (from silica body removal to degradation of cellulose) would be quite complex; therefore, this study is restricted to the consideration of silica body removal, which is believed to be the first process that occurs in OPEFB due to applied deformation or chemical constraints.

Fig. 1. (a) Silica bodies embedded in OPEFB fibre (re-drawn from Law et al. (2007)), (b) stress-strain curve of OPEFB fibre

There are few studies available in the literature that involve the study of micro-mechanics of silica bodies and OPEFB fibre, as well as their contributions to the mechan-ical behaviour of OPEFB. In contrast, micromechanical studies on other natural fibres such as wood and plant cell walls are well established (Somerville et al. 2004; Burgert 2006; Qi 2009, 2011; Burgert and Dunlop 2011; Hayot et al. 2012). It is believed that silica bodies contribute to the strength and rigidity of OPEFB (Nascimento et al. 2012). To illustrate this, Fig. 1(b) shows a suggested stress-strain curve of OPEFB, which can be separated into three regions: elastic, plastic/debonding, and fracture.

In the elastic region, the bonding of silica bodies is perfect, and no failure between the interfaces can be observed. In the plastic/debonding region, debonding between the interfaces is believed to cause the curve to deviate from the elastic line (dotted line in Fig. 1(b)). Debonding then continues to accumulate until complete failure occurs in the fracture region. By treating OPEFB as a heterogeneous structure, where the fibre acts as the matrix and silica bodies as fillers, the micromechanical theory of interface debonding of filler and matrix by Meddad and Fisa (1997) can be applied. OPEFB reportedly displays a stress-strain curve similar to Fig. 1(b), as obtained by Yusoff et al. (2009) from uniaxial tension tests, who found an elastic region strain limit of ~0.04 and failure strain of ~0.13.

Silica body removal from piassava fibre material is relatively easy; it can be done via abrasion, chemical treatment, or mechanical loading (Nascimento et al. 2012). However, Bahrin et al. (2012) reported that it is difficult to remove silica bodies from OPEFB fibre, as this must be performed using high-pressure steam or chemical treatments. This is in agreement with the results obtained by Yunus et al. (2010), where the silica bodies from OPEFB could only be dislodged completely from the OPEFB fibre using a combination of acid hydrolysis at 100 C and ultrasonic pretreatments. These findings suggest that the behaviour at the interface between silica bodies and OPEFB fibre is important for the removal of silica bodies from OPEFB fibre.

There are a very limited number of publications that have reported on silica body removal mechanisms, as well as the relationship between silica bodies and the fibre components in the OPEFB. As reported in previous works (Currie and Perry 2007; Fang and Ma 2006; Lins et al. 2002), silica bodies act as a defensive barrier that protects against bacterial and fungal attacks. It is commonly understood that these biological attacks may only take place when the hydrolase enzymes attach to the exposed amorphous region of the fibre. Therefore, the removal of silica bodies could open up the siliceous pathway and expose more of the amorphous region of the fibres, resulting in better enzymatic hydrolysis performance.

This work therefore investigated the microstructure of silica bodies/protrusions on OPEFB fibre surfaces using scanning electron microscopy (SEM). The information from image analysis results (such as the geometry of the silica bodies and volume fraction) was used in finite element modelling of OPEFB, treating OPEFB as a heterogeneous material. Details of the development of the model in the commercially available finite element software Abaqus (2009) will also be discussed. The investigated parameters include the effect of silica body geometry, possible anisotropy/orthotropy, and debonding between the interface of the silica body and OPEFB fibres. This study will provide an understanding of silica body removal from natural fibres like OPEFB.

MATERIALS AND METHODS

Oil palm empty fruit bunches (OPEFB) were obtained from Besout Palm Oil Mill (Sungkai, Perak, Malaysia; 352’59.34’’ N, 10116’35.87’’ E). The samples were physically pressed to remove oil and moisture before being shredded to sizes between 15 and 20 cm. The shredded samples were then kept in environmentally controlled conditions at -20 C prior to SEM analysis.

Scanning electron microscopy (SEM) analysis was performed using an electron microscope (S-3400N, Hitachi, Japan). The OPEFB fibres were cut to sizes ranging from 0.2 to 0.5 cm. The samples were then mounted on an aluminium stub using double-sided adhesive tape and were sputter-coated with platinum prior to morphological assessment (E-1010, Hitachi, Japan). The SEM micrographs were obtained with an acceleration voltage of 15 to 25 kV. Images from the SEM analysis of OPEFB fibres are shown in Fig. 2. It was observed that silica bodies (fillers) were embedded in the matrix (OPEFB fibre). The geometry of the fillers was circular with spikes. Similar findings were observed by Law et al. (2007) and Bahrin et al. (2012). The filler volume fraction was obtained using SEM image analysis in ImageJ software (Rasband 2012). This was performed by converting the image into a binary (black and white) image and calculating the difference between the black and white areas.

Fig. 2. SEM analysis of silica bodies from OPEFB fibres: (a) 500x magnification, (b) 4000x magnification, (c) 5000x magnification, (d) 10,000x magnification

For OPEFB, volume fraction values of 15% ± 4 were obtained from at least eight SEM images at different magnifications. Note that in this work, it is assumed that the area fraction (obtained from SEM images) is the same as the volume fraction of the fibre, based on the assumption by Underwood (1970). However, an accurate volume fraction (e.g. corrected volume fraction) needs to be investigated in the future using a 3D analysis method, since it is possible that the volume fraction is not uniform throughout the cross section of the fibre.

RESULTS AND DISCUSSION

Model Development

OPEFB fibre will be regarded as the matrix and the silica bodies as the filler from this section onward, unless otherwise specified. A 2-D single-particle model of the OPEFB was produced using the commercial finite element software Abaqus with Abaqus/Standard procedure (Abaqus 2009). The single-particle model is presented in Fig. 3(a) and consists of filler embedded in a matrix. The spikes in the filler were produced based on the SEM images shown in the previous section (Fig. 2). The effect of the spikes on the mechanical behaviour of OPEFB will be discussed in detail later in this section. Any sharp edges on the spikes and filler were tapered (see Fig. 3(a)), as sharp edges will affect the convergence of the finite element modelling results.

Fig. 3. (a) The microstructure finite element model of OPEFB and simulation image results of 10-spike filler (b), and circular filler (c) models. The S22 colour scale indicates principal stress in the y direction (values in MPa).

The filler volume fraction for the model was set at 15%, which was obtained from the image analysis performed in the previous section. Plane stress elements (CPS element type) and a ‘static’ time step were used throughout the analysis. The model was loaded under uniaxial tension, compression, and simple shear modes at a true strain rate of 0.1/s. The following boundary conditions were used for evaluation of uniaxial tension and uniaxial compression:

where δ, the applied displacement, is > 0 under uniaxial tension and < 0 under uniaxial compression; u and