Experimental and numerical characterization of mechanical properties of hemp fiber reinforced composites using multiscale analysis approach
Abstract
Synthetic fibers are mainly considered as a source of reinforcement in many composites that have been studied so far. As a continuous effort is being made in the entire world to develop more sustainable products and systems, fibers obtained from natural sources (hemp, flax, jute, etc.) are being considered as substitutes to the synthetic fibers which are commonly used these days. In this study, the tensile, flexural and interlaminar shear strength properties of hemp fiber composites, bonded with an epoxy matrix, are characterized. Three-point bending tests were conducted on the hemp fiber composites and their failure modes were inspected using micrographs and documented. The short-beam shear method was utilized to investigate the interlaminar shear strength of these composites. Hemp fiber composites were found to have considerably lower flexural strength as compared to typical glass fiber/epoxy composites. Lastly, multiscale analyses were performed for the tensile case to predict the response of these composites by conducting simulations at fiber tows and matrix level rather than using typical finite element approach of homogenized properties at ply level with Altair’s multiscale designer software (MDS). In the multiscale macro-simulations, a unit cell with twill weave configuration was developed, and the response of the unit cell under tensile loadings was compared to the experiment results. Excellent Correlation was achieved between numerical and experimental results for unnotched tension/compression MDS macro-simulation. A parametric study was conducted to investigate the effect of fiber tow minor radius on the tensile response of the hemp/epoxy composites keeping rest of the geometric parameters of unit cell similar. The smaller minor tow radius predicted higher yield and ultimate strengths as compared to larger ones. A stochastic analysis was also performed to investigate the effects of variations in the geometric unit cell parameters on the young’s modulus of hemp/epoxy composite.
Keywords
Hemp fiber Mechanical properties Multiscale analysis1 Introduction
Currently, most composites utilized in the industry are composed of synthetic-type fibers, such as glass fiber and carbon fiber, and thermoset epoxies. Synthetic fibers have fantastic mechanical properties and can be manufactured to very precise specifications for their respective applications. Thermoset epoxies have high strengths and great durability. These advantages, however, come at a cost. The product typically cannot be completely reused or recycled. Instead, at the end of the product’s life, disposal is required to some extent, which is limited to landfill or incineration [1].
In contrary, natural fibers that come from renewable resources are eco-friendlier. Some natural fibers that have been of main interest include sisal, flax, hemp, jute and bamboo. Hemp plants contain strong and stiff plant fibers within the stem, which help hold the plant upright, meaning they have the necessary characteristics to be useful as a composite material reinforcement [2]. The key factors in the determination of a natural fiber’s mechanical properties are its cellulous, hemicellulous and lignin content [3]. The benefits of reinforcing composites with hemp fibers include low cost, low density, high stiffness-to-weight ratio and easy processability [3, 4, 5]. The industrial sector and academic community have shown interest in natural fibers and their composites due to these advantages. Applications of natural fiber composites include components such as seat backs, pillar covers, parcel shelves, dashboards and headliner panels [6, 7, 8, 9]. Natural fiber composites can be utilized in sports commodities [10], the construction industry [11] and other consumer products.
Continuum damage mechanics models have been utilized for characterizing the failure in composites for a few decades now. However, their use to predict the response of natural fiber composites is fairly recent. Panamoottil et al. [12] predicted the tensile failure of thermoplastic and thermoset matrix-based flax fabric composites by combining experimentally determined damage rules with discrete representations of fabric geometry. The analytical/semi-analytical models that have been used for natural fiber composites are based on point stress criterion that is semiempirical [13] and modified point stress criterion [14, 15]. These damage criterions can only be applied within the limits from which their test data are derived. Bougherara et al. [16] used an energy-based damage model to investigate the damage growth of angle-ply [± 45]_{16} flax-reinforced epoxy composites. They found out that the above-mentioned damage model was able to predict the damage rate in both fiber and matrix directions under applied loads. Pan and Zhong [17] conducted research activity to predict material property degradation of sisal fiber composites due to moisture absorption through the proposed micromechanical model. Modified Mori–Tanaka method was used in this study after introducing a damage variable to consider mechanical property degradation.
Zhong et al. [18] reviewed the studies corresponding to microstructures, mechanical behavior and numerical modeling of plant fiber composites. Misnon et al. [19] studied the characteristics of woven hemp fiber for composite reinforcement and found that woven hemp fabrics are suitable for composite reinforcement. Panamoottil et al. [20] predicted the failure of unidirectional flax fabrics in a polypropylene matrix under quasi-static tensile loading by applying an anisotropic continuum damage modeling approach. They showed that modeling the geometric model closely to actual fabric geometry and using an experimentally-determined material degradation model can yield good mechanical response of the composite. Panamoottil et al. [21] proposed a hierarchical framework to investigate the macroscale properties of composites based on the properties of microscale. They used this framework to obtain properties of impregnated fiber tows from the characteristics of the fibers, the matrix material and the interface between them. Russo et al. [22] compared the performance and damage extent in jute, flax and basalt fibers composites impregnated in a polypropylene matrix and subjected to flexural loads. Vishwas et al. [23] performed a comparative study investigating the damage response of glass–epoxy (GE), jute–epoxy (JE) laminates and jute–rubber–jute (JRJ) sandwich composites using ABAQUS/CAE finite element software. No delamination was observed in JRJ sandwich composites as opposed to GE and JE laminates. This study also indicated that due to the compliant nature of rubber, JRJ sandwich composites absorbed more energy than GE laminate composites. Xiong et al. [24] recently developed multiscale constitutive models to investigate the mechanical properties of plain woven composites. The predicted results showed that the yarn twist angle has significant effects on the elastic properties of the composites.
It was found out that in terms of numerical simulations, the plant fiber composites have not received sufficient attention as synthetic fibers composites [25, 26]. It was also found after detailed literature consultation that research studies involving the mechanical characterization of hemp fiber-reinforced epoxy composites at unit cell level are rather scant. In light of these facts, this study was developed to examine the mechanical behavior of hemp fiber–epoxy composites. In this study, the tensile, flexural and interlaminar shear strength properties of hemp fiber composites, bonded with an epoxy matrix, are characterized in order to have a better understanding of the response of hemp/epoxy composites under mechanical loadings. The main aim of this research study was to find out the possibility of replacing the synthetic fibers (Eglass) with the natural fibers (hemp) under similar mechanical loading and having the same fiber volume fraction. It was found out that hemp might only be a suitable replacement in non-structural, low load-bearing applications to prevent early product failure and frequent replacement. The multiscale analysis simulation performed in this research study helps to predict the response of the system at fiber tow and matrix scale rather than at homogenized ply level. Thus, the practical multiscaling approach considers the effects of unit cell geometric parameters on the mechanical response of the hemp/epoxy composites and predicts the response of the system more accurately and consistent with experimental results.
2 Materials and methods
3 Tensile tests
Tensile tests on hemp/epoxy laminate specimens were done as per ASTM D3039 test method. Hemp/epoxy laminate of 250 mm × 250 mm with five layers of hemp fiber cloth (thickness 1.91 ± 0.04 mm) was cured for tensile tests. Laminates were cut into 200 mm × 25 mm test coupons with 38 mm grip length on each side. The rectangular cross section of test coupons is maintained to avoid failure near the grip. Due to the soft nature of hemp/epoxy composites, to avoid enormous pressure on the coupon, grit paper was used on the laminate in the gripping. Extensometer is used to obtain strain from the coupon. The hemp fiber composites were tested to determine the tensile properties in the longitudinal direction. From the load–displacement data of machine, the stress–strain curve was plotted. The values of tensile modulus, maximum strength and failure strain were calculated from the experimental plots.
4 Shear tests
5 Three-point bend tests
6 Multiscale analysis
Multiscale designer is an accurate and efficient tool for building multiscale material models and performing material characterization analysis for components developed from any heterogeneous material including, but not limited to, continuous and chopped fiber composites, honeycomb cores, lattice structures, reinforced concrete, soil, bones, etc. The entire multiscale simulation takes benefit from the natural division at various length scales between microstructure, fiber/matrix, tow/matrix ply, laminate and component which are found in composite structures. This natural separation allows the multiscale technique to be employed at particular length scale for any one entity (e.g., fiber tows) to compute its properties, developing a constitutive model through homogenization and giving the response to the higher level for characterizing the mechanical behavior of the larger component (e.g., individual plies). Therefore, in the multiscale technique, an analysis is executed by passing the information between various length scales and not by coupling different global–local simulation techniques. In this study, the tensile response of the hemp/epoxy fiber composite was modeled by using twill weave unit cell capabilities of multiscale designer software. The steps involved in multiscale analysis using MDS software are given as follows.
6.1 Unit cell definition
Geometric attribute values used in this study
Geometric attributes | |
---|---|
Tow major radius; w | 5 mm |
Tow minor radius; z | 0.6 mm |
Tow center spacing in x-axis; a | 12 mm |
Tow center spacing in y-axis; b | 12 mm |
Tow volume fraction (%) | 55 |
6.2 Linear material characterization
- The isotropic material stiffness matrixwhere$$\left[ {\begin{array}{*{20}l} {\delta + 2\varphi } \hfill & \delta \hfill & \delta \hfill & 0 \hfill & 0 \hfill & 0 \hfill \\ \delta \hfill & {\delta + 2\varphi } \hfill & \delta \hfill & 0 \hfill & 0 \hfill & 0 \hfill \\ \delta \hfill & \delta \hfill & {\delta + 2\varphi } \hfill & 0 \hfill & 0 \hfill & 0 \hfill \\ 0 \hfill & 0 \hfill & 0 \hfill & \varphi \hfill & 0 \hfill & 0 \hfill \\ 0 \hfill & 0 \hfill & 0 \hfill & 0 \hfill & \varphi \hfill & 0 \hfill \\ 0 \hfill & 0 \hfill & 0 \hfill & 0 \hfill & 0 \hfill & \varphi \hfill \\ \end{array} } \right]$$(4)$$\delta = \frac{\nu E}{{\left( {1 + \nu } \right)\left( {1 - 2\nu } \right)}}\;{\text{and}}\;\varphi = \left( {\frac{E}{{2\left( {1 + \nu } \right)}}} \right)$$(5)$${\text{Stability}}\;{\text{Conditions}}:\;E > 0\;{\text{and}}\; - 1 < \nu < 0.5$$(6)
- Material stiffness matrix for transversely isotropic linear material law (23-plane of isotropy)where$$\left[ {\begin{array}{*{20}l} n \hfill & l \hfill & l \hfill & 0 \hfill & 0 \hfill & 0 \hfill \\ l \hfill & {k + m} \hfill & {k - m} \hfill & 0 \hfill & 0 \hfill & 0 \hfill \\ l \hfill & {k - m} \hfill & {k + m} \hfill & 0 \hfill & 0 \hfill & 0 \hfill \\ 0 \hfill & 0 \hfill & 0 \hfill & m \hfill & 0 \hfill & 0 \hfill \\ 0 \hfill & 0 \hfill & 0 \hfill & 0 \hfill & p \hfill & 0 \hfill \\ 0 \hfill & 0 \hfill & 0 \hfill & 0 \hfill & 0 \hfill & p \hfill \\ \end{array} } \right]$$(7)$$G_{23} = \left( {\frac{{E_{2} }}{{2\left( {1 + \nu_{23} } \right)}}} \right) ;\quad \nu_{21} = \frac{{\nu_{12} E_{2} }}{{E_{1} }} ;k = - \frac{1}{{\left\{ {\left( {\frac{1}{{G_{23} }}} \right) - \left( {\frac{4}{{E_{2} }}} \right) + \frac{{4\nu_{12}^{2} }}{{E_{1} }}} \right\}}};\quad l = 2k\nu_{12}$$(8)$$n = E_{1} + \frac{{l^{2} }}{k} ;\quad m = G_{23} \;{\text{and}}\;p = G_{12}$$(9)
Fiber and matrix phase properties for linear material characterization
Fiber micro-properties (GPa) | Matrix micro-properties (GPa) | ||
---|---|---|---|
Parameter | Value | Parameter | Value |
E_{1} | 20 | E (GPa) | 3.4 |
E_{2} | 3.5 | ν | 0.36 |
ν_{12} | 0.28 | ||
ν_{23} | 0.28 | ||
G_{12} | 4.24 | ||
E_{1_C} | 1.5 |
6.3 Reduced-order model computation
The computational cost was reduced to considerably low levels through the reduced-order calculations in the nonlinear material characterization and subsequent nonlinear macro-analysis. The total number of degrees-of-freedom in the unit cell is decreased by using the reduced-order model (ROM) technique [28]. A series of influence function problems are solved, and calculated tensors are obtained as a material database of the reduced-order unit cell. Phase average theory [29] is a classic example of reduced-order model. In the reduced-order unit cell model, the optimal influence functions depend on considering that whether a particular failure mode is a matrix or fiber dominant within each inclusion phase of the unit cell; the option to make this choice for each inclusion phase of the unit cell is provided in MDS. In this study, for the Tow_X and Y fiber phases of the unit cell, the xx deformation modes were considered as inclusion dominant, and the remaining five deformation modes were taken as matrix dominant.
6.4 Nonlinear material characterization
After the computation of the reduced-order model data, the nonlinear material characterization step was performed. The inverse homogenization approach was used to compute the nonlinear macro-homogenized properties of the unit cell. In the inverse characterization approach, the nonlinear macro-homogenized properties are defined (typically based on experimentally observed test data from one or more coupon tests) and the unknown nonlinear micro-properties are characterized by solving the inverse optimization problem. For an inverse characterization of the mechanical solution sequence, macroscopic stress–strain experimentally observed test data from different coupon tests can be considered simultaneously in constructing the inverse optimization objective function. The nonlinear material characterization (simulation) with the inverse homogenization approach was performed by assigning each micro-phase a damage law and entering the necessary nonlinear parameter values. In the nonlinear material characterization, matrix phase is modeled with the orthotropic damage and plasticity continuum damage model, whereas the Tow_x and Tow_y phases were modeled with the orthotropic damage bilinear continuum damage model.
- Elastic stress–strain relationship$$\sigma_{ij} = \left( {1 - W} \right)L_{ijkl} \left( {\varepsilon_{k} = \varepsilon_{kL}^{p} } \right)$$(17)
- Scalar equivalent strain or the volumetric strain (J_{1})$$\hat{\varepsilon } = \varepsilon_{11} + \varepsilon_{22} + \varepsilon_{33}$$(18)
- Damage criterion$$g\left( {\hat{\varepsilon },r} \right)\text{ := }\hat{\varepsilon } - r \le 0$$(19)
- Damage evolution$$\dot{w} = \gamma H\left( {\hat{\varepsilon }} \right)$$(20)$$\dot{r} = \gamma$$(21)
- Kuhn–Tucker complementarity conditions$$\gamma \ge 0,g\left( {\hat{\varepsilon },r} \right) \le 0,\gamma g\left( {\hat{\varepsilon },r} \right) = 0$$(22)
- Consistency condition$$\gamma \dot{g}\left( {\hat{\varepsilon },r} \right) = 0 \quad {\text{if}}\quad g\left( {\hat{\varepsilon },r} \right) = 0$$(23)
Tow and matrix phase properties for nonlinear material characterization
Tow-x and tow-y micro-properties | Matrix micro-properties | ||||||||
---|---|---|---|---|---|---|---|---|---|
Parameter | Value | Active | Lower bound | Upper bound | Parameter | Value | Active | Lower bound | Upper bound |
σ_{0 (1)} (GPa) | 0.12 | ✓ | 0.1 | 0.16 | σ_{Y} | 0.0326 | ✓ | 0.0286 | 0.050 |
ε_{1 (1)} | 0.2 | ✓ | 0.16 | 0.3 | σ_{1} | 0.0532 | ✓ | 0.070 | 0.12 |
σ_{0 (2)} (GPa) | 0.1 | ✓ | 0.007 | 0.12 | δ 42 | 1000 | ✓ | 800 | 1200 |
ε_{1 (2)} | 0.017 | ✓ | 0.0119 | 0.018 | H | 0.0034 | ✓ | 3 | 10 |
σ_{0 (1)_C} (GPa) | 0.16 | θ | 1 | ||||||
σ_{0 (2)_C} (GPa) | 0.13 | β | 0.15 | ||||||
D_{12} | 0.0 | ε_{p0} | 1.0 | ||||||
D_{23} | 0.0 | ε_{p1} | 1.1 | ||||||
σ_{mean} | 0.0283 | ||||||||
J_{1} | 0.008 | ||||||||
C | 0 | ||||||||
W_max | 1.0 |
7 Results and discussion
7.1 Experimental results
7.1.1 Tensile properties
Comparative analysis for mechanical properties of flax and hemp/epoxy composites
Parameter | Hemp/epoxy | Flax/epoxy |
---|---|---|
Tensile strength (MPa) | 70 | 68.29 |
Young’s modulus (GPa) | 6.7 | 5.870 |
7.1.2 Interlaminar shear properties
7.1.3 Flexural properties
7.2 Multiscale numerical results
Homogenized linear regime material properties
Tension data (GPa) | Compression data (GPa) | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|
Homogenized modulus and Poisson’s ratio | Homogenized modulus and Poisson’s ratio | ||||||||||
E_{x} | E_{y} | E_{z} | G_{yz} | G_{xz} | G_{xy} | E_{x} | E_{y} | E_{z} | G_{yz} | G_{xz} | G_{xy} |
6.674 | 6.674 | 1.726 | 0.533 | 0.533 | 0.634 | 2.155 | 2.155 | 1.607 | 0.524 | 0.524 | 0.633 |
ν_{xy} | ν_{xz} | ν_{yx} | ν_{yz} | ν_{zx} | ν_{zy} | ν_{xy} | ν_{xz} | ν_{yx} | ν_{yz} | ν_{zx} | ν_{zy} |
---|---|---|---|---|---|---|---|---|---|---|---|
0.121 | 0.425 | 0.121 | 0.425 | 0.110 | 0.110 | 0.305 | 0.314 | 0.305 | 0.314 | 0.234 | 0.234 |
7.2.1 Unnotched tension/compression (UNT/C)
Input parameters for UNT/C macro-simulation
Parameter | Value |
---|---|
Loading rate (/s) | 0.03 |
Maximum strain | 0.03 |
Delta temperature | 0 |
Minimum thermal increments | 0 |
Minimum mechanical increments | 300 |
Probability distribution parameters
Parameter | Mean value | # of samples | Distribution | Standard deviation |
---|---|---|---|---|
Tow major radius | 5 | 17 | Normal | 0.22 |
Tow minor radius | 0.6 | 68 | Normal | 0.25 |
Tow center spacing X | 12 | 100 | Log-normal | 0.35 |
Tow center spacing Y | 12 | 100 | Log-normal | 0.35 |
Tow volume fraction | 52 | 50 | Normal | 5 |
Optimum geometric parameters of unit cell considering value of E_{x} having maximum probability
Parameter (mm) | Value |
---|---|
Tow major radius | 5.15 |
Tow minor radius | 0.515 |
Tow center spacing X | 11.7 |
Tow center spacing Y | 11.7 |
Tow volume fraction % | 57.5 |
8 Conclusions
Mechanical properties of hemp/epoxy composites are characterized in this investigation. Tensile and interlaminar stress–strain plots have a bilinear nature with elastic proportionality limit being relatively smaller in the tensile plots. The average values of tensile and flexural moduli were found to be 6.7 GPa and 7.35 GPa, respectively. Hemp fiber, though it greatly improves the flexural capabilities of pure epoxy and is less expensive than glass fiber, was found to have much less desirable mechanical properties than typical glass fiber/epoxy composites (ultimate tensile strength and Young’s modulus). This shows that hemp may only be a suitable replacement in non-structural, low load-bearing applications to prevent early product failure and frequent replacement. Areas, where hemp is currently being used, include vehicle interiors, packaging, construction and horse bedding. The multiscale analysis was conducted on tensile behavior of hemp/epoxy composites by developing twill weave unit cell model and thereby considering the effects of fiber tows and matrix on the material response. Excellent correlation was achieved by using a multiscale analysis approach. Stochastic analysis showed the dominance of different geometric parameters on E_{x }of composite with the considered standard deviation for these parameters.
Notes
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
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