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Tensile Behavior of [0/90]_{7} Hemp/Elium Biocomposites after Water Aging: In-Situ Micro-CT Testing and Numerical Analysis

^{*}

## Abstract

**:**

_{7}hemp/Elium composite after three different conditionings: “Ambient storage”, “Saturated at 60 °C” and “15 wet/dry cycles”. Instrumented repeated progressive tensile loading tests were carried out and showed an unexpected increase in the secant modulus for the aged samples at the end of the test. An in-situ micro-CT tensile test was then performed on a “15 wet/dry cycles” aged sample. The analysis of the tomographic images showed the damage development with interfacial debonding and matrix cracks in the specimen volume, and also the decrease in the curvature radius of the warp yarns during tensile loading facilitated by the plasticization of the resin. Finite element calculations were thus performed and demonstrated that the increase in the modulus is directly linked to the straightening of warp yarns, showing that the evolution of the modulus on a macroscopic scale can be explained by the deformations of the yarns on a microscopic level. These results allow us to better understand the mechanical behavior and the damage mechanisms that occur in biocomposites during tensile testing after water aging.

## 1. Introduction

_{s}and [+45/−45]

_{s}) of flax/Greenpoxy composite. They showed that Young’s modulus of composites decreases up to 26% for [0/90]

_{s}specimens and 45% for [+45/−45]

_{s}ones after 30 days of aging [29]. Wet/dry cycles have also been studied by some authors to reproduce the variation of humidity and temperature on flax fiber-reinforced composites [30,31,32]. For example, Mak and Fam worked on unidirectional flax-reinforced epoxy laminates subjected to twelve wet/dry cycles composed of 23 days of immersion in distilled room temperature water followed by 5 days in an oven at 60 °C. They showed a loss in tensile properties of the studied composites with a reduction of 12% in strength and 19% in Young’s modulus [30].

_{7}hemp/Elium samples and compared with the ambient storage: immersion in water until saturation and wet/dry cycling. To estimate their influence on the mechanical performances of the biocomposite, the evolution of the tensile modulus was analyzed by performing repeated progressive tensile loading tests. This type of mechanical test consists in cyclically loading a sample while increasing the stress until failure. It allows us to extract from the stress-strain curve the secant modulus for each cycle. Initially developed for carbon fiber reinforced composites by Lemaitre [33], it was then applied in some studies to determine the modulus evolution in plant-reinforced composites [18,34]. In this study, in addition to the repeated progressive loading tests, an in-situ micro-computed tomography (micro-CT) tensile test was also performed. This test allowed us to investigate the microstructural phenomena occurring during tensile loading in the volume of the sample. Few studies deal with the use of X-ray micro-CT on plant fiber composites. For example, Madra et al. [35] worked on the measurement of orientation and dimension of short fibers in composites and Perrier [21] used micro-CT to quantify damage in a woven hemp/epoxy composite. However, to the best of the authors’ knowledge, there are no published works concerning in-situ micro-CT tests on plant fiber composite. The novelty of this work also lies in the fact that this experimental investigation was completed with a finite element calculation to simulate the tensile behavior of the woven hemp/Elium composite. In literature, different numerical models have been developed for plant fiber composites [36,37,38]. The specificity of the model proposed in this work is that it takes into account the influence of the yarn undulation in the woven biocomposite.

## 2. Materials and Methods

#### 2.1. Tested Material

^{2}are used. The matrix is a thermoplastic polymer named Elium 188 developed by the company Arkema. The vacuum infusion process was applied to manufacture the biocomposite plates. After manufacturing, samples were cut from the plates into rectangular specimens with the following overall dimensions: 140 mm in length, 20 mm in width and 4 mm in thickness. The orientation of the warp yarns is parallel to the tensile axis in order to obtain [0/90]

_{7}hemp/Elium samples. The properties of the raw materials are summarized in Table 1. The fiber volume fraction of samples was determined by geometry and weight measurements and is equal to 39.7 ± 1.6%.

#### 2.2. Aging Conditions

_{7}specimens to determine their influence on the evolution of the tensile modulus during mechanical tests. To compare with the aging conditions, the “Ambient storage” is used as the reference state. It consists of the storage of the samples at room temperature and humidity: 21 ± 2 °C and 48 ± 5% of relative humidity. The first aging condition is named “Saturated at 60 °C”: samples are immersed in water at 60 °C until reaching the maximum water uptake (12 days). The second aging condition, called “15 wet/dry cycles”, consists of immersing samples in water at 60 °C for 12 days and drying them for 2 days at 40 °C, fifteen times successively.

#### 2.3. Ex-Situ Repeated Progressive Tensile Loading

_{σi}/E

_{0}

#### 2.4. In-Situ Micro-CT Tensile Test

_{7}hemp/Elium sample (Figure 2c). The test was made with a crosshead speed of 0.5 mm/min as for the ex-situ test. It has to be noticed that the rotation axis of the tomograph is the same as the tensile loading axis of the testing machine.

#### 2.5. Image Processing

#### 2.6. Finite Element Calculation

## 3. Results and Discussion

#### 3.1. Modulus Evolution

_{7}hemp/Elium samples for the “Ambient storage”, “Saturated at 60 °C” and “15 wet/dry cycles” conditionings (Figure 4). The different stress levels applied during the ex-situ repeated progressive loading tests were chosen so that the evolution of the secant modulus is described from the elastic domain to the failure of the sample.

#### 3.2. In-Situ Tensile Test

_{7}hemp/Elium sample after an aging of “15 wet/dry cycles”. One loading-unloading cycle was performed under the micro-CT system. The maximum applied stress was chosen to be in the increasing part of the modulus evolution curve (Figure 5). Therefore, three micro-CT acquisitions were carried out during the in-situ tensile test, one at each following state:

- before loading: the sample was mounted on the in-situ tensile testing machine and the displacement of the crosshead was held at zero,
- loaded: the sample was loaded and when the applied stress reached 80% of the ultimate stress (37.6 MPa in accordance with Table 2), the displacement of the crosshead was blocked,
- unloaded: the sample was unloaded and when the applied stress was close to zero, the displacement of the crosshead was blocked.

_{7}hemp/Elium sample used for the in-situ test. The maximum stress reached was equal to 47.7 MPa, which is almost the same value as the one obtained during ex-situ tests (47.1 MPa, see Table 2).

#### 3.3. Damage Quantification

#### 3.4. Evolution of the Curvature Radius of the Yarns

#### 3.4.1. Micro-CT Analysis

_{1}) and under loading (R

_{2}). It was observed that the curvature radius is multiplied by two between the unloaded and the loaded states; from about 0.5 mm to 1.0 mm. Figure 8 provides a schematic explanation of the damage and the curvature radius evolution in the tested composite. Before loading and aging, the curvature radius of the warp yarns is equal to R1 and there is no damage (Figure 8a). After the fifteen wet/dry cycles, some debonding at the yarn/matrix interfaces appear and the curvature radius is unchanged (Figure 8b). When the “15 wet/dry cycles” sample is subjected to a tensile loading up to 80% of the ultimate stress, the damage is far more developed, with both debonding and matrix cracks (Figure 8c). The significant development of damage explains the decrease in the modulus observed during ex-situ tensile tests up to about 50% of ultimate stress (Figure 5). Figure 8c also shows the significant increase in the curvature radius of the warp yarns (R

_{2}). On another scale, Placet et al. [45] demonstrated that the re-alignment of microfibrils leads to the increase in the modulus of a single hemp fiber during tensile testing. Can this explanation be transposed to the composite scale? Is the straightening of the warp yarns the cause of the increase in the modulus of the woven composite? In order to answer this question, a 3D numerical model of the tested composite was created.

#### 3.4.2. Finite Element Calculations

_{7}hemp/Elium sample. Therefore, two different geometries were created: one with the curvature radius R

_{1}, measured before loading, and one with the curvature radius R

_{2}, measured under loading. Figure 9 presents a longitudinal section of the modeled volume for each of the geometries. It shows the transverse strain fields (ε

_{yy}) calculated in the two cases for a displacement of 0.02 mm applied at one end of the sample.

## 4. Conclusions

_{7}hemp/Elium biocomposite. Repeated progressive loading tests instrumented with extensometers were carried out on the studied material for three different conditionings: “Ambient storage”, “Saturated at 60 °C” and “15 wet/dry cycles”. For the two water aging conditions, samples showed an increase in their secant modulus after reaching about 50% of their ultimate stress, while the ambient stored sample showed a continuous decrease in its modulus. Further investigations were then carried out to understand this phenomenon. A specific mechanical testing machine was used in order to perform an in-situ micro-CT tensile test on a [0/90]

_{7}“15 wet/dry cycles” aged sample. Three tomographic acquisitions were realized on this sample: before loading, loaded at 80% of its ultimate stress, and unloaded. The reconstructed volumes were segmented thanks to an artificial intelligence-based algorithm and the damage ratio was quantified at each state. Results showed that the water absorption fatigue led to about 2% of damage. Then, at 80% of the ultimate stress, the damage ratio reached almost 9%, before decreasing to 7% when the sample was unloaded. Moreover, an in-depth analysis of micro-CT images showed that, for “15 wet/dry cycles” aged samples, besides the damage development with interfacial debonding and matrix cracks, the curvature radius of the warp yarns increased during the tensile loading. Finite element calculations were thus performed on a portion of the composite, taking into account the different radii of curvature measured. The results showed an increase in Young’s modulus when the warp yarns straightened. It demonstrates that the evolution of the modulus in a woven biocomposite subjected to tensile loading is a competition between the damage development, which leads to the modulus decrease, and the warp yarn straightening, corresponding to a modulus increase. This study, by combining experimental results from in-situ micro-CT testing with finite element modeling, allows a better understanding of the complex mechanical behavior of biocomposites after water aging.

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

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**Figure 1.**Stress-strain schematic curve (solid blue line) of a repeated progressive tensile loading test and method of determination of the secant moduli E

_{σi}, identified by the dotted lines.

**Figure 2.**(

**a**) Picture of the tensile device for in-situ micro-CT acquisitions. (

**b**) Scheme of the MUTTOM tensile testing machine. (

**c**) The [0/90]

_{7}hemp/Elium sample gripped in the tensile testing machine jaws.

**Figure 3.**Principle of the assembly of the numerical model: (

**a**) weft yarns, (

**b**) warp yarns, (

**c**) polymer matrix, (

**d**) portion of [0/90] hemp/Elium composite with boundary conditions.

**Figure 4.**Tensile stress-strain curves of [0/90]

_{7}hemp/Elium samples for “Ambient storage”, “Saturated at 60 °C” and “15 wet/dry cycles” conditionings.

**Figure 5.**Evolutions of the tensile modulus versus the applied stress measured on [0/90]

_{7}hemp/Elium samples for the three conditionings.

**Figure 6.**3D segmentation for large and small volumes of the in-situ micro-CT tested hemp/Elium sample (

**a**,

**b**) before loading, (

**c**,

**d**) loaded at 37.6 MPa and (

**e**,

**f**) unloaded.

**Figure 7.**Micro-CT images and zooms of the aged [0/90]

_{7}hemp/Elium sample tested in situ (

**a**) before loading and (

**b**) loaded at 36.7 MPa.

**Figure 8.**Schemes of the evolution of damage and of the curvature radius of warp yarns (

**a**) before loading and aging, (

**b**) after “15 wet/dry cycles” and before loading, (

**c**) after “15 wet/dry cycles” and under loading.

**Figure 9.**Transverse strains (ε

_{yy}) in a portion of the [0/90] hemp/Elium sample calculated by finite element modeling for (

**a**) initial curvature radius (R

_{1}) and (

**b**) curvature radius measured under loading (R

_{2}).

**Figure 10.**Nodes considered for calculating the Young’s modulus value of the simulated material using Reaction Force in the tensile loading direction.

Material | Density (g·cm^{−3}) | Young’s Modulus (MPa) | Ultimate Stress (MPa) | References | |
---|---|---|---|---|---|

Longitudinal | Transverse | ||||

Hemp yarn | 1.48 | 23,000 | 1264 | 601 | [39,40] |

Elium 188 | 1.01 | 3300 | 76 | [41] |

**Table 2.**Initial Young’s modulus, ultimate stress, and maximum strain values of [0/90]

_{7}hemp/Elium samples for “Ambient storage”, “Saturated at 60 °C” and “15 wet/dry cycles” conditionings.

Mechanical Properties | Ambient Storage | Saturated at 60 °C | 15 Wet/Dry Cycles |
---|---|---|---|

Initial Young’s modulus (MPa) | 7323 ± 368 | 2428 ± 125 | 2392 ± 179 |

Ultimate stress (MPa) | 61.8 ± 5.0 | 56.7 ± 2.8 | 47.1 ± 2.4 |

Maximum strain (%) | 3.4 ± 1.0 | 7.6 ± 0.9 | 6.5 ± 0.4 |

**Table 3.**Total and residual strains along the X-axis and Y-axis measured during the in-situ and ex-situ tensile tests for an applied stress of 80% of the ultimate stress for [0/90]

_{7}hemp/Elium after aging of “15 wet/dry cycles”.

X-Axis | Y-Axis | |||
---|---|---|---|---|

In-Situ | Ex-Situ | In-Situ | Ex-Situ | |

Loaded state: total strain (%) | 5.38 ± 0.07 | 4.98 ± 0.1 | −1.10 ± 0.13 | −1.11 ± 0.10 |

Unloaded state: residual strain (%) | 3.70 ± 0.21 | 2.40 ± 0.25 | −0.51 ± 0.05 | −0.50 ± 0.05 |

**Table 4.**Damage quantification for large and small volumes determined from in-situ micro-CT tested samples.

Damage Quantification (%) | ||
---|---|---|

Large Volume (1620 mm ^{3} = 5 × 10^{8} Voxels) | Small Volume (3.38 mm ^{3} = 1 × 10^{6} Voxels) | |

Before loading | 2.0 | 2.4 |

Loaded at 37.6 MPa | 8.9 | 8.5 |

Unloaded | 6.0 | 6.7 |

**Table 5.**Comparison of Young’s modulus values calculated by finite element modeling in function of the curvature radius of the warp yarns.

Model Geometry with R_{1} | Model Geometry with R_{2} | Deviation (%) | |
---|---|---|---|

Curvature radius (mm) | 0.5 | 1.0 | 100 |

Young’s modulus (MPa) | 5137 | 5577 | 8.6 |

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**MDPI and ACS Style**

Drouhet, Q.; Touchard, F.; Chocinski-Arnault, L.
Tensile Behavior of [0/90]_{7} Hemp/Elium Biocomposites after Water Aging: In-Situ Micro-CT Testing and Numerical Analysis. *Micro* **2023**, *3*, 496-509.
https://doi.org/10.3390/micro3020033

**AMA Style**

Drouhet Q, Touchard F, Chocinski-Arnault L.
Tensile Behavior of [0/90]_{7} Hemp/Elium Biocomposites after Water Aging: In-Situ Micro-CT Testing and Numerical Analysis. *Micro*. 2023; 3(2):496-509.
https://doi.org/10.3390/micro3020033

**Chicago/Turabian Style**

Drouhet, Quentin, Fabienne Touchard, and Laurence Chocinski-Arnault.
2023. "Tensile Behavior of [0/90]_{7} Hemp/Elium Biocomposites after Water Aging: In-Situ Micro-CT Testing and Numerical Analysis" *Micro* 3, no. 2: 496-509.
https://doi.org/10.3390/micro3020033