Experimental Method for Tensile Testing of Unidirectional Carbon Fibre Composites Using Improved Specimen Type and Data Analysis
Abstract
:1. Introduction
2. Materials and Methods
2.1. Protective Layer
2.2. Protected Carbon Composites
2.3. Test Specimens
2.3.1. Geometry and Tabs
2.3.2. Preparation
2.3.3. Experimental Design
2.4. Composite Characterization
2.4.1. Fibre Volume Fraction
2.4.2. Composite Volume Fractions
2.4.3. Tensile Properties
2.5. Stress–Strain Curve Analysis
2.5.1. Second-Order Polynomial Fitting
2.5.2. Back-Calculation of Stresses
2.6. Single Fibre Tensile Testing
3. Results and Discussion
3.1. Analysis of Stress–strain Curves
- Single failure mode, where failure of the central carbon composite layer is leading to immediate failure of the protective glass composite layers.
- Double failure mode, where failure of the central carbon composite layer is leading to a drop of load, and the load is then carried by the protective glass composite layers until their failure at a later stage of the tensile test.
3.2. Effect of Specimen Geometry
3.3. Effect of Specimen Protection
3.4. Combined Effect of Specimen Geometry and Protection
4. Conclusions
- An excellent agreement is found between the experimental stress–strain curves and the second-order polynomial fitted curves. Accordingly, it is demonstrated that the fitted stress–strain curves can be used as practical operational curves for further analysis.
- A good agreement between back-calculated stress–strain curves of protected carbon composites and measured stress–strain curves of unprotected carbon composites is found. Similarly, a good agreement is found between back-calculated stress–strain curves of carbon fibres and measured stress–strain curves of single carbon fibres. Only the failure points differ between the curves. Altogether, this validates the efficacy of the applied analysis of stress–strain curves and method of back-calculation.
- Initial stiffness (E0) that takes into account the shape of the whole stress–strain curve was determined based on the practical operational stress–strain curves. Initial Stiffness of unprotected carbon composites with the three different specimen geometries were found to be identical at 153 GPa. This is expected since a change in specimen geometry should have no effect on the determined stiffness.
- Comparing butterfly specimens to straight-sided ones, the effect of changing specimen geometry was an increase in strain to failure from 1.31 to 1.42%. For the elongated X-butterfly specimens, strain to failure was furthermore increased to 1.44%.
- It can be speculated that the non-identical tested materials volume of the three different specimen geometries in itself may result in variation of strain to failure due to a materials size effect. Based on Weibull model calculations, it is found that the potential size effect phenomenon is relatively small compared to the effect of specimen geometry.
- The protected carbon composites showed higher strain to failure compared to the unprotected carbon composites. The relative increment of strain to failure was in the range of 4–6% for the three specimen geometries. The largest difference was seen for the X-butterfly specimens where the strain to failure was increased from 1.44 to 1.53%.
- An extensive broom-like failure mode was observed for all specimens. In the protected specimens, however, failure was observed to be more restricted in the gauge section. This observation indicates that the protective layers help promote failure of the carbon composite in the gauge section.
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
Appendix A
Material | ||||||
---|---|---|---|---|---|---|
Epoxy adhesive | 3.3 | 80 | 0.040 | 0.0015 | 3.4 | −70.3 |
Vinyl ester matrix | 3.6 | 95 | 0.061 | 0.0015 | 3.7 | −70.4 |
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Specimen Geometries | Composite Volume Fractions | ||
---|---|---|---|
VC | VG | VA | |
Straight | 31.0 ± 0.01 | 66.6 ± 0.2 | 2.4 ± 0.2 |
Butterfly | 29.6 ± 0.01 | 67.4 ± 0.2 | 3.0 ± 0.3 |
X-Butterfly | 31.1 ± 0.01 | 67.6 ± 0.2 | 1.3 ± 0.2 |
Carbon Composite | Specimen Geometry | ||
---|---|---|---|
Straight-Sided | Butterfly | X-Butterfly | |
Unprotected, C | 153 ± 3 | 153 ± 3 | 153 ± 4 |
Protected, G/C/G | 77 ± 2 | 75 ± 1 | 79 ± 1 |
Carbon Composite | Specimen Geometry | ||
---|---|---|---|
Straight-Sided | Butterfly | X-Butterfly | |
Unprotected, C | 1.31 ± 0.09 | 1.42 ± 0.06 | 1.44 ± 0.04 |
Protected, G/C/G | 1.37 ± 0.04 | 1.48 ± 0.06 | 1.53 ± 0.07 |
Gauge length (mm) | 30 | 40 | 50 |
Strain to failure (%) | 1.69 ± 0.25 | 1.60 ± 0.24 | 1.48 ± 0.28 |
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Kumar, R.; Mikkelsen, L.P.; Lilholt, H.; Madsen, B. Experimental Method for Tensile Testing of Unidirectional Carbon Fibre Composites Using Improved Specimen Type and Data Analysis. Materials 2021, 14, 3939. https://doi.org/10.3390/ma14143939
Kumar R, Mikkelsen LP, Lilholt H, Madsen B. Experimental Method for Tensile Testing of Unidirectional Carbon Fibre Composites Using Improved Specimen Type and Data Analysis. Materials. 2021; 14(14):3939. https://doi.org/10.3390/ma14143939
Chicago/Turabian StyleKumar, Rajnish, Lars P. Mikkelsen, Hans Lilholt, and Bo Madsen. 2021. "Experimental Method for Tensile Testing of Unidirectional Carbon Fibre Composites Using Improved Specimen Type and Data Analysis" Materials 14, no. 14: 3939. https://doi.org/10.3390/ma14143939