# Effect of Fibre Orientation on Impact Damage Resistance of S2/FM94 Glass Fibre Composites for Aerospace Applications: An Experimental Evaluation and Numerical Validation

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## Abstract

**:**

## 1. Introduction

^{®}fibre metal laminate, which is installed in parts of the fuselage of the Airbus A380 [2]. The first commercial aircraft to contain a composite structure made from glass fibre was the Boeing 707 jet in the 1950s, where it comprised about 2% of the structure. Nowadays, the content of composites in modern commercial aircrafts such as the Boeing 787 Dreamliner and the Airbus A350 XWB has reached just over 50% of their weight. However, composites are prone to various failure modes in the fibre and matrix. For example, delamination between adjacent plies, fibre kinking and breakage, matrix cracking, and debonding are the main failure modes. From those damages reported earlier, delamination is the most critical form of damage that occurs in laminates subjected to low-velocity impact due to the weak performance of fibres to the overall strength of the composite in the impact direction, especially for thin laminates [3,4]. Indeed, composite structures tend to have weak resistance to impact damage by foreign objects [5]. Damage caused by impact can affect the load-carrying capacity, particularly when the structure is under compression [5]. Damage in aeronautical composite structures may occur during the taking off and landing of the aircraft due to high-velocity impact from bird strike, metal fragments, hailstone etc. It can also occur due to low-velocity impact during aircraft ground service from accidental falling objects such as hammers, boxes etc. The severity of the impact damage on composite structures can vary from full penetration to barely visible impact damage. The latter can be critical for the safety and integrity of the aircraft structure since visual inspection will not detect subsurface damage. The definition of a low-velocity impact itself is based on the speed of the falling weight or the level of damage on the structure, as reported by several researchers in the past [3,6,7,8]. Some researchers considered a low impact test is that of which the speed of the falling object was less 10 m/s [3,7]. In another definition, it was that which occurs at impact speeds below 100 m/s [3,6]. The damage in composites due to low-velocity impacts can be critical at the micro-level as it could lead to a severe reduction in the material post-impact residual strength and stiffness [3]. Therefore, studying impact damage is important, especially with modern aircraft, which have an increasing percentage of composites in their structures. There are four main failure mechanisms (failure modes) that occur in fibre reinforced composites due to low-velocity impact loading [9]. The first failure mechanism occurs in the matrix due to tension, compression, or shear loading. The matrix failure mode results in cracking parallel to the fibres and debonding between the fibres and the matrix. The main reason for these failures is related to the property mismatch between the composite constituents (i.e., the fibre and the matrix). The second failure mechanism occurs in fibres subjected to tensile (fibre breakage) or compression (fibre buckling) loading. The third failure mechanism is due to the interlaminar stresses which are responsible for delamination. The fourth failure mechanism occurs when the impactor fully perforates the laminate, a phenomenon that is more common at the ballistic impact range. Core buckling and shearing can be considered as a fifth failure mechanism, but this occurs in composite sandwich structures [10]. Nevertheless, previous studies reported that perforation damage in low velocity impact loading is mainly affected by the laminate thickness for CFRP laminates and by glass fibre treatment in GFRP laminates [11]. There are many studies which devoted their efforts to study the low-velocity impact behaviour of E-glass and carbon fibres. However, only a handful of studies can be found in the open literature which investigated the damage in S2 glass fibre composites due to low velocity impact, an essential prerequisite to increase the use of S2 glass fibre composites in industry. The ability of the fibre to store energy elastically is of great importance. According to Satishkumar et al. [12], S2 glass fibres have the highest young’s modulus, tensile strength, and percentage elongation at break, among many other types of glass fibres such as (A, C, D, E, R, EGR, and AR). This means that fibres with a higher modulus of elasticity and failure strain can better resist damage due to low-velocity impact loading and absorb higher elastic energy [11].

## 2. Materials and Methods

#### 2.1. Composite Plates Manufacturing and Sample Preparation

^{®}Industrial Materials Limited

^{®}, Heanor, U.K [29,30,31,32,33]. The panels were then cut into nine square plates (70 mm × 70 mm each) to be used in the impact tests according to BS EN ISO 6603-1:2000. The BS EN ISO 6603-1:2000 standard states that the samples should be squared, have a minimum size of 60 mm, should be placed on the top of a 40 ± 2 mm punctured hole clamping system, and be impacted using a 20 mm diameter spherical striker (impactor). The prepregs were stacked in different arrangements to achieve the desired fibre orientations in each plate, as shown in Table 1. The large panels were cured in an autoclave for 5 h at elevated temperatures of 120 °C and under a pressure of six bars according to the supplier guidelines [28]. The designated prepreg orientation employed in the plates was to mimic specific aircraft structures based on those used in standard grades of GLARE

^{®}laminates due to the main beneficial characteristics which those orientations provide, such as fatigue and strength [0/0], impact [0/90/90/0], shear and off-axis properties [+45/−45] [29]. Moreover, The general manufacturing process of composite material components restricts the stacking sequence combination to laminates with 90°, ±45°, 0° oriented plies [34].

#### 2.2. Setup of Impact Machine and Test Parameters

#### 2.3. Computerised Tomography

## 3. Numerical Model

#### 3.1. Intralaminar Damage Model

_{11}, E

_{22}, E

_{33}, G

_{12}, G

_{13}, and G

_{23}are the elastic and shear modulus in the fibre and transverse directions, respectively, and ν

_{12}, ν

_{13}, and ν

_{23}are Poisson’s ratios in plane 1–2, 2–3 and 1–3, respectively [41].

#### 3.2. Interlaminar Damage Model at Interfaces

_{nn}, G

_{1}/E

_{ss,}and G

_{2}/E

_{tt}normalised elastic and shear modulus, and ${G}_{n}$, ${G}_{s}$, ${G}_{t}$ are the normal, shear (first and second directions) fracture energy modes. Finally, the linear degradation law function of the dissipated energy was used along with the Benzeggagh–Kenane (BK) law for the mixed opening mode [43].

#### 3.3. Numerical Implementation

## 4. Results and Discussion

#### 4.1. Load Time Response

#### 4.2. Load Displacement Response

#### 4.3. Absorbed Energy Time Response

## 5. Conclusions

- Plates fabricated using${\left[0\right]}_{32}$fibre orientation was the least resistant to impact at all tested energy levels. The samples were severely damaged and failed purely due to shear stresses without delaminating.
- The impact tests showed that the plates with ${\left[0/90/90/0\right]}_{8s}$ configuration absorbed more energy with less penetration depth than the plates with other stacking configurations.
- CT scans revealed that delamination was the main failure mechanism in plates fabricated using ${\left[0/90/90/0\right]}_{8s}$, ${\left[+45/-45\right]}_{16s}$, and ${\left[0/90/+45/-45\right]}_{8s}$ orientation.
- Finite element models showed good agreement with experimental data and accurately predicted the failure modes in the plates due to impact.
- The use of cohesive elements between each ply of the laminate is a very useful technique to capture the delamination between differently oriented plies, but also to prove that the delamination onset occurs first when the fibres have different orientations between adjacent plies.
- From the four different plates tested, it was found that plates with ${\left[0/90/90/0\right]}_{8s}$ stacking sequence showed better performance under the impact, whereas the unidirectional plates showed poor performance at all energy levels and were comparable to the ${\left[0/90/+45/-45\right]}_{8s}$ plates. The ${\left[+45/-45\right]}_{16s}$ performed satisfactorily at low and medium energy, which makes it the second suitable candidate for the studied loading condition.
- The presented findings in this study incorporating experimental and validated numerical modelling will help researchers in aerospace engineering in understanding the influence of various important parameters on the impact damage characteristics of S2/FM94 glass/epoxy composite laminates to be used in aircraft structures.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**(

**a**) Ply configuration for one of the glass fibre plates used in the study ${\left[+45/-45\right]}_{16s}$ (

**b**) Manufacturing setup of the workpiece inside the autoclave.

**Figure 5.**Load vs. time curves for S2/FM94 glass fibre plates with different fibre orientation systems under (

**a**) 75 J, (

**b**) 150 J, and (

**c**) 225 J impact energy.

**Figure 8.**Load–displacement curves for S2/FM94 glass fibre plates with different fibre orientation systems under (

**a**) 75 J, (

**b**) 150 J, and (

**c**) 225 J impact energy.

**Figure 9.**Absorbed energy vs. time curves for S2/FM94 glass fibre plates with different fibre orientation systems under (

**a**) 75 J, (

**b**) 150 J and (

**c**) 225 J impact energy.

**Figure 10.**FE results for the absorbed energy vs. time curves for S2/FM94 glass fibre plates with different fibre orientation systems under 150 J impact energy.

**Figure 11.**3D CT scan views of samples after impact using an energy of 225 J (

**a**) ${\left[0/90/+45/-45\right]}_{8s}$ (

**b**) ${\left[0/90/90/0\right]}_{8s}$ (

**c**) ${\left[+45/-45\right]}_{16s}$ (

**d**) ${\left[0\right]}_{32}$

**Figure 12.**Cross-sectional micrographs of the S2/glass fibre plates impacted at different energy levels.

**Figure 13.**Comparison of damaged specimens (front side) after the impact at 150 J and FE model results for (

**a**) ${\left[0\right]}_{32}$ (

**b**) ${\left[+45/-45\right]}_{16s}$, (

**c**) ${\left[0/90/90/0\right]}_{8s},$ and (

**d**) ${\left[0/90/+45/-45\right]}_{8s}$ composite laminate.

**Figure 14.**Numerical prediction of delamination damage at the top surface of (

**a**) ${\left[0\right]}_{32}$ (

**b**) ${\left[+45/-45\right]}_{16s}$ (

**c**) ${\left[0/90/90/0\right]}_{8s},$ and (

**d**) ${\left[0/90/+45/-45\right]}_{8s}$ composite laminate.

**Figure 15.**Damage contour with delamination through the thickness of the cross-section of the (

**a**) ${\left[0/90/90/0\right]}_{8s},$ and (

**b**) ${\left[0/90/+45/-45\right]}_{8s}$ plates.

Plate Number | Plate 1 | Plate 2 | Plate 3 | Plate 4 |
---|---|---|---|---|

Ply orientation | ${\left[0/90/+45/-45\right]}_{8s}$ | ${\left[0/90/90/0\right]}_{8s}$ | ${\left[+45/-45\right]}_{16s}$ | ${\left[0\right]}_{32}$ |

Impact Level | Level 1 | Level 2 | Level 3 |
---|---|---|---|

Impact Energy (J) | 75 | 150 | 225 |

$\mathbf{}{\mathit{X}}_{\mathit{T}}$ $\mathbf{}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | $\mathbf{}{\mathit{X}}_{\mathit{C}}$ $\mathbf{}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathit{Y}}_{\mathit{T}}$ $\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathit{Y}}_{\mathit{C}}$ $\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathit{S}}_{\mathit{L}}$ $\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathit{S}}_{\mathit{T}}$ $\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ |
---|---|---|---|---|---|

2430 | 2000 | 50 | 150 | 76 | 50 |

Density (Kg/m ^{3}) | E_{11} (GPa) | E_{22} (GPa) | E_{33} (GPa) | G_{12} (GPa) | G_{13} (GPa) | G_{23} (GPa) | ν_{12} | ν_{13} | ν_{23} |
---|---|---|---|---|---|---|---|---|---|

1980 | 53.98 | 9.412 | 9.412 | 5.548 | 3 | 5.548 | 0.0575 | 0.0575 | 0.33 |

**Table 5.**Fracture energies for fibre and matrix tension and compression failure modes for S2-FM94 glass fibre adhesive epoxy.

$\mathbf{}{\mathit{G}}_{\mathit{f}\mathit{t}}^{\mathit{C}}$(kJ/m^{2}) | ${\mathit{G}}_{\mathit{f}\mathit{c}}^{\mathit{C}}\phantom{\rule{0ex}{0ex}}(\mathbf{kJ}/{\mathbf{m}}^{2})$ | ${\mathit{G}}_{\mathit{m}\mathit{t}}^{\mathit{C}}\phantom{\rule{0ex}{0ex}}(\mathbf{kJ}/{\mathbf{m}}^{2})$ | ${\mathit{G}}_{\mathit{m}\mathit{c}}^{\mathit{C}}\phantom{\rule{0ex}{0ex}}(\mathbf{kJ}/{\mathbf{m}}^{2})$ |
---|---|---|---|

12.5 | 12.5 | 1.0 | 1.0 |

$\mathbf{}{\mathit{t}}_{\mathit{n}}^{0}$$\mathbf{}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathit{t}}_{\mathit{s}}^{0}$ $\mathbf{}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathit{t}}_{\mathit{t}}^{0}$ $\mathbf{}\left(\mathbf{M}\mathbf{P}\mathbf{a}\right)$ | ${\mathit{G}}_{\mathit{n}}$ (kJ/m^{2}) | ${\mathit{G}}_{\mathit{s}}$ (kJ/m^{2}) | ${\mathit{G}}_{\mathit{t}}$ (kJ/m^{2}) | E/E_{nn}(GPa/mm) | G_{1}E_{ss}(GPa/mm) | G_{2}E_{tt}(GPa/mm) |
---|---|---|---|---|---|---|---|---|

50 | 50 | 60 | 4.0 | 4.0 | 4.0 | 10^{5} | 10^{5} | 10^{5} |

Setup | Impactor | Plate |
---|---|---|

Mesh size | 2 × 2 mm^{2} | 1 × 1 mm^{2} |

Element type | R3D3, R3D4 | SC8R, COH3D8 |

Number of elements | 1431 | 475,272 |

Material | Steel | S2 glass fibre composite |

Body type | Discrete rigid non-deformable | Deformable |

Solver | Dynamic Explicit |

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**MDPI and ACS Style**

Giasin, K.; Dhakal, H.N.; Featheroson, C.A.; Pimenov, D.Y.; Lupton, C.; Jiang, C.; Barouni, A.; Koklu, U.
Effect of Fibre Orientation on Impact Damage Resistance of S2/FM94 Glass Fibre Composites for Aerospace Applications: An Experimental Evaluation and Numerical Validation. *Polymers* **2022**, *14*, 95.
https://doi.org/10.3390/polym14010095

**AMA Style**

Giasin K, Dhakal HN, Featheroson CA, Pimenov DY, Lupton C, Jiang C, Barouni A, Koklu U.
Effect of Fibre Orientation on Impact Damage Resistance of S2/FM94 Glass Fibre Composites for Aerospace Applications: An Experimental Evaluation and Numerical Validation. *Polymers*. 2022; 14(1):95.
https://doi.org/10.3390/polym14010095

**Chicago/Turabian Style**

Giasin, Khaled, Hom N. Dhakal, Carol A. Featheroson, Danil Yurievich Pimenov, Colin Lupton, Chulin Jiang, Antigoni Barouni, and Ugur Koklu.
2022. "Effect of Fibre Orientation on Impact Damage Resistance of S2/FM94 Glass Fibre Composites for Aerospace Applications: An Experimental Evaluation and Numerical Validation" *Polymers* 14, no. 1: 95.
https://doi.org/10.3390/polym14010095