Investigation of maintainability of fiberglass and carbon fiber plastics during tensile testing

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Introduction

For every product maintenance and repair is an integral part of the life cycle. Polymer composite materials (CM) are widely used in various industries including aviation. The increase in the volume fraction of the use of composites in the construction of an aircraft airframe is largely due to their high strength characteristics. At the same time, during operation, the problem of repairing composite structures to extend the life cycle of products becomes acute.

Composite material is a material consisting of two or more components with significantly different physical and/or chemical properties [1]. Laminar structures are assembled in such a way that the orientation of the fibers provides the required mechanical properties. In multilayer structures thin and high-strength shells are separated by properties of light honeycombs.

The most serious damage to composites reinforced with high-strength glass and carbon fibers occurs as a result of impact. The defects visible on the surface are less significant than the damage inside the layered structure. During operation of aviation equipment the presence of internal damage such as delamination can lead to a rapid loss of bearing capacity.

The main types of damage to composite materials include: delamination, cracking and dents (Fig. 1).

Figure 1. Main types of CM damage: a) delamination, b) formation of a local crack, c) cracking and d) rupture [2]

Any process of repairing composite structures begins with an assessment of damage, both visually and using special technical means. Some damage to composites is obvious and easy to assess, but in many cases the damage may appear small at first, although the actual damage is much more severe. Impact damage to the fiber may appear as a small dent in the reinforced composite surface. The decision to repair a structure or to scrap it is determined taking into account the amount of repair necessary to restore the required structural characteristics of the composite material. Other important factors in assessing the feasibility of a repair are the cost of repair, the location and accessibility of the damage, and the availability of suitable repair materials. The initial damage assessment determines the method of repairing the composite structure. Simple repairs, such as overlays, usually do not affect the structural integrity of the structural member. Complicated repair is necessary when the damage is significant and requires restoring the structural characteristics of the composite. When choosing repair materials the best option is to use original fibers, fabrics and resins. Any alternative will require careful consideration of composite operating conditions and component compatibility. The proposed repair scheme must meet all of the original design requirements for the structure. A quality check is always required before being returned to service. A range of non-destructive tests are used to comprehensively inspect repaired parts. Particular attention is paid to the quality of the restored area and, in particular, the boundary between the original and the repaired areas.

Experimental Study

In this paper an experimental study of the maintainability of fabric glass and carbon plastics is carried out. The tests were carried out in the laboratory of composite materials and structures.

As a repair method, the manufacture of composite overlays was considered. Since the connection of the lining with the damaged sample is adhesive, it was decided to study samples of glass and carbon fiber reinforced plastics with impact damage for tensile strength. It was also necessary to investigate the effectiveness of using a patch made of composite material.

The studies were carried out on samples of two types: carbon plastic based on CC420T twill fabric and epoxy binder [3]; and fiberglass based on plain weave T10 fabric and a two-component epoxy binder (Fig. 2).

Figure 2. CC420T carbon fabric a) and T10 glass fabric b)

The tests were carried out in several stages. First, samples without damage were tested to determine the initial tensile strength characteristics of the composites (Fig. 3).

Figure 3. Stages of an experimental study

Next, the material samples were impacted using a special setup assembled in the laboratory with a weight of 584 g from a height of 1.5 m. After the impact some of the samples were tensile tested.

In this study the thickness of the specimens is not relevant, so patching is a sufficient repair method. The surfaces of the samples with impact damage were subjected to mechanical processing (grinding) at the points of contact between the original part and the patch. An epoxy adhesive was applied to the prepared surface providing not only a good mechanical but also a chemical connection between the patch and the sample. Next, a patch was installed and the samples were glued at a temperature of +40°C to increase the strength characteristics of the adhesive joint. After curing of the epoxy adhesive the samples were tensile tested.

Research results

When determining the initial strength characteristics, carbon fiber showed greater tensile strength and withstood a load of 80 kN, while fiberglass - 56 kN (Fig. 4).

Figure 4. Displacement versus applied load for a) carbon fiber and b) fiberglass

However, when the critical load was reached, the samples of carbon fabric were destroyed in several places at the boundary of the interweaving of threads (Fig. 5a). Fiberglass under maximum load was evenly destroyed in 1 place (Fig. 5b).

Figure 5. Failure patterns of a) carbon fiber and b) fiberglass without damage in tensile tests.

Upon impact damage to carbon fiber occurred at the fiber interlacing boundary where the load was mainly on the less durable binder (Fig. 6a). Damage to the CFRP fibers is only visible from the impact side. In Figure 6a it is highlighted in white. No visual deformations are observed on the reverse side of the carbon fiber specimens. A small depression is formed in the fiberglass in the impact zone (Fig. 6b). At the same time elliptical spots are observed inside the samples diverging in different directions which indicates internal damage - delamination.

Figure 6. Impact damage to the CM

The loss of strength during tensile tests after impact for carbon fiber was up to 80%, while for fiberglass it was up to 20% (Fig. 7).

Figure 7. Displacement versus applied load after impact for a) carbon fiber and b) fiberglass

The rupture of carbon fiber specimens occurred along the line of impact damage (Fig. 8a). Fiberglass was deformed uniformly in the impact zone (Fig. 8b). The fiberglass samples show how the ellipse formed upon impact was stretched.

Figure 8. Failure patterns of a) carbon fiber and b) fiberglass with tensile failure.

The installation of composite patches made it possible to significantly improve the strength characteristics of carbon fiber (Fig. 9a). Due to the high rigidity of the carbon fabric samples the patch was torn off only at the moment when the maximum loads were reached and the destruction proceeded in the stress concentration zones. In fiberglass the patch came off at small values (Fig. 9b). Fracture in fiberglass occurred uniformly in the impact zone.

Figure 9. Strength comparisons of a) carbon fiber and b) fiberglass 

The obtained test results show that the use of composite overlays allows you to quickly and effectively increase the strength of composites in places of damage.

Conclusion

In the course of this study it was found that striking even from an insignificant height affects the strength of composite materials. In this case damage is usually hidden and can affect the use of composite products after damage. The effectiveness of the use of patches for the repair of composite materials was also shown.

The work was carried out with financial support from the Ministry of Science and Higher Education of the Russian Federation under the project FSSS-2020-0016.

About the authors

Максим Александрович Одинцов

Author for correspondence.
Email: odintsov.m.al.01@gmail.com
Russian Federation

Svetlana Olegovna Davydova

Email: davidova.so@ssau.ru

Svetlana Alexandrovna Pavlova

Email: pavlova.sa@ssau.ru

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