Prepregs for Aerospace Repair and Maintenance

A reinforcement (tows, weaved, or unidirectional) that has been impregnated with a known volume of partially cured (catalyzed resin) matrix is known as prepreg. For the fabrication of high-performance composite constructions (automotive, aerospace, ballistics, sports, etc.), prepregs act as the fundamental building component. Prepregs are said to be the most advanced technique used in composite production. Almost majority of the aramid materials used for commercial and military aircraft constructions are provided in prepreg form. Prepregs are also utilized for electrical circuit boards, industrial applications, and applications involving ground transportation (where the reinforcement may take the form of a paper or nonwoven sheet rather than a fabric) (Jabbar and Nasreen 2021).

Prepregs are commonly constructed from glass, carbon, or aramid fibers and are thermoset resin-coated for increased strength. When preparing aerospace composites, it is typically designed with an adhesive on one side to make application simpler (McKeen 2020).

Manufacturing of Prepregs

Prepregs are most frequently produced using the following techniques:

• Solvent Drip Technique

By dipping cloth into a predetermined resin, prepregs are created. The cloth is then wrapped around the item to give it form in the following stage. If you were dealing with a wing component, for instance, you would wrap the fabric cloth around it several times until you achieved the necessary number. Some prepregs are also vacuum treated utilizing pressure chambers before, during, or after curing to improve their qualities.

• Hot Melt Method

The hot-melt method is another approach to make prepregs. Spreading resin onto a metal sheet is the first step in this procedure; the resin is then melted by heating the metal sheet in an oven. The cloth is then spread out and pushed on the metal sheet. The cloth will have absorbed resin from being sandwiched between the two as it cools. During this process, your composite material will absorb more resins the more pressure you exert. Your composite material is ready for use once it has cooled, often in an oven.

Advantages of Prepregs

The rigorous field of aerospace engineering necessitates the use of lightweight materials without sacrificing strength. Prepregs are used to swiftly produce aircraft components that are safe for passenger safety, sturdy, and lightweight (PAC 2021).

In aircraft engineering, prepreg composites are a fantastic substitute for metal aerospace components. Compared to their competitors, they provide a number of benefits, including:

• High ratio of strength to weight
• Cheaper than steel or aluminum in price
• Quick turnaround time
• Excellent resistance to corrosive, abrasive, and solvent or chemical damage.

Usage of Prepregs in Aerospace Industry

Prepregs are utilized in aircraft engineering because they are robust, lightweight, and long-lasting, which makes them perfect for applications in the field where weight is frequently a concern. To increase the endurance of aerospace composites, prepregs are also used. Aerospace engineers can utilize them more easily than other composite materials since they have an adhesive on one side. Aerospace composites offer engineers a quicker production schedule than metal or plastic alternatives, making prepreg manufacture easier over time.

• Methods of Aerospace Component Repair with prepregs

One of the most important usages of prepregs in the aerospace industry is in the composite structures repair processes. One of the common procedures for repairing composite structures is prepreg scarf repair, which involves removing the damaged region, cutting and laminating uncured prepreg so that it fits the area to be fixed, and applying heat under a vacuum to speed up curing. The preparation of the damaged region, the creation of the repair patch, and the heating and curing processes all take a lot of time with this procedure.

Another common repair technique is wet lay-up. However, this approach necessitates the lamination of fibers impregnated with resin on the job site, which can lead to quality variances, such as the existence of unfilled gaps, depending on the worker's expertise.

One final repair technique makes use of pre-cured mending prepreg patches. To obtain high bonding quality while employing this procedure, the repair patch's shape often has to match the shape of the region to be fixed.

• Compatibility with other materials

Prepregs are mainly used in the repair of aerospace components that are manufactured from composite material structures. Metallic structures in general cannot be repaired with prepreg repair procedures that are mentioned in the above section (SUHARA, et al. 2016).

Compatibility with molding techniques is also very important. The development of liquid thermoplastic resins and adhesives that can be cured at room temperature, like Elium resin, has been one of the main research topics in this field in recent years as significant efforts have been made to shift towards sustainability in the composites sector. Elium, in contrast to other thermoplastic resins, is a liquid resin with a viscosity similar to that of traditional thermoplastic resins. As a result, it can be cured at room temperature and is compatible with liquid composite molding procedures (Khan, Hafeez and Umer 2023).

Performance after repair

If correct processing of the prepreg is carried out, strong mechanical properties can be achieved for repairs that are 400 times faster that traditional composite structures repairs. In order to assure dependable performance, testing would consequently need to be carried out under the harsh environmental conditions faced in service (Hubert, et al. 2018).

In order to test the prepregs, a repair to be made using these prepregs and then several tests can be conducted to ensure the required performance in met. Some of these tests are listed as follows:

• Optical Microscopy:

Through optical pictures and digital microscopy, the quality of the scarf surfaces and the restored specimens may be examined. The scarf geometries' surface and cross-sectional pictures need to be carefully examined for any signs of machining damage. Following repair, cross-sectional photographs taken along the whole length of the repaired samples' scarves must be used to evaluate the integrity of the scarf joints.

• Analysis via Thermogravimetry

To evaluate the thermoplastic resin's thermal stability and pinpoint the essential repair and degradation temperatures, the TGA of liquid thermoplastic resin can be used. At an appropriate purge flow rate and heating rate, multiple TGA scans may be carried out in a nitrogen atmosphere. Using alumina pans, the analysis may be carried out in a temperature range from 30 °C to 1000 °C.

• DMA, or Dynamic mechanical analysis

An Artemis device can be used to carry out the DMA scans. The temperature ramp sequence may be used to complete the DMA scans in 3-point bending clamps while maintaining a constant span. Under normal circumstances, many scans can be carried out by adjusting the temperature, albeit the heating rate needs to be consistent. You may maintain a steady applied frequency and dynamic amplitude as well. The ASTM D5023-15 test standard must be followed when conducting the tests.

• Flexural Analysis

Bending tests can be used to examine the flexural characteristics of the composite samples that have been repaired. A load cell-equipped Universal Testing Machine (UTM) can be used to conduct the testing. The tests need to be conducted in compliance with ASTM D6272 test requirements. It is necessary to maintain a steady loading span and support span. Additionally, the loading rate must remain consistent. In comparison to the unrepaired, pristine composites, the performance of the restored samples may be assessed in terms of residual flexural strength. To see complete delamination, the scarf joints must be positioned on the sample's bottom side, or the tension area.

• Fractographic Analysis

After flexural testing, the microstructure of the repaired and unaltered samples may be examined using SEM analysis to determine the relationship between the flexural performance and repair quality and the failure mechanisms. The FEI Quanta microscope may be used to obtain the SEM pictures. The SEM micrographs must also be compared to current research on various failure mechanisms in CFRP composite laminates (Khan, Hafeez and Umer 2023).

References

Hubert, Pascal, Timotei Centea, Lessa Grunefelder, Steven Nutt, James Kratz, and Arthur Lévy. 2018. "Out-of-Autoclave Prepreg Processing." Elsevier 63-94.

Jabbar, Madeha, and Adeela Nasreen. 2021. "Composite fabrication and joining." Composite Solutions for Ballistics.

Khan, Tayyab, Farrukh Hafeez, and Rehan Umer. 2023. "Repair of Aerospace Composite Structures Using Liquid Thermoplastic Resin." Polymers .

McKeen, Laurence W. 2020. "Effect of Radiation on the Properties of Laminates and Composites." The Effect of Radiation on Properties of Polymers.

PAC. 2021. "What is a Prepreg in Composites?" Pacific Aerospace Corporation. December 21. Accessed August 20, 2023. https://www.pacificaerospacecorp.com/what-is-a-prepreg-in-composites/#:~:text=A%20Prepreg%20is%20a%20material,composite%20to%20improve%20its%20properties.

SUHARA, MASAYOSHI, TAKAYUKI SHIMIZU, KOICHI HASEGAWA, TOSHIKAZU SHIGETOMI, MASAKAZU KAMIBAYASHI, and YUKIHIRO SATO. 2016. "Development of Quick Repair Method for ." Mitsubishi Heavy Industries Technical Review.