Damage Monitoring in Carbon Fiber-Reinforced Composites

Damage Monitoring in Carbon Fiber Reinforced Composites Using Digital Image Processing

Use of Carbon Fiber Reinforced Composites in the Aerospace Industry

Demand for carbon fiber reinforced polymers (CFRP) has been increasing in recent years. Fiber-reinforced composites possess superior mechanical properties such as high strength-to-weight ratio, stiffness-to-weight ratio, fracture toughness and corrosion resistance. Due to these advantages, fiber-reinforced composites are widely used in various design and manufacturing sectors including aerospace, space, marine, automotive, medical, sports equipment, wind turbine blades, defense and transportation. For example, the majority of modern aircraft fuselages are designed and manufactured from CFRP materials as shown in Figure 1.

Defects Observed in Machining Processes of Carbon Fiber Reinforced Composites

Approximately 40% of machining operations in the aerospace industry are performed with conventional drilling operations. Although some specialized and non-conventional drilling methods such as water jet, laser and electrical discharge machining have been industrialized appropriately for composite materials, conventional mechanical drilling with drill tools remains the most common method for drilling holes in composite plates. Despite their excellent mechanical properties, carbon fiber reinforced polymers are highly susceptible to machining-induced damage as shown in Figure 2. The main damage types can be described as delamination, peeling and fiber pullout under high localized stress concentration. Machining-induced damage such as delamination, hole shrinkage and fiber pullout generally occurs when the stress induced in plate layers exceeds inter-laminar strength. Delamination is the primary damage type observed in composite materials machining that compromises structural integrity. It is known that delamination and microcracking significantly affect the performance of composite materials and the behavior of assembled parts and reduce their performance, whereas drilling-induced delamination not only reduces surface finish and assembly tolerance, but also affects the fatigue resistance properties of the hole. For this reason, drilling-induced delamination is known as one of the most critical damage types causing severe losses in industry. Over the past decade, the rate of parts rejected due to drilling-induced delamination damage observed in composite plates in the final machining operation has been significantly high. Furthermore, even with the recent emergence of delamination repair technology, it has been reported that repairing drilling-induced delamination can take 5 to 6 hours, while repairing a large oval hole in aircraft assembly can take only 10 to 15 minutes. For this reason, the recent emergence of delamination repair technology has enabled significant research work on hole-induced delamination. Assessment of drilling-induced delamination is of great importance for delamination control. Depending on delamination geometry, different delamination evaluation formulas exist (as shown in Figure 3). Various techniques have been used to measure delamination after drilling of composites, and quantitative evaluation is required to assess the effect of both basic cutting parameters and drill bit geometry.   [caption id="attachment_135242" align="aligncenter"] Figure 3. Various delamination analysis methods in carbon fiber reinforced composites.[/caption]

Investigation of Machining-Induced Defects Using Image Processing

In-process detection of machining-induced production defects in carbon fiber reinforced composites has gained importance with Industry 4.0. For this purpose, digital image processing-based evaluation methods are used in many industries thanks to advances in imaging technologies that are also suitable for monitoring composite machining processes in measuring delamination and dimensional quality. Due to the mechanical behavior of carbon fiber reinforced composites, irregular shapes, dimensional variations and fiber pullout can be observed in machining operations. Therefore, a robust and efficient mathematical approach must be used to apply image processing methods. Image processing is an approach that aims to analyze the relationship between processing parameters and machining-induced damage and enables analysis from digitized images through a series of different steps that serve this purpose. The proposed image processing approach includes several steps from obtaining the original image to contour definition as summarized in Figure 4. Morphological image processing is applied to create a distinguishable contour. After creating a distinguishable contour, the variation between the expected part contour and the measured contour is calculated. These variations are interpreted as machining-induced delamination. The image processing techniques used in this experimental study were found to be suitable and consistent for delamination evaluation studies. From the processed images, the relationship between cutting parameters and changes in force and dimensions is calculated. In this way, optimized process parameters are defined to reduce machining-induced damage. It can be concluded that image digitization and processing is a useful method for evaluating machining-induced damage. [caption id="attachment_135245" align="aligncenter"] Figure 4. Defect detection using digital image processing[/caption]

Acknowledgment

This work is part of the COMACH project supported by SMART EUREKA Cluster and TÜBİTAK with grant numbers S0120 and 9190018, respectively.

References

Figure 1: Machining CFRP/GFRP composite material. (n.d.). Retrieved from https://www.machining4.eu/Technology Figure 2: Canadian Metalworking. (2014, December 16). End mills for machining CFRP. Canadian Metalworkings. Retrieved January 4, 2022, & Machining Carbon Fibre Materials. studylib.net Figure 3: Daxi Geng, Yihang Liu, Evaluation and suppression during drilling of composite laminates: A review, Composite Structures, Volume 216, 2019, Pages 168-186 Burcu Bilgiç Faculty of Engineering and Natural Sciences Sabancı University Assoc. Prof. L. Taner Tunç Faculty of Engineering and Natural Sciences Integrated Manufacturing Technologies Research and Application Center Sabancı University