1 Chapter I Introduction I.1 Research Background Composite material implementations are increasing, specifically in aerospace applications. Figure I.1 represents the increase in composite applications in several types of Boeing aircraft (A.S., 2004). Recently, in the aerospace industry, composite materials, i.e., carbon fiber reinforced polymers (CFRPs), have been utilized to construct main component of aircraft structures (Galos, 2020). For example, more than 50% of the airframe structures of the Boeing 787 (Hale, 2006) and Airbus A350 XWB (Kinsley-Jones, 2006) are made of CFRPs. This is due to the fact that composite structures enable optimization in mechanical performance. Thus, several studies aim at the development of composite optimization to achieve weight reduction, enhance damage tolerance, stiffness, and strength (Dursun & Soutis, 2014). Figure I.1. Combination of Materials used in Boeing Aircrafts (A.S., 2004) A composite material consists of two or more constituent materials. Typically, composites are constructed from fibers and matrix. The configuration of composite materials varies based on fiber orientation, as illustrated in Figure I.2. The behavior of composites differs when a load is applied to different fiber configurations. 2 Unidirectional composite exhibits the greatest strength compared to other configurations when the load is aligned with the fiber orientations. However, their strength decreases considerably when the load is applied in other orientations, relying primarily on the matrix. Bidirectional composites have a lower ultimate strength but can bear loads in two directions. As the fiber orientation becomes more statistically distributed throughout the composite, the ultimate strength decreases, but the overall properties become more uniform in all loading orientations (Okhuysen, 2004; Lim, 1997). Hence, the fiber orientation primarily contributes to the structural stiffness of composite. (a) (b) (c) (d) Figure I.2. Different Types of Fiber Configuration in Composites: (a) Unidirectional, (b) Random, (c) Bidirectional, and (d) Multi- directional (Lim, 1997) Besides the fiber orientation, the presence of failure, such as delamination and cracking as depicted in Figure I.3, can significantly affect the stiffness of the structure. For instance, the axial stiffness of composite laminates is reduced due to the presence of transverse matrix crack (Yuan & Selek, 1993; Praveen & Reddy, 1995). Composite failures impacting structural stiffness can be triggered by various conditions. In an aircraft application, operating conditions subject the structure to complex combined load cases, including interactions between aerodynamic and structural dynamic loads. Additionally, failures can sometimes be initiated by the existence of initial defects resulting from the manufacturing process (McKeen, 2016). The combination of initial damage and load cycles, in this case aerodynamic 3 and structural dynamic load, can lead to stiffness degradation due to fatigue (Pakdel & Mohammadi, Stiffness degradation of composite laminates due to matrix cracking and induced delamination during tension-tension fatigue, 2019). (a) (b) Figure I.3. Type of Composite Failures: (a) Delamination (Mohammadi et al., 2024) and (b) Crack (Awaja at al., 2016) Furthermore, the presence of damage can significantly influence the dynamic characteristics of a structure, including its natural frequencies and mode shapes, which define the structural response to dynamic forces. These properties offer crucial insights into the vibrational behavior of the structure under external forces, playing an essential role in applications such as assessing structural stability and predicting responses to dynamic loads. Damage affects dynamic characteristics through the alteration of structural stiffness. Therefore, it is crucial to consider structural damage, such as cracks, in structures like aircraft that must withstand dynamic loads. In addition to influencing the dynamic characteristics, changes in stiffness due to the presence of cracks can lead to catastrophic incidents. A notable example was reported by the National Transportation Safety Board (NTSB). In 2011, the crash of a North American P-51D Mustang was attributed to aerodynamic flutter. According to the aircraft accident brief released by the NTSB, the incident was triggered by a reduction in stiffness of the elevator trim tab, which induced aerodynamic flutter at racing speed (NTSB, 2012). Flutter is a dynamic aeroelastic instability characterized by undamped, periodic oscillations due to the interaction of aerodynamic, structural, and inertial forces as illustrated in Figure I.4. Hence, 4 structural stiffness plays a vital role in withstanding aeroelastic instabilities such as flutter. Figure I.4. Flutter Illustration This accident highlights the interrelationship between structural damage and aeroelasticity. In the aeroelasticity aspects, the aero-structural instability can be influence by several factors, such as the reduction of structural stiffness. As previously mentioned, the fiber orientation and crack existence can be the main cause for the structural stiffness alteration. This condition can lead to transformation of aeroelastic instabilities speed, such as divergence and flutter. Based on Castravete and Ibrahim, the stiffness significantly affects the flutter boundary (Castravete & Ibrahim, 2008). When flutter occurs at a lower-than-expected speed due to the presence of crack, it indicates that the flutter boundary region has shifted downward. Both divergence and flutter can be catastrophic, leading to sudden destruction of the vehicle. Thus, it is vital for the aircraft designers to considering such aeroelastic instability problems. In today's era of technological advancements, dynamic and aeroelastic characteristics can be determined through computational processes. One computational method widely employed involves the Finite Element Method (FEM), utilized for structural simulations.