PSI - Issue 52

Angela Russo et al. / Procedia Structural Integrity 52 (2024) 535–542 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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while also being lightweight enough to reduce fuel costs [1,2]. Composite materials are used in the construction of aircraft, spacecraft, and satellites, as well as in the manufacture of components such as wings, fuselages, and landing gear. The use of composite materials has allowed aerospace engineers to design more efficient and reliable structures, as well as reduce the overall weight of the aircraft. Additionally, composite materials are often more resistant to corrosion and fatigue than traditional metals, making them a more reliable choice for aerospace applications. However, composite materials can be susceptible to damage from impacts, fatigue, and other environmental factors [3]. Impact damage is one of the most common types of damage to composite materials. Impact damage can occur when the material is struck by a heavy object or when it is subjected to a sudden force. This type of damage can cause cracks, delamination, and other structural damage [4-6]. Delamination is a common failure mode in composite materials. It occurs when the layers of the composite material separate from each other, resulting in a weakened structure. In addition to mechanical damage, such as impacts, vibrations, or excessive loads, delamination can be also caused by a variety of factors, including inadequate adhesion between layers, improper curing of the composite material, or exposure to extreme temperatures. The effects of delamination can be serious and costly. Delamination weakens the material, making it more susceptible to further damage and failure. Furthermore, delamination may lead to reduced performance of the structure and a decrease in its service life [7]. Numerical simulation is a powerful tool for investigating the behaviour of delaminated composite structures, enabling engineers to accurately predict the behaviour of a structure under various loading conditions and to identify potential failure points. Moreover, simulation can be used to identify the most effective repair techniques and to optimize the design of the structure. Furthermore, numerical simulation can be employed to study the effects of delamination on the mechanical properties of the composite structure, such as the stress and strain distribution in the structure, as well as the effect of delamination on the stiffness and strength of the structure. The propagation of delamination in materials can be modelled by using fracture mechanics with either the stress intensity factor (SIF) or strain energy release rate (SERR) approaches, with the latter becoming more commonly used due to difficulties in calculating SIF for non-homogeneous materials. The Virtual Crack Closure Technique (VCCT) is a reliable approach in Fracture Mechanics which can be utilized to analyse the growth of cracks, allowing for the computation of the strain energy release rate [8]. The VCCT is used to predict the delamination of composite materials under various loading conditions. This technique is based on the concept of virtual crack closure, which states that the crack can be closed by applying a virtual force to the crack faces. This force is applied in such a way that the crack is closed and the material is restored to its original state. The VCCT-based delamination growth methods are often coupled with the Fail-Release (FR) approach, where delamination is modelled as two sub-laminates connected through contact elements. When the delamination conditions are met, the contacts are released, and the delaminated area propagates. However, modelling the delamination growth can be challenging because of the complex shapes related to crack front growth and it can lead to under or overestimation of the delaminated area. To overcome these issues, the SMXB methodology has been used, being able to achieve mesh and time step insensitivity [9]. This paper presents a parametric study on the mechanical behaviour of a single stiffened composite panel, considering an initial debonded region at the Skin/Stringer interface. The study focuses on the influence of the initial debonded region length on the damage behaviour of the panel. The mechanical behaviour has been assessed in the ANSYS environment, utilizing the SMXB numerical tool, developed by the authors. The results of the study are used to identify the most critical parameters and to provide design guidelines for the optimization of the panel ’ s performance. In Section 2, the test case is introduced, while in Section 3 the results are presented and discussed. 2. Test case description The compression behaviour of a single stiffened composite panel with an artificial skin-stringer debonding has been numerically investigated by utilizing the SMart-time XB (SMXB) tool. The analysed stiffened panel featured a T-shaped stiffener bonded to a skin panel. Two configurations, characterized by different length of an artificial single stringer debonding equal to 80 mm (SS#1 configuration), 120 mm (SS#2 configuration), and 140 mm (SS#3