Low-Velocity Impact Analysis of Stiffened Composite Plates Using Higher-Order Layer-Wise Models

Carbon fiber-reinforced composite materials are increasingly used in fields where high mechanical strengths and low specific weights are required, such as the aerospace sector. The use of composites may require advanced modeling approaches due to some challenging features of this class of materials. For instance, the orthotropy of composite materials leads to low out-of-plane strength if compared with in-plane strength, making them vulnerable to impact phenomena. During maintenance, landing, and take-off, foreign objects can hit parts of the aircraft at low velocity and cause damage inside the structure that is not visible or barely visible to the human eye. The onset of damage inside the structure can significantly reduce its strength and stiffness capabilities and significantly threaten the whole aircraft's safety. An impact can cause various damages, e.g., indentation, intralaminar, and interlaminar damage. In the case of reinforced structures, an impact phenomenon may cause a debonding between the panel and the reinforcement, severely limiting the strength contribution of the reinforcement to the structure. Over the last decades, many authors have studied the behavior of stiffened panels subjected to impact, highlighting how the presence of the reinforcement and the point of impact strongly influence the dynamic and damage behavior of the structure. Furthermore, another critical aspect to consider is the difference in elastic properties between two differently oriented plies and the discontinuity due to the presence of the reinforcements, generating a concentration of stresses inducing damage and debonding. The vast majority of the scientific community studies the behavior of reinforced structures using the finite element method based on the classical plate or shell models. However, this approach does not accurately evaluate transverse axial and shear stresses. Other authors obtained a good match between the experimental and numerical data by modeling the individual plies with three-dimensional elements and studying the effects of delamination using cohesive elements. This approach can provide good results but may require high computational costs. This work adopts higher-order structural theories based on the Carrera Unified Formulation to provide an efficient and accurate mathematical model. The aim is the accurate description of the kinematic of the reinforced structure using beam and shell elements, thus, avoiding the more expensive 3D elements. The focus is on the evaluation of the 3D stress field and the evaluation of the damage onset via failure indexes. The results obtained with the proposed model are compared with commercial codes and literature data to highlight the soundness of the proposed solution and its numerical efficiency.