If hot bonding of the composite patch to the cracked plate leads to an improvement in the adhesion energy between these two protagonists, it is a source of residual stresses in the plate, the adhesive and the patch near their interfaces. Using FEM, this study aims to improve the performance of hot plate-patch joining and patch-crack interactions by optimizing the patch shape. Thus, all the patch shapes developed in this study arise from an initially rectangular shape. This results in six patch shapes: elliptical, orthogonal, star, H, double arrow and butterfly. This optimization is analyzed in terms of improvement in both the fracture energy gain of the plate (stabilization of the SIF with the evolution of the crack size Delta K), the mechanical energy gain of the adhesive (reduction of the risk of rupture of the adhesive by a drop in the level of shear stresses in the adhesive Delta tau) and of the mass gain of the patch Delta m (reduction of the risk of peeling and delamination and of debonding by an improvement of the aerodynamic resistance of the patch). These three gains, Delta K, Delta tau, Delta m, developed for the first time in this study, constitute characteristic criteria for the performance of composite patch repair. This is where the originality of this work lies. This study highlights that improving the fracture energy gain alone is not a sufficient condition for the effectiveness of hot repair. Thus, the simultaneous satisfaction of these three criteria is a necessary condition for the durability and performance of the repair. This therefore constitutes the originality of this study which lies in the development of these criteria. It appears from this study that the repair using an optimized double arrow-shaped patch leads to the satisfaction of these criteria. This shape simultaneously ensures stabilization of the three components of the repair: the plate by stabilizing the crack, the adhesive by reducing shear stresses, the patch by reducing its mass. The risks of damage (debonding, peeling, delamination) due to these stresses and the size and thickness of the patch are clearly minimized. It also emerges from this study that, contrary to previous works, hot bondings of the patch to the cracked plate doesn't in any way affect the performance of the hot repair using an optimized patch.
This study investigates the thermo-mechanical behavior of cracked and porous Al-12Si/Al2O3 metal-ceramic composites under varying thermal and mechanical conditions. A predicted model is developed to analyze stress distribution, Young's modulus, and Poisson's ratio while incorporating the effects of porosity and ceramic content. The results reveal that higher porosity weakens stress transfer and reduces stiffness, while increased ceramic content enhances rigidity but also intensifies stress localization. Young's modulus decreases significantly with temperature and porosity. Poisson's ratio decreases with rising temperature, confirming reduced lateral deformation resistance in more porous structures. The proposed model is validated against the Equivalent Constraint Model (ECM), showing strong agreement in stiffness degradation trends. This study extends previous research by integrating Knudsen's model into stress distribution analysis, providing a novel approach to quantify porosity effects in thermo-mechanical behavior. The findings contribute to optimizing metal-ceramic composites for high-temperature applications in aerospace and automotive industries, ensuring enhanced thermal and mechanical stability.
One of the primary causes of a decrease in strength in adhesive joints is the presence of stress concentrations at the ends of the overlap region. To address this issue effectively, the bi-adhesive technique has been introduced to minimize stress concentrations at the overlap ends. Several research efforts have been devoted to studying single lap adhesive joints. However, this type of assembly exhibits stress concentrations at the ends of the joint, in contrast to its inactive core. The aim of this research is to present a numerical analysis of single lap biadhesive joints using cohesive zone models with four different adhesives. Two adhesives of the same chemical composition, one rigid and one flexible, were used. The overlap length parameters for each adhesive area were considered. Thus, the placement of both rigid and flexible adhesives was optimized. The combination of both adhesives in a single lap adhesive joint, with the more rigid adhesive positioned at the core of the joint, provided enhanced strength to the assembly with a 37% improvement. The variation in overlap length of the adhesives in the bi-adhesive joints affected the joint toughness. Among them, the best results were obtained when covering 40% to 60% of the overlap length with the more rigid adhesive at the core of the joint.
This paper investigates the elastic instability behavior of sandwich beams featuring functionally graded skins and functionally graded porosity distribution of ceramic (Type-A) or metal (Type-B) core. Employing a high-order quasi-3D beam theory and the principle of virtual work, we derive the governing stability equations. The analysis considers three distinct porosity distributions (FGP) across the thickness, capturing variations in elastic modulus. The functionally graded porosity (FGP) distributions in functionally graded material (FGM) sandwich beams offer significant mechanical advantages over traditional porosity patterns. FGP facilitates the graded customization of stiffness, strength, and vibration response through the thickness, thereby optimizing weight and energy absorption while minimizing adverse effects, such as excessive deflection or diminished load capacity. This methodology enhances the overall structural performance and design. A parametric study evaluates how slenderness ratio, porosity volume fraction, aspect ratio, power-law grading index, and boundary conditions affect the critical buckling load (CBL). Numerical solutions are obtained and compared with existing higher-order shear deformation theories and full 2D/3D models, confirming the accuracy and robustness of the present approach.
A sea-skimming unmanned aerial vehicle (UAV) is a specialized type of aircraft capable of performing tasks at low altitudes above sea level. However, under extreme sea conditions, wave disturbances introduce significant errors in measuring the relative distance between the UAV and the sea surface, which complicates the control system and degrades flight performance. While existing research primarily relies on filtering or adaptive control methods, their effectiveness in handling non-Gaussian disturbances remains limited. In this paper, a mathematical model for the altitude and pitch motion of a sea-skimming UAV is established, taking into account external disturbances caused by sea wave height. A control method integrating Backstepping theory with a nonlinear disturbance observer (NDO) is designed to ensure system stability. The key innovation of this work lies in using the NDO to estimate and compensate for aggregated disturbances, thereby enabling the Backstepping controller to achieve exponential stability without a parameter adaptation process. Furthermore, a quasi-adaptive coefficient function is incorporated into the NDO to adjust its gains in real time, improving estimation performance. The stability of the proposed method is rigorously proven using Lyapunov theory. Simulation results demonstrate the effectiveness of the NDO-based Backstepping control scheme, showing that it maintains a tracking accuracy of 98.52% even under severe level 7 sea condition.
