Morphological Characterization and Failure Analysis of the Ultrasonic Welded Single-Lap Joints.

Ultrasonic welding technology represents an advanced method for joining thermoplastic composites. However, there exists a scarcity of systematic investigations into welding parameters and their influence on the morphological characteristics and quality of the welded regions. Furthermore, a comprehensive experimental understanding of the welded joint failure mechanisms remains deficient. A robust model for simulating the failure behavior of welded joints under loading has yet to be formulated. In this study, ultrasonic welded specimens were fabricated using distinct welding control methods and varied parameter combinations. Diverse experimental methodologies are employed to assess the morphological features of the welded areas, ascertain specimen strength, and observe welding interface failure modes. Based on a cohesive model, a finite element model is developed to predict the strength of the ultrasonic welded joints and elucidate the failure mechanisms. The results showed that, under identical welding parameters, the specimens welded with a high amplitude and low welding force exhibit superior welding quality. The specimens produced under displacement control exhibit minimal dispersion in strength. The proposed finite element model effectively prognosticates both welded joint strength and failure modes.

A Hyper-Pseudoelastic Model of Cyclic Stress-Softening Effect for Rubber Composites.

Rubber composites are hyperelastic materials with obvious stress-softening effects during the cyclic loading-unloading process. In previous studies, it is hard to obtain the stress responses of rubber composites at arbitrary loading-unloading orders directly. In this paper, a hyper-pseudoelastic model is developed to characterize the cyclic stress-softening effect of rubber composites with a fixed stretch amplitude at arbitrary loading-unloading order. The theoretical relationship between strain energy function and cyclic loading-unloading order is correlated by the hyper-pseudoelastic model directly. Initially, the basic laws of the cyclic stress-softening effect of rubber composites are revealed based on the cyclic loading-unloading experiments. Then, a theoretical relationship between the strain energy evolution function and loading-unloading order, as well as the pseudoelastic theory, is developed. Additionally, the basic constraints that the strain energy evolution function must satisfy in the presence or absence of residual deformation effect are derived. Finally, the calibration process of material parameters in the hyper-pseudoelastic model is also presented. The validity of the hyper-pseudoelastic model is demonstrated via the comparisons to experimental data of rubber composites with different filler contents. This paper presents a theoretical model for characterizing the stress-softening effect of rubber composites during the cyclic loading-unloading process. The proposed theoretical model can accurately predict the evolution of the mechanical behavior of rubber composites with the number of loading-unloading cycles, which provides scientific guidance for predicting the durability properties and analyzing the fatigue performance of rubber composites.

Application of Carbon Fiber-Reinforced Ceramic Composites in Active Thermal Protection of Advanced Propulsion Systems: A Review.

Thermal protection is one of the crucial issues for the advanced propulsion systems of Reusable Launch Vehicles. New service requirements for materials, such as high strength, low density, low thermal expansion, high thermal stability, etc., are raised for the thermal structure with the increasing demand of flight Mach numbers and thrust-to-weight ratio. Carbon fiber-reinforced ceramic composites, which generally meet the aforementioned requirements, show great potential for various applications and they have been widely applied in the thermal protection for hypersonic vehicles. This paper gives a comprehensive and systematic review of current research status for carbon fiber-reinforced ceramic composites applied in the thermal structure of advanced propulsion systems. Three aspects are presented and discussed: the ceramic composites fabrication and the property characterization, the thermal performance of composite thermal structure used in practical engines, and the numerical methods for predicting mechanical and thermal properties of composites as well as the physicochemical phenomenon in the cooling channels. Firstly, the main manufacturing processes for the carbon-reinforced ceramic composites are presented and the corresponding advantages and disadvantages are analyzed. The high-temperature oxidation and ablation behaviors of composites are demonstrated and the improvement of oxidation and ablation resistance by introducing the ultra-high-temperature ceramics into C/C composites is discussed in detail. Then, several typical applications of carbon fiber-reinforced ceramic composites (mainly C/SiC), including the work of RCI, JAXA and NASA, have been reviewed and analyzed. After that, the current research status of macroscale equivalent and multiscale numerical methods for predicting the mechanical and thermal properties of composites as well as the complex physicochemical phenomenon occurring in hydrocarbon fuels are sorted out. Finally, several potential prospects are pointed out for the future research on the thermal protection of advanced propulsion systems based on the carbon fiber-reinforced ceramic composites.