RESUMO
Pores are common defects generated during fabrication, which restrict the application of carbon/carbon (C/C) composites. To quantitatively understand the effects of pores on mechanical strength, this paper proposes a representative volume element model of unidirectional (UD) C/C composites based on the finite element method. The Hashin criterion and exponential degraded rule are used as the failure initiation and evolution of pyrolytic carbon matrices, respectively. Interfacial zones are characterized using the cohesive constitutive. At the same time, periodic boundary conditions are employed to study transverse tensile, compressive, and shear deformations of UD C/C composites. Predicted results are compared with the experimental results, which shows that the proposed model can effectively simulate the transverse mechanical behaviors of UD C/C composites. Based on this model, the effects of microstructural parameters including porosity, pore locations, the distance between two pores, pore clustering, and pore shapes on the mechanical strength are investigated. The results show that porosity markedly reduces the strength as porosity increases. When the porosity increases from 4.59% to 12.5%, the transverse tensile, compressive, and shear strengths decrease by 35.91%, 37.52%, and 30.76%, respectively. Pore locations, the distance between two pores, and pore clustering have little effect on the shear strength of UD C/C composites. For pore shapes, irregular pores more easily lead to stress concentration and matrix failure, which greatly depresses the bearing capacity of UD C/C composites.
RESUMO
Thermal protective textiles are crucial for safeguarding individuals, particularly firefighters and steelworkers, against extreme heat, and for preventing burn injuries. However, traditional firefighting gear suffers from statically fixed thermal insulation properties, potentially resulting in overheating and discomfort in moderate conditions, and insufficient protection in extreme fire events. Herein, an innovative soft robotic textile is developed for dynamically adaptive thermal management, providing superior personal protection and thermal comfort across a spectrum of environmental temperatures. This unique textile features a thermoplastic polyurethane (TPU)-sealed actuation system, embedded with a low boiling point fluid for reversible phase transition, resembling an endoskeleton that triggers an expansion within the textile matrix for enhanced air gap and thermal insulation. The thermal resistance improves automatically from 0.23 to 0.48 Km2 W-1 by self-actuating under intense heat, exceeding conventional textiles by maintaining over 10 °C cooler temperatures. Additionally, the knitted substrate incorporated into the soft actuators can substantially mitigate convective heat transfer, as evidenced by the thermal resistance tests and the temperature mapping derived from numerical simulations. Moreover, it boasts significantly increased moisture permeability. The thermoadaptation and breathability of this durable all-fabric system signify considerable progress in the development of protective clothing with high comfort for dynamic and extreme temperature conditions.
RESUMO
Using compression textiles to exert an appropriate and steady pressure on human limbs is a primary treatment method in the medical area. Compression pressure is a crucial parameter that determines the treatment efficacy. However, there is a lack of pressure-sensing fabrics that can both apply and measure the pressure of compression textiles, particularly the theoretical study of the prediction of the pressure and sensing performance of such a sensing fabric. In this study, based on the developed elastic pressure-exerting and -sensing fabrics and a setup test protocol simulating the pressure-exerting process, the relationships between the displacement of the press head, resultant fabric extension, and pressure were theoretically explored. Two finite element (FE) models, continuum and discontinuous models, were first established to predict the pressure behavior of elastic pressure-exerting and -sensing fabrics. The simulation results present good agreement with the experimental results wherein the pressure generated increases with the increase of the fabric strain in a nonlinear form. Furthermore, with the above FE models for the relationship between fabric extension and pressure generated, as well as the measured electrical resistance of the sensing fabric, a model for the electrical resistance of the sensing fabric can thus be established. Among pressure-sensing fabrics in three different structures, the sensing fabric in sateen exhibits better pressure prediction accuracy and a faster response to the pressure change. Finally, a series of numerical simulations were conducted to investigate the effects of the press head diameter, the unit cell crimp factor of fabric and the fabric pretension on the fabric extension, the resultant pressure, and electrical resistance change. The simulation results show that the pressure decreases with the increase of the press head diameter. The crimp factor and pretension of the sensing fabric also have a significant effect on the pressure and electrical resistance change generated. This simulation approach provides a new theoretical understanding of the pressure behavior and mechanism of pressure-sensing fabrics for future smart compression textiles.
RESUMO
The components of carbon/carbon (C/C) composites have significant influence on the thermal and mechanical properties, so a quantitative characterization of component is necessary to study the microstructure of C/C composites, and further to improve the macroscopic properties of C/C composites. Considering the extinction crosses of the pyrocarbon matrix have significant moving features, the polarized light microscope (PLM) video is used to characterize C/C composites quantitatively because it contains sufficiently dynamic and structure information. Then the optical flow method is introduced to compute the optical flow field between the adjacent frames, and segment the components of C/C composites from PLM image by image processing. Meanwhile the matrix with different textures is re-segmented by the length difference of motion vectors, and then the component fraction of each component and extinction angle of pyrocarbon matrix are calculated directly. Finally, the C/C composites are successfully characterized from three aspects of carbon fiber, pyrocarbon, and pores by a series of image processing operators based on PLM video, and the errors of component fractions are less than 15%.
