مقاله انگلیسی رایگان در مورد روش سلول واحد در محاسبه حرارتی کامپوزیت ها – الزویر 2018

For a high-speed vehicle like hypersonic one, the surface temperature may reach a value of 1600℃ or even higher [1, 2] in a very short time of hundreds of seconds. Under this condition, a reliable and efficient thermal protection system (TPS) is required, and the thermal characteristics of relevant TPS composites should be deeply studied. The effective thermal properties of composites can be efficiently calculated by a representative volume element (RVE) model. According to the structure of composites, there are two types of RVE can be formulated. For composites with random phase distributions such as needled composites (randomly distributed short fibers) [3, 4], fiber layers in proton exchange membrane fuel cell (randomly distributed short fibers and pores) [5-8], porous materials (random pores) [9-11], granular composite (random reinforcing granula) [12] and thermal barrier coatings (random pores) [13, 14], the RVE is formulated based on statistical parameters (e.g., phase volume fraction) of the composite structure. Due to the structure stochasticity, such RVE is an approximate rather than accurate model. For another type of composite with certain geometric symmetries such as textile reinforced composites [15-26] and idealized foam materials [27-30], the RVE can be formulated based on structure symmetries. Such RVE is the so-called unit cell (UC). In composites, any complex symmetric structures can be decomposed into three types of symmetric structures: translation along an axis, reflection about a plane and rotation about an axis for a certain angle (mainly 180°) [31]. The translational symmetry is always used to formulate a full UC first, and the other two symmetries can then be used to reduce the unit cell size. Each UC needs corresponding boundary condition (BC) to represent the macro structure. The derivation of BC should be based on rigorous mathematical and thermo-physical considerations [32]. In most above Refs. [15-19, 22, 23], only translational symmetries are employed to formulate unit cells, and the periodic relative temperature or periodic temperature gradient are the appropriate BC for such unit cells. In the authors’ works about plain woven [20], satin woven [32, 33], three-dimensional four-directional braided (3D4d) [34] composites, the reflectional and 180° rotational symmetries are used to formulate unit cells, and the results show that such two symmetries will reduce the UC size while lead to more complicated BC. Compare with the RVE of randomly structured composites, a UC model of symmetrically structured composites can be theoretically accurate in representing the macro composite; however, the formulation of an accurate UC is relatively complicated. In general, it involves construction of geometric configuration and derivation of BC, and the process is closely related to the structure symmetries and macro thermal condition (heat flux/temperature gradient field) of the composite.