DESIGN OF THE THERMOMECHANICAL CLAMP JOINT OF MATERIALS WITH SHAPE MEMORY EFFECT FOR UNMANNED AERIAL VEHICLE

89 АВИАЦИОННАЯ И РАКЕТНО-КОСМИЧЕСКАЯ ТЕХНИКА 05.07.01 – Аэродинамика и процессы теплообмена летательных аппаратов; 05.07.02 – Проектирование, конструкция и производство летательных аппаратов; 05.07.03 – Прочность и тепловые режимы летательных аппаратов; 05.07.05 – Тепловые электроракетные двигатели и энергоустановки летательных аппаратов; 05.07.07 – Контроль и испытание летательных аппаратов и их систем; 05.07.09 – Динамика, баллистика, управление движением летательных аппаратов; 05.07.10 – Инновационные технологии в аэрокосмической деятельности


INTRODUCTION
When designing small-sized unmanned aerial vehicles (UAVs) with a mid-ship diameter less than 800 mm, in addition to the classic tasks of reducing weight and increasing the strength of the design, problems of ensuring the article high technological effectiveness arise.
Currently, the two types of joints: point and contour, are primarily used in UAV designs for connecting compartments to each other. The use of point (flange) joints leads to the necessity of making numerous threaded holes and flanges, which increases the structural mass [1]. Therefore, the contour joint is prevalent for connecting small diameter UAV compartments. These joints are comprised of telescopic and clamp (tape) connections. A telescopic one has a number of drawbacks -big length, the requirement of high accuracy for frames surfaces, the complex structural assembly [2]. As a result, it is more efficient to use clamp (tape) joints to connect UAV compartments of small diameter. The disadvantages of this joint involve available tightening bolts that are necessary to ensure the sufficient tension of the clamp, which affects the UAV performance. A possible solution to this problem may be SME materials utilization in the construction. The effect of shape memory is the property of the extensive class of materials, which possess the reversibility of inelastic deformation [3][4][5][6][7][8][9][10][11]. SME alloys have been widely utilized in aerospace engineering, which is represented in works [12][13][14][15], e.g. as special couplings that provide the required tension. The use of SME detachable clamps for missile bodies will not only allow us to ensure all the forces transfer from one part of the UAV to another in conformity with the conditions of strength and rigidity, but also to fulfill the key technological requirements i.e., to automate a process of assembly, improve its accuracy, create the possibility of highquality compartments joining without special tools and cooperative processing of compartments mating surfaces.
This article solves the designing problem of a clamp joint for small-diameter UAV compartments using SME materials, which have high manufacturability and meet the strength conditions.

SOLUTION ALGORITHM
The design of the studied clamp (tape) demountable joint is shown in Figure 1. A clamp tape is an open envelope made up of SME material. When heated, its diameter decreases to the specified to provide tightness and absence of clearances in the structure. The application of SME clamp ensures absence of after-assembly residual stress and makes it possible to fulfill a hidden threaded coupling that is flush with the article caliber, which has a positive effect on the UAV performance. Compartments are connected by means of a projection on one of the frames [16] in order to limit radial displacements. Thus, the clamp is utilized to prevent axial movements and shells turnover along the axis of rotation. AMg6 aluminum (which has breaking strength σ в = 300 MPa) and titanium nickel TN-1(σ в = 800 MPa) were selected as the model materials for the UAV body and clamp, respectively.
At the first stage of designing, it is necessary to establish an initial set of clamp geometric parameters stemming from the limitations imposed on the UAV body ( fig. 2). After identifying the parameters for the purpose of reducing the joint unit mass and increasing its strength performance, it is essential to accomplish a task of parametric structure optimization [17]. At the same time, establishing the optimal parameters for the clamp joint, under which the joint will satisfy the strength, resistance to aerodynamic heating requirements and have the minimal mass, is required.
where M is the bending moment; N is the longitudinal force; J is the inertia moment; φ is the circumferential coordinate; δ is the wall thickness; is the average radius.
In the loading case, the clamp is the element responding merely to tensile loads. Load s exerts pressure q from the frame side on the contact surface with the clamp and friction forces within the contact plane ( fig. 4, б). The projection of these forces on the horizontal line should be equal to s: If the value of q is known, you can find the resulting vertical component в Since the friction coefficient depends on many factors and can be very small, it can be accepted as μ = 0, and the load can be determined by the following formulas: Distributed forces в ( fig. 4, a), acting around the clamp circumference, cause the clamp elongation. The value of the tensile forces can be found on the basis of the equilibrium condition, mentally cutting the clamp by a horizontal diametrical section ( fig. 5). The acting force on the clamp element will be equal to в , where is the central angle corresponding to the element. Having taken the sum of the forces vertical components acting on half a clamp, we obtain the following equilibrium equation: Tensile stress in the clamp, which determines the condition of strength, can be obtained by means of dividing the force of Nφ by the cross-sectional area of the clamp п : Further, a stress analysis is executed. The factor of safety: = в ⁄ is equal to 1.5. If the condition (8) is not met, the checking calculation is repeated under a new set of parameters. Figure 6 shows the algorithm of designing computation for the clamp demountable joint, which can be utilized to compute joints of UAV compartments with different diameters.   Figure 7 illustrates that the tensile force increases steadily as the inclination angle of surface θ does. Herewith, the radius of the clamp connection R influences its increase: the greater the value of R, the lower the value of N φ .  Figure 8 gives that the pressure from the frame side on a clamp increases steadily along with the angle increase θ. Likewise, in Figure 9, a dependence of the value q в on the value of R is noticeable: the greater the value of R, the lower the value of q в , which indicates that when choosing an optimal diameter of a clamp connection from a variety of options, from the point of view of strength, a clamp joint with the maximum value of R is preferable.   Figure 10 illustrates the area within which the strength condition for TN-1type material with SME (this area is below the tolerance limit) is met. In accordance with Figure 10, one can find the maximum allowable values of clamp surface inclination angles for different radii of clamp fitting under which the strength condition for the nitinol clamp is fulfilled: • θ ≤ 24° for R = 0.5 m; • θ ≤ 27° for R = 0.6 m; • θ ≤ 30° for R = 0.7 m; • θ ≤ 32° for R = 0.8 m.
At the angles of θ, exceeding the specified values, the tensile force and the pressure, acting on a clamp from the side of frame, fall outside the allowable maximum load, as it is depicted in Figure 10.
Based on the computation results of the ANSYS software solutions, a parametric finite element clamp model was designed. Using the "Response Surface Optimization" module, the analysis of the model with the parameters of the clamp geometry, physical and mechanical characteristics of the material and the maximum values of stresses and deflection under limitations was carried out: where σ доп and w доп are the tolerance values of stresses and deflection, w max is the maximal deflection value.
Multi-purpose search was adopted as an optimization algorithm. As a result of optimization, the region of compromise solutions, which visualization is represented in Figure 11, was obtained. The criterion of minimum mass was selected as the major optimization one. According to this criterion, the optimal design and engineering solution of the clamp construction was selected from a variety of the obtained compromises. The value of the optimal parameters of a clamp joint is given in Table 1.
The resulting set of parameters allows you to design the working structure of a clamp joint with the lowest mass.

CONCLUSION
The design problem of a clamp joint of small diameter, using SME materials, was set and solved.