فصلنامه مکانیک هوافضا
سال بیستم شماره 3 (پیاپی 77، پاییز 1403)

In the present research, the experimental investigation of the female die forming of metallic plates with and without the use of venting hole in the central part of the plate has been performed. In order to apply the load to the specimen, repeated underwater explosive loading was used, so that in the 1st loading, the mass of the explosive charge was 4 gr and in the subsequent loadings, 8 grs were used. The results of the research showed that in the process of female die forming with venting hole, a protrusion or discontinuity is created in the central part, which leads to an increase of 28, 73 and 90% of the maximum thinning along the radial length of the plate in the 2nd, 3rd and 4th loadings in comparison to the 1st loading. Also, for the thinning in the center in the same situation, an increase of 27% and a decrease of 8% and 6% were observed, which was an unusual behavior caused by the presence of the venting hole. In this case, the filling rate of the specimen was 24%. This is despite the fact that in the process of female die forming without venting hole, the filling rate, maximum thinning and thinning in the center increased by 30%, increased by 6% and decreased by 10%, respectively. One of the important points is that the maximum amount of thinning in the primary bending region is lower compared to the center of the plate when using female die forming without venting hole, which is the opposite result obtained in the die forming with venting hole, so that in that case, the maximum thinning in the center of the area was higher than the region with initial bending. Therefore, it is very efficient to use the idea of female die forming without venting hole for sheet metal forming under repeated underwater explosive loading.

The so-called severe plastic deformation processes have received great attention from researchers due to the production of materials with desirable physical and mechanical properties through grain refinement. In this work, the corrosion behavior, hardness, and microstructure of 6061 aluminum alloy (Al6061) have been investigated after processing by constrained groove pressing (CGP). The results showed that the average grain size of the annealed sample, which was 42 micrometers, reached 24, 17, and 16 micrometers in the first, second, and fourth passes, respectively. Also, the hardness of the initial, first, second, and fourth passes was 33, 48, 56, and 57 HV, respectively. Therefore, the average hardness value increases dramatically at first, and then the increasing trend slows down in subsequent passes. Finally, the corrosion resistance of samples increases with CGP operation, so that the corrosion current of the first, second, and fourth pass samples shows a decrease of 45%, 59%, and 65%, respectively, as compared to the initial sample. The corrosion current reduction due to the CGP application is due to the decrease of surface defects such as holes, cracks, and open porosity, which was proven by the scanning electron microscope obtained from the corrosion surface of the samples. Finally, the sample weight reduction (the weight difference before and after the corrosion test) at different CGP conditions confirmed the results of the polarization curves.

This paper addresses the design and practical implementation of a constrained extended state observer for integration between an inertial navigation system (INS) and a global navigation satellite system (GNSS) navigation system in the presence of model uncertainties and inertial sensor errors. The use of physical constraints in the observer design can achieve an accurate and reliable model for the inertial navigation system during GNSS outages. The proposed method provides an estimate of this variable alongside the state variables of the inertial navigation system by considering the term including model uncertainties and inertial sensor errors as a new state variable. Additionally, the use of motion and environmental constraints in the proposed observer, including non-holonomic constraints and altitude constraints, improves estimation accuracy, reduces error accumulation, and increases the dynamic stability of the estimates. The performance of the proposed observer is evaluated through vehicle tests in a real-world test environment. The results indicate that the model uncertainties and inertial sensor errors in the inertial navigation system can be estimated in real-time. Thus, the designed observer can provide reliable estimates of the state variables by using GNSS information during its availability and utilizing physical constraints during GNSS outages. Furthermore, the proposed algorithm is fast due to its low computational load, making it suitable for practical implementation.

The aim of the current research is to achieve a fine grain structure (less than 200 micrometers) in Ti-48Al-2Cr-2Nb Intermetallic by a three-step heat treatment and to investigating the parameters affecting this process. For this purpose, after the homogenization at 1175 C for 24 hours for eliminating the cast structure, three-stage heat treatment including the first stage (single-phase annealing at 1400 C for one hour with cooling in furnace and air environments), the second stage (cyclic operations including repeated heating up to 1400 C for three, five and ten cycles and cooling in air and water environments) and the third stage (Two-phase annealing at temperatures of 1175, 1225 and 1275 C  for one hour) was performed. Finally, single-phase annealing at 1400°C for one hour and cooling in the furnace, then cyclic heat treatment with five cycles at the same temperature with cooling in air and annealing at 1225°C for one hour is suggested as the optimal process to create a semi-lamellar structure with the smallest colony size.

The expansion of capabilities and the continuous development of additive manufacturing technology have led to a reduction in costs for custom product manufacturing and have enabled the production of complex structures for micro-drones. Weight is generally one of the primary design features of micro-drones, often considered a trade-off against durability and other functionalities. However, ultra-lightweight structures can be achieved through additive manufacturing and topology optimization without compromising structural integrity. This study examines the use of these two technologies for designing and fabricating optimized lightweight micro-drones. Generative design using artificial intelligence algorithms performs topology optimization for optimal load distribution, demonstrating its efficiency in various simulations. This research addresses the complexities of micro-drone design and the interdependence between geometry, manufacturing methods, and materials. In this study, a square-frame micro-drone was constructed using Fused Deposition Modeling (FDM) with PETG filament. The selected material's properties were determined through mechanical testing and literature review. Subsequently, topology optimization was conducted to create a lightweight body structure with an X configuration. The optimized design was 3D printed and validated through load testing to verify finite element simulation results. The experimental and simulation results indicated that the combination of topology optimization and 3D printing can be safely and reliably used for the rapid design and production of micro-drones. The final optimized model produced through additive manufacturing could withstand a load 100 times its own weight, demonstrating superior performance compared to previous designs.

In this paper, free vibration behavior of three-skin conical shells with arbitrary boundary conditions is studied based on semi-analytical and numerical methods. The three-layer shell consists of identical outer-inner composite face sheets and a lattice core with regular hexagonal cells made of composite helical and circumferential ribs. For this purpose, t, the dynamic equations along with the boundary conditions of such sandwich shells are derived using the equivalent stiffness method, Donnell’s classic shell theory, and Hamilton’s principle. The frequency equation is presented by solving the integral form of these governing equations based on the Galerkin method. The vibration mode shapes in the form of forward or backward waves in the circumferential direction and standing waves in the direction of the cone length (Euler-Bernoulli beam modal functions) are used in order to obtain natural frequencies. Also, ABAQUS FE simulations are carried out to verify the vibration behavior predicted by the Galerkin solution method. Finally, parametric studies are performed to investigate the effects of geometric dimensions and boundary conditions on the response quantities. Numerical results show that there is a very good agreement between the natural frequencies obtained by Galerkin and ABAQUS FE methods (i.e., the maximum difference in the results is less than 10%). Also, the effects of face sheet thickness and the rib width on the natural frequencies of three-layer composite lattice conical shell with different supports are significant.

This article focuses on the derivation of nonlinear dynamic equations and form-finding using genetic algorithms for class 1 tensegrity structure. The structures under consideration feature a triangular cross-section, with a sphere enclosing them. The nonlinear dynamic equations of the system are obtained by applying the Lagrangian approach and the finite element method, considering the nodal positions as the generalized coordinates. The proposed approach illustrates how to develop large-scale, detailed dynamic models with different tensegrity structures.  The form-finding approach employs a simple framework capable of identifying both regular and irregular tensegrity configurations without limitation on dimensions. Stable tensegrity structures are created by applying specific restrictions to random configurations. The genetic algorithm and multi-objective functions are used to determine the fitness function and create these structures. Three separate scenarios with both defined and random connection matrices and member types—bars and cables—evaluate the performance of the proposed method for arbitrary architectures. The resulting models are validated with regard to force density. The vibration behavior of the final models under harmonic loads is investigated via modal analysis. The simulation results demonstrate the efficacy of the proposed method in precisely determining the vibration characteristics of tensegrity structures through the application of an intelligent form-finding methodology. This method is capable of managing both regular and irregular tensegrity structures in stochastic conditions. This approach is capable of handling both regular and irregular tensegrity structures in stochastic conditions.

The flow in the draft tube of water turbines at the point of maximum load operation is associated with many instabilities. In addition to the loss of efficiency caused by the vortex breakdown, the resulting low frequency pressure fluctuations can cause the resonance phenomenon and reduce the life of the turbine. In the current study, the process of forming the maximum load vortex core (torch) in the transition from the optimal operating point to the maximum load has been studied and the transient regimes formed during this transition process have been discussed and studied. It is clear that the core of the torch vortex is the result of the bubble vortex breaking phenomenon and its main factor is the formation of a stagnation point and the creation of a recirculation flow in the axis of the draft tube. This recirculation flow is created due to the loss of axial momentum in the conical region of the draft tube. The results of the vortex core show that in the stages of growth and formation of the burner vortex core, the core diameter of the optimal charge base grows first. Then, waves will be created on the surface of the vortex core structure, which with the passage of time and the strength of the rotation in the flow field, will break the integrated structure of the core and will be divided into several parts. Then the diameter and length of the vortex core structure increases and affects many parts of the draft tube cross-section, the length of this structure will progress to the draft tube diffuser. Therefore, it is clear that the main place of effect of the torch is the end of the conical area and the elbow inlet of the draft tube. By examining the numerical results, it was determined that with the passage of 1.5 to 2 seconds after the start of the transition process, there are significant changes in the rotational, axial and radial velocity components to will exist.