Browsing by Author "Gayir, C. E."

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Open Access
Experimental and numerical modal characterization for additively manufactured triply periodic minimal surface lattice structures: Comparison between free-size and homogenization-based optimization methods
(Wiley, 2023-06) Özdemir, Mirhan; Simsek, U.; Kuşer, Engin; Gayir, C. E.; Celik, A.; Şendur, Polat; Mechanical Engineering; ŞENDUR, Polat; Özdemir, Mirhan; Kuşer, Engin
Homogenization-based topology optimization (HMTO) is one of the most extensively used grading methods to generate functionally graded lattice structures (FGLs). However, it requires a precharacterization of the lattices, which is time-consuming. As a remedy, free-size optimization-based graded lattice generation (FOGLG) is explored as an alternative method to generate the FGLs. This article builds on the authors’ previous work in which the HMTO and FOGLG approaches are studied to improve the dynamic characteristic of a design by using a single lattice type, namely, double gyroid (DG) structure. To show applicability of the proposed methods, different lattice types including diamond (D), gyroid (G), and I-WP are employed to create FGLs herein. The frequency response analysis is performed, and the results from HMTO and FOGLG are compared in terms of their accuracy and efficiency. The optimized designs are then reconstructed by relative density mapping (RDM) and enhanced relative density mapping (ERDM) methods. The fabricated test samples made of cobalt–chromium using the direct metal laser melting (DMLM) technique are then experimentally validated using a laser vibrometer. The results reveal that HMTO and FOGLG can be used on the lattice types with a variety of configurations and relative densities.
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An integrated homogenization–based topology optimization via RBF mapping strategies for additively manufactured FGLS and its application to bandgap structures
(Springer Nature, 2020) Şimşek, Uğur; Gayir, C. E.; Kiziltas, G.; Şendur, Polat; Mechanical Engineering; ŞENDUR, Polat; Şimşek, Uğur
The manufacturing of lattice structures has been greatly facilitated thanks to the advances in additive manufacturing. Functionally graded lattice (FGL) structures, a major class of such structures, developed using topology optimization (TO) are known to have superior mechanical characteristics such as high stiffness to weight ratio. A new design methodology using an integrated TO process is proposed for the development of FGL structures in this research. For that purpose, a material-penalization formula derived by the application of homogenization is integrated into the TO process. As a result, relative densities of the TO are mapped directly. This approach is more advantageous compared with the alternative techniques as there is no need to post-process the optimization results. Therefore, the degradation of the optimization results from post-processing is eliminated. Then, radial basis functions (RBFs) are used to create the geometry of the FGLs efficiently. The proposed methodology is demonstrated on a case study, where a cantilever beam with a desired bandgap characteristic is designed. Numerical results using the proposed method show that the first and second bending frequencies with the resulting optimized geometry are within 3% and 12% of the original TO design, whereas using method 1 the calculated relative errors are 24% and 74% and method 2 these errors are calculated as 8% and 34%, respectively. These comparative results indicate that the geometry created by the new method is superior to other design strategies as evidenced by the improved compatibility level between the bandgap performance results of the original unpenalized TO and structures generated using alternative techniques.
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Modal characterization of additively manufactured TPMS structures: comparison between different modeling methods
(Springer Nature, 2020-10-14) Şimşek, Uğur; Akbulut, Aykan; Gayir, C. E.; Başaran, Cansu; Şendur, Polat; Mechanical Engineering; ŞENDUR, Polat; Şimşek, Uğur; Akbulut, Aykan; Başaran, Cansu
The use of lattice structures has received increasing interest in various engineering applications owing to their high strength to weight ratio. Advances in additive manufacturing technologies enabled the manufacturing of highly complex lattice structures such as triply periodic minimal surface (TPMS) models in recent years. The application of simulation tools is expected to enhance the performance of these designs further. Therefore, it is vital to understand their accuracy and computational efficiency. In this paper, modal characterization of additively manufactured TPMS structures is studied using five different modeling methods for a beam, which is composed of primitive, diamond, IWP, and gyroid unit cells. These methods include (1) shell modeling, (2) solid modeling, (3) homogenization, (4) super-element modeling, and (5) voxelization. The modal characterization is performed by using modal analysis, and the aforementioned models are compared in terms of their computational efficiency and accuracy. The results are experimentally validated by performing an experimental modal testing on a test specimen, made of HS188, and manufactured by direct metal laser melting. Finally, the relationship between the modal characteristics and volume fraction is derived by carrying out a parametric study for all types of TMPS structures considered in this paper. The complex modal characteristics of different TPMS types suggest that they can be jointly used to meet the ever-challenging design requirements using the modeling guidelines proposed in this study.
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A new sensitivity-based mapping scheme for topology optimization of graded TPMS designs
(Springer, 2023-12) Parlayan, O.; Özdemir, Mirhan; Gayir, C. E.; Şimşek, Uğur; Kiziltas, G.; Özdemir, Mirhan; Şimşek, Uğur
Graded TPMS topologies display excellent mechanical and thermal properties. Design schemes targeting optimal performance exist, but final reconstructed designs still suffer from performance degradation. To overcome this challenge, we propose an automated design framework based on the integration of a homogenization-based topology optimization scheme and a new mapping strategy. Optimized designs obtained using a modified SIMP technique are reconstructed as graded gyroid structures. Unlike mapping strategies using relative density values prior to TPMS infill, for the first time, we make use of readily available adjoint sensitivities for mapping optimal densities to graded gyroid structures. Results show that the proposed framework delivers performance-preserving graded designs when compared to original optimized designs obtained using OptiStruct and superior performance in comparison to standard density-based mapping methods. The resulting graded design is manufactured using additive manufacturing, and three-point bending tests are performed confirming simulation results and demonstrating the applicability of the presented design scheme.
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A novel design framework for generating functionally graded multi-morphology lattices via hybrid optimization and blending methods
(Elsevier, 2023-05-25) Özdemir, Mirhan; Simsek, U.; Kiziltas, G.; Gayir, C. E.; Celik, A.; Şendur, Polat; Mechanical Engineering; ŞENDUR, Polat; Özdemir, Mirhan
Owing to its excellent mechanical properties, triply periodic minimum surfaces (TPMS) lattice structures have recently gained more interest in engineering applications. The superior properties of these structures make it easier to achieve engineering design goals such as strength and weight. However, technological advancements compel the designer to enhance the traditional TPMS design qualities. Hybridization of different lattice types emerges as a strong candidate for enhancing overall design performance. Therefore, a hybrid optimization scheme based on genetic algorithms (GA) and anisotropic homogenization-based topology optimization is considered to generate a functionally graded multi-morphology for a Messerschmitt–Bölkow–Blohm (MBB) beam design in this paper. The GA is performed to identify the best lattice morphology, including Diamond (D), Gyroid (G), I-WP, and Primitive (P), and their relative densities prior to topology optimization (TO). Once the best lattice morphology of the design domain is obtained via the GA, the homogenization-based topology optimization is applied to grade the multi-morphology lattice to improve the design performance further. The final step is the reconstruction of the graded multi-morphology using a novel blending algorithm. The reconstructed MBB beams are made of cobalt-chromium (CoCr) alloy and are then manufactured using the laser sintering method, direct metal laser melting (DMLM) technique. Destructive metallographic and non-destructive metrological techniques are utilized to assure manufacturing quality. An impact hammer test is conducted on the fabricated beams to validate and compare the proposed graded multi-morphology geometry with graded and uniform single lattice morphologies. Experimental results show that the stiffness of the graded multi-morphology structure designed by the proposed hybrid optimization is 4.5 % and 13.0 % higher than the graded form of D and P-type single lattice morphologies, respectively. Also, it is observed that the graded form single lattice morphologies deliver superior performance than their uniform encounters namely D and P-type lattice structures.
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Parametric studies on vibration characteristics of triply periodic minimum surface sandwich lattice structures
(Springer Nature, 2020-10-07) Şimşek, Uğur; Arslan, T.; Kavas, B.; Gayir, C. E.; Şendur, Polat; Mechanical Engineering; ŞENDUR, Polat; Şimşek, Uğur
Additive manufacturing has opened new avenues for the manufacturing of structures to achieve challenging engineering tasks. Gyroid, a unique example of such structures, exhibits many attractive properties, such as high stiffness-to-weight ratio and impact characteristics. This study aimed to evaluate the dynamic performance of gyroid structures made from HS188 using direct metal laser melting. The frequency response predictions of a finite element-based model of the gyroid sandwich structure were first validated against the modal testing in terms of its natural frequencies and mode shapes using the Dewesoft software. Subsequently, the effects of the plate and gyroid wall thickness on the dynamic characteristics of the structure were investigated by varying these across their expected limit ranges as part of a parametric study using the validated finite element model. The findings from the parametric study were validated against modal testing. Moreover, the performance of the aforementioned structure was compared with that of a solid structure with the same mass. The simulation results indicated that the dynamic characteristics of the gyroid structure can be improved considering the structure's frequency response by using parametric models. It was concluded that simulation and optimization tools will play a crucial role in additive manufacturing techniques to attain optimal mechanical properties of complex structures.