Mohammad Javad Khoshgoftar

This research explores the dynamic characteristics of composite nano-beams with a hybrid cellular structure (HCS) core, composed of two segments with distinct unit cell configurations, and face sheets reinforced with carbon nanotube (CNT) composites. By considering three-layered sandwich beams with aluminum cores of varying unit cell angles, the study explores a broad spectrum of achievable Poisson’s ratios. The top and bottom face sheets incorporate CNTs, distributed either uniformly or in a functionally graded manner. The governing equations are derived using Eringen’s nonlocal elasticity framework and the modified theory of shear deformation, with solutions obtained via the Galerkin method. A detailed parametric analysis is conducted to evaluate the effects of CNT content, arrangement configurations, hybrid core cellular angles, nonlocal parameters, and slenderness ratio (L/h) on the dimensionless natural frequencies of sandwich nanobeams with hybrid cellular cores. A key contribution of this study is the presentation of natural frequencies for nanobeams with hybrid cellular cores and composite face sheets reinforced with functionally graded CNTs, derived from advanced theoretical formulations. These findings offer new insights into design optimization and highlight the potential applications of hybrid cellular sandwich nanobeams in cutting-edge engineering systems.

Bone tissue damage resulting from trauma, tumor, or bone infection is one of the most common problems that humans face in everyday life. Tissue engineering is a developing method for the reconstruction and repair of damaged tissues, and a scaffold should have properties such as biocompatibility, mechanical properties suitable for the type of damaged tissue, and suitable surface characteristics for better cell adhesion. This study presents a precise and practical process for designing a scaffold used in bone tissue repair, such as the femur, and the effect of unit cell geometry on the mechanical properties and surface characteristics of the scaffold is investigated. CT scan images of the human femur area are obtained, and 3D models of the cortical and cancellous parts of the femur are constructed. The outer surface of the damaged parts is designed with ideal geometry; six types of unit cells with different geometries are designed, and the changes in the Young's modulus are determined as a function of changes in the unit cell porosity. The cubic–star‐shaped unit cell is chosen as the suitable unit cell for the scaffold structure, with a side length of 0.88 mm and a pore size of 0.6 mm.

Because of the larger specimen dimensions and more significant ligament for crack growth, the drop-weight tear test (DWTT) has become more widely used than other impact tests. On the other hand, very little information is available from DWTT experiments on weld metal (WM). In the present study, in addition to the standard chevron notch (CN) DWTT specimen with a notch depth (ND) of 5.1 mm, API X65 specimens with 10- and 15-mm ND were tested on the WM and also the base metal (BM). Fracture energies versus the ND were plotted, based on which tests can be performed successfully with lower-capacity DWTT machines to predict the absorbed energy for each ND, by specimens with increased ND and reduced absorbed energy. The standard requires thickness reduction for specimens thicker than 19 mm. With the method proposed in the present study, high-grade specimens can be tested with their original thickness. In the DWTT, regardless of specimen materials (BM or WM), about 44 % of the absorbed energy is spent on crack initiation, and 56 % is spent on crack growth. In all specimens, the BM absorbed energy was more than that of the WM. Therefore, this study proposes that the API standard, which does not include DWTT on the WM, should require performing DWTT on WM specimens. Hence, the results of the current study can form the basis for further studies that can initiate the development of new standards on the WM.

This paper presents an analytical model to investigate the static behaviour of sandwich plates comprised of two isotropic face sheets and a honeycomb core. Through-thickness transverse shear stresses were considered using a unified displacement field with which various plate theories were implemented, i.e., exponential, third-order, hyperbolic, sinusoidal, fifth-order, Mindlin, and the classic plate theory. The equilibrium equations of a simply-supported sandwich panel were derived using the principle of virtual work and Navier solution was obtained under static transverse loading. After validating of the model, various mechanical and geometrical parameters were varied to characterise the behaviour of the structure under regular and auxetic response. It was found that the auxeticity of the core strongly affects the mechanical response, e.g., in controlling deflection, in-plane anisotropy, and Poisson’s ratio. Cell wall angle was found to be most critical parameter that can be used to adjust anisotropy, out-of-plane shear modulus, transverse shear stress distribution, and deflection of the panel. Also the cell aspect ratio controls the sensitivity of the core response to other geometrical variations. In terms of the higher-order theories, the deflection-dependent parameter of the unified formulation seems to have more control of maximum deflection compared to independent rotations. Auxeticity of the core showed some benefits in controlling anisotropy, deflection and providing additional out-of-plane shear rigidity. Overall, since there is not one-to-one relationship between specific values of Poisson’s ratio, anisotropy, and shear rigidity, careful design considerations must be invested to obtain a correct mechanical response.

فراماده ها موادی هستند که خواص غیرمعمول از خود نشان میدهند. مواد آکزتیک به عنوان یک دسته از مواد فراماده، ساختارهایی دارای نسبت پوآسون منفی‌اند. این مواد در هنگام اعمال تنش کششی بر خلاف مواد معمول در راستای عمود بر نیروی وارده ضخیم‌تر و هنگام اعمال تنش فشاری نازک‌تر می‌شوند. رفتار آگزتیک یک خاصیت مستقل است که می‌تواند از سطح‌های میکروسکوپیک و ماکروسکوپیک و حتی سطح‌های ملکولی و سلولی به دست آید. ساختارها و مواد بسیاری با این ویژگی شناخته شده‌اند. این ویژگی منحصر به فرد کاربردهای بالقوه‌ای در زمینه‌های نظامی، هوافضا، پزشکی و سنسورهای هوشمند و بسیاری از صنایع دیگر ایجاد کرده است. با این حال هنوز مشکلات بسیاری برای استفاده گسترده از این مواد وجود دارد. در این مقاله به بررسی جامعی از ساختارها، خصوصیات منحصر به فرد و کاربرد مواد آگزتیک پرداخته شده است. علاوه بر این، برخی از آخرین پیشرفت‌هایی که در مورد این مواد انجام شده است بیان می‌شود. مواد با نسبت پوآسون منفی پتانسیل بسیار زیادی برای استفاده در سازه‌های مختلف مانند سازه‌های سبک و یا کاربردهای بایومتریال دارند.

The sandwich panel structures have been widely used in many industrial applications because of their high mechanical properties. The middle layer of these structures is very important factor in controlling and enhancing their mechanical performance under various loading scenarios. The re-entrant lattice configurations, are prominent candidates that can be used as the middle layer in such sandwich structures because of several reasons namely the simplicity in tuning their elastic (e.g., values of Poisson’s ratio and elastic stiffness) and plastic (e.g., high strength-to-weight ratio) properties by only adjusting the geometrical features of the constituting unit cells. Here, we investigated the response of a three-layered sandwich plate with a re-entrant core lattice under flexural bending using analytical (i.e., zig-zag theory), computational (i.e., finite element) and experimental tests. We also analyzed the effects of different geometrical parameters (e.g., angle, thicknesses, and length to the height ratio of unit cells) of re-entrant lattice structures on the overall mechanical behavior of sandwich structures. We found that the core structures with auxetic behavior (i.e., negative Poisson’s ratio) resulted in a higher bending strength and a minimum out-of-plane shear stress as compared to those with conventional lattices. Our results can pave way in designing advanced engineered sandwich structures with architected core lattices for aerospace and biomedical applications.