The Effect of Functional Gradient Material Distribution and Patterning on Torsional Properties of Lattice Structures Manufactured Using MultiJet Fusion Technology

Functionally graded lattice structures have attracted much attention in engineering due to their excellent mechanical performance resulting from their optimized and application-specific properties. These structures are inspired by nature and are important for a lightweight yet efficient and optimal functionality. They have enhanced mechanical properties over the uniform density counterparts because of their graded design, making them preferable for many applications. Several studies were carried out to investigate the mechanical properties of graded density lattice structures subjected to different types of loadings mainly related to tensile, compression, and fatigue responses. In applications related to biomedical, automotive, and aerospace sectors, dynamic bending and rotational stresses are critical load components. Therefore, the study of torsional properties of functionally gradient lattice structures will contribute to a better implementation of lattice structures in several sectors. In this study, several functionally gradient triply periodic minimal surfaces structures and strut-based lattice structures were designed in cylindrical shapes having 40% relative density. The HP Multi Jet Fusion 4200 3D printer was used to fabricate all specimens for the experimental study. A torsional experiment until the failure of each structure was conducted to investigate properties of the lattice structures such as torsional stiffness, energy absorption, and failure characteristics. The results showed that the stiffness and energy absorption of structures can be improved by an effective material distribution that corresponds to the stress concentration due to torsional load. The TPMS based functionally gradient design showed a 35% increase in torsional stiffness and 15% increase in the ultimate shear strength compared to their uniform counterparts. In addition, results also revealed that an effective material distribution affects the failure mechanism of the lattice structures and delays the plastic deformation, increasing their resistance to torsional loads.

Effect of Initial Temperature on the Microstructure and Properties of Stellite-6/Inconel 718 Functional Gradient Materials Formed by Laser Metal Deposition

Stelite-6/Inconel 718 functionally gradient materials (FGM) is a heat-resisting functional gradient material with excellent strength performance under ultra-high temperatures (650–1100 °C) and, thus, has potential application in aeronautic and aerospace engineering such as engine turbine blade. To investigate the effect of initial temperature on the microstructure and properties of laser metal deposition (LMD) functional gradient material (FGM), this paper uses the LMD technique to form Stelite-6/Inconel 718 FGM at two different initial temperatures: room temperature and preheating (300 °C). Analysis of the internal residual stress distribution, elemental distribution, microstructure, tensile properties, and microhardness of 100% Stelite-6 to 100% Inconel 718 FGM formed at different initial temperatures in a 10% gradient. The experimental results prove that the high initial temperature effectively improves the uneven distribution of internal residual stresses. Preheating slows down the solidification time of the melt pool and facilitates the escape of gases and the homogeneous diffusion of elements in the melt pool. In addition, preheating reduces the bonding area between the gradient layers, enhancing the metallurgical bonding properties between the layers and improving the tensile properties. Compared with Stellite-6/Inconel 718 FGM formed at room temperature, the mean yield strength, mean tensile strength, and mean elongation of Stellite-6/Inconel 718 FGM formed at 300 °C are increased by 65.1 Mpa, 97 MPa, and 5.2%. However, the high initial temperature will affect the hardness of the material. The average hardness of Stellite-6/Inconel 718 FGM formed at 300 °C is 26.9 HV (Vickers hardness) lower than that of Stellite-6/Inconel 718 FGM formed at 20 °C.

Direction-Dependent Properties in Inverted Carbon Nitride Colloidal Glasses with Gradient Porosity

It is well known that the step from a dense packing of colloidal beads to the inverted systems was important for the optimization of photonic crystal properties. Inverted opals made of high-refractive index semiconductors have attracted great attention due to their supreme optical features such as the occurrence of a photonic band-gap and because of an astonishing behavior in photocatalysis or for photovoltaics caused by so-called slow photons. It is much less known that photonic glasses, despite being disordered, exhibit unique optical properties too like random lasing or high-contrast structural colors. In analogy to opals and inverted opals, one can expect that inverted colloidal glasses may lead to an amplification of photonic properties as well or even to the emergence of unexpected features. An inverted photonic glass is characterized by a dense packing of monodisperse voids with colloidal dimensions without any long-range order. The preparation of inverse photonic glasses has rarely been reported by now and cases for materials composed of a semiconductor as a pore-wall material are unknown. The synthesis of porous carbon nitride (C₃N₄) with inverted colloidal glass structure is demonstrated here using a template approach. The formation of the template with glass-like order is achieved by analytical ultracentrifugation (AUZ) of size-selected silica colloids, followed by infiltration of a precursor sol, transformation to carbon nitride and the final removal of the template. The use of AUZ is particularly important because it even allows to use a mixture of differently sized template particles, which are gradually fractionated. Monoliths with optimized morphological features exhibiting a gradient porosity and highly accessible pores are obtained. The result are materials with a graded structure. What makes such functional gradient material interesting is, a dependence of the optical features on the position can be expected. In addition, the method presented here allows to synthesize materials with adjustable composition ranging from carbon over nitrogen-doped carbon to C₃N₄ with either graphitic or polymeric structure. Therefore, the optical band gap is highly adjustable and tunable with regards to the photonic properties, as confirmed by optical absorption and photoluminescence measurements.

Direction-Dependent Properties in Inverted Carbon Nitride Colloidal Glasses with Gradient Porosity

It is well known that the step from a dense packing of colloidal beads to the inverted systems was important for the optimization of photonic crystal properties. Inverted opals made of high-refractive index semiconductors have attracted great attention due to their supreme optical features such as the occurrence of a photonic band-gap and because of an astonishing behavior in photocatalysis or for photovoltaics caused by so-called slow photons. It is much less known that photonic glasses, despite being disordered, exhibit unique optical properties too like random lasing or high-contrast structural colors. In analogy to opals and inverted opals, one can expect that inverted colloidal glasses may lead to an amplification of photonic properties as well or even to the emergence of unexpected features. An inverted photonic glass is characterized by a dense packing of monodisperse voids with colloidal dimensions without any long-range order. The preparation of inverse photonic glasses has rarely been reported by now and cases for materials composed of a semiconductor as a pore-wall material are unknown. The synthesis of porous carbon nitride (C₃N₄) with inverted colloidal glass structure is demonstrated here using a template approach. The formation of the template with glass-like order is achieved by analytical ultracentrifugation (AUZ) of size-selected silica colloids, followed by infiltration of a precursor sol, transformation to carbon nitride and the final removal of the template. The use of AUZ is particularly important because it even allows to use a mixture of differently sized template particles, which are gradually fractionated. Monoliths with optimized morphological features exhibiting a gradient porosity and highly accessible pores are obtained. The result are materials with a graded structure. What makes such functional gradient material interesting is, a dependence of the optical features on the position can be expected. In addition, the method presented here allows to synthesize materials with adjustable composition ranging from carbon over nitrogen-doped carbon to C₃N₄ with either graphitic or polymeric structure. Therefore, the optical band gap is highly adjustable and tunable with regards to the photonic properties, as confirmed by optical absorption and photoluminescence measurements.

STRESS CONCENTRATION IN A HOMOGENEOUS PLATE WITH A CIRCULAR HOLE REINFORCED BY AN INCLUSION FROM A FUNCTIONAL-GRADIENT MATERIAL

Computer simulation of the stress-strain state of a thin rectangular homogeneous isotropic plate with a circular hole, reinforced by an annular inclusion made of a functional-gradient material (FGM) has been carried out. The influence of the geometric (width) and mechanical (elastic modulus) parameters of the inclusion is investigated when various laws of change in the elastic modulus of the FGM are set on the stress concentration around the hole. Recommendations for the use of inclusions are given. If there is a "hard" annular inclusion in a homogeneous plate with a hole, it is expedient to use an FGM with a nonlinear law of change in the modulus of elasticity in comparison with an inclusion made of a homogeneous material. Despite the fact that the inclusion leads to a slight increase in the stress concentration factor in comparison with a homogeneous material, it makes it possible to increase the rigidity of the system as a whole. The width of FGM inclusions affects the nature of the stress distribution: the wider the inclusion, the more smoothly the stress redistribution in the main matrix occurs.

Thermoelastic plastic deformation of a functional gradient material under conditions of central symmetry

В работе рассматривается ряд краевых задач теории термоупругопластического деформирования материала в условиях тороидальной симметрии. Рассмотрен процесс упругой разгрузки предварительно нагретого объекта. Рассчитаны поля остаточных напряжений и перемещений. Получены точные формулы для аналитического решения поставленной краевой задачи A number of boundary value problems of the theory of thermoelastoplastic deformation of a material under conditions of toroidal symmetry are presented. The process of elastic unloading of a heated object is considered. The fields of residual stresses and displacements are calculated. Exact formulas are obtained for the analytical solution of the stated boundary value problem.

An artificial neural network (ANN) solution to the prediction of age-hardening and corrosion behavior of an Al/TiC functional gradient material (FGM)

In this theoretical study, the prediction of the corrosion resistance and age-hardening behavior of an Al/TiC functional gradient material (FGM) has been investigated by using the artificial neural network (ANN). The input parameters have been selected as TiC volume fraction of the composite layers, aging periods of the composite, environmental conditions, and applied potential during the corrosion tests. Current and microhardness were used as the one output in the proposed network. Also, a new three-layered composite has been imaginarily designed to demonstrate the predictive capability and flexibilities of the ANN model as a case study. Artificially aging (T6) process and potentiodynamic scanning (PDS) tests were used for heat-treating and corrosion response of the FGS, respectively. The results showed that the generated PDS curves of the FGM and calculated corrosion parameters of the case study are quite near and in acceptable limits for similar composites obtained values in experimental studies. Besides, this study has been a great success in predicting peak-aging times and its corresponding hardness values more precisely.

Dynamic Fracture Analysis of Functional Gradient Material Coating Based on the Peridynamic Method

Functional gradient materials (FGMs) have tremendous potential due to their characteristic advantage of asymptotic continuous variation of their properties. When an FGM is used as a coating material, damage and failure of the interface with the substrate component can be effectively inhibited. In order to study the dynamic crack propagation in FGM coatings, a new method, peridynamics (PD), was used in the present study to simulate dynamic fractures of FGM coatings bonded to a homogeneous substrate under dynamic loading. The bond-based PD theory was employed to study crack propagation and branching in the FGM coating. The influences of the coating gradient pattern, loading, and the geometry and size of the structure on crack curving and propagation under impact loading were investigated. The numerical results show that different forms of the elastic modulus of graded material, the geometry of the structure, and the loading conditions have considerate effects on crack propagation in FGM coatings, but a specific form of elastic modulus had a limited effect on the dynamic fracture of FGM coating.

ON THE LOSS OF STABILITY OF A TWO-LAYERED PLATE MADE OF A FUNCTIONAL-GRADIENT MATERIAL WITH A NON-UNIFORM FIELD OF PRE-STRESSES

In the framework of three-dimensional nonlinear elasticity we consider linear instability of a composite plate made of functionally graded material and having initial stresses. The plae consists of two layers which were obtained as a result of flattening of an annual sector of an elastic cylinder. This deformation results in appearance of internal stresses. Thus, the plate becomes initially stressed. The initial stresses depend on the thickness coordinate, so we get inhomogeneous stress field. We have two types of inhomogeneities, the first is the inhomogeneity of the initial stresses whereas the second is the material inhomogeneity.We use the incompressible neo-Hookean material model as a constitutive relation. Despite of relatively simple form this model describes properly severe deformations of some rubber-like materials. For incompressible materials the flattening constitutes one of the so-called universal deformations, that is such deformation which is independent on the choice of constitutive relation. The material inhomogeneity is described through a dependence of the shear modulus on the thickness coordinate. Such inhomogeneity could be related to the manufacturing of the material or to further treatment. The stability was analysed using the linearization approach. We superimpose infinitesimal deformations on the finite initial one. The linearized boundary-value problem was derived and its nontrivial solutions were obtained. The solution was obtained in series of trigonometric functions. This helps to automatically satisfy a part of boundary conditions. We consider the influence of the inhomogeneity and initial stresses. We show that the initial stresses may significantly change critical deformations. For example, the loss of stability is possible due to initial stresses only.

ON THE LOSS OF STABILITY OF A TWO-LAYERED PLATE MADE OF A FUNCTIONAL-GRADIENT MATERIAL WITH A NON-UNIFORM FIELD OF PRE-STRESSES

In the framework of three-dimensional nonlinear elasticity we consider linear instability of a composite plate made of functionally graded material and having initial stresses. The plae consists of two layers which were obtained as a result of flattening of an annual sector of an elastic cylinder. This deformation results in appearance of internal stresses. Thus, the plate becomes initially stressed. The initial stresses depend on the thickness coordinate, so we get inhomogeneous stress field. We have two types of inhomogeneities, the first is the inhomogeneity of the initial stresses whereas the second is the material inhomogeneity.We use the incompressible neo-Hookean material model as a constitutive relation. Despite of relatively simple form this model describes properly severe deformations of some rubber-like materials. For incompressible materials the flattening constitutes one of the so-called universal deformations, that is such deformation which is independent on the choice of constitutive relation. The material inhomogeneity is described through a dependence of the shear modulus on the thickness coordinate. Such inhomogeneity could be related to the manufacturing of the material or to further treatment. The stability was analysed using the linearization approach. We superimpose infinitesimal deformations on the finite initial one. The linearized boundary-value problem was derived and its nontrivial solutions were obtained. The solution was obtained in series of trigonometric functions. This helps to automatically satisfy a part of boundary conditions. We consider the influence of the inhomogeneity and initial stresses. We show that the initial stresses may significantly change critical deformations. For example, the loss of stability is possible due to initial stresses only.