The main objective of the present numerical analysis is to predict the nonlinear frequency ratios associated with the nonlinear free vibration response of porous composite plates at microscale in the ...presence of different microstructural gradient tensors. To achieve this end, by taking cubic-type elements into account, isogeometric models of porous composite microplates are obtained with and without a central cutout and relevant to various porosity patterns of distribution along the plate thickness. The established unconventional models have the capability to capture the effects of various unconventional gradient tensors continuity on the basis of a refined shear deformable plate formulation. For the simply supported microsized uniform porous functionally graded material (U-PFGM) plate having the oscillation amplitude equal to the plate thickness, it is revealed that the rotation gradient tensor causes to reduce the frequency ratio about 0.73%, the dilatation gradient tensor causes to reduce it about 1.93%, and the deviatoric stretch gradient tensor leads to a decrease of it about 5.19%. On the other hand, for the clamped microsized U-PFGM plate having the oscillation amplitude equal to the plate thickness, these percentages are equal to 0.62%, 1.64%, and 4.40%, respectively. Accordingly, it is found that by changing the boundary conditions from clamped to simply supported, the effect of microsize on the reduction of frequency ratio decreases a bit.
•Detailed experimental failure analysis of single-lap thin-ply composite bolted joint is conducted.•The interrupted observation of failed specimens is carried out by X-ray micro-CT and SEM.•The ...bearing failure modes of the thin-ply laminates are similar to the conventional thickness composite. Furthermore, the fiber-matrix splitting is more serious although the delamination of the cross-section is near suppression.•Mechanical test results show that the thin-ply laminate bolted joint has higher bearing capacity and longer progressive damage stages compared with the traditional thickness composite joints.
Composite bolted joints are widely used on primary and secondary load-bearing structures of aircrafts. However, investigating the damage progression and failure of composite bolted joints under high bearing loads is challenging due to the geometric, contact and material nonlinearities. In this work, an extensive experimental study has been carried out to investigate and understand the damage evolution and failure of single-lap thin-ply laminated composites bolted joints under quasi-static loading. Quasi-isotropic carbon/epoxy laminates with stacking sequence 45/0/-45/904s were selected for fabricating the test specimen. The specimens were observed using X-ray computed tomography (CT) scanning and SEM imaging at different stages of the loading process to evaluate internal damage and deformation characteristics. The results indicate that the bearing failure of composite bolted joints can be interpreted as an accumulated damage process with local compressing, and mainly includes four stages: damage initiation, damage evolution, non-linear softening and catastrophic failure. The major failure modes of the thin-ply laminates are found to be similar to those of traditional thickness composite, including fiber breakage, matrix cracking, delamination, fiber kinking and fiber-matrix splitting. However, the major difference is that the delamination growth in the bearing area in thin-ply composites is suppressed compared with the traditional thickness composites. Therefore, the obtained experimental data provides valuable information for developing mechanism-based failure models of single-lap thin-ply composite bolted joints.
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CFRP/Ti bolted joints are increasingly used in aircraft structures. Optimizing the joint design is vital for overall composite structure designs. Therefore, a progressive damage model was developed ...for investigating the effects of clearance and interference sizes on the damage and failure of CFRP/Ti double-lap, single-bolt joints under quasi-static loads, in which the improved three dimensional Hashin failure criterion and Tan degradation rules were used through an ABAQUS user-define-field (USDFLD) subroutine. The corresponding quasi-static tensile tests and fatigue tests were also conducted. Joints strength were evaluated and failure mechanism was discussed. Numerical results showed that the matrix compression failure dominated the joint failure mode. Joint ultimate strength decreased gradually with the increase of clearance sizes, while joint bearing strength and stiffness exhibited an increase with interference sizes at first and then decreased rapidly due to the initial installation damage. Moreover, the maximum strength was achieved at the interference size of 0.5%. Those results were in well agreement with corresponding experimental results. In addition, interference sizes were also revealed a correlation with the fatigue life of the joints. The study presented here will be useful for optimization of composite structure designs.
As an advanced manufacturing technology that has been developed in recent years, three-dimensional (3D) printing of macromolecular materials can create complex-shaped components that cannot be ...realized by traditional processing. However, only a few types of macromolecular materials are suitable for 3D printing: the structure must have a single function, and manufacturing macromolecular functional devices is difficult. In this study, using poly lactic acid (PLA) as a matrix, conductive composites were prepared by adding various contents of multi-walled carbon nanotubes (MWCNTs). The printability and properties of MWCNT/PLA composites with different MWCNT proportions were studied by using the fused deposition modeling (FDM) processing technology of 3D printing. The experimental results showed that high conductivity can be realized in 3D-printed products with a composite material containing 5% MWCNTs; its conductivity was 0.4 ± 0.2 S/cm, its tensile strength was 78.4 ± 12.4 MPa, and its elongation at break was 94.4% ± 14.3%. It had a good melt flow rate and thermal properties, and it enabled smooth printing, thus meeting all the requirements for the 3D printing of consumables.
The rational pre-stretching can contribute to obtaining better mechanical properties. This paper studies the effect of creep stain, mechanical properties, and microstructures of 7055 alloy under ...different pre-stretching conditions. The results show that compared with solid-quenched alloy, the 7055-T6 alloy is the optimal scheme to attain more creep strain, and the range of pre-stretching from 1.6% to 3.3% is suitable for creep-aged 7055-T6 alloy to obtain better mechanical properties. Further examination by TEM test shows that pre-stretching promotes the formation of dislocations, which provides superior nucleation regions for ή phase resulting in a higher strength alloy. Meanwhile, a unified creep-aging constitutive model for 7055-T6 alloy is established which can be used to accurately predict its creep behavior under the different pre-stretching.
Thin-ply laminated composites have recently gained increasing attention in the aerospace engineering field, due to enhanced design possibilities and positive size effects with regard to decreasing ...ply thickness. In this paper, the mechanical behavior of thin-ply laminate and TC21 titanium alloy hybrid joints with double-lap bolted structure under quasi-static loading was studied experimentally. X-ray computed tomography (CT) scanning and SEM microcosmic imaging at special stages of the whole loading process were used to clarify the progression of bearing damage and fastening hole deformation characteristics of laminates. A more gradual damage and failure in bearing was observed in all specimens from the stress-displacement curves. The final failure of hybrid joints is a combined mode of bolt tensile cracking and hole bearing deformation. More importantly, the common delamination of standard-ply composite laminated plates in the bearing failure plane and tensile failure plane was suppressed in thin-ply laminates, resulting in higher bearing failure strength and more damage accumulation. Therefore, the novelty of this work contributes to detailed experimental observations of the bearing failure mechanisms, which provide valuable information for developing accurate mechanism-based failure models to be used in simulations.
In the present study, the isogeometric numerical solving process incorporating non-uniform rational B-splines is put to use to analyze the size-dependent thermal postbuckling behavior of porous ...functionally graded (FG) microplates having a central cutout with different shapes. Accordingly, the modified couple stress continuum elasticity is employed within the framework of a hybrid-type quasi-3D higher-order plate theory to take the through-thickness deformations into consideration by only four variables. On the basis of a refined power-law function together with the Touloukian scheme, the porosity-dependent as well as temperature-dependent material properties are achieved. The couple stress-based thermal postbuckling equilibrium paths are acquired corresponding to various geometrical and material parameters and different boundary conditions. It is found that the gap between thermal postbuckling equilibrium paths relevant to various patterns of the porosity dispersion is a bit higher for the couple stress-based case than the classical one. Furthermore, it is indicated that a central cutout causes to change the trend of the load-deflection response that leads to decrease the initial thermal postbuckling strength, while it enhances the microplate strength in deep thermal postbuckling region.
Composite bolted joints are being used extensively in primary load-bearing structures of modern aircrafts. However, simulating progressive damage and failure of the bolted composite structures under ...high bearing loading is still challenging. In this paper, numerical simulation and experimental verification on the bearing failure of single-lap and double-lap thin-ply laminated composite bolted joints were carried out for the entire loading process. 3D explicit finite element models have been developed using Abaqus/Explicit together with a 3D physically-based intralaminar damage model implemented in a VUMAT subroutine. An interface cohesive-zone model based on the Benzeggagh-Kenane (B-K) law was used to simulate delamination initiation and evolution. For the intralaminar damage, the initiation was determined based on Pinho’s failure criteria while the evolution was regularized with the crack-band approach to alleviate mesh-size dependence. Element deletion and in-situ effect as practical numerical strategy are discussed and analyzed in detail. Element deletion is used to prevent excessive element distortion and capture corresponding post-peak bearing failure behavior. In-situ effect has been proved to have a key influence on the damage initiation and progression of thin-ply composite matrix. The final simulation results are shown to agree well with experimental data and reproduce the mechanical response curves. More importantly, the model can accurately capture the local crushing of bolt hole in the later loading stage. This study shows that the numerical model can be helpful for the design and optimization of composite bolted structures.
The phenomenon of high-temperature oxidation in magnesium alloys constitutes a significant obstacle to their application in the aerospace field. However, the incorporation of active elements such as ...alloys and rare earth elements into magnesium alloys alters the organization and properties of the oxide film, resulting in an enhancement of their antioxidation capabilities. This paper comprehensively reviews the impact of alloying elements, solubility, intermetallic compounds (second phase), and multiple rare earth elements on the antioxidation and flame-retardant effects of magnesium alloys. The research progress of flame-retardant magnesium alloys containing multiple rare earth elements is summarized from two aspects: the oxide film and the matrix structure. Additionally, the existing flame-retardancy models for magnesium alloys and the flame-retardant mechanisms of various flame-retardant elements are discussed. The results indicate that the oxidation of rare earth magnesium alloys is a complex process determined by internal properties such as the structure and properties of the oxide film, the type and amount of rare earth elements added, the proportion of multiple rare earth elements, synergistic element effects, as well as external properties like heat treatment, oxygen concentration, and partial pressure. Finally, some issues in the development of multi-rare earth magnesium alloys are raised and the potential directions for the future development of rare earth flame-retardant magnesium alloys are discussed. This paper aims to promote an understanding of the oxidation behavior of flame-retardant magnesium alloys and provide references for the development of rare earth flame-retardant magnesium alloys with excellent comprehensive performance.
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