•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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Titanium alloys have become the material of choice for marine parts manufacturing due to their high specific strength and excellent resistance to seawater corrosion. However, it is still challenging ...for a single titanium alloy to meet the comprehensive specifications of a structural component. In this study, we have applied a molecular dynamics approach to simulate the aging phase transformation, K-TIG welding process, and mechanical properties of the TC4-TA17 (Ti6Al4V-Ti4Al2V) alloy. The results show that during the aging phase transformation process, changes in the structure of the titanium alloys are mainly manifested in the precipitation of a new phase from the sub-stable β-phase, and after the state stabilization, the α-phase content reaches 45%. Moreover, during the melting and diffusion process of TC4-TA17, aluminum atoms near the interface diffuse, followed by titanium atoms, while relatively few vanadium atoms are involved in the diffusion. Finally, the results of tensile simulations of the TC4-TA17 alloy after welding showed that stress values can reach up to 9.07 GPa and that the mechanical properties of the alloy in the weld zone are better than those of the single alloys under the same conditions. This study will provide theoretical support for the optimization of process parameters for TC4-TA17 alloy welding.
•Dynamic cold expansion (DCE) based on electromagnetic load was proposed.•DCE showed lower resistance than static cold expansion (SCE).•DCE samples show evener hole diameter than SCE samples along ...axial direction.•DCE specimens exhibited more fatigue life improvement than SCE specimens.•DCE specimens show different crack propagation compared to SCE specimens.
In this study, a novel dynamic cold expansion (DCE) method based on electromagnetic load was developed to enhance the fatigue life of open hole structures. The experiments were conducted to investigate the enhance properties of DCE method, and the corresponding static cold expansion (SCE) method was also investigated as comparison. The expansion resistance was evaluated. The geometric dimension varieties of holes after cold expansion were measured. Moreover, the fatigue life of specimens after DCE and SCE were assessed, and the fatigue failure mechanism was discussed. Results showed that DCE method can obviously reduce expansion resistance compared with SCE method and generate more uniform hole diameter distribution along the hole axial direction after cold expansion. In fatigue tests, specimens worked by DCE exhibited the higher fatigue life than that worked by SCE, especially under high load level. Furthermore, DCE method was also observed to have significant effects on fatigue crack initiation and propagation of specimens.
CFRP/Ti pinned joints are widely applied on main load-bearing structures of aircrafts. Understanding the dynamic behavior of the joints is important for optimal joint designs. Therefore, a dynamic ...test platform based on electromagnetic loading method was developed to investigate the dynamic behavior of CFRP/Ti single-lap pinned joints under tensile dynamic loading. Bearing and failure loads of the joints were evaluated, and the dynamic failure mechanism was discussed. The test results demonstrated that both bearing and failure loads decreased with impact velocity, which is consistent with the literature. Failure modes of the joints were dominated by CFRP failure. High-rate joints failed first in bearing failure mode, but ultimately experienced tearing-out failure. Moreover, much fiber and matrix fracture was observed during dynamic loading progress. A study of interference size also revealed a correlation with the dynamic behavior of the joints, which will help to optimize joint designs.
Interference fit has advantages in improving fatigue behaviors of composite bolted joints; however, interference fit bolt insertion tends to cause damages in laminates weakening joint mechanical ...properties. Therefore, an experimental study was conducted to investigate bolt insertion damages of Carbon Fiber Reinforced Polymer (CFRP)/CFRP interference fit bolted joints. Mechanical behaviors of joints were also evaluated experimentally under both quasi-static loads and cyclic loads. Scanning Electron Microscope (SEM) and high-resolution X-ray micro-CT scan were used to examine micro damages in laminates. Damage and failure behaviors of joints were characterized. The results demonstrated that the hole entrance in upper laminate and the laminate boundary near the hole wall were the most critical regions for damages during bolt insertions. However, the influence of those damages on quasi-static failure loads and fatigue failure modes of joints was minimal. Delamination and matrix cracking occurred first in laminates following fiber and matrix fracture in quasi-static tensile tests. Interference fit could improve the fatigue resistance of the laminate hole; however, the bolt seemed to suffer a more critical local fatigue loading condition. This paper can contribute to composite structure designs, especially in understanding damage and failure behaviors of composite bolted joints.
Low‐contents/absence of non‐electrochemical activity binders, conductive additives, and current collectors are a concern for improving lithium‐ion batteries' fast charging/discharging performance and ...developing free‐standing electrodes in the aspects of flexible/wearable electronic devices. Herein, a simple yet powerful fabricating method for the massive production of mono‐dispersed ultra‐long single‐walled carbon nanotubes (SWCNTs) in N‐methyl‐2‐pyrrolidone solution, benefiting from the electrostatic dipole interaction and steric hindrance of dispersant molecules, is reported. These SWCNTs form a highly efficient conductive network to firmly fix LiFePO4 (LFP) particles in the electrode at low contents of 0.5 wt% as conductive additives. The binder‐free LFP/SWCNT cathode delivers a superior rate capacity of 161.5 mAh g−1 at 0.5 C and 130.2 mAh g−1 at 5 C, with a high‐rate capacity retention of 87.4% after 200 cycles at 2 C. The self‐supporting LFP/SWCNT cathode shows excellent mechanical properties, which can withstand at least 7.2 MPa stress and 5% strain, allowing the fabrication of high mass loading electrodes with thicknesses up to 39.1 mg cm−2. Such self‐supporting electrodes display conductivities up to 1197 S m−1 and low charge‐transfer resistance of 40.53 Ω, allowing fast charge delivery and enabling near‐theoretical specific capacities.
A simple yet powerful fabricating method for the massive production of mono‐dispersed ultra‐long single‐walled carbon nanotubes (SWCNTs) in N‐methyl‐2‐pyrrolidone solution, benefiting from the electrostatic dipole interaction and steric hindrance of dispersant molecules, is reported. These SWCNTs form a highly efficient conductive network to firmly fix LiFePO4 particles in the electrode at low contents of 0.5 wt% as conductive additives.
Interference strengthening technology is a common method of improving the fatigue life of metal assemblies, and is particularly used to prolong service life of metal structures. Composite materials ...have lower interlaminar strength and lower stretch ratio making it difficult to implement interference-fit joining, especially interference-fit riveting. The primary difficulty is controlling the interference to prevent damaging to the composite structure. The expansion of the rivet shaft by conventional riveting techniques is non-uniform and easy to damage the composite structure. Hence, interference-fit riveting is not recommended in composite structures. However, experimental results have shown that interference strengthening technology can be effectual in composite structures. The paper describes an electromagnetic riveting (EMR) technique which can create a well-distributed shaft expansion, and can be used to conduct interference-fit riveting in composites structures. This paper gives out that using an especial washer and appropriate clearance between shaft and aperture wall can restrict shaft expansion and prevent damaging to composites, and also studies the configuration of the washer and the clearance of riveting composite laminates. The paper also reports on an investigation involving a comparison of shaft expansion between hydraulic squeeze riveting and EMR. The experiments show that EMR technique can achieve an interference-fit riveting in fiber composite structures.
With the increasing applications of novel materials and structures in new-generation aircraft, conventional joining techniques in aircraft component assembly are greatly challenged. To meet those ...challenges, the electromagnetic riveting (EMR) technique was developed as an advanced joining tool, which exhibits obvious advantages in the assembly of new-generation aircraft. In this paper, the riveting principle of EMR was analyzed, and its development history and status were presented in detail. Then, equipment features of three typical EMR systems were given. Moreover, three important applications of EMR were covered, i.e., composite structure riveting, titanium rivet and large-size aluminum rivet riveting, and interference fit bolt installation. Specially, a novel strengthening method for mechanical linking holes based on EMR was also presented, which can significantly improve the fatigue behaviors of mechanical joints. Finally, open questions in the EMR field were discussed, and some recommendations for future work were also made. This paper can be useful for optimizing the joint designs of aircraft components and improving the level of aircraft maintenance.
In this paper, Tungsten Inert Gas Welding (TIG) and single-sided welding and double-sided forming have been used to weld the 7A05-T6/5A06-O dissimilar aluminum alloy circular welded joint of a ...ring-stiffened closed cylindrical sandwich shell. Microstructural characterization and mechanical properties of welded joints were investigated by use of scanning electron microscopy (SEM), backscatter electron diffraction (EBSD) and transmission electron microscopy (TEM), respectively. Hardness distribution and tensile properties of the welded joints were examined. The results showed the failure of the welded joints produced in the fusion zone (FZ). The tensile strength and yield strength of the welded joints were, respectively, 78.87% and 97.24% of the 5A06-O base metal (BM), and the elongation reached 84.29% of 7A05-T6 base metal. Welding high heat input led to the coarse grain size in the fusion zone, and the long-term similar quenching effect can lead to the full dissolution of the strengthening zone of the fusion zone resulting in the reductions of strength and hardness. Around 7A05 heat-affected zone (HAZ), there is an obvious hardening zone and softening zone, the solid solution precipitates into the Rayleigh brilliant η′ (MgZn₂) phase, which results in natural aging strengthening, thus obtaining high hardness. η′ (MgZn₂) enhanced phase dissolved fully and the dislocation density decreased rapidly in the HAZ region resulted in a softening zone with a lower hardness at about (10⁻18) mm from the center of the weld center.
•Dynamic installation method (DI) of bolt in interference-fit composite structure based on electromagnetic load was proposed.•DI method significantly improves the topography characteristics of the ...bolt-hole contact interface.•The mechanism of lower interference installation damage of DI method was investigated.•DI specimens show more favorable stress distribution for improved joint life compared to SI specimens.
Dynamic installation (DI) of composite fasteners can reduce interface damage caused by interference and further improve joint performance. Understanding the low-damage mechanisms during DI based on electromagnetic force is critical. In this paper, a 3-D nonlinear progressive damage model based on continuum damage mechanics (CDM) was developed to predict the geometric dimensional variations of the bolt, the damage behavior and stress distribution of the bolt-hole contact interface after interference-fit connection. Static installation (SI) analysis was also considered as a comparative study. Experimental bolt-hole interface damage behavior validated the FEA results. The results show that DI can significantly reduce the installation resistance by thinning and elongating the bolt, so as to remarkably reduce or avoid severe damage to composite laminates, especially for large interference sizes. The low installation damage of the DI specimens results in more uniform residual stresses and higher amplitudes at the hole edges, which may be more conducive to achieving a more uniform stress distribution in the joints under external cyclic loading, thus helping to reduce the incidence of failure and extend the operating horizons of structures.
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