In order to solve the forming problems such as difficult deformation and easy cracking of high strength steel square tubes, the local induction heating technology has been introduced based on the traditional cold forming process. An electromagnetic-temperature multi-field coupling model for local induction heating is established by using ANSYS parametric design language based on the magnetic vector potential and physical environment method. A numerical simulation is conducted at different heating process parameters, and the optimized parameters are used for experimental research. The simulation results show that the induction heating efficiency can be significantly improved by using magnetizers to increase the magnetic field intensity. As the heating frequency increase, the heating speed of the outer fillet area and the temperature difference between the outer and inner fillet area increase. As the heating power increases, the high temperature area and the peak temperature increase, but the outer fillet area is more prone to be overheated. The experimental results show that a high strength steel square tube with an extremely small fillet radius, increased corner thickness, but with no crack defect can be obtained by using the optimized heating process parameters. The average error between the simulated and measured heating temperature is about 7.57%, indicating that the finite element model has a good prediction accuracy.
In order to further improve the strength between carbon fiber reinforced thermoplastic (CFRTP) and lightweight metals, a novel ultrasonic self-fusion riveting method is proposed, in which carbon fiber reinforced polyamide 6(CF/PA6) is melted by ultrasonic welding and pressed into the prefabricated hole on the aluminum alloy plate to realize the join between CF/PA6 and aluminum alloy. The joining mechanism is mechanical self-locking. The results show that the overall mechanical performances increase with the increase of the number of holes. The optimal welding energy is 2 000 J and the maximum shear strength is (58.9 ± 7.1) MPa. According to the welding power and welding displacement signal, the ultrasonic self-fusion riveting process can be divided into the pressing stage, the energy director embedding stage, and the hole filling stage. When the energy is constant, with the increase of the number of holes, the energy director will be embedded into CF/PA6 earlier, and the welding time will be shortened. Compared with the welding energy, the number of holes has a greater impact on the welding process. Because ultrasonic self-fusion riveting mainly realizes the join between CFRTP and metal through mechanical self-locking, this method is not limited by metal types and has a broad application prospect.
To explore the potential of attapulgite as thermal barrier materials, bulk attapulgite samples were prepared by pressureless sintering. The effects of sintering temperature on the phase composition, porosity, microstructure, and thermal conductivity of bulk attapulgite were investigated. With increasing sintering temperature, bulk attapulgite transforms from predominant quartz phase (700 ℃) to coexistence of quartz and enstatite phases (800—900 ℃), and to coexistence of quartz, enstatite and cristobalite phases (1000—1200 ℃). Meanwhile, the microstructure of the bulk attapulgite changes from random, loose packed fiber-like porous morphology, to dense structure with a random distribution of MgO·SiO2 grains inside the SiO2 matrix to result in a significant decrease in porosity. The thermal conductivity of bulk attapulgite increases with increasing temperature. When sintered at 700 ℃, bulk attapulgite presents a temperature-independent thermal conductivity with an ultra-low value of 0.16 W/(m·K) at room temperature. Attapulgite, with its natural abundance and low cost, along with the ultra-low thermal conductivity, has a great potential as thermal barrier materials.
Diamond has an extremely high thermal conductivity, making it to have a great potential as a heat dissipation material. Based on the hot filament chemical vapor deposition (HFCVD) technique, diamond thick films were deposited on silicon carbide substrates by using the multi-step method in this paper. The scanning electron microscopy (SEM) and Raman spectroscopy were adopted for characterizing the samples. The influences of filament power, carbon concentration, and reactive pressure on the growth rate and quality of the diamond films were systematically studied. It is found that the diamond film with the best quality is synthesized by adopting a filament power of 1 600 W, a methane/hydrogen flux ratio of 18/300 (nucleation stage) and 14/300 (growth stage), and a reactive pressure of 4 kPa. The corresponding growth rate is 1.4 μm/h.
In order to investigate the mechanical properties of uncured rubber, uniaxial and cyclic tensile experiments are conducted on uncured rubber at different strain rates. From the experimental results, the rate-dependent nonlinear mechanical behaviors of uncured rubber can be clearly observed. With strain rate increasing, the stress level, hysteresis and Mullins effect get enhanced, and the residual strain decreases. To characterize the nonlinear visco-elastoplastic mechanical behaviors of uncured rubber, a three-network (TN) constitutive model that contains a hyperelastic network and two nonlinear viscoplastic networks is proposed. The eight-chain model is used to characterize the hyperelastic behavior while the Bergstr?m-Boyce flow model is applied in the viscoplastic networks to capture the nonlinear viscous flow. The proposed constitutive model is implanted into the finite element software Abaqus with which, the multistep tensile relaxation test is simulated. The simulation result is satisfactorily consistent with experimental results, which verifies the effectiveness of the TN model. Finally, the simplified molding process of a tire tread is simulated, which further verifies the stability of the TN model.
As a high-performance thermoplastic composite material, AS4/PEEK has been widely used in aerospace, military, automotive, and other fields. After conducting the off-axial tensile test of unidirectional AS4/PEEK laminates with different angles, the relevant stress-strain curves and tensile strengths, as well as fracture plane angles are obtained. In simulation, a 3D elastic-plastic model where the parameters are determined by trust-region reflective algorithm is used to describe the nonlinear mechanical behavior of AS4/PEEK laminates. In combination with the LaRC05 criterion and the crack zone theory, a user material subroutine VUMAT based on Abaqus is developed and applied to the numerical simulation of off-axis tensile test. The numerical results show that the 3D elastic-plastic damage constitutive model can accurately simulate the plastic effect of AS4/PEEK laminates and the tensile strength predicted by the numerical method agrees well with those from the test. The proposed 3D elastic-plastic damage model provides an accurate and effective method for the comprehensive analysis of plastic deformation and damage of thermoplastic composites.
When encountering heavy rain, the membrane surface with a relatively small slope is easy to accumulate water. Accurately simulating the deformation of the membrane surface at this time will help ensure the safety of the structure. The linear constitutive model of membrane material used in existing research is not suitable for simulating the deformation of membrane structure in rainstorm. This study conducts a uniform load test on a membrane structure to simulate the mechanical behavior of the membrane surface in rainstorm and obtains the deformation form of the structure when water is accumulated. The linear constitutive model and the double broken line constitutive model of membrane material are used in the finite element model of the membrane structure for load analysis. By comparing the deformation of the finite element model and the actual structure, it selects the constitutive model suitable for simulating the deformation of the membrane structure in heavy rain. The numerical simulation results show that the structural deformation simulated by the double broken line constitutive model is closer to the deformation measured in the experiment than the linear constitutive model. The research results can provide a reference for the selection of the membrane constitutive model and the analysis of membrane structure in rainstorm.
To reveal the influence of material selection on joint properties in the joint design between carbon fiber reinforced polymer (CFRP) and steels, experiments of uniaxial tension and normal tension were performed on the bonded, riveted, and rivet-bonding joints, which were made of CFRP laminates and DC05, HC260Y, DP590, DP780, DP1180, and PHS1500 steel sheets. The failure modes and tensile strength of these joints were analyzed. The thickness ratio and bearing coefficient ratio of the CFRP parts and adjacent steel parts were summarized. The results show that the failure mode of the CFRP/steel joint depends on the mechanical properties of the material of the weaker side. During joint design, the strength and stiffness of the CFRP parts and adjacent parts should be as similar as possible. The thickness ratio of the CFRP parts and adjacent steel parts is recommended to be between 1.37 and 1.91, and the load bearing coefficient ratio is recommended to be between 0.9 and 1.52.
In order to further control the springback of advanced high-strength steel in stamping,a method to reduce the bending springback by increasing the variable closing pressure (VCP) through servo hydraulic equipment without increasing the material forming margin was proposed. For U-shaped benchmark parts of 1 180 MPa dual-phase (DP1180) steel formed by stamping, the feasibility of increasing VCP to control springback was analyzed by using the finite element method, and the mechanism of increasing VCP to reduce springback was studied from the perspective of residual stress and dislocation. The results show that the proposed method can reduce the residual stress and the dislocation density of the formed material, thus increasing the inelastic recovery and achieving the effect of reducing the springback angle of the target part by 22.3%. Because this method is only applied at the end of part forming, which does not affect the plastic flow in the material forming process, there is no need to increase the additional material forming ability and shaping process, which can be applied to actual industry without changing the production rhythm, and has a broad application prospect.
