Effect of hygrothermal aging on moisture diffusion and tensile behavior of CFRP composite laminates

Carbon Fiber Reinforced Polymer (CFRP) composites are widely used in aircraft structures, because of their superior mechanical and lightweight properties. CFRP composites are often exposed to hygrothermal environments in service. Temperature and moisture can affect the material properties of composites. In order to make clear the moisture diffusion behavior and the properties degradation of composites, the TG800/E207 composite laminates with four stacking sequences [0]₁₆, [90]₁₆, [±45]_4s, and [(+45/0/0/–45)_s]_s are designed and manufactured. Moisture absorption tests are carried out at 80 ℃, 90 %RH. It is shown that the moisture absorption curves of composite laminates present a three-stage. A modified Fickian model was proposed to capture the diffusion behavior of TG800/E207 composite laminates. The relationships among the non-Fickian parameters, the environmental parameters and the stacking sequences of CFRP were correlated and compared. Results showed that the modified Fickian curve is sensitive to the diffusivity of Stage I and Stage II. Compared with unaged specimens, the maximum tensile stress for [0]₁₆, [90]₁₆, [±45]_4s, and [(+45/0/0/–45)_s]_s decreased by 14.94 %, 28.15 %, 11.96 %, and 26.36 %, respectively. The strains at failure for [0]₁₆, [90]₁₆, [±45]_4s, and [(+45/0/0/–45)_s]_s decreased by 55.38 %, 62.65 %, 46.41 %, and 31.71 %, respectively. The elastic modulus for [0]₁₆, [90]₁₆, [±45]_4s, and [(+45/0/0/–45)_s]_s increased by 90.93 %, 94.57 %, 49.22 %, and 8.22 %, respectively. [90]₁₆ sample has the minimum saturated moisture content and the maximum strength degeneration.     

Microstructural formation and mechanical performance of friction stir double-riveting welded Al-Cu joints

A novel friction stir double-riveting welding (FSDRW) technology was proposed in order to realize the high-quality joining of upper aluminum (Al) and lower copper (Cu) plates, and this technology employed a Cu column as a rivet and a specially designed welding tool with a large concave-angle shoulder. The formations, interfacial characteristics, mechanical properties and fracture features of Al/Cu FSDRW joints under different rotational velocities and dwell times were investigated. The results showed that the well-formed FSDRW joint was successfully obtained. The cylindrical Cu column was transformed into a double riveting heads structure with a Cu anchor at the top and an Al anchor at the bottom, thereby providing an excellent mechanical interlocking. The defect-free Cu/Cu interface was formed at the lap interface due to the sufficient metallurgical bonding between the Cu column and the Cu plate, thereby effectively inhibiting the propagation of crack from the intermetallic compound layer at the lap interface between the Al and Cu plates. The tensile shear load of joint was increased first and then decreased when the rotational velocity and dwell time of welding tool increased, and the maximum value was 5.52 kN. The FSDRW joint presented a mixed mode of ductile and brittle fractures.     

Recent advances and trends in roll bonding process and bonding model: A review

This review presents a thorough survey of the roll bonding process with a focus on the bimetallic bars/tubes as well as the bonding models and criteria. The review aims to provide insight into cold, hot and cryogenic bonding mechanisms at the micro and atomic scale and act as a guide for researchers working on roll bonding, other joining processes and bonding simulation. Meanwhile, the shortcomings of roll bonding processes are presented from the aspect of formable shapes, while bonding models are shown from the aspect of calculation time, convergence, interface behavior of dissimilar materials as well as hot bonding status prediction. Two well-accepted numerical methodologies of bonding models, namely the contact algorithm and cohesive zone model (CZM) of bonding models and in simulations of the bonding process are highlighted. Particularly, recent advances and trends in the application of the combination of mechanical interlocking and metallurgical bonding, special energy fields, gradient structure, novel materials, green technology and soft computing method in the roll bonding process are also discussed. The challenges for advancing and prospects of the roll bonding process and bonding model are presented in an attempt to shed some light on the future research direction.     

Aspects related to variability of radiative cooling by NO in lower thermosphere,TEC and O/N2 correlation,and diffusion of NO into mesosphere during the Halloween storms

Nitric Oxide is a very important trace species which plays a significant role acting as a natural thermostat in Earth’s thermosphere during strong geomagnetic activity. In this paper, we present various aspects related to the variation in the NO Infrared radiative flux (IRF) exiting the thermosphere by utilizing the TIMED/SABER (Thermosphere Ionosphere Mesosphere Energetics and Dynamics/ Sounding of the Atmosphere using Broadband Emission Radiometry) observational data during the Halloween storm which occurred in late October 2003. The Halloween storm comprised of three intense-geomagnetic storms. The variability of NO infrared flux during these storm events and its connection to the strength of the geomagnetic storms were found to be different in contrast to similar super storms. The connection between the quantum of energy outflux from the upper atmosphere into space in terms of NO IRF and the duration of storms is established. The NO radiative cooling, and the closely correlated depletion in O/N₂ ratio are controlled by the Joule heating intensity (proxied by AE-index). The collisional excitation rate of NO, calculated using the modelled datasets of WACCM-X (Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension), correlates well with the observed pattern of radiative emission by NO. Observational datasets from TIMED/GUVI (Global Ultra-Violet Imager) and MIT Haystack observatory madrigal GNSS (Global navigation satellite system) total electron content (TEC) database shows that the TEC and O/N₂ enhancement in low-mid northern hemispheric latitudes are mainly controlled by the z-component of Interplanetary magnetic field (IMF-B_z). The penetration of eastward electric field during the storm events is found to be responsible for the overall enhancement of TEC. The contribution of enhanced day-side TEC in observed variation of O/N₂ ratio by GUVI is also reported. It is also seen that during substorms related events the night-time polar region experiences more cooling due to NO than the daytime polar region. The connections between the mid- and low-latitude enhancement in NO IRF with the propagation of LSTIDs (Large-scale traveling ionospheric disturbances) in combination with the O/N₂ variability, and the altitudinal variation in NO flux with the progression of the storm is also investigated. This study presents the evidence on the role of diffusion processes in the large scale enhancement of NO in the mesospheric altitudes.     

Hot deformation behavior and strain compensation constitutive model of equiaxed fine grain diffusion-welded micro-duplex TC4 titanium alloy

In this work, two-stage diffusion bonding of micro-duplex TC4 titanium alloy was carried out to study the flow behavior and constitutive models of the bonding joint and the base metal after the same thermal cycling during the hot forming process. Microstructure and mechanical properties test were used to verify the good quality of the equiaxed fine grain diffusion-welded TC4 alloy. Quasi-static tensile experiment was carried out at temperatures ranging from 750–900 °C and strain rates of 0.0001–0.1 s⁻¹. The joint showed the weak dynamic recovery at strain rates of 0.01–0.1 s⁻¹ and temperatures of 750–850 °C. At strain rates of 0.0001–0.001 s⁻¹ and temperatures of 850–900 °C, the flow stress of joint presented steady-state characteristics. Different deformation conditions lead to the remarkable difference of dynamic softening performance between the joint and heat-treated base metal, but the flow stress in elastic and strain hardening stages exhibited similar behavior. The strain compensated Arrhenius-type constitutive models of TC4 joint and heat-treated base metal were developed respectively. The fifth-order polynomial functions between the material property correlation coefficients and strain were obtained. The models have shown good correlation, with correlation coefficient values of 0.984 and 0.99. The percentage average absolute relative error for the models were found to be 10% and 9.46%, respectively.     

燃料电池接触电阻与温度相关性研究

研究双极板与气体扩散层接触电阻的形成机理，分析影响该接触电阻的因素，包括温度、载荷以及粗糙度等参数。通过建立数学模型以及利用仿真工具来预测接触电阻数值与变化规律，并进行金属双极板和碳纸气体扩散层的界面接触电阻测试用于模型验证，模型计算结果与实验结果吻合较好。