Effect Mechanisms of Hygrothermal Environments on Failure of Single-Lap and Double-Lap CFRP-Aluminum Bolted Joints

The high demands for load-carrying capability and structural efficiency of composite-metal bolted joints trigger in-depth investigations on failure mechanisms of the joints in hygrothermal environments.However,few studies have been presented to exhaustively reveal hygrothermal effects on the failure of CFRP-metal bolted joints,which differ from CFRP-CFRP or metal-metal bolted joints because of the remarkably different material properties of CFRPs and metals.In this paper,hygrothermal effects on tensile failures of single-lap and double-lap CFRP-aluminum bolted joints were experimentally and numerically investigated.A novel numerical model,in which a hygrothermal-included progressive damage model of composites was established and elastic-plastic models of metals were built,was proposed to predict the failures of the CFRP-metal bolted joints in hygrothermal environments and validated by corresponding experiments.Different failure mechanisms of single-lap and double-lap CFRP-aluminum bolted joints,under 23°C/Dry and 70°C/Wet conditions,were revealed,respectively.It follows that both the collapse failures of the single-lap and double-lap bolted joints were dominated by the bearing failure of the CFRP hole laminate in the two conditions,indicating that the hygrothermal environment did not change the macro failure modes of the joints.However,the hygrothermal environment considerably shortened the damage propagation processes and reduced the strength of the joints.Besides,the hygrothermal environment weakened the load-transfer capability of the single-lap joint more severely than the double-lap joint because it aggravated the secondary bending effects of the single-lap joint obviously.

Bearing Failure Optimization of Composite Double-Lap Bolted Joints Based on a Three-Step Strategy Marked By Feasible Region Reduction and Model Decoupling

To minimize the mass and increase the bearing failure load of composite double-lap bolted joints,a three-step optimization strategy including feasible region reduction,optimization model decoupling and optimization was presented.In feasible region reduction,the dimensions of the feasible design region were reduced by selecting dominant design variables from numerous multilevel parameters by sensitivity analyses,and the feasible regions of variables were reduced by influence mechanism analyses.In model decoupling,the optimization model with a large number of variables was divided into various sub-models with fewer variables by variance analysis.In the third step,the optimization sub-models were solved one by one using a genetic algorithm,and the modified characteristic curve method was adopted as the failure prediction method.Based on the proposed optimization method,optimization of a double-lap single-bolt joint was performed using the ANSYS®code.The results show that the bearing failure load increased by 13.5%and that the mass decreased by 8.7%compared with those of the initial design of the joint,which validated the effectiveness of the three-step optimization strategy.

Model of CEL for 3D Elements in PDMs of Unidirectional Composite Structures

Progressive damage models(PDMs)have been increasingly used to simulate the failure process of composite material structures.To accurately simulate the damage in each ply,3D PDMs of composite materials have received more attention recently.A characteristic element length(CEL),which is an important dimensional parameter of PDMs for composite materials,is quite difficult to obtain for 3D elements,especially considering the crack directions during damage propagation.In this paper,CEL models for 3D elements in PDMs of unidirectional composite structures are presented,and their approximate formulae are deduced.The damage in unidirectional composite materials can be divided into fiber cracks and inter-fiber cracks.The fiber crack and inter-fiber crack directions are considered in the CEL derivations,and thus,the CELs of 3D elements that have various damage modes and damage directions could be obtained relatively precisely.Static tensile and compressive tests of open-hole laminates were conducted,and the corresponding numerical analyses by the progressive damage method,including the proposed CEL models and those models from the literature,were performed.The numerical results are in good agreement with the experimental results,which proves the fidelity and effectiveness of the proposed CEL models.In addition,the proposed CEL models have better performance in improving the mesh independence of the numerical models.

Current Situation and Countermeasures of Weather Modification Ground Operation in Heilongjiang Province

Heilongjiang Province is the granary of China,which plays a key role in ensuring the national food security.The total grain output of Heilongjiang Province has ranked first in China for 12 consecutive years.In the past four years,it has been stable at more than 75 billion kg,a record high.One bowl of rice in every nine bowls in China comes from Heilongjiang.The work of weather modification and disaster prevention and reduction is an important measure to ensure the development of agricultural production,and is the key of meteorological services for agriculture.Based on the actual work of artificial weather modification in Heilongjiang Province,this paper analyzes the current situation of ground operation in Heilongjiang Province,studies and judges the safety production,and puts forward reasonable countermeasures.The purpose is to improve the ground operation ability of artificial weather modification and provide safe and scientific services for agricultural production.

IPO创新承诺与企业创新

在推进国家创新驱动发展战略的背景下,资本市场如何影响企业创新是一个值得深究的学术话题。企业创新活动受其创新意愿和创新能力两个方面的影响,既有文献多围绕创新能力进行研究,鲜有文献关注创新意愿这一维度。本文从招股说明书中募集资金是否投向创新入手,考察IPO创新承诺对企业创新的影响。以A股市场2006~2017年IPO成功的企业为样本研究发现,IPO创新承诺能够显著促进企业创新。进一步,我们发现IPO创新承诺提高了企业过会概率,减弱了企业融资面临的不确定性,有利于企业保持创新活动的连贯性,进而推动创新,但这一效应仅在上市后的两年内显著,且无法实现高质量创新的持续。此外,我们还考察了IPO创新承诺的经济后果,研究发现,创新承诺可以降低IPO抑价幅度,并提升企业上市后的业绩。上述结果表明,尽管IPO创新承诺向市场传递出积极信号,但长期来看企业的创新表现并未得到延续,这意味着,IPO核准制在甄别企业创新能力方面缺乏效率。

Development of Fe-Mn-Si-Cr-Ni shape memory alloy with ultrahigh mechanical properties and large recovery strain by laser powder bed fusion

This work systematically studied the effect of volumetric energy density E on the densification,mi-crostructures,tensile mechanical properties,and shape memory performance of a Fe-Mn-Si-Cr-Ni shape memory alloy(SMA)fabricated by laser powder bed fusion(L-PBF).An E of 90-265 J/mm3 is suggested to fabricate the Fe-Mn-Si-Cr-Ni SMA with minor metallurgical defects and a high relative density of above 99%.The increase in E can promote the formation of the primaryγaustenite and the solid phase trans-formation from the primaryδferrite to theγaustenite,which helps to achieve a nearly complete y austenitic microstructure.The increase in E also contributes to fabricating the Fe-Mn-Si-Cr-Ni SMA with superior comprehensive mechanical properties and shape memory performance by L-PBF.The Fe-Mn-Si-Cr-Ni SMA with a combination of good ductility of around 30%,high yield strength of above 480 MPa,an ultrahigh ultimate tensile strength of above 1 GPa,and large recovery strain of about 6%was manu-factured by L-PBF under a high E of 222-250 J/mm^(3).The good shape memory effect,excellent compre-hensive mechanical properties,and low cost of Fe-Mn-Si-Cr-Ni SMAs,as well as the outstanding ability to fabricate complex structures of L-PBF technology,provide a solid foundation for the design and fabri-cation of novel intelligent structures.

Parameter studies and evaluation principles of delamination damage in laminated composites

Delamination represents one of the most severe failure modes in composite laminates,especially when they are subjected to uniaxial compression loads.The evaluation of the delamination damage has always been an essential issue of composite laminates for durability and damage tolerance in engineering practice.Focusing on the most typical and representative elliptical delamination issue,an analytical model simultaneously considering the conservative buckling process and non-conservative delamination propagation process is implemented.Various computational cases considering different delamination depths,directions,aspect ratios,and areas are established,and the predicted results based on the analytical model are carefully compared.Effects of these geometrical delamination parameters on the buckling,delamination propagation,and failure behaviors of composite laminates are thoroughly analyzed,and innovative evaluation principles of the delamination damage have been concluded.It is found that the delamination area is the key factor that truly affecting the failure behaviors of delaminated composites,and the local/global buckling and failure loads show clear linearity with the delamination area,whilst the delamination depth and direction only have slight effects.

Variation of crystal orientation during epitaxial growth of dendrites by laser deposition

A nickel-based superalloy was deposited onto a single crystal substrate based on epitaxial laser metal forming （E-LMF）. The microstructure development in two depositions has been researched. For the first time, the crystal orientation of dendrites varying beyond 20° was found when the dendrites deflected in deposition. In addition, a new grain boundary was found between different orientation dendrites in a grain, and the detected grain boundary angle was 23°. The result shows that flowing field in laser pool is responsible for this phenomenon.

Uncertainty evaluation for bearing fatigue property of CFRP double-lap,single-bolt joints

The considerable uncertainty in mechanical properties of composite bolted joints not only prevents advanced composite materials from efficient applications,but also threatens the safety and reliability of the aircraft structures.In this paper,the uncertainty in bearing fatigue properties of a CFRP double-lap,single-bolt joint was evaluated by combing a Progressive Fatigue Damage Model(PFDM)with the interval analysis method.In the PFDM,a residualstrain-based gradual material degradation model and a strain-based fatigue failure criterion were combined with a micromechanics-based sudden material degradation model to predict fatigue properties of the joint.Based on the interval analysis,the key uncertain parameters,which were firstly picked out from eighteen structural parameters of the joint,were described by estimated intervals,and the envelope cases were determined to estimate the lower and upper bounds of fatigue properties of the joint.The predicted results have the same tendency with the experimental results in literatures,which indicates that the PFDM combined with the interval analysis shows potential in efficiently evaluating the fatigue reliability of the complex bolted joints with an adequate accuracy.

原绿球藻对环境胁迫的生理和分子响应机制

原绿球藻(Prochlorococcus)作为海洋丰度最高的浮游植物,对海洋生态系统的物质循环和能量流动起着重要的驱动作用。原绿球藻生长和光合作用活性容易受到环境胁迫的影响,进而影响整个海洋生态系统的稳定性。因此,研究原绿球藻应对环境胁迫的响应机制具有重要的生态意义。原绿球藻主要通过分化出不同的生态型来适应不同光照和营养盐的海洋环境,但仍然会很难快速适应各种突如其来的海洋环境变化。本文从原绿球藻应对环境胁迫的角度,探讨了其生理和分子响应机制的最新研究进展,包括光系统I循环电子传递在光照变化时发挥的重要作用,通过RNA快速响应而调控基因表达应对环境胁迫,以及在辅助异养细菌的保护下应对活性氧的胁迫等。本文也展望了原绿球藻对环境胁迫响应的生理和分子机制的未来研究方向,旨在为原绿球藻抗逆机制的深入研究提供参考。

Unveiling exceptional sinterability of ultrafine a-Al2O_(3) nanopowders

Scalable pressureless sintering of nanocrystalline alumina(Al2O_(3))ceramics is a challenging problem with great scientific and technological interest.This challenge was addressed in our recent works utilizing ultrafine a-Al2O_(3) nanopowders with exceptional sinterability combined with two-step sintering technique.Here the sintering mechanism and kinetic parameters(grain boundary diffusivity and its activation energy)were analyzed from constant heating-rate sintering experiments by three different sintering models and compared with existing sintering data in the literature.We found that the lowtemperature sintering of 4.7 nm a-Al2O_(3) nanopowders can be well explained by conventional sintering mechanism via grain boundary diffusion,with reasonable activation energy of 4e5 eV that is smaller than that of coarse Al2O_(3) powders and enhanced diffusivity.However,unphysically small activation energy could be obtained if an inappropriate model was used.Lastly,successful two-step sintering was demonstrated under different heating rates.Our work illustrates that the exceptional sinterability of ultrafine a-Al2O_(3) nanopowders are most likely contributed by small size(short diffusion distance),large surface area(large sintering driving force)and good dispersity rather than new sintering mechanism,and highlights the importance of fast firing and the non-equilibrium nature for the low-temperature sintering of such nanopowders.