疲劳裂纹扩展门槛值的研究进展

Ⅰ.引言 大量的研究和疲劳裂纹扩展的试验表明,对于存在一定尺寸裂纹及缺陷的材料或构件,只有当裂纹尖端的应力强度因子达到或超过某一值时,裂纹才会在交变应力的作用下扩展。当裂纹尖端的应力强度因子小于这一值时,裂纹在交变应力作用下不发生扩展。这个应力强度因子值,就是界限应力强度因予幅值△K_(th),在疲劳研究中称为裂纹扩展的门槛值。 门槛值△K_(th)和疲劳裂纹扩展速率da/dN一样,是反映带裂纹或缺陷构件抗疲劳性能的     

平均应力对疲劳曲线的影响

本文通过试验和计算以及用有效应力参数法研究了平均应力对疲劳曲线的影响,用计算机算出国产LY12CZ 材料的有效应力参数公式中的常数m.1.有关疲劳曲线及其影响因素表示应力振幅S_α或最大应力S_(max)与疲劳寿命N     

The mean first passage time of a three-level atomic optical bistable system subjected to noise

The transient properties of a three-level atomic optical bistable system in the presence of multiplicative and additive noises are investigated. The explicit expressions of the mean first-passage time (MFPT) of the transition from the high intracavity intensity state to the low one are obtained by numerical computations. The impacts of the intensities of the multiplicative noise D M and the additive noise D A , the intensity of correlation between two noises λ , and the intensity of the incident light y on the MFPT are discussed, respectively. Our results show: (i) for the case of no correlation between two noises (λ = 0.0), the increase in D M and D A can lead to an increase in the probability of the transition to the low intracavity intensity state, while the increase in y can lead to a retardation of the transition; and (ii) for the case of correlation between two noises (λ = 0.0), the increase in λ can cause an increase in the probability of the transition, and the increase in D A can cause a retardation of the transition firstly and then an increase in the probability of the transition, i.e., the noise-enhanced stability is observed for the case of correlation between two noises.     

海洋平台结构中的疲劳问题

近三十年来,人们将注意力逐渐转向海洋,因为人们认识到海洋不仅是海上交通和军事活动的重要场所,而且海洋还蕴藏着巨大的资源,包括数以千亿吨计的石油资源和矿物资源。世界上许多国家都投入大量的人力和物力进行大规模的海洋开发。70年代以来,海洋科学技术获得了迅速的发展,相信不久,人类将进入海洋开发的新时代。      

表面催化反应模型中关联噪声诱导非平衡相变

建立含有关联噪声的双分子-单分子(DM)表面催化反应延迟反馈模型,该模型能同时显示一级和二级非平衡动力学相变,即在一级和二级非平衡动力学相变之间的反应窗口展现.讨论双分子在DM延迟反馈模型中两种吸附机制,即局域和随机吸附模型.研究结果表明:1)外部噪声及两噪声关联性致使反应窗口的宽度收缩;2)内部噪声对非平衡动力学相变行为的影响依赖两噪声关联性,即当两噪声负关联,内部噪声致使反应窗口的宽度变宽;而当两噪声正关联时,内部噪声致使反应窗口的宽度收缩;3)关联噪声致使反应窗口变化对DM模型中一级和二级非平衡动力学相变研究具有重要的科学意义.     

无机非铅钙钛矿Cs3Bi2I9的电子和光学性质

有机-无机卤化钙钛矿材料因具有优异的光电性质而被广泛应用于太阳电池中,然而材料及器件的稳定性及含铅问题却严重制约其生产发展.与杂化钙钛矿相比,无机非铅钙钛矿Cs₃Bi₂I₉因具有更强的稳定性和环境友好性受到人们的广泛关注.Cs₃Bi₂I₉具有单斜、三角和六方3种晶型,目前,对Cs₃Bi₂I₉的理论和实验研究主要集中在六方相.本文基于密度泛函理论的第一性原理对Cs₃Bi₂I₉单斜、三角和六方相的电子性质、载流子有效质量(m~*)、稳定性和光学性质进行了理论研究.结果表明,3种晶相具有相近的稳定性,三角相因具有较小的直接带隙(1.21 eV)性质成为最具研究潜力的对象.3种晶相的m~*均具有沿a,b方向相同和沿c方向不同的特点,三角相的电子有效质量最小、且沿a方向的电子有效质量小于c方向.相比单斜和六方相,三角相Cs_3<...     

新型二维铜族硫族化合物的研究进展

二维材料因其纳米级尺寸、超凡的电子特性、出色的机械性能等,被视为下一代信息器件的理想材料。随着外延生长等自下而上制备方法以及材料计算方法的发展和提高,越来越多的体相非层状的二维材料被制备和发现。近年来在信息器件等领域具有应用潜力的体相非层状的二维铜族(11族)硫族化合物得到广泛关注,铜族元素位于化学元素周期表中的11族,主要包括铜、银和金等元素。本文综述了二维铜族硫族化合物的研究进展,以化学计量比为主线,将二维铜族硫族化合物分为MX单层(M=Cu,Ag,Au;X=O,S,Se,Te)、M₂X单层以及其他化学计量比的铜族硫族化合物单层。从实验合成和理论模拟两个角度出发,对二维铜族硫族化合物原子结构、能带结构、电学特性等进行介绍。从现有的报道来看,体相非层状的新型二维铜族硫族化合物不仅包含半导体、金属等电子性质,还表现出拓扑、磁性等丰富的新奇物性,是一类值得关注的新型二维功能材料。     

Transport enhancement and efficiency optimization in two heat reservoir ratchets

We study a Brownian motor moving in a sawtooth potential in the presence of an external driving force and two heat reservoirs. Based on the corresponding Fokker--Planck equation, the analytical expressions of the current and efficiency in the quasi-steady-state limit are obtained. The effects of temperature difference and the amplitude of the external driving force on the current and efficiency are discussed, respectively. The following is our findings. (i) The current increases with both δ and A. In other words, δ and A enhance the transport of the Brownian motor. (ii) The competition between the temperature difference and the amplitude of the external driving force can lead to efficiency optimization. The efficiency is a peaked function of temperature, i.e., δ>0 and a lower amplitude value of the external driving force is necessary for efficiency optimization. (iii) The efficiency increases with δ, and decreases with A. δ and A play opposite roles with respect to the efficiency, which indicates that δ enhances the efficiency of energy transformation while A weakens it.     

Entropic noise induced stability and double entropic stochastic resonance induced by correlated noises

For the activated dynamics of a Brownian particle moving in a confined system with the presence of entropic barriers, this paper investigates a periodic driving and correlations between two noises. Within the two-state approximation, the explicit expressions of the mean first passage time (MFPT) and the spectral power amplification (SPA) are obtained, respectively. Based on the numerical computations, it is found that: (i) The MFPT as a function of the noise intensity exhibits a maximum with the positive correlations between two noises (λ>0), this maximum for MFPT shows the characteristic of the entropic noise induced stability (ENIS) effect. The intensity λ of correlations between two noises can enhance the ENIS effect. (ii) The SPA as a function of the noise intensity exhibits a double-peak by tuning the noise correlation intensity λ, i.e., the existence of a double-peak behaviour is the identifying characteristic of the double entropic stochastic resonance phenomenon.