一类压缩型映射的不动点定理

在完备的度量空间中,讨论了一类新型的非线性压缩映射ρ(Tx,Ty)≤a(ρ(x,y))ρ(x,Tx)+b(ρ(x,y))ρ(y,Ty)+c(ρ(x,y))ρ(x,y)通过构造迭代序列,指出该映射的不动点的存在性和唯一性,并给出相应的误差估计式,拓展和改进了有关文献的范围.     

基于临界状态的砂土本构模型研究

砂土孔隙比及所受压力是其力学特性的重要影响因素. 本文基于砂土临界状态线特性分析,采用以e-(p/p_a)ξ平面内的线性关系描述其等向压缩线. 通过对比分析两种不同压缩线函数 与临界状态线函数之间的关系提出更适合描述砂土在等向压缩下的参考压缩线,并给出了基于参考压缩线的等向硬化规律. 建议了适用于 描述砂土剪切特性的屈服面函数,并给出利用等向压缩和等p路径确定屈服面形状参数μ的方法. 将不同应力比对应的压缩线作为砂土状态参量参考线,以获取潜在强度M_f与特征状态应力比M_c,进而描述砂土压缩与剪切特性;基于等向压缩与等p路径建立了当前应力比与状态参量参考线之间的相关关系,从而实现了砂土状 态参量参考线由参考压缩线向临界状态线平稳过渡. 建立的砂土本构模型共11个参数,均能够通过常规土工试验或经验获取. 基于模型预测与Toyoura砂的等向压缩、三轴不排水剪切试验及排水剪切试验的对比结果,本文建立的砂土本构模型很好地描述了Toyoura 砂在不同孔隙比和不同压力下的压缩与剪切特性.      

酚醛层压材料的冲击力学行为及本构模型

为了研究酚醛层压材料的冲击力学行为并获得本构模型，利用万能试验机和整形修正的分离式霍普金森压杆(SHPB)装置，对材料试样进行了应变率范围为10-3~103 s-1的单轴压缩实验，得到了不同加载应变率下的应力应变曲线，对其在准静态、动态载荷下的压缩破坏机理进行了初步探讨。结果表明，酚醛层压材料具有较强的应变率效应，与准静态(1.67×10-3 s-1)时相比，在动态载荷(7×102 s-1)下，峰值应力增加了约10倍；破坏应变减少了约一半；在准静态和动态加载条件下试样力学性能的差异是由于纤维基体界面特性以及不同应变率下破坏模式的不同；采用朱-王-唐本构方程描述了酚醛层压材料力学行为，拟合得到了本构方程的系数，在加载过程中，理论计算值与实验结果吻合较好。     

循环孔隙水压下混凝土常规三轴压缩损伤破坏特性分析

本文进行了孔隙水压不同循环次数(0次,10次,50次,100次和200次)以及不同应变速率下(10~(-5)/s,10~(-4)/s,10~(-3)/s和10~(-2)/s)混凝土常规三轴压缩试验,分析了混凝土峰值应变的变化规律、应力-应变曲线及损伤特性。结果表明:相同循环次数孔隙水压下,峰值应变随应变速率增加,整体呈现出增加的趋势;而相同应变速率下,峰值应变随孔隙水压循环次数的变化规律并不明显;在中低应变速率(10~(-5)~10~(-3)/s)下,混凝土的损伤变化受孔隙水压循环次数影响较大;当循环次数达到200次时,孔隙水压作用对混凝土产生较大的损伤。通过对循环孔隙水作用下混凝土动态损伤破坏机理的分析可知:混凝土的破坏过程实际上是内部裂纹不断形成、扩展、贯通,材料损伤不断产生、累积的过程;当损伤达到一定程度,混凝土发生宏观破坏,失去承载力。     

Enhancing signal-to-noise ratio and resolution in low-field NMR relaxation measurements using post-acquisition digital filters

The traditional way to enhance signal-to-noise ratio (SNR) of nuclear magnetic resonance (NMR) signals is to increase the number of scans. However, this procedure increases the measuring time that can be prohibitive for some applications. Therefore, we have tested the use of several post-acquisition digital filters to enhance SNR up to one order of magnitude in time domain NMR (TD-NMR) relaxation measurements. The procedures were studied using continuous wave free precession (CWFP-T₁) signals, acquired with very low flip angles that contain six times more noise than the Carr–Purcell–Meiboom–Gill (CPMG) signal of the same sample and experimental time. Linear (LI) and logarithmic (LO) data compression, low-pass infinity impulse response (LP), Savitzky–Golay (SG), and wavelet transform (WA) post-acquisition filters enhanced the SNR of the CWFP-T₁ signals by at least six times. The best filters were LO, SG, and WA that have high enhancement in SNR without significant distortions in the ILT relaxation distribution data. Therefore, it was demonstrated that these post-acquisition digital filters could be a useful way to denoise CWFP-T₁, as well as CPMG noisy signals, and consequently reducing the experimental time. It was also demonstrated that filtered CWFP-T₁ method has the potential to be a rapid and nondestructive method to measure fat content in beef and certainly in other meat samples.     

MDYB-3有机玻璃在不同应变率下的一维屈服行为

对MDYB-3有机玻璃进行了多组不同应变率(10-3~3 000 s-1)下的压缩实验, 得到准静态下的屈服应力与动态下的峰值应力。沿其增强与面内2个方向进行准静态压缩实验, 以分析定向拉伸对屈服应力的影响, 修正了Ree-Eyring模型与Cooperative模型以描述定向有机玻璃的屈服行为。采用Johnson-Cook模型描述屈服后的黏塑性行为。结果表明Cooperative屈服模型比Ree-Eyring屈服模型更接近实验结果, 且能准确描述准静态屈服应力。动态压缩下的峰值应力为失效应力, 说明试样在1 500 s-1以上应变率下未达到屈服应力时已经发生破坏。Johnson-Cook模型对于单条曲线拟合良好, 但无法准确描述材料的应变率相关性。     

基于Oda裂隙结构张量的流纹岩各向异性试验研究

岩石内天然存在长度、倾角和形态不同的裂隙,造成岩石的各向异性特征。为揭示岩石内天然随机裂隙发育特征对岩石物理力学特性的影响规律,以泥巴山隧址区采集裂隙性流纹岩为研究对象,首先对试样裂隙进行素描统计分析;然后基于Oda裂隙结构张量,获得天然随机分布裂隙的几何统计参数;最后对裂隙性流纹岩试样分别进行单轴和常规三轴压缩试验,得到不同应力路径下流纹岩的应力-应变曲线及物理力学参数。分析Oda裂隙结构张量定义的各向异性参数与试验获得的力学参数之间的规律,研究结果表明:(1)Oda裂隙结构张量适用于天然随机分布裂隙的几何统计分析,各向异性参数A(F)越大,裂隙优势方向越明显;(2)单轴压缩下,随着各向异性参数I1和A(F)的增大,流纹岩各向异性程度增大,弹性模量减小,泊松比增大;(3)常规三轴压缩下,流纹岩弹性模量和泊松比随各向异性参数改变的规律较不明显,Oda裂隙结构张量不再适用。     

用浮阻力模型研究Richtmyer-Meshkov不稳定性诱导混合

采用浮阻力模型对激波管低压缩和激光加载高压缩情况下的Richtmyer-Meshkov不稳定性诱导混合现象进行了研究。通过与实验和理论分析结果进行比较发现:为了达到好的吻合, Richtmyer-Meshkov不稳定性情况下阻力系数的取值范围(2.0~5.36)比Rayleigh-Taylor不稳定性情况下的值(3.3~4.0)宽得多; 而在Richtmyer-Meshkov不稳定性情况下, 高压缩时阻力系数的不确定度(约为3.36)明显高于低压缩时的值(约为1.46), 模型的进一步完善还有待于更精确实验的验证。研究显示:指数律经验公式中指数随工况的不同而显著变化, 目前工程设计中采用指数律经验公式是粗糙的。     

Effects of graded porous structure on local strain distribution under compression in silicone rubber

Silicone rubber samples with gradually changing pore sizes within the range of 70–610 μm are produced using an improved spacer method. The samples are scanned using an X‐ray computed tomography to evaluate their graded structure as compared to uniform rubber. A compressive test reveals that graded porous silicone rubber has characteristic stress–strain curves whose slope changes within a specific strain range depending on the porous structure. Analysis results of local strain based on a digital image correlation of the graded porous silicone rubber under compression demonstrate that the characteristic stress–strain properties are caused by shifts in the main deformation region in the graded structure. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 1033–1042     

Compressing strongly connected subgroups in social networks: An entropy-based approach

To detect and study cohesive subgroups of actors is a main objective in social network analysis. What are the respective relations inside such groups and what separates them from the outside. Entropy-based analysis of network structures is an up-and-coming approach. It turns out to be a powerful instrument to detect certain forms of cohesive subgroups and to compress them to superactors without loss of information about their embeddedness in the net: Compressing strongly connected subgroups leaves the whole net’s and the (super-)actors’ information theoretical indices unchanged; i.e., such compression is information-invariant. The actual article relates on the reduction of networks with hundreds of actors. All entropy-based calculations are realized in an expert system shell.