Measurement of rheological properties of soft biological tissue with a novel torsional resonator device

A new device for measuring the rheological properties of soft biological tissues is presented. The mechanical response is characterized for harmonic shear deformations at high frequencies (up to 10 kHz) and small strains (up to 0.2%). Experiments are performed using a cylindrical rod excited to torsional resonance. One extremity of the rod is in contact with the soft tissue and adherence is ensured by vacuum clamping. The damping characteristics and the resonance frequency of the vibrating system are inferred from the control variables of a phase stabilization loop. Due to the contact with the soft tissue, and depending on the rheological properties of the tissue, changes occur in the Q-factor and in the resonance frequency of the system. The shear modulus of the soft tissue is determined from the experimental results with an analytical model. The reliability of the proposed technique is evaluated through repeatability tests and comparative measurements with synthetic materials. The results of measurements on bovine organs demonstrate the suitability of the experimental procedure for the characterization of biological tissues and provide some insight in their rheological properties at frequencies in the range 1–10 kHz.     

软土基坑变形失稳形态模拟试验研究

通过不同土性、地下水条件下软土基坑开挖变形失稳的模拟试验, 研究了软土基坑开挖变形发展直至失稳破坏的全过程。通过试验研究, 初步认为软土基坑坑壁在无支护或支护刚度较小的情况下, 其坑壁破坏形态呈抛物线型; 在基坑开挖范围内若存在有砂性土, 且地下水位较高时, 易于发生流砂渗透破坏, 并导致了地表沉陷, 但侧向变形相对较小; 而对于因承压水引起的坑底土体隆起变形, 若不考虑土体的强度特性, 计算结果是偏于安全的。     

基于微结构指标主成分分析的沉桩挤土效应研究

前期研究已认识到,土的宏观力学性质及其表现从本质上应取决于土的微观结构。在结构性较强的软土中沉桩,桩周土体内部结构会发生显著变化,土体的强度与变形性质是这种内在变化的宏观表现,研究土体微观结构与宏观力学行为变化之间的关系,对认知土体的力学性质,从微观出发去认识沉桩挤土效应的机理,指导工程实践具有重要地理论和现实意义。本文基于天津滨海地基土实际静压桩工程,在沉桩的不同时刻、沿桩身的不同位置取桩周土体原状土样进行室内三轴固结不排水剪切试验,得到土体强度指标参数,同时进行对应的微观结构试验,得到垂直与水平方向的10个微结构指标。采用主成分分析方法,在微结构指标中提取3个主成分,较好地分析了土体微结构特征。研究表明: 3个主成分与黏聚力之间存在较好的相关关系,而与内摩擦角之间的相关性相对较弱; 第一主成分对各微结构信息的提取比较充分,第二、第三主成分是对第一主成分未反映信息的进一步补充。同时主成分分析表明,土体微结构性质对强度性质起控制作用,在沉桩过程中,近地表和下部土层宏观力学指标表现出了相反的变化规律。主成分分析方法较好地表述了土体的微结构性质,为进一步从微观入手解释沉桩挤土效应机理提供了有力依据。     

沿海软粘土取土质量的对比分析

本文在简要介绍固定活塞式薄壁取土器取土技术基础上，详尽分析了在连云港、赤湾港和广深路所进行的软粘土取土技术对比试验结果。试验结果表明，土样扰动对于室内所测定的力学性指标影响极大，但对物理性指标影响不大。研究结果还有力地证明，只要操作正确，采用薄壁取土器能取得高质量土样，而采用敞口式厚壁取土器取出的土样扰动较大。     

连云港海相软土工程特性及处治方法探讨

根据连云港地区历年来多项工程的地质资料,并与我国部分沿海地区软土进行对比,系统分析了连云港海相软土的基本物理力学性质指标统计特征,特别是抗剪强度特性、固结变形特性和蠕变特性,并总结了该地区海相软土主要物理力学性质指标之间的相关关系,最后介绍了适合于海相软土的地基处理措施。     

Mechanical Properties of Ballistic Gelatin at High Deformation Rates

The characterization of soft or low impedance materials is of increasing importance since these materials are commonly used in impact and energy absorbing applications. The increasing role of numerical modeling in understanding impact events requires high-rate material properties, where the mode of loading is predominantly compressive and large deformations may occur at high rates of deformation. The primary challenge in measuring the mechanical properties of soft materials is balancing the competing effects of material impedance, specimen size, and rate of loading. The traditional Split Hopkinson Pressure Bar approach has been enhanced through the implementation of polymeric bars to allow for improved signal to noise ratios and a longer pulse onset to ensure uniform specimen deformation. The Polymeric Split Hopkinson Pressure Bar approach, including the required viscoelastic bar analysis, has been validated using independent measurement techniques including bar-end displacement measurement and high speed video. High deformation rate characterization of 10% and 20% ballistic gelatin, commonly used as a soft tissue simulant, has been undertaken at nominal strain rates ranging from 1,000 to 4,000/s. The mechanical properties of both formulations of gelatin exhibited significant strain rate dependency. The results for 20% gelatin are in good agreement with previously reported values at lower strain rates, and provide important mechanical properties required for this material.     

Torsion of Incompressible Fiber-Reinforced Nonlinearly Elastic Circular Cylinders

Torsion of solid cylinders in the context of nonlinear elasticity theory has been widely investigated with application to the behavior of rubber-like materials. More recently, this problem has attracted attention in investigations of the biomechanics of soft tissues and has been applied, for example, to examine the mechanical behavior of passive papillary muscles of the heart. A recent study in nonlinear elasticity was concerned specifically with the effects of strain-stiffening on the torsional response of solid circular cylinders. The cylinders are composed of incompressible isotropic nonlinearly elastic materials that undergo severe strain-stiffening in the stress-stretch response. Here we investigate similar issues for fiber-reinforced transversely-isotropic circular cylinders. We consider a class of incompressible anisotropic materials with strain-energy densities that are of logarithmic form in the anisotropic invariant. These models reflect stretch induced strain-stiffening of collagen fibers on loading and have been shown to model the mechanical behavior of many fibrous soft biological tissues. The consideration of anisotropy leads to a more elaborate mechanical response than was found for isotropic strain-stiffening materials. The classic Poynting effect found for rubber-like materials where torsion induces elongation of the cylinder is shown to be significantly different for the transversely-isotropic materials considered here. For sufficiently large anisotropy and under certain conditions on the amount of twist, a reverse-Poynting effect is demonstrated where the cylinder tends to shorten on twisting The results obtained here have important implications for the development of accurate torsion test protocols for determination of material properties of soft tissues.     

Stress Concentration in a Soft Ferromagnetic with an Elliptic Inclusion in a Homogeneous Magnetic Field

The paper addresses a stress–strain problem for an infinite soft ferromagnetic body with an elliptic inclusion. The body is in a homogeneous magnetic field B ₀₁. The basic stress–strain characteristics and induced magnetic field in the body and inclusion are determined and their features in the neighborhood of the inclusion are studied. The magnetoelastic and Maxwell stresses are plotted against the ratio of ellipse axes and the latitude angle. Maximum stresses versus magnetic induction and mechanical and magnetic properties of the material are tabulated     

基于离散元法的刚性履带低含水软地面附着特性数值分析

针对低含水软地面低黏性、高摩擦的力学特性,以刚性履带在干燥壤土行驶这一典型工况为试验条件,基于离散单元法选取合理的颗粒细观参数,在三轴剪切试验的基础上建立并验证了土壤数值仿真模型。通过数值牵引试验,对刚性履带低含水软地面的附着特性进行了离散元细观分析。分析表明,土壤附着力随履刺的向后排列而递减;垂直于履带行驶方向的附着作用不可忽略;离散元法能更好地还原低含水软地面的力学特性,揭示了土壤颗粒在动态荷载下非连续的本质,适合于干燥、松散的低含水软地面数值研究。     

Development of a micro-indentation device for measuring the mechanical properties of soft materials

Indentation is a simple and nondestructive method to measure the mechanical properties of soft materials, such as hydrogels, elastomers and soft tissues. In this work, we have developed a micro-indentation system with high-precision to measure the mechanical properties of soft materials, where the shear modulus and Poisson's ratio of the materials can be obtained by analyzing the load–relaxation curve. We have validated the accuracy and stability of the system by comparing the measured mechanical properties of a polyethylene glycol sample with that obtained from a commercial instrument. The mechanical properties of another typical polydimethylsiloxane sample submerged in heptane are measured by using conical and spherical indenters, respectively. The measured values of shear modulus and Poisson's ratio are within a reasonable range.     

天然生物材料及其摩擦学

评述了几种天然生物材料的特性及其摩擦学研究现状,分析了天然生物复合材料、角蛋白材料、土壤动物体表、植物叶的结构和表面形态及性能,介绍了天然生物材料的摩擦与磨损性能及生物防粘减阻和生物润滑技术,并探索性地提出了摩擦学仿生概念。     

Measuring soil properties to predict tractive performance of an agricultural drive tire

The coefficient of traction for a 9.5–16 R-1 bias ply tire was measured and compared with predictions using equations developed by Janosi and Hanamoto [Proc. 1st Int. Conf. on Mechanics, p. 707–736 (1961)]; Wismer and Luth [J. Terramechanics10, 49–61 (1973)] Gee-Clough et al. [J. Terramechanics15, 81–84 (1978)] and Brixius [ASAE Paper No. 87–1622 (1987)]. For the soft soil condition, with a cone index of 120 kPa, Gee-Clough's equation predicted the coefficient of traction better, but predictions using the Brixius equation were better for soil with a cone index of 225 kPa. An experimental device was developed to simultaneously measure the horizontal and vertical stress-strain relationships of soil. The use of resultant stress from the experimental device data failed to show any improvement in coefficient of traction prediction over using the cone index. The resultant of the normal and shear stress from the experimental device data did not adequately represent the soil properties involved in terrain-vehicle mechanics.     

某公路路基填土工程地质性质研究

盱眙地区软土成因复杂,矿物成分特殊,本文通过对其矿物成分、化学成分、颗粒分析、物理指标、膨胀性、压缩性等试验研究,对该软土的工程性质取得一些认识。结果认为该类软土中粘土矿物含量高、亲水性强,填土液塑限高、有机质含量高;在含水量高时碾压成饼状,含水量低时坚硬难以破碎,不能直接做路基填料,采用常规的掺灰处理效果不理想,是一种特殊性软土。     

Global and local large-deformation response of sub-micron,soft- and hard-particle filled polycarbonate

Since polymers play an increasingly important role in both structural and tribological applications, understanding their intrinsic mechanical response is key. Therefore in the last few decades much effort has been devoted into the development of constitutive models that capture the polymers' intrinsic mechanical response quantitatively. An example is the Eindhoven Glassy Polymer model. In practice most polymers are filled, e.g. with hard particles or fibers, with colorants, or with soft particles that serve as impact modifiers. To characterize the influence of type and amount of filler particles on the intrinsic mechanical response, we designed model systems of polycarbonate with different volume fractions of small, order 100 nm sized, either hard or soft particles, and tested them in lubricated uniaxial compression experiments. To reveal the local effects on interparticle level, three-dimensional representative volume elements (RVEs) were constructed. The matrix material is modeled with the EGP model and the fillers with their individual mechanical properties. It is first shown that (only) 32 particles are sufficient to capture the statistical variations in these systems. Comparing the simulated response of the RVEs with the experiments demonstrates that in the small strain regime the stress is under-predicted since the polymer matrix is modeled by using only one single relaxation time. The yield- and the large strain response is captured well for the soft-particle filled systems while, for the hard-particles at increased filler loadings, the predictions are less accurate. This is likely caused by polymer–filler interactions that result in accelerated physical aging of the polymer matrix close to the surfaces. Modifying the S_a-parameter, that captures the thermodynamic state of the polymer matrix, allows us to correctly predict the macroscopic response after yield. The simulations reveal that all rate-dependencies of the different filled systems originate from that of the polymer matrix. Finally, an onset is presented to predict local and global failure based on critical events on the microlevel, that are likely to cause the over-prediction in the large-strain response of the hard-particle filled systems.     

里下河古潟湖相软土基本工程特性与形成机理的关系

淮盐高速公路穿越的里下河地区,广泛分布有高含水量、高压缩性古潟湖相软土,工程性质很差,该层软土的工程性质与里下河古潟湖的形成机理密切相关。本文在对潟湖形成一般模式总结的基础上,结合苏北地区第四纪以来沉积环境及区域地质地貌变化情况,得出里下河古潟湖相软土的形成机理,并对淮沿高速公路盐线典型地段土样工程性质进行分析,根据土的工程性质的垂直变化,判断古潟湖相软土的分布情况,进而寻找古潟湖相软土工程性质与其形成机理之间的关系。     

Reducing sliding resistance of soil against bulldozing plates by unsmoothed bionics surfaces

Unsmoothed bulldozing plates were designed and tested by imitating the surface morphology of the top of the head of the dung beetle (Ohthophagus lenzii harold) on the D-optimum theory. The results showed that the bulldozing resistance of the unsmoothed plates was, on average, reduced by 13.02% and that of the optimum plate was reduced by 18.09% in comparison with a conventional (that is, smooth) plate. The mechanism of reducing sliding resistance of the unsmoothed surfaces was discussed.     

Evaluating the mechanical behavior of arterial tissue using digital image correlation

In this study, digital image correlation (DIC) was adopted to examine the mechanical behavior of arterial tissue from bovine aorta. Rectangular sections comprised of the intimal and medial layers were excised from the descending aorta and loaded in displacement control uniaxial tension up to 40 percent elongation. Specimens of silicon rubber sheet were also prepared and served as a benchmark material in the application of DIC for the evaluation of large strains; the elastomer was loaded to 50 percent elongation. The arterial specimens exhibited a non-linear hyperelastic stress-strain response and the stiffness increased with percent elongation. Using a bilinear model to describe the uniaxial behavior, the average minor and major elastic modulii were 192±8 KPa and 912±40 KPa, respectively. Poisson's ratio of the arterial sections increased with the magnitude of axial strain; the average Poisson's ratio was 0.17±0.02. Although the correlation coefficient obtained from image correlation decreased with the percent elongation, a correlation coefficient greater than 0.8 was achieved for the tissue experiments and exceeded that obtained in the evaluation of the elastomer. Based on results from this study, DIC may serve as a valuable method for the determination of mechanical properties of arteries and other soft tissues.     

Multi-scale Structural Modeling of Soft Tissues Mechanics and Mechanobiology

Soft tissues account for a major fraction of the body volume and mass. They are present in all non-skeletal organs, being responsible for protecting the body, maintaining internal homeostasis, and allowing for mobility. Their function in different organs is highly diverse, as are their properties which are optimally suited for their specific tasks. From a mechanical perspective, specificity of structure and properties is acquired via evolutionary adaptation of the tissue composition and multi-scale structure. In modeling tissue mechanics and mechano-biology, it is thus natural to seek the structural determinants of tissues and their evolution (the “structural approach”). Earlier models were exclusively phenomenological, based either on the general principles of non-linear continuum mechanics or alternatively, on empirical mathematical expressions that fit specific response patterns. In the late 1970’s, structural models were introduced to tissue mechanics (Lanir in J. Biomechanics 12(6): 423–436, 1979; Lanir in J. Biomechanics 16(1): 1–12, 1983). Ever since, a gradually increasing number of structural models have been developed for different types of tissues, and today, it is the method of choice (Cowin and Humphrey in J. Elasticity 61: ix–xii, 2000). The structural approach was recently extended to incorporate a mechanistic formulation of mechano-biological pathways by which tissue structures remodel during growth (Lanir in Biomech Model Mechanobiol, 14(2): 245–266, 2015). Here, the characteristic features of soft tissue structures and their constitutive modeling are reviewed. The presentation starts with a brief survey of the multi-scale and multi-phasic soft tissues structure. The global mechanical characteristics of soft tissues and of their constituents are then briefly reviewed. These two aspects form the basis for structural constitutive formulation via the multi-scale structure-function link. Based on established criteria for model validity, predictions of the formulated theory are contrasted against measured response characteristics. Using this structure-function relationship, the evolutionary pathway by which tissue structure and mechanics remodel during growth to adapt to their physiological function, is laid down. The review concludes with an account of the state of the art, the big picture, and future research challenges in tissue mechanobiological modeling.     

Comparison of Low Impedance Split-Hopkinson Pressure Bar Techniques in the Characterization of Polyurea

The split-Hopkinson pressure bar (SHPB) technique has been widely employed for over fifty years in characterizing the high strain-rate properties of many common engineering materials. Historically, however, this technique has had limited success in characterizing soft materials, since their low mechanical impedances can increase delays in attaining dynamic equilibrium and result in transmission pulses with extremely low signal-to-noise ratios. Due to interest in improving characterization of soft materials at high strain rates, numerous modifications to the traditional SHPB technique have been proposed. These include: using more sensitive piezoelectric gauges, employing hollow transmission bars, utilizing lower impedance polymeric pressure bars, and the use of pulse shaping techniques. To date, there has been no comparative studies or consensus within the SHPB community as to which approach is most advantageous. The goal of this investigation is to compare a number of these techniques, specifically the use of PMMA pressure bars and a hollow aluminum transmission bar (both with and without pulse shaping), alongside more traditional solid aluminum pressure bars in the characterization of polyurea, a common low impedance polymer. The advantages and disadvantages of each technique in generating high strain-rate stress-strain curves are discussed.     

Experimental study on the microstructure and nanomechanical properties of the wing membrane of dragonfly

Detailed investigations on the microstructure and the mechanical properties of the wing membrane of the dragonfly are carried out. It is found that in the direction of the thickness the membrane was divided into three layers rather than a single entity as traditionally considered, and on the surfaces the membrane displays a random distribution rough microstructure that is composed of numerous nanometer scale columns coated by the cuticle wax secreted. The characteristics of the surface structure are measured and described. The mechanical properties of the membranes taken separately from the wings of live and dead dragonflies are investigated by the nanoindentation technique. The Young’s moduli obtained here are approximately two times greater than the previous result, and the reasons that yield the difference are discussed. The project supported by the National Natural Science Foundation of China (10372102 and 10672164).