砌浆层状复合材料珍珠层的应变率效应

为了分析具有砌浆结构的层状复合材料的应变率效应,以珍珠层为研究对象,采用纳米压入法测试珍珠层力学性能,利用连续刚度测量法得到不同加载速率下材料的硬度值和弹性模量。利用扫描电子显微镜观察珍珠层不同方向的砌浆微结构形貌,并结合微观结构对比分析不同压入深度和不同应变率两种工况下,珍珠层表层与横断面方向的力学性能。结果表明:在相同加载条件下,珍珠层表层方向的弹性模量小于其横断面方向的弹性模量,而表层方向硬度值则大于横断面方向的硬度值;当应变率恒定时,珍珠层弹性模量与硬度随压入深度增加而增加,当压入深度达到750nm后,弹性模量不再随压入深度变化而变化;当压入深度恒定时,硬度值、弹性模量和弹性回复率均随着应变率的增加而变大。     

Modeling of a biological material nacre: Waviness toughness model

Hard biological materials such as nacre, bone, and teeth exhibit high values of toughness although it is meanly made of a ceramic material. Ceramic materials are brittle and fail in a catastrophic manner therefore they have low values of toughness. Researchers have been curious in examining the reasons behind such performance. It has been found that the staggered structure of these materials, that is brittle tablets embedded into a soft matrix, is the main reason that allows for multiple toughening mechanisms to operate at different length scales. In addition, it has been shown that the tablets are not flat. There is some waviness that generates hardening and spreading of nonlinear deformation leading to high values of toughness. The effect of waviness was not included in previous analytical toughness models. In the present work, a toughness model based on J-integral approach is developed that considers the waviness. Crack resistance curve, denoted by R-curve, is obtained that agrees with the experimental results. The developed toughness model aids in the design and optimization of nacre-like materials.     

Distributed damage creates flaw tolerance

A qualitative micromechanical fracture mechanics model is presented that shows how a structure that is sensitive to the presence of a single crack or hole can be rendered flaw tolerant by the presence of an interacting distribution of such flaws. The simple model was inspired by the ductile fracture experienced by the under-designed gusset plates recovered from the I-35W Bridge collapse and by the experimentally measured increase in toughness of concrete damaged by fire.     

The toughening mechanism of nacre and structural materials inspired by nacre

Abstract

The structure and the toughening mechanism of nacre have been the subject of intensive research over the last 30 years. This interest originates from nacre’s excellent combination of strength, stiffness and toughness, despite its high, for a biological material, volume fraction of inorganic phase, typically 95%. Owing to the improvement of nanoscale measurement and observation techniques, significant progress has been made during the last decade in understanding the mechanical properties of nacre. The structure, microscopic deformation behavior and toughening mechanism on the order of nanometers have been investigated, and the importance of hierarchical structure in nacre has been recognized. This research has led to the fabrication of multilayer composites and films inspired by nacre with a layer thickness below 1 μm. Some of these materials reproduce the inorganic/organic interaction and hierarchical structure beyond mere morphology mimicking. In the first part of this review, we focus on the hierarchical architecture, macroscopic and microscopic deformation and fracture behavior, as well as toughening mechanisms in nacre. Then we summarize recent progress in the fabrication of materials inspired by nacre taking into consideration its mechanical properties.     

Montmorillonite based artificial nacre prepared via a drying process

Taking advantage of the specific behaviour of Na/Ca montmorillonite in aqueous dispersion, textured films were prepared by progressive evaporation of dispersions with low concentrations of delaminated platelets. The increase of the concentration of ions during evaporation changes the nature of the clay mineral platelet face interactions from repulsive to attractive. After complete drying, a dense brick-like structure is obtained when a sodium salt is used as deflocculant.

The bending strength of the textured film is strongly affected by cracks formation during drying, specially when the sample thickness increases. After optimisation, crack-free textured samples with a 140 μm thickness, 20 cm × 25 cm area and 120 MPa bending strength were obtained.     

The dynamics of nacre self-assembly

We show how nacre and pearl construction in bivalve and gastropod molluscs can be understood in terms of successive processes of controlled self-assembly from the molecular- to the macro-scale. This dynamics involves the physics of the formation of both solid and liquid crystals and of membranes and fluids to produce a nanostructured hierarchically constructed biological composite of polysaccharides, proteins and mineral, whose mechanical properties far surpass those of its component parts.     

A microstructural approach to flaw size dependence of strength in engineering ceramics

In this work, microstructural effects on the flaw size dependence of ceramic strength were investigated from aspects of stress analysis in the grain just ahead of the crack tip and also R-curve behaviour. In the analysis, it was assumed that the stress averaged in one grain just ahead of the crack tip, in ceramics, might control the fracture from a flaw. A microstructurally modified fracture criterion using the averaged stress was established by introducing the R-curve due to the grain bridging effect for longer cracks. A new R-curve of an exponential type was proposed for the fracture criterion. The criterion could adequately express the central trend in the dispersal of experimental results in the strength versus flaw size relation. To explain the scatter of results, the size distribution and the crystallographic anisotropy of the grain ahead of the crack tip were examined as dominant factors. The lower bound of strength scatter was estimated from the largest grain size, and the strength dispersion was reduced by decreasing the range of grain size variation. In FEM simulations, each element was regarded as one grain with a different crystallographic orientation, which was randomly selected by using a series of quasi-uniform random numbers. It was revealed that the scatter of strength due to crystallographic variations was smaller than the strength dispersion caused by a distributed grain size.     

From ultratough artificial nacre to elastomer: Poly(n-butyl acrylate) grafted graphene oxide nanocomposites

A biologically inspired, multilayer laminate structural design is deployed into composite films of poly(n-butyl acrylate) (PBA) graft graphene oxide (GO) synthesized by Ce(IV)/HNO₃ redox system in aqueous solution. Artificial hybrid films are fabricated by vacuum-assisted filtration macroscopic assembly method. Using nacre as the brick-and-mortar model construct free-standing membranes, here GO is similar to brick and PBA acts as mortar revealing the similar function of biopolymers in the natural nacre. Owing to the low T_g of PBA, the polymer chains could move freely at room temperature, enhancing the extensibility and flexibility. Meanwhile, the chemical structure of free-standing membranes was studied by Raman spectrum and XPS. The morphologies were charactered by XRD, SEM and TEM which are compact with the mechanical properties of the films. Interestingly, by tuning grafted PBA contents from 3.5 wt% to 77 wt%, quite wide range of mechanical properties (tensile strength from 20 to 180 MPa, Young’s modulus from 0.1 to 7 GPa, toughness from 0.8 to 4.3 MJ/m³, elongation from 1.2 to 24.5%) were obtained. At the same time, we found that the nanocomposite membranes can be adjusted from mimic nacre-liked film with high strength to a homogenous dispersed elastomer.     

Relationship between fiber flaw spectra and the fragmentation process: A computer simulation investigation

The single-filament-composite (SFC) fragmentation test can be utilized to provide quantitative information on the fiber strength distribution and the fiber/matrix interface shear stress, which are important properties that control the performance of fiber composites. An accurate interpretation of the fragmentation data, however, is difficult owing to the stochastic nature of the fragmentation process, as well as the complex interplay between the fiber flaw strength variation and the stress transfer zones about every broken flaw. In this work we have developed a computer simulation approach that models the fragmentation process by explicitly incorporating considerations regarding the strength and spatial distributions of the flaws. The effect of stress variation along the fiber length is accounted for by incorporating specific fiber-loading models. From the simulation it is demonstrated that the fragmentation data may be used to produce a rough sketch of the underlying flaw strength spectrum. An examination of the fragment size statistics suggests that appropriate analysis of the SFC data may be utilized to detect the occurrence of matrix/fiber interface yielding or debonding. An alternative methodology for mapping out the flaw strength distribution by means of a multiple-long-fiber failure test is also presented for comparison purposes.     

Utilization of prior flaw information in ultrasonic NDE: An analysis of flaw scattering amplitude as a random variable

Probabilistic approaches to flaw detection, classification, or characterization often assume prior knowledge of the flaw distribution. It is implicit that there is a scattering amplitude distribution associated with the flaw distribution. In a number of previously published probabilistic analyses, it has been assumed that scattering amplitude is an uncorrelated, Gaussian random variable with zero mean and known variance. In the work reported here, these assumptions are evaluated for the case of a lognormal distribution of spherical flaws. The correlation, mean, variance, and nature of the scattering amplitude distribution are considered as a function of frequency and as a function of the breadth of the assumed flaw distribution. It is shown for the assumed flaw distributions that scattering amplitude is not uncorrelated and does not have zero mean. It is shown that errors in estimating the flaw distribution variance affect both the scattering amplitude mean and variance. Using both analytical and numerical procedures, the scattering amplitude distribution is shown to be lognormal at long wavelength for a lognormal distribution of spherical scatterers. At high frequency, the distribution is shown to have a bimodal character.     

Underpinning of a simple blunt flaw fracture initiation relation

With the cohesive process zone representation of the micro-mechanistic processes that are associated with fracture as a basis, the author is involved in a wide-ranging research programme, the objective being to extend the fracture mechanics methodology for sharp cracks to blunt flaws, so as to take credit for the blunt flaw geometry. In earlier work, a Mode I fracture initiation relation has been derived, subject to the restriction that the process zone size s is small compared with the flaw depth (length) and any characteristic dimension other than the flaw root radius . The relation gives the critical elastic flaw-tip peak stress _pcr, and has been derived using a two-extremes procedure, whereby the separate, and indeed exact, solutions for small and large s/ values are blended together to give an all-embracing relation that is valid for all s/ values. _pcr is expressed in terms of the process zone material parameters and geometrical parameters but, for a wide range of flaw geometry parameters, _pcr essentially depends on only one geometrical parameter . This paper provides underpinning for the general thrust of the two-extremes procedure by appealing to exact results for the complete spectrum of s/ values from analyses of appropriate Mode III models. Results obtained by applying the two-extremes procedure are shown to be in very good agreement with the exact results.     

Impedance of single-turn coil surrounding a conducting cylinder with a flaw

The impedance of a single-turn coil which surrounds a conducting cylinder with a flaw is calculated employing Green's function technique. The Born approximation is used in order to know how the impedance change due to the presence of a flaw depends on the conductivity and size of a flaw.     

三角帆蚌珍珠质层结构和珍珠质涂层的研究

利用扫描电镜和光学显微镜对三角帆蚌贝壳和珍珠的珍珠质层微观结构进行了分析研究, 发现贝壳的珍珠质层中存在异常的结构带, 主要有柱状珍珠质带, 针状晶体带以及棱柱状晶体带. 其中柱状珍珠质带中, 单片文石板片的厚度超过1μm, 是正常珍珠质中文石板片厚度的两倍. 而对正常珍珠的珍珠质层的大量观察却未发现类似的异常结构. 分析认为这可能是因为贝壳珍珠质的矿化微环境与珍珠的珍珠质矿化微环境不同导致的. 并利用圆柱形珍珠囊在钛金属牙种植体表面制备的珍珠质涂层具有沿整个圆周面均匀生长的特点.     

仿珍珠层自组装制备有机/无机纳米复合薄膜

模仿珍珠层结构, 采用蒸发诱导自组装的方法, 在石英片表面制备了聚三缩丙二醇双丙烯酸酯( PTPG-DA) / SiO₂ 纳米复合薄膜, 采用FT-IR、XRD 和TEM 等分析技术对薄膜结构进行了表征, 测试了其摩擦力学行为, 并初步讨论了纳米复合薄膜的形成机理。结果表明, 所制备的薄膜具有有机/ 无机有序交替的层状纳米复合结构, 其聚合前的层间距为2. 65 nm , 聚合后的层间距为2. 35 nm。聚合后的纳米复合薄膜具有较好的减摩性能。      

Effect of magnetostriction on fracture of a soft ferromagnetic medium with a crack-like flaw

The magnetic‐induction field in the vicinity of an elliptical inclusion embedded in an infinite soft ferromagnetic medium is determined based on complex potential theory. By using a constitutive relation of magnetostriction for isotropic materials, the stress field in the vicinity of an elliptical flaw is obtained. Furthermore, the stress field at the tip of a slender elliptical crack is determined for the case in which only an external magnetic field perpendicular to the major axis of the ellipse is applied at infinity. The results indicate that the stress field in the neighbourhood of the tip is governed by the magnetostriction and permeability of the soft ferromagnetic material. The induction magnetostrictive modulus is a key parameter in determining which of the two mechanisms, i.e., magnetostriction and magnetic‐force‐induced deformation, is dominant in determining the stress field in the neighbourhood of the tip of a crack‐like flaw. With regard to the influence of the magnetic field on the apparent toughness of a soft ferromagnetic body with a crack‐like flaw, soft ferromagnetic materials can be roughly divided into two categories: one possesses a large induction magnetostrictive modulus and the other has a small modulus. An approximate criterion for categorizing the materials is presented. For the benefit of engineering design, the expressions of the stress‐intensity factor for these two categories of soft ferromagnetic materials are presented. The results show that the stress‐intensity factor is affected not only by the flaw geometry, but also by the permeability of the medium inside the flaw.     

Fracture strength of various lay-ups of carbon/epoxy laminates using the modified inherent flaw model

Modifications are made in the inherent flaw model of Waddoups, Eisenman and Kaminski (known as the WEK model) for accurate prediction of notched tensile strength of composite laminates containing a sharp notch. To examine the adequacy of the model, fracture data of center cracked carbon/epoxy composite laminates with various lay-ups are considered. The notched strength estimates are found to be close to the test results.     

鲍鱼壳珍珠层无机文石片的层状微结构研究

贝壳珍珠层是软体动物壳的最内层,经过若干世纪的自然进化,贝壳珍珠层形成了优良的微结构,并使贝壳具有了相当高的强度、刚度及断裂韧性.本文利用扫描电镜(SEM)观察了鲍鱼贝壳珍珠层的主要微结构特征,发现其是由层状的无机文石片和有机胶原蛋白质组成的生物陶瓷复合材料.根据发现的贝壳珍珠层层状微结构特征,建立贝壳珍珠层三维有限元模型,并用此模型分析了珍珠层的拉伸屈服极限与无机文石片拉伸屈服极限及其厚度的关系,研究表明珍珠层的屈服极限随无机文石片屈服极限的增加和无机文石片厚度的减小而增加.     

Change in impedance of a single-turn coil due to a flaw in a conducting half space

The problem of detection and characterization of a flaw in a conducting half-space using an eddy-current coil oriented parallel to the interface is examined. An expression is derived for a first order approximation for the change in complex impedance due to a flaw located within the conducting medium. The overall impedance is a function of the radius and lift-off distance of the test coil and the conductivity of the material. An analytical expression is derived for the change in impedance as a function of the electric fields at the position of the flaw. It is found to be an integral over the volume of the flaw of the electric fields found with and without the flaw being present. The limiting case of a degenerate point flaw may be examined in greater detail by allowing the field in the presence of the flaw to be approximated by the unperturbed field. For flaws small enough that the field does not vary much over its volume, the field may be even further approximated by using just the value of the field at the position of the centroid of the flaw. Plots are shown to illustrate the behavior of the change in impedance as a function of the radial range of the flaw and the depth of the flaw centroid, using previously derived expressions for the fields for the unflawed case.     

珍珠母及其仿生复合材料力学行为的研究进展

珍珠母是由天然文石晶片和有机基质构成的一种两相生物复合材料.其中,文石晶片通过典型交错层叠方式镶嵌在连续的有机基质中,形成高度有序的分级结构,使珍珠母呈现出远优于其组份材料的力学性能.因此,受到力学、材料学和生物学领域研究学者们的广泛关注.本文首先介绍了珍珠母材料的微结构特征及其基本变形机制和力学性能,然后分别从理论分...