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.     

Mechanical analysis and toughening mechanisms of a multiphase recycled CFRP

The mechanical response of a recycled CFRP is investigated experimentally. A complex multiscale microstructure is revealed, with both dispersed fibres (with fractured-sections) and fibre-bundles. The specific properties of the recyclate compare favourably with those of aluminium and glass–fibre composites. Micromechanical studies show that tensile failure follows the pre-existing fractured-sections on the dispersed-fibres, while compressive failure occurs by shear-banding. Fracture toughness measurements coupled with SEM evidence how bundles considerably toughen the composite by complex failure mechanisms. This analysis can guide the optimisation of recycling processes and support the development of design methods for recycled CFRP; it also provides insight on the mechanical response of other multiphase short-fibre reinforced materials.     

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。      

Epoxy nanocomposites - fracture and toughening mechanisms

This study focuses to provide information about reinforcing influences of nanoparticles exerted on the mechanical and fracture mechanical properties of epoxy resins, particularly with regard to fracture and toughening mechanisms. A comprehensive study was carried out on series of nanocomposites containing varying amounts of nanoparticles, either titanium dioxide (TiO₂) or aluminium oxide (Al₂O₃). Nanocomposites were systematically produced by applying high (shear) energy during a controlled dispersion process, in order to reduce the size of agglomerates and to gain a homogeneous distribution of individual nanoparticles within the epoxy resin. The mechanical performance of the nanocomposites was then characterized by flexural testing, dynamic mechanical analysis (DMA), and furthermore, by fracture mechanics approaches (LEFM) and fatigue crack growth testing (FCP). The microstructure of specimens and the corresponding fracture surfaces were examined by TEM, SEM and AFM techniques in order to identify the relevant fracture mechanisms involved, and to gain information about the dispersion quality of nanoparticles within the polymer. It was found that the presence of nanoparticles in epoxy induces various fracture mechanisms, e.g. crack deflection, plastic deformation, and crack pinning. At the same time, nanoparticles can overcome the drawbacks of traditional tougheners (e.g. glass beads or rubber particles) by simultaneously improving stiffness, strength and toughness of epoxy, without sacrificing thermo-mechanical properties.     

Evaporation-induced self-assembly of organic–inorganic ordered nanocomposite thin films that mimic nacre

The present study attempts to incorporate acrylate-based polymers into ordered lamellar organic–inorganic nanocomposite thin films composed of alternating Poly(TPGDA)/ITO layers. The films were prepared by dip-coating from a homogeneous solution containing the soluble inorganic metal salts (InCl₃·4H2O and SnCl₂·2H₂O), surfactant, cross-linkers, organic monomers, and initiators, thus leading to composite lamellar nanocomposite materials through evaporation-induced self-assembly method. The final polymer/ITO nanocomposite thin film was obtained by a separate free-radical polymerization step, initiated by UV exposure. Structures and composition of the films were characterized using FTIR, XRD, UV–Vis spectrophotometer and TEM. The results indicated that the films were composed of organic and inorganic layers with orderly interlaced arrangement.     

Morphology and toughening mechanisms in clay-modified styrene-butadiene-styrene rubber-toughened polypropylene

Clay-modified styrene-butadiene-styrene (SBS) rubber is utilized to toughen polypropylene (PP). The SBS rubber is found to have good compatibility with clay particles. SBS rubber helps to finely disperse clay particles in PP matrix. Mode-I fracture mechanisms are investigated using optical microscopy and transmission electron microscopy techniques. Rubber particle cavitation and matrix shear yielding are found to be the main toughening mechanisms in PP/SBS system. In the case of PP/SBS/clay system, widespread rubber particle cavitation, which appears to be facilitated by the presence of clay particle inclusions inside the SBS rubber particles, takes place in the PP matrix. This, in turn, leads to the formation of a bigger shear yielded zone in PP matrix. As a result, an enhanced toughness is observed.     

Strengthening and toughening mechanisms in microfiber reinforced cementitious composites

Materials with quasi-brittle stress strain curves exhibit desirable properties such as enhanced durability, flaw tolerance and toughness. This study reveals that steel microfiber reinforced cement based composites exhibit such quasi-brittle behavior. Mechanical properties of steel microfiber reinforced cement based composites are obtained through flexure and splitting tension tests. The cracking process and crack fiber interactions that lead to the quasi-brittle behavior in these composites were investigated. The strength and toughness enhancement is associated with crack wake mechanisms. Aggregate bridging and pullout and secondary crack formations associated with microfiber bridging sites are predominant during the strain hardening regime. Multiple secondary microcracks perpendicular to the fiber/matrix interface is the dominant failure mode beyond peak load in the strain softening regime.     

Impact performance and toughening mechanisms of toughness-tailored polypropylene impact copolymers

The impact performance and resistance of toughness-tailored polypropylene (PP) impact copolymer (ICP) blends were examined using a range of microscopic techniques. The double-notch four-point bending (DN-4PB) Charpy impact technique was used to clearly study the impact-enhancement of the ICP blends responsible for the observed toughening effect. The incorporation of ethylene–octene copolymer with a lower molecular weight into the PP matrix induced the formation of massive crazes and shear yielding, leading to a more ductile PP matrix. A detailed investigation showed that well-oriented talc particles act in the PP matrix as stress concentrators to initiate massive crazes by crack deflection and bifurcation. These toughening mechanisms are responsible for the improved impact resistance. The DN-4PB Charpy impact technique used in this study provides an effective methodology for probing the impact fracture mechanisms of toughness-tailored ICP blends.     

Modelling of the toughening mechanisms in rubber-modified epoxy polymers

A mathematical model has been developed to quantify the relationships between the microstructure and fracture properties of multiphase rubber-toughened epoxy polymers. Good agreement between predictions from the model and experimental results have been found. The model also reveals that localized plastic shear banding in the epoxy matrix, running between the rubbery particles, is the dominating mechanism under all testing conditions. Plastic void growth in the epoxy matrix is the other main toughening mechanism. This latter mechanism is initiated by internal cavitation of the rubbery particle, or by debonding at the particle-matrix interface, and is particularly significant at higher test temperatures.     

An analysis of competing toughening mechanisms in layered and particulate solids

The relative potency of common toughening mechanisms is explored for layered solids and particulate solids, with an emphasis on crack multiplication and plasticity. First, the enhancement in toughness due to a parallel array of cracks in an elastic solid is explored, and the stability of co-operative cracking is quantified. Second, the degree of synergistic toughening is determined for combined crack penetration and crack kinking at the tip of a macroscopic, mode I crack; specifically, the asymptotic problem of self-similar crack advance (penetration mode) versus $90^{\circ }$ symmetric kinking is considered for an isotropic, homogeneous solid with weak interfaces. Each interface is treated as a cohesive zone of finite strength and toughness. Third, the degree of toughening associated with crack multiplication is assessed for a particulate solid comprising isotropic elastic grains of hexagonal shape, bonded by cohesive zones of finite strength and toughness. The study concludes with the prediction of R-curves for a mode I crack in a multi-layer stack of elastic and elastic–plastic solids. A detailed comparison of the potency of the above mechanisms and their practical application are given. In broad terms, crack tip kinking can be highly potent, whereas multiple cracking is difficult to activate under quasi-static conditions. Plastic dissipation can give a significant toughening in multi-layers especially at the nanoscale.     

Mechanical properties and toughening mechanisms of polypropylene/barium sulfate composites

The effect of interfacial interaction on the mechanical performance of a group of polypropylene (PP)/barium sulfate (BaSO₄) composites were studied. It was found that PP can be toughened with specially treated BaSO₄ particles. The interfacial modification contributes to the toughening in two aspects. The first is to provide a proper interfacial adhesion and control the interfacial debonding occurs at well-timed stages. This ensures the inorganic particles transfer the stress and stabilizes the cracks at the initial stage of the deformation, and satisfy the stress conditions for plastic deformation of matrix ligaments subsequently via debonding. The second is that the modified interface between PP matrix and filler particles increases the nucleating ability of the fillers and retards the motion of the PP chains. This leads to the formation of PP crystals with less perfection and smaller size in the matrix and promotes plastic deformation of the matrix after the debonding occurs.     

基于扩展有限元的陶瓷复合材料多重增韧机制

为了提高陶瓷材料的断裂韧性和可靠度,改善材料抵御破坏的能力,将优化的多重增韧机制应用到氧化铝基陶瓷材料的开发中。相变增韧机制可以耗散部分能量,降低裂纹尖端处的应力集中程度,阻止或延缓裂纹扩展速率。当增强相分布较为合理、材料的致密度较高时,裂纹偏转与桥接增韧机制可以有效地削弱裂纹扩展动力,提高材料的断裂韧性。利用扩展有限元（X-FEM）手段讨论了裂纹扩展问题,为分析陶瓷复合材料的多重增韧机制提供了新思路。     

Microstructures and toughening mechanisms of organoclay/polyethersulphone/epoxy hybrid nanocomposites

Hybrid nanocomposites (HNCs) with high fracture toughness were successfully prepared by incorporating polyethersulphone (PES) and organoclay into epoxy resin. Their microstructures were studied. They were composed of homogeneous PES/epoxy matrices and micron-scale organoclay agglomerates. These agglomerates consisted of smaller tactoid-like regions which were comprised of ordered exfoliated nanolayers. The toughening mechanisms of the two tougheners were also studied and then related to their microstructures. For one thing, the PES which was dissolved in the epoxy resin homogeneously improved the ductility of the epoxy resin and made it easier to deform. For another, the organoclay agglomerates induced crack front bowing, crack bridging, crack deflection, crack bifurcation and plastic deformation of the matrices on the micron-scale, respectively. These toughening processes were achieved by the ordered exfoliated nanolayers with various orientations, which debonded from the matrices, bridged the cracks and induced the plastic deformation of the matrices on the nanoscale.     

改性介孔硅-ESO/聚丙烯三元体系复合材料增强增韧机制

以改性介孔硅为主要补强剂,聚丙烯(PP)为基体树脂,环氧大豆油(ESO)为增塑剂和稳定剂,通过熔融挤出注塑方法制备了改性介孔硅-ESO/PP体系复合材料。通过XRD、SEM、光学显微镜(OM)、偏光显微镜(PLM)及力学性能测试对介孔硅与ESO增强增韧PP的机制进行分析。结果表明:改性介孔硅、ESO二者同时填充PP制备改性介孔硅-ESO/PP复合材料时,三者界面以物理交联或化学接枝的结合方式形成了空间网状结构,改性介孔硅均匀分散在PP基体中;改性介孔硅添加量一定时,随ESO用量增加,改性介孔硅-ESO/PP复合材料弯曲强度稍微下降,但抗冲击强度、硬度都得到提高,当改性介孔硅用量为20%(与PP的质量比)、ESO用量为2.5%(与PP的质量比)时,综合性能较好。改性介孔硅本身的高模量及粒子在聚合物熔体中的异向成核促进了基体树脂结晶,以及良好的界面结合及分散性,这是增强的主要原因。ESO分子插入聚合物分子链间,削弱了聚合物分子链间的移动性,一定程度降低了基体结晶度等是增韧的主要原因。改性介孔硅使α晶型PP转变为具有更高冲击强度β晶型PP,与ESO二者协同作用,能增强增韧,但是过多的ESO使介孔硅粒子集中在材料的表面,导致其表面硬度增大。     

Effect of cross-linking density on the toughening mechanisms of rubber-modified thermosets

Solid rubbers have been incorporated into thermosets using the compositional quenching process. Electron micrographs reveal that rubber particles a few micrometres or less in size have been dispersed in the matrix. Two model systems were studied: an epoxy resin cured with primary amines which represents a tight network structure, and a phenoxy resin cross-linked with methylene diisocyanate which represents a loose network. The present study indicates that a small amount of a cross-linking agent can reduce the degree of matrix yielding with a resultant drop in impact strength. SEM fractography provides complementary information on the transition from ductile fracture to brittle fracture.     

碳纤维无纺布对CFRP层板层间的增韧作用及机制

为了揭示短纤维无纺布对碳纤维增强树脂基复合材料(CFRP)层板层间韧性的影响规律,测试了不同面密度(1.95、3.90、7.80和15.60 mg/cm2)和不同纤维平均长度(0.8 mm和4.3 mm)的碳纤维无纺布增韧的CFRP层板I型层间断裂韧性。实验结果表明:对于不同短纤维增韧的CFRP层板,平均长度为0.8mm的短纤维增韧效果优于平均长度为4.3mm的短纤维,并且面密度为7.8mg/cm2、厚度约为150μm、平均长度为0.8mm的碳纤维无纺布显著提高了CFRP层板的层间断裂韧性,与未改性的CFRP层板相比,其能量释放率最大可提高99%。光学显微镜观察结果表明环氧基体中长度为0.8mm的短纤维具有三维交织结构,该结构可以有效地阻止裂纹的扩展;SEM观察结果表明短纤维从环氧基体中的脱粘和拔出以及短纤维周围环氧基体的塑性变形是CFRP层板的主要增韧机制。研究结论为层板短纤维增韧技术的应用奠定了基础。     

The interlaminar fracture behaviour and toughening mechanisms of new carbon fibre-reinforced bismaleimide composites

The interlaminar fracture behaviour of carbon fibre-reinforced bismaleimide (BMI) composites prepared by using a new modified BMI matrix has been investigated by various methods. Laminates of three typical stacking sequences were evaluated. Double cantilever beam, end-notch flexure and edge-delamination tension tests were conducted under conventional conditions and in a scanning electron microscope. The strain energy release rates in Mode I and Mode II, G_Ic and G_IIc, as well as the total strain energy release rate, G_mc, have been determined and found to be higher than those for laminates with an epoxy matrix. Dynamic delamination propagation was also studied. The toughening mechanisms are discussed.     

Crystal orientation of non-magnetic materials by imposition of a high magnetic field

From the view point of learning from the nature, the controlling of crystal orientation is accounted to be a major subject for materials processing. This paper reviews the researches on the crystal orientation by use of a high magnetic fieldand belongs to the category of researches for mimicking structures, namely the crystal orientation, which nature or livingbodies are forming. Regarding to the crystal orientation, several methods such as unidirectional solidification and epitaxial growth and so on have been developed hitherto. On the other hand the magnetization force that is familiar with the force to attract iron to a magnet, has been recognized to be effective even in non-magnetic materials when those are placed under a high magnetic field, which has become rather conveniently available by developing superconducting technologies in these days. In this paper, main results obtained when the imposition of a high magnetic field was accompanied to several materials processing such as electrodeposition, vaperdeposition, solidification, baking, slip-casting and precipitation, arereviewed from the view point of crystal orientation of non-magnetic materials.     

Crystal orientation of non-magnetic materials by imposition of a high magnetic field

From the view point of learning from the nature, the controlling of crystal orientation is accounted to be a major subject for materials processing. This paper reviews the researches on the crystal orientation by use of a high magnetic field and belongs to the category of researches for mimicking structures, namely the crystal orientation, which nature or living bodies are forming. Regarding to the crystal orientation, several methods such as unidirectional solidification and epitaxial growth and so on have been developed hitherto. On the other hand the magnetization force that is familiar with the force to attract iron to a magnet, has been recognized to be effective even in non-magnetic materials when those are placed under a high magnetic field, which has become rather conveniently available by developing super-conducting technologies in these days. In this paper, main results obtained when the imposition of a high magnetic field was accompanied to several materials processing such as electrodeposition, vaperdeposition, solidification, baking, slip-casting and precipitation, are reviewed from the view point of crystal orientation of non-magnetic materials.