Observations on the fracture and deformation behaviour during annealing of residually stressed polycrystalline aluminium oxides

Highly residually stressed polycrystalline aluminium oxides were found to exhibit residual stress relaxation, as evidenced by changes in load-bearing ability, at temperatures as low as about 850° C. This temperature is much too low for such relaxation to occur by dislocation, Nabarro—Herring or Coble creep. Irreversible changes in specimen dimension coupled with SEM-fractography revealed that the stress relaxation resulted from creep by intergranular cavitation and crack propagation. In one aluminium oxide, such cavitation and crack propagation appeared to take place in a stable mode along a viscous glassy grain boundary phase. In high-purity fine-grained aluminium oxide, crack propagation occurred in a frequently totally catastrophic and highly unstable manner. This latter material was also observed to exhibit spontaneous fatigue during isothermal anneal. Implications of the findings of this study for the use of thermal anneals to promote residual stress relaxation in structural ceramic materials are discussed.     

In situ observation of nanograin rotation and deformation in nacre

Nacre is a natural nanocomposite material with superior mechanical strength and toughness. What roles do the nanoscale structures play in the inelasticity and toughening of nacre? Can we learn from this to produce nacre-like nanocomposites? Here we report in situ dynamic atomic force microscope observations of nacre with aragonite nanograins (nanoparticles) of an average grain size of 32 nm, which show that nanograin rotation and deformation are the two prominent mechanisms contributing to energy dissipation in nacre. The biopolymer spacing between the nanograins facilitates the grain rotation process. The aragonite nanograins in nacre are not brittle but deformable.     

Cavitation and cavity-induced fracture during superplastic deformation

The characteristics of fracture by cavitation in superplastic materials are reviewed. Particular attention is paid to the theoretical developmental aspects of cavity nucleation, cavity growth and cavity interlinkage. Various factors, including grain boundary sliding, impurity atoms or particles, phase proportion, deformation temperature, strain rate, strain and grain size, are discussed. Finally, methods for controlling cavitation during superplastic deformation are summarized, and problems which require further work are also presented.     

Nanoscale damage during fracture in silica glass

We report here atomic force microscopy experiments designed to uncover the nature of failure mechanisms occuring within the process zone at the tip of a crack propagating into a silica glass specimen under stress corrosion. The crack propagates through the growth and coalescence of nanoscale damage spots. This cavitation process is shown to be the key mechanism responsible for damage spreading within the process zone. The possible origin of the nucleation of cavities, as well as the implications on the selection of both the cavity size at coalescence and the process zone extension are finally discussed.     

Multidimensional model and criterion of ductile fracture during plastic deformation

Translated from Problemy Prochnosti, No. 9, pp. 14–18, September, 1988.     

Mechanisms of deformation and fracture of multilayer coatings during thermal cycling

Damage to multilayer coatings under thermal cyclic loading is investigated. The mechanisms of crack formation are established and studied as a function of the composition of the coatings, the base metal, and the form of cycle. On the basis of these results, we suggest a composition of multilayer coating with high heat resistance.Science and Production Department, Polzunov Central Design and Technological Institute, St. Petersburg. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 6, pp. 48–55, November–December, 1993.     

Micro- and nano-scale deformation processes during scratch damage in high density polyethylene

Abstract

Atomic force microscopy and scanning electron microscopy techniques have been used to examine the surface deformation experienced by high density polyethylene during the scratch test. The scratch deformation process involves stretching of fibrils and microfibrils resulting in the formation of surface openings. At the molecular level the chains of molecules unfold and align in the direction of the moving indenter. In the scratch test, the scratch velocity may suggest that low strain rates are valid, but the local strain rates can be many orders of magnitiude higher as exemplified by atomic force microscopy. A number of modes of deformation are encountered during scratching. They include deformation bands, crazing, tearing, microcracking, regular cracking, and grooving. Crazing-tearing is the predominant mode of scratch deformation. It is envisaged that the sequence of tearing along the craze involves formation of deformation bands, development of craze, followed by tearing. Atomic force and scanning electron microscopy of scratch surface damage indicated that the nature and modes of scratch deformation are qualitatively similar to the case of uniaxial tensile deformation, implying similarities in the deformation behaviour between scratch and tensile deformation.     

用SEM研究碳纤维的表面及断口形貌

采用SEM观察不同纺丝工艺的日本和国产碳纤维的表面及断口形貌,从结构上对比分析它们的差异,结果表明,表面缺陷减少,内部致密性提高,轴向微孔尺寸减小都是碳纤维强度提高的主要原因。国产干喷湿纺碳纤维表面缺陷和内部缺陷明显减少,有助于提高纤维拉伸强度。另外,洁净的生产环境及精细加工的设备有利于碳纤维缺陷的减少,使纤维拉伸强度提高的空间变大。     

Assessment of interface damage during the deformation of carbon nanotube composites

The deformation micromechanics of single-walled carbon nanotubes in a polymeric matrix was studied through the use of Raman spectroscopy. The variation of stress sensitive G′ band positions was used to detect the interfacial adhesion between the nanotubes and the matrix when the composites were subjected to a cyclic deformation process. It was found that the level of the interfacial adhesion decreases with the maximum loading strain and the repeated loading cycles. The debonding phenomenon was saturated by the third cycle of loading of the composites up to 1.0% strain. A hysteresis loop was observed to develop due to the change of the stress transfer efficiency between the loading and the unloading steps when the sample was deformed over 0.4% strain. By analysing the loop area, the energy dissipated in the deformation of the composite materials was investigated and the extent of the interface damage was also assessed.     

Observations and predictions of fracture in asbestos-cement composites

Direct observation byin situ scanning electron microscopy of the failure process in asbestos—cement composites indicates that multiple microcracking and extensive fibre pull-out are dominant in this material under load. From this experimental base, suitably modified analytical treatments are shown to give good predictions of mechanical strength.     

Systematic evaluation of the damage to a material during plastic deformation

Aluminum alloy D16 is used as an example to systematically evaluate the damage to metal on the basis of characteristics of its damping capacity (logarithmic vibration decrement), acoustic emission, and weight as determined by a new method developed by the authors. It is shown that the damage sustained by metallic materials during static loading can be estimated from the change in the vibration decrement and certain acoustic-emission parameters. These conclusions are substantiated by similar relations expressing the dependence of these parameters on the dilatation of the material in an evaluation of its volume change. Translated from Problemy Prochnosti, No. 5, pp. 23–30, May, 1996.     

Cyclic deformation and fracture of polymers

The cyclic stress-strain behaviour of a wide variety of rigid polymers has been studied. Three classes of fatigue response can be defined, each class displaying a characteristic evolutionary pattern in the stress-strain relation as deformation proceeds from the initial fatigue cycle to fatigue-crack propagation. Ductile polymers undergo a marked decrease in deformation resistance prior to crack formation; the detailed mechanism by which this “softening” develops can be related to the material microstructure and thermomechanical history. Amorphous polymers with a moderate degree of ductility soften slightly; in these materials crazing plays a dominant role in both the cyclic stress-strain response and the structural fatigue resistance. Brittle and nearly-brittle polymers are essentially stable in cyclic deformation; the fatigue resistance of these materials is very sensitive to strain amplitude in cyclic deformation.     

Simulations of deformation and damage processes of SiCp/Al composites during tension

The deformation, damage and failure behaviors of 17 vol.% SiCp/2009Al composite were studied by microscopic finite element (FE) models based on a representative volume element (RVE) and a unit cell. The RVE having a 3D realistic microstructure was constructed via computational modeling technique, in which an interface phase with an average thickness of 50 nm was generated for assessing the effects of interfacial properties. Modeling results showed that the RVE based FE model was more accurate than the unit cell based one. Based on the RVE, the predicted stress-strain curve and the fracture morphology agreed well with the experimental results. Furthermore, lower interface strength resulted in lower flow stress and ductile damage of interface phase, thereby leading to decreased elongation. It was revealed that the stress concentration factor of SiC was ～2.0: the average stress in SiC particles reached ～1200 MPa, while that of the composite reached ～600 MPa.     

Plastic deformation and fracture in coarse-grained aluminum

Plastic deformation and fracture in aluminum polycrystalline aggregate were investigated experimentally. A series of tensile specimens with a single edge crack were made of coarse-grained aluminum plates. The in-plane moiré technique was used to quantitatively obtain the deformation field around the crack tip. The strain field ahead of the crack tip prior to crack growth, as well as grain rotations during the course of plastic deformation, were evaluated from the corresponding moiré fringe patterns. The results of this study show that for small plastic deformation, grain rotation starts to take place at the very beginning of the plastic deformation and increases proportionally with plastic strain. The plastic strain ahead of the crack tip prior to crack growth drops significantly with decreasing average grain size of the specimen. Grain boundary sliding was also observed at some of the grain boundaries where the resolved shear stress had reached a critical value. The results also show that the crack propagated with maximum velocity at the center of a grain and assumed much slower velocity near grain boundaries or grain boundary junctions. The influence of the deformation rate is also discussed in terms of the stress relaxation.     

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.     

Emergence of fibrous fan morphologies in deformation directed reformation of hyperelastic filamentary networks

Recently, the authors generalized a theory for modelling the scission and reforming of crosslinks in isotropic polymeric materials to include materials in which elastic fibers are embedded in an elastic matrix. The fibers were assumed to dissolve with increasing deformation and then to immediately reassemble in a direction defined as part of the model. The model was illustrated in detail for uniaxial stretching along the direction of the fibers. Fiber reassembly was along the original fiber direction and did not result in a change in fiber alignment. The present work examines the implications of this model when the direction of reassembly is uncorrelated with the original fiber direction. In particular, the fibers are assumed to reassemble in the direction of maximum principal stretch of the matrix. The specific case is treated when the deformation is simple shear and the initial fiber direction is perpendicular to the direction of shear. The resulting fiber elongation with increasing shear results in fiber dissolution over a constitutively determined interval of the amount of simple shear. Newly formed fibers align in the current principal direction of maximum stretch, which is a direction that changes with the amount of simple shear. The resulting interval of alignment angles generates a fan-like fiber morphology at each material point. The formation and structure of the fan is described. In addition, the relation between the shear and normal stresses and the amount of shear is discussed, both during loading and unloading. It is shown that there can be a state of permanent set that is related to the original shape by triaxial extension and shear.