Fracture processes and mechanisms of crack growth resistance in human enamel

Human enamel has a complex micro-structure that varies with distance from the tooth’s outer surface. But contributions from the microstructure to the fracture toughness and the mechanisms of crack growth resistance have not been explored in detail. In this investigation the apparent fracture toughness of human enamel and the mechanisms of crack growth resistance were evaluated using the indentation fracture approach and an incremental crack growth technique. Indentation cracks were introduced on polished surfaces of enamel at selected distances from the occlusal surface. In addition, an incremental crack growth approach using compact tension specimens was used to quantify the crack growth resistance as a Junction of distance from the occlusal surface. There were significant differences in the apparent toughness estimated using the two approaches, which was attributed to the active crack length and corresponding scale of the toughening mechanisms.     

Effect of switching stresses on domain evolution during quasi-static crack growth in a ferroelastic single crystal

The present paper demonstrates the effect of switching stresses on domain evolution and fracture toughening during quasi-static crack growth in elastically isotropic ferroelastic single crystals with transversally isotropic ferroelastic strains. With a simple switching algorithm and crack propagation procedure, domain evolution is simulated in an exemplary material with semi-infinite crack under mode I loading, starting from a mono-domain configuration. Domain reorientation is found to be strongly affected by switching stresses, which therefore have to be considered in the context of domain evolution modelling and fracture toughening. Before the onset of crack growth a needle-like domain is formed at the tip of the stationary crack, but this does not effect the crack tip stress intensity factor. Elongation of this domain during the onset of crack growth causes a large increase of the fracture toughness. Domain separation in a later stage results in toughness reduction. The subsequent domain evolution indicates a periodic formation of needle-like domains as observed in soft ferroelastic materials.     

Toughening effects quantification in glass matrix composite reinforced by alumina platelets

A borosilicate glass matrix composite containing alumina platelets was considered to investigate toughening mechanisms and crack tip behavior in dispersion reinforced brittle matrix composites. Fracture toughness was determined by applying the chevron notched specimen technique, and fractographic analysis was employed to reveal the active toughening mechanisms with increasing content of reinforcement. A roughness-induced shielding effect has been quantified to prove the relation between fracture toughness and fracture surface roughness. Theoretical calculations of the fracture toughness enhancement based on a modified crack deflection model developed by Faber and Evans, combined with the influence of the increase in Young’s modulus, were found to be in good agreement with experimental data. An effect of residual stresses upon toughening of the investigated composite is discussed.     

R-curve behavior of BaTiO3- and PZT ceramics under the influence of an electric field applied parallel to the crack front

The fracture resistance curves (R-curves) of BaTiO₃ and commercial PZT–PIC 151 were measured with compact tension specimens under the influence of an electric field applied parallel to the crack front. A strong influence of the electric field on the starting and plateau value was found as well as on the length of the R-curve. Generally a toughness increase was detected with increasing electric field. The toughening effect is estimated from the change in crack tip stress intensity induced by ferroelastic domain switching near the crack tip using the weight function formalism developed for stress-induced transformation toughening of zirconia ceramics. In order to obtain a quantitative prediction of toughening, ferroelastic and ferroelectric properties were measured.     

Phase assemblage effects on the fracture and fatigue characteristics of magnesia-partially stabilized zirconia

A detailed investigation on the relationships between phase assemblage and fracture and fatigue characteristics of Mg-PSZ has been conducted. In doing so, three completely different microstructural conditions were first attained through different thermal treatments and then their flexural strength, fracture toughness and crack growth resistance and fatigue crack growth (FCG) behaviour were evaluated. The obtained results are discussed considering the interplay between microstructural features and dominant crack-microstructure interaction and its influence on the operation of given toughening and mechanical fatigue mechanisms for each phase assemblage studied. FCG resistance, under both sustained and cyclic loading, is found to be closely related to the corresponding fracture toughness of each phase assemblage. However, real mechanical fatigue effects are estimated to be, once they are rationalized with respect to particular environmental-assisted cracking behaviours, an exclusive function of crack path type. Finally, different cyclic fatigue mechanisms for Mg-PSZ are pinpointed depending upon the prevalent transgranular or intergranular FCG morphology.     

Effects of rate on crack growth in a rubber-modified epoxy

The rate-dependent fracture behavior of a 10-phr rubber-modified epoxy was investigated using double-cantilever-beam tests at various crosshead speeds. Dramatic rate effects were observed in the R-curve behavior and in the relationship between the applied energy-release rate and the crack velocity. Furthermore, a transition between fracture with toughening mechanisms operating (kinetic crack growth) and brittle behavior (dynamic crack growth) was observed. This transition depended on the crack velocity and applied energy-release rate. Such behavior is expected to depend on how the intrinsic toughness and/or the extrinsic toughening mechanisms are influenced by strain rate. It was shown that the size of the process zone was only weakly dependent on the crack velocity until the onset of dynamic fracture. Furthermore, the extent of void growth was virtually independent of the crack velocity in the kinetic regime. These results appear to rule out the notion that crack-tip shielding is significantly affected by rate effects in this rubber-modified epoxy. Rather, the rate effects may arise from a rate-dependent intrinsic toughness. It was observed that the intrinsic toughness decreased significantly with increasing crack velocity. The crack instability was shown to be associated with an abrupt cessation of the development of the process zone, with both cavitation and void growth being totally suppressed.     

Effects of dislocation dynamics and microstructure on crack growth mechanisms in two-phase titanium aluminide alloys

The fracture behaviour of two-phase titanium aluminide alloys was characterized by fracture toughness tests performed in a wide temperature range on chevronnotched three point bending bars. Temperature and rate dependent deformation processes were characterized by temperature and strain rate cycling tests. The alloy investigated had compositions and microstructures which are currently being considered for engineering applications. The paper considers the effects of microstructure and crack tip plasticity on the crack growth resistance. The temperature dependence of the fracture toughness was rationalized in terms of micro-processes which determine the glide resistance of the dislocations in the plastic zone of crack tips. The implications of such observations for the engineering application of the materials are addressed briefly.     

Effect of Delamination on Mechanical Properties of Al-Li Alloy Modified by Rare Earth Element Ce

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杂质及Ce对8090Al—Li合金内,外韧化水平的影响

本文研究了杂质及Ｃｅ对８０９０Ａｌ－Ｎｉ合金内，外韧化水平的影响。结果表明，Ｆｅ，Ｓｉ主Ｎａ，Ｋ杂质有一定外韧化效果，但严重降低内韧化水平，在含料多杂质的材料中添加微量Ｃｅ，能够提高内韧化水平，但却降低外韧化水平。     

Investigations on the microstructure and room temperature fracture toughness of directionally solidified NiAl–Cr(Mo) eutectic alloy

The microstructures and room temperature fracture toughness of directionally solidified NiAl-xCr-6Mo (x = 28, 32 and 36 at%) alloys were investigated. Fully eutectic microstructure could be obtained in the alloys over a wide composition range. High temperature gradient could increase the planar/cellular transition rate and expand the eutectic coupled growth zone. The volume fraction of Cr(Mo) strengthening phase increased with the increasing content of Cr, accordingly, the fracture toughness of NiAl–Cr(Mo) alloys also gradually increased. The fracture toughness of 26.15 MPa m^1/2 was obtained in the NiAl-36Cr-6Mo hypereutectic alloy solidified at withdrawal rate of 10 μm/s and temperature gradient of 600 K/cm, which is the highest value in the NiAl–Cr–Mo alloy system until now. Well-aligned microstructure was beneficial to the enhancement of the fracture toughness, while the existence of primary phase seriously deteriorated the toughness. All the directionally solidified NiAl–Cr(Mo) alloy failed as brittle quasi-cleavage fracture. Some toughening mechanisms, such as crack bridging, crack nucleation, crack blunting, crack deflection, interface debonding and shear ligament toughening as well as linkage of microcracks were observed. In addition, mobile dislocation generated from the interface also had significant influence on the toughness.     

Micro- and macrostructural factors in DRA fracture resistance

Fracture resistance continues to be a critical, heavily researched property for discontinuously reinforced aluminum (DRA) materials because of its importance in many of the structural applications for which these materials are candidates. Over the last ten years, significant progress has been made in understanding and improving the fracture resistance ofD RA materials, largely through the control of microstructural features by judicious alloy and reinforcement selection coupled with process modification and control. This article reviews recent progress in microstructural understanding and describes directions being pursued for further improvement, especially in particle-reinforced systems. In addition, methods to improve fracture resistance that are more macrostructural in nature are being developed. These involve the combination of reinforced and unreinforced regions on a macroscale in planar and axisymmetric geometries in order to capitalize on extrinsic toughening mechanisms.     

Room—temperature mechanical properties of WSi2／MoSi2 composites

Five Kinds of WSi2/MoSi2 composites were successfully prepared by mechanical alloying,IP and high temperature sintering techniques.And their hardness and fracture toughness were measured by the Vickers indentation fracture mode through an Hv-10A type sclerometer.The microstructure and morphology were investigated by a JSM-5600IV scanning electron microscope.Results show that the addition of 50% WSi2(in mole fraction)has remarkable hardening and toughening effects on the MoSi2 matrix.whose hardness value and fracture toughness value are increased about 60% and 86%,respectively.For WSi2/MoSi2 comosite,the hardening mechanisms are fine-grain and the second phase particles strengthening,and the toughening mechanisms include fine-grain,grain drawing,crack deflection,microbridge and bowing toughening.     

Micromechanics-based model for trends in toughness of ductile metals

Relations between fracture toughness and microstructural details have been calculated for ductile materials based on a dilatational plasticity constitutive model that has recently been proposed. The model generalizes the Gurson model to account for both void growth and coalescence with explicit dependence on void shape and distribution effects. Based on a small scale yielding formulation of crack growth, toughness trends are determined as a function of yield stress, strain-hardening, initial porosity, void shape and spacing as well as void spacing anisotropy. Distinctions are drawn between the engineering fracture toughness, which is typically associated with 0.2 mm of crack growth, and the theoretical toughness based on coalescence of the crack tip with the first void ahead of it. Comparison with one set of experimental data for a steel is made for which a fairly complete characterization of the microstructure is available.     

Fracture Toughness of Functionally Graded Steels

In this study, fracture toughness of functionally graded steels in both crack divider and crack arrester configurations has been studied. Spot-welded plain carbon steel and austenitic stainless steel with different thicknesses and arrangements were used as electrodes of electroslag remelting to produce functionally graded steels. Fracture toughness of the specimens in crack divider configuration was found to depend on the arrangements of the primary electrodes’ pieces together with the type of the containing phases. In crack arrester configuration, the fracture toughness was found to depend on the crack tip position and the distance of the crack tip with respect to the bainitic or martensitic intermediate layers.     

Quantitative evaluation of fracture toughness-microstructural relationships in alpha-beta titanium alloys

The fracture toughness of two alpha-beta titanium alloys containing an alpha platelet in a transformed beta matrix has been examined in terms of the microstructural parameters controlling the fracture initiation and propagation in the alloys. Equations have been formulated that show that the highest toughness values of both alloys were associated with the finest platelet spacings and the thickest alpha platelets. It is proposed that the fracture initiation process in both alloys is controlled by the distance between the platelets, the fracture toughness of the alloys being dependent on the distance between active centers of void nucleation, i.e., as a function of the alpha platelet thickness and spacing between the platelets. Seven models of ductile fracture relating fracture toughness to mechanical property and microstructural parameters have been compared in their ability to predict the toughness of the alloys after solution treatments, which produce varying platelet thickness and inter-platelet spacings. The principle has been adopted following Rice and Rosengren and Hutchinson (HRR)^[1,2] that there must be a 1/x energy singularity at the crack tip, which also prescribes the stress and strain distribution ahead of a crack tip. Any model not incorporating these requirements should be rejected.     

通过调整润湿性开发具有高抗损伤性能的仿珍珠贝壳层状2024Al/B4C复合材料

为了解决复合材料中B4C陶瓷相难以被金属铝润湿的问题,利用TiH2和B4C的原位反应引入TiB2,进而调节其润湿性和界面结合.通过将熔融合金压力浸渗到冷冻铸造法制备的多孔陶瓷支架中,制备具有层状结构的2024Al/B4C?TiB2复合材料.与2024Al/B4C复合材料相比,加入TiH2后复合材料的抗弯强度和裂纹扩展韧...     

氧化铝陶瓷增韧的研究现状

Al2O3陶瓷具有耐高温、抗氧化、耐磨损等优良特性，但韧性低一直是阻碍其进一步应用的重要原因。本文分别通过颗粒弥散相增韧、纤彬晶须增韧、相变增韧、复合增韧以及自增韧等方法对Al2O3陶瓷的增韧研究进行了分类叙述，总结了高韧性氧化铝陶瓷的制备、增韧机理、影响因素的研究进展。     

Fatigue and fracture toughness of epoxy nanocomposites

The fatigue and failure mechanisms of epoxy composites have been researched extensively because of their commercial importance in fields demanding materials with high specific strength. Particulate, sheets, short and long fibers with dimensions in the micrometer and nanometer range are the major fillers which have been studied for enhancing the fatigue resistance of epoxies. The nano and micro scale dimensions of the fillers give rise to unexpected and fascinating mechanical properties, often superior to the matrix including fracture toughness and fatigue crack propagation resistance. Such properties are dependent on each other (e.g., the fatigue properties of the polymer composites have been found to be strongly influenced by its toughness). This article is a review of the various developments in this field and the underlying mechanisms which are responsible for performance improvements in such composites.     

Fracture behavior of α-zirconium phosphate-based epoxy nanocomposites

The fracture behaviors of α-zirconium phosphate (α-ZrP) based epoxy nanocomposites, with and without core-shell rubber (CSR) toughening, were investigated. The state of exfoliation and dispersion of α-ZrP nanofiller in epoxy were characterized using X-ray scattering and various microscopy tools. The level of enhancement in storage moduli of epoxy nanocomposite against neat epoxy is found to depend on the state of exfoliation of α-ZrP as well as the damping characteristics of the epoxy matrix. The fracture process in epoxy nanocomposite is dominated by preferred crack propagation along the weak intercalated α-ZrP interfaces, and the presence of α-ZrP does not alter the fracture toughness of the epoxy matrix. However, the toughening using CSR can significantly improve the fracture toughness of the nanocomposite. The fracture mechanisms responsible for such a toughening effect in CSR-toughened epoxy nanocomposite are rubber particle cavitation, followed by shear banding of epoxy matrix. The ductility and toughenability of epoxy do not appear to be affected by the incorporation of α-ZrP. Approaches for producing toughened high performance polymer nanocomposites are discussed.     

The Multiscale Origins of Fracture Resistance in Human Bone and Its Biological Degradation

Akin to other mineralized tissues, human cortical bone can resist deformation and fracture due to the nature of its hierarchical structure, which spans the molecular to macroscopic length scales. Deformation at the smallest scales, mainly through the composite action of the mineral and collagen, contributes to bone’s strength or intrinsic fracture resistance, while crack-tip shielding mechanisms active on the microstructural scale contribute to the extrinsic fracture resistance once cracking begins. The efficiency with which these structural features can resist fracture at both small and large length scales becomes severely degraded with such factors as aging, irradiation, and disease. Indeed, aging and irradiation can cause changes to the cross-link profile at fibrillar length scales as well as changes at the three orders of magnitude larger scale of the osteonal structures, both of which combine to inhibit the bone’s overall resistance to initiation and growth of cracks.