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.     

C1-controlled creep crack growth by grain boundary cavitation

Quasi steady-state creep crack growth is widely associated with the nucleation and growth of voids on grain boundaries ahead of the crack tip. In this paper, a micromechanics-based constitutive law is used to study the velocity-dependent fracture toughness of porous solids under extensive creep conditions. Void growth and coalescence in the fracture process zone is modeled by a nonlinear viscous microporous strip of cell elements. Under steady-state crack growth, two dissipative processes contribute to the macroscopic fracture toughness: the work of separation in the fracture process zone, and creep dissipation in the background material. Under extensive creep conditions, the competition between these two processes produces an inverted U-shaped C¹–velocity curve. The effects of rate sensitivity, initial porosity as well as hydrogen attack on fracture toughness are studied. The numerically simulated fracture toughness vs. crack velocity curves show good agreement with existing experimental results.     

Study on dynamic fracture behavior of TA15ELI ahoy under mode-Ⅱ loading by caustics method

The dynamic fractttre behavior of TA15ELI alloy with lath-like microstructure was studied by caustics method. Specimens with double-side pre-notch were tested under the plane-stress condition at mode-Ⅱ loading with a drop hammer system. Caustics information recorded in films illustrated the histories of both crack length and stress intensity factor. The dynamic fracture toughness and crack growth velocity of mixed feature of mainly plastic mode for TA15ELI alloy in dynamic mode-Ⅱ fracture. Shear localization was observed in the vicinity of the crack initiation area.     

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.     

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.     

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。     

Study on dynamic fracture behavior of TA15ELI alloy under mode-Ⅱ loading by caustics method

The dynamic fracture behavior of TA15ELI alloy with lath-like microstructure was studied by caustics method.Specimens with double-side pre-notch were tested under the plane-stress condition at mode-Ⅱ loading with a drop hammer system.Caustics information recorded in films illustrated the histories of both crack length and stress intensity factor.The dynamic fracture toughness and crack growth velocity of TA15ELI with lath-like microstructure were determined to be 279 MPa·m1/2 and 32.6 m/s,respectively.SEM fractograph analysis showed a mixed feature of mainly plastic mode for TA15ELI alloy in dynamic mode-Ⅱ fracture.Shear localization was observed in the vicinity of the crack initiation area.     

Crack branching instability and directional stability in dynamic fracture of a tough bulk metallic glass

We report the ductile fracture behavior of a tough Zr-based bulk metallic glass (BMG) in the dynamic regime, where a rapid moving crack initiates by a Mode II shearing and then advances through plastic voids growth and linking. A clear fractographic evolution from river-like vein pattern to chevron-like zone where microscopic dimples and voids coexist, followed by crack microbranching pattern, was observed on the dynamic fracture surfaces. A terminal crack velocity of about 0.5 of the Rayleigh wave velocity was determined for the onset of such microbranching instability. Furthermore, the dynamic branched daughter cracks are favored to continue in their original mother crack plane, having “directional stability”. Our findings may increase the understanding of the dynamic fracture of BMGs with both glassy nature and metallic bonding character.     

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.     

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.     

Strain rate effect on toughening of nano-sized PEP–PEO block copolymer modified epoxy

A bisphenol A-based epoxy was modified with an amphiphilic poly(ethylene-alt-propylene)–b-poly(ethylene oxide) (PEP–PEO) block copolymer as a toughening agent. PEP–PEO molecules self-assemble into spherical micelles in epoxy with an average diameter of 15 nm and give rise to 180% improvement in fracture resistance. The fracture and tensile behaviors of the PEP–PEO-modified epoxy were investigated at loading rates ranging from 0.51 to 508 mm min^?1. The toughened epoxy exhibits mechanical properties that are significantly more rate dependent than the neat epoxy material. As expected, a higher test rate leads to a more brittle behavior of the material and a lower fracture toughness value. With careful systematic study of their micromechanical deformation processes, the observed strain rate dependence is explained. The implications of the current findings on nano-sized rubber toughening of epoxy are also discussed in detail.     

Aging and fracture of human cortical bone and tooth dentin

Mineralized tissues, such as bone and tooth dentin, serve as structural materials in the human body and, as such, have evolved to resist fracture. In assessing their quantitative fracture resistance or toughness, it is important to distinguish between intrinsic toughening mechanisms, which function ahead of the crack tip, such as plasticity in metals, and extrinsic mechanisms, which function primarily behind the tip, such as crack bridging in ceramics. Bone and dentin derive their resistance to fracture principally from extrinsic toughening mechanisms, which have their origins in the hierarchical microstructure of these mineralized tissues. Experimentally, quantification of these toughening mechanisms requires a crack-growth resistance approach, which can be achieved by measuring the crack-driving force (e.g., the stress intensity) as a function of crack extension (“R-curve approach”). Here this methodology is used to study the effect of aging on the fracture properties of human cortical bone and human dentin in order to discern the microstructural origins of toughness in these materials.     

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.     

Laminated particulate-reinforced aluminum composites with improved toughness

An architectural approach for toughening discontinuously reinforced aluminum (DRA) alloys is described. It is based upon exploiting the higher apparent toughness of thin DRA lamina to obtain a laminate of higher thickness and toughness. The laminated composite consisted of alternate layers of a 7093/SiC/15p DRA, and an unreinforced aluminum–manganese alloy, 3003. Fracture toughness tests in the crack divider configuration showed a toughness improvement of 79% in an underaged condition and an improvement of 53% in the peak-aged condition compared to the monolithic DRA. Fractographic observations of the primary void size, and its close correspondence with the fracture surface of thin specimens, provided evidence that the individual DRA lamina indeed experienced sufficient loss of constraint in the thickness direction. This was further confirmed by observations of delamination in the fast fracture domain. However, the bond strength was quite good, as evidenced by very little delamination in the fatigue crack growth region, and by a lack of such damage in tension specimens. In essence, the laminate behaved as a smart structure, being resistant to failure under normal conditions, but allowing full loss of constraint in the severe stress–strain field ahead of a loaded crack. Modeling efforts were consistent with a reduction of hydrostatic stress, through the loss of out-of-plane constraint for the laminas, although the predicted level of toughness improvement was lower than observed. Overall, this study clearly demonstrates that the fracture toughness of laminated composites can be engineered based on the understanding of constraint effects associated with specimen thickness in the DRA composite.     

Interactions between domain switching and crack propagation in poled BaTiO3 single crystal under mechanical loading

In this paper, the interactions between the crack propagation and corresponding domain switching in ferroelectric single crystal under mechanical loading were investigated. An experimental setup with a polarized light microscope (PLM) was designed and constructed to in situ observe the crack propagation in poled BaTiO₃ specimens subjected to three-point-bending loading. The observed domain switching was stimulated by the intensive stress field near the crack tip, and the theoretical R-curve taking into account the domain switching toughening agrees well with the experimental results quantitatively. It is confirmed from the actual switched zone that the 90° domain switching is the major mechanism of the fracture toughening, and the apparent fracture toughness increases by 150% in the BaTiO₃ single crystal specimen.     

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.     

Testing study of subcritical crack growth rate and fracture toughness in different rocks

1 Introduction The time effect of rock engineering stability is attached importance to with enlarging of the rock engineering scale. The instability instances in-situ and rock laboratory examinations show that the rock instability is owing to crack growt…     

韧-脆转变温度区间内焊接接头断裂韧度预测

首先对采用同种焊接材料施焊的X80及X65管线用钢焊接接头,在韧脆转变温度区间测试了其断裂韧度,试验研究了强度匹配、试样几何形式对接头断裂行为的影响。然后根据局部法由标准三点弯曲试样的断裂韧度值,得出反映材料的脆性断裂控制参量m、σu,再根据该参量对同种接头及不同强度匹配接头双边裂纹拉伸试样的断裂行为进行了定量预测,当考虑韧脆转变温度区间内的延性裂纹扩展后,预测结果与试验结果相当吻合。表明,局部法能很好地描述材料的断裂行为。     

High cycle fatigue and fracture behavior of 2124-T851 aluminum alloy

The high cycle fatigue properties and fracture behavior of 2124-T851 aluminum alloy were investigated roundly, including the fatigue crack growth rate, fracture toughness and fatigue S--N curve. Furthermore, the fatigue crack growth rate was analyzed by fitting the curves. And the microstructure of the alloy was studied using by optical microscopy, transmission electron microscopy and X-ray diffractometry, scanning electron microscop1/2 The results show that the fatigue strength and the fracture toughness of 2124-T851 thick plate are 243 MPa and 29.6 MPa.m at room temperature and R=0. 1, respectively. At high cycle fatigue condition, the characteristics of fatigue facture were observed obviously. And the higher the stress amplitude, the wider the space between the fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area and the greater the ratio between the intermittent fracture area and the whole fracture area.     

焊接接头高应变率下的动态断裂韧度分析

对采用Hopkinson气炮加载－Charpy V形冲击试样试验方法对焊接接头高应变率下的动态断裂韧度数据进行了分析研究，通过对试样断口形貌、冲击吸收功、断裂韧度及其影响因素进行分析，表明在高应变率加载条件下，焊接接头冲击试样断口形貌有特殊的表现，延伸带或钝化区的尺寸大小反映了裂纹萌生前的塑性变形大小，也反映了裂纹萌生功的变化趋势。同时说明焊接接头在高速动态冲击载荷作用下，其断裂韧度参量受组配强度、冲击速度及环境温度等因素影响显著，焊接热影响区的断裂韧度值关系到焊接接头断裂韧度的好坏。