考虑应变梯度效应的三点弯梁模型解析研究--第一部分:局部化带的传播

为分析应变软化和由此带来的应变局部化问题,将梯度塑性理论引入裂纹带模型。以拉应变局部化区域代替裂纹带,在三点弯梁裂纹带(具有一定尺寸的带宽由特征长度确定)内部存在着不均匀分布的拉应变,这与实验结果相符。对拉应变进行积分,得到了拉应变局部化区域的张拉位移的理论表达式,结果表明:该位移与拉应力成线性规律,拉应变局部化区域的宽度越大,弹性模量越小或降模量越小,则该位移越大。此外,采用应力平衡条件、应变软化的本构关系及平截面假定,还得到了拉应变局部化区域的扩展规律,结果表明:下降模量越大、三点弯梁高度越小及弹性模量越小,则在相同的拉应力的情况下,拉应变局部化区扩展的长度越小;抗拉强度对拉应变局部化区扩展长度的最大值没有影响。此外,还研究了梁中部横截面内中性轴到具有最大承载能力的点的距离的变化规律。     

考虑应变梯度效应的三点弯梁模型解析研究--第二部分:局部化带的位移及转角

基于梯度塑性理论,分析了应变软化及真实裂纹扩展阶段的局部化带的张拉位移和转角。在弹性阶段,可以由弹性理论来确定二者的关系。真实裂纹出现后,利用平衡条件、几何条件及梯度以来的应变软化本构关系,得到了真实裂纹长度与局部化带长度的关系。当真实裂纹刚出现时,局部化带长度达到最大值。在任何阶段,局部化带到中性轴的距离单调降低,局部化带的张拉位移和转角受梁深、带宽、弹模及下降模量等的影响。弹模及下降模量越大,带宽越小,则局部化带的张拉位移和转角都增加。而且,在前两个阶段,张拉位移都线性增加,但在后两个阶段,转角都非线性增加。     

Fracture mechanisms of aluminium alloy AA7075-T651 under various loading conditions

The fracture behaviour of the aluminium alloy AA7075-T651 is investigated for quasi-static and dynamic loading conditions and different stress states. The fracture surfaces obtained in tensile tests on smooth and notched axisymmetric specimens and compression tests on cylindrical specimens are compared to the fracture surfaces that occur when a projectile, having either a blunt or an ogival nose shape, strikes a 20 mm thick plate of the aluminium alloy. The stress state in the impact tests is much more complex and the strain rate significantly higher than in the tensile and compression tests. Optical and scanning electron microscopes are used in the investigation. The fracture surface obtained in tests with smooth axisymmetric specimens indicates that the crack growth is partly intergranular along the grain boundaries or precipitation free zones and partly transgranular by void formation around fine and coarse intermetallic particles. When the stress triaxiality is increased through the introduction of a notch in the tensile specimen, delamination along the grain boundaries in the rolling plane is observed perpendicular to the primary crack. In through-thickness compression tests, the crack propagates within an intense shear band that has orientation about 45° with respect to the load axis. The primary failure modes of the target plate during impact were adiabatic shear banding when struck by a blunt projectile and ductile hole-enlargement when struck by an ogival projectile. Delamination and fragmentation of the plates occurred for both loading cases, but was stronger for the ogival projectile. The delamination in the rolling plane was attributed to intergranular fracture caused by tensile stresses occurring during the penetration event.     

(001)面双轴应变锗材料的能带调控

本文基于形变势理论构建(001)面双轴应变Ge材料的能带结构模型。计算结果表明(001)面双轴应变可以将Ge的能带从以L能谷为导带底的间接带半导体调控到以Δ4能谷为导带底的间接带半导体或者以Г能谷为导带底的直接带半导体。同时室温下Ge的带隙与应变的关系可用四段函数来表示:当压应变将Ge材料调控为以Г能谷为导带底的间接带半导体后,每增加1%的压应变,禁带宽度将线性减小约78.63meV;当张应变将Ge材料调控为直接带半导体后,张应变每增加1%,禁带宽度将线性减小约177.98meV;应变介于-2.06%和1.77%时,Ge将被调控为以L能谷为导带底的间接带半导体,禁带宽度随着压应变每增加1%而增加11.66meV,随着张应变每增加1%而线性减小约88.29meV。该量化结果可为研究和设计双轴应变Ge材料及其器件提供理论指导和实验依据。     

晶须取向对HAP/SiCw复合生物陶瓷力学性能的影响

本文用原位复合法合成了纳米HAP/SiCw复合微粉,用热压烧结法制取了HAP/SiCw复合生物陶瓷,并研究了晶须取向对其力学性能的影响.试验结果表明,当复合材料所受拉应力方向与热压面平行或剪应力方向与热压面垂直时材料的力学性能最好,当所受拉应力方向与热压面垂直或剪应力方向与热压面平行时材料的力学性能最差.这一结论对HAP/SiCw复合生物陶瓷的实际应用有重要的指导意义.     

MIXED-MODE FRACTURE MECHANISMS NEAR THE FATIGUE THRESHOLD OF AISI 316 STAINLESS STEEL

Abstract— Near threshold, mixed mode (I and II), fatigue crack growth occurs mainly by two mechanisms, coplanar (or shear) mode and branch (or tensile) mode. For a constant ratio of ΔK_I/ΔK_II the shear mode growth shows a self-arrest character and it would only start again when ΔK_I and ΔK_II are increased. Both shear crack growth and the early stages of tensile crack growth, are of a crystallographic nature; the fatigue crack proceeds along slip planes or grain boundaries. The appearance of the fracture surfaces suggest that the mechanism of crack extension is by developing slip band microcracks which join up to form a macrocrack. This process is thought to be assisted by the nature of the plastic deformation within the reversed plastic zone where high back stresses are set up by dislocation pile-ups against grain boundaries. The interaction of the crack tip stress field with that of the dislocation pile-ups leads to the formation of slip band microcracks and subsequent crack extension. The change from shear mode to tensile mode growth probably occurs when the maximum tensile stress and the microcrack density in the maximum tensile plane direction attain critical values.     

Finite element limit analyses of under-matched tensile specimens

This paper quantifies the effect of under-matching on plastic limit loads and fully plastic stress triaxialities for mismatched flat plate (in plane strain and plane stress) and round bar tensile specimens, via parametric finite element (FE) limit analyses based on elastic-perfectly-plastic materials. It is found that the effect of the strength mismatch ratio (the ratio of the yield strength of the weld zone to that of the base material) on plastic limit loads and fully plastic stress triaxialities can be significant for plane strain plate and round bar specimens, but much less significant for plane stress plate specimens. Its effect is dependent significantly on the slenderness of the weld zone (defined by the ratio of the weld zone width to the specimen width). The effect of the slenderness of the weld zone is apparent only when the weld width is less than the specimen width or diameter. In particular, the stress triaxiality in the softer weld zone can increase significantly with decreasing the ratio of the weld zone width to the specimen width (radius). Based on the present limit load results, a simple method to extract intrinsic tensile properties of the under-matched weld zone from test results of under-matched tensile specimens is proposed.     

一类非均布荷载作用下厚壁圆筒平面问题的应力解析解

采用复变函数法给出了表面受有一类非均布径向压力作用下厚壁圆筒平面问题的应力解析解,发现随着非均布侧压系数的增大,圆筒内某些区域的切向应力和(或)径向应力会由压应力变为拉应力,并且切向拉应力远大于径向拉应力。在不存在拉应力区的前提下,反算出非均布侧压系数的范围,并给出了不同非均布侧压系数所对应的拉应力区。最后用数值方法验证了理论分析的正确性。     

反应磁控溅射ZnO薄膜的高温退火研究

ZnO薄膜是一种新型的II VI族直接能带化合物半导体材料 ,有可能实现短波长的探测器 ,LED和LD等光电子器件。用磁控溅射法在硅衬底上生长ZnO薄膜 ,由于薄膜与衬底之间较大的应力失配 ,以及由于较快的生长速率 ,薄膜中存在较多的Zn间隙原子和O空位 ,在薄膜中存在应力。通过高温退火 ,可以使应力得到弛豫 ,降低O空位和Zn间隙原子的浓度 ,提高薄膜的化学计量比和改善薄膜的结晶质量。本实验用XRD和AFM研究了高温退火对ZnO薄膜的晶体性能和表面的影响。对ZnO薄膜在退火处理后c轴方向的应力性质的转变作了机理上的探讨。     

Effect of material and geometric parameters on deformations near the notch-tip of a dynamically loaded prenotched plate

We analyze plane strain thermomechanical deformations of a prenotched rectangular plate impacted on one side by a prismatic body of rectangular cross-section and moving parallel to the axis of the notch. Both the plate and the projectile are made of the same material. Strain hardening, strain-rate hardening and thermal softening characteristics of the material are modeled by the Johnson–Cook relation. The effect of different material parameters, notch-tip radius, impact speed and the length of the projectile on the maximum tensile principal stress and the initiation and propagation of adiabatic shear bands at the notch-tip is analyzed. It is found that for high impact speeds or enhanced thermal softening, two shear bands, one at −10° to the notch ligament and the other at −128° to it, propagate from the notch tip. Otherwise, only one shear band nearly parallel to the notch-ligament initiates at the notch-tip. The notch-tip distortion for high strength materials is quite different from that for low strength materials. The maximum tensile principal stress occurs at a point on the upper surface of the notch-tip and for every set of values of material parameters and impact speeds studied equals about 2.3 times the yield stress of the material in a quasistatic simple tension or compression test. We assume that the brittle failure occurs when the maximum tensile principal stress equals twice the yield stress of the material in a quasistatic simple tension test and a shear band initiates when the effective plastic strain at a point equals 0.5. The effect of material and geometric parameters on the time of initiation of each failure mode is computed. It is found that for low impact speeds (<30 m/s) a material will fail due to the maximum tensile principal stress exceeding its limiting value, and at high impact speeds due to the initiation of a shear band at the notch-tip. Results are also computed for a C-300 steel with material parameters given by Zhou et al. For an impact speed of 50 m/s, the shear band speed and the maximum effective plastic strain-rate before a material point melts are found to be 350 m/s and 5×10⁷/s respectively. Key words: Failure-mode transition, shear bands, thermoviscoplasticity, numerical simulations. This revised version was published online in July 2006 with corrections to the Cover Date.     

Deformation mechanisms of carbon nanotube fibres under tensile loading by in situ Raman spectroscopy analysis

Deformation mechanisms of carbon nanotube (CNT) fibres under tensile loading are studied by means of in situ Raman spectroscopy to detect the CNT deformation and stress distributions in the fibres. The G' band in the Raman spectrum responds distinctly to the tensile stress in Raman shift, width and intensity. The G' band changes with the tensile deformation of the fibre at different stages, namely elastic deformation, strengthening and damage-fracture. It is deduced that the individual CNTs only deform elastically without obvious damage or bond breaking. The yield and fracture of fibres can be due to the slippage among the CNTs.     

Interaction of crazes with pre-existing shear bands in glassy polymers

Shear bands have been grown in bulk specimens of P₃O(poly 2,6 diphenyl 1,4 phenylene oxide) and in thin films of two blends of polystyrene with poly(xylenyl ether). The subsequent interaction of crazes with these shear bands has been characterized by transmission electron microscopy. For the case of shear bands grown under the plane stress conditions of thin films, it is found that the bands act as preferential sites for craze nucleation. A fairly regularly-spaced array of short crazes grows within the shear bands and these crazes may thicken sufficiently to coalesce. When the crazes reach the end of the shear band they emerge and propagate into the unoriented polymer matrix. Within the shear band the craze growth direction does not lie normal to the tensile axis, but is rotated due to the molecular orientation of the shear band. The direction of craze growth is also affected under the plane strain conditions of bulk specimens. In this case the craze is diverted along the shear band before re-emerging into the matrix. Measurements of the craze fibril extension ratio, , within the shear band show an increase over typical values obtained outside the shear band in the same polymer. This high value of leads to an increased likelihood of craze break-down and crack nucleation within the shear band.     

应力生长FeCoSiB非晶薄膜的磁各向异性研究

采用应力生长方法，制备出受压应力和张应力作用的FeCoSiB非晶磁弹性薄膜，研究了薄膜中的应变大小对FeCoSiB薄膜的磁滞回线、剩磁、应力诱导各向异性场等磁特性的影响．结果表明，无应变薄膜在薄膜面内呈现各向同性，而有应变的薄膜呈现出明显的各向异性。张应力诱导的各向异性与应力方向平行，而压应力形成垂直于应力方向的磁各向异性。各向异性场随应变的增大而线性增大。     

SHEAR FATIGUE CRACK GROWTH: A LITERATURE SURVEY

A fatigue crack is often initiated by a localized cyclic plastic deformation in a crystal where the active slip plane coincides with the plane of maximum shear stress. Once a crack is initiated, the crack will propagate on the maximum shear plane for a while and, in the majority of the cases, will eventually change to the plane of the applied tensile stress. The “shear” and “tensile” modes of fatigue crack propagation are termed stage I and stage II fatigue crack growth. They are also known as mode II and mode I fatigue crack growth. However, the mechanism of the tensile mode fatigue crack propagation is shear in nature. Considerable progress has been made recently in the understanding of mode II fatigue crack growth. This paper reviews the various test methods and related data analyses. The combined mode I and mode II elastic crack tip stress field is reviewed. The development and the design of the compact shear specimen are described and the results of fatigue crack growth tests using the compact shear specimens are reviewed. The fatigue crack growth tests and the results of inclined cracks in tensile panels, center cracks in plates under biaxial loading, cracked beam specimens with combined bending and shear loading, center cracked panels and the double edge cracked plates under cyclic shear loading are reviewed and analyzed in detail.     

The influence of microstructural anisotropy on the mode of plate failure during projectile impact

Changes in the failure mode of rolled steel plate during projectile impact were examined in terms of the angle between the planes of microstructural inhomogenieties in the steel and the plane of the target. Two failure modes were identified: (1) discing failure involving multiple shear band formation in the rolling plane with separation along the shear bands to form a disc segment, and (2) tensile star cracking with the propagation of a tensile tear through the rear surface of the plate. Both the ballistic resistance and the predominant mode of failure were critically dependent on the orientation of the microstructural banding in the plate.     

Prediction of grain boundary stress fields and microcrack initiation induced by slip band impingement

Slip localization is widely observed in metallic polycrystals undergoing cyclic deformation or post-irradiation tensile deformation, whatever their crystallographic structure. Hence, strong strain localization occurs in thin slip bands (SBs) inducing by the way local stress concentrations at their intersections with grain boundaries (GBs). Many GB stress field formulae based on the dislocation pile-up theory have been proposed since the pionnering work of Stroh and others. These allow the use of the Griffith criterion for prediction GB fracture initiation. However, recent observations show that assuming that slip is localized on a single atomic plane leads to unrealistic results. In fact, a large number of slip planes are plastically activated and then finite slip band thickness should be accounted for. Numerous crystalline finite element (FE) computations have been carried out using considering a slip bands with low critical resolved shear stress embedded in an elastic matrix. The computed GB normal and shear stress fields:

are considerable lower than the pile-up ones and exhibit strong dependency on the slip band thickness close to the SB corner

but are in fair agreement with the solution predicted by the pile-up theory far away.

Since the pile-up theory leads to the overestimation of the local GB stress fields, the main goal of the current paper is to perform analytical model of GB stress components based upon FE calculations. The effect of various parameters can be understood in the framework of matching asymptotic expansions which is usually applied to cracks with V notches of finite thickness. Finally, the predicted remote stresses to GB fracture in pre-irradiated austenitic stainless steels subjected to tensile loading in various environment are compared to experimental data and the pile-up based predictions.     

Analysis of stress-induced phase transformations in elastoplastic materials with strain-softening behavior under plane shear

Possible two-phase piecewise-homogeneous deformations in elastoplastic materials with strain-softening behavior under plane shear are analyzed. Discontinuities of stress and deformation gradient across interfaces between phases are considered and continuity of traction and displacement across interfaces and the Maxwell relation is imposed. The governing equations are obtained. The analysis is reduced to finding a minimum value of the loading at which governing equations have a unique, real, physically acceptable solution. It is found that for a plate with constant thickness under plane shear two-phase piecewise-homogeneous deformations are possible, and the Maxwell stress, the stresses and strains in both phases, the jumps of stress and deformation gradient across interfaces and the inclination angle of the localized deformed band can all be determined. As an illustration, a NiTi alloy plate under plane shear is numerically analyzed. The inclination angle of the martensite band is predicted to be 90°, and this predicted value can be applied to explain why no locally deformed spiral martensite band was observed in experiments on thin-walled NiTi alloy tubes under torsion.     

Investigation of the multiaxial fatigue behaviour of 316 stainless steel based on critical plane method

In this work, the multiaxial behaviour of 316 stainless steel is studied under the lens of critical plane approach. A series of experiments were developed on dog bone–shaped hollow cylindrical specimens made of type 316 stainless steel. Five different loading conditions were assessed with (a) only tensile axial stress, (b) only hoop stress, (c) combination of axial and hoop stresses with square shape, (d) combination of tensile axial and hoop stresses with L shape, and (e) combination of compressive axial and hoop stresses with L shape. The fatigue analysis is performed with four different critical plane theories, namely, Wang‐Brown, Fatemi‐Socie, Liu I, and Liu II. The efficiency of all four theories is studied in terms of the accuracy of their life predictions and crack failure plane angle. The best fatigue life predictions were obtained with Liu II model, and the best predictions of the failure plane were obtained with Liu I model.     

Failure of Zr<Subscript>50</Subscript>Ti<Subscript>16.5</Subscript>Cu<Subscript>15</Subscript>Ni<Subscript>18.5</Subscript> amorphous metallic ribbon

The deformation and fracture behavior of Zr₅₀Ti_16.5Cu₁₅Ni_18.5 bulk amorphous metal in the form of a thin ribbon have been determined in tensile test at room temperature. The fracture is localized in a major shear band and the fracture angle between the tensile stress axis and the fracture plane is close to 45°. Fractographic observations have revealed that the fracture surface of the amorphous metallic glass consists mainly of a vein-like pattern morphology. We present a scheme of three zones of fracture surface morphology: progressive smooth sliding region (A), dominating vein like pattern (B), and river-like ripples (C). __________ Translated from Problemy Prochnosti, No. 1, pp. 28–31, January–February, 2008.     

Ductility and toughenability study of epoxy resins under multiaxial stress states

The local strains in unmodified and rubber-modified epoxies under multiaxial stress states were examined. Matrix ductility was varied by using epoxide resins of different epoxide monomer molecular weights. The stress state was altered from a plane strain case to a plane stress case by varying the thickness of the test specimens. It was confirmed that, in the case of unmodified resins, the thinner specimens which experienced nearly uniaxial tensile stress exhibited much higher local strains at failure than the thicker counterparts which experienced highly triaxial tensile stress. Also, the cross-link density was reduced as monomer molecular weight increased, thus the increase in local plastic strain due to the stress state change also became greater. Furthermore, it was found that rubber modification markedly increased the plastic strain to failure, irrespective of the specimen dimensions, and that the extent of this plastic strain increased as cross-link density was lowered. These results are consistent with the concept that the cavitation of rubber particles relieves the initial multiaxial constraint in a thick specimen, induces a stress state closer to plane stress throughout the specimen, and consequently enables the matrix to deform to a larger extent. The results also show clearly that the toughenability of a matrix resin is not independent of the stress state and the matrix ductility.