Al-5.4Zn-2.6Mg-1.4Cu合金板材的低周疲劳行为

进行Al-5.4Zn-2.6Mg-1.4Cu合金板材的室温低周疲劳实验,对比研究了轴向平行于轧制方向(RD方向)和垂直于轧制方向(TD方向)试样的低周疲劳行为。结果表明：对于0.4%~0.8%的外加总应变幅,RD和TD方向合金试样的循环应力响应行为均呈现出循环稳定;对于相同的外加总应变幅,TD方向合金的循环应力幅值高于RD方向,而RD方向合金的疲劳寿命高于TD方向。对于RD和TD方向,Al-5.4Zn-2.6Mg-1.4Cu合金的塑性应变幅、弹性应变幅与载荷反向周次均呈线性关系。在低周疲劳加载条件下,裂纹在疲劳试样的自由表面以穿晶方式萌生和扩展。     

双向玻纤织物复合材料双轴拉伸载荷下的力学行为

为研究双向玻纤织物复合材料在复杂应力状态下的力学行为,设计双轴加载十字型试样,对其进行不同载荷比的双轴拉伸实验,对比分析了材料在双轴拉伸载荷下的拉伸模量、拉伸强度及失效模式。结果表明:双向玻纤织物复合材料单轴拉伸行为表现为后期非线性、脆性断裂,双轴拉伸载荷下非线性现象更为显著;双轴拉伸模量随载荷的增大而增加,双轴拉伸载荷对材料的拉伸模量具有一定的强化作用;材料的双轴拉伸强度存在双向弱化效应,等比例双轴拉伸时,双轴拉伸强度最低,仅为单轴强度的60.5%;试样破坏发生于中心实验区域,材料不同载荷比的破坏形式有所不同,分别主要表现为纤维断裂、基体失效和玻纤布分层。     

交变应力下A356铝合金疲劳行为及微结构演变

对处于交变应力下A356铝合金的单轴疲劳寿命以及变形行为进行了研究,并与单一应力加载下的加载情况进行了对比.发现先进行高应力加载后换用低应力加载将会显著延长合金的疲劳寿命.合金的循环应变值主要与合金的循环加载应力幅值有关.此外,利用透射电镜的方法观察了在不同循环加载历史条件下合金中微观结构的变化情况,尤其是位错以及位错带的演变规律.并发现了沉淀物附近的位错塞积现象.     

轧制AZ31镁合金板材(4mm)动态压缩性能与失效行为

为了研究轧制AZ31镁合金板材(4mm)在高应变速率下的动态力学性能和失效行为,采用分离式霍普金森压杆装置(SHPB)在室温下应变速率为500～2600s-1范围内对其进行了动态压缩实验,并利用金相显微镜(OM)和扫描电镜(SM)对冲击后的试样进行了显微分析.探讨了轧制AZ31镁合金板材沿轧制方向(RD)、横向(TD)和法向(ND)的动态压缩性能和失效行为.结果表明:轧制AZ31镁合金4mm板材动态压缩性能存在各向异性.沿RD和TD方向压缩的动态性能相同,沿ND方向压缩的动态断裂强度最大.AZ31镁合金4mm板材的动态压缩断裂机制为解理断裂.变形机制为沿RD和TD方向高速压缩时,{101-2}<112-0>拉伸孪晶参与变形;沿ND方向高速压缩时,{101-1}<112-0>压缩孪晶参与变形.     

带状组织对微合金钢应变局部化及应变强化行为的影响

目的 研究不同加载方向下带状结构对微合金钢塑性变形及应变强化行为的影响。方法 将X80管线钢加热到1200 ℃后分别进行炉冷和空冷热处理,得到多边形铁素体/退化珠光体和粗/细铁素体晶粒带状组织以用于实验研究。将单轴微拉伸实验和数字图像相关方法相结合,通过零变形实验分析系统精度,获取RD、45°和TD取向带状组织试样的拉伸变形规律。同时,在DIC全场变形测量结果与实验数据统计分析结果的基础上,提出了可描述应变强化程度的参量K_f,用于对比和分析不同加载方向下带状结构对应变强化行为的影响。结果 零变形实验中位移平均值和标准差的最大值分别为0.004 pixels和0.006 pixels,系统精度较高。加载方向会影响带状组织的塑性变形演化情况,45°试样中产生了明显与带状取向一致的应变分布特征。在相同应力下45°试样首先达到了较大的应变强化程度,其抗变形能力最低。结论 不均匀带状组织对局部变形的影响程度与2个组成带的力学性能差异有直接联系,参量K_f可在一定程度上描述应变局部化引起的各向异性特征。     

双轴拉伸下高聚物的时间-应力等效原理试验研究

时间-应力等效原理(TSSP)能有效简化高聚物的黏弹性本构模型和力学性能测试。以往研究主要集中在单向应力状态,但工程构件通常处于双向或三向复杂应力状态。为此,首先利用有限元方法对双轴十字型试样进行合理设计与优化,使试样中心测试区的应力和应变基本满足均匀分布,并依此加工出硅橡胶试样。然后,对硅橡胶试样进行不同应力水平及应力比的双轴蠕变试验,获得一系列短期蠕变试验曲线。基于柯西应力和左柯西-格林变形张量定义合适的应力度量和变形度量以探讨双轴拉伸下的TSSP。按照主曲线移位方法,将不同应力水平下的短期性能曲线沿对数时间轴移位构建出宽广时域的主曲线。利用应力移位因子方程对移位结果进行非线性回归分析,获得相应的方程参数。研究结果可为复杂应力状态下黏弹性高聚物的长期力学性能加速表征和长期寿命预估提供理论基础和重要参考。     

双向拉伸Hopkinson斜杆加载方法的探索与研究EI北大核心CSCD

为了实现对材料的双轴同步拉伸加载,现已有通过双向电磁脉冲驱动,以及对称双曲杆产生的双向拉伸波的方式,对试样进行双向动态拉伸加载。试图探索一种结构简单,造价低廉,加工难度小的双向拉伸Hopkinson斜杆加载装置。为了理解斜杆对弹性压缩波传播规律的影响,对斜杆撞击进行了ABAQUS有限元仿真计算和理想验证试验分析。结果发现,斜杆夹角在低于60°和高于90°时,方波平台段出现前高后低的情形,导致波形失真,同时在夹角大于105°之后,失真更为严重。还对多轴加载的同步性问题和双轴加载的数据获取进行了讨论和分析。结果表明,加载波的不同步会导致试样上加载阶跃,从而加载应变率阶跃,这会影响率敏感材料的试验准确性。为了验证该装置的有效性,搭建验证装置对双向拉伸杆上的拉伸波形进行试验验证,取得了较好的试验结果。     

基于不同应力状态的铝合金损伤与变形探究

通过晶界迁移理论对铝合金在零度,三十度,四十五度,六十度以及九十度的拉伸和卸载试验,研究铝合金在在不同的应力状体下变形的晶界迁移以及与这些现象有关的裂纹和断裂。结果表明:铝合金在不同的应力状态下的工程应力明显不同。随着加载角度的不断增加,断裂应变能随驱使晶界移迁的位错弹性能增大的趋势,在四十五度加载时以〈111〉为轴旋转40°的晶界移动最快,需要驱使晶界移迁的位错弹性能最少,断裂应变能达到最小,九十度加载时界面偏析的溶质原子对界面起钉扎作用,阻碍界面迁移,第二相粒子的阻碍作用更为显著,需要驱使晶界移迁的位错弹性能最大,断裂应变能达到最大。     

高温后混凝土在非比例加载下的双轴受压性能研究

混凝土在双向非比例加载下的受压试验研究不多。该文利用大型混凝土静、动三轴试验系统,对常温20℃及200℃―600℃高温后的混凝土,进行了非比例加载的双轴压试验,侧应力等级分别为0、0.2fc、0.4fc、0.6fc四种。测得了混凝土的强度及应变,并根据试验结果,系统地探讨了有侧应力作用时,混凝土在不同温度后的双轴受压强度和变形等力学性能,在此基础上,建立了以八面体应力空间表示的混凝土在非比例加载下的双轴受压破坏准则。这些结论,为受高温后的混凝土复杂结构的设计、分析提供了理论依据。     

E玻璃纤维增强环氧树脂基复合材料轴向拉伸力学性能的应变率效应

采用MTS-810材料试验机、Zwick-HTM5020高速拉伸试验机及分离式Hopkinson拉杆（SHTB）实验装置,并结合数字图像相关性（Digital image correlation,DIC）分析方法,对E玻璃纤维增强环氧树脂基复合材料棒材在10^-3~2 400 s^-1应变率范围内的轴向拉伸力学性能进行了较系统的实验研究,获得了不同应变率下材料的应力-应变曲线,揭示了应变率对材料的拉伸强度和断裂应变的影响规律。通过显微分析拉伸试样的断口形貌,揭示了试样的断裂机制及对应变率的依赖性。实验结果表明：E玻璃纤维增强环氧树脂基复合材料的力学性能具有强烈的应变率效应,归一化拉伸强度随着应变率对数线性增加,而归一化断裂应变则随着对数应变率线性减小;断口显微分析显示：E玻璃纤维增强环氧树脂基复合材料的轴向拉伸断裂模式依赖于应变率,低应变率加载下试样发生沿45°方向的剪切断裂,随着应变率增大,试样断裂模式逐渐过渡到沿轴向的拉伸断裂,特别是在高应变加载下,观察到大量的玻璃纤维丝被拉断,同时环氧树脂基体也发生严重的碎裂现象,这反映了基体材料与玻璃纤维之间相互约束作用在增强。     

A different approach for considering the effect of non-proportional loading path on the forming limit diagram of AA5083

In this paper, the effects of strain path change on the forming limit diagram (FLD) of AA5083 sheet were investigated. The aim is to predict the forming limit curve (FLC) with non-proportional loading path by ductile fracture criteria. For this purpose, some square blanks were pre-strained by uniaxial tension in rolling direction (RD) and transverse direction (TD), and some others were pre-strained by biaxial stretching over a hemispherical punch. Then, the FLD test specimens were prepared by trimming the pre-strained blanks with the longitudinal axis in the RD and TD directions. The out-of-plane formability test was used for obtaining the FLD. The commercial finite element software ABAQUSE 6.9 was used for simulation in accordance with the experimental procedure. For trimming in the simulation environment, a program was written in MATLAB 7.6 that could determine the elements and introduce their properties to the new simulation model. Ductile fracture criteria were used for predicting the failure, and the Hill’79 criterion was used for applying the anisotropic coefficients. The results show that pre-straining in biaxial tension generally reduces the FLC and shifts it to the right-hand side of the FLD, whereas pre-straining in uniaxial tension raises the FLC and shifts it to the left-hand side. The numerical results were compared with the experimental findings, and relatively good agreement was achieved.     

Low‐cycle fatigue behavior of rolled WE43‐T5 magnesium alloy

This paper describes the main results from an investigation into the strength and low‐cycle fatigue (LCF) behavior of a rolled plate of WE43 Mg alloy in its T5 condition at room temperature. The alloy was found to exhibit small tension/compression yield asymmetry and small anisotropy being stronger in transverse direction (TD) than in rolling direction (RD) along with some anisotropy in strain hardening. The LCF tests were conducted under strain‐controlled conditions with the strain amplitudes ranging from 0.6% to 1.4% without the mean strain component. While the stress amplitudes during the LCF were higher for tests along TD than RD, the LCF life was similar for both directions. As revealed by electron microscopy, the fractured surfaces under tension consisted mainly of microvoid coalescence with some transgranular facets, while those fractured in LCF showed a combination of intergranular fracture and transgranular facets with minor content of microvoid coalescence.     

Effect of initial texture on fatigue properties of extruded ZK60 magnesium alloy

The effect of initial texture on cyclic deformation behavior of extruded ZK60 magnesium (Mg) alloy was experimentally investigated under strain‐controlled loading with the strain amplitudes at 4%, 1%, and 0.35%. The testing specimens were taken from extrusion direction (ED), transverse direction (TD), and a material precompressed to 9.4% along the ED (ED_?9.4%). At a high strain amplitude of 4%, the cyclic deformation modes of ED and ED_?9.4% specimens are similar, and they experience twinning exhaustion → slip and detwinning exhaustion → slip during each loading cycle. At a medium strain amplitude of 1%, twinning‐detwinning is involved in the cyclic deformation, but different deformation mechanisms were observed in the 3 different specimens. Partial twinning‐complete detwinning mode dominates the cyclic deformation in the ED specimen, while partial detwinning‐retwinning mode occurs in the ED_?9.4% specimen. For the TD specimen, both basal slip and tension twinning occur during cyclic deformation. At a low strain amplitude of 0.35%, dislocation slips dominate the deformation for the ED specimen with a few observable tension twins. For the ED_?9.4% specimen, initially twined texture increases the ductility of the material and enhances fatigue life as compared with the other 2 specimens.     

拉-压及拉-扭载荷下6063合金的疲劳行为

对6063锻造铝合金进行了不同应力幅值下拉-压及拉-扭复合疲劳试验,并用透射电子显微镜观察了疲劳失效试样的位错结构。结果表明:在循环加载过程中,循环硬化占据主要地位,循环硬化的速率和程度对应力幅值和加载路径有依赖性;相同应力幅值下,试样在拉-扭复合加载下失效比拉-压失效形成更为复杂的位错结构,且位错密度更高;位错之间及位错与析出相的交互作用是材料发生循环硬化的主要原因,循环硬化程度越高,疲劳寿命越短。     

Analysis of anisotropy mechanism in the mechanical property of titanium alloy tube formed through hot flow forming

Anisotropy of mechanical property is an important feature influencing the service performance of tita-nium(Ti)alloy tube component.In this work,it is found that the hot flow formed Ti alloy tube exhibits higher yield strength along circumferential direction(CD),and larger elongation along rolling direction(RD),presenting significant anisotropy.Subsequently,the quantitative characteristics and underlying mechanism of the property anisotropy were revealed by analyzing the slip,damage and fracture behav-ior under the combined effects of the spun{0002}basal texture and fibrous microstructure for different loading directions.The results showed that the prismatic slip in primary α grain is the dominant defor-mation mechanism for both loading directions at the yielding stage.The prismatic slip is harder under CD loading,which makes CD loading present higher yield strength than RD loading.Additionally,the yield anisotropy can be quantified through the inverse ratio of the averaged Schmid Factor of the activated prismatic slip under different loading directions.As for the plasticity anisotropy,the harder and slower slip development under CD loading causes that the CD loading presents larger external force and normal stress on slip plane,thus leading to more significant cleavage fracture than RD loading.Moreover,the micro-crack path under RD loading is more tortuous than CD loading because the fibrous microstructure is elongated along RD,which may suppress the macro fracture under RD loading.These results suggest that weakening the texture and fibrous morphology of microstructure is critical to reduce the differences in slip,damage and fracture behavior along different directions,alleviate the property anisotropy and optimize the service performance of Ti alloy tube formed by hot flow forming.     

Biaxial cyclic deformation of an epoxy resin: Experiments and constitutive modeling

Biaxial (proportional and non-proportional) cyclic tests were conducted on thin-walled tubular specimens to investigate deformation behavior of an epoxy resin, Epon 826/Epi-Cure Curing Agent 9551. The focus was placed on the biaxial stress-strain response and their dependency on the load control mode, stress or strain range and loading path. Experimental results indicated that under strain-controlled equi-biaxial (proportional) cyclic loading, mean stress relaxation occurred in both axial and hoop directions, whereas under stress-controlled equi-biaxial cyclic loading, ratcheting strains accumulated in both principal directions. Under strain- or stress-controlled non-proportional cyclic loading, anisotropy in stress-strain responses was induced in both axial and hoop directions, and the axial and hoop hysteresis loops rotated in opposite directions. This was particularly evident at high stress or strain levels. The experimental results were further used to evaluate the predictive capabilities of a nonlinear viscoelastic constitutive model. Qualitative and quantitative comparison with the test data indicated a good agreement in predicting the complex stress-strain response under biaxial cyclic loading with various loading paths, applied stress or strain ranges and loading control modes.     

Orientation dependencies of mechanical response, microstructure and texture evolution in hot compression of AZ31 magnesium alloy processed by equal channel angular extrusion

The anisotropic mechanical behavior during hot compression of an AZ31 Mg alloy processed by equal channel angular extrusion (ECAE) was evaluated and then discussed in correlation with the concurrent microstructure and texture evolution. The results revealed apparent orientation-dependencies in the mechanical responses, microstructure, and texture development in uniaxial compression along two perpendicular directions. Compression along the transverse direction (TD) led to a higher hardening rate, higher peak stress, and earlier softening than those obtained in compression along the extrusion direction (ED). This can be attributed to the differences in the initial textures prior to compression along the two directions, which led to a more significant contribution of tensile twinning at the early stage of straining and consequently more extensive dynamic recrystallization in loading along TD than along ED. These results suggest that the deformation behavior in compressive loading of the ECAE-processed Mg alloy is highly anisotropic, which needs to be taken into account in their applications.     

Biaxial tension fatigue response of concrete

Concrete structures such as rigid airport pavements are subjected to repeated high-amplitude loads resulting from passing aircraft. The resulting stress-state in concrete is a biaxial combination of compression and tension. It is of interest to understand the response of plain concrete to such loading conditions, which will enable development of realistic material models for implementation in mechanistic pavement design procedures.The objective of this work is to characterize the quasi-static and low-cycle fatigue response of concrete subjected to biaxial stresses in the biaxial tension region, where the principal tensile stress is larger than or equal in magnitude when compared with the principal compressive stress. An experimental investigation of material behavior in the biaxial tension region is conducted. The experimental setup consists of the following test configurations: (a) notched concrete beams tested in three-point bend configuration, and (b) hollow concrete cylinders subjected to torsion.Failure of concrete in the biaxial tension region is shown to be a local phenomenon under quasi-static and fatigue loading, wherein the specimen fails owing to a single crack. The crack propagation is studied using the equivalent elastic crack concept. It is observed that the crack growth rate in constant amplitude fatigue loading exhibits a two-phase process: a deceleration phase followed by an acceleration stage. The crack growth in the acceleration stage is shown to follow Paris law. The model parameters obtained from uniaxial fatigue tests are shown to be sufficient for predicting the considered biaxial fatigue response.     

Evolution of mechanical properties for a dual-phase steel subjected to different loading paths

The evolution of the mechanical properties of a dual-phase (DP590) steel sheet after being prestrained by uniaxial tension, plane strain and equal biaxial stretching was investigated. Specimens were first loaded using the three prestraining modes. Then, from the prestrained specimens, a few sub-sized samples were machined along the rolling direction and the transverse direction for further uniaxial tension testing. Six loading paths were provided. Equal biaxial stretching was performed using a cruciform specimen. The evolution of work hardening performance, elastic modulus, yield stress and tensile stress under the six loading paths were discussed in detail. The results indicate that loading paths can affect the latent work hardening performances, strain hardenability, yield stress and tensile stress evolution as well as the elastic modulus decrease during plastic deformation. The uniaxial tension–uniaxial tension path results in a cross-softening phenomenon, the largest yield stress enhancement and a mild maximum tensile stress increase. The equal biaxial stretching-uniaxial tension path leads to a cross-hardening phenomenon, the least yield stress enhancement and the largest tensile strength increase maximum tensile strength. The elastic modulus of DP590 steel not only changes with the accumulated plastic strain but also varies with the loading paths. The largest decrease of the elastic modulus equal biaxial stretching–uniaxial tension can reach 12.7% beyond 8% equivalent strain, which is 5.2% greater than that in the monotonic uniaxial tension path.